X-Planes: Bell X-22

The Bell X-22A had four propellers, each inside a tilting duct, two forward and two aft. It first flew on 17 March 1966, one of the two aircraft was destroyed five months later, and the survivor went on flying into the 1980s, until October 1984 in the summary of the Wikipedia entry and until 1988 in the account of the Vertical Flight Society. Even the earlier date makes a research career of about eighteen years, twice the nine years and four months over which the X-15 flew. This article is the twenty-third in the X-Planes series, following the X-1, the X-2, the X-3, the X-4, the X-5, the X-6, the X-7, the X-8, the X-9, the X-10, the X-11, the X-12, the X-13, the X-14, the X-15, the X-16, the X-17, the X-18, the X-19, the X-20, and the X-21.

It did not spend those years being an X-22. From 1968 it carried a variable-stability system that let it imitate the handling of other aircraft in flight, and what it produced was not data about four-ducted-propeller aeroplanes but data about how any vertical-takeoff aircraft ought to handle, a career that NASA TM-81156 and the Vertical Flight Society both describe. The most successful vertical-takeoff research aircraft of the period succeeded by ceasing to be a prototype and becoming an instrument.

The standard inventory entry is Jenkins Landis and Miller 2003 American X-Vehicles, An Inventory X-1 to X-50 and the vehicle compilation is Miller 2001 The X-Planes, X-1 to X-45. The programme’s retrospective is The Bell X-22A V/STOL, Variable Stability Research Airplane, Lessons Learned.

The Research Question

The series has now met four ways of getting an aeroplane off the ground vertically. The X-13 stood on its tail. The X-14 deflected jet thrust. The X-18 tilted its whole wing and the X-19 tilted four propellers mounted on two tandem wings. The X-22 tilts four propellers as well, but it puts each one inside a duct, as the Vertical Flight Society describes it, and almost everything that distinguishes this aeroplane follows from that decision.

A shroud around a propeller is not a fairing. It changes the momentum balance, it carries part of the thrust on its inner surface, and in forward flight it behaves like a short annular wing. Its period theory is in Kruger 1949, Lerbs 1953, Thwaites 1953, Van Niekerk 1958, Theodorsen 1960, Ordway et al 1960, Morgan 1961, and Greenberg and Ordway 1964, its static tests begin with Platt 1948, and the large-scale tests of the period are summarised in Mort 1966. The full literature appears below under ducted propellers, shrouds and lift fans.

The question this article asks is what the duct is worth and what it costs, because the answer turns out to be large in both directions and the two effects arrive at different points in the flight.

The period record behind that question is very large, and this article’s weight sits on it. Between the war and the end of the 1960s the vertical-takeoff problem was worked over by many laboratories with wind tunnels, across ducted and open propellers, tilting wings, deflected slipstreams and jet lift. Examples are the ducted-propeller assault transport studies in Zabinsky 1956, the static thrust of open propellers in Coward 1955, tilt-wing design in McCormick and Mallen 1957, the deflected slipstream in Kuhn and Grunwald 1960, jet lift in Smith 1961, and the state of the art as the NASA conferences and the national survey of the period recorded it in NASA Conference on V/STOL 1960, and Campbell 1964. The period records are spread across the clusters below, with the largest shares in the clusters on ducted propellers and on vertical and short takeoff configurations. Every aircraft in this part of the series was designed on top of it.

Programme Origin

The Navy requested two prototypes powered by four ducted fans in 1962 and ordered the X-22A in 1964, to give insight into the tactical use of vertical-takeoff troop transports. Bell had already flown the X-14 and had extensive experience of vertical-takeoff aircraft, in the Wikipedia entry, and the four-duct tandem layout had been under investigation in wind tunnels for several years before the contract, with its tunnel literature, early and recent, in Larry A. Young, Ronald C Busan et al, Siena K S Whiteside and Beau P Pollard, Goodson and Yaggy 1960, Tapscott and Kelley 1960, Kelley 1962, McKinney and Newsom 1962, Clark et al 1963, Newsom 1963, Curtiss 1965, Biggers et al 1966, Freeman and Newsom 1966, Xiang et al 2016, Geuther et al 2020, Zhang et al 2020, Lee et al 2021, Meng et al 2026.

The X-22A was the Navy’s part of the Tri-Service Assault Transport Program, in the history kept by the Vertical Flight Society, and its layout was chosen by arithmetic about engines. A NASA design study of a modern tiltduct, Whiteside and Pollard, reports from the Bell design study that cross-shafted arrangements of two, three, four and five ducts were considered, and that four were chosen on engine weight and availability with the thrust-to-weight ratio needed for an engine-out hover in view. The variable-stability system was designed into the aircraft before it flew, as Beilman’s 1965 description of the X-22A variable stability system shows.

Two aircraft were built. The first was rolled out on 25 May 1965 and made its first hovering flight in March 1966, according to the Vertical Flight Society. It was written off in August of the same year after a hard landing, which the Wikipedia entry attributes to the failure of a propeller control and the Vertical Flight Society to a hydraulic failure after about three hours of flying. The two accounts are compatible if the hydraulic failure was in the propeller pitch actuation, but no primary account of the accident is available. The second aircraft carried the programme on, and the Wikipedia entry reports that technicians made it flight capable with components stripped from the first.

The Aircraft

Four ducts of 7 feet diameter, each containing a three-bladed propeller. The rear wing spans 39 feet 3 inches and the forward one 22 feet 11 inches over the ducts, the aircraft is 39 feet 7 inches long, and it weighs 10,478 pounds empty against a gross weight of 15,287 and a maximum takeoff weight of 17,644, in the specification of the Wikipedia entry.

The duct itself is documented at full scale. A full-scale duplicate of the X-22A ducted propeller was tested in the Ames 40 by 80 foot wind tunnel, in Gamse and Mort 1967, and their table of dimensions gives a duct chord of 49 inches, a propeller tip clearance of 0.4 inch, a net area of 37.84 square feet at the propeller, a net exit area of 44.08 square feet and an exit vane of 31.8 square feet.

Four General Electric YT58-GE-8D turboshaft engines of 1,250 shaft horsepower each drive all four propellers through a cross-shafted transmission, so that any engine can drive any duct, as both the Vertical Flight Society and NASA TM-81156 describe. The 1,250 horsepower is also the design power per duct at which Gamse and Mort 1967 evaluated the duct’s thrust.

\[A_{d} = \frac{\pi D^{2}}{4} = \frac{\pi \times 7^{2}}{4} = 38.48\ \text{ft}^{2}\]

Four of those give 153.9 square feet of disc, and at 14,600 pounds, the three-engine hover weight assumed here, which no primary document read for this article states,

\[\frac{W}{A} = \frac{14{,}600}{153.9} = 94.8\ \text{lbf/ft}^{2}\]

That is a very high disc loading. A transport helicopter sits roughly between five and ten, the CH-47D Chinook at 8.8 pounds per square foot in the table of disc loadings, although light helicopters sit lower and the heaviest higher. The X-19 used large propellers and sat far below it, at 25.7 pounds per square foot, but the X-18, at 82.1, was not far short of it. The X-22 is closer to a lift jet than to a rotorcraft, and the consequences run through everything that follows.

Sizing From First Principles

What the Shroud Buys

Take a propeller inside a duct whose exit area is $\sigma$ times the disc area. Continuity and momentum across the disc give the thrust and the ideal power in terms of the exit velocity,

\[T = \rho \sigma A V_{e}^{2} \qquad P = \tfrac{1}{2} \rho \sigma A V_{e}^{3}\]

Eliminating $V_e$,

\[P = \frac{1}{2} \frac{T^{3/2}}{\sqrt{\rho \sigma A}} \qquad \Rightarrow \qquad T \propto \sigma^{1/3}\]

An open propeller is the case $\sigma = 1/2$, because its wake contracts to half the disc area far downstream, and substituting that recovers the familiar $T^{3/2}/\sqrt{2\rho A}$. So a duct that merely prevents the contraction, with $\sigma = 1$, gives

\[\frac{T_{\text{duct}}}{T_{\text{open}}} = (2\sigma)^{1/3} = 2^{1/3} = 1.26\]

Twenty-six percent more static thrust for the same power, from a ring of metal that does no work. A mildly diffusing duct does better still. The configuration exists for that gain, which makes a seven-foot propeller viable at ninety-five pounds per square foot.

The X-22A duct is one of those. Its measured area ratio is

\[\sigma_{d} = \frac{A_{e}}{A_{p}} = \frac{44.08}{37.84} = 1.165\]

with $A_e$ the net exit area and $A_p$ the net area at the propeller, from the table in Gamse and Mort 1967, and the same ideal relation then gives

\[\frac{T_{\text{duct}}}{T_{\text{open}}} = (2\sigma_{d})^{1/3} = 2.330^{1/3} = 1.33\]

The hover arithmetic below keeps $\sigma = 1$ and lets the figure of merit absorb the difference.

The tunnel measured the real thing. Gamse and Mort 1967 report a maximum figure of merit of 81 percent based on the net exit area and a maximum propulsive efficiency of 74 percent. For free-stream speeds up to about 100 knots the measured thrust at the design 1,250 horsepower follows ideal ducted momentum theory with a propeller efficiency of 90 percent. Above that speed it falls away from the ideal curve, and they attribute the shortfall to the drag of the duct, centre body and struts, most of which is not skin friction.

The ideal power to hover 14,600 pounds on 153.9 square feet is 3,750 horsepower for an open rotor and

\[P_{\text{ideal}} = \tfrac{1}{2} \frac{W^{3/2}}{\sqrt{\rho A}} = 2{,}651\ \text{hp}\]

for the ducted case, which is $1/\sqrt{2}$ of it.

What the Quoted Margin Means

The programme is described by the Vertical Flight Society as having 35 percent excess power so that it could hover with one engine out. The NASA survey of in-flight simulators, NASA TM-81156, states the same number as a vertical thrust-to-weight ratio of 1.35 that allows a single-engine-out hover. The two readings give very different margins, and the arithmetic shows how different.

Four engines give 5,000 shaft horsepower and three give 3,750. If the installed power exceeds what hovering requires by 35 percent, then what remains after losing an engine is

\[\frac{3}{4} \times 1.35 = 1.0125\]

One and a quarter percent. On the power reading the number is the smallest that makes a three-engine hover arithmetically possible, with nothing to spare.

On the thrust reading, thrust at a fixed figure of merit scales as power to the two-thirds, so losing a quarter of the power leaves

\[\frac{T_{3}}{W} = 1.35 \left( \frac{3}{4} \right)^{2/3} = 1.35 \times 0.8255 = 1.114\]

an eleven percent thrust margin with an engine out, the margin the NASA report’s sentence describes. The NASA wording is the primary statement, so the thrust reading is the documented one, and the power reading that follows tests the secondary figure without describing the aircraft.

The power reading is still checkable, because it fixes the figure of merit. If hovering at 14,600 pounds requires 5,000 divided by 1.35, then

\[\text{FM} = \frac{P_{\text{ideal}}}{P_{\text{required}}} = \frac{2{,}651}{3{,}704} = 0.716\]

That is a thoroughly ordinary figure of merit for a ducted lift fan, so the reading passes the first check. The second is stronger. Feeding 0.716 back through momentum theory gives the weight that three engines can lift,

\[W_{3} = W \left( \frac{P_{3} \, \text{FM}}{P_{\text{ideal}}} \right)^{2/3} = 14{,}721\ \text{lbf}\]

against the 14,600 assumed here. Agreement to 0.83 percent shows that the 35 percent and the 14,600 pounds are consistent with each other on the power reading, but because no primary document read here states the 14,600 pounds, the agreement checks two secondary figures against each other and not against the aircraft.

The full-scale tunnel figure separates the readings only partly. A figure of merit referred to the net exit area converts to the disc-area basis used here through the ideal power, which varies as the inverse square root of the area,

\[\text{FM}_{\text{disc}} = \text{FM}_{\text{exit}} \sqrt{\frac{A_{e}}{A_{d}}} = 0.81 \sqrt{\frac{44.08}{38.48}} = 0.867\]

On the power reading the installed figure of merit of 0.716 is 83 percent of the tunnel’s best, and the difference would have to be accounted for by transmission losses, download on the wing and fuselage, and duct interference. On the thrust reading, the tunnel’s best figure of merit and the rated 5,000 horsepower give a four-engine thrust of

\[T_{4} = \left( 2\,\text{FM}_{\text{disc}}\, P \sqrt{\rho A} \right)^{2/3} = 20{,}261\ \text{lbf}\]

and a thrust-to-weight ratio of 1.35 at 15,008 pounds, essentially the 15,000 pound design gross weight that Gamse and Mort 1967 use to set their trim conditions. Three engines would then hover 16,725 pounds. That agreement assumes no installation loss at all, so it is an upper bound, and the record read here does not settle which reading the programme intended. What it does settle is that the NASA survey gives the 35 percent as thrust.

On the power reading, with the figure of merit of 0.716, the same arithmetic is unkind about the rest of the weight range.

condition	weight, lbf	power required, shp	four engines	three engines
three-engine vertical takeoff	14,600	3,704	+35.0%	+1.3%
gross	15,287	3,968	+26.0%	−5.5%
maximum takeoff	17,644	4,920	+1.6%	−23.8%

On that reading, at maximum takeoff weight the X-22 could not hover on three engines and had 1.6 percent in hand on four. Vertical takeoff was available at the bottom of the weight range and nowhere near the top, the recurring fact about every vertical-takeoff aircraft in this series, here put in numbers.

The Price in Downwash

High disc loading is not free at the bottom of the aircraft either. The exit velocity follows from the thrust per duct,

\[V_{e} = \sqrt{\frac{T}{\rho \sigma A_{d}}} = \sqrt{\frac{3{,}650}{0.0023769 \times 38.48}} = 200\ \text{ft/s}\]

One hundred and thirty-six miles per hour, straight down, from four ducts at once.

Disc loading is the parameter that sets all of this, and the relations are short. The induced velocity and the power loading follow from it alone,

\[v_{e} = \sqrt{\frac{W/A}{\rho}} \qquad \frac{T}{P} = \frac{2\,\text{FM}}{v_{e}}\]

so that lifting a pound costs power in proportion to the square root of the disc loading.

$W/A$, lbf/ft²	$v_e$, ft/s	lbf per horsepower
5	45.9	17.2
10	64.9	12.1
40	129.7	6.1
94.8	199.7	3.9

A transport helicopter of the period sat between five and ten and lifted about three and a half times as much per horsepower. The measurement and prediction of hover performance across that range is in Dyke 1963, Shivers 1967, Kocurek and Tangler 1977, Felker 1993, Ramasamy et al 2010, Overmeyer and Martin 2017, and Jimenez Garcia and Barakos 2018, with the rest under hover performance below, and that literature quantifies the penalty for choosing a small, heavily loaded disc. The penalty is the standing charge for choosing ducts, and it is paid in every second of hover. The dynamic pressure in that jet is 47 pounds per square foot. The ground erosion, the recirculation and the hazard to anyone nearby are all consequences of the same choice that made the duct worth having, and they are treated in Morse and Newhouse 1960, Obryan 1960, Morse and Newhouse 1961, O’Bryan 1961, George et al 1964, Kuhn 1965, Pruyn and Taylor 1971, and Ison 2024, and in the downwash cluster below.

What the Shroud Charges

A duct held at an angle to the oncoming air captures a mass flow and turns it onto the duct axis. Turning momentum requires a force, and the reaction on the duct is a normal force,

\[N_{\text{mom}} = \dot{m} V_{\infty} \sin\alpha \qquad \dot{m} = \rho A_{d} \left( V_{\infty} \cos\alpha + v_{i} \right)\]

The rounded lip adds a suction force, taken here to be of the same order, so the total is assumed to be roughly twice the momentum term. The duct is behaving as an annular wing, and the theory and measurement of that are Goodson and Grunwald 1962, Mort 1965, Curtiss and Howard C. 1969, Mann 1974, Walker 1981, Lee and Kwok 1983, Gill and D’Andrea 2020.

This has to be evaluated along a conversion, not on a grid. A duct angle of forty-five degrees at two hundred feet per second is not a flight condition, and tabulating it produces a normal force larger than the aeroplane.

The Corridor Can Be Derived, Not Assumed

No wing area appears in the sources read for this article, which looks like a dead end and is not one. At the end of a conversion the ducts are horizontal and the wings carry the whole weight, so the conversion speed fixes the area,

\[S = \frac{2W}{\rho V_{\text{conv}}^2 C_{L,\max}} = \frac{2 \times 14{,}600}{0.0023769 \times 203^2 \times 1.4} = 213\ \text{ft}^2\]

That number has to survive a geometric check and it does. Two thirds of it on the 39.25 foot rear span is a mean chord of 3.62 feet, and the remaining third on the 22.92 foot forward span is 3.10 feet. No chord appears in those sources either, so the check is one of plausibility rather than agreement.

With an area the corridor follows from the force balance. Writing $i$ for the duct angle above the horizontal,

\[T \sin i + L = W \qquad T \cos i = D\] \[\tan i = \frac{W - L}{D} \qquad T = \sqrt{(W-L)^2 + D^2}\]

with $L = q S C_L$ and $D = q (S C_{D_w} + f)$, where $f$ is the equivalent flat-plate area of the four ducts computed below and the wing drag coefficient $C_{D_w}$ is the 0.10 assumed here. The trim of a vertical-takeoff aircraft through its conversion has a period literature, in Seibel 1963, Englar and Kirkpatrick 1969, Cheng and Pei 2023, Lee et al 2026, and the quantity it turns on is how much lift the wing is developing at each speed, which for a propeller aircraft depends on more than the wing. The slipstream changes the lift over whatever part of the wing it washes, and that effect has been measured since the 1930s, in Stuper 1938, Franke and Weinig 1939, Brenckmann 1958, Kuhn and Grunwald 1960, Winston and Huston 1962, Goland et al 1964, Butler et al 1966, and Rizk 1980, with more under propeller slipstream and wing interaction below.

Thrust required falls monotonically through the conversion, because the wing takes the lift faster than the drag builds, so the corridor was never thrust-limited. Whatever made the X-22 hard to convert, it was not power.

$V$, ft/s	knots	duct angle	$\dot m$, slug/s	$N$, lbf	$N/T$
20	11.8	81.1	18.6	733	0.20
60	35.5	63.4	20.7	2,224	0.61
100	59.2	45.7	24.7	3,528	0.97
120	71.1	36.8	27.1	3,890	1.07
140	82.9	27.9	29.6	3,881	1.06
180	106.6	10.2	34.5	2,197	0.60
200	118.5	1.3	36.6	340	0.09

The normal force peaks at about the thrust of its duct, and it peaks in the middle of the conversion. The table samples the conversion at seven speeds and the true maximum lies between two of its rows, at 130 feet per second, where the normal force reaches 3,941 pounds against a duct thrust of 3,650, a ratio of 1.08.

The linear schedule is close to what was flown. The Calspan account of the third research programme, Lebacqz and Aiken 1975, states that for the X-22A the centre of the allowable transition corridor is well approximated by a linear relationship between duct angle and flight velocity, and notes that tilt-duct aircraft have a fairly narrow corridor bounded by buzz and buffet.

The Corridor With the Wing at Full Lift

The linear schedule is not the only one a force balance permits. Evaluating the same normal force along the derived corridor, where the wing pulls its full lift coefficient at every speed and the ducts make up the difference, gives something much worse. The ducts stay at large angles far later than a linear ramp suggests, and the momentum estimate climbs to 8,266 pounds per duct at 180 feet per second, or 2.26 times the whole aircraft weight over four ducts.

That is not a credible force and saying so is the point. More than twice the weight, applied sideways in mid-conversion, would be unflyable and unmissable. So the model is wrong there, and the useful question is where it stops being right.

The momentum-turning argument assumes the duct captures its streamtube and turns all of it. That holds while the crossflow component is small compared with the velocity the duct itself induces, so the validity parameter is

\[\mu = \frac{V_{\infty} \sin i}{v_{i}}\]

$V$, ft/s	knots	duct angle	$\mu$	$N/T$	status
40	23.7	89.6	0.20	0.40	model holds
60	35.5	89.2	0.30	0.60	model holds
100	59.2	87.2	0.50	1.02	marginal
140	82.9	82.1	0.69	1.52	model invalid
180	106.6	60.7	0.79	2.26	model invalid
200	118.5	11.2	0.20	0.77	model holds

The parameter passes 0.5 at 100 feet per second and comes back below it at 195. The momentum model is therefore valid at both ends of the conversion and invalid in the middle, and the middle is exactly where a tilt aircraft is hard to fly.

Momentum does not cap the force in that band. The lip cannot hold the flow round it at that incidence and separates, and this known weakness of every ducted fan now has a computed boundary. The honest statement is that the normal force reaches about the duct’s thrust where the model can be trusted, and that beyond it the binding limit is lip stall. Over four ducts that is of the order of the aircraft weight, applied perpendicular to the duct axes, at the speed where the wings are not yet working and the ducts are no longer pointing at the ground.

The measured record places lip separation in descent rather than in level conversion. On the full-scale duct, Gamse and Mort 1967 found from pressure distributions that separation over the whole upstream lip, the kind that changes forces and power, occurs on a representative four-duct configuration only at low power and high duct angle, as in descending flight, while partial separation in level flight brings no large change in force. The large-scale tandem model of Biggers et al 1966 showed duct stall only on the front ducts and limited the estimated descent rate of a 6,500 pound aeroplane of that layout to about 600 feet a minute at a duct incidence near 60 degrees. In flight, the initial Navy tests reported by Rhodes in 1969 and summarised in NASA CR-1901 identified lip separation as the cause of duct buzz, a high-frequency vibration through the airframe, and suggested a descent-angle limit of about 10 degrees at low speed. At 40 knots, or 67.5 feet per second, a 10 degree path is a sink rate of

\[\dot{h} = 60\, V \sin\gamma = 60 \times 67.5 \times \sin 10^{\circ} = 703\ \text{ft/min}\]

which is the same order as the tunnel estimate.

The measured pitching moment peaks earlier than the momentum normal force. Biggers and his colleagues found the untrimmed pitching moment largest at about 60 degrees of duct incidence, and Whiteside and Pollard summarise the earlier Ames semispan tests as placing the pitch-up maximum near 45 knots at a duct angle near 60 degrees. On the linear schedule, with the ducts vertical at rest and horizontal at 203 feet per second,

\[i = 90^{\circ} \left( 1 - \frac{V}{V_{\text{conv}}} \right) \qquad V_{60^{\circ}} = \frac{203}{3} = 67.7\ \text{ft/s} = 40\ \text{kt}\]

against 130 feet per second, or 77 knots, where the momentum estimate of the normal force peaks. The moment the controls must trim arrives in the first third of the conversion, and the momentum-turning model, which places the whole force at one point on the lip, does not locate it.

That is the charge for the twenty-six percent. The shroud that makes the hover affordable is an annular wing that has to be flown through the conversion, and the aeroplane which needs the shroud most in hover is the one that has the most trouble in between.

The Ducts Are Also the Drag

The maximum speed in the specification of the Wikipedia entry, 278 knots at 10,000 feet, provides a check on all of this from a direction the hover calculation does not touch, although the record disagrees about whether it was flown. The Vertical Flight Society gives a maximum of 315 miles an hour, or 274 knots, Wikipedia’s own text says that the required 283 knots was never reached, and NASA TM-81156 gives 150 knots as the maximum speed of the aircraft in its research configuration. Taking 278 knots, an installed 5,000 shaft horsepower and the propulsive efficiency of 0.80 assumed here,

\[D = \frac{\eta P}{V} = \frac{0.80 \times 5{,}000 \times 550}{469.2} = 4{,}689\ \text{lbf}\] \[f = \frac{D}{q} = \frac{4{,}689}{193.3} = 24.3\ \text{ft}^2\]

The X-22 has 24.3 square feet, and the lift-to-drag ratio at that condition is 3.26.

Now the same number from geometry. Four ducts of seven-foot diameter present 153.9 square feet of frontal area, so the equivalent flat plate implies a duct drag coefficient of

\[C_{D,\text{duct}} = \frac{f}{4 A_{d}} = \frac{24.3}{153.9} = 0.158\]

which is derived and cannot be checked, since the full-scale duct test gives no drag coefficient on frontal area in its text. Estimating the drag of external bodies on an aeroplane in this way is old, standardised work, in Bonner et al 1975, Neal 1975, Smetana 1975, Corsiglia et al 1979, Corsiglia et al 1981, Malone and Peavey 1999.

The measured propulsive efficiency is lower than the one assumed. Gamse and Mort 1967 report a maximum of 74 percent for the full-scale duct, and with that value the same relations give

\[D = \frac{0.74 \times 5{,}000 \times 550}{469.2} = 4{,}337\ \text{lbf} \qquad f = \frac{4{,}337}{193.3} = 22.4\ \text{ft}^{2} \qquad C_{D,\text{duct}} = \frac{22.4}{153.9} = 0.146\]

so the duct drag coefficient lies between 0.146 and 0.158 on the two efficiencies. Their tunnel result also shows where the drag comes from. The drag measured with the propeller removed was much larger than the estimated skin friction of the shroud, and they conclude that a duct designed for high speed could have had a significantly higher propulsive efficiency.

That is the third face of the same trade. The shroud buys twenty-six percent of thrust in the hover, charges an annular wing’s normal force in the conversion, and then charges again in cruise by presenting four times thirty-eight square feet of frontal area to the airstream for the rest of the flight.

Dependent Systems

The Elevons Are in the Jet

In hover there is no free-stream dynamic pressure at all, so a conventional control surface has nothing to work with. The duct exits have plenty.

\[q_{\text{jet}} = \tfrac{1}{2} \rho V_{e}^{2} = 47.4\ \text{lbf/ft}^{2}\]

The duct normal force acts near the lip rather than at the duct centre, because the lip is where the flow is turned, so each duct contributes a nose-up couple of about

\[M = \frac{N \ell}{2}\]

for a duct of length $\ell$. At the largest defensible normal force and the four-foot duct, whose chord is 49 inches in the table of Gamse and Mort 1967, that is 7,900 foot-pounds per duct and 31,500 foot-pounds over four, and the tandem layout does not cancel it, because both duct pairs tilt the same way and both couples act nose-up about their centres.

Something has to trim that, and the X-22A uses two effectors whose roles exchange through the conversion. The ducts carry spanwise elevons across their exits, immersed in the propeller slipstream, and the propellers have collective and differential blade pitch. In the hover, according to NASA TM-81156 and Freeman and Newsom 1966, pitch and roll come from differential blade pitch between front and rear and between left and right. Yaw comes from differential elevon deflection in Freeman and Newsom, because a vane in a vertical jet turns the jet sideways, while NASA TM-81156 assigns hover yaw to left and right differential blade pitch, and the documents read do not reconcile the two. A mechanical mixer proportions the pilot’s commands between propellers and elevons as a function of duct angle, and in forward flight pitch comes from differential deflection of the front and rear elevons. In the large-scale tunnel model of Biggers et al 1966, longitudinal trim came from differential fore and aft exit-vane deflection between cruise and 40 degrees of duct incidence and from differential fore and aft thrust above 50 degrees.

The elevons are large. The exit vane of the full-scale duct is 31.8 square feet, and the elevons of the 0.18-scale model of Freeman and Newsom scale up by the square of the length ratio to

\[S_{e} = 0.82 \times 5.56^{2} = 25.3\ \text{ft}^{2} \qquad S_{e} = 1.06 \times 5.56^{2} = 32.8\ \text{ft}^{2}\]

for the front and rear ducts. At the lift coefficient of 0.9 assumed here, a vane of 31.8 square feet in the hover jet gives

\[F = q_{\text{jet}} S_{e} C_{L_{e}} = 47.4 \times 31.8 \times 0.9 = 1{,}357\ \text{lbf}\]

which is more than a third of the duct thrust, a ratio of 0.37, turned sideways. With that force, one vane per duct is enough for yaw in the hover.

The free stream does not reach that dynamic pressure until about 200 feet per second, which is the end of the conversion, so through the conversion the elevons depend on the propeller slipstream. The design provided control power beyond trim in a 35 knot wind of 0.60 radian per second squared in pitch, 1.00 in roll and 0.56 in yaw, in Freeman and Newsom, whose free-flight model pilots needed about 110, 80 and 30 percent of the scaled values for the manoeuvres they flew. By 1979 the variable-stability aircraft had nominal hover control powers of 3.20, 3.45 and 0.75 radians per second squared in pitch, roll and yaw, according to NASA TM-81156.

The Cross-Shaft

Four engines drive four propellers through a common transmission, treated in Leishman 1966, Overfield and Crawford 1967, Envia 2016, Casadei et al 2019, Yan and Chen 2019, Yu et al 2020, Leelaburanathanakul et al 2021, Hoogreef and Soikkeli 2022, Kang et al 2023, Zhang et al 2024, Aroussi 2025, Hung and Dai 2025, Nabadova and Sabziev 2025, Jokar and Khoshnood 2026, Mokotoff and Cinar 2026.

Cross-shafting adds no power. What it changes is the failure mode. Without it, losing an engine loses a duct, and losing one of four ducts on a machine with a rear semi-span of 19.6 feet applies

\[M = T_{\text{duct}} \, y = 3{,}650 \times 19.62 = 71{,}600\ \text{ft}\cdot\text{lbf}\]

of rolling moment that has to be trimmed out by the remaining three. With cross-shafting the same failure removes a quarter of the power and none of the symmetry.

The shaft itself is undramatic in torque. At the 700 foot per second tip speed assumed here the propellers turn at about 1,910 revolutions per minute, inside the range of 1,200 to 2,590 at which Mort and Gamse ran the full-scale duct, so each absorbs

\[Q = \frac{P}{\omega} = \frac{1{,}250 \times 550}{200} = 3{,}440\ \text{ft}\cdot\text{lbf}\]

and with one engine out the three survivors deliver 938 horsepower to each of four propellers, so 312 horsepower has to cross the airframe, a quarter of one engine.

It was not undramatic in heat. By 1979 the engine and fan systems were limited operationally to hover at air temperatures below 80 degrees Fahrenheit by gearbox temperature limits, which NASA TM-81156 notes precluded significant amounts of hover in the summer, and allowance for an engine-out hover limited operation further according to atmospheric conditions.

Sizing a drive train of this kind is rotorcraft engineering rather than aeroplane engineering, treated in Doblhoff 1956, Ward 1970, Cormier 1979, Dean 1979, Mancini 1983, White 1983, Albrecht 1988, Tokarski et al 1994.

The cost of cross-shafting is not the shaft. It is that every gearbox must accept power from either direction, and that the train becomes a single path whose failure takes everything. The first aircraft was lost to the failure of a part that the test pilot Stanley Kakol, in the account of the Wikipedia entry, called the only non-redundant component in the power chain, which is the same lesson from the other side.

This is the X-18’s fatal deficiency, answered. The X-18 had two engines with no interconnection, and its flight testing ended after an engine failure in one account and a propeller pitch control problem that put it into a spin in another, which is the asymmetry rather than the shortfall. The X-22 was designed so that the asymmetry cannot happen, and the price is a transmission that has to carry full power to any duct from any engine.

The Flight Test Record

First flight 17 March 1966. The first aircraft was lost on 8 August 1966 in a hard landing, the date given by the Wikipedia entry, after about three hours of flying in which it had been tested to transition angles of up to 30 degrees and speeds of up to 100 knots, according to the Vertical Flight Society. The aircraft was written off.

The second aircraft flew on, performing hundreds of complete transitions and, on 30 July 1968, a hover above 8,000 feet that the Vertical Flight Society records as a record. Its variable-stability and control system, from the Cornell Aeronautical Laboratory, later Calspan, in the Wikipedia entry, was demonstrated in early 1968, and from that point its purpose changed. It stopped being a prototype of a configuration and became a flying simulator of configurations that did not exist, in Harper and Robert P. 1955, Creer and McNeill 1956, Marchese 1963, Newell et al 1963, Garren and Kelly 1965, Person and Robbins 1965, Determination of flight characteristics 1967, Rhoads 1967, Szalai 1967, Eney 1968, Klopfstein 1968, Malcom 1969, Bell Aerospace Co Buffalo Ny 1970, Rhoads 1970, Schuler et al 1972, Beilman 1973, Smith et al 1973, Lebacqz 1974, Smith 1974, Lebacqz and Aiken 1975, Motyka 1975, Aiken et al 1977, Corliss et al 1977, Parker 1986, Henschel and Chetty 1989, Sato and Satoh 2011, Sato 2020, Scholten et al 2020, Chandrasekaran et al 2021, Network Solution for Real-time Synchronization in Flight Simulator 2022, Yahui et al 2024, Yi and Feng 2025.

How the Variable-Stability System Worked

The system was a response-feedback design, the most common mechanization of the period in Lebacqz 1974, in which sensed motion variables command control deflections in proportion, so that changing the gains changes the closed-loop aircraft. NASA TM-81156 describes it in detail. It is a four-axis system, in thrust, pitch, roll and yaw, primarily analogue with an on-board digital computer, with electrohydraulic servos driving the right-hand flight controls through the same linkages the right-hand pilot moves in normal flight. The evaluation pilot sits on the left, mechanically disconnected from the right-hand controls, and flies through artificial-feel servos whose force gradients run from 1.5 pounds per inch to a stiff stick in pitch and roll. The safety pilot on the right sets up each configuration in flight. Feedback gains are programmable with airspeed from 30 knots rearward to 150 knots forward, using an omnidirectional airspeed system with a second sensor on the nose boom for the vertical component in hover. A limited-authority feedforward system acts as a vernier on blade pitch and elevon angle, because a closed-loop variable-stability aircraft needs its actuators placed very precisely. The ducts rotate together between 0 and 95 degrees at no more than 5 degrees a second.

A response-feedback aircraft does not know exactly what it is simulating. The resulting dynamics are not known in advance, so each configuration has to be identified from its flight records, which is the subject of Lebacqz 1974. The Calspan team used a Kalman filter technique to estimate the derivatives the system had produced, among them the angle-of-attack stability and pitch rate damping varied in the first programme and every roll and yaw derivative varied in the second.

The limits are stated as plainly as the capabilities. The variable-stability system could not be used above about 125 knots, which excluded the upper end of the conversion. It had only limited uncoupled direct force in the longitudinal and vertical directions and none sideways. Hover was restricted by gearbox temperature as described above. And the airframe had an established service life of 500 flight hours, of which 80 were expected to remain at the end of 1980, in NASA TM-81156, so the aircraft would by then have flown about

\[500 - 80 = 420\ \text{h}\]

against the about 200 hours the Vertical Flight Society gives for its career. The two cannot both be right, and the NASA figure is the primary one.

The Research Programmes

NASA TM-81156 summarises the programmes flown through 1979, and the Calspan papers report them in detail. The first three were sponsored by an interagency steering group of the Navy, the Air Force, NASA Langley and the Federal Aviation Administration, and the later ones by the Navy alone.

The first programme measured short-term longitudinal dynamics for the steep approach of a short takeoff and landing, or STOL, aircraft. Two pilots made 50 evaluations of 18 combinations of short-period frequency and damping at 65 and 80 knots, on approach paths of 6 to 10 degrees, visual and on instruments, reported in Schuler et al 1972 and Smith et al 1973. The account in Lebacqz 1974 gives sixteen combinations at a 9 degree glide slope, and the two counts are not reconciled in the documents read. Essentially no data of this kind existed for STOL approaches before it, and supplying them for the V/STOL flying-qualities specification, MIL-F-83300, was a main purpose. It showed that steep instrument approaches could be flown satisfactorily with only instrument landing system information, given good short-term dynamics, and the results were applied in the development of the YC-14 and YC-15, in NASA TM-81156.

The second measured lateral-directional dynamics and roll control power. Three pilots flew 109 evaluations of 17 sets of dynamics at a 7.5 degree glide slope and 65 knots, varying the roll mode time constant, Dutch roll frequency, roll due to sideslip and yaw due to aileron, with roll authority limited electrically for some sets. Two findings bear directly on the specification. Satisfactory flying qualities needed about twice the roll damping that MIL-F-83300 specified, and about half its roll control power, in NASA TM-81156. Since the roll mode time constant is the inverse of the roll damping derivative,

\[\tau_{R} = -\frac{1}{L_{p}}\]

doubling the damping halves the time constant. The same programme expanded the system into the hover and made the first vertical landings performed with the X-22A’s own variable-stability system engaged, in NASA TM-81156.

The third flew instrument transitions to the hover, in Lebacqz and Aiken 1975. The task was a hooded approach from 100 knots, capturing a 7.5 degree glide slope and decelerating at about 0.05 g to a hover at 100 feet. Decelerating uniformly from 100 knots, or 168.8 feet per second, at that rate takes

\[s = \frac{V^{2}}{2a} = \frac{168.8^{2}}{2 \times 0.05 \times 32.17} = 8{,}854\ \text{ft} \qquad t = \frac{V}{a} = 105\ \text{s}\]

which is consistent with the deceleration beginning about 8,000 feet out in still air, as the paper gives it, and continuing through the flare. On the glide slope at 100 knots the aircraft descends at

\[\dot{h} = 60\, V \sin\gamma = 60 \times 168.8 \times \sin 7.5^{\circ} = 1{,}322\ \text{ft/min}\]

Thirty-seven evaluations covered five display formats, from position information alone to four-axis control directors, against five levels of control augmentation, in a flight experiment of 45 hours, in NASA TM-81156. The central result is a trade between control and display, in which more augmentation needs less display and the reverse. Translational velocity had to be displayed for any acceptable system, and because the basic X-22A has low natural height damping near the hover, a thrust director was needed whenever the pilot also had to manage the duct angle. The programme also devised a duct-angle command, the Independent Thrust Vector Inclination Command, which follows the linear corridor described earlier, in Lebacqz and Aiken 1975.

The fourth simulated the AV-8B for a decelerating instrument approach. Forty-three evaluations of 22 configurations, drawn from six flight control schemes and seven head-up display formats, showed that no display gave consistently satisfactory ratings with the proposed rate stability augmentation, while rate-command and attitude-command systems did with several displays, and the resulting display formats were being considered for the AV-8B. The fifth, begun in April 1978 and planned through 1980, addressed hover and low-speed criteria for visual and instrument flight and then transition criteria, in NASA TM-81156.

The four completed programmes add up to

\[50 + 109 + 37 + 43 = 239\]

piloted evaluations in NASA TM-81156, each a configuration flown and rated.

The final flights are dated October 1984 in the summary of the Wikipedia entry and 1988 in its text and in the account of the Vertical Flight Society. No primary document read here records the end of flying.

Comparison With Ground Prediction

The four-duct tandem layout had been in wind tunnels since at least 1960, and the reports on it, from the period to the present, are Larry A. Young, Ronald C Busan et al, Siena K S Whiteside and Beau P Pollard, Goodson and Yaggy 1960, Tapscott and Kelley 1960, Kelley 1962, McKinney and Newsom 1962, Clark et al 1963, Newsom 1963, Curtiss 1965, Biggers et al 1966, Freeman and Newsom 1966, Xiang et al 2016, Geuther et al 2020, Zhang et al 2020, Lee et al 2021, Meng et al 2026. On the gross behaviour the tunnels were right. The thrust augmentation, the general shape of the conversion, and the pitching behaviour were all anticipated.

The full-scale duct test anticipated the duct’s performance well. Gamse and Mort 1967 measured a figure of merit and a propulsive efficiency that bracket the figures this article derives, found the thrust following ideal ducted theory to about 100 knots, and located the upstream lip separation that flight later reported as duct buzz in descent.

The free-flight model anticipated the handling problems. The 0.18-scale model of Freeman and Newsom 1966 had unstable control-fixed pitching and rolling oscillations in hover, with periods of 3.4 and 3.0 seconds that scale with the square root of the length ratio,

\[T_{\text{full}} = T_{\text{model}} \sqrt{5.56} \qquad 3.4 \times 2.358 = 8.0\ \text{s} \qquad 3.0 \times 2.358 = 7.1\ \text{s}\]

slow enough that the pilots found them easy to control. Near the ground the fan slipstreams met between the fore and aft ducts and formed an upward flow that went erratically into one fan or the other, the model gained 20 percent in lift from an upload on the fuselage, and the disturbances were severe enough that the authors concluded artificial stabilisation would be needed in ground effect. Through most of the conversion the model had an unstable Dutch roll and very low directional stability, and stall of the upper outside duct surfaces caused wing dropping on the approach, although the authors note that small ducts stall differently from full-scale ones.

The ground work was weakest where it usually is, in the interference between parts. The rear ducts operate in air the forward ducts have already worked on, and the wake of a duct is not the wake of an open propeller. The large-scale model of Biggers et al 1966 measured some of it. The static thrust of the complete model matched that of four isolated ducts, so interference in the hover was negligible, but in the conversion duct stall appeared only on the front ducts, because their downwash delayed stall on the rear ones, and the interference lowered the configuration’s stall boundary below that of an isolated duct.

Flight added a characteristic the tunnels did not isolate. The Calspan pilots met a low natural height damping near the hover in the basic aircraft, with vertical damping that fell as the duct angle increased, and the guidance in Lebacqz and Aiken 1975 compensated with a gain that rose with duct angle.

What the Data Changed

The X-22 settled that the configuration flies. Nothing in eighteen years of operation suggested that a four-duct tandem vertical-takeoff aircraft was uncontrollable or unsafe, which is a stronger negative result than the X-18 or the X-19 produced.

What it did not settle is whether anyone wanted one. The arithmetic of the useful load is short and unkind,

\[\frac{W_{\text{payload}}}{W} = \frac{1{,}200}{14{,}600} = 0.082\]

against an empty weight of 10,478 pounds and a useful load of 4,122, most of which is fuel. The 1,200 pound maximum payload is from the specification in the Wikipedia entry, and the Vertical Flight Society gives the same figure, with six passengers as the alternative. Weight estimation is the discipline that decides whether a configuration survives contact with a requirement, and its methods are set out in Torenbeek 1972, Ascani 1974, Hayase 1974, Hayase 1974, Hayase 1974, Hayase 1974, Hayase 1974, Hayase 1974, Hiyama 1974, Martindale et al 1974, Wildermuth et al 1974, Wildermuth et al 1974, DeBilzan 1975, Onat and Tolle 1979, Hajela and Chen 1988. The disc loading that the duct makes affordable also produces a 136 mile per hour downwash, a severe weight sensitivity in the hover, and a payload of 1,200 pounds on an aircraft of 15,000. The configuration works and the mission arithmetic is unattractive, a different verdict from the one the earlier tilt aircraft received.

The largest thing it changed is not about ducts at all. The variable-stability system turned the aircraft into a source of handling-qualities data for vertical-takeoff aircraft generally. That literature runs from the proposed V/STOL criteria of Curry et al 1965, and Anderson 1966 through the specification of Key 1971 and the simulator work that supported its further development in Vinje and Miller 1973 to the later Navy criteria of Clark 1982 and the retrospectives of Ashkenas 1984, and Harper and Cooper 1986, and the cluster below on handling and flying qualities criteria holds it, with the aircraft’s programme reports, such as Smith 1974, and Aiken et al 1977, in the first cluster. Most of the titles in this survey that name the X-22A concern flying qualities, variable stability or simulation, and none names the duct, as the cluster on the aircraft below shows.

That influence is most direct in the specification work. MIL-F-83300, the military specification for the flying qualities of piloted V/STOL aircraft, was generated around 1970, in Key 1971, and the X-22A’s first programme was flown in large part to supply it with STOL approach data that did not otherwise exist. Its second programme then found the specification’s roll damping requirement too low by about a factor of two and its roll control power requirement about twice what satisfactory flying qualities needed, according to NASA TM-81156. Those are corrections in both directions, measured in flight, and they are what a variable-stability aircraft is for.

The Contemporary Literature

Cluster	Records
The X-22A and the ducted-propeller assault transport	26
Variable-stability aircraft and flight simulation	44
The duct at incidence and the lip	20
Engine inlets in crosswind and inlet distortion	26
Ducted fan, rotor and vertical-takeoff noise	185
Downwash, ground effect and recirculation	77
Transmissions, drive shafts and power transfer	12
Weight, sizing, performance and cost estimation	24
Drag of nacelles, ducts and external bodies	14
Electric, hybrid-electric and distributed propulsion	156
eVTOL aircraft, urban air mobility and certification	88
Ducted propellers, shrouds and lift fans	356
Transition and conversion	66
Handling and flying qualities criteria	204
Multirotor and unmanned vertical-takeoff vehicles	86
Flight control, displays and guidance for vertical takeoff	105
Propeller slipstream and wing interaction	26
Hover performance and hovering flight	57
Vertical and short takeoff configurations	81
Propellers, rotors and rotorcraft	38
Total	1,691

Ducted fans are in the middle of the largest revival they have ever had, which the cluster on ducted propellers, shrouds and lift fans below counts, in work such as Ohanian et al 2012, Deng et al 2020, Zhang and Barakos 2021, Jin et al 2018, Akturk and Camci 2022, and Liu et al 2025. The broader vertical-takeoff field around them, from the control of multirotor vehicles to urban air mobility, is gathered in the clusters on multirotor and unmanned vehicles and on eVTOL aircraft, and is surveyed in Saied et al 2024, Xu et al 2025, and Darvishpoor et al 2025.

Almost nothing in that revival changes the aerodynamics. The momentum theory is the same, the thrust augmentation is the same, and the three charges the shroud levies are the same three this article computes. What changed is the machinery, and the machinery changed everything about how the configuration is used.

Electric Propulsion Dissolved the Cross-Shaft

The X-22’s most consequential single decision was the cross-shafted transmission, and it was forced. Four turboshafts driving four propellers cannot be allowed to fail independently, because losing one duct applies 71,600 foot-pounds of rolling moment, so every gearbox had to accept power from either direction and the whole train became one failure path.

An electric machine removes the requirement rather than solving it, in Distributed Electric Propulsion 2018, Weng et al 2021, Li et al 2022, Darmstad et al 2024, Siddavatam et al 2024, Chen et al 2025, Xiao et al 2025, and Ma et al 2025, and in the cluster below on electric, hybrid-electric and distributed propulsion. Motors can be sized with margin individually, they can be fed from separate buses, and a failure removes one rotor’s thrust without any mechanical coupling to the others. The redundancy moves from the shaft to the wiring.

And it changes how thrust is controlled. Every propeller on a shafted system turns at the same speed, so the X-22 changed one duct’s thrust by changing its blade pitch, and that differential blade pitch was its primary pitch and roll control in the hover, with the elevons in its duct exits providing yaw in Freeman and Newsom 1966. An electric rotor can instead change thrust by changing speed, and a fixed-pitch rotor controlled that way needs no variable-pitch hub. The control principle is the X-22’s, and the mechanism that delivered it is what has gone.

The Lip Still Stalls

A ducted fan is weak at the lip, which separates at large incidence or in a crosswind, in Jacocks and Kneile 1975, Mugridge 1975, Groeneweg 1977, Seidel and Matwey 1983, Motycka 1985, Sedlock 1985, J P Longley and E M Greitzer 1992, Xu et al 2015, Minaker and Defoe 2019, Minaker and Defoe 2019, Akturk and Camci 2022, Chennuru et al 2025.

This article computes where the momentum model stops describing the flow, and the X-22A record shows where the lip actually let go, in descent at low power and high duct angle, with a buzz felt through the whole aircraft, in Gamse and Mort 1967 and NASA CR-1901. Nothing about that has moved. A modern ducted electric vertical takeoff and landing aircraft, or eVTOL, hovering in a crosswind meets the same separation at the lip, and recent work such as Xu et al 2015 and Akturk and Camci 2022 treats lip separation as a problem still to be controlled.

The Normal Force in Transition Is Still the Hard Part

The duct as an annular wing at incidence is Goodson and Grunwald 1962, Mort 1965, Milla and Blick 1966, Curtiss and Howard C. 1969, Mann 1974, Walker 1981, Lee and Kwok 1983, and Gill and D’Andrea 2020, and the conversion it complicates is Nardizzi et al 1971, Huntley 1972, Hill 1983, Jiang and Zhang 2021, Pfeifle and Fichter 2023, Li et al 2020, and Zhou et al 2025, with the rest in the cluster on transition and conversion below.

What has changed is the tool rather than the answer. The article’s momentum estimate with a lip factor is what could be done with a slide rule, and it fails in the middle of the conversion for reasons the article states. Modern work computes the flow, and it still treats the pitching moment of a lift fan in transition as something to be reduced, in Jiang and Zhang 2021. The X-22 met that with elevons in its jets at low duct angles and differential blade pitch at high ones, in Freeman and Newsom 1966 and NASA TM-81156. A modern aircraft can meet it with rotor speed, and the transition control problem is now posed as an allocation problem, in Pfeifle and Fichter 2023, and no longer as a question about where to put a surface.

Hover Efficiency Has Not Improved and Cannot

The disc-loading relations in this article are momentum theory, so they are not going to be superseded, and the modern hover measurements are Helf et al 1973, Velkoff 1981, Lee and Dassonville 2020, Son and Kim 2023.

A ducted lift fan at ninety-five pounds per square foot pays about three and a half times what a helicopter pays per pound lifted, and it always will. Electric propulsion changes the source of the power and not the amount of it.

Downwash Became a Public Problem

The 136 mile per hour downwash this article computes is now a matter for the safety standards of the places such aircraft land, in Ison 2024, resting on measurements of downwash and outwash such as Chaderjian et al 2002, Ramasamy and Yamauchi 2017, Tan et al 2018, Mehrabi and Davari 2020, and Wu et al 2026, and on the rest of the cluster on downwash, ground effect and recirculation below.

High disc loading near people is the constraint that the X-22 never had to answer to, because it was a research aircraft flown by test crews, away from the public. An aircraft expected to operate from a city rooftop has to account for what its jets do to anyone standing nearby, and the arithmetic that produces the jet velocity is the same arithmetic that produces the hover efficiency. The two cannot be traded against each other. They are the same number.

Noise Became a Requirement

Ducted fan acoustics, gathered in the cluster on ducted fan, rotor and vertical-takeoff noise below, run from the period analysis of ducted turbomachinery noise in Zorumski and Lester 1974 to the shape of the duct inlet in Shur et al 2018, acoustic liners in Azimi et al 2014, and Sato et al 2024, ducted lift fans designed for low noise in Zhang et al 2024, and Zhao et al 2025, and the community noise of urban air mobility in Rizzi and Rafaelof 2021, and Raza and Stansbury 2025.

The duct is helpful here, because it shields the blades and gives somewhere to put a liner, and that is a real reason to choose the configuration that did not weigh in 1962. It is also the clearest case in this survey of a modern advantage that the X-22 could not have collected, since nobody was choosing a military research aircraft’s configuration for its noise.

The Field Has Moved to Certification

The X-22 spent eighteen years measuring how vertical-takeoff aircraft ought to handle, and the modern version of that question is certification, in Cardoso et al 2022, Lim et al 2023, Norsetter and Smith 2024, Pleffken 2026, and Shubert et al 2026. Beneath it lies handling-qualities work for eVTOL and multicopter aircraft such as Bahr et al 2021, Li et al 2024, Favaro et al 2025, Wang et al 2025, and Jusko and Berger 2026, and the wider literature on handling and flying qualities criteria below.

This was the aircraft’s legacy, and it should be stated plainly. Its in-flight simulation work supplied data to the V/STOL flying-qualities specifications of its own period, in NASA TM-81156, and the certification work cited above addresses the same question for a new class of aircraft.

In-Flight Simulation Outlived the Configuration

Variable-stability aircraft are Harper and Robert P. 1955, Creer and McNeill 1956, Drinkwater and Rolls 1962, Marchese 1963, Newell et al 1963, Garren and Kelly 1965, Person and Robbins 1965, Determination of flight characteristics 1967, Rhoads 1967, Szalai 1967, Eney 1968, Klopfstein 1968, Malcom 1969, Michaels 1969, Bell Aerospace Co Buffalo Ny 1970, Rhoads 1970, Schuler et al 1972, Beilman 1973, Smith et al 1973, Lebacqz 1974, Smith 1974, Lebacqz and Aiken 1975, Motyka 1975, Aiken et al 1977, Corliss et al 1977, Corliss et al 1978, Parker 1986, Henschel and Chetty 1989, Sato and Satoh 2011, Ko and Park 2014, Sato 2020, Scholten et al 2020, Chandrasekaran et al 2021, Fang et al 2022, Network Solution for Real-time Synchronization in Flight Simulator 2022, Yahui et al 2024, Yi and Feng 2025.

The technique the X-22 was converted to use outlived the aircraft built for it. Ground-based simulation improved, and by 1979 the remaining in-flight simulation capability for vertical-takeoff work was two fixed-wing aircraft and a few helicopters, with no replacement for the X-22A yet identified, in NASA TM-81156. The X-22 was the last of a line as well as the best of it.

What the Survey Shows

The duct’s arithmetic was correct in 1962 and is correct now. Twenty-six percent more static thrust for a ring of metal, an annular wing’s normal force through the conversion, frontal area in cruise, and a hover bill set by disc loading. None of that has changed, and none of it is in dispute.

What came back is not the X-22’s aeroplane but the X-22’s component, attached to a machine that has no shafts, changes thrust by changing speed, and is being asked to fly near people. The three charges are unchanged and two of the answers are new.

And the thing the X-22 was actually for turned out to be the thing that lasted. The configuration was not adopted, and the programme was cancelled, in the Wikipedia entry. The handling-qualities data was, in the flying-qualities specifications of its period in NASA TM-81156, and the question it answered for eighteen years, by pretending to be other aeroplanes, is the one a modern ducted eVTOL must answer for certification.

The X-22A and the ducted-propeller assault transport

Reports and papers whose titles name the X-22A, together with the 1956 studies of a ducted-propeller assault transport from which the four-duct configuration descends.

Of the 16 titles that name the X-22A, 11 name flying qualities, variable stability, simulation, identification or displays, and none names the duct. They run from 1963 to 1990. The aircraft’s ducted propulsion is documented in tunnel reports whose titles do not name it, of which the full-scale duct test of Mort and Gamse is the most important, and that is the shape the Source Base describes.

26 records. [Aiken et al 1977] [Beilman 1973] [Bell Aerospace Co Buffalo Ny 1956] [Bell Aerospace Co Buffalo Ny 1956] [Bell Aerospace Co Buffalo Ny 1956] [Bell Aerospace Co Buffalo Ny 1970] [Henderson 1964] [Hiller Aircraft Corp Palo Alto Ca 1956] [Lebacqz 1974] [Lebacqz and Aiken 1975] [Lebacqz and Aiken 1975] [Lebacqz et al 1974] [Macey 1956] [Marchese 1963] [Michaels 1969] [Pfeiff 1965] [Schuler et al 1972] [Sing and Knauer 1956] [Smith 1974] [Smith et al 1973] [Stearns et al 1956] [The Bell X-22A V/STOL, Variable Stability Research Airplane, Lessons Learned] [Vollo and Brassaw 1956] [Walz 1978] [Zabinsky 1956] [Zabinsky and Laszewski 1956]

Variable-stability aircraft and flight simulation

Variable-stability aircraft, in-flight simulators and the ground simulators set beside them, including the other airborne V/STOL simulators of the X-22’s period.

The technique outlived the aircraft. Of the 44 records, 7 date from 1965 or earlier and 37 from 1966 or later, 12 of them from 2015 or later.

44 records. [Aláez et al 2022] [Chandrasekaran et al 2021] [Corliss et al 1977] [Corliss et al 1978] [Creer and McNeill 1956] [Davies 1975] [Determination of flight characteristics 1967] [Drinkwater and Rolls 1962] [Ellis 1971] [Ellis et al 2002] [Eney 1968] [Fang et al 2022] [Gallagher 1971] [Garren and Kelly 1965] [Hamel 2017] [Hamidani and Kunz 2018] [Harper and Robert P. 1955] [Henschel and Chetty 1989] [Kereliuk and Sinclair 1979] [Key et al 1965] [Klopfstein 1968] [Ko and Park 2014] [Komoda et al 1989] [Malcom 1969] [McGREGOR and Smith 1966] [Motyka 1975] [Network Solution for Real-time Synchronization in Flight Simulator 2022] [Newell et al 1963] [Parker 1986] [Person and Robbins 1965] [Portapas 2025] [Rhoads 1967] [Rhoads 1970] [Sato 2020] [Sato and Satoh 2011] [Scamps and Gibbens 2003] [Scholten et al 2020] [Scholten et al 2020] [Sims and Kimberlin 1995] [Sinacori 1970] [Szalai 1967] [Vinje and Miller 1973] [Yahui et al 2024] [Yi and Feng 2025]

The duct at incidence and the lip

The ducted propeller or fan at angle of attack, in non-axial inflow and in crosswind, and separation at the duct lip.

The subject the conversion depends on is small and recent. Of the 20 records, 10 date from 2015 or later and two from 1965 or earlier, the shrouded-propeller tunnel tests of the NACA and NASA that measured the loads on an isolated duct through the whole range of incidence.

20 records. [Akturk and Camci 2010] [Akturk and Camci 2022] [Akturk and Camcı 2014] [An et al 2012] [Cantos et al 2026] [Cao et al 2024] [Cerny and Breitsamter 2020] [Chen et al 2026] [Goodson and Grunwald 1962] [Hall and Delaney 1991] [Hwang et al 1994] [Lieblein et al 1973] [Parlett 1955] [Pflimlin et al 2006] [Pflimlin et al 2007] [Roh et al 2018] [Roh et al 2020] [Wang et al 2024] [Wang et al 2026] [Xu et al 2015]

Engine inlets in crosswind and inlet distortion

The inlets of engine nacelles and turbomachines in crosswind and with distorted inflow, kept because their lip separates for the reason a duct lip does.

26 records. [Chen et al 2021] [Chennuru et al 2025] [Dickinson et al 2024] [J P Longley and E M Greitzer 1992] [Jacocks and Kneile 1975] [Koch 2012] [Kokubo et al 2024] [Kokubo et al 2026] [Lavi and Headley 1967] [Li et al 2027] [Lin et al 2023] [Minaker and Defoe 2019] [Minaker and Defoe 2019] [Motycka 1985] [Nichols et al 2019] [Nichols et al 2020] [Nichols et al 2021] [Nichols et al 2022] [Nichols et al 2024] [Nichols et al 2026] [Sedlock 1985] [Seidel and Matwey 1983] [Song et al 2025] [Song et al 2025] [Ting et al 1975] [Trapp and da Motta Girardi 2010]

Ducted fan, rotor and vertical-takeoff noise

Noise of ducted fans and shrouded propellers, of rotors and of vertical-takeoff aircraft, the community response to it, and the noise of fans in ducts outside aircraft, whose acoustics is the same.

The duct’s acoustic advantage is a modern subject. Of the 185 records, 109 date from 2015 or later and 5 from 1965 or earlier.

185 records. [Aalmoes et al 2025] [Abdelhamid 1974] [Abdelhamid 1974] [Adámek et al 2021] [Agarwal 1997] [Ahmed et al 2025] [Aircraft Noise and Certification 2019] [Alexander A Svetgoff and Kevin Walczyk] [Araghizadeh et al 2024] [Atassi and Grady 2003] [Azimi et al 2014] [Bai et al 2025] [Balakrishnan and Srinivasan 2019] [Bauer and Redmann 2023] [Bauer et al 2021] [Bekofske et al 1977] [Bennett and Fitzpatrick 2007] [Bielak et al 2002] [Boucher et al 2023] [Boxwell et al 1978] [Bu et al 2026] [Bulaga 2005] [Buǧday and Saetti 2024] [Cantos et al 2024] [Chen et al 2015] [Chen et al 2023] [Cho et al 2024] [Cole and England 1967] [de Laborderie and Moreau 2016] [Deming 1940] [Dittmar 1978] [Dittmar 1988] [Dittmar et al 1993] [Doherty and Butzel 1979] [Dunn 1997] [Dunn and Farassat 1998] [Envia 2016] [Envia 2016] [Eversman 1992] [Eversman and Roy 1993] [Falarski 1972] [Farazi and Zou 2024] [Kilcoyne and Huang 2025] [Merino-Martinez and Schade 2025] [Schoder et al 2023] [Simon et al] [Fricke and Bissell 1968] [Glegg et al 1997] [Go et al 2022] [Go et al 2023] [Go et al 2023] [Goethert and Borchers 1974] [Goethert and Borchers 1975] [Goldschmidt and Ukeiley 2025] [Goudswaard et al 2024] [Groeneweg 1977] [Hanson 2001] [Hargest 1966] [He et al 2026] [Henderson et al 2005] [Henschel et al 1974] [Hilton et al 1960] [Hubbard 1950] [Hummel et al 2026] [Huo et al 2025] [Jdiobe et al 2024] [Jia and Li 2025] [Jiang et al 2019] [Johnson 1974] [Johnson-Groh 2024] [Jones et al 1979, Evaluation of two inflow control] [Jung et al 2025] [Kevin Li and Lee 2021] [Khorrami and Fares 2019] [Killeen et al 2023] [Kobayashi 1978] [Koch 2010] [Koch et al 2011] [Koch et al 2012, Tone Noise Predictions for a Spac] [Koppelberg et al 2023] [Krishnamurthy and Rizzi 2022] [L Danielle Koch et al 2012] [Koch et al] [Lee et al 2025] [Lendvai and Benedek 2023] [Lendvai and Benedek 2024] [Lewy 2002] [Li et al 2023] [Liu et al 2021] [Liu et al 2024] [Liu et al 2026] [Longhouse 1972] [Longhouse 1973] [Lowson 1972] [Dunn et al 1996] [Ma] [Majić et al 2016] [Margetts 1987] [Marques et al 2026] [McArdle et al 1981] [McLemore and Pegg 1980] [Meng et al 2026] [Miller and Beranek 1958] [Miller and Papamoschou 2024] [Miller et al 2023] [Modeling of Ducted Acoustic 2021] [Moreau et al 2022] [Morhy Cavalcante et al 2026] [Moulton and Brown 1992] [Mugridge 1975] [Musser et al 2023] [Netjasov 2012] [Yao et al 2026] [Zawodny et al 2025] [Nt et al 2024] [Nöding and Bertsch 2021] [Oh et al 2026] [Ohtsuta and Akishita 1990] [Oleson and Patrick 1998] [Page et al 2021] [Palleja-Cabre et al 2024] [Palleja-Cabre et al 2025] [Papamoschou 2023] [Park et al 2017] [Park et al 2022] [Park et al 2022] [Pascioni et al 2026] [Posey et al 2004] [Qingyi et al 2023] [Rao et al 1974] [Raza and Stansbury 2025] [Richards 1955] [Rizzi and Rafaelof 2021] [Rizzi et al 2026] [Sato et al 2024] [Schade et al 2024] [Schmidt et al 2021] [Schmitz and Stepniewski 1973] [Schmähl et al 2021] [Sharma et al 1987] [Shur et al 2016] [Shur et al 2018] [Simon 2024] [Smith et al 2000] [Sofrin and Cicon 1987] [Sofrin and Cicon 1987] [Son et al 2025] [Sorenson et al 2023] [Sow et al 2025] [Spence and Spence 1997] [Sun et al 2024] [Sun et al 2026] [Sutliff 2019] [Sutliff and Nagel 1993] [Tangler et al 1973] [Thalheimer 2021] [Time Domain Sound Spectrum Measurements in Ducted Axial Fan under Stall Region 2015] [Tinney and Valdez 2026] [Truong and Papamoschou 2014] [Truong and Papamoschou 2015] [Tweed 2000] [Wang 2017] [Wang and Li 2025] [Wang and Ma 2026] [Wang et al 2005] [Wang et al 2023] [Wang et al 2024] [Wang et al 2025] [Wang et al 2026] [Woodward and Glaser 1981] [Woodward et al 1984] [Woodward et al 1992] [Wu et al 2025] [Yadegari et al 2024] [Yang et al 2022] [Yeow 1974] [Yusri et al 2026] [Zhang and Barakos 2021] [Zhang et al 2024] [Zhao and Yang 2023] [Zhao and Yang 2026] [Zhao et al 2025] [Zhao et al 2026] [Zhong et al 2023] [Zorumski and Lester 1974]

Downwash, ground effect and recirculation

Downwash, outwash and ground erosion, ground effect and suckdown, recirculation and exhaust ingestion, and the agricultural spraying literature in which rotor downwash is the working medium.

The problem has two literatures with a gap between them. Of the 77 records, 15 date from 1965 or earlier, 15 from 1966 to 1989, 3 from 1990 to 2014 and 44 from 2015 or later.

77 records. [Adarkar and Hall 1969] [Ai et al 2020] [Arabshahi and Pankajakshan 1994] [Bai and Zhou 2023] [Bardera Mora and Matías García 2022] [Brinkman and Johnson 2021] [Brinkman et al 2020] [Butler and Thomas 1964] [Chaderjian et al 2002] [Chang et al 2023] [Chen et al 2025] [Curtiss 1965] [Doğru et al 2016] [Editor 2016] [George et al 1964] [Georgiev 2025] [Hall 1967] [He and Leang 2025] [He et al 2026] [Higgins et al 1966] [Hoad and Gentry 1975] [Hoad and Gentry 1977] [Hover Over from Dover 1968] [Huang and Hu 2018] [Huston and Winston 1960] [Hwang and Kwon 2019] [Irmawan et al 2023] [Ison 2024] [Kalra and Baeder 2017] [Kato and Suzuki 2019] [Kemp 1969] [Kim and Yee 2020] [Kuhn 1965] [Kuhn 1979] [Kuhn and Grunwald 1960] [Kuhn and Grunwald 1961] [Lee et al 2026] [Levinsky et al 1968] [Li et al 2024] [Louisse and Marshall 1976] [Mehrabi and Davari 2020] [Milluzzo and Kuerbitz 2017] [Milluzzo and Leishman 2017] [Minervino et al 2016] [Morse and Newhouse 1960] [Morse and Newhouse 1961] [Morse and Newhouse 1961] [Neumann et al 2020] [Newsom and Tosti 1962] [O’Bryan 1961] [Obryan 1960] [Otsuka et al 2024] [Page and Aiken 1971] [Pasquali et al 2020] [Pruyn and Taylor 1971] [Ramasamy and Yamauchi 2017] [Renselaer 1975] [Renselaer 1977] [Rogers et al 1969] [Sagaga and Lee 2025] [Shouji et al 2021] [Spray-on Landing Field Aids VTOL Planes 1965] [STOL Operation Transition-in-Ground Effect 2007] [Su et al 2024] [Tan et al 2018] [Trivkovic et al 2016] [White et al 1960] [Wu et al 2026] [Yang et al 2022] [Yang et al 2022] [Zhang et al 2018] [Zhang et al 2021] [Zhao et al 2022] [Zhao et al 2023] [Zheng et al 2018] [Zhou et al 2026] [Zhu et al 2024]

Transmissions, drive shafts and power transfer

Transmissions, gearboxes, drive shafts and their bearings, almost all of them for rotorcraft.

12 records. [Albrecht 1988] [Aroussi 2025] [Caldwell 1940] [Cormier 1979] [Dean 1979] [Deckert and Rolls 1974] [Leishman 1966] [Mancini 1983] [Overfield and Crawford 1967] [Tokarski et al 1994] [Ward 1970] [White 1983]

Weight, sizing, performance and cost estimation

Weight estimation programmes and methods, sizing, performance estimates and operating cost.

24 records. [Ascani 1974] [Bonner et al 1975] [Burnell and Fan 1957] [DeBilzan 1975] [Gallagher and Stoessel 1967] [Hajela and Chen 1988] [Hayase 1974] [Hayase 1974] [Hayase 1974] [Hayase 1974] [Hayase 1974] [Hayase 1974] [Hiyama 1974] [Irvin and Swan 1956] [Martindale et al 1974] [Mokotoff and Cinar 2026] [Onat and Tolle 1979] [Otsuka et al 2015] [Park 2025] [Saelman 1963] [Taflan et al 2024] [Torenbeek 1972] [Wildermuth et al 1974] [Wildermuth et al 1974]

Drag of nacelles, ducts and external bodies

Drag of nacelles, ducted bodies and other external bodies, and its estimation.

14 records. [Callaghan et al 1973] [Corsiglia et al 1979] [Corsiglia et al 1981] [Definitions of the Thrust of a Jet Engine and of the Internal Drag of a Ducted Body 1955] [Gilman and Burdges 1967] [Jiang and Zhang 2021] [Kuhn 1959] [Levine 1957] [Malone and Peavey 1999] [Milla and Blick 1966] [Neal 1975] [Smetana 1975] [Takita et al 2019] [Tinkler 1962]

Electric, hybrid-electric and distributed propulsion

Electric, hybrid-electric and distributed propulsion, electric ducted fans, motors, batteries and the power electronics that drive them.

This literature did not exist when the X-22 flew. Of the 156 records, 148 date from 2015 or later, and the one record from before 1966 is a 1952 whirl test of a propeller driven by an electric motor.

156 records. [Akash et al 2021] [Alfares 2026] [Ali and Maqsood 2024] [Alvarez et al 2023] [Ambroziak et al 2023] [Arace et al 2026] [Bacchini and Cestino 2019] [Chakraborty et al 2023] [Chu et al 2025] [Jiang et al 2025] [Junaidin and Wicaksono 2022] [Li et al 2025] [Niță et al 2026] [Sedunin et al 2026] [Simmons and Murphy] [Brancaleoni et al 2025] [Bravo et al 2021] [Brucculeri et al 2019] [Bustamante Alarcon et al 2023] [Bustamante Alarcon et al 2025] [Chakraborty and Mishra 2023] [Chen et al 2025] [Choi and Kim 2025] [Choi et al 2014] [Choi et al 2026] [Chu et al 2025] [Cole et al 2021] [Comunian et al 2025] [Cusati et al 2023] [Darmstad et al 2024] [Darrah et al 2020] [Dayhoum et al 2024] [de Vries et al 2019] [Distributed Electric Propulsion 2018] [Donateo and Çinar 2022] [Dorfling and Traub 2023] [Dzulfikar et al 2025] [Ebus et al 2022] [Ebus et al 2024] [Eqbal et al 2023] [Fan et al 2023] [Fang and Callegaro 2021] [Farras et al 2021] [Finger et al 2018] [Finger et al 2020] [Fredericks et al 2013] [Friedrich and Robertson 2015] [Gao et al 2025] [Gholamian and Beik 2026] [Goshtasbi et al 2025] [He et al 2020] [He et al 2025] [Hossain and Bodson 2022] [Hu and Booker 2022] [Hu et al 2026] [Huang et al 2025] [Hui et al 2025] [Ison 2024] [Horton et al] [Jin et al 2018] [Junaidin 2023] [Junaidin and Cahyono 2019] [Kamarudin 2020] [Kang et al 2023] [Kang et al 2025] [Kliza et al 2025] [Kou et al 2021] [Kozakiewicz and Grzegorczyk 2021] [Kruger and Uranga 2024] [Krznar et al 2021] [Krznar et al 2021] [Kubo et al 2009] [Lee and Yee 2024] [Lee et al 2022] [Legrand et al 2025] [Lei and Zhao 2023] [Li et al 2022] [Li et al 2024] [Li et al 2024] [Li et al 2024] [Li et al 2025] [Li et al 2026] [Litt 2024] [Liu et al 2024] [Liu et al 2025] [Ma et al 2020] [Ma et al 2022] [Ma et al 2025] [DeVore et al 2022] [Myasnikov and Ilyin 2023] [Nandy and Kim 2025] [Neogi et al] [Ng et al 2021] [Odukoya and Jo 2026] [Park and Park 2026] [Pavel 2022] [Perry et al 2023] [Pouryoussefi et al 2023] [Qian et al 2023] [Qiao and Zhou 2026] [Qin et al 2023] [Busan et al] [Safieh and Rauleder 2024] [Safwat et al 2026] [Saif et al 2025] [Sajid et al 2026] [Sarjito and Mulyadin 2024] [Sarup 2025] [Sarup 2026] [Serrano et al 2021] [Serrano et al 2022] [Shang et al 2025] [Shi et al 2024] [Showers 1952] [Siddavatam et al 2024] [Su et al 2026] [Suiçmez and Kutay 2023] [Sun et al 2025] [Sun et al 2025] [Takeyeldein 2026] [Teichert et al 2026] [Tyan et al 2017] [Van et al 2019] [Vechtel and Buch 2022] [Vepa 2018] [Vieira et al 2019] [Vratny and Hornung 2018] [Wachłaczenko 2025] [Wahid et al 2025] [Walvekar and Chakravarthy 2023] [Wang et al 2023] [Wang et al 2024] [Watthewaduge and Bilgin 2023] [Wei et al 2026] [Weinstein et al 2026] [Weintraub et al 2022] [Weng et al 2021] [Wiston and Yan 2025] [Wu et al 2023] [Xiao et al 2024] [Xiao et al 2025] [Xie et al 2023] [Xin and Lee 2024] [Yan et al 2019] [Yang et al 2026] [Yildiz 2022] [Yingtao et al 2025] [Yoon and Jeong 2025] [Yuan et al 2026] [Zhang et al 2023] [Zhao et al 2022] [Zhao et al 2023] [Zhong et al 2026] [Zhu et al 2022] [Zong et al 2021] [Zong et al 2022]

eVTOL aircraft, urban air mobility and certification

The eVTOL aircraft as a vehicle and as a transport system, covering configurations, operations, vertiports, public acceptance, safety and certification, with the earlier literature of VTOL transport.

It is almost entirely recent. Of the 88 records, 74 date from 2015 or later.

88 records. [Adeline et al 2023] [Al-Rubaye et al 2023] [Alkaabneh et al 2026] [Alomar and Shafqat 2026] [Altouq et al 2025] [Asher 1969] [Bahr et al 2021] [Simmons and Hatke 2021] [Simmons] [Simmons 2023] [Simmons and Busan] [Bogdanović et al 2024] [Brown and Harris 2020] [Bruno Garcia Franciscone and Elton Fernandes 2023] [Campbell 1965] [Cardoso et al 2022] [Chen et al 2026] [Choi and Suk 2026] [Ci̇zreli̇oğullari et al 2022] [Darvishpoor et al 2025] [Caudle et al] [Favaro et al 2025] [Favaro et al 2025] [Fu et al 2025] [Fu et al 2025] [Goyal and Cohen 2022] [Guo et al 2026] [Han and Pei 2026] [Hearn et al 2023] [Jiang et al 2025] [Kim and Zhang 2025] [Kirste and Stumpf 2025] [Koumoutsidi et al 2022] [Kwak 2026] [Laarmann et al 2023] [Lebacqz and Scott 1985] [Lee et al 2021] [Li et al 2026] [Lim et al 2023] [Liu et al 2026] [Mak et al 2025] [Mall et al 2026] [Marzouk 2025] [McKercher et al 2024] [Radotich] [Min et al 2026] [Mirković et al 2026] [Ngcukayitobi 2026] [Norsetter and Smith 2024] [Oyama et al 2024] [Pasupuleti 2025] [Pearlman 1968] [Pleffken 2026] [Pooladsanj et al 2026] [Pradeep and Wei 2019] [Pukhova et al 2021] [Qian et al 2026] [Qiao and Zhou 2025] [Rao and Chimata 2024] [Ren and Wang 2025] [Rigas et al 2024] [Rolls 1965] [Salvador 2022] [Savino and Zanotti 2025] [Schade 1968] [Schatrow et al 2025] [Shao et al 2021] [Shubert et al 2026] [Whiteside and Pollard] [Simpson 1968] [Spühler et al 2025] [Stapleford et al 1974] [Sun et al 2026] [Takacs and Haidegger 2022] [Wang et al 2023] [Wang et al 2025] [Wang et al 2025] [Wang et al 2026] [Xu et al 2025] [Xue et al 2026] [Yan et al 2026] [Yang et al 2026] [Liu and Wang] [Yu et al 2024] [Zhang et al 2024] [Zhang et al 2024] [Zhao and Cao 2026] [Zhou et al 2024]

Ducted propellers, shrouds and lift fans

Ducted propellers, shrouded rotors, ducted fans and lift fans, covering their theory, tunnel tests and design, and the modelling and control of ducted-fan vehicles.

The revival is larger than the period literature, and the interval between them is thin. Of the 356 records, 59 date from 1965 or earlier, 38 from 1966 to 1989, 86 from 1990 to 2014 and 162 from 2015 or later.

356 records. [A Variable-speed Control Method to Reduce Ducted-fan Thrust Fluctuations for Flying Car Utilization 2024] [Aboelezz et al 2022] [Abrego et al 2002] [Abrego et al 2002] [Acheson 2011] [Advanced Ducted Propulsor In-Flight] [Afjeh 1991] [Ahmadi et al 2025] [Ahn and Lee 2004] [Ai et al 2020] [Ai et al 2025] [Albachten 1956] [Ale Isaac Khoueini] [Alesin et al 2020] [Applin et al 1994] [Armutcuoglu et al 2004] [Avanzini et al 2001] [Avanzini et al 2003] [Bai et al 2025] [Bangalee et al 2025] [Beck et al] [Beeler 1965] [Betz 1966] [Bigarella 2025] [Biggers et al 1966] [Black and Wainauski 1967] [Boldman et al 1991] [Boldman et al 1993] [Bontempo and Manna 2016] [Brabec and Helmich 2012] [Brizzolara et al 2009] [Brogan et al 1961] [Brown 1950] [Buba et al 2024] [Bulat and Chernyshov 2026] [Bulat et al 2023] [Cai et al 2015] [Cai et al 2019] [Cai et al 2022] [Cao et al 2005] [Cao et al 2021] [Cao et al 2022] [Carmichael and McNay 1961] [Castles et al 1960] [Chambers and Grafton 1968] [Chen et al 1991] [Chen et al 2017] [Chen et al 2018] [Cheng and Pei 2020] [Cheng and Pei 2022] [Cheng and Pei 2022] [Cheng and Pei 2023] [Cheng and Pei 2024] [Cheng and Pei 2026] [Cheng et al 2020] [Chiu et al 2025] [Cho 2025] [Clark et al 1963] [Colman et al 2011] [Cook and Hickey 1967] [Cousins et al 2026] [Cui et al 2025] [Curtiss and Howard C. 1969] [Dallas and Irvin 1956] [Davis 1977] [Dayhoum et al 2024] [Dayhoum et al 2025] [Dayhoum et al 2025] [Deng et al 2018] [Deng et al 2020] [Denisenko et al 2021] [Du et al 2025] [Ducted fan] [Ducted Fan Model ala 2026] [Echavarria and Poroseva 2015] [Effati and Banazadeh 2013] [Emami and Banazadeh 2015] [Eversman and Moehring 1987] [Falarski and Mort 1971] [Fan et al 2017] [Fan et al 2018] [Fan et al 2018] [Fan et al 2020] [Fan et al 2022] [Fang et al 2025] [Farassat et al 2001] [Fazeli Asl and Moosapour 2017] [Flight Characteristics of Quad Ducted Fan Helicopter with Thrust Vectoring Nozzles 2014] [Franz and Hauser] [Freeman and Newsom 1966] [Fu et al 2023] [Gamse and Mort 1967] [Gao and Xu 2022] [Gentili et al 2009] [Gill and D’Andrea 2020] [Gilmore and Grahame 1959] [Gong et al 2021] [Goodson and Yaggy 1960] [Graf et al 2008] [Grafton and Newsom 1971] [Grant et al 2015] [Gray 1961] [Greenberg and Ordway 1964] [Griffith and Moller 1990] [Guerrero et al 2003] [Guo and Zhou 2022] [Guo and Zhou 2022] [Guo and Zhou 2026] [Głowacki et al 2015] [Hall and Delaney 1990] [Hall et al 1991] [Ham and Moser 1958] [Han et al 2014] [Han et al 2019] [Han et al 2024] [Harada 2018] [He et al 2013] [Heyson 1974] [Hickling 1963] [Hoehne and Hoffman 1961] [Hook et al 2011] [Hou et al 2015] [Hough 1961] [Huo et al 2015] [Huo et al 2019] [Iek et al 1992] [Iek et al 1993] [Iek et al 1993] [Jensen] [Jang et al 2012] [Janus et al 1992] [Jasudavisius 2021] [Jeong et al 2019] [Jiang et al 2015] [Jiang et al 2018] [Jiang et al 2025] [Johnson 1958] [Kaji et al 2017] [Kaji et al 2018] [Kang et al 2021] [Kantner et al] [Kaskel et al 1963] [Keith and Srivastava 1997] [Keller and Holliger 1959] [Kelley 1962] [Kondor and Moore 2005] [Liu and Li 2024] [Liu et al 2019] [Lu et al 2025] [Marconi and Naldi 2012] [Mort 1966] [Kim et al 2018] [Kim et al 2019] [Kim et al 2020] [Kirby 1963, Flight-and Force-test Investigati] [Kondor et al 2003] [Kriebel 1964] [Kriebel and Mendenhall 1966] [Kriebel et al 1963] [Kruger 1949] [Kuantama and Tarca 2017] [Naldi et al 2010] [Parlett 1961] [Pflimlin et al 2010] [Pütsep et al 2023] [Rostami and Farajollahi 2021] [Wang et al 2015] [Wang et al 2018] [Wang et al 2026] [Yilmaz et al 2015] [Young] [Lawrence 1965] [Lee and Kwok 1983] [Lei et al 2024] [Lerbs 1953] [Li 2008] [Li and Liu 2023] [Li et al 2021] [Li et al 2023] [Li et al 2024] [Lin et al 2019] [Liu 2023] [Liu and Mi 2025] [Liu et al 2012] [Liu et al 2013] [Liu et al 2019] [Liu et al 2022] [Liu et al 2023] [Liu et al 2025] [Lovell 1955] [Lu] [Luo et al 2023] [Luo et al 2024] [Luo et al 2024] [Luo et al 2024] [Mahakalkar et al 2024] [Maldonado and Fernandes 2024] [Maldonado et al 2024] [Mann 1974] [Manzoor et al 2020] [Manzoor et al 2022] [Manzoor et al 2022] [Mao et al 2024] [Martin 1964] [Martin and Tung 2004] [McCormick 1956] [McCune 1993] [McKinney and Newsom 1962] [McKinney et al 1966] [McNay 1958] [Meyerhoff and Zvengrowski 1962] [Mi 2020] [Miwa and Marubashi 2014] [Miwa et al 2012] [Miwa et al 2013] [MODELLING AND CONTROLLER DESIGN OF A NOVEL DUCTED FAN UNMANNED AIRCRAFT 2021] [Mohammadi and Boroomand 2014] [Morgan 1961] [Morse 1956] [Mort 1965] [Moser 1959] [Mouille 1970] [Muehlebach and D’Andrea 2017] [Mumasha et al 2026] [Murthy et al 2002] [Myers et al 2010] [Naldi et al 2015] [Nardi et al 1999] [Nelson 1956] [Nelson 1959] [Newsom 1963] [Nguyen and Ngo 2017] [Pettingill et al] [Niwa and Suzuki 1991] [Ohanian et al 2009] [Ohanian et al 2011] [Ohanian et al 2012] [Okan et al 2002] [Ordway and Greenberg 1961] [Ordway et al 1960] [Otsuka et al 1969] [Ozdemir and Horn 2009] [Pahonie et al 2018] [Pangaribuan and Hartono 2024] [Park et al 2013] [Park et al 2015] [Parlett 1961] [Pawłucki 2016] [Peddle et al 2009] [Performance validation of the 1999] [Pflimlin et al 2007] [Phanindra et al 2020] [Platt 1948] [Przedpelski 1965] [Qing et al 2019] [Raeisi 2021] [Rajagopalan and Zhang 1989] [Maki and Giulianetti 1966] [Regier et al 1947] [Ritschl et al 2003] [Roberts 1964] [Ruzicka et al 2004] [Ryals et al 2026] [Sargent and Anemaat 2018] [Savage et al 2026] [Schaller] [Sellers et al 1977] [Seo and Kim 2016] [Sheehy 1973] [Sheng and Sun 2015] [Sheng et al 2015] [Sheng et al 2015] [Shi et al 2022] [Shiau et al 2024] [Shuai and Xiao-hui 2011] [Simmonds 1967] [Simmonds 1967] [Simmonds 1967] [Sinacori and Lange 1961] [Singh and Dinavahi 2011] [Smith 1960] [Srivastava 1992] [Srivastava et al 1996] [Stiltner and Ohanian 2012] [Stockman 1970] [Szafran et al 2014] [Taher et al 2026] [Takallu 2009] [Takeda et al 2020] [Tang et al 2026] [Tapscott and Kelley 1960] [Taylor 1958] [Tekinalp et al 2009] [Theodorsen 1960] [Theodorsen 1960] [Theodorsen and Nomicos 1961] [Thwaites 1951] [Thwaites 1953] [Tian et al 2026] [Tobias and Horn 2008] [Tran et al 2021] [Tran Minh 2025] [Valarezo 1990] [Van Niekerk 1958] [Vu et al 2026] [Wahl and Julian 1956] [Wainauski 1968] [Walker 1981] [Walters and Ashworth 1974] [Wang and Qi 2011] [Wang et al 2007] [Wang et al 2025] [Weetman and Cromack 1973] [Weir 1988] [West 1969] [White and Phelan 1987] [Winter 1980] [Wonseok Lee and Hyochoong Bang 2007] [Worobel and Peracchio 1968] [Xiang et al 2015] [Xiang et al 2016] [Xu et al 2026] [Xu et al 2026] [Yaggy and Mort 1961] [Yang et al 2004] [Yang et al 2011] [Yauwenas et al 2022] [Yilmaz et al 2013] [Yin and Pei 2025] [Yin and Pei 2026] [Yoeli 2002] [Yoeli 2005] [Yoshikawa et al 2019] [Yu 2023] [Yuska and Diedrich 1972] [Zakaria et al 2019] [Zamri 2011] [Zeng et al 2025] [Zhang and Barakos 2021] [Zhang and Fan 2012] [Zhang et al 2016] [Zhang et al 2020] [Zhang et al 2020] [Zhang et al 2021] [Zhang et al 2022] [Zhang et al 2022] [Zhang et al 2023] [Zhang et al 2025] [Zhang et al 2025] [Zhang et al 2026] [Zhao and Bil 2008] [Zhao et al 2013] [Zhao et al 2026] [Zhao et al 2026] [Zheng et al 2026] [Zondervan et al 2006]

Transition and conversion

Transition and conversion between hover and wingborne flight, from the period flight tests to the transition control of modern unmanned aircraft.

66 records. [Aroussi 2026] [Burton et al 2026] [Chen 2023] [Cheng et al 2026] [Clark et al 1982] [Coles 1968] [Comer et al 2026] [Cong et al 2024] [Daud Filho and Belo 2023] [Di et al 2025] [Englar and Kirkpatrick 1969] [Girard 1959] [Hill 1983] [Huntley 1972] [INAV-Based Adaptive Transition Control for QuadPlane VTOL UAVs in Urban Wind Gust Environments 2025] [Johnson and Craig 1974] [Kikumoto et al 2022] [Kuhn 1981] [Laillatul and Darwito 2026] [Lee et al 2026] [Li et al 2020] [Liu et al 2018] [Lombaerts et al 2022] [Lovell and Parlett 1956] [Ma and Wang 2018] [Magee and Taylor 1971] [Magee and Taylor 1971] [Marchini] [Margason and Hoad 1979] [May et al 2024] [Mineck and Margason 1974] [Naldi and Marconi 2009] [Naldi and Marconi 2013] [Nardizzi et al 1971] [Otis 1962] [Pfeifle and Fichter 2023] [Pirera 1983] [Pobikrowska and Goetzendorf-Grabowski 2025] [Rae Jr. 1972] [Ramnath 1970] [Rampy 1966] [Rehan et al 2026] [Setiawarman and Sasongko 2026] [Shimizu and Miwa 2018] [Shimizu and Miwa 2019] [Skelton 1968] [Smith 1958] [Smith 1961] [Spreemann 1960] [Spreemann 1966] [Spreemann 1968] [Thirumaleshwar Hegde et al 2020] [Trenka 1967] [Wang et al 2019] [Wang et al 2026] [Winston 1970] [Xia et al 2026] [Yan et al 2023] [Yang et al 2024] [YAV-8B reaction control system 1993] [Yue et al 2026] [Yuksek and Inalhan 2022] [Zhao et al 2023] [Zhao et al 2024] [Zhao et al 2026] [Zhou et al 2025]

Handling and flying qualities criteria

Handling and flying qualities criteria and specifications for V/STOL aircraft, helicopters, rotorcraft and eVTOL aircraft, with the conventional-aircraft criteria and pilot rating methods they rest on.

The cluster is weighted to the X-22’s working life. Of the 204 records, 99 date from 1966 to 1989, and six titles name MIL-F-83300.

204 records. [Abdulrahim et al 2017] [Adams and Hatch 1971] [Air Force Test Pilot School Edwards Afb Ca 1969] [Air Force Test Pilot School Edwards Afb Ca 1981] [Air Force Test Pilot School Edwards Afb Ca 1988] [Air Force Test Pilot School Edwards Afb Ca 1988] [Air Force Test Pilot School Edwards Afb Ca 1988] [Air Force Test Pilot School Edwards Afb Ca 1989] [Air Force Test Pilot School Edwards Afb Ca 1990] [Air Force Test Pilot School Edwards Afb Ca 1990] [Air Force Test Pilot School Edwards Afb Ca 1990] [Air Force Test Pilot School Edwards Afb Ca 1991] [Air Force Test Pilot School Edwards Afb Ca 1991] [Air Force Test Pilot School Edwards Afb Ca 1992] [Alexander 2025] [Alscher et al 1966] [Anderson 1960] [Anderson 1960] [Anderson 1966] [Anderson 1970] [Anderson 1979] [Anderson 1985] [Anderson and Foster 1984] [Anderson and Schroers 1972] [Antonakis 2025] [Antonakis 2025] [Ashkenas 1965] [Ashkenas 1965] [Ashkenas 1984] [Bachelder et al 2021] [Bailes et al 1975] [Barbini et al 1970] [Baughman and Longeauay 2015] [Berger et al 2010] [Besco 1965] [Bivens 1985] [Bivens 1986] [Black and Moorhouse 1979] [Blanken et al 2000] [Boeing Vertol Co Philadelphia Pa 1983] [Boothe et al 1974] [Bowes and Miller 1976] [Brauser 1982] [Breul 1963] [Campos and Marques 2021] [Cao et al 2022] [Cappetta 1993] [Cappetta and Johns 1990] [Carlson 1958] [Cavallo et al 1990] [Chalk and Key 1969] [Chalk and Saunders 1969] [Chalk et al 1969] [Chalk et al 1974] [Chance Vought Corp Dallas Tx 1979] [Cheatham and Hackler 1966] [Chowhan et al 2019] [Blanken and Whalley 1993] [Citurs and Caton 1985] [Clark 1980] [Clark 1982] [Clarke and Griffin 2001] [Clarke et al 2003] [Corless and Blanken 1983] [Corliss 1982] [Corliss 1983] [Craig et al 1978] [Crim 1959] [Crombie and Moorhouse 1980] [Crother et al 1973] [Curry et al 1965] [Curtiss 1971] [Daughaday and Duwaldt 1955] [Difranco 1970] [DiFranco 1971] [Doman 1995] [Dooley 1977] [Drinkwater and Feistel 1965] [Dudgeon et al 1998] [Dussart et al 2019] [Ford 1989] [Franklin 2000] [Franklin 2002] [Garkushenko and Vinogradov 2016] [Garrard and Liebst 1990] [Garren 1962] [Gerdes and Hynes 1971] [Gerken 1979] [Goldstein 1982] [Gorham 1977] [Green 1972] [Griffin and Bellaire 1968] [Harper and Cooper 1986] [Harper and Sardanowsky 1969] [Harper et al 1969] [Harper et al 1970] [Hebbar and Pashilkar 2016] [Helicopter Handling Qualities 1982] [Henriquez Huecas et al 2022] [Hess 2016] [Higgins and Shomber 1965] [Hoffman 1969] [Hoffman 1969] [Hoffman 1971] [Hoffman et al 1970] [Hoh and Mitchell 1983] [Hoh et al 1981] [Holladay 1966] [Holzhauser et al 1965] [Huff 1949] [Hung and Dai 2025] [Ioannis and Ioannis 2026] [Jacobson and Joshi 1977] [Jacobson and Joshi 1978] [Ji et al 2022] [Johnston and Friend 1965] [Johnston et al 1965] [Johnston et al 1974] [Jusko and Berger 2026] [Key 1971] [Key 1982] [Kidd and Bull 1963] [Kozhanov et al 2022] [Kriechbaum and Lollar 1968] [Kriechbaum and Stineman 1972] [Kuhn et al 1964] [Lacey 1977] [Lantzsch et al 2014] [Lateral-Directional Flying Qualities in 2002] [Lawrence et al 2016] [Lawrence et al 2018] [Lebacqz and Scott 1984] [Levison 1982] [Li et al 2026] [Liao et al 2025] [Lollar and Kriechbaum 1968] [Lollar and Matous 1963] [Longitudinal Flying Qualities in 2002] [Low and Garrard 1991] [Malpica and Lusardi 2013] [Malpica et al 2011] [Malpica et al 2012] [Martin 1963] [McCormick 1969] [Memon et al 2021] [Miller 1970] [Miller and Clark 1965] [Mitchell and Hoh 1983] [Mitchell et al 2003] [Miyajima 1969] [Momiyama 1977] [Moorhouse et al 1989] [Nagata et al 1973] [Nagy and Kirsten 1976] [Nettleton 1965] [Nonnenmacher and Jones 2016] [Place et al 1974] [Pomfret and Clarke 2009] [Portapas and Cooke 2020] [Rauer 2019] [Reeder 1958] [Reschak 1970] [Reschak 1971] [Rich 1969] [Ritter et al 1971] [Rotorcraft handling qualities design 2011] [Rylko et al 2025] [Saetti and Rogers 2024] [Safe Coordinated Maneuvering of Teams of Multirotor Unmanned Aerial Vehicles: A Cooperative Control Framework for Multivehicle, Time-Critical Missions 2016] [Salem et al 2014] [Shomber and Gertsen 1967] [Shubert and Jones 2025] [Slaughter 1958] [Smith 1966] [Smith 1977] [Smith 2023] [Smith and Geddes 1979] [Srinathkumar 2011] [Srinathkumar 2015] [Stapleford and Walton 1966] [Swaim 1970] [Swaim and Yen 1979] [Tapscott 1960] [Tapscott 1964] [Tischler and Hoh 1982] [Truong et al 2016] [Vinje 1968] [Walter et al 2020] [Walter et al 2022] [Wang and Chen 2024] [Wei et al 2023] [Welter 1966] [Wernecke 1958] [Westbrook 1964] [Whitbeck and Hofmann 1978] [Wilhelm and Schafranek 1986] [Willebeek-LeMair and Rhinehart 2023] [Wilson et al 1993] [Wright and Lappos 1981] [Yan and Chen 2019] [Yu et al 2020] [Yuan et al 2022] [Zanoni et al 2022] [Zhou et al 2017]

Multirotor and unmanned vertical-takeoff vehicles

Multirotor and other unmanned vertical-takeoff vehicles, mostly their modelling and control.

All 86 records date from 2015 or later.

86 records. [Abdessameud and Janabi-Sharifi 2015] [Abdul Ghaffar et al 2021] [Ahmadi Dastgerdi and Nabavi-Chashmi 2026] [Ahmed and Xiong 2024] [Akshat Deshpande and Shyamkant Limaye 2016] [Aláez et al 2023] [Badea et al 2026] [Bangura and Mahony 2017] [Barra et al 2020] [Bezerra and Santos 2022] [Bouzid et al 2017] [Chen and Pérez-Arancibia 2024] [Chen et al 2019] [Choi et al 2017] [Chung and Son 2020] [Chung and Son 2023] [Chung et al 2019] [Czyba et al 2018] [Dargham et al 2015] [De Wagter and Meulenbeld 2019] [Derawi et al 2015] [Ducard and Allenspach 2021] [Edmonds and Yi 2021] [Erasmus and Jordaan 2020] [Faizal et al 2018] [Fernandez et al 2023] [Gong et al 2025] [Habibnia and Pascoa 2021] [Hafezi et al 2022] [Halder and Benedict 2018] [Hashim 2022] [Hayama et al 2021] [Hegde 2020] [Invernizzi et al 2020] [Iriarte et al 2021] [Islam et al 2017] [Islam et al 2020] [Ispiryan 2024] [Jabbari Asl 2019] [Jung et al 2019] [Kulathunga and Klimchik 2023] [Lee and Son 2021] [Lei et al 2020] [Lei et al 2021] [Lei et al 2023] [Lei et al 2023] [Li et al 2018] [Lin and Supsukbaworn 2017] [Lin et al 2024] [Logan et al 2017] [Lovas 2026] [Manzoni et al 2024] [Mendes and Egbue 2018] [Milić et al 2025] [Miyazaki et al 2020] [Moeini et al 2021] [Murata et al 2025] [Nabadova and Sabziev 2025] [Noboni et al 2025] [Nugroho et al 2022] [Porter et al 2016] [Reut and Tsukanov 2025] [Riboldi and Rolando 2023] [Ruggia 2025] [Runco and Benedict 2023] [Saied et al 2019] [Saied et al 2019] [Saied et al 2024] [Slim et al 2021] [Strampe and Klingauf 2026] [Takita and Kashitani 2016] [Takita et al 2018] [Tolba and Shirinzadeh 2022] [Ram Rohit 2024] [Wang 2019] [Wang and Zhang 2018] [Wang et al 2017] [Whidborne and Cooke 2017] [Wijaya et al 2023] [Wokanubun et al 2023] [Yamaguchi et al 2026] [Yu and Kwon 2017] [Zimmer and Gagnon 2023] [Öznalbant and Kavsaoğlu 2018] [聂 2017] [鲁 2020]

Flight control, displays and guidance for vertical takeoff

Flight control systems, stability augmentation, displays, guidance and instrument approach for vertical-takeoff aircraft and helicopters.

105 records. [A piloted simulator investigation 1979] [Alford and Luoma 1960] [Anderson 1967] [Comer et al 2025] [Arizaga et al 2021] [Axten et al 2024] [Bird 1970] [Bird 1970] [Bird and McLarty 1970] [Boothe and Ledder 1973] [Champine 1971] [Chu and Berman 2025] [Clark 1975] [Craig 1961] [Craig and Heffley 1972] [Demko and Boschma 1979] [Di Carlo et al 1971] [Douglas Aircraft Co Inc Santa Monica Ca 1963] [Downing and Bryant 1979] [Drinkwater et al 1963] [Drinkwater et al 1965] [Duda et al 2010] [Dukes 1970] [Ema 1992] [Experimental Investigation of Control-Display Requirements for VTOL Instrument Transition 1978] [Fay 1963] [Faye 1960] [Foster et al 1987] [Franklin et al 1986] [Frost et al 2002] [Fry et al 1966] [Gallagher and O’Donnell 1965] [Garren and Kelly 1966] [Garren et al 1969] [Garren et al 1970] [Gentry et al 1966] [Gerdes 1964] [Gerdes 1977] [Guy 1976] [Hammond and Margason 1964] [Hegarty et al 1965] [Hill 1966] [Hiller Aircraft Corp Palo Alto Ca 1965] [Hoh and Ashkenas 1981] [Holzhauser et al 1971] [Holzhauser et al 1972] [Hoogreef and Soikkeli 2022] [Hua et al 2015] [Cook and Gregory] [Johannsen and Govindaraj 1978] [Jokar and Khoshnood 2026] [Kaufman 1965] [Kelly et al 1973] [Kelly et al 1974] [Kelly et al 1979] [Kirby and Newsom 1964] [Klinar and Craig 1961] [Laskowitz 1961] [Lazer and Kuatbayeva 2026] [Lebacqz 1979] [Lebacqz 1982] [Lebacqz 1983] [Lebacqz and Aiken 1977] [Lebacqz et al 1981] [Leelaburanathanakul et al 2021] [Liu et al 2022] [McGee 1977] [McGee et al 1981] [McIntyre 1963] [Merrick 1984] [Merrick and Gerdes 1978] [Merrick et al 1990] [Mitchell et al 2014] [Miwa and Maruhashi 2016] [Morris et al 1964] [Mueller 1965] [Ostheimer and Giguere 1963] [Patton 1972] [Pauli 1965] [Pauli 1969] [Pauli et al 1970] [Renselaer 1974] [Ricardo Jr and Santos 2023] [Roscoe et al 1984] [Schwartz and Shearer 1964] [Seher-Weiß and Jones 2021] [Seibel 1963] [Selman 2023] [Shivers 1967] [Spreemann 1958] [Spreemann and Kuhn 1956] [Stendahl 1985] [Stortz and Franklin 1996] [Takahashi et al 2017] [The role of human 1986] [Tosti 1962] [VTOL control and display 1988] [Wachtell 1970] [Wilson et al 2012] [Winick 1968] [Ye et al 2020] [Young 1969] [Yuan 1970] [Zhao and Liu 2018] [Çakan 2026]

Propeller slipstream and wing interaction

Propeller slipstream and its interaction with the wing, from deflected-slipstream V/STOL tests to the transonic wing in a slipstream.

None of the 26 records is later than 1989.

26 records. [Bencze et al 1978] [Brenckmann 1958] [Butler et al 1966] [Cummings 1968] [Fink 1963] [Franke and Weinig 1939] [Goland et al 1964] [Hayes et al 1958] [Howard and Miley 1989] [Howard et al 1985] [Kirby 1956] [Kuhn 1957] [Lan 1975] [Mallen and Dancik 1959] [Winston and Huston 1962] [Miley et al 1985] [Miley et al 1986] [Newsom 1967] [Rizk 1980] [Rizk 1981] [Schouten 1982] [Stuper 1938] [Takagi and Inumaru 1967] [Turner and Drinkwater 1963] [Vidal et al 1960] [Welge and Crowder 1978]

Hover performance and hovering flight

Hover performance of rotors, propellers and ducted rotors, and the stability of hovering flight.

57 records. [Arnold 2025] [Bramwell 1956] [Cameron and Sirohi 2019] [Cameron et al 2016] [Chaderjian 2023] [Choi et al 2022] [Coward 1955] [Dai et al 2025] [Eilertson 1977] [Erickson et al 1965] [Felker 1993] [Goland 1956] [Hammond 1966] [Han et al 2021] [Helf et al 1973] [Hohenemser 1956] [Hwang and Kwon 2015] [Jacobellis et al 2021] [Jacobson and Smith 2018] [Jimenez Garcia and Barakos 2018] [Johnson and Sirohi 2024] [Kirk and Pitt 1969] [Kocurek and Tangler 1977] [Kozapalli and Warmbrodt 1997] [Lee and Dassonville 2020] [Lee and Kim 2020] [Lucassen and Sterk 1965] [Mihaila et al 2026] [Mortimer et al 2026] [Chaderjian] [Overmeyer and Martin 2017] [Park and Kwon 2021] [Payne 1955] [Pohly et al 2021] [Ramanujam and Abhishek 2022] [Ramasamy et al 2010] [Rand and Khromov 2017] [Shahmiri 2017] [Sheng 2018] [Shivers 1961] [Shrestha et al 2016] [Son and Kim 2023] [Szolnoky Cunha and Ortiz 2024] [Tian et al 2026] [Velkoff 1981] [Wang et al 2018] [Webb and Willer 1952] [White and Crimi 1961] [Koning et al] [Koning et al] [Xu et al 2025] [Zhao 2018] [Zhao and Li 2018] [Zhao et al 2014] [Zhao et al 2019] [Zhou et al 2022] [Zhu et al 2023]

Vertical and short takeoff configurations

Vertical and short takeoff configurations other than the ducted propeller, covering tilt wings, deflected slipstreams, jet lift and lift engines, and programmes such as the XC-142A and XV-4.

81 records. [Adamson 1968] [American Helicopter Society Inc Washington Dc 1965] [Beppu et al 1966] [Boeing Co Morton Pa Vertol Div 1956] [Brown and Fisher 1968] [Campbell 1968] [Chambers and Grafton 1966] [Chambers and Grafton 1970] [Chance Vought Corp Dallas Tx 1963] [Conference on V/Stol Aircraft 1960] [Curtiss et al 1961] [Curtiss et al 1967] [Decker 1966] [Dyke 1963] [Edenborough 1968] [Egerton and Fitzpatrick 1967] [Garner 1967] [General Electric Co Cincinnati Oh 1964] [Giurca 2025] [Goodson 1966] [Grima 2016] [Ground tests of piasecki 59K VTOL 1958] [Hall 1965] [Hargraves 1961] [Heyson and Grunwald 1966] [Hebert et al 1970] [Hebert et al 1970] [Hesse 1965] [Hill and Waters 1974] [Huston and Winston 1969] [Irvine 1968] [Jacobson 1965] [Hassell and Kirby 1966] [Campbell 1964] [Kanishk Mahaveer Jain 2025] [Goodson 1966] [Kirby 1960] [Kirby 1961] [Kirby et al 1963] [Lacey et al 1979] [Laskowitz 1963] [Lavi 1967] [Liu 1962] [Lockheed-Georgia Co Marietta 1969] [Loewy and Yntema 1958] [Margason 1970] [Marsh 1967] [McCormick and Mallen 1957] [Mort and Yaggy 1963] [NASA Conference on V/STOL 1960] [Nicholson and lowry 1966] [O’Malley 1959] [Payne and Henry E. 1961] [PlCKERELL and Cresswell 1968] [Putman 1961] [Kuhn and McKinney 1965] [Kirby 1960] [Roberts 1965] [Rolls 1960] [Schwaiger and Wills 2016] [Scientific and Technical Information Division 1966] [Shimizu and Miwa 2019] [Shimizu and Miwa 2020] [Skifstad 1970] [Smith 1959] [Smith 1961] [Snyder 2014] [Spillman 1967] [Stepniewski 1957] [Stepniewski and Schneider 1964] [Geuther et al 2020] [Strand and Levinsky 1969] [Stuart 1957] [Swaim 1969] [Swaim and Connor 1968] [Trenka 1966] [Vertical Takeoff and Landing 2014] [VTOL - 1968 1969] [VTOL BOOSTER 1961] [Ward 1960] [Zosimovych 2022]

Propellers, rotors and rotorcraft

Propellers, rotors and rotorcraft design outside the hover-performance literature.

38 records. [Army Aviation Board Fort Rucker Al 1964] [Avant and Cunningham 1952] [Briardy and Head 1968] [Cardinale 1969] [Casadei et al 2019] [Crist and Symes 1981] [Doblhoff 1956] [Doman 1957] [Drees 1963] [Dutton 1967] [Ekquist 1965] [Fay and Johnstone 1960] [Ham and Madden 1965] [Hiller Aircraft Corp Palo Alto Ca 1965] [Hiller Aircraft Corp Palo Alto Ca 1965] [Hiller Aircraft Corp Palo Alto Ca 1965] [Hiller Aircraft Corp Palo Alto Ca 1965] [Hiller Aircraft Corp Palo Alto Ca 1965] [Kidwell and Wray 1964] [Kolesar et al 1970] [Lerbs 1955] [Lieberman and Hoffman 1964] [McCormick 1956] [McDonnell Aircraft Corp St Louis Mo 1950] [Morduchow et al 1953] [Nelson 1965] [Peters et al] [Zawodny et al] [Roberts 1956] [Schroers and Antoniou 1966] [Sikorsky 1960] [Simpson 1967] [Sissingh 1941] [Smith 1968] [Snyder 1950] [Stange 1959] [Tung 1950] [Yamazaki 1965]

Where the Framing Breaks Down

The thrust augmentation figure assumes an ideal duct with no internal losses and an assumed exit area ratio. A real duct has friction, tip clearance and a diffuser that may separate, and the figure of merit derived here absorbs all of it into a single number.

The normal force calculation is the weakest part of the article. It is momentum turning multiplied by an assumed lip factor of two, and the sensitivity is large, giving a peak of anywhere between 0.54 and 1.35 times the aircraft weight over four ducts as that factor runs from one to two and a half. With $k$ the lip factor and $N_{\text{mom}}$ the momentum term at the peak, half of the 3,941 pounds computed with $k = 2$,

\[\frac{4 N_{\max}}{W} = \frac{4 k N_{\text{mom}}}{W} = \frac{4 k \times 1{,}971}{14{,}600} = 0.54\, k\]

The magnitude is not robust, and neither is the location, since the measured pitching moment peaks near 60 degrees of duct incidence in Biggers et al 1966, well before the momentum estimate’s peak.

The conversion corridor is derived rather than assumed, from a wing area implied by the conversion speed, since no data sheet read here gives one. That derivation rests on an assumed maximum lift coefficient of 1.4 and on the assumption that the wings carry the whole weight at the conversion speed, and the resulting area is checked against the two spans, with no published figure to compare.

The article states the weakness of the normal force openly. The momentum model overestimates it badly wherever the crossflow parameter exceeds about one half, which is most of the middle of the conversion, and the article marks the number the model produces there as not credible and does not rely on it.

The tandem interference enters only through the tunnel and model results reported above, which show it protecting the rear ducts from stall and unsettling the hover near the ground. No relation in the article models it.

Several figures are assumptions with no source, and each is labelled where it is used. They are the 14,600 pound three-engine hover weight, the propulsive efficiency of 0.80 at maximum speed, the wing drag coefficient of 0.10 in the corridor calculation, the maximum lift coefficient of 1.4, the lip factor of two, the 700 foot per second tip speed and the elevon lift coefficient of 0.9.

The Source Base

The X-22 is well documented on the flying-qualities side and documented on the ducts mainly by tunnel tests, of which the full-scale test of its own duct is the most important. The programme’s own retrospective is The Bell X-22A V/STOL, Variable Stability Research Airplane, Lessons Learned, written twenty-four years after first flight, and the tri-service description of the aircraft is contemporary with the contract.

The primary documents behind the account of the vehicle are NASA reports. NASA TM-81156 describes the aircraft, its variable-stability system and its research programmes through 1979. Gamse and Mort 1967 report the full-scale duct, Biggers et al 1966 and Freeman and Newsom 1966 the large-scale and free-flight models, Lebacqz 1974 and Lebacqz and Aiken 1975 two of the Calspan programmes, and NASA CR-1901 the initial military flight tests at second hand. The programme’s retrospective of 1990 and the 1969 report of the initial military tests are not freely available, the NASA records of Aiken et al 1977 and Smith 1974 carry no full text, and the Defense Technical Information Center reports cited here, among them Bell Aerospace Co Buffalo Ny 1970, were not retrievable. The accident, the first flight of the second aircraft, the last flight and the maximum speed therefore rest on secondary accounts that disagree with each other.

The imbalance is a consequence of what the aircraft became. Eighteen years of variable-stability work generated a large literature about handling qualities in which the X-22 is the instrument rather than the subject, and comparatively little was published about the ducted propulsion after the configuration stopped being a candidate for production.

The research survey holds 1,691 research records drawn from the NASA Technical Reports Server, the Defense Technical Information Center and the journal literature indexed by Crossref, of which 499 are report primaries, being 29.5 percent, by the identifier rule set out under The Definition of Primary. The median publication year is 2013. Of the 1,659 dated records, 258 were published in 1965 or earlier, before the aircraft flew, being 15.6 percent, and 809 were published in 2015 or later, being 48.8 percent.

The Definition of Primary

Primacy is a property of the document, and this survey derives it from the identifier. A record is a report primary when its address is on the NASA Technical Reports Server or it carries a digital object identifier under the 10.21236 prefix, which Crossref registers for Defense Technical Information Center reports. The vehicle’s own record is primary under that rule only in part. Of the 16 titles that name the X-22A, 10 are report primaries. The papers presented to the American Helicopter Society and to the American Institute of Aeronautics and Astronautics, among them the programme’s 1990 retrospective, and a 1973 journal description of the aircraft as an in-flight simulator are not, so 29.5 percent is a lower bound on how much of the survey is first-hand. The NASA survey of in-flight simulators and the NASA contractor report on terminal guidance, on which much of the account of the aircraft rests, are curated references rather than survey records.

Thin Subjects

Seven subjects that this article’s derivations depend on have thin literatures. Each count below is the number of research records in this survey whose title carries at least one of the listed terms, with a hyphen read as a space. Where a row joins two groups of terms with the word together, a title must carry a term from each group.

Subject	Title terms counted	Records
the normal force and pitching moment of a duct at incidence	normal force, pitching moment together with duct, ducted, shroud, shrouded, fan, propfan	1
thrust augmentation by a shroud	thrust augmentation, augmentation ratio, static thrust together with duct, ducted, shroud, shrouded	3
the cross-shafted transmission	cross shaft, cross shafting, cross shafted, interconnect, interconnected, shaft coupled	1
duct buzz	buzz	0
height damping and height control in the hover	height damping, vertical damping, height control, heave damping, vertical rate command	4
vanes and elevons in a duct exit	vane, vanes, elevon, elevons, deflector together with duct, ducted, shroud, shrouded	2
hover with an engine out	engine out, engine failure, engine inoperative, one engine together with hover, VTOL, V/STOL, vertical, tilt rotor, eVTOL, helicopter	7

The force the conversion turns on is rarely named. One title names a pitching moment, for an annular lift fan aircraft, and none names a normal force, although tunnel reports in the incidence cluster measure the loads on a duct at angle of attack, of which those are the components. The estimate of that force above therefore rests on the momentum relation and on the full-scale duct test.

The shroud’s thrust gain is measured but not named. Three titles name static thrust together with a duct or shroud, and none uses the phrase thrust augmentation, so the twenty-six percent rests on the momentum relation derived above.

The cross-shaft has almost no literature of its own. One title names a shaft-coupled drive, a 1977 analysis of control concepts for gas-coupled and shaft-coupled lift fan systems, and of the 12 records of the transmission cluster only a 1974 study of integrated propulsion and energy transfer control for lift-fan aircraft concerns the interconnection of separate lift units, while the rest concern drive trains, gearboxes and shafts.

Duct buzz has none. No title in the survey names buzz, and the account of it above rests on the initial Navy flight tests as summarised in NASA CR-1901.

Height damping is a helicopter subject in the literature. Four titles name height control or a vertical rate command. Three concern helicopters or rotorcraft, and the fourth is a 1964 piloted motion simulator study of height-control requirements for vertical-takeoff aircraft. The low natural height damping that the Calspan pilots met in the X-22A is documented in the programme’s own reports rather than in a literature of its own.

The exit vane appears twice. Two titles name vanes in a duct, a 1959 study of a ducted fan with turning vanes and a 2021 computation of a ducted fan and vane configuration in hover, and no title names an elevon.

Engine-out hover is a recent subject. Seven titles name an engine failure or an inoperative engine together with a vertical-flight vehicle, all of them published in 2019 or later, and none names a cross-shafted or interconnected drive, which is the arrangement that makes the X-22A’s engine-out case a loss of power rather than of symmetry.

What the Survey Admits

A record is admitted only when a person reading its title finds it on this article’s subject, which is the ducted propeller, the vertical-takeoff problem around it, and the handling-qualities work the X-22 served. Many words this subject depends on also name something else, and a record that shares only such a word is excluded.

A ducted propeller is a marine technology, an aeronautical one and a wind-energy one, and a title search on the phrase returns all three. The Kort nozzle is a duct around a ship’s propeller and its literature is large, old and entirely irrelevant here. Diffuser-augmented wind turbines and tidal turbines are ducted rotors whose momentum theory is the same and whose application is not. Marine, tidal and wind-turbine records are excluded from the reference base, and often only the venue separates them, because their titles are indistinguishable from the aeronautical ones.

Shared words also mislead in the other direction. A paper on the United States Marine Corps and vertical takeoff is not marine engineering and belongs here, and a paper on Building Credible Flight Models is not building aerodynamics. The phrase tandem propeller, by contrast, also describes a submarine.

Five more words are shared, and four of the five are shared inside engineering rather than outside it. Trim means the aircraft’s balance, the interior fittings, the engine setting and the attitude of a planing hull, so the word alone gathers door seals and engine test sets. Inlet belongs to propulsion and to coastal geomorphology, where a tidal inlet is the channel between barrier islands, so inlet flow is also a subject in the literature of sediment transport. Figure of merit is the hover efficiency of a rotor and also a performance metric in thermoelectric materials, electronic devices, optics and nuclear-radiation detectors. Electric propulsion means an aircraft motor here and an ion thruster in the spacecraft literature, where it covers subjects such as optimal low-thrust interplanetary trajectories. And transition is the conversion from hover to wingborne flight here and the change from a laminar to a turbulent boundary layer in aerodynamics generally, so a study of transition on a hovering rotor blade belongs to hover performance and a study of transition on a slender re-entry body does not belong at all. The ducted-fan engine of the 1940s and 1950s is the turbofan, and ducted propagation is a phenomenon of radio and ocean acoustics.

Excluded besides the homonym families are community noise studies that contain no aircraft, ducts that carry steam, radiation or fuel sprays, the disc loading of hummingbird wings and the aerodynamics of flapping wings, wing-in-ground-effect craft, supersonic and swept-wing aerodynamics and wave drag, jet mixing, pulsejets, ducted rockets and rocket nozzles in ducts, the handling qualities of road vehicles, room and duct silencing acoustics, signal-processing noise reduction, machines that use a ducted fan only as an actuator, such as a crane, a pipeline robot and a wall-climbing robot, and records that are not works, meaning figures, tables, supplementary files, peer-review reports, notices of erratum, correction, retraction or withdrawal, and journal front matter. A review of a book, a dictionary or short encyclopedia entry and a notice such as an editorial, an announcement or a news item are not research works either and are excluded. A document deposited twice under two identifiers is counted once. Doubtful records are kept where the problem is shared, such as the downwash load of an agricultural spraying aircraft, a cross-Channel hovercraft and an exterior noise test facility, because each bears on a consequence of high disc loading that the X-22 also produced. Engine nacelle inlets in crosswind are kept because their lip separates for the reason a duct lip does.

Every research title has been read for relevance, so the off-topic share that remains is a matter of reading judgement. A reading of titles cannot show that a work is relevant in substance, so this is a statement about titles only. Three further doubtful classes are kept besides those just named. The aeroacoustics of axial fans in ducts outside aircraft, such as ventilation and server-rack fans, is kept because the fan and duct physics is the same. The handling qualities of spacecraft and lunar landers are kept because their criteria were developed by the same method as the vertical-takeoff criteria. Noise studies of aircraft in general, such as airport and landing-gear noise, are kept because they frame the community-noise problem that ducted fans are now asked to meet.

Epistemic State

Established and not in dispute. The aircraft first flew in 1966, one of two was lost in 1966, and the survivor operated as a variable-stability in-flight simulator from 1968 and was scheduled to fly research programmes through 1980 on a 500-hour service life.

Documented in primary reports. The full-scale duct’s area ratio of 1.165, figure of merit of 81 percent on exit area, propulsive efficiency of 74 percent and lip stall boundaries. The thrust-to-weight ratio of 1.35. The variable-stability system’s architecture and limits, and the results of the four research programmes completed by 1979.

Disputed among secondary accounts. The cause of the 1966 accident, the first flight of the second aircraft, the last flight in 1984 or 1988, the career hours, and whether the specified maximum speed was reached.

Computed here from quoted figures. The 26 percent thrust augmentation follows from momentum theory given the exit area ratio. The figure of merit of 0.716 follows from reading the quoted 35 percent as excess power, and reproduces the assumed three-engine hover weight to 0.83 percent. The documented reading, a thrust-to-weight ratio of 1.35, leaves an eleven percent thrust margin on three engines, and the tunnel’s best figure of merit makes it reachable at the 15,000 pound design weight only with no installation loss.

Not supported. That the 35 percent was chosen as the bare minimum for three-engine hover. That reading requires the figure to be excess power, and the primary statement gives it as thrust to weight.

Weakly supported. The magnitude of the duct normal force, which depends on an assumed lip factor, and the position of its peak, which falls later in the conversion than the measured pitching-moment peak.

Not addressed. A model of the tandem interference, and the structural and weight cost of the cross-shafted transmission.

Out of Scope

The detailed design of the transmission. The variable-stability system’s control laws, which are a subject in their own right. The tri-service requirement that produced the programme and the politics of its cancellation. Comparison with the tilt-rotor line that eventually succeeded, which belongs to a later article.

Conclusion

The duct is the aeroplane. It gains twenty-six percent more static thrust for nothing but a ring of metal, enough for a seven-foot propeller to carry ninety-five pounds per square foot, and it charges for that in the conversion by acting as an annular wing whose normal force peaks at about the duct thrust in the middle of the manoeuvre.

The margin depends on what the 35 percent measured. Read as excess power it leaves one and a quarter percent after an engine fails, and read as thrust to weight, which is how the NASA survey states it, it leaves eleven percent. The full-scale duct’s measured figure of merit makes the thrust reading at the 15,000 pound design weight possible only with almost no installation loss, so the design was at best tight, and how tight remains open on the record read here.

And the aeroplane outlived its question. It was built to find out whether four ducted propellers made a practical transport, and the answer was that they made a practical aircraft and an unattractive transport. What kept it flying for eighteen years was that somebody realised a machine which can hover and convert, and whose control system can be reprogrammed to feel like something else, is more valuable as a laboratory than as a prototype.

None of the four aircraft before it in this series was rescued that way. The X-18 is generally assumed to have been scrapped, the X-19 crashed, the X-20 was cancelled and the X-21 was left derelict and used as a photographic target, in the Wikipedia entry on the X-21. The X-22 was repurposed, and it is the only one of them still whole, stored at the Niagara Aerospace Museum according to the Wikipedia entry.

References

Books

• Jenkins Landis and Miller 2003 American X-Vehicles, An Inventory X-1 to X-50
• Miller 2001 The X-Planes, X-1 to X-45

Reference

• Ad Hoc Study Group for Aeronautics, V/STOL Flight Simulation, NASA TM-81156, 1979
• Beilman, X-22A Variable Stability System, 1965
• Bell X-22A
• Disk loading
• Northrop X-21
• Vertical Flight Society, Vertipedia, Bell X-22A
• Wolkovitch, Lamont and Lochtie, Fundamental Limitations on V/STOL Terminal Guidance Due to Aircraft Characteristics, NASA CR-1901, 1971

Related Post

• X-1
• X-10
• X-11
• X-12
• X-13
• X-14
• X-15
• X-16
• X-17
• X-18
• X-19
• X-2
• X-20
• X-21
• X-3
• X-4
• X-5
• X-6
• X-7
• X-8
• X-9
• X-Planes series

Research

• A piloted simulator investigation 1979
• A Variable-speed Control Method to Reduce Ducted-fan Thrust Fluctuations for Flying Car Utilization 2024
• Aalmoes et al 2025
• Abdelhamid 1974
• Abdelhamid 1974
• Abdessameud and Janabi-Sharifi 2015
• Abdul Ghaffar et al 2021
• Abdulrahim et al 2017
• Aboelezz et al 2022
• Abrego et al 2002
• Abrego et al 2002
• Acheson 2011
• Adams and Hatch 1971
• Adamson 1968
• Adarkar and Hall 1969
• Adeline et al 2023
• Advanced Ducted Propulsor In-Flight
• Adámek et al 2021
• Afjeh 1991
• Agarwal 1997
• Ahmadi Dastgerdi and Nabavi-Chashmi 2026
• Ahmadi et al 2025
• Ahmed and Xiong 2024
• Ahmed et al 2025
• Ahn and Lee 2004
• Ai et al 2020
• Ai et al 2020
• Ai et al 2025
• Aiken et al 1977
• Air Force Test Pilot School Edwards Afb Ca 1969
• Air Force Test Pilot School Edwards Afb Ca 1981
• Air Force Test Pilot School Edwards Afb Ca 1988
• Air Force Test Pilot School Edwards Afb Ca 1988
• Air Force Test Pilot School Edwards Afb Ca 1988
• Air Force Test Pilot School Edwards Afb Ca 1989
• Air Force Test Pilot School Edwards Afb Ca 1990
• Air Force Test Pilot School Edwards Afb Ca 1990
• Air Force Test Pilot School Edwards Afb Ca 1990
• Air Force Test Pilot School Edwards Afb Ca 1991
• Air Force Test Pilot School Edwards Afb Ca 1991
• Air Force Test Pilot School Edwards Afb Ca 1992
• Aircraft Noise and Certification 2019
• Akash et al 2021
• Akshat Deshpande and Shyamkant Limaye 2016
• Akturk and Camci 2010
• Akturk and Camci 2022
• Akturk and Camcı 2014
• Al-Rubaye et al 2023
• Albachten 1956
• Albrecht 1988
• Ale Isaac Khoueini
• Alesin et al 2020
• Alexander 2025
• Alexander A Svetgoff and Kevin Walczyk
• Alfares 2026
• Alford and Luoma 1960
• Ali and Maqsood 2024
• Alkaabneh et al 2026
• Alomar and Shafqat 2026
• Alscher et al 1966
• Altouq et al 2025
• Alvarez et al 2023
• Aláez et al 2022
• Aláez et al 2023
• Ambroziak et al 2023
• American Helicopter Society Inc Washington Dc 1965
• An et al 2012
• Anderson 1960
• Anderson 1960
• Anderson 1966
• Anderson 1967
• Anderson 1970
• Anderson 1979
• Anderson 1985
• Anderson and Foster 1984
• Anderson and Schroers 1972
• Antonakis 2025
• Antonakis 2025
• Applin et al 1994
• Arabshahi and Pankajakshan 1994
• Arace et al 2026
• Araghizadeh et al 2024
• Arizaga et al 2021
• Armutcuoglu et al 2004
• Army Aviation Board Fort Rucker Al 1964
• Arnold 2025
• Aroussi 2025
• Aroussi 2026
• Ascani 1974
• Asher 1969
• Ashkenas 1965
• Ashkenas 1965
• Ashkenas 1984
• Atassi and Grady 2003
• Avant and Cunningham 1952
• Avanzini et al 2001
• Avanzini et al 2003
• Axten et al 2024
• Azimi et al 2014
• Bacchini and Cestino 2019
• Bachelder et al 2021
• Badea et al 2026
• Bahr et al 2021
• Bai and Zhou 2023
• Bai et al 2025
• Bai et al 2025
• Bailes et al 1975
• Balakrishnan and Srinivasan 2019
• Bangalee et al 2025
• Bangura and Mahony 2017
• Barbini et al 1970
• Bardera Mora and Matías García 2022
• Barra et al 2020
• Bauer and Redmann 2023
• Bauer et al 2021
• Baughman and Longeauay 2015
• Beck et al
• Beeler 1965
• Beilman 1973
• Bekofske et al 1977
• Bell Aerospace Co Buffalo Ny 1956
• Bell Aerospace Co Buffalo Ny 1956
• Bell Aerospace Co Buffalo Ny 1956
• Bell Aerospace Co Buffalo Ny 1970
• Bencze et al 1978
• Bennett and Fitzpatrick 2007
• Beppu et al 1966
• Berger et al 2010
• Besco 1965
• Betz 1966
• Bezerra and Santos 2022
• Bielak et al 2002
• Bigarella 2025
• Biggers et al 1966
• Bird 1970
• Bird 1970
• Bird and McLarty 1970
• Bivens 1985
• Bivens 1986
• Black and Moorhouse 1979
• Black and Wainauski 1967
• Blanken and Whalley 1993
• Blanken et al 2000
• Boeing Co Morton Pa Vertol Div 1956
• Boeing Vertol Co Philadelphia Pa 1983
• Bogdanović et al 2024
• Boldman et al 1991
• Boldman et al 1993
• Bonner et al 1975
• Bontempo and Manna 2016
• Boothe and Ledder 1973
• Boothe et al 1974
• Boucher et al 2023
• Bouzid et al 2017
• Bowes and Miller 1976
• Boxwell et al 1978
• Brabec and Helmich 2012
• Bramwell 1956
• Brancaleoni et al 2025
• Brauser 1982
• Bravo et al 2021
• Brenckmann 1958
• Breul 1963
• Briardy and Head 1968
• Brinkman and Johnson 2021
• Brinkman et al 2020
• Brizzolara et al 2009
• Brogan et al 1961
• Brown 1950
• Brown and Fisher 1968
• Brown and Harris 2020
• Brucculeri et al 2019
• Bruno Garcia Franciscone and Elton Fernandes 2023
• Bu et al 2026
• Buba et al 2024
• Bulaga 2005
• Bulat and Chernyshov 2026
• Bulat et al 2023
• Burnell and Fan 1957
• Burton et al 2026
• Busan et al
• Bustamante Alarcon et al 2023
• Bustamante Alarcon et al 2025
• Butler and Thomas 1964
• Butler et al 1966
• Buǧday and Saetti 2024
• Cai et al 2015
• Cai et al 2019
• Cai et al 2022
• Caldwell 1940
• Callaghan et al 1973
• Cameron and Sirohi 2019
• Cameron et al 2016
• Campbell 1964
• Campbell 1965
• Campbell 1968
• Campos and Marques 2021
• Cantos et al 2024
• Cantos et al 2026
• Cao et al 2005
• Cao et al 2021
• Cao et al 2022
• Cao et al 2022
• Cao et al 2024
• Cappetta 1993
• Cappetta and Johns 1990
• Cardinale 1969
• Cardoso et al 2022
• Carlson 1958
• Carmichael and McNay 1961
• Casadei et al 2019
• Castles et al 1960
• Caudle et al
• Cavallo et al 1990
• Cerny and Breitsamter 2020
• Chaderjian
• Chaderjian 2023
• Chaderjian et al 2002
• Chakraborty and Mishra 2023
• Chakraborty et al 2023
• Chalk and Key 1969
• Chalk and Saunders 1969
• Chalk et al 1969
• Chalk et al 1974
• Chambers and Grafton 1966
• Chambers and Grafton 1968
• Chambers and Grafton 1970
• Champine 1971
• Chance Vought Corp Dallas Tx 1963
• Chance Vought Corp Dallas Tx 1979
• Chandrasekaran et al 2021
• Chang et al 2023
• Cheatham and Hackler 1966
• Chen 2023
• Chen and Pérez-Arancibia 2024
• Chen et al 1991
• Chen et al 2015
• Chen et al 2017
• Chen et al 2018
• Chen et al 2019
• Chen et al 2021
• Chen et al 2023
• Chen et al 2025
• Chen et al 2025
• Chen et al 2026
• Chen et al 2026
• Cheng and Pei 2020
• Cheng and Pei 2022
• Cheng and Pei 2022
• Cheng and Pei 2023
• Cheng and Pei 2024
• Cheng and Pei 2026
• Cheng et al 2020
• Cheng et al 2026
• Chennuru et al 2025
• Chiu et al 2025
• Cho 2025
• Cho et al 2024
• Choi and Kim 2025
• Choi and Suk 2026
• Choi et al 2014
• Choi et al 2017
• Choi et al 2022
• Choi et al 2026
• Chowhan et al 2019
• Chu and Berman 2025
• Chu et al 2025
• Chu et al 2025
• Chung and Son 2020
• Chung and Son 2023
• Chung et al 2019
• Citurs and Caton 1985
• Ci̇zreli̇oğullari et al 2022
• Clark 1975
• Clark 1980
• Clark 1982
• Clark et al 1963
• Clark et al 1982
• Clarke and Griffin 2001
• Clarke et al 2003
• Cole and England 1967
• Cole et al 2021
• Coles 1968
• Colman et al 2011
• Comer et al 2025
• Comer et al 2026
• Comunian et al 2025
• Conference on V/Stol Aircraft 1960
• Cong et al 2024
• Cook and Gregory
• Cook and Hickey 1967
• Corless and Blanken 1983
• Corliss 1982
• Corliss 1983
• Corliss et al 1977
• Corliss et al 1978
• Cormier 1979
• Corsiglia et al 1979
• Corsiglia et al 1981
• Cousins et al 2026
• Coward 1955
• Craig 1961
• Craig and Heffley 1972
• Craig et al 1978
• Creer and McNeill 1956
• Crim 1959
• Crist and Symes 1981
• Crombie and Moorhouse 1980
• Crother et al 1973
• Cui et al 2025
• Cummings 1968
• Curry et al 1965
• Curtiss 1965
• Curtiss 1971
• Curtiss and Howard C. 1969
• Curtiss et al 1961
• Curtiss et al 1967
• Cusati et al 2023
• Czyba et al 2018
• Dai et al 2025
• Dallas and Irvin 1956
• Dargham et al 2015
• Darmstad et al 2024
• Darrah et al 2020
• Darvishpoor et al 2025
• Daud Filho and Belo 2023
• Daughaday and Duwaldt 1955
• Davies 1975
• Davis 1977
• Dayhoum et al 2024
• Dayhoum et al 2024
• Dayhoum et al 2025
• Dayhoum et al 2025
• de Laborderie and Moreau 2016
• de Vries et al 2019
• De Wagter and Meulenbeld 2019
• Dean 1979
• DeBilzan 1975
• Decker 1966
• Deckert and Rolls 1974
• Definitions of the Thrust of a Jet Engine and of the Internal Drag of a Ducted Body 1955
• Deming 1940
• Demko and Boschma 1979
• Deng et al 2018
• Deng et al 2020
• Denisenko et al 2021
• Derawi et al 2015
• Determination of flight characteristics 1967
• DeVore et al 2022
• Di Carlo et al 1971
• Di et al 2025
• Dickinson et al 2024
• Difranco 1970
• DiFranco 1971
• Distributed Electric Propulsion 2018
• Dittmar 1978
• Dittmar 1988
• Dittmar et al 1993
• Doblhoff 1956
• Doherty and Butzel 1979
• Doman 1957
• Doman 1995
• Donateo and Çinar 2022
• Dooley 1977
• Dorfling and Traub 2023
• Douglas Aircraft Co Inc Santa Monica Ca 1963
• Downing and Bryant 1979
• Doğru et al 2016
• Drees 1963
• Drinkwater and Feistel 1965
• Drinkwater and Rolls 1962
• Drinkwater et al 1963
• Drinkwater et al 1965
• Du et al 2025
• Ducard and Allenspach 2021
• Ducted fan
• Ducted Fan Model ala 2026
• Duda et al 2010
• Dudgeon et al 1998
• Dukes 1970
• Dunn 1997
• Dunn and Farassat 1998
• Dunn et al 1996
• Dussart et al 2019
• Dutton 1967
• Dyke 1963
• Dzulfikar et al 2025
• Ebus et al 2022
• Ebus et al 2024
• Echavarria and Poroseva 2015
• Edenborough 1968
• Editor 2016
• Edmonds and Yi 2021
• Effati and Banazadeh 2013
• Egerton and Fitzpatrick 1967
• Eilertson 1977
• Ekquist 1965
• Ellis 1971
• Ellis et al 2002
• Ema 1992
• Emami and Banazadeh 2015
• Eney 1968
• Englar and Kirkpatrick 1969
• Envia 2016
• Envia 2016
• Eqbal et al 2023
• Erasmus and Jordaan 2020
• Erickson et al 1965
• Eversman 1992
• Eversman and Moehring 1987
• Eversman and Roy 1993
• Experimental Investigation of Control-Display Requirements for VTOL Instrument Transition 1978
• Faizal et al 2018
• Falarski 1972
• Falarski and Mort 1971
• Fan et al 2017
• Fan et al 2018
• Fan et al 2018
• Fan et al 2020
• Fan et al 2022
• Fan et al 2023
• Fang and Callegaro 2021
• Fang et al 2022
• Fang et al 2025
• Farassat et al 2001
• Farazi and Zou 2024
• Farras et al 2021
• Favaro et al 2025
• Favaro et al 2025
• Fay 1963
• Fay and Johnstone 1960
• Faye 1960
• Fazeli Asl and Moosapour 2017
• Felker 1993
• Fernandez et al 2023
• Finger et al 2018
• Finger et al 2020
• Fink 1963
• Flight Characteristics of Quad Ducted Fan Helicopter with Thrust Vectoring Nozzles 2014
• Ford 1989
• Foster et al 1987
• Franke and Weinig 1939
• Franklin 2000
• Franklin 2002
• Franklin et al 1986
• Franz and Hauser
• Fredericks et al 2013
• Freeman and Newsom 1966
• Fricke and Bissell 1968
• Friedrich and Robertson 2015
• Frost et al 2002
• Fry et al 1966
• Fu et al 2023
• Fu et al 2025
• Fu et al 2025
• Gallagher 1971
• Gallagher and O’Donnell 1965
• Gallagher and Stoessel 1967
• Gamse and Mort 1967
• Gao and Xu 2022
• Gao et al 2025
• Garkushenko and Vinogradov 2016
• Garner 1967
• Garrard and Liebst 1990
• Garren 1962
• Garren and Kelly 1965
• Garren and Kelly 1966
• Garren et al 1969
• Garren et al 1970
• General Electric Co Cincinnati Oh 1964
• Gentili et al 2009
• Gentry et al 1966
• George et al 1964
• Georgiev 2025
• Gerdes 1964
• Gerdes 1977
• Gerdes and Hynes 1971
• Gerken 1979
• Geuther et al 2020
• Gholamian and Beik 2026
• Gill and D’Andrea 2020
• Gilman and Burdges 1967
• Gilmore and Grahame 1959
• Girard 1959
• Giurca 2025
• Glegg et al 1997
• Go et al 2022
• Go et al 2023
• Go et al 2023
• Goethert and Borchers 1974
• Goethert and Borchers 1975
• Goland 1956
• Goland et al 1964
• Goldschmidt and Ukeiley 2025
• Goldstein 1982
• Gong et al 2021
• Gong et al 2025
• Goodson 1966
• Goodson 1966
• Goodson and Grunwald 1962
• Goodson and Yaggy 1960
• Gorham 1977
• Goshtasbi et al 2025
• Goudswaard et al 2024
• Goyal and Cohen 2022
• Graf et al 2008
• Grafton and Newsom 1971
• Grant et al 2015
• Gray 1961
• Green 1972
• Greenberg and Ordway 1964
• Griffin and Bellaire 1968
• Griffith and Moller 1990
• Grima 2016
• Groeneweg 1977
• Ground tests of piasecki 59K VTOL 1958
• Guerrero et al 2003
• Guo and Zhou 2022
• Guo and Zhou 2022
• Guo and Zhou 2026
• Guo et al 2026
• Guy 1976
• Głowacki et al 2015
• Habibnia and Pascoa 2021
• Hafezi et al 2022
• Hajela and Chen 1988
• Halder and Benedict 2018
• Hall 1965
• Hall 1967
• Hall and Delaney 1990
• Hall and Delaney 1991
• Hall et al 1991
• Ham and Madden 1965
• Ham and Moser 1958
• Hamel 2017
• Hamidani and Kunz 2018
• Hammond 1966
• Hammond and Margason 1964
• Han and Pei 2026
• Han et al 2014
• Han et al 2019
• Han et al 2021
• Han et al 2024
• Hanson 2001
• Harada 2018
• Hargest 1966
• Hargraves 1961
• Harper and Cooper 1986
• Harper and Robert P. 1955
• Harper and Sardanowsky 1969
• Harper et al 1969
• Harper et al 1970
• Hashim 2022
• Hassell and Kirby 1966
• Hayama et al 2021
• Hayase 1974
• Hayase 1974
• Hayase 1974
• Hayase 1974
• Hayase 1974
• Hayase 1974
• Hayes et al 1958
• He and Leang 2025
• He et al 2013
• He et al 2020
• He et al 2025
• He et al 2026
• He et al 2026
• Hearn et al 2023
• Hebbar and Pashilkar 2016
• Hebert et al 1970
• Hebert et al 1970
• Hegarty et al 1965
• Hegde 2020
• Helf et al 1973
• Helicopter Handling Qualities 1982
• Henderson 1964
• Henderson et al 2005
• Henriquez Huecas et al 2022
• Henschel and Chetty 1989
• Henschel et al 1974
• Hess 2016
• Hesse 1965
• Heyson 1974
• Heyson and Grunwald 1966
• Hickling 1963
• Higgins and Shomber 1965
• Higgins et al 1966
• Hill 1966
• Hill 1983
• Hill and Waters 1974
• Hiller Aircraft Corp Palo Alto Ca 1956
• Hiller Aircraft Corp Palo Alto Ca 1965
• Hiller Aircraft Corp Palo Alto Ca 1965
• Hiller Aircraft Corp Palo Alto Ca 1965
• Hiller Aircraft Corp Palo Alto Ca 1965
• Hiller Aircraft Corp Palo Alto Ca 1965
• Hiller Aircraft Corp Palo Alto Ca 1965
• Hilton et al 1960
• Hiyama 1974
• Hoad and Gentry 1975
• Hoad and Gentry 1977
• Hoehne and Hoffman 1961
• Hoffman 1969
• Hoffman 1969
• Hoffman 1971
• Hoffman et al 1970
• Hoh and Ashkenas 1981
• Hoh and Mitchell 1983
• Hoh et al 1981
• Hohenemser 1956
• Holladay 1966
• Holzhauser et al 1965
• Holzhauser et al 1971
• Holzhauser et al 1972
• Hoogreef and Soikkeli 2022
• Hook et al 2011
• Horton et al
• Hossain and Bodson 2022
• Hou et al 2015
• Hough 1961
• Hover Over from Dover 1968
• Howard and Miley 1989
• Howard et al 1985
• Hu and Booker 2022
• Hu et al 2026
• Hua et al 2015
• Huang and Hu 2018
• Huang et al 2025
• Hubbard 1950
• Huff 1949
• Hui et al 2025
• Hummel et al 2026
• Hung and Dai 2025
• Huntley 1972
• Huo et al 2015
• Huo et al 2019
• Huo et al 2025
• Huston and Winston 1960
• Huston and Winston 1969
• Hwang and Kwon 2015
• Hwang and Kwon 2019
• Hwang et al 1994
• Iek et al 1992
• Iek et al 1993
• Iek et al 1993
• INAV-Based Adaptive Transition Control for QuadPlane VTOL UAVs in Urban Wind Gust Environments 2025
• Invernizzi et al 2020
• Ioannis and Ioannis 2026
• Iriarte et al 2021
• Irmawan et al 2023
• Irvin and Swan 1956
• Irvine 1968
• Islam et al 2017
• Islam et al 2020
• Ison 2024
• Ison 2024
• Ispiryan 2024
• J P Longley and E M Greitzer 1992
• Jabbari Asl 2019
• Jacobellis et al 2021
• Jacobson 1965
• Jacobson and Joshi 1977
• Jacobson and Joshi 1978
• Jacobson and Smith 2018
• Jacocks and Kneile 1975
• Jang et al 2012
• Janus et al 1992
• Jasudavisius 2021
• Jdiobe et al 2024
• Jensen
• Jeong et al 2019
• Ji et al 2022
• Jia and Li 2025
• Jiang and Zhang 2021
• Jiang et al 2015
• Jiang et al 2018
• Jiang et al 2019
• Jiang et al 2025
• Jiang et al 2025
• Jiang et al 2025
• Jimenez Garcia and Barakos 2018
• Jin et al 2018
• Johannsen and Govindaraj 1978
• Johnson 1958
• Johnson 1974
• Johnson and Craig 1974
• Johnson and Sirohi 2024
• Johnson-Groh 2024
• Johnston and Friend 1965
• Johnston et al 1965
• Johnston et al 1974
• Jokar and Khoshnood 2026
• Jones et al 1979, Evaluation of two inflow control
• Junaidin 2023
• Junaidin and Cahyono 2019
• Junaidin and Wicaksono 2022
• Jung et al 2019
• Jung et al 2025
• Jusko and Berger 2026
• Kaji et al 2017
• Kaji et al 2018
• Kalra and Baeder 2017
• Kamarudin 2020
• Kang et al 2021
• Kang et al 2023
• Kang et al 2025
• Kanishk Mahaveer Jain 2025
• Kantner et al
• Kaskel et al 1963
• Kato and Suzuki 2019
• Kaufman 1965
• Keith and Srivastava 1997
• Keller and Holliger 1959
• Kelley 1962
• Kelly et al 1973
• Kelly et al 1974
• Kelly et al 1979
• Kemp 1969
• Kereliuk and Sinclair 1979
• Kevin Li and Lee 2021
• Key 1971
• Key 1982
• Key et al 1965
• Khorrami and Fares 2019
• Kidd and Bull 1963
• Kidwell and Wray 1964
• Kikumoto et al 2022
• Kilcoyne and Huang 2025
• Killeen et al 2023
• Kim and Yee 2020
• Kim and Zhang 2025
• Kim et al 2018
• Kim et al 2019
• Kim et al 2020
• Kirby 1956
• Kirby 1960
• Kirby 1960
• Kirby 1961
• Kirby 1963, Flight-and Force-test Investigati
• Kirby and Newsom 1964
• Kirby et al 1963
• Kirk and Pitt 1969
• Kirste and Stumpf 2025
• Klinar and Craig 1961
• Kliza et al 2025
• Klopfstein 1968
• Ko and Park 2014
• Kobayashi 1978
• Koch 2010
• Koch 2012
• Koch et al
• Koch et al 2011
• Koch et al 2012, Tone Noise Predictions for a Spac
• Kocurek and Tangler 1977
• Kokubo et al 2024
• Kokubo et al 2026
• Kolesar et al 1970
• Komoda et al 1989
• Kondor and Moore 2005
• Kondor et al 2003
• Koning et al
• Koning et al
• Koppelberg et al 2023
• Kou et al 2021
• Koumoutsidi et al 2022
• Kozakiewicz and Grzegorczyk 2021
• Kozapalli and Warmbrodt 1997
• Kozhanov et al 2022
• Kriebel 1964
• Kriebel and Mendenhall 1966
• Kriebel et al 1963
• Kriechbaum and Lollar 1968
• Kriechbaum and Stineman 1972
• Krishnamurthy and Rizzi 2022
• Kruger 1949
• Kruger and Uranga 2024
• Krznar et al 2021
• Krznar et al 2021
• Kuantama and Tarca 2017
• Kubo et al 2009
• Kuhn 1957
• Kuhn 1959
• Kuhn 1965
• Kuhn 1979
• Kuhn 1981
• Kuhn and Grunwald 1960
• Kuhn and Grunwald 1961
• Kuhn and McKinney 1965
• Kuhn et al 1964
• Kulathunga and Klimchik 2023
• Kwak 2026
• L Danielle Koch et al 2012
• Laarmann et al 2023
• Lacey 1977
• Lacey et al 1979
• Laillatul and Darwito 2026
• Lan 1975
• Lantzsch et al 2014
• Laskowitz 1961
• Laskowitz 1963
• Lateral-Directional Flying Qualities in 2002
• Lavi 1967
• Lavi and Headley 1967
• Lawrence 1965
• Lawrence et al 2016
• Lawrence et al 2018
• Lazer and Kuatbayeva 2026
• Lebacqz 1974
• Lebacqz 1979
• Lebacqz 1982
• Lebacqz 1983
• Lebacqz and Aiken 1975
• Lebacqz and Aiken 1975
• Lebacqz and Aiken 1977
• Lebacqz and Scott 1984
• Lebacqz and Scott 1985
• Lebacqz et al 1974
• Lebacqz et al 1981
• Lee and Dassonville 2020
• Lee and Kim 2020
• Lee and Kwok 1983
• Lee and Son 2021
• Lee and Yee 2024
• Lee et al 2021
• Lee et al 2022
• Lee et al 2025
• Lee et al 2026
• Lee et al 2026
• Leelaburanathanakul et al 2021
• Legrand et al 2025
• Lei and Zhao 2023
• Lei et al 2020
• Lei et al 2021
• Lei et al 2023
• Lei et al 2023
• Lei et al 2024
• Leishman 1966
• Lendvai and Benedek 2023
• Lendvai and Benedek 2024
• Lerbs 1953
• Lerbs 1955
• Levine 1957
• Levinsky et al 1968
• Levison 1982
• Lewy 2002
• Li 2008
• Li and Liu 2023
• Li et al 2018
• Li et al 2020
• Li et al 2021
• Li et al 2022
• Li et al 2023
• Li et al 2023
• Li et al 2024
• Li et al 2024
• Li et al 2024
• Li et al 2024
• Li et al 2024
• Li et al 2025
• Li et al 2025
• Li et al 2026
• Li et al 2026
• Li et al 2026
• Li et al 2027
• Liao et al 2025
• Lieberman and Hoffman 1964
• Lieblein et al 1973
• Lim et al 2023
• Lin and Supsukbaworn 2017
• Lin et al 2019
• Lin et al 2023
• Lin et al 2024
• Litt 2024
• Liu 1962
• Liu 2023
• Liu and Li 2024
• Liu and Mi 2025
• Liu and Wang
• Liu et al 2012
• Liu et al 2013
• Liu et al 2018
• Liu et al 2019
• Liu et al 2019
• Liu et al 2021
• Liu et al 2022
• Liu et al 2022
• Liu et al 2023
• Liu et al 2024
• Liu et al 2024
• Liu et al 2025
• Liu et al 2025
• Liu et al 2026
• Liu et al 2026
• Lockheed-Georgia Co Marietta 1969
• Loewy and Yntema 1958
• Logan et al 2017
• Lollar and Kriechbaum 1968
• Lollar and Matous 1963
• Lombaerts et al 2022
• Longhouse 1972
• Longhouse 1973
• Longitudinal Flying Qualities in 2002
• Louisse and Marshall 1976
• Lovas 2026
• Lovell 1955
• Lovell and Parlett 1956
• Low and Garrard 1991
• Lowson 1972
• Lu
• Lu et al 2025
• Lucassen and Sterk 1965
• Luo et al 2023
• Luo et al 2024
• Luo et al 2024
• Luo et al 2024
• Ma
• Ma and Wang 2018
• Ma et al 2020
• Ma et al 2022
• Ma et al 2025
• Macey 1956
• Magee and Taylor 1971
• Magee and Taylor 1971
• Mahakalkar et al 2024
• Majić et al 2016
• Mak et al 2025
• Maki and Giulianetti 1966
• Malcom 1969
• Maldonado and Fernandes 2024
• Maldonado et al 2024
• Mall et al 2026
• Mallen and Dancik 1959
• Malone and Peavey 1999
• Malpica and Lusardi 2013
• Malpica et al 2011
• Malpica et al 2012
• Mancini 1983
• Mann 1974
• Manzoni et al 2024
• Manzoor et al 2020
• Manzoor et al 2022
• Manzoor et al 2022
• Mao et al 2024
• Marchese 1963
• Marchini
• Marconi and Naldi 2012
• Margason 1970
• Margason and Hoad 1979
• Margetts 1987
• Marques et al 2026
• Marsh 1967
• Martin 1963
• Martin 1964
• Martin and Tung 2004
• Martindale et al 1974
• Marzouk 2025
• May et al 2024
• McArdle et al 1981
• McCormick 1956
• McCormick 1956
• McCormick 1969
• McCormick and Mallen 1957
• McCune 1993
• McDonnell Aircraft Corp St Louis Mo 1950
• McGee 1977
• McGee et al 1981
• McGREGOR and Smith 1966
• McIntyre 1963
• McKercher et al 2024
• McKinney and Newsom 1962
• McKinney et al 1966
• McLemore and Pegg 1980
• McNay 1958
• Mehrabi and Davari 2020
• Memon et al 2021
• Mendes and Egbue 2018
• Meng et al 2026
• Merino-Martinez and Schade 2025
• Merrick 1984
• Merrick and Gerdes 1978
• Merrick et al 1990
• Meyerhoff and Zvengrowski 1962
• Mi 2020
• Michaels 1969
• Mihaila et al 2026
• Miley et al 1985
• Miley et al 1986
• Milić et al 2025
• Milla and Blick 1966
• Miller 1970
• Miller and Beranek 1958
• Miller and Clark 1965
• Miller and Papamoschou 2024
• Miller et al 2023
• Milluzzo and Kuerbitz 2017
• Milluzzo and Leishman 2017
• Min et al 2026
• Minaker and Defoe 2019
• Minaker and Defoe 2019
• Mineck and Margason 1974
• Minervino et al 2016
• Mirković et al 2026
• Mitchell and Hoh 1983
• Mitchell et al 2003
• Mitchell et al 2014
• Miwa and Marubashi 2014
• Miwa and Maruhashi 2016
• Miwa et al 2012
• Miwa et al 2013
• Miyajima 1969
• Miyazaki et al 2020
• Modeling of Ducted Acoustic 2021
• MODELLING AND CONTROLLER DESIGN OF A NOVEL DUCTED FAN UNMANNED AIRCRAFT 2021
• Moeini et al 2021
• Mohammadi and Boroomand 2014
• Mokotoff and Cinar 2026
• Momiyama 1977
• Moorhouse et al 1989
• Morduchow et al 1953
• Moreau et al 2022
• Morgan 1961
• Morhy Cavalcante et al 2026
• Morris et al 1964
• Morse 1956
• Morse and Newhouse 1960
• Morse and Newhouse 1961
• Morse and Newhouse 1961
• Mort 1965
• Mort 1966
• Mort and Yaggy 1963
• Mortimer et al 2026
• Moser 1959
• Motycka 1985
• Motyka 1975
• Mouille 1970
• Moulton and Brown 1992
• Muehlebach and D’Andrea 2017
• Mueller 1965
• Mugridge 1975
• Mumasha et al 2026
• Murata et al 2025
• Murthy et al 2002
• Musser et al 2023
• Myasnikov and Ilyin 2023
• Myers et al 2010
• Nabadova and Sabziev 2025
• Nagata et al 1973
• Nagy and Kirsten 1976
• Naldi and Marconi 2009
• Naldi and Marconi 2013
• Naldi et al 2010
• Naldi et al 2015
• Nandy and Kim 2025
• Nardi et al 1999
• Nardizzi et al 1971
• NASA Conference on V/STOL 1960
• Neal 1975
• Nelson 1956
• Nelson 1959
• Nelson 1965
• Neogi et al
• Netjasov 2012
• Nettleton 1965
• Network Solution for Real-time Synchronization in Flight Simulator 2022
• Neumann et al 2020
• Newell et al 1963
• Newsom 1963
• Newsom 1967
• Newsom and Tosti 1962
• Ng et al 2021
• Ngcukayitobi 2026
• Nguyen and Ngo 2017
• Nichols et al 2019
• Nichols et al 2020
• Nichols et al 2021
• Nichols et al 2022
• Nichols et al 2024
• Nichols et al 2026
• Nicholson and lowry 1966
• Niwa and Suzuki 1991
• Niță et al 2026
• Noboni et al 2025
• Nonnenmacher and Jones 2016
• Norsetter and Smith 2024
• Nt et al 2024
• Nugroho et al 2022
• Nöding and Bertsch 2021
• O’Bryan 1961
• O’Malley 1959
• Obryan 1960
• Odukoya and Jo 2026
• Oh et al 2026
• Ohanian et al 2009
• Ohanian et al 2011
• Ohanian et al 2012
• Ohtsuta and Akishita 1990
• Okan et al 2002
• Oleson and Patrick 1998
• Onat and Tolle 1979
• Ordway and Greenberg 1961
• Ordway et al 1960
• Ostheimer and Giguere 1963
• Otis 1962
• Otsuka et al 1969
• Otsuka et al 2015
• Otsuka et al 2024
• Overfield and Crawford 1967
• Overmeyer and Martin 2017
• Oyama et al 2024
• Ozdemir and Horn 2009
• Page and Aiken 1971
• Page et al 2021
• Pahonie et al 2018
• Palleja-Cabre et al 2024
• Palleja-Cabre et al 2025
• Pangaribuan and Hartono 2024
• Papamoschou 2023
• Park 2025
• Park and Kwon 2021
• Park and Park 2026
• Park et al 2013
• Park et al 2015
• Park et al 2017
• Park et al 2022
• Park et al 2022
• Parker 1986
• Parlett 1955
• Parlett 1961
• Parlett 1961
• Pascioni et al 2026
• Pasquali et al 2020
• Pasupuleti 2025
• Patton 1972
• Pauli 1965
• Pauli 1969
• Pauli et al 1970
• Pavel 2022
• Pawłucki 2016
• Payne 1955
• Payne and Henry E. 1961
• Pearlman 1968
• Peddle et al 2009
• Performance validation of the 1999
• Perry et al 2023
• Person and Robbins 1965
• Peters et al
• Pettingill et al
• Pfeiff 1965
• Pfeifle and Fichter 2023
• Pflimlin et al 2006
• Pflimlin et al 2007
• Pflimlin et al 2007
• Pflimlin et al 2010
• Phanindra et al 2020
• Pirera 1983
• Place et al 1974
• Platt 1948
• PlCKERELL and Cresswell 1968
• Pleffken 2026
• Pobikrowska and Goetzendorf-Grabowski 2025
• Pohly et al 2021
• Pomfret and Clarke 2009
• Pooladsanj et al 2026
• Portapas 2025
• Portapas and Cooke 2020
• Porter et al 2016
• Posey et al 2004
• Pouryoussefi et al 2023
• Pradeep and Wei 2019
• Pruyn and Taylor 1971
• Przedpelski 1965
• Pukhova et al 2021
• Putman 1961
• Pütsep et al 2023
• Qian et al 2023
• Qian et al 2026
• Qiao and Zhou 2025
• Qiao and Zhou 2026
• Qin et al 2023
• Qing et al 2019
• Qingyi et al 2023
• Radotich
• Rae Jr. 1972
• Raeisi 2021
• Rajagopalan and Zhang 1989
• Ram Rohit 2024
• Ramanujam and Abhishek 2022
• Ramasamy and Yamauchi 2017
• Ramasamy et al 2010
• Ramnath 1970
• Rampy 1966
• Rand and Khromov 2017
• Rao and Chimata 2024
• Rao et al 1974
• Rauer 2019
• Raza and Stansbury 2025
• Reeder 1958
• Regier et al 1947
• Rehan et al 2026
• Ren and Wang 2025
• Renselaer 1974
• Renselaer 1975
• Renselaer 1977
• Reschak 1970
• Reschak 1971
• Reut and Tsukanov 2025
• Rhoads 1967
• Rhoads 1970
• Riboldi and Rolando 2023
• Ricardo Jr and Santos 2023
• Rich 1969
• Richards 1955
• Rigas et al 2024
• Ritschl et al 2003
• Ritter et al 1971
• Rizk 1980
• Rizk 1981
• Rizzi and Rafaelof 2021
• Rizzi et al 2026
• Roberts 1956
• Roberts 1964
• Roberts 1965
• Rogers et al 1969
• Roh et al 2018
• Roh et al 2020
• Rolls 1960
• Rolls 1965
• Roscoe et al 1984
• Rostami and Farajollahi 2021
• Rotorcraft handling qualities design 2011
• Ruggia 2025
• Runco and Benedict 2023
• Ruzicka et al 2004
• Ryals et al 2026
• Rylko et al 2025
• Saelman 1963
• Saetti and Rogers 2024
• Safe Coordinated Maneuvering of Teams of Multirotor Unmanned Aerial Vehicles: A Cooperative Control Framework for Multivehicle, Time-Critical Missions 2016
• Safieh and Rauleder 2024
• Safwat et al 2026
• Sagaga and Lee 2025
• Saied et al 2019
• Saied et al 2019
• Saied et al 2024
• Saif et al 2025
• Sajid et al 2026
• Salem et al 2014
• Salvador 2022
• Sargent and Anemaat 2018
• Sarjito and Mulyadin 2024
• Sarup 2025
• Sarup 2026
• Sato 2020
• Sato and Satoh 2011
• Sato et al 2024
• Savage et al 2026
• Savino and Zanotti 2025
• Scamps and Gibbens 2003
• Schade 1968
• Schade et al 2024
• Schaller
• Schatrow et al 2025
• Schmidt et al 2021
• Schmitz and Stepniewski 1973
• Schmähl et al 2021
• Schoder et al 2023
• Scholten et al 2020
• Scholten et al 2020
• Schouten 1982
• Schroers and Antoniou 1966
• Schuler et al 1972
• Schwaiger and Wills 2016
• Schwartz and Shearer 1964
• Scientific and Technical Information Division 1966
• Sedlock 1985
• Sedunin et al 2026
• Seher-Weiß and Jones 2021
• Seibel 1963
• Seidel and Matwey 1983
• Sellers et al 1977
• Selman 2023
• Seo and Kim 2016
• Serrano et al 2021
• Serrano et al 2022
• Setiawarman and Sasongko 2026
• Shahmiri 2017
• Shang et al 2025
• Shao et al 2021
• Sharma et al 1987
• Sheehy 1973
• Sheng 2018
• Sheng and Sun 2015
• Sheng et al 2015
• Sheng et al 2015
• Shi et al 2022
• Shi et al 2024
• Shiau et al 2024
• Shimizu and Miwa 2018
• Shimizu and Miwa 2019
• Shimizu and Miwa 2019
• Shimizu and Miwa 2020
• Shivers 1961
• Shivers 1967
• Shomber and Gertsen 1967
• Shouji et al 2021
• Showers 1952
• Shrestha et al 2016
• Shuai and Xiao-hui 2011
• Shubert and Jones 2025
• Shubert et al 2026
• Shur et al 2016
• Shur et al 2018
• Siddavatam et al 2024
• Sikorsky 1960
• Simmonds 1967
• Simmonds 1967
• Simmonds 1967
• Simmons
• Simmons 2023
• Simmons and Busan
• Simmons and Hatke 2021
• Simmons and Murphy
• Simon 2024
• Simon et al
• Simpson 1967
• Simpson 1968
• Sims and Kimberlin 1995
• Sinacori 1970
• Sinacori and Lange 1961
• Sing and Knauer 1956
• Singh and Dinavahi 2011
• Sissingh 1941
• Skelton 1968
• Skifstad 1970
• Slaughter 1958
• Slim et al 2021
• Smetana 1975
• Smith 1958
• Smith 1959
• Smith 1960
• Smith 1961
• Smith 1961
• Smith 1966
• Smith 1968
• Smith 1974
• Smith 1977
• Smith 2023
• Smith and Geddes 1979
• Smith et al 1973
• Smith et al 2000
• Snyder 1950
• Snyder 2014
• Sofrin and Cicon 1987
• Sofrin and Cicon 1987
• Son and Kim 2023
• Son et al 2025
• Song et al 2025
• Song et al 2025
• Sorenson et al 2023
• Sow et al 2025
• Spence and Spence 1997
• Spillman 1967
• Spray-on Landing Field Aids VTOL Planes 1965
• Spreemann 1958
• Spreemann 1960
• Spreemann 1966
• Spreemann 1968
• Spreemann and Kuhn 1956
• Spühler et al 2025
• Srinathkumar 2011
• Srinathkumar 2015
• Srivastava 1992
• Srivastava et al 1996
• Stange 1959
• Stapleford and Walton 1966
• Stapleford et al 1974
• Stearns et al 1956
• Stendahl 1985
• Stepniewski 1957
• Stepniewski and Schneider 1964
• Stiltner and Ohanian 2012
• Stockman 1970
• STOL Operation Transition-in-Ground Effect 2007
• Stortz and Franklin 1996
• Strampe and Klingauf 2026
• Strand and Levinsky 1969
• Stuart 1957
• Stuper 1938
• Su et al 2024
• Su et al 2026
• Suiçmez and Kutay 2023
• Sun et al 2024
• Sun et al 2025
• Sun et al 2025
• Sun et al 2026
• Sun et al 2026
• Sutliff 2019
• Sutliff and Nagel 1993
• Swaim 1969
• Swaim 1970
• Swaim and Connor 1968
• Swaim and Yen 1979
• Szafran et al 2014
• Szalai 1967
• Szolnoky Cunha and Ortiz 2024
• Taflan et al 2024
• Taher et al 2026
• Takacs and Haidegger 2022
• Takagi and Inumaru 1967
• Takahashi et al 2017
• Takallu 2009
• Takeda et al 2020
• Takeyeldein 2026
• Takita and Kashitani 2016
• Takita et al 2018
• Takita et al 2019
• Tan et al 2018
• Tang et al 2026
• Tangler et al 1973
• Tapscott 1960
• Tapscott 1964
• Tapscott and Kelley 1960
• Taylor 1958
• Teichert et al 2026
• Tekinalp et al 2009
• Thalheimer 2021
• The Bell X-22A V/STOL, Variable Stability Research Airplane, Lessons Learned
• The role of human 1986
• Theodorsen 1960
• Theodorsen 1960
• Theodorsen and Nomicos 1961
• Thirumaleshwar Hegde et al 2020
• Thwaites 1951
• Thwaites 1953
• Tian et al 2026
• Tian et al 2026
• Time Domain Sound Spectrum Measurements in Ducted Axial Fan under Stall Region 2015
• Ting et al 1975
• Tinkler 1962
• Tinney and Valdez 2026
• Tischler and Hoh 1982
• Tobias and Horn 2008
• Tokarski et al 1994
• Tolba and Shirinzadeh 2022
• Torenbeek 1972
• Tosti 1962
• Tran et al 2021
• Tran Minh 2025
• Trapp and da Motta Girardi 2010
• Trenka 1966
• Trenka 1967
• Trivkovic et al 2016
• Truong and Papamoschou 2014
• Truong and Papamoschou 2015
• Truong et al 2016
• Tung 1950
• Turner and Drinkwater 1963
• Tweed 2000
• Tyan et al 2017
• Valarezo 1990
• Van et al 2019
• Van Niekerk 1958
• Vechtel and Buch 2022
• Velkoff 1981
• Vepa 2018
• Vertical Takeoff and Landing 2014
• Vidal et al 1960
• Vieira et al 2019
• Vinje 1968
• Vinje and Miller 1973
• Vollo and Brassaw 1956
• Vratny and Hornung 2018
• VTOL - 1968 1969
• VTOL BOOSTER 1961
• VTOL control and display 1988
• Vu et al 2026
• Wachtell 1970
• Wachłaczenko 2025
• Wahid et al 2025
• Wahl and Julian 1956
• Wainauski 1968
• Walker 1981
• Walter et al 2020
• Walter et al 2022
• Walters and Ashworth 1974
• Walvekar and Chakravarthy 2023
• Walz 1978
• Wang 2017
• Wang 2019
• Wang and Chen 2024
• Wang and Li 2025
• Wang and Ma 2026
• Wang and Qi 2011
• Wang and Zhang 2018
• Wang et al 2005
• Wang et al 2007
• Wang et al 2015
• Wang et al 2017
• Wang et al 2018
• Wang et al 2018
• Wang et al 2019
• Wang et al 2023
• Wang et al 2023
• Wang et al 2023
• Wang et al 2024
• Wang et al 2024
• Wang et al 2024
• Wang et al 2025
• Wang et al 2025
• Wang et al 2025
• Wang et al 2025
• Wang et al 2026
• Wang et al 2026
• Wang et al 2026
• Wang et al 2026
• Wang et al 2026
• Ward 1960
• Ward 1970
• Watthewaduge and Bilgin 2023
• Webb and Willer 1952
• Weetman and Cromack 1973
• Wei et al 2023
• Wei et al 2026
• Weinstein et al 2026
• Weintraub et al 2022
• Weir 1988
• Welge and Crowder 1978
• Welter 1966
• Weng et al 2021
• Wernecke 1958
• West 1969
• Westbrook 1964
• Whidborne and Cooke 2017
• Whitbeck and Hofmann 1978
• White 1983
• White and Crimi 1961
• White and Phelan 1987
• White et al 1960
• Whiteside and Pollard
• Wijaya et al 2023
• Wildermuth et al 1974
• Wildermuth et al 1974
• Wilhelm and Schafranek 1986
• Willebeek-LeMair and Rhinehart 2023
• Wilson et al 1993
• Wilson et al 2012
• Winick 1968
• Winston 1970
• Winston and Huston 1962
• Winter 1980
• Wiston and Yan 2025
• Wokanubun et al 2023
• Wonseok Lee and Hyochoong Bang 2007
• Woodward and Glaser 1981
• Woodward et al 1984
• Woodward et al 1992
• Worobel and Peracchio 1968
• Wright and Lappos 1981
• Wu et al 2023
• Wu et al 2025
• Wu et al 2026
• Xia et al 2026
• Xiang et al 2015
• Xiang et al 2016
• Xiao et al 2024
• Xiao et al 2025
• Xie et al 2023
• Xin and Lee 2024
• Xu et al 2015
• Xu et al 2025
• Xu et al 2025
• Xu et al 2026
• Xu et al 2026
• Xue et al 2026
• Yadegari et al 2024
• Yaggy and Mort 1961
• Yahui et al 2024
• Yamaguchi et al 2026
• Yamazaki 1965
• Yan and Chen 2019
• Yan et al 2019
• Yan et al 2023
• Yan et al 2026
• Yang et al 2004
• Yang et al 2011
• Yang et al 2022
• Yang et al 2022
• Yang et al 2022
• Yang et al 2024
• Yang et al 2026
• Yang et al 2026
• Yao et al 2026
• Yauwenas et al 2022
• YAV-8B reaction control system 1993
• Ye et al 2020
• Yeow 1974
• Yi and Feng 2025
• Yildiz 2022
• Yilmaz et al 2013
• Yilmaz et al 2015
• Yin and Pei 2025
• Yin and Pei 2026
• Yingtao et al 2025
• Yoeli 2002
• Yoeli 2005
• Yoon and Jeong 2025
• Yoshikawa et al 2019
• Young
• Young 1969
• Yu 2023
• Yu and Kwon 2017
• Yu et al 2020
• Yu et al 2024
• Yuan 1970
• Yuan et al 2022
• Yuan et al 2026
• Yue et al 2026
• Yuksek and Inalhan 2022
• Yuska and Diedrich 1972
• Yusri et al 2026
• Zabinsky 1956
• Zabinsky and Laszewski 1956
• Zakaria et al 2019
• Zamri 2011
• Zanoni et al 2022
• Zawodny et al
• Zawodny et al 2025
• Zeng et al 2025
• Zhang and Barakos 2021
• Zhang and Barakos 2021
• Zhang and Fan 2012
• Zhang et al 2016
• Zhang et al 2018
• Zhang et al 2020
• Zhang et al 2020
• Zhang et al 2021
• Zhang et al 2021
• Zhang et al 2022
• Zhang et al 2022
• Zhang et al 2023
• Zhang et al 2023
• Zhang et al 2024
• Zhang et al 2024
• Zhang et al 2024
• Zhang et al 2025
• Zhang et al 2025
• Zhang et al 2026
• Zhao 2018
• Zhao and Bil 2008
• Zhao and Cao 2026
• Zhao and Li 2018
• Zhao and Liu 2018
• Zhao and Yang 2023
• Zhao and Yang 2026
• Zhao et al 2013
• Zhao et al 2014
• Zhao et al 2019
• Zhao et al 2022
• Zhao et al 2022
• Zhao et al 2023
• Zhao et al 2023
• Zhao et al 2023
• Zhao et al 2024
• Zhao et al 2025
• Zhao et al 2026
• Zhao et al 2026
• Zhao et al 2026
• Zhao et al 2026
• Zheng et al 2018
• Zheng et al 2026
• Zhong et al 2023
• Zhong et al 2026
• Zhou et al 2017
• Zhou et al 2022
• Zhou et al 2024
• Zhou et al 2025
• Zhou et al 2026
• Zhu et al 2022
• Zhu et al 2023
• Zhu et al 2024
• Zimmer and Gagnon 2023
• Zondervan et al 2006
• Zong et al 2021
• Zong et al 2022
• Zorumski and Lester 1974
• Zosimovych 2022
• Çakan 2026
• Öznalbant and Kavsaoğlu 2018
• 聂 2017
• 鲁 2020