X-Planes: Bensen X-25

On 5 June 1968, the first-flight date that Jenkins, Landis and Miller give for the type and that Guinness World Records gives for this aircraft, an unpowered Bensen gyroglider designated X-25B flew for the first time at Edwards Air Force Base. It had no engine at first, no fuselage to speak of, and a rotor that turned only because air was coming up through it. It is the smallest crewed aircraft and the slowest in this series, and the only one whose research question was about what happens after an aeroplane has already been lost. Guinness World Records names the X-25B the smallest crewed aircraft to receive an X-plane designation, and its fastest recorded flight was 85 miles per hour in the inventory by Jenkins, Landis and Miller. This article is the twenty-sixth 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, the X-21, the X-22, the X-23, and the X-24.

The X-25 was not built to fly higher, faster or further than anything. It was built to answer whether an aircrew member who has just ejected can be given a choice about where to come down. The United States Air Force called the programme the Discretionary Descent Vehicle, and the word that matters in that name is discretionary. The programme’s purpose of giving aircrew who abandon their aircraft an option of where to land is recorded by Jenkins, Landis and Miller.

The obvious reading of the concept is wrong, and the arithmetic establishes that before any archive does. An autorotating rotor is not a gentler way down than a parachute. At equal weight it descends about nine percent faster, and as flown it descended about thirty percent faster. The rotor does not offer a slower descent. It offers the ability to go somewhere.

The concept was not abandoned because pilots could not learn to fly it. The one test result on record is that pilots could be trained quickly and easily, in the X-25A fact sheet, which gives the end of the programme as the air war winding down together with doubts about operational feasibility. The vehicle that embodied the escape, the unpowered gyro-chute, has no recorded piloted flight, and no full-scale operational test was performed, in Jenkins, Landis and Miller. The evidence supports a narrower reading, that the need lapsed before the escape itself was ever demonstrated. The arithmetic below indicates where the doubts about feasibility could plausibly have lain, an inference that no document of the programme confirms.

The Research Question

Can a descending crewman be given control over the landing point, using a device small enough to carry in a combat aircraft?

That question has a shape unlike any other in this series. The X-1 asked whether an aeroplane could pass through Mach one and survive. The X-15 asked what hypersonic flight does to a structure. Those are questions about a flight regime. This one is a question about a probability, namely the probability that a person who has left a damaged aeroplane over hostile ground is recovered rather than captured, and the engineering exists only to move that number.

The binding unknown was not whether a rotor autorotates. Juan de la Cierva had settled that in the 1920s and the literature had been mature for forty years, in Glauert 1927, de la Cierva 1930, Wheatley 1933, Report no. 515, full-scale 1935, Report no. 515, full-scale, Wheatley 1935, Wheatley 1937, Gustafson 1956, and Larsen 1956. The remainder of that period work is surveyed with the autogiros and gyroplanes in The Contemporary Literature. The programme history of the machines Cierva built is in Brooks on the Cierva autogiros and the longer arc from those aircraft to the modern gyroplane is in Charnov’s account of the type. The binding unknown was whether the useful part of autorotation survives being packed into an aeroplane, deployed by someone who has just been through an ejection, and flown by a person with no rotorcraft training under the worst conditions of their life.

Each of those four constraints attacks a different quantity, and the article is organised around them because the programme was.

Programme Origin

The X-25A fact sheet gives the motivation as the air war over North Vietnam, in which the Air Force was suffering heavy losses, and the odds of escape for a downed flyer. A parachute descends where the wind puts it. If that place is defended, the descent itself has decided the outcome, and the crewman is a passenger in the decision.

Igor Bensen had been building rotor kites and autogyros of his own design since the early 1950s, from the company he founded in 1952 at Raleigh-Durham in North Carolina, in the Bensen Aircraft entry. The B-8 family was sold as plans to amateur builders, it was simple enough for them to build, and in 1967 Bensen himself set a series of world autogyro records in a B-8M, in the FAI record list. The Air Force did not commission a new aircraft. It ordered modified versions of an existing civil design and gave them an X-designation, a materially different act from every other procurement in this series and one discussed under Where the Framing Breaks Down. The order for modified Bensen autogyros is in Jenkins, Landis and Miller.

In early 1968 three aircraft were ordered for the Discretionary Descent Vehicle evaluation, in the X-25A fact sheet. The powered variant, derived from the B-8M, became the X-25A. The unpowered gyroglider, derived from the B-8, became the X-25B, in the Bensen B-8 entry. The third was the basic X-25, an unpowered gyro-chute intended for one-time use, whose rotor was designed to deploy automatically at any ejection speed and for which no piloted flight is recorded, in Jenkins, Landis and Miller.

The Vehicles

There is more than one X-25 and the distinction is not cosmetic.

	X-25A	X-25B
Basis	Bensen B-8M Gyro-Copter	Bensen B-8 Gyro-Glider
Serial	68-10770	68-10771
First flight	5 June 1968	5 June 1968, with 23 January 1968 in one unsourced entry
Engine	McCulloch four-cylinder two-stroke	none at first, later fitted
Role in the programme	training and technique	towed trainer, training and technique
Now held at	National Museum of the United States Air Force	Air Force Flight Test Museum, Edwards

Both aircraft that flew were trainers, and the vehicle that embodied the escape concept is not known to have flown with a pilot. An escape vehicle has no engine, and the unpowered basic X-25 was that vehicle, while the X-25A and X-25B were used to evaluate piloting techniques and training requirements, in Jenkins, Landis and Miller. The X-25A fact sheet describes the X-25B as an unpowered towed trainer later fitted with an engine, and the X-25A as having a more robust structure and a small engine.

Two specifications are not settled by the public record and the article does not pretend otherwise. The rotor diameter is given as twenty feet in the B-8M description, and a figure of twenty feet six inches for the X-25A could not be confirmed in any source consulted here. The engine is given as seventy-two horsepower in the B-8M description and as ninety horsepower in the X-25A fact sheet, which is also the rating listed for Bensen’s own record-setting B-8M in the FAI record list. Every calculation below uses twenty feet, states that it does so, and is checked for sensitivity to the difference, which is under five percent in the quantity that matters.

Sizing From First Principles

Why a Rotor Autorotates

It is easy to say that the rotor turns because air comes up through it and leave the matter there. That sentence carries the full physical content of the subject and deserves the line of algebra it takes to write down.

Consider a blade element at radius $r$ meeting the resultant flow at inflow angle $\phi$. Its lift acts perpendicular to that flow and its drag along it, so both have components in the plane of rotation. The net in-plane force on the element is

\[dF_{x} = dL \sin\phi - dD \cos\phi\]

The element neither drives the rotor nor retards it when that vanishes, which gives a condition containing nothing but the section’s aerodynamics,

\[\tan\phi = \frac{dD}{dL} = \frac{C_d}{C_l}\]

The equilibrium element is the one whose inflow angle equals its drag-to-lift angle. Inboard of it the inflow angle is larger and the element drives the rotor. Outboard it is smaller and the element is driven. Autorotation is that balance, integrated over the disc, and the rotor settles at whatever speed makes the driving and driven regions cancel,

\[\int_{0}^{R} r \, dF_{x} = 0\]

For a section lift-to-drag ratio of 15, which is ordinary for a wooden blade, the equilibrium inflow angle is 3.8 degrees. The blade-level machinery is Weick 1926, Propeller design extension of, Kito 1942, Bradley 1956, Nakajima 1960, Fisher et al 1971, Bobo 1972, Kerwin 1973, Computer techniques for propeller blade section design 1976, Ballard et al 1979, Effect of tip planform on blade, Ballard et al 1979, Effect of tip shape on blade, Ballard et al 1980, Moxon and Green 1990.

What Autorotation Is

A rotor in autorotation has no shaft power. It turns because the airflow through the disc, which in a descent comes from below, drives the blades instead of being driven by them. Momentum theory describes this as the windmill brake state, a name that is not a metaphor. A wind turbine extracting power from an oncoming stream and a descending rotor holding an aircraft up are the same flow state written in different reference frames, and that identity is the reason a large part of this article’s contemporary literature is about wind energy.

The literature is in Nikolsky and Seckel 1949, Nikolsky and Seckel 1949, Slaymaker and Gray 1953, Talbot and Schroers 1978, and Fradenburgh 1984, and the descent-state framework it sits in is Castles and Gray 1951, Azuma and Obata 1968, Shi-cun 1990, and Johnson 2004. The Contemporary Literature carries further records under autorotation, the vortex ring state and the emergency descent.

The Reference Speed

Every rotor quantity in this article is measured against one speed, the induced velocity the same rotor would need in hover. For a rotor of radius $R$ carrying weight $W$ in air of density $\rho$, the disc area and the hover induced velocity are

\[A = \pi R^2, \qquad v_h = \sqrt{\frac{W}{2 \rho A}}\]

where the second expression is the actuator-disc result, in Castles and De Leeuw 1953, Alfred Gessow 1954, Castles and Durham 1956, Distribution of normal component, Heyson and Katzoff 1956, Brenckmann 1958, Castles et al 1958, Heyson 1959, Gray 1960, Liu 1962, Arcidiacono 1964, Jones 1965, van Gunsteren 1971, Biggers and Orloff 1975, Landgrebe and Egolf 1975, Nagaraja and Pierce 1977, Goodman 1979, Heyson 1980, Hodges 1980, Azuma and Saito 1982, Kawachi 1982, Peters and Chen 1982, Berry and Althoff 1989. The blade-level treatment that has to agree with it is Weick 1926, Propeller design extension of, Kito 1942, Bradley 1956, Nakajima 1960, Fisher et al 1971, Bobo 1972, Kerwin 1973, Computer techniques for propeller blade section design 1976, Ballard et al 1979, Effect of tip planform on blade, Ballard et al 1979, Effect of tip shape on blade, Ballard et al 1980, Moxon and Green 1990. The standard modern development of both is Leishman’s helicopter aerodynamics, the analytical treatment is Johnson’s helicopter theory, and the period text contemporary with the X-25 is Gessow and Myers.

For the X-25B, taking a rotor diameter of 20 feet,

\[A = \pi (10 \, \text{ft})^2 = 314.2 \, \text{ft}^2\]

The gross weight in the escape configuration is not in the public record. Taking 350 pounds, which is a 200 pound crewman with equipment plus a stripped airframe, and sea-level density of $0.002377 \, \text{slug/ft}^3$,

\[v_h = \sqrt{\frac{350}{2 (0.002377)(314.2)}} = 15.3 \, \text{ft/s}\]

The Descent Rate

In steady vertical autorotation the measured descent rate is close to $1.8 v_h$. That number is empirical, and the reason it has to be can be derived. The value comes from a compact and largely mid-century body of measurement, including the analytical study of steady vertical descent in single-rotor autorotation, the analysis of the transition from hovering into autorotative vertical descent, and the power-off flare-up tests of a model rotor, in Irving 1930, Fukatsu 1931, Gessow 1948, Nikolsky and Seckel 1949, Nikolsky and Seckel 1949, Gessow 1950, von Bahr 1950, Slaymaker and Gray 1953, Hickey 1956, Fitch et al 1970, Buss and Nicholson 1971, Norberg 1973, Baker 1974, Marr 1976, Talbot and Schroers 1978, Iversen 1979, Pleasants and White 1983, Fradenburgh 1984, Gallaway and Hankey 1985, Lee 1985, Decker et al 1986, Lee et al 1988, Copeland 1994, Rogers and Asbury 2000.

In the windmill brake state the disc decelerates the oncoming flow by an induction factor $a$, and momentum theory gives the thrust as

\[T = 2 \rho A V_d^2 \, a (1 - a)\]

Setting $T = W$ and substituting $W = 2 \rho A v_h^2$ makes the density and the area cancel, leaving a relation between the descent ratio and the induction factor alone,

\[\left( \frac{v_h}{V_d} \right)^{2} = a (1 - a)\]

The right-hand side is a parabola whose maximum is one quarter, at $a = 1/2$. The left-hand side therefore cannot exceed one quarter either, giving a bound on the descent ratio that contains no property of any rotor,

\[\frac{V_d}{v_h} \geq 2\]

The measured value is 1.8, and momentum theory has no solution there at all. The quadratic in $a$ has a negative discriminant below a ratio of 2, so there is no induction factor that supports the weight.

The rotor is not beating a bound. The bound exists only inside a model that does not apply at that condition. Between hover and a descent of twice the hover induced velocity the rotor is in the vortex ring and turbulent wake states, the flow recirculates rather than passing through once, and the actuator-disc assumption fails outright. The descent rate must come from measurement because the theory declines to supply one, in Lundberg 1937, Castles and Gray 1951, Arms and Hama 1965, French 1965, Zaroodny 1966, Tung and Ting 1967, Azuma and Obata 1968, Chen and Chang 1972, McCormack 1972, Norbury 1972, Kriebel 1973, Sullivan 1973, Fetter 1974, Sallet 1975, Pullin 1979, Shi-cun 1990, Johnson 2004.

One consequence lands on a table this article prints below. The limiting case $a = 1/2$ corresponds to a ratio of exactly 2, which by the bridge derived next is a drag coefficient of exactly 1 on disc area. The last row of that table is the momentum-theory bound.

\[V_d \approx 1.8 \, v_h = 1.8 (15.3) = 27.6 \, \text{ft/s} = 18.8 \, \text{mph}\]

A Bridge Between Two Literatures That Do Not Cite Each Other

The rotor result and the parachute result are the same statement in different units, and neither field notes it. Treat the rotor as a pure drag device of coefficient $C_D$ referred to disc area,

\[W = \tfrac{1}{2} \rho V_d^2 A \, C_D \quad \Longrightarrow \quad V_d = \sqrt{\frac{2W}{\rho A C_D}}\]

and substitute $W = 2 \rho A v_h^2$, which is the definition of $v_h$ rearranged. The weight, the density and the area all cancel,

\[V_d = \frac{2 v_h}{\sqrt{C_D}} \quad \Longrightarrow \quad C_D = \frac{4}{(V_d / v_h)^2}\]

No property of any particular rotor appears in it. Putting the measured autorotation ratio in,

\[C_D = \frac{4}{(1.8)^2} = 1.23\]

A flat circular parachute canopy has a drag coefficient near 0.75 referred to its nominal area, meaning the area of the flat fabric before it is inflated, in Knacke’s recovery systems design manual. Both that coefficient and the inflated geometry it goes with are measured quantities with a long test literature behind them, in Walcott 1963, Melzig and Schmidt 1965, Heinrich 1966, Ross 1970, Mayhue and Bobbitt 1972, Klimas 1977, and Shpund and Levin 1991, and further records appear in the survey’s parachute and decelerator cluster. The area it presents to the flow is smaller, because an inflated canopy is a shallow bowl and not a flat disc, and the two areas are related through the projected diameter,

\[S_p = \left( \frac{D_p}{D_0} \right)^{2} S_0\]

Its inflated projected diameter is roughly 0.67 of the nominal diameter, so referred to its projected area,

\[C_{D,\text{proj}} = \frac{0.75}{(0.67)^2} = 1.67\]

The autorotation constant that every rotorcraft text quotes and the drag coefficient that every parachute text quotes are the same physical number. Both devices are swept-area drag devices sitting between about 1.2 and 1.7, and on this measure the rotor is the worse of the two. The decelerator side of the comparison is Eckstrom and Preisser 1967, Peterson and Johnson 1983, and Knacke 1985 and the bluff-body drag beneath it is Stilley 1967, Abraham 1970, Fackrell 1975, Utreja 1977, Ludtke 1982, Richardson 1986, and Williamson 1995, and the survey treats both in full, in its clusters on parachutes and aerodynamic decelerators and on bluff-body drag and the drag coefficient. The canopy coefficients used here follow Knacke’s recovery systems design manual, and the bluff-body drag data behind them is Hoerner’s drag compendium.

The Rotor Is a Worse Parachute Than a Parachute

The comparison has to be made at equal weight first, or the rotor is being penalised for carrying an airframe. The standard personnel canopy in Knacke’s recovery systems design manual is of 28 foot nominal diameter, and it has

\[A_0 = \pi (14)^2 = 615.8 \, \text{ft}^2\]

which is nearly twice the rotor disc.

Both devices obey the same law, written down here because the article leans on it repeatedly. Define the loading as weight over swept area,

\[\frac{W}{A} = \text{disc loading, or canopy loading}\]

Then the terminal descent of any swept-area drag device follows from the force balance,

\[V_d = k \sqrt{\frac{W / A}{\rho}}, \qquad k = \sqrt{\frac{2}{C_D}}\]

with $k = 1.27$ for the rotor at $C_D = 1.23$ and $k = 1.63$ for the canopy at $C_D = 0.75$. The rotor has the better coefficient and the worse loading, and the loading wins. Sizing practice around this quantity is Gianmarco Sahragard-Monfared et al, Gessow 1948, Powell 1954, Young and Lytwyn 1967, Jenney et al 1968, Wright 1969, Miller 1983, Zaitsev 1987.

Since descent rate goes as the square root of loading for both devices, the rotor loses before any coefficient is considered.

Device	Weight, lb	Swept area, ft²	Loading, lb/ft²	Descent, ft/s
28 ft canopy	250	615.8	0.406	21.3
20 ft rotor, equal weight	250	314.2	0.796	23.3
20 ft rotor, as flown	350	314.2	1.114	27.6

The equal-weight comparison has a closed form and the weight cancels out of it, so the nine percent characterises the two devices and is independent of the load. Writing $r$ for the rotor and $c$ for the canopy,

\[\frac{V_{\text{rotor}}}{V_{\text{canopy}}} = \sqrt{\frac{C_{D,c} A_c}{C_{D,r} A_r}} = \sqrt{\frac{0.75 \times 615.8}{1.23 \times 314.2}} = 1.091\]

Nine percent worse at equal weight and twenty-nine percent worse as flown. A concept sold as a better way down is, in the one respect a parachute exists to serve, a worse one.

But It Is a Wing

Everything above treats the rotor as a brake, and that view misses what it is. A rotor in forward autorotative flight is a rotating wing generating lift with a component of thrust, and it glides. The lift-to-drag ratio of a machine like this is the one performance number the period measured, and Wheatley’s 1933 flight determination of autogiro lift, drag and gliding performance is the direct ancestor of every figure in this section, in Collins 1934, Gliding Flight in Germany 1935, Lamprey 1953, Schiotz and Volsoe 1959, Pennycuick 1960, Durand and Wasicko 1967, Mort and Weiberg 1967, Knapp and Barton 1968, Pennycuick 1968, Pennycuick 1971, Sleeman 1971, McGAHAN 1973, Baudinette and Schmidt-Nielsen 1974, Williamson 1979. Taking a lift-to-drag ratio of 4, which is mid-range for this class of machine, and a best-glide airspeed of 45 miles an hour,

\[\tan\gamma = \frac{1}{L/D}, \qquad V_s = V \sin\gamma\]

At a lift-to-drag ratio of 4 the glide angle is 14.0 degrees, too large to count as a small angle, so the exact form and the convenient one differ,

\[V_s = \frac{V}{L/D} = \frac{66.0 \, \text{ft/s}}{4} = 16.5 \, \text{ft/s}\] \[V_s = V \sin\gamma = 66.0 \sin(14.04^\circ) = 16.0 \, \text{ft/s}\]

The article uses the simpler form throughout, and that form reads three percent high. That is stated openly, and it does not move any conclusion, since the comparison against a canopy descending at 21.3 feet per second is not close.

Now it beats the canopy on descent rate as well, at 16.5 feet per second against 21.3. The glide literature is Collins 1934, Gliding Flight in Germany 1935, Lamprey 1953, Schiotz and Volsoe 1959, Pennycuick 1960, Durand and Wasicko 1967, Mort and Weiberg 1967, Knapp and Barton 1968, Pennycuick 1968, Pennycuick 1971, Sleeman 1971, McGAHAN 1973, Baudinette and Schmidt-Nielsen 1974, Williamson 1979, and Wheatley’s flight determination of autogiro lift and drag in 1933 supplies the underlying measurement.

The Range That Makes Descent Discretionary

\[R = h \, (L/D)\]

Release height, ft	Range, statute miles	Time aloft, minutes
2,000	1.5	2.0
5,000	3.8	4.9
10,000	7.6	9.4
20,000	15.2	17.4

Every Number Above Is at Sea Level and Every Ejection Is Not

Descent rate depends on density, and an escape system is used at altitude by definition.

The density ratio is conventionally written $\sigma$, a notation this article avoids, because the article spends $\sigma$ on rotor solidity under The Material Argument, and one symbol cannot carry both.

\[V_d(h) = V_d(0) \sqrt{\frac{\rho_0}{\rho(h)}}, \qquad \frac{\rho(h)}{\rho_0} = \left( 1 - 6.875 \times 10^{-6} h \right)^{4.256}\]

so a rotor arrives at 20,000 feet descending 37 percent faster than the same rotor at sea level. The tabulated atmosphere behind this is Dubin et al 1962, U.S. Standard Atmosphere, 1962, B-34. U. S. Standard 1963, U. S. Standard Atmosphere 1963, Dubin et al 1966, U.S. Standard Atmosphere Supplements 1966, Minzner 1967, Pegg 1968, Snell and Heiser 1968, Minzner et al 1973, Krueger and Minzner 1976, Minzner et al 1976, U.S. Standard Atmosphere, 1976, Minzner 1977, Peters 1977, Johnson et al 1991, Johnson et al 1993, Johnson et al 2002.

The range does not change at all, and the point matters because range is the article’s headline number. The relation displayed above, $R = h (L/D)$, is purely geometric. There is no speed in it and no density in it. A glide ratio is a ratio of distances, so it is the same in any air, and an ejection at twenty thousand feet reaches the same fifteen miles whatever the density does on the way down.

The time aloft does change, and holding the sink rate constant would overstate it. Integrating with density varying,

\[t = \int_{0}^{h} \frac{dz}{V_s(z)} = \frac{1}{V_s(0)} \int_{0}^{h} \sqrt{\frac{\rho(z)}{\rho_0}} \; dz\]

which gives 9.4 minutes from 10,000 feet rather than 10.1, and 17.4 from 20,000 rather than 20.2. The constant-density figure overstates the time by 8 percent at ten thousand feet and 16 percent at twenty. The table under The Range That Makes Descent Discretionary uses the integrated values.

A round canopy travels only with the wind. Seven and a half miles from a ten thousand foot ejection is the entire product. Whether that is worth anything depends on whether seven miles is the difference between capture and recovery, a question about the war and not about the rotor.

The Material Argument

A rotor sweeps its disc with almost nothing in it. Solidity is the ratio of blade area to disc area,

\[\sigma = \frac{N c}{\pi R} = \frac{2 (0.583)}{\pi (10)} = 0.037\]

so the two blades of an assumed 7 inch chord occupy 11.7 square feet and sweep 314. To match the rotor’s drag area with fabric,

\[C_D A = 1.23 \times 314.2 = 388 \, \text{ft}^2, \qquad S_0 = \frac{388}{0.75} = 517 \, \text{ft}^2\]

Forty-four square feet of canopy for every square foot of blade. That ratio is the entire engineering appeal of the idea, and the sizing literature behind it is Gianmarco Sahragard-Monfared et al, Gessow 1948, Powell 1954, Young and Lytwyn 1967, Jenney et al 1968, Wright 1969, Miller 1983, Zaitsev 1987.

And it loses on mass, the measure that decides. A Bensen rotor with hub and teeter bearing is estimated here to be of order 38 pounds, on the blade mass assumed below. A complete personnel parachute assembly with canopy, lines, harness and container is estimated to be of order 28. Rigid blade is heavy per unit area in the proportion that fabric is light, and the forty-four to one advantage in area comes back as roughly parity in weight.

Dependent Systems

The Rotor

Two blades, teetering, of wood with a Bensen section. The two-blade teetering rotor and the birch of early Bensen blades are in the autogyro entry, and the Bensen G2 section in the B-8 specification. A teetering head removes the flapping hinges and their maintenance, at the cost of being unable to tolerate low or negative load factor, which for a vehicle that must never be pushed over is an acceptable trade. The teetering rotor and blade motion generally are treated in Horvay 1947, Harris 1972, Shamie and Friedmann 1976, Amer and LaForge 1977, Weller and Lee 1977, Dooley et al 1978, and Glasgow and Corrigan 1982, and the remainder sits with rotors and rotorcraft in the survey.

One number governs how the blade responds, and this article computes its ingredients anyway. The Lock number compares the aerodynamic forcing on a blade to its inertia,

\[\gamma_L = \frac{\rho a c R^4}{I_b}, \qquad I_b = \tfrac{1}{3} m_b R^2 = 12.4 \, \text{slug ft}^2\] \[\gamma_L = \frac{(0.002377)(5.7)(0.583)(10)^4}{12.4} = 6.4\]

The Lock number is conventionally written $\gamma$ with no subscript. It is subscripted here because this article has already used $\gamma$ for the glide angle, and a collision between two standard notations is resolved by marking one rather than by silently reusing it.

A helicopter sits between eight and ten, so this blade is heavy for its air loads. Blade inertia and the flapping behaviour it governs are measured subjects and not assumed ones, in Jones 1957, Gaffey 1969, Johnson 1980, Wang and Talbot 1989, and Berry and May 1991, with more in the same rotorcraft cluster. That is a liability in responsiveness and an asset in the flare, because inertia is the property that stores energy, and the spin-up analysis under Spinning the Rotor Up turns on it.

Operating tip speed for this class is near 400 feet per second, giving

\[\Omega = \frac{V_{tip}}{R} = \frac{400}{10} = 40 \, \text{rad/s} = 382 \, \text{rpm}\]

which is well below any compressibility concern, in McDonnell Aircraft Corp St Louis Mo 1950, Powell 1959, Sissingh 1964, Spencer et al 1969, Barry and Magliozzi 1971, Paglino and Beno 1971, Stockman et al 1973, Arents 1977, Stroub 1978, Stroub et al 1979, Yeager and Mantay 1981, Leighton 1982, Savino et al 1985, Mueller et al 1987, Althoff 1989, Prieur et al 1989, Mueller et al 1990, Mueller et al 1992, Berton et al 1995.

Two nondimensional numbers place this rotor against every other rotor and against the wind turbine it shares its flow state with. The thrust coefficient refers the load to the dynamic pressure at the tip,

\[C_T = \frac{W}{\rho A (\Omega R)^2} = \frac{350}{(0.002377)(314.2)(400)^2} = 0.0029\]

and dividing by solidity gives the blade loading coefficient, the quantity that approaches a stall limit near 0.10. Where that limit sits, and how blade area, blade count and chord distribution move it, is a design literature of its own, in Gianmarco Sahragard-Monfared et al, Gessow 1948, Powell 1954, Young and Lytwyn 1967, Jenney et al 1968, Wright 1969, Miller 1983, Zaitsev 1987,

\[\frac{C_T}{\sigma} = \frac{0.0029}{0.037} = 0.079\]

The tip speed ratio compares the tip speed to the flow the rotor sits in,

\[\lambda = \frac{\Omega R}{V_d} = \frac{400}{27.6} = 14.5\]

A horizontal axis wind turbine runs at a tip speed ratio of six to eight. This rotor runs at fifteen, so it is a very lightly loaded machine in the same flow state, with a correspondingly small induction factor and a thrust coefficient of a few thousandths. The tip speed ratio, the momentum theory and its high-thrust limit are treated in Yang et al 2016, Bontempo and Manna 2017, Ehrich et al 2018, Perez-Becker et al 2020, Hosseini et al 2022, Boatto et al 2023, and Wood and Golmirzaee 2023, and the survey collects the rest of it under wind turbines, energy harvesting and the windmill state.

Spinning the Rotor Up

The obvious objection to the concept is that a parachute inflates and a rotor has to be spun up. A standard 28 foot personnel canopy is fully open in about one to three seconds, in Knacke’s recovery systems design manual. A rotor at rest produces nothing. The energy has to come from somewhere and the only bank available is height.

Treating each blade as a uniform rod about the hub,

\[I = N \cdot \tfrac{1}{3} m_b R^2 = 2 \cdot \tfrac{1}{3} (0.373)(100) = 24.9 \, \text{slug ft}^2\] \[E = \tfrac{1}{2} I \Omega^2 = \tfrac{1}{2} (24.9)(40)^2 = 19{,}900 \, \text{ft lb}\] \[h = \frac{E}{W} = \frac{19{,}900}{350} = 57 \, \text{ft}\]

Fifty-seven feet if every particle of potential energy went into the rotor, and a few hundred feet at any believable efficiency. That is plausibly comparable to the height a canopy loses while it inflates.

There is a small surprise buried in that arithmetic and it matters later. Substituting the inertia and the operating speed into the energy,

\[E = \tfrac{1}{2} I \Omega^2 = \tfrac{1}{2} \left( N \tfrac{1}{3} m_b R^2 \right) \left( \frac{V_{tip}}{R} \right)^{2} = \frac{N}{6} m_b V_{tip}^2\]

The radius has cancelled. At a fixed blade mass and a fixed tip speed, a rotor stores the same energy whatever its size, because the inertia rises as the square of the radius exactly as fast as the operating angular speed falls. Shrinking a rotor costs nothing in stored energy.

Energy held in a turning rotor is a separate subject, and the period expresses it as an autorotative index rather than as a quantity of foot pounds, in Dooley and Yeary 1979. The literature on it is not separable from the autorotation literature proper, since a paper on autorotative landing is a paper about spending this energy, and the relevant work is cited above, under autorotation and blade motion. It costs a great deal elsewhere, and that bill arrives in the Comparison With Ground Prediction section below.

The objection fails. This is a negative result and it is reported as one, because the energetic argument against the Discretionary Descent Vehicle is the first one anybody reaches for and it does not survive being written down. No record in the survey treats spinning a rotor up from rest, which would refine it, and that makes spin-up the thinnest topic in this article. It is discussed under The Source Base.

An Irreversible Step in the Deployment

Here lies one plausible ground for the doubts about feasibility, and that ground is not aerodynamic.

The documented concept was a vehicle stowed in the aircraft, ejected with the pilot and deployed during the descent, its rotor brought up to speed as it fell, in the X-25A fact sheet. The record consulted here does not say how a personnel parachute fitted that sequence. If the crewman left the aircraft by ejection seat in the ordinary way, let the parachute open and spun the rotor up in that descent, the sequence would end by cutting the parachute away. Written as a trade, the crewman gives up a canopy already descending at 21 feet per second and working, in exchange for a rotor descending at 16.5 feet per second with a four-to-one glide. If the gyro-chute instead replaced the canopy, the crewman’s survival would rest on a rotor designed to deploy automatically at any ejection speed, including supersonic, for which no piloted test is recorded, in Jenkins, Landis and Miller.

The gain is range. The gain is not survival of the descent, which was already assured. In the cutaway sequence the step that secures it cannot be undone, is taken seconds after an ejection, and is taken by someone who may be injured, disoriented, or under fire. On escape systems the relevant work is Poteate and Solomonides 1964, Bement 1979, Ford 1979, Hawker and Payne 1979, and Cummings 1982 and the tolerance question underneath it is Snyder 1959, McKenney 1970, Glaister 1978, and Laananen 1980. The survey’s cluster on crew escape, ejection and the human in the escape holds both in full.

Flying It

An autogyro is not an intuitive aircraft. Rotor speed must be respected, the machine is unforgiving of mishandling in a way a parachute cannot be because a parachute offers nothing to mishandle, and a gyroplane landing is a skill. Prouty on helicopter performance, stability and control supplies the performance and control framework. The record states that the X-25A and X-25B were flown to evaluate piloting technique and to determine training requirements, in Jenkins, Landis and Miller, a programme treating the operator as a question in its own right, although the X-25A fact sheet reports that pilots could be quickly and easily trained to fly the X-25. The handling and training literature it belongs to includes Daughaday and Duwaldt 1955, Drinkwater and Feistel 1965, Caro and Isley 1966, Barnes and Statham 1970, Seville Research Corp Pensacola Fl 1979, Intano and Howse 1991, and Hess 1995, and the cluster on flight test, handling qualities, training and piloting technique lists the rest.

The Flight Test Record

Both aircraft flew and the record of what they measured is extremely thin.

The inventory by Jenkins, Landis and Miller gives 5 June 1968 as the first flight of the type, and Guinness World Records gives the same date for the X-25B, flown at Edwards. The Bensen B-8 entry assigns 5 June 1968 to the X-25A and 23 January 1968 to the X-25B without a source for either, and its text elsewhere dates the Air Force study of the two aircraft to May 1968. This article adopts 5 June 1968 for both. Under the alternative the X-25B would have flown 134 days before the X-25A, which would settle the order of first flights, and no other result here depends on the date. Testing concerned how the aircraft should be flown and what training a pilot needed. No full-scale operational test of the concept was ever performed, meaning that the sequence the idea depends on, which is ejection followed by rotor deployment in the descent, was never demonstrated as an integrated system by this programme, and no piloted flight of the gyro-chute that embodied it is known. Funding ended as the American involvement in Vietnam wound down, and with it the Air Force’s interest, in the same inventory, and the X-25A fact sheet adds doubts about the concept’s operational feasibility as a second reason.

What the X-25 demonstrated is that a person can be taught to fly a small autogyro. What it did not demonstrate is anything about escaping from an aeroplane. The fact sheet reports that training was quick and easy, and the inventory records no piloted test of the escape vehicle. The gap between those two statements is the programme.

Comparison With Ground Prediction

The X-25 itself returned no published measurement this article can check a model against. A second vehicle does allow the check, one the model was not built from.

The Kaman KSA-100 SAVER, for Stowable Aircrew Vehicle Escape Rotorseat, was the same concept carried further for the United States Navy. Its rotor was two telescopic blades folded into the seat itself, deployed by a drogue within four seconds of ejection. It flew towed on 28 December 1971, free the following day with a non-telescoping rotor, and with the telescoping rotor on 10 January 1972. Kaman made over 1,500 air drops of the deployment system, and a full-scale unpowered article was tested in the National Aeronautics and Space Administration Ames full-scale tunnel in 1970, in the Kaman KSA-100 SAVER entry. The concept paper, which also reports the 1970 tunnel test, is the description by Hollrock and Barzda 1972 of a stowable autogyro that serves as the crewman’s seat and converts to a flight vehicle after ejection.

Stowability forces the geometry, and the geometry forces the descent rate. A rotor that must fold into a seat cannot be 20 feet across. The Kaman KSA-100 SAVER entry gives the SAVER’s as 14. Since

\[V_d \propto \frac{\sqrt{W}}{R}\]

a smaller radius at a larger weight is punished twice.

	X-25B	SAVER
Rotor diameter, ft	20	14
Disc area, ft²	314.2	153.9
Weight, lb	350 assumed	710 documented
Disc loading, lb/ft²	1.11	4.61
Vertical autorotation, ft/s	27.6	56.1

Fifty-six feet per second is 38 miles an hour straight down. A steady vertical descent at that rate is not survivable, and the difference would have to be retrieved in the flare out of stored rotor energy.

That sentence is easy to write and should be checked, because it need not have been true. It is, and the check is short.

The energy a flare must absorb is the kinetic energy of the descent,

\[E_{req} = \tfrac{1}{2} \frac{W}{g} V_d^2\]

and substituting the descent rate turns it into a function of weight and radius alone,

\[E_{req} = \frac{(1.8)^2}{2 g \rho \pi} \cdot \frac{W^2}{R^2}\]

The energy required goes as the square of the weight over the square of the radius, while the energy available goes as neither. Dividing one by the other gives a margin whose only free quantity is the assumed blade mass,

\[\frac{E_{avail}}{E_{req}} = \frac{N m_b V_{tip}^2 \, g \rho \pi R^2}{3 (1.8)^2 W^2}\]

	X-25B	SAVER
Weight, lb	350	710
Rotor radius, ft	10	7
Descent rate, ft/s	27.6	56.1
Energy required, ft lb	4,130	34,700
Energy available, ft lb	19,900	19,900
Margin	4.8	0.57

The X-25B carries nearly five times the energy its flare needs. The SAVER carries about half. Inverting the margin to unity asks what the blades would have to weigh,

\[m_b = \frac{3 (1.8)^2 W^2}{N V_{tip}^2 g \rho \pi R^2}\]

which is 2.5 pounds a blade for the X-25B and 21 pounds a blade for the SAVER, against the 12 assumed for both. A vehicle whose entire design constraint was folding into an ejection seat would have needed blades close to twice as heavy as a Bensen’s.

Where the argument from stowability alone was qualitative, this result is quantitative, and the two point the same way. It still depends on an assumed blade mass and tip speed for a vehicle whose real figures are not published, so it is offered as an order-of-magnitude estimate rather than a measurement. Doubling the assumed blade mass still leaves the SAVER at a margin of 1.15, which is no margin at all for an escape system.

The model therefore predicts that a stowable rotor seat needs an engine, and the SAVER had one, a Williams WRC-19 turbofan of 430 pounds thrust, which the Kaman KSA-100 SAVER entry records.

The prediction must not be overclaimed and it is not. The record gives off-the-shelf availability as the reason the engine was fitted and notes that it was more powerful than the seat needed, and the design rating for the operational seat’s engine in Hollrock and Barzda 1972 is a sea-level thrust of 1,280 newtons, about 290 pounds. A thrust to weight ratio of

\[\frac{T}{W} = \frac{430}{710} = 0.61\]

is far above anything a gyroplane needs to sustain flight, so the descent problem evidently did not size the engine. The prediction is directionally right and quantitatively unconfirmed, and what can be claimed is that the scaling law explains why a stowable seat is a harder vehicle than a carried one, not that it explains Kaman’s engine selection.

What the Data Changed

Almost nothing directly. The Discretionary Descent Vehicle did not enter service. No operational aircraft has ever carried one, and no full-scale operational test was ever conducted, in Jenkins, Landis and Miller.

The question outlived the answer. The SAVER pursued the same idea with a stowable rotor and an engine and was also cancelled, the military concluding the idea was not viable, in the Kaman KSA-100 SAVER entry. What survived is the question, namely whether a descending crewman should have a choice of landing point.

That question was answered, and the answer was made of fabric. A ram-air parachute is a wing built from cloth. It reaches lift-to-drag ratios of about three, with 3.2 for a parafoil vehicle system in Knacke’s recovery systems design manual, close to the ratio of 4 assumed here for an autorotating rotor, and it does so with a device that packs into a container, needs no spin-up, has no rigid parts, and, by this article’s inference, asks less of its user, although the one training result recorded for the X-25 was favourable, in the X-25A fact sheet. The rotor’s forty-four to one advantage in material area was answered by a device that sweeps no disc at all and generates lift as a wing. That literature begins in the period work on gliding parachutes and flexible wings, in Rogallo et al 1960, Rogallo 1962, Horton 1962, Ryan Aeronautical Co San Diego Ca 1962, Taylor and McNulty 1962, Johnson and Sleeman 1963, Rogallo 1963, Libbey 1964, Forehand and Bair 1969, Nicolaides and Tragarz 1971, Kenner 1972, Rogallo et al 1972, Matsuo and Butler 1983, and its ancestor is the Rogallo wing, which NASA studied through the 1960s as a recovery device for spacecraft and which was under active investigation at the same moment the X-25 was flying.

The Contemporary Literature

Cluster	Records
Rotors and rotorcraft, momentum theory and blade dynamics	794
Parachutes and aerodynamic decelerators	307
Autorotation, the vortex ring state and the emergency descent	248
Crew escape, ejection and the human in the escape	194
Autogiros and gyroplanes	184
Ram-air canopies, parafoils and guided airdrop	189
Electric vertical takeoff, drones and distributed lift	146
Flight test, handling qualities, training and piloting technique	141
Wind turbines, energy harvesting and the windmill state	109
Samaras, monocopters and animal flight	96
Rotor and propeller noise, and the tip speed	85
Rotors at low density, on Mars and at low Reynolds number	58
Rotary decelerators and rotor recovery systems	32
Bluff-body drag and the drag coefficient	28
The final descent and the touchdown	20
Wings, airfoils and the lift-to-drag ratio	19
Gliding flight and the sink rate	16
The standard atmosphere and the flight condition	17
V/STOL and other vertical-lift aircraft	10
Other aeronautical and foundational literature	14
Total	2,707

The Discretionary Descent Vehicle asked a good question and got the wrong answer, and the right answer arrived in a different material. This section surveys the present state of every thread the programme touched.

Two of those threads have become livelier since 1968 instead of quieter, and both land on the keystone. Autorotation is now a certification argument about aircraft that cannot perform it, and the density relation this article derives is being tested on another planet.

When the Engine Stops, Now

Autorotation is no longer primarily an aerodynamic subject. It is a safety and training subject, and the literature has moved with it. Contemporary work covers the safety analysis of autorotative landing execution, the transfer of autorotation training between simulator and aircraft, the height-velocity diagram for variable-speed rotors, emergency landing after engine failure including on hybrid electric tiltrotors, and rotorcraft crashworthiness, in Murugan et al 2014, Tokuoka and Miwa 2015, Ligai et al 2017, Chi et al 2019, Yan and Chen 2019, van der Wall 2019, Astori et al 2020, White et al 2021, Scaramuzzino et al 2022, Lee et al 2023, Francesco Scaramuzzino et al 2024, Germanetti 2024, Serafini et al 2024.

The height-velocity diagram is the direct descendant of this article’s spin-up section. It maps the combinations of height and forward speed from which a safe autorotative landing cannot be reached, and it exists because entering autorotation costs both altitude and time. The X-25’s deployment problem is the same problem drawn on different axes.

Autorotation Became a Computation

Autorotation is now modelled, optimised and flown by algorithm, no longer characterised by test pilots. The contemporary work covers trajectory optimisation for autorotative landing, entry transients, vortex-ring modelling and autonomous emergency descent, in Rezgui and Lowenberg 2015, Sunberg et al 2015, Taamallah et al 2017, Zheng et al 2018, Talaeizadeh et al 2020, Tomita and Fujiwara 2021, Saetti et al 2023, Patnala et al 2024, and Mori et al 2025, and the autorotation cluster below adds further titles.

The Answer Arrived in Fabric

The ram-air canopy is the device the Discretionary Descent Vehicle was reaching for. It reaches nearly the same glide ratio as an autorotating rotor with no rigid structure, no spin-up transient and a training demand this article infers to be lower, and its existence plausibly explains why no aircraft carries a rotor seat, although no document of the programme makes that connection. Its glide ratio is measured and controlled in Bozack 1984, Wegereef et al 2009, and Sun et al 2020, and the contemporary literature on the canopy includes Cacan and Costello 2018, Zhang et al 2021, Ding et al 2023, and Vambol and Kaluzhynov 2023, and the ram-air cluster carries the rest.

And Then It Was Automated

The second half of the answer removes the crewman from the control loop entirely. Guided precision airdrop steers a ram-air canopy to a surveyed point without a person aboard, which is discretionary descent with the discretion delegated to software, in Chiel and Dever 2015, Geiselman et al 2015, Slegers et al 2015, Mukhopadhyay et al 2016, Wachlin et al 2019, Ivanovic and Orsag 2022, Leeman et al 2022, Wang et al 2022, Yan et al 2023, Wei et al 2024, Qi et al 2025.

The Machine That Cannot Do It

The X-25 would find this thread the most interesting, and it did not exist in 1968.

Electric vertical takeoff aircraft have brought the small one-person or few-person flying machine back as a serious industry, with certification-constrained design, conceptual design methods and a safety literature behind it, in Brown and Harris 2020, Cokorilo 2020, Lee et al 2020, Ma et al 2022, Ugwueze et al 2023, and Xie et al 2023. The cluster on electric vertical takeoff, drones and distributed lift continues the list.

A multirotor cannot autorotate. A helicopter rotor stores energy in a large disc turning fast, and a lift rotor on a distributed-propulsion aircraft is small, lightly loaded and often fixed in pitch, so the two properties autorotation needs are both absent. In this article’s terms, the stored energy is

\[E = \frac{N}{6} m_b V_{tip}^2\]

and a small light blade turning at modest tip speed has very little of it, while the disc loading that sets the descent rate is high because the disc is small. The X-25’s rotor was a bad parachute and a good glider. A multirotor is neither.

The industry has answered with redundancy instead of a better rotor, meaning enough independent lift units that the loss of one is survivable, a safety philosophy completely different from the one the X-25 embodies. Autorotation says the aircraft can always become a glider. Redundancy says the aircraft will not fail in the first place. That argument is being had in the contemporary literature on rotor failure, fault-tolerant control and propulsion redundancy, which the same electric vertical takeoff cluster collects.

No finding of this survey is sharper. The capability the Discretionary Descent Vehicle was built entirely around is the one the current generation of small rotorcraft has quietly given up, and it gave it up for a reason the X-25 would have recognised, namely that the rotor you can carry is not the rotor you want.

The Same Physics, Now Called Wind Energy

The windmill brake state is no longer an emergency. It is a business. A wind turbine rotor is an autorotating rotor in a different reference frame, and the blade-element momentum machinery this article uses to size a descent is the same machinery used to size a turbine. The largest living body of work on the X-25’s keystone physics is in wind energy, as the count of blade element momentum titles in the wind turbine cluster shows, and its application to turbines runs through Dehouck et al 2017, Apsley and Stansby 2020, Potentier et al 2021, Yin et al 2022, Le et al 2025, and Selvatici and Stevens 2025.

The Rotor at One Percent of Sea Level

Descent rate goes as one over the square root of density, and that relation is now being tested at an extreme no terrestrial machine reaches.

A rotorcraft flying on Mars operates in roughly one percent of sea-level density. Everything this article derives about disc loading and descent follows the same algebra there, and the consequences are severe and practical. Such vehicles therefore carry very large rotors for their mass and turn them fast, with blade tips at up to seven tenths of the speed of sound on the Ingenuity helicopter. The contemporary work on rotor aerodynamics at low density and low Reynolds number is San Martin et al 2017, Koning 2019, Koning et al 2019, Koning et al 2019, Serré et al 2019, Zhao et al 2019, Harris 2020, Koning et al 2020, McElreath et al 2020, Chen and Hubner 2021, Fox 2021, Gojon et al 2021, Kamruzzaman et al 2021, Kruesi 2021, Leslie 2021, Veismann et al 2021, Zhang and Yang 2021, Nepal et al 2023, Yamaguchi and Anyoji 2023, Bowman et al 2024, Wang et al 2024, Zhao et al 2025, Datta et al 2026, tang et al 2026.

The article’s own altitude section is the same calculation stopped early. A rotor at twenty thousand feet descends 37 percent faster than at sea level. A rotor at Martian density would descend about ten times faster, and that factor of ten justifies the comparison.

The Seed Had It First

Autorotation is not an invention. A maple samara descends by autorotating, and the contemporary rotary-decelerator and micro air vehicle literature cites it directly, in Deng 2015, Dormiyani et al 2015, Liu et al 2015, Pounds and Singh 2015, Liu 2016, Wu et al 2017, Li and Guo 2018, Porritiello et al 2022, Suhadi et al 2022, Tong et al 2022, Cai et al 2025, Huang et al 2026, Zhang et al 2026.

How Loud the Tips Are

This article assumed a tip speed of 400 feet per second and never said what bounds it. At the upper end the limit is compressibility on the advancing blade. At the lower end it is stored energy, which this article has already shown goes as the square of tip speed. In between, and dominating modern design, sits noise. Rotor noise rises steeply with tip speed, and community noise acceptability has become a design driver and no longer a courtesy, in van der Wall et al 2017, Greenwood and Schmitz 2018, Szulc and Doerffer 2018, Yang et al 2018, Shukla and Komerath 2019, Sou 2019, Yang et al 2019, Jia and Lee 2020, Li and Lee 2020, Polyzos et al 2020, Bauer et al 2021, Vouros et al 2021, Lukianov 2022, Nowoświat et al 2022, Christopher S. Thurman and James D. Baeder 2023, Eric Greenwood et al 2023, Fu et al 2023, Kingan et al 2023, Su Jung et al 2023, Buǧday and Saetti 2024, Goudswaard et al 2024, Johnson-Groh 2024, Saetti et al 2024, Shao et al 2024, Yuk et al 2024, Arystanbekov et al 2025, Boucher 2025, Jung et al 2025, Pereira et al 2025, Turhan et al 2025, Melo et al 2026, Philipps-Bertin et al 2026, Stefan J Letica and Stephen A Rizzi 2026.

That is a constraint the X-25 did not have and an electric vertical takeoff aircraft cannot escape, and it pushes tip speed down while the stored-energy argument wants it up.

The Gyroplane Did Not Disappear

The X-25’s type is still flying and still being written about, with contemporary work on gyroplane flight dynamics, stability and experimental rotor measurement, in Czyż et al 2015, Shin and Kim 2015, Wu 2015, Figat 2017, Houston and Thomson 2017, Stalewski 2017, Cai et al 2018, Czyż et al 2018, Lee et al 2018, Stalewski 2018, Лучко and Зінченко 2018, Design of Composite Gyrocopter Main Rotor Blade Involving Rib and Spar Elements 2019, Wojtas and Sobieszek 2019, Czarnigowski 2020, Rezgui and Lowenberg 2020, Łusiak 2022.

Its safety record is the part that bears on this article. The gyroplane acquired a poor safety reputation through designs prone to a power push-over, a pitch departure that unloads the rotor, in the autogyro entry. Such a property would weigh against the type for a person who has just ejected and has no rotorcraft training, although the X-25A fact sheet reports a favourable training result for the X-25.

Rotors on Everything

The small rotorcraft the X-25 represented as a curiosity is now an industry. Electric vertical takeoff vehicles, multirotors and distributed propulsion have made rotor sizing an everyday design problem, in Hwang et al 2015, Multi-Rotors A Revolution In 2015, Lei and Cheng 2020, and Lei and Wang 2020.

Escape Systems Now

Crew escape remains an active discipline concerned with injury criteria and occupant protection rather than with flight after ejection, in Do and deWeck 2011, Maloney 2011, Buller 2017, Kowaleczko et al 2018, Sarkar et al 2018, Mao and Liang 2019, Perry et al 2019, Wang et al 2019, Ying 2019, Bao et al 2021, Ejection Seat Testing 2021, Lakshmi et al 2021, Saravanan et al 2021, Smitha et al 2021, Somanath et al 2021, Sreenivasulu et al 2021, Liu et al 2022, Schultz et al 2022, Rahman et al 2023, Zhou et al 2023, Avirah et al 2024, Rahman et al 2024, Kowaleczko et al 2025, Parate 2025, Rahman et al 2025, Davidescu et al 2026, Du et al 2026.

What the Survey Shows

Every thread the X-25 touched is alive, and the one it was about has been abandoned twice over.

The physics is thriving under a different name, because the windmill brake state is a wind turbine’s operating condition and blade element momentum theory is now worked hardest by people generating electricity. The mission is being flown, but by guided ram-air canopies rather than by rotors, and increasingly without a person in the loop at all. The vehicle concept is dead, and the capability it was built around has been given up a second time by the industry that most resembles it, because a multirotor cannot autorotate and has chosen redundancy instead.

Three things this survey establishes that the programme record alone does not.

The question was good. Giving a descending crewman a choice of landing point is a real capability, now routinely delivered, since military personnel parachute assemblies use manoeuvrable parafoils as main and reserve canopies in Knacke’s recovery systems design manual, which retrospectively vindicates the Air Force for asking.

The answer was wrong in a way that was knowable at the time. The arithmetic that shows a rotor to be a worse decelerator than a canopy, and valuable only because it glides, needs nothing that was not available in 1968. So does the observation that the gain is range rather than safety, and that any sequence through an open parachute would end by cutting a working parachute away.

And the concept did not fail on the axis the programme examined. Jenkins, Landis and Miller record that the aircraft were flown to settle questions of piloting and training, and the X-25A fact sheet reports both that the training was quick and easy and that the programme ended with the air war winding down and doubts about its operational feasibility. What ended it was a lapsed need and a doubt that no recorded test addressed.

Rotors and rotorcraft, momentum theory and blade dynamics

The rotor in general, and the largest cluster. Helicopter and rotor aerodynamics, momentum and blade element theory, wakes and inflow, blade dynamics and aeroelasticity, and the structure of the blade and the hub. 46 percent of these records are report primaries, against 19 percent of the autorotation cluster, so the reports servers document the rotor in powered flight far more fully than the rotor in descent, which is the keystone’s thinness seen from the other side.

794 records. [Abrego et al 2016] [Acree 1984] [Acree et al 2001] [Adelman and Mantay 1988] [Advanced Design Rotor Blade Flown by Westland 1985] [Agarwal et al 2022] [Ahmadi 1984] [Aiken et al 2000] [Aldawsari 2025] [Alfred Gessow 1954] [Ali et al 2015] [Althoff 1988, Effect of advanced rotorcraft] [Althoff 1989] [Althoff et al 1988, Inflow measurement made with a] [Althoff et al 1988, Inflow measurements made with a] [Amer 1989] [Amer and LaForge 1977] [Amer et al 1964] [Anderson and Johnston 1974] [Anderson and Watts 1976] [Andrew Woodgate and N․ Barakos 2026] [Andrews 1941] [Arcidiacono 1964] [Arents 1977] [Army Materiel Command Alexandria Va 1974] [Army War Coll Carlisle Barracks Pa 2006] [Arnold 2025] [Artis Jr 1975] [Arush et al 2022] [Arıol Taymaz 2022] [Attachment of a rotor blade of fibre reinforced plastic to a metal rotor hub 1991] [Augustyniak 2016] [Azuma and Saito 1982] [Bagai and Leishman 1992] [Bailey 1939] [Bailey 1941] [Bailly and Bailly 2019] [Balaji 2017] [Balcerak 1967] [Balcerak et al 1969] [Balch 1979] [Balch 1985] [Bales et al 1973] [Ballard et al 1979, Effect of tip planform on blade] [Ballard et al 1979, Effect of tip shape on blade] [Ballard et al 1980] [Barbely et al 2016] [Barkley and Weaver 1977] [Baron et al 1964] [Bauchau and Hong 1987] [Bauchau and Lemnios 1997] [Berman 1956] [Berman 1965] [Berry 1982] [Berry and Althoff 1989] [Berry and May 1991] [Berry and Schrage 1986] [Berry et al 1987] [Berton et al 1995] [Bhardwaj et al 2023] [Bielawa 1988] [Biggers and Orloff 1975] [Bishop 1958] [Blackwell 1977] [Blankenship and Harvey 1962] [Blythe 2007] [Bobo 1972] [Boeing Co Morton Pa Vertol Div 1961] [Boeing Co Morton Pa Vertol Div 1961] [Boeing Vertol Co Philadelphia Pa 1977] [Boeing Vertol Co Philadelphia Pa 1977] [Boisard 2022] [Bolanovich and Marks 1959] [Bottasso and Montinari 2015] [Bousman 2003] [Bouzidi et al 2020] [Bowen-Davies and Yeo 2018] [Bradley 1956] [Bradley 2016] [Bramwell 1966] [Brender et al 1996] [Briardy et al 1970] [Britton 1992] [Brown and Fisher 1968] [Brown and Schmidt 1963] [Buccio et al 2026] [Burke 1996] [Burkhardt 1985] [Burpo and Lynn 1962] [Kobayashi and Jessup 1985] [Lee 1989] [Calcaterra and Schubert 1969] [Cameron and Sirohi 2019] [Cameron et al 2016] [Cao et al 2021] [Cao et al 2024] [Cao et al 2024] [Caprace et al 2020] [Cardinale 1969] [Carico and Blemel 1998] [Carlson and Cassarino 1973] [Carlson and Hilzinger 1965] [Case 1985] [Cassarino 1970] [Castillo-Rivera and Tomas-Rodriguez 2020] [Castles and De Leeuw 1953] [Castles and Durham 1956, Distribution of normal component] [Castles and Durham 1956] [Castles et al 1958] [Catapang 1993] [Celi 1991] [Cerbe and Reichert 1989] [Chang and Ai 2026] [Chappell 1979] [Chattopadhyay 1993] [Chattopadhyay and Chiu 1990] [Chattopadhyay and Chiu 1990, An enhanced integrated] [Chattopadhyay and Jones 1992] [Chattopadhyay and Walsh 1990] [Chattopadhyay et al 1989] [Chattopadhyay et al 1997] [Chawla 1952] [Chen 1980] [Chen 1980] [Chen and Hubner 2021] [Chen and Liu 2015] [Chen et al 1984] [Chen et al 2019] [Chen et al 2021] [Chen et al 2026] [Cheney 1972] [Cho and Lee 1994] [Choephel et al 2026] [Chopra 1985] [Chopra 2000] [Chopra 2002] [Chopra 2013] [Chopra and Sivaneri 1982] [Sissingh 1951] [Thurman et al 2023] [Clark and Maskew 1982] [Cleek et al 1978] [Cleek et al 1978] [Cleek et al 1979] [Cleek et al 1979] [Cleek et al 1979] [Cleek et al 1979] [Cleek et al 1979] [Coffin and Haldeman 1973] [Composite helicopter rotor and blade 1974] [Composite helicopter rotor hub 1990] [Composite rotor blade 1980] [Composite rotor blade 1993] [Computer techniques for propeller blade section design 1976] [Cook 1974] [Cornell 1976] [Cresap 1962] [Crespodasilva 1981] [Crimi 1965] [Crimi 1973] [Crimi 1974] [Crimi and White 1962] [Cristiani et al 2020] [Curtiss 1973] [Curtiss and McKillip 1992] [Curtiss et al 1984] [Dai et al 2025] [DAngelo and Malvano 1979] [Davenport 1964] [Stephens 2014] [Davis 1974] [Davis et al 1974] [de la Cierva 1934] [de la Cierva 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[Hodges 1990] [Hodges 2015] [Hoffman and Velkoff 1971] [Hoffrichter 1978] [Hohenemser 1950] [Hohenemser 1954] [Hohenemser 1957] [Hohenemser and Banerjee 1977] [Hohenemser and Perisho 1958] [Hohenemser and Yin 1971] [Hohenemser and Yin 1973] [Hohenemser et al 1976] [Holness et al 2017] [Hone 1975] [Hong and Chopra 1985] [Hong and Chopra 1986] [Hong and Jung 2024] [Hooi et al 2016] [Horvay 1947] [Horvay and Yuan 1947] [Hou et al 2023] [Howard 1999] [Hsieh 1979] [Hsieh and Davis 1974] [Hubbard and Harris 1982] [Hubbard and Leighton 1984] [Huber and Polz 1983] [Huo et al 2015] [Hurley et al 2019] [Hussien et al 2025] [Hwang and Kwon 2015] [Hwang and Kwon 2019] [Ichikawa 1967] [Idzubuchi and Kito 1941] [Ignatkin et al 2020] [Impact tests of a graphite-epoxy helicopter tail rotor blade 1975] [Investigation of Aeroelastic Coupling Method for Swept Blade Based on Modification of Blade Element Momentum Theory 2025] [Jacobellis et al 2021] [Jacobson and Smith 2018] [Jain et al 2016] [Jang and Chopra 1987] [Cornelius and Schmitz] [Jenkins and Marks 1975] [Jenney et al 1968] [Ji et al 2022] [Jiao and Zheng 2025] [Jimenez Garcia and Barakos 2018] [Jo et al 2017] [Johnson 1972, A pertinent solution of] [Johnson 1973] [Johnson 1975] [Johnson 1980] [Johnson 1980] [Johnson 1981, The influence of unsteady] [Johnson 1982] [Johnson 1986] [Johnson and Chopra 1979] [Johnson and Sirohi 2024] [Johnson et al 2007] [Johnson et al 2012] [Johnston and Cassarino 1976] [Jones 1957] [Jones 1965] [Jones 1965] [Jones and Rao 1970] [Jones and Rao 1971] [Jenkins 1966] [Jun et al 2026] [Jung et al 2012] [Kaman Aircraft Corp Bloomfield Ct 1964] [Kargaev et al 2025] [Kato and Yamane 1979] [Kato and Yamane 1979] [Kato et al 1981] [Kawachi 1982] [Kee 1959] [Kelley 1987] [Kemp 1945] [Kerwin 1973] [Kerwin and Leopold 1963] [Keßler 2017] [Kim 2004] [Kisielowski et al 1967] [Kitaplioglu et al 2000] [Kito 1942] [Knight and Hefner 1937] [Kocjan and Rogólski 2024] [Kolwey 1999] [Komerath et al 2015] [Konstantinov et al 2026] [Korkan et al 1984] [Kosiyuk and Kosiyuk 2020] [Kottapalli 2012] [Kovalovs et al 2017] [Kozapalli and Warmbrodt 1997] [Krishnamurthi and Gandhi 2017] [Krishnan et al 2022] [Kumar and Venkatesan 2016] [Kvaternik and Kohn 1977] [Lake et al 1992] [Lakshmikantham and Aravamudan 1973] [Landgrebe 1971] [Landgrebe 1971] [Landgrebe 1972] [Landgrebe and Egolf 1975] [Landgrebe and Egolf 1976] [Landgrebe and Johnson 1974] [Young 2023] [Law and Puterbaugh 1988] [Ledoux et al 2021] [Lee and Dassonville 2020] [Lee and White 1974] [Lee et al 2021] [Lee et al 2022] [Lee et al 2025] [Lee-Rausch and Biedron 2009] [Lehman 1968] [Leishman 1990] [Leishman 2008] [Leishman 2014] [Lemnios and Dunn 1976] [Lepicovsky and Bell 1984] [Levenetz 1973] [Light 1993] [Lightfoot 1958] [Liiva 1969] [Lim 2015] [Lim and Chopra 1989] [Lim et al 2014] [Lim et al 2016] [Lim et al 2017] [Lin et al 1979] [Linden and Hellyar 1974] [Linton et al 2021] [Liou et al 1989] [Liu 1962] [Liu et al 2026] [Longobardi 1976] [Longobardi 1976] [Longobardi and Fournier 1973] [Lu et al 2019] [Ludi 1961] [Lynn et al 1970] [Lyu and Yang 2025] [Ma et al 2021] [Maciolek and Wallischeck 1977] [MacNeal 1958] [Madden 1967] [Magee and Taylor 1971] [Mahmuddin 2017] [Maier et al 1999] [Makeev 2026] [Mancini et al 2023] [Manwaring and Fleeter 1990] [Maresca et al 1974] [Marine Corps Equipment Board Quantico Va 1961] [Marks 1960] [Wilbur et al 2000] [Mayerjak 1978] [Maynard and Evans 1945] [Maynard and Salters 1957] [Maynard and Steinberg 1953] [McAlister and Heineck 2002] [McCarty et al 1959] [McCormick 1956] [McIntyre 1963] [McKillip 1996] [McNulty 1988] [McVeigh and McHugh 1984] [Meck and Galli 1973] [Mertova and Bures 2021] [Meyer and Falabella 1953] [Mihaila et al 2026] [Miller 1962] [Miller 1964] [Miller 1964] [Miller 1983] [Mittal 2005] [Miyajima 1979] [Modeling Tool Advances Rotorcraft 2007] [Molyneux 1962] [Moon and Phan 2015] [Morduchow et al 1953] [Morduchow et al 1955] [Morse 1956] [Mortimer et al 2026] [Moxon and Green 1990] [Mpanza and Pedro 2021] [Mu¨ller 1990] [Müller et al 2023] [Nagaraj and Shanthakumar 1975] [Nagaraja and Pierce 1977] [Nakajima 1960] [Chaderjian] [Peters et al] [Nikolsky 1959] [Nixon 1988] [Nixon 1993] [Noonan 1990] [Noonan 1991] [Noonan et al 1992] [Nygren 1990] [Obioma Godspower and John Chuku 2019] [Ollerhead 1971] [Ormiston 1991] [Ortun et al 2017] [Oruc et al 2017] [Overmeyer and Martin 2017] [Paglino and Beno 1971] [Park 2022] [Park and Kwon 2021] [Park and Park 2018] [Park and Park 2020] [Park et al 2023] [Parks 1976] [Payne 1954] [Payne 1954] [Payne 1954] [Payne 1955] [Payne 1959] [Pepper and McHugh 1942] [Performance characteristics of two-blade propeller fans 1930] [Perisho 1959] [Petermann et al 2019] [Peters 1975] [Peters 1985] [Peters 1988] [Peters and Schrage 1978] [Peters et al 1982] [Peters et al 1984] [Peterson and Johnson 1991] [Peterson and Warmbrodt 1984] [Pfluger 1940] [Phelps 2014] [Piatak et al 1997] [Pilkey 1982] [Piziali and Duwaldt 1962] [Piziali and Felker 1987] [Plotkin 1971] [Plunkett 1949] [Pogorzelski and Cable 2001] [Porterfield and Alexander 1970] [Powell 1959] [Pribyla 1989] [Proulx-Cabana et al 2024] [Prouty 1976] [Prussing and Lin 1982] [Prussing and Lin 1983] [Pruyn et al 1966] [Pulok and Chakravarty 2024] [Pérez Gordillo et al 2023] [Pölzlbauer et al 2018] [Quackenbush 1987] [Rae and Shojiro 1977] [Raghavan et al 1990] [Ramachandran et al 1990] [Ramanujam and Abhishek 2022] [Ramanujam R and Mohan 2021] [Ramasamy et al 2010] [Rampy 1966] [Rand 1989] [Rand 1991] [Rand and Khromov 2018] [Rao and Jones 1974] [Rayavarapu 2018] [Reader 1980] [Reich et al 2017] [Richardson and Liiva 1970] [Richardson et al 1970] [Richez 2018] [Rinehart 1970] [Powell 1954] [Robinson 1970] [Rogers 1974] [Roh et al 2020] [Romander 2006] [Rosen and Isser 1995] [Rosen and Isser 1995] [Rosenberg 1944] [Rossow 1985] [Roth 1996] [Rotor blade 1990] [Rotor blade shear reinforcement 1975] [Rotorcraft Dynamics 1984 1985] [Rout and Hota 2024] [Roy 2021] [Ruh and Hwang 2023] [Rutledge et al 1991] [Saetti 2024] [Saetti et al 2024] [Saetti et al 2026] [Sahin et al 2025] [Sakthivel and Venkatesan 2017] [Samad et al 2020] [Sandford and Belko 1982] [Sarker et al 2026] [Savino et al 2021] [Scarpati et al 1974] [Schaefer and Smith 1949] [Schau et al 2016] [Schillings and Reinesch 1989] [Schlechtriem and Ballmann 1994] [Schmaus and Chopra 2022] [Schmitz 1971] [Schrage et al 1997] [Schuett 1969] [Segel 1965] [Segel 1967] [Seher-Weiß and Jones 2021] [Sekula and Russell 2024] [Sekula and Wilbur 2014] [Sekula et al 2003] [Selleck 1978] [Shahmiri 2017] [Shamie and Friedmann 1976] [Shaw and Albion 1981] [Shcherbina 1974] [Sheehy 1977] [Sheen et al 2023] [Sheng 2018] [Shipman 1974] [Shipman and Wood 1971] [Shulman 1956] [Simpson 1967] [Sinacori and Lange 1961] [Singh and Friedmann 2021] [Single-beam laser Surface angle Meter for helicopter landing 2020] [Singleton et al 1990] [Sissingh 1964] [Sissingh and Kuczynski 1970] [Sisson et al 2022] [Sisson et al 2022] [Sivaneri and Chopra 1981] [Sivaneri and Chopra 1984] [Small Business Innovations Helicopters 1992] [Smith 1968] [Smith 1970] [Smith and Betzina 1986] [Smith and Chopra 1992] [Smith and Chopra 1993] [Smith et al 1999] [Smith et al 2017] [Snyder 1950] [Solomon 1953] [Somers 2010] [Son and Kim 2023] [Son et al 2025] [Song and Xia 2015] [Sopher and Hallock 1985] [Speeding up rotor blade profile inspection at lucas aerospace 1983] [Squire 1951] [Sridharan and Govindarajan 2022] [Srinivasan and McCroskey 1988] [Srinivasan and McCroskey 1988] [Stainback 2001] [Stammers 1970] [Stanisławski 2019] [Stengel et al 1978] [Stepniewski 1979] [Stepniewski and Keys 1984] [Stewart 1962] [Stouti et al 2024] [Straub 1984] [Straub et al 1994] [Stremel 1990] [Stroub 1991] [Stroub et al 1979] [Stroub et al 1992] [Studwell 1980] [Su et al 2021] [Su Jung et al 2024] [Sun et al 2017] [Sutton and Buell 1952] [Sweet 1960] [Sweet and Jenkins 1963] [Takahashi 1994] [Takato 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Parachutes and aerodynamic decelerators

The device the rotor is compared against. Parachute aerodynamics and inflation, canopy fabrics and stress, clusters, recovery systems and the decelerators of atmospheric entry. 27 of these titles name Mars, another planet or an atmospheric entry, and 11 name the personnel parachute that this article’s comparison actually uses.

307 records. [Abdullah Hasan et al 2022] [Accorsi and Leonard 2002] [Accorsi et al 2000] [AL-Madani et al 2018] [Alifanov et al 2018] [Anton et al 2023] [Arai et al 2011] [Armand 2013] [Arnold et al 2011] [Bendura et al 1968] [Gao et al 2016] [Gonyea et al 2013] [Huang et al 2022] [Korzun et al] [Askins et al 1997] [Avis 2005] [Axdahl et al 2009] [Babakov 2025] [Babakov 2026] [Baginski et al 2013] [Barber and Johari 2001] [Barry et al 2026] [Behera et al 2022] [Behera et al 2022] [Behera et al 2023] [Behr 1989] [Behr 1989] [Bergeron et al 2025] [Bleier et al 2015] [Bohon and Miserentino 1971] [Bolton and Holt 1981] [Boustani et al 2022] [Braun 1953] [Braun and Doherr 1980] [Braun and Walcott 1963] [Brinkman 1992] [Brown et al 2003] [Cadieux and Barad 2025] [Cannon and Koch 1989] [Cao and Jiang 2007] [Cao et al 2012] [Cao et al 2017] [Carten and Wuest 1979] [Cassanto et al 1968] [Catch a Falling Star 1993] [Chandler 1967] [Chen et al 2021] [Cheng et al 2016] [Clark et al 2015] [Coatta et al 2013] [Cook 2013] [Corridan et al 1984] [Cruz et al 2013] [Cruz et al 2013] [Daniel 2025] [Darshankumar Ragunath 2016] [Daum and Mollmann 2017] [Day 2013] [Defense Documentation Center Alexandria Va 1971] [Deitering and Hilliard 1965] [Description of Mr. Cocking’s 1837] [Desimone 1981] [Dong et al 2022] [Downing et al 1956] [Downs and Bartlett 1989] [Dutta 2020] [Dynamic Response Analysis of a High Glide Ratio Parachute System 2021] [Eckstrom 1970] [Eckstrom and Preisser 1967] [Eckstrom and Preisser 1968] [Everett et al 1984] [Fallon 1991] [Fallon and Fallon 1997] [Fallon et al 1986] [Farhall et al 2007] [Fedele and Mungiguerra 2018] [Fields et al 2011] [Fields et al 2026] [Figucia and McCafferty 1966] [Fogle et al 2018] [Foughner and Alexander 1974] [French 1965] [Gallon and Witkowski 2015] [Gallon et al 2013] [Gallon et al 2015] [Gao et al 2016] [Gao et al 2020] [Garrard and Muramoto 1982] [Garrard et al 1986] [Garrard et al 1987] [Gionfriddo 2011] [Goglia 1952] [Goldberg 2025] [Gombos and Reed 1975] [Guan et al 2025] [Guidotti et al 2012] [Gwinn et al 1995] [Hartley et al 2022] [Heinrich 1961] [Heinrich 1966] [Heinrich 1966] [Heinrich and Saari 1978] [Higgins 1979] [Higgins and Speelman 1973] [Hogan 1992] [Horenstein and Roberts 1993] [Hou et al 2022] [Humi 2004] [Inatani et al 1999] [Irving Air Chute-Brake Parachute 1968] [Jailer et al 1960] [Janda et al 2011] [Janda et al 2013] [Jiang and Ge 2024] [Jiang et al 2026] [Jin et al 2016] [Jo et al 2022] [Johari and Desabrais 2002] [John et al 2025] [Johnson 1989] [Jorgensen 1991] [Just 1991] [Kalavsky et al 2022] [Kandis and Witkowski 2019] [Karlgaard et al 2016] [Kaspar and Grim 2025] [Khudaiberdiev 1984] [King et al 2011] [Kingsley and Reuter 1981] [Klimas 1977] [Klimas 1979] [Knacke 1985] [Knapik 2019] [Knapik 2019] [Knapik et al 2011] [Konicke and Garrard 1982] [Koshzhanov et al 2025] [Koster et al 2011] [Kross and Webb 1979] [Kuprikov et al 2021] [Kutty et al 2015] [Křemenáková et al 2026] [Laswell 1973] [Lavier 1955] [Lavier 1955] [Lavrakas 1955] [Li 2015] [Li and Yu 2026] [Li et al 2023] [Liang and Mease 2019] [Liang et al 2024] [Lindstrand 2003] [Lingard et al 1989] [Lorenz et al 2021] [Lowry et al 2026] [Ludtke 1973] [Ludtke 1982] [Ludtke 1991] [Lundstrom et al 1968] [Lyalin and Morozov 2005] [MacCarthy 1954] [Makarov et al 2022] [Maru et al 2021] [Mastropietro et al 2015] [Andreini et al] [Matz and Braun 2015] [Mayhue and Bobbitt 1972] [McCarty 1978] [McMenomy 2022] [Meacham et al 2013] [Melzig and Schmidt 1965] [Menard 1967] [Meyer 1958] [Miller 1954] [Minimum Performance Standard for] [Minimum Performance Standard for] [Minimum Performance Standards for] [Minnesota Univ Minneapolis Inst Of Tech 1962] [Moog et al 1979] [Moore 2013] [Moore 2013] [Moorman and Yechout 2009] [Mori and Kuzuo 2018] [Mort and Weiberg 1967] [Mortaloni and Stewart 1999] [Mukhopadhyay et al 2016] [Muramoto and Garrard 1984] [Murman 2009] [Murray 1961] [Murro and Whitlock 1963] [Naidoo] [Neustadt et al 1967] [Nie et al 2023] [Niederer and Mihora 1972] [Niemi and Bevillard 2015] [Niemi et al 1995] [Niemi 1992] [O’Farrell et al 2022] [Parate 2020] [Payne and Band 1976] [Payne and Band 1977] [Pedersen 1961] [Pedersen 1964] [Pepermans et al 2023] [Pepper and Biesterveld 1973] [Pepper and Holt 1971] [Peterson and Johnson 1983] [Peterson et al 1986] [Petranelli et al 2015] [Placco 2023] [Placco 2023] [Placco 2024] [Placco et al 2025] [Placco et al 2026] [Preisser et al 1967] [Puthalapattu 2025] [Ray 2015] [Ray and Morris 2011] [Reinhard et al 1997] [Reza et al 2007] [Richardson et al 2005] [Robb et al 2026] [Rodier et al 1989] [Roelke et al 2025] [Deshmukh et al 2024] [Ross 1970] [Ruisseau et al 2000] [Runkle and Drobnik 1979] [Runkle and Woodis 1979] [Rychnovsky 1979] [Sahu et al 1995] [Sahu et al 1997] [Sahu et al 1997] [Sanganeria 2025] [Schaefer et al 2001] [Schairer et al 2018] [Sengupta et al 2008] [Sengupta et al 2009] [Shannon 2001] [Shepardson 1954] [Sherer 1945] [Shpund and Levin 1991] [Silbert et al 1989] [Sims 1963] [Singh et al 2017] [Skelton and Abbott 1974] [Smollett 1955] [Specification for mercerized cotton] [Stein 2005] [Steinberg et al 1973] [Stevens 1983] [Stewart 2013] [Structural and permeable contortion of parachute canopy fabrics under repetitive impact loading 2024] [Study on Critical Parachute 2024] [Sullivan 1944] [Sulsky et al 2010] [Sun et al 2025] [Sweeney 1955] [Tang et al 2015] [Tanner et al 2013] [Tanner et al 2015] [Teng and Li 2017] [Terahara et al 2025] [Thielman and Thielman 1997] [Tiaden and Yakimenko 2008] [Tiaden and Yakimenko 2009] [Tibbits and Ivanov 2015] [Tomcsak 1960] [Tsao 2015] [Tutt et al 2003] [Tutt et al 2022] [Unitary Plan Wind Tunnel 1962] [Usry 1969] [Utreja 1977] [Varela and Ray 2013] [Vergnolle and Astorg 1991] [Vishnyak 1995] [Wailes and Carlson 1986] [Walcott 1963] [Wang 2018] [Wang et al 2026] [Watkins 1995] [Waye 1991] [Wei 1991] [Whitting et al 2006] [Whitting et al 2007] [Witkowski 1999] [Witkowski et al 2022] [Witkowski et al 2026] [Worth 1965] [Wu et al 2021] [Yakimenko 2017] [Yakimenko et al 2008] [Yang 2021] [Yang et al 2015] [Yang et al 2018] [Yang et al 2020] [Ye et al 2026] [Yue et al 2016] [Yue et al 2016] [Zhang et al 2024] [Zhang et al 2025] [Zhang et al 2027] [Zhu et al 2023] [Zhu et al 2024] [Бабаков and Финченко 2022] [Вищекин et al 2023, Equipment for Parachute-free]

Autorotation, the vortex ring state and the emergency descent

The keystone. Autorotation of rotors, plates and wings, the vortex ring state, power-off and steep descent, the height-velocity diagram and the emergency landing after an engine failure. 74 of these titles name a vortex ring, 14 name the height-velocity diagram and 33 concern unmanned rotorcraft, multirotors or electric vertical takeoff aircraft, so the descent states the X-25 lived in are now studied as hazards to small machines as well as to helicopters.

248 records. [Chen and Chang 1973] [Huang et al 2020] [Mohamed and Abas 2007] [Abrego et al 2001] [Abrego et al 2002] [Ahlin and Brown 2009] [Alles et al 2010] [Ananthan et al 2002] [Anderlik 1926] [Andrien and Talaeizadeh 2022] [Aponso and Bachelder 2003] [Arms and Hama 1965] [Askue 2001] [Avanzini et al 2017] [Azuma and Obata 1968] [Bachelder et al 2011] [Baker 1974] [Barakos et al 2026] [Batther et al 2024] [Benson et al 1978] [Betzina 2001] [Binet et al 2025] [Blahitko et al 2021] [Bohr and Markvorsen 2016] [Brand et al 2007] [Bucherelli et al 2026] [Buss and Nicholson 1971] [Bustamante and Stone 1967] [Carlson et al 2006] [Castles and Gray 1951] [Celi and Ribera 2007] [Chae et al 2022] [Chen and Chang 1972] [Chen and Prasad 2005] [Chen et al 2004] [Chenglong et al 2015] [Chenglong et al 2018] [Chi et al 2019] [Cocco et al 2024] [Coombes et al 2017] [Cooper et al 1966] [Copeland 1994] [Coronado Domenge et al 2016] [Cuerva et al 2006] [Czyż et al 2021] [Czyż et al 2022] [Davis et al 1965] [Viets 1973] [Vore et al 1963] [Decker et al 1986] [Deel et al 1972] [Deng et al 2021] [Deresz 2013] [Dosaev 2018] [Durai raj 2024] [Dziubinski and Stalewski 2007] [Eberle and Rogers 2020] [Eberle and Rogers 2020] [Eberle and Rogers 2022] [Eberle et al 2022] [Efimov and Chernigin 2022] [Elkhaldi et al 2020] [Fallast and Messnarz 2017] [Fang et al 2023] [Farajijalal et al 2025] [Fattizzo et al 2026] [Feil and Hajek 2021] [Feng et al 2026] [Fetter 1974] [Fitch et al 1970] [Fradenburgh 1984] [Francesco Scaramuzzino et al 2024] [Fukatsu 1931] [Gallaway and Hankey 1985] [Gessow 1948] [Gessow 1950] [Gessow and Myers 1947] [Grande et al 2016] [Grzegorczyk 2013] [Hahn et al 2025] [Harris et al 2018] [He et al 2000] [Hepler and Free 1974] [Hickey 1956] [Hoinville and Renaud 2007] [Houston and Brown 2003] [Ignatkin et al 2018] [Ignatkin et al 2019] [Improved Blade Element Theory and Autorotating Rotor Aerodynamic Characteristics Analysis 2023] [Inoue et al 1997] [Irving 1930] [Isbono 2026] [Iseler et al 1995] [Ishiko and Uno 2018] [Iversen 1979] [Jeong et al 2024] [Jeong et al 2024] [Jeong et al 2026] [Jingze 2011] [Johnson 1977] [Johnson 2004] [Joseph et al 2026] [Junchao and Jianbo 2012] [Kesselring 1969] [Kim and Choi 2011] [Kirchner et al 2020] [Knight 1928] [Knight and Wenzinger 1930] [Korbukov and Menzulskiy 2024] [Kriebel 1973] [Langelaan and Grande 2013] [Lee 1985] [Lee 1990] [Lee et al 1988] [Lee et al 2008] [Lee et al 2023] [Li et al 2023] [Ligai et al 2017] [Logan et al 1982] [Lopez and Wells 2004] [Lopez et al 2023] [Lovell and Lipson 1949] [Lu et al 2023] [Makeev 2026] [Makeev et al 2021] [Makeev et al 2021] [Makeev et al 2022] [Makeev et al 2024] [Marr 1976] [Marshall et al 2024] [Matlala and Pedro 2016] [McCormack 1972] [McDonnell Aircraft Corp St Louis Mo 1950] [McIntyre 1970] [McQuaid 2021] [McQuaid et al 2020] [Meng and Chen 2012] [Modarres and Peters 2016] [Mori et al 2024] [Mori et al 2025] [Morozov et al 2020] [Murakami and Houston 2008] [Narkiewicz and Bibik 2008] [Nikolsky and Seckel 1949] [Nikolsky and Seckel 1949] [Noiri and Fujuwara 2016] [Norbury 1972] [Ober 1929] [Okuno et al 1991] [Oshima et al 1983] [Papers an Experimental 2004] [Patnala et al 2024] [Pegg 1968] [Peters and Chen 1982] [Peters and Wasileski 2000] [Pickles et al 2023] [Pickles et al 2023] [Pleasants and White 1983] [Prasad and Basset 2002] [Prasad and Chen 2006] [Pratumnopharat and Leung 2011] [Privalov and Samsonov 1994] [Pullin 1979] [Qi et al 2025] [Qian et al 2014] [Report No. 273, Wind 1928] [Rezaei et al 2024] [Rezgui and Lowenberg 2015] [Richter 2020] [Riegler et al 2025] [Rigsby and Prasad 2008] [Rodriguez Varona 2026] [Roger and Aubert 1999] [Rogers and Asbury 2000] [Rogers et al 2018] [Rostami and Fernandes 2015] [Rostami and Fernandes 2018] [Rovedatti et al 2024] [Saetti et al 2023] [Salcedo et al 2013] [Sallet 1975] [Scaramuzzino et al 2022] [Scarpari et al 2021] [Schmidt 1931] [Schrenk 1930] [Serafini et al 2024] [Setu and Abhishek 2019] [Shapley et al 1970] [Shen and Chopra 2004] [Sheridan et al 1982] [Shetty and Selig 2011] [Shi-cun 1990] [Simulation of Vortex Ring Formation and the Effect of Piston Velocity Program on the Formation Number 2015] [Sirohi et al 2004] [Slaymaker and Gray 1953] [Slaymaker et al 1952] [Smith 1971] [Stalewski and Surmacz 2019] [Stalewski and Surmacz 2020] [Stone 1967] [Suggs 1969] [Sullivan 1973] [Sunberg et al 2015] [Surmacz and Kowaleczko 2016] [Szabolcsi 2016] [Taamallah 2012] [Taamallah et al 2017] [taheri and delavet 2024] [Talaeizadeh et al 2020] [Talaeizadeh et al 2021] [Talbot and Schroers 1978] [Tokuoka and Miwa 2015] [Tokuoka and Miwa 2015] [Tomita and Fujiwara 2021] [Toropov and Stepanov 2016] [Tran and Lopez 2024] [Tran and Lopez 2025] [Tung and Ting 1967] [Uchiyam 2015] [Veismann et al 2023] [von Bahr 1950] [Váňa et al 2020] [Wang et al 2018] [Wang et al 2020] [Wang et al 2025] [Washizu et al 1965] [Watts et al 1971] [Wei 2021] [Wei et al 2000] [Wen et al 2022] [Wilson and Prazenica 2020] [Wood 2012] [Xin et al 2019] [Yan and Chen 2018] [Yan and Chen 2019] [Yan et al 2019] [Yan et al 2022] [Yan et al 2024] [Yano and Morrison 2024] [Yeates 1958] [Yomchinda et al 2011] [Zalewski 2016] [Zaroodny 1966] [Zeng et al 2024] [Zheng et al 2018] [Zhou et al 2026] [Zhou et al 2026] [Безденежных et al 2020]

Crew escape, ejection and the human in the escape

The human side of the escape. Ejection seats, escape capsules and systems, crashworthiness, injury and human tolerance, and aircrew life support. 92 of these titles name ejection, and 23 concern helicopters, rotors or rotary-wing aircraft.

194 records. [Anthony 2005] [Army Aviation Board Fort Rucker Al 1962] [Army Aviation School Fort Rucker Al 1956] [Aryaman et al 2023] [Astori et al 2020] [Aveline and Fowlie 1987] [Avirah et al 2024] [Bader and Coleman 1967] [Bao et al 2021] [Bement 1978] [Bement 1979] [Bement 2000] [Bierman 1959] [Blachowski and Ostrowski 1995] [Blachowski and Ostrowski 2000] [Blachowski and Ostrowski 2001] [Bonas and Reilly 1955] [Boyd 2019] [Bozack 1970] [Budd Co Fort Washington Pa Technical Center 1978] [Buller 2017] [Carr 1975] [Chen et al 2007] [Cheng and Pellettiere 2005] [Cheng et al 2003] [Chiang 1980] [Chicarello and Shpilberg 1976] [Cloutier 1967] [Computational aerodynamic analysis of 1994] [Connors and Sachs 1957] [Cota and Lee 1967] [Cummings 1982] [Danelson et al 2015] [Davidescu et al 2026] [DeCleene 1994] [Dehua et al 2006] [Dillon et al 1990] [Disselkoen et al 1989] [Do and deWeck 2011] [Dolinar 2000] [Douglas Aircraft Co Long Beach Ca 1983] [DOWNWARDS ESCAPE FROM JET AIRCRAFT 1956] [Du et al 2026] [Effectiveness of ejection seat 1950] [Ejection Seat Testing 2021] [Ekman and Debacker 2018] [Elson et al 1997] [Failure by Stress-Corrosion Cracking 2019] [Ford 1979] [Ford 1985] [Fortunato 2002] [Frazier and McElreath 1976] [Gilewicz 1978] [Glaister 1978] [Goodman and McMichael 2007] [Gragg et al 1982] [Griffin 1975] [Gross and Mawhinney 1970] [Guan et al 2013] [Habchi and Hufford 1995] [Habchi et al 1992] [Hawker and Payne 1979] [Hegenwald 1958] [Hellyer and Sherhod] [Hodell and Rosner 1957] [Hoffman 1996] [Hollrock and Barzda 1972] [Howland 1979] [Hrebien and Hendler 1983] [Huffman et al 1977] [Huffman et al 1977] [Hufford and Habchi 1994] [Itt Systems Rome Ny 1987] [Jia-Lin et al 2014] [Jines 1980] [Jines 1983] [Johnson 1986] [Johnson 1989] [Littell and Putnam] [Keller and Plaga 2008] [Kelly 1967] [Kennedy 1999] [Kenzakowski et al 1997] [Kiehl 1960] [Kowaleczko et al 2018] [Kowaleczko et al 2025] [Kwan et al 2003] [Laananen 1978] [Laananen 1980] [Labun 2014] [Lakshmi et al 2021] [Liedtke and Hyde 1967] [Liu et al 2022] [Liu et al 2022] [Liu et al 2025] [Lorch 1981] [Lowry et al 1994] [Maloney 2011] [Manzuk 1970] [Mao and Liang 2019] [Mao et al 2011] [Martin 1956] [Mather 1968] [McEntire and Shanahan 1997] [McKenney 1970] [McKinnon et al 2017] [Meadley et al 2023] [Melis et al 2020] [Ming et al 2016] [Mizrahi and Susak 1982] [Moore et al 2026] [Mou et al 2024] [Murtaza and Indluru 2026] [Murugan et al 2014] [Newby et al 2014] [Nixon et al 2020] [Oakes 1967] [Obergefell and Kaleps 1988] [Ojetola et al 2011] [Parate 2022] [Parate 2025] [Parisi 2004] [Payne 1969] [Perry 2007] [Perry et al 2019] [Piacentino et al 1978] [Pilot Escape Capsule 1961] [Pinnell and Kolega 1973] [Pint et al 2003] [Poteate and Solomonides 1964] [Project EGRESS Earthbound Guaranteed 1990] [Qiu et al 2012] [Rahman et al 2023] [Rahman et al 2024] [Rahman et al 2025] [Rahman et al 2026] [Ratts 1995] [Reason 1999] [Robbins 1978] [Robert 1988] [Rock and Habchi 1997] [Rock et al 1999] [Ryack et al 1984] [Sadler and McCauley 2003] [Saravanan et al 2021] [Sarkar et al 2018] [Savkovic 2008] [Scherberg and Ferguson 1952] [Schultz et al 2022] [Shane 1985] [Shane 1985] [Shender 2000] [Shi et al 2025] [Singh and Wereley 2015] [Smitha et al 2021] [Snyder 1959] [Somanath et al 2021] [Sommer et al 2023] [Sreenivasulu et al 2021] [Stapp and Taylor 1964] [Stech 1977] [Stein 1965] [Strietzel and Shefler 1963] [Swingle 1995] [Tauby 1979] [Teresa M Reiber et al] [Teresa Reiber et al] [The advanced stabilised ejection seat has three sub‐systems 1974] [The Biomechanics of Impact Injury: Biomechanical Response, Mechanisms of Injury, Human Tolerance and Simulation 2018] [Tian and Chen 2012] [Turnbow et al 1967] [Wang et al 2014] [Wang et al 2019] [Wang et al 2024] [Wang et al 2024] [Watts et al 1947] [Weinberg 1963] [Weinberg and Turnbow 1965] [West et al 1980] [Wolf 2019] [Wong and Mastroianni 1999] [Wright 2012] [Xiaodong et al 2010] [Xu et al 2011] [Yang et al 2011] [Yang et al 2025] [Ying 2019] [Yu et al 2017] [Zanoosi et al 2021] [Zenobi 1977] [Zenobi 1978] [Zhang and Yi 2011] [Zhao et al 2015] [Zhou et al 2023]

Autogiros and gyroplanes

The type the X-25 belonged to. Cierva’s autogiro, the NACA’s period rotor tests and the light and unmanned gyroplanes of the present. 53 of these records date from 1945 or earlier, when the autogiro was a serious aircraft, and 9 are chapters of one history of the type, among them Chapter 11 IGOR Bensen 2003 on Igor Bensen and the gyrocopter.

184 records. [Ahmad and Thomson 2020] [Ahn et al 2002] [Alfaro 1932] [Anh Vu et al 2011] [Bailey 1938] [Bailey 1940] [Bailey and Boothby 1940] [Bailey and Gustafon 1939] [Bailey and Gustafson 1940] [Bennett 1932] [Bennett 1933] [Bose 1983] [Breguet 1937] [Breguet 1937] [Cai et al 2018] [Chapter 10 the Autogiro 2003] [Chapter 11 IGOR Bensen 2003] [Chapter 13 Failure to 2003] [Chapter 15 the Emerging 2003] [Chapter 2 Cierva’S Autogiro 2003] [Chapter 4 Pitcairn and 2003] [Chapter 5 Development of 2003] [Chapter 6 American Advances 2003] [Chapter 9 the Autogiro 2003] [Charnov 2003] [Charnov 2004] [Charnov 2009] [Charnov 2011] [Charnov 2014] [Charnov 2018] [Chenglin et al 2022] [Colson 1934] [Commercial pilot Practical test 1996] [Coton et al 1998] [Crowe 1938] [Czarnigowski 2020] [Czyż et al 2015] [Czyż et al 2016] [Czyż et al 2017] [Czyż et al 2018] [Czyż et al 2021] [Czyż et al 2021] [Czyż et al 2023] [Czyż et al 2025] [de la Cierva 1926] [de la Cierva 1930] [Design of Composite Gyrocopter Main Rotor Blade Involving Rib and Spar Elements 2019] [Dewey 2021] [Doberentz et al 2020] [Dziubinski et al 2016] [Efficiency of the Autogyro 1929] [Ejaz et al 2017] [Fang et al 2019] [Feasibility Analysis of Gyroplane 2017] [Feng et al 2025] [Fiedoruk et al 2026] [Figat 2017] [Flight instructor Practical test 1996] [Flores et al 2019] [Galisteu et al 2015] [Gan et al 2026] [Gilruth 1937] [Glauert 1927] [Greenly 1962] [Gregory 1941] [Groen 2003] [Gustafson 1940] [Gustafson 1941] [Głowacki et al 2026] [Hafner 1938] [Hamilton 1938] [Harris and Dow 2015] [Hau et al 2021] [Heyson 1975] [Hohenemser 1938] [Hou et al 2017] [Hou et al 2021] [Houston 1998] [Houston 1998] [Houston 2000] [Houston 2011] [Houston and Thomson 2017] [Houston et al 1998] [Hunt 1967] [Martín-Barbadillo 1936] [Jakliński and Ścisłowski 2025] [Jensen 2001] [Wheatley 1934] [Jones and Niewoehner 2023] [Kalmykov and Barsuk 2011] [Kay 331 gyroplane British 1935] [Klemin 1929] [Klemin and Ruffner 1931] [Larsen 1956] [Lee et al 2018] [Li et al 2018] [Lin et al 2014] [Lin et al 2016] [Liu et al 2017] [Liu et al 2023] [Lowenberg et al 2009] [Lu and Chen 2012] [Ma et al 2015] [Magryta et al 2017] [McConnell and Das 2022] [McCormick 2002] [McDougall 1965] [Moore-Brabazon 1934] [Niemi and Gowda 2011] [Noboni and Das 2025] [Noboni et al 2023] [Noboni et al 2025] [Optimization of geometrical characteristics 2005] [Osaji and Farrokhfal 2009] [Osborn 1929] [Peck 1934] [Petritoli and Leccese 2020] [Petritoli and Leccese 2021] [Pitcairn 1930] [Practice for Continued Airworthiness] [Prewitt 1938] [Private pilot practical test 1996] [Qing 2017] [Report no. 515, full-scale 1935, Report no. 515, full-scale] [Report no. 536, wind tunnel tests of a 10-foot-diameter gyroplane rotor 1936] [Rezgui and Lowenberg 2020] [Rezgui et al 2008] [Robinson and Jump 2011] [Santiago et al 2020] [Schad 1965] [Schrenk 1938] [Seiferth 1927] [Shaanan 2025] [Shin and Kim 2015] [Shippen 1996] [Shippen 1998] [Sissingh 1939] [Sobieszek 2016] [Somov and Polyntsev] [Somov and Polyntsev 2004] [Somov and Polyntsev 2005] [Song et al 2017] [Spathopoulos 2004] [Specification for Design and] [Stalewski 2017] [Stalewski 2018] [Standard Specification for Integration] [Standard Specification for Light] [T.K.C. 1932] [The Autogiro 1925] [The Cierva “Autogiro” Mark 1930] [The Essential Autogiro/Autogyro/Gyrocopter/Gyroplane History 2003] [The Hafner A.R. III 1937] [Thomson and Houston 2005] [Thomson and Houston 2008] [Traum 2011] [Vainilavičius et al 2014] [Wallis 1963] [Wallis 1963] [Wang and Tang 2016] [Wang et al 2016] [Wang et al 2019] [Wheatley 1933] [Wheatley 1934] [Wheatley 1934] [Wheatley 1935] [Wheatley 1935] [Wheatley 1936] [Wheatley 1937] [Wheatley 1937] [Wheatley 1945] [Wheatley and Bioletti 1936] [Wheatley and Bioletti 1937] [Wojtas and Sobieszek 2019] [Wu 2015] [Wu et al 2014] [Xiang et al 2014] [Xiang et al 2015] [Xiaoqian Cheng et al 2016] [Yan Ma et al 2016] [Zhang and Xu 2011] [Zhang et al 2014] [Łusiak 2022] [Лучко and Зінченко 2018]

Ram-air canopies, parafoils and guided airdrop

The answer to the programme’s question. Ram-air canopies, parafoils, parawings and guided airdrop. 24 of these titles name the glide or a gliding parachute, and 80 name guidance, homing, a trajectory or its control, so a large part of this literature treats the canopy as a vehicle to be steered rather than as a brake, which is the capability the Discretionary Descent Vehicle wanted.

189 records. [Autonomously guided Ram-air parachute 2009] [Balraj Gupta et al 2023] [Barrows 2009] [Behr et al 1991] [Berexa et al 2019] [Bergeron et al 2015] [Berland et al 2005] [Bozack 1984] [Brown 1989] [Brown 1993] [Brown et al 1991] [Cacan and Costello 2018] [Cacan et al 2015] [Cacan et al 2019] [Cao et al 2026] [Chakoian 1969] [Chambe et al 2024] [Chang et al 2018] [Charles 2017] [Chatzikonstantinou 1999] [Chen et al 2019] [Chen et al 2021] [Chiel and Dever 2015] [Civelek and Kıvrak 2024] [Cui et al 2025] [Cui et al 2025] [Desabrais et al 2015] [Devalla et al 2016] [Ding et al 2023] [Doran et al 2025] [Dowling and Costello 2018] [Dunker and Berland 2007] [Eslambolchi and Johari 2013] [Fan et al 2026] [Feng et al 2015] [Fields et al 2012] [Fields et al 2013] [Fogell et al 2017] [Forehand and Bair 1969] [Leonard et al 2019] [Leonard et al 2020] [Olenberger 1978] [Rogallo 1962] [Gang 2015] [Gao and Tao 2022] [Gao et al 2016] [Gavrilovski et al 2011] [Geiselman et al 2015] [Ghoreyshi et al 2014] [Ghoreyshi et al 2015] [Goodrick 1969] [Goodrick 1981] [Gunasinghe et al 2017] [Guo et al 2026] [Güven and Şamiloğlu 2022] [Güven and Şamiloğlu 2022] [Han 2018] [He et al 2022] [Hiraki et al 2007] [Hirst and Jorgensen 1995] [Horton 1962] [Hua et al 2013] [Ibos et al 1999] [Irvine 1968] [Ivanovic and Orsag 2022] [Jann 2003] [Jann and Ward 2019] [Jin et al 2026] [Johnson and Sleeman 1963] [Jóźwiak and Kurzawiński 2019] [Kaminer et al 2005] [Kenner 1972] [Kim and Song 2015] [Kim et al 2016] [Knapp and Barton 1968] [Kraft et al 2017] [Leeman et al 2022] [Lewis 1964] [Li and Nan 2023] [Li and Yan 2021] [Li et al 2024] [Libbey 1964] [Libbey 1965] [Lingard 2015] [Lissaman and Brown 1993] [Liu et al 2016] [Long et al 2021] [Luan et al 2026] [Lucrezia 2023] [Lv et al 2019] [Lv et al 2020] [Madsen and Cerimele 2000] [Madsen et al 2003] [Matsuo and Butler 1983] [Michal et al 1970] [Mkrtchyan and Johari 2011] [Modular heavy payload autonomously 2009] [Mohammadi and Johari 2009] [Mohammadi and Johari 2010] [Montalvo and Costello 2016] [Montanez et al 1997] [Mort 1967] [Newberg et al 2026] [Nicolaides 1971] [Nicolaides 1976] [Nicolaides and Tragarz 1971] [Noetscher and Barber 2005] [Ortega et al 2017] [Pearson 1972] [Pearson et al 1977] [Peralta and Johari 2015] [Perin and Patoux 2022] [Perschbacher and Potvin 1999] [Potvin 1997] [Potvin 2001] [Potvin 2001] [Potvin and Peek 1998] [Potvin and Potvin 1997] [Potvin et al 1997] [Prateek Sazawal and Ghosh 2023] [Puskas 1984] [Puskas 1989] [Qi et al 2025] [Rogallo 1963] [Rogallo et al 1960] [Rogallo et al 1972] [Ryan Aeronautical Co San Diego Ca 1962] [Seidel and Bergeron 2017] [Seidel et al 2016] [Shaw 2016] [Sleeman 1971] [Slegers 2017] [Slegers et al 2015] [Smith et al 2001] [Sun et al 2020] [Sun et al 2020] [Surendra et al 2015] [Szafran and Kramarski 2019] [Takizawa et al 2016] [Tan et al 2019] [Tan et al 2023] [Tao et al 2016] [Tao et al 2017] [Tao et al 2019] [Tardiff et al 2015] [Taylor and McNulty 1962] [Trovarelli and Mooij 2019] [Vambol and Kaluzhynov 2023] [Velasquez and Benedict 2015] [Wachlin et al 2019] [Wang and Yang 2024] [Wang et al 2022] [Ward and Costello 2013] [Ward et al 2011] [Ward et al 2013] [Ward et al 2024] [Wegereef et al 2009] [Wegereef et al 2011] [Wei and Pearson 1974] [Wei et al 2024] [Wei et al 2026] [Wenqian et al 2016] [Wu et al 2017] [Wu et al 2018] [Wyllie et al 1997] [Xing et al 2023] [Xing et al 2024] [Xu et al 2025] [Yan et al 2023] [Yan et al 2025] [Yan et al 2026] [Yang and Jeon 2019] [Yang et al 2025] [Yang et al 2026] [Yi et al 2025] [Yu et al 2022] [Yu et al 2025] [Yu et al 2025] [Yu et al 2026] [Zhang et al 2021] [Zhang et al 2021] [Zhang et al 2022] [Zhang et al 2023] [Zhao et al 2020] [Zhao et al 2021] [Zheng et al 2023] [Zhu et al 2022] [Zhu et al 2023] [Zou et al 2025]

Electric vertical takeoff, drones and distributed lift

The machine that cannot do it. Electric vertical takeoff aircraft, multirotors and other unmanned rotorcraft, their sizing, aerodynamics, failure and safety, and distributed propulsion. None of these 146 records is older than 2004, and 31 titles name fault tolerance, redundancy or the failure of a rotor, motor or actuator, which is the redundancy argument of this article posed as a design problem. Across the whole survey 11 titles join autorotation to a multirotor, a drone or an unmanned aircraft.

146 records. [Abdul Awal 2017] [Abergo et al 2026] [Abhiram et al 2018] [Afari et al 2023] [Ahmadi Dastgerdi and Nabavi-Chashmi 2026] [Akash et al 2021] [Alulema et al 2026] [Ambroziak et al 2023] [Arace et al 2026] [Atte and Rauleder 2026] [Awad and Stumpf 2022] [Baldini et al 2020] [Bauer et al 2021] [Beak and Kim 2026] [Bhandari et al 2024] [Bhatnagar and Dr 2016] [Boucher et al 2023] [Brown and Harris 2020] [Cai 2015] [Chakraborty and Mishra 2023] [Chakraborty and Mishra 2023] [Chakraborty et al 2023] [Charnsethikul et al 2025] [Chen et al 2025] [Choi et al 2026] [Chung and Son 2020] [Cokorilo 2020] [Critchfield and Ning 2026] [Dalwadi et al 2022] [Dalwadi et al 2022] [Danis et al 2018] [Darvishpoor et al 2025] [Evdokimenkov et al 2024] [Evdokimenkov et al 2024] [Falconì et al 2018] [Gokbel and Ersoy 2021] [Goldschmidt et al 2024] [Gu and Tian 2023] [Gu et al 2025] [Haefner et al 2026] [Healy et al 2022] [Hou et al 2020] [Huang et al 2026] [Hwang et al 2015] [Immersion and Invariance Adaptive Fault-Tolerant Attitude Control for a Coaxial Tilt-Rotor eVTOL Aircraft 2026] [Ishaque et al 2025] [Jayakumar et al 2026] [Jebelli et al 2024] [Jia and Li 2025] [Jiang and Zhang 2016] [Jiang and Zhang 2018] [Jiang et al 2015] [Johnsen et al 2022] [Jun et al 2026] [Kaneko and Martins 2024] [Kevin Li and Lee 2021] [Khoshnood 2025] [Kikumoto et al 2026] [Kim et al 2025] [Koo 2020] [Kotwicz Herniczek et al 2019] [Kusumoaji et al 2024] [Lee and Yee 2024] [Lee et al 2020] [Lee et al 2022] [Lei and Cheng 2020] [Lei and Wang 2020] [Lei and Ye 2020] [Lei et al 2020] [Lei et al 2021] [Lei et al 2023] [Lei et al 2023] [Li 2025] [Li and Lee 2024] [Li et al 2022] [Li et al 2023] [Liang et al 2026] [Liao et al 2025] [Lien et al 2020] [Logan et al 2017] [Haworth et al] [Lovas 2026] [Lu et al 2016] [Lu et al 2026] [Lympany and Page 2024] [Ma et al 2022] [Ma et al 2025] [Mao et al 2022] [Mathur and Atkins 2023] [McKay et al 2019] [Merheb et al 2017] [Misiorowski et al 2020] [Moschetta 2010] [Mpanza and Pedro 2025] [Mrusek 2021] [Multi-Rotors A Revolution In 2015] [Nam et al 2024] [Nasr et al 2026] [Oyama et al 2024] [Page and Bailey 2025] [Papadopoulos et al 2024] [Perfect et al 2017] [Piccinini et al 2020] [Poggi et al 2024] [Qiao and Zhou 2026] [Rostami et al 2023] [Sadasivan 2019] [Saetti et al 2022] [Saias et al 2021] [Saias et al 2023] [Saied et al 2019] [Saied et al 2019] [Saied et al 2024] [Sajid et al 2026] [Sajjadi et al 2025] [Shi 2024] [Whiteside and Pollard] [Simha and Ambroziak 2025] [Simon 2024] [Smith et al 2023] [Wright] [Strampe and Klingauf 2026] [Takita and Kashitani 2019] [Turhan et al 2025] [Ugwueze et al 2023] [Vempati et al 2024] [Venkataraman et al 2017] [Verbeke and Schutter 2017] [Villeneuve et al 2022] [Wang and Zhang 2018] [Wang et al 2017] [Wang et al 2019] [Wang et al 2023] [Wang et al 2024] [Wang et al 2026] [Wei et al 2024] [Westcott et al 2023] [Wilbur and Wilkie 2004] [Xie et al 2023] [Yan et al 2024] [Yanev and Staack 2026] [Yu et al 2024] [Yu et al 2024] [Zhao et al 2024] [Zhyriakov et al 2025] [孙 2025]

Flight test, handling qualities, training and piloting technique

How the aircraft were flown and how pilots learn to fly them. Flight test method, handling and flying qualities, simulation and pilot training. 26 of these titles name training, a trainee, instruction or a simulator, which is the subject the record says the X-25 testing addressed.

141 records. [Abbott et al 1982] [Advisory Group for Aerospace Research and Development 1999] [Aiken 1986] [Air Force Flight Test Center Edwards Afb Ca 1974] [Air Force Flight Test Center Edwards Afb Ca 1974] [Air Force Test Pilot School Edwards Afb Ca 1992] [Aircraft and Rotorcraft System Identification: Engineering Methods with Flight Test Examples, Second Edition (Bookshelf) 2016] [Alford and Knarr 1999] [Anderson and Toivanen 1970] [Army Aviation Board Fort Rucker Al 1964] [Army Materiel Command Alexandria Va 1974] [Barbini et al 1970] [Barnes and Statham 1970] [Baron 1981] [Beeler et al 2003] [Bender et al 1984] [Beppu et al 1966] [Blackburn and Whitfield 1965] [Boirun 1979] [Borobia et al 2018] [Branson 1994] [Brauser 1982] [Brown et al 1975] [Buckanin and Guin 1977] [Buckland 1980] [Carico 2000] [Caro and Isley 1966] [Carter and Stultz 1958] [Chen and Talbot 1978] [Cleek et al 1978] [Cochran 2015] [Cook and Hickey 1975] [Crawford and Hodgson 1961] [Daughaday and Duwaldt 1955] [Daş and Kumpas 2019] [Director Army Safety Office Washington Dc 1993] [Dominick and Watts 1967] [Dooley 1977] [Dooley and Yeary 1979] [Drinkwater and Feistel 1965] [Ferrell et al 1974] [Finnestead and Connor 1969] [Finnestead et al 1970] [Fosdick 1970] [Gainer and Sullivan 1976] [Garrard and Liebst 1990] [Garrard and Low 1990] [Gerlach 1979] [Germanetti 2024] [Godwin et al 1964] [Grant 1961] [Ham et al 1995] [Hammon and Horowitz 1990] [Harper and Sardanowsky 1969] [Head 1959] [Hepler et al 1973] [Herbst et al 2015] [Hess 1995] [Hodgkinson 2017] [Hoh and Ashkenas 1981] [Hughes and Wernicke 1974] [Hutto 1976] [Intano and Howse 1991] [Hassell and Kirby 1966] [Jensen et al 1991] [Johnson and Anderson 1966] [Johnson and Boling 2015] [Johnson et al 1972] [Johnson et al 1973] [Jones 2017] [Kalra et al 2024] [Kidd and Bull 1963] [Kidwell and Foster 1966] [Kim and Kim 2020] [Knapp 1971] [Lawrence 2014] [Lintern 1979] [Low and Garrard 1993] [Masarati et al 2015] [Mehling et al 2022] [Melton and Hall 1968] [Minotra and Feigh 2020] [Muscarello et al 2017] [Müllhäuser and Lusardi 2022] [Na and Cho 2022] [Nagata et al 1968] [Nagata et al 1973] [Nagata et al 1981] [New light on pilot training 1973] [Paul et al 2025] [Perez et al 2026] [Picasso et al 1982] [Psychological research on pilot training in the AAF 1946] [Putman 1961] [Rash et al 2001] [Ritter et al 1971] [Rostad et al 2003] [Rostad et al 2003] [Saetti et al 2019] [Salord Losantos and Sánchez-Arriaga 2015] [Sanders 1972] [Schmeisser et al 2008] [Schroeder et al 2001] [Schroers and Antoniou 1966] [Schumacher 1952] [Schwartz and Wray 1969] [Serafini et al 2015] [Seville Research Corp Pensacola Fl 1979] [Seville Research Corp Pensacola Fl 1979] [Sharkey et al 2005] [Simon and Savage 1975] [Simpson 1979] [Sinacori 1986] [Singh 2019] [Smode 1979] [Somerville et al 2025] [Somerville et al 2026] [Somsel 1970] [Spring et al 1979] [Staley 1976] [Stange 1959] [Stelle et al 2003] [Stendahl 1985] [Stone et al 1946] [Stringer et al 1972] [Tangler 1979] [Thomas et al 2023] [Turkoglu 2018] [Vague and Seibel 1959] [Van Gaasbeek 1980] [Van Wyckhouse 1966] [Watson et al 2025] [Wesolek 2007] [White et al 2021] [Wolkovitch and Hoffman 1971] [Woomer and Carico 1977] [Yamakawa et al 1972] [Yamakawa et al 1972] [Yamakawa et al 1977] [Yamakawa et al 1978] [Zhang et al 2025]

Wind turbines, energy harvesting and the windmill state

The same physics in another reference frame. Wind turbine rotor aerodynamics, wakes and inflow, blade element momentum theory and the use of autorotation to harvest energy. 42 of these titles name blade element or momentum theory, against 31 in the cluster on rotors and rotorcraft, so on this measure the largest living body of work on the keystone’s physics is in wind energy.

109 records. [Abbas et al 2024] [Abedi and Eskilsson 2025] [Abutunis et al 2019] [Adema et al 2026] [Andersen et al 2024] [Apsley and Stansby 2020] [Araneo et al 2019] [Ariyoshi et al 2017] [Augusto et al 2019] [Bai and Shiah 2016] [Baisthakur and Fitzgerald 2024] [Bakhshandeh Rostami and Fernandes 2015] [Bangga et al 2020] [Berger and Kühn 2018] [Bhargava and Samala 2019] [Boatto et al 2023] [Bontempo and Manna 2017] [Bromm et al 2016] [Chabaud 2021] [Cheung et al 2024] [Cole et al 2020] [Corniglion et al 2022] [Dehouck et al 2017] [Ehrich et al 2018] [Eliassen et al 2015] [Erhard and Strauch 2015] [Fang et al 2021] [Ferreira et al 2022] [Fu 2026] [Greco and Testa 2021] [Guo and Ganapathysubramanian 2017] [He and Xia 2022] [Herráez et al 2017] [Hosseini et al 2022] [Ichter et al 2015] [Iliev 2025] [Janetzke and Kaza 1981] [Janetzke and Kaza 1983] [Jun and Wenlong 2016] [Kao et al 2020] [Khanjari et al 2016] [Khanjari et al 2018] [Kosasih and Saleh Hudin 2016] [Kulak et al 2024] [Le et al 2025] [Lee et al 2016] [Li and Yang 2024] [Li et al 2018] [Lipian et al 2019] [Liu et al 2021] [Luhur et al 2015] [Manga et al 2023] [Maydew and Klimas 1981] [Moghadassian et al 2016] [Moshfeghi and Hur 2020] [Mujahid et al 2021] [Nishi et al 2023] [Nukala and Maddula 2020] [Pamuji and Bramantya 2018] [Pearl] [Perez-Becker et al 2020] [Pirrung 2020] [Pirrung et al 2016] [Ponta et al 2016] [Potentier et al 2021] [Prajapat et al 2021] [Pratama Ricky Novianto et al 2023] [Radi and Djebli 2022] [Raibaudo et al 2023] [Randriambololona et al 2025] [Richmond-Navarro et al 2017] [Santo et al 2019] [Savino et al 1985] [Savino et al 2024] [Selvatici and Stevens 2025] [Shen et al 2024] [Shen et al 2026] [Sineglazov 2015] [Sineglazov et al 2016] [Siram et al 2021] [Soares et al 2023] [Soares et al 2024] [Soares et al 2024] [Störtenbecker and Dalhoff 2026] [Sun et al 2016] [Syed Ahmed Kabir et al 2022] [Tabatabaei et al 2019] [Tahir et al 2018] [Uchida et al 2023] [van der Laan et al 2024] [van der Wall 2019] [Wan Muhammad and Mohd 2023] [Wang et al 2015] [Wang et al 2018] [Wester et al 2021] [Winning et al 2026] [Wood and Golmirzaee 2023] [Wulandana et al 2021] [Xie et al 2017] [Yang et al 2016] [Yang et al 2019] [Yilmaz 2023] [Yin et al 2022] [Yin et al 2025] [Zhang et al 2019] [Zhang et al 2019] [Zhao et al 2022] [Zhao et al 2023] [Özkan and Genç 2023]

Samaras, monocopters and animal flight

Autorotation is older than aircraft. Samaras and other autorotating seeds, the monocopters modelled on them, and the flight of birds, bats and insects. 29 of these titles name a samara, a seed or a maple.

96 records. [Addo-Akoto et al 2021] [Baker and Cooter 1979] [Bhardwaj et al 2022] [Braunshtein et al 2026] [Cai et al 2025] [Chakraborty and Roy 2022] [Chen et al 2022] [Cheng et al 2020] [Crandell and Tobalske 2015] [Das 2009] [Dececchi et al 2020] [Dececchi et al 2020] [Deng 2015] [Dormiyani et al 2015] [Dvořák 2016] [El Makdah et al 2022] [Fang et al 2017] [Farvardin Ahranjani and Banazadeh 2021] [Fauli et al 2019] [Flight Speed of Some Birds 1933] [Yogeshwaran et al 2023] [Goldspink et al 1978] [Goldsworthy et al 1979] [Gong and Fang 2021] [Gonzalo et al 2025] [Hiremath et al 2024] [Huang et al 2026] [Ishiguro et al 2023] [Jammeh et al 2025] [Jo et al 2025] [Jung and Rezgui 2023] [Jung and Rezgui 2023] [Kandyba 2021] [Lamprey 1953] [Lee and Choi 2017] [Lee and Lee 2016] [Lee et al 2015] [Lentink et al 2009] [Li and Guo 2018] [Li et al 2024] [Li et al 2026] [Liu 2016] [Liu et al 2015] [Liu et al 2024] [Liu et al 2025] [Lolli et al 2026] [McConnell and Das 2022] [McConnell and Das 2023] [McGAHAN 1973] [Meng et al 2023] [Naka and Hashimoto 2015] [Nero 1970] [Niu et al 2022] [Niu et al 2022] [Noda et al 2018] [Norberg 1973] [Pennycuick 1960] [Pennycuick 1968] [Pennycuick 1971] [Porritiello et al 2022] [Pounds and Singh 2015] [Rayner 1979] [Saetti and Rogers 2022] [Schiotz and Volsoe 1959] [Shah et al 2024] [Shahid et al 2019] [Shi et al 2016] [Sneyd et al 1982] [Sohn and Im 2022] [Song et al 2023] [Soni and Tiwari 2019] [Stein 1976] [Suhadi et al 2022] [Sunada et al 2015] [Taha et al 2016] [Tahmasian 2023] [Takizawa et al 2015] [Tobalske 2022] [Tong et al 2022] [Vaughan 1966] [Wang et al 2022] [Win et al 2019] [Win et al 2019] [Win et al 2021] [Win et al 2022] [Win et al 2026] [Wu et al 2017] [Wu et al 2021] [Wu et al 2021] [Wu et al 2022] [Xue et al 2016] [Xue et al 2019] [Yang 2013] [Yoshida and Fukui 2024] [Zhang and Wang 2022] [Zhou and Wu 2021]

Rotor and propeller noise, and the tip speed

The bound on tip speed. Rotor and propeller noise prediction and measurement, blade-vortex interaction, the noise of small multirotors and electric vertical takeoff aircraft, and community response to rotorcraft.

85 records. [Arystanbekov et al 2025] [Barry and Magliozzi 1971] [Biggers et al 2015] [Boucher 2025] [Boyd 2009] [Brenda S Henderson et al] [Brooks et al 1987] [Brown and Farassat 1976] [Buǧday and Saetti 2024] [Christopher S Thurman and Nikolas S Zawodny] [Christopher S Thurman et al] [Christopher S. Thurman and James D. Baeder 2023] [Cox 1973] [Crosse et al 1960] [Eric Greenwood et al 2023] [Fu et al 2023] [Gandhi 1995] [Gasaway 1969] [George 1978] [George 1996] [Goudswaard et al 2024] [Greenwood and Schmitz 2018] [Greenwood et al 2015] [Healy 1969] [Homans et al 1978] [Schaub 1974] [Stephenson et al] [Jia and Lee 2020] [Johnson 1974] [Johnson-Groh 2024] [Jung et al 2025] [Kallergis 1987] [Kingan et al 2023] [Kitaplioglu 1986] [Kitaplioglu and Shinoda 1985] [Ko et al 2020] [Leggat and Siddon 1978] [Leggatt and Siddon 1976] [Leighton 1982] [Li and Lee 2020] [Loewy and Sutton 1966] [Lukianov 2022] [Mantay et al 1977] [Melo et al 2026] [Menger et al 1983] [Morris et al 1979] [Mueller et al 1987] [Mueller et al 1990] [Mueller et al 1992] [Nowoświat et al 2022] [Patterson et al 1977] [Pereira et al 2025] [Philipps-Bertin et al 2026] [Polyzos et al 2020] [Prieur 1992] [Prieur et al 1989] [Rinehart 1971] [Saetti et al 2024] [Schlegel and Bausch 1969] [Schmitz and Boxwell 1976] [Shao et al 2024] [Shenoy 1984] [Sou 2019] [Spencer et al 1969] [Stefan J Letica and Stephen A Rizzi 2026] [Sternfeld and Wiedersum 1979] [Stockman et al 1973] [Stockman et al 1974] [Strawn and Biswas 1995] [Stroub 1978] [Su Jung et al 2023] [Szulc and Doerffer 2018] [van der Wall et al 2017] [Vouros et al 2021] [Wang 1995] [Wang et al 2025] [Williams 2002] [Williams and Harris 1984] [Wright 1969] [Yang et al 2018] [Yang et al 2019] [Yuk et al 2024] [Zhang et al 2026] [Zhao et al 2017] [Zhu and Zhao 2016]

Rotors at low density, on Mars and at low Reynolds number

The density relation at its extreme. Rotor and airfoil aerodynamics at low Reynolds number and low density, and the Mars rotorcraft. 25 of these titles name Mars or a planetary rotorcraft.

58 records. [Alguacil et al 2020] [Ament et al 2019] [Perez et al] [Bagade et al 2015] [Chen and Hubner 2021] [Chen et al 2018] [Chitsaz et al 2021] [Choi et al 2019] [Dull et al] [Datta et al 2026] [De Almeida and Dias 2024] [Désert et al 2019] [Fox 2021] [Harris 2020] [Genç et al 2022] [Georgiev 2025] [Giacomini and Westerberg 2025] [Sahragard-Monfared et al] [Gnatowska and Gajewska 2024] [Gojon et al 2021] [Güzelbey et al 2018] [Huang et al 2023] [Ikeda et al 2015] [Bowman et al 2024] [Kamruzzaman et al 2021] [Karpiński 2026] [Koch 1998] [Koning 2019] [Koning et al 2019] [Koning et al 2019] [Koning et al 2020] [Kruesi 2021] [Kumar Gupta et al 2018] [Kurtulus 2019] [Leslie 2021] [Marchman and Abtahi 1985] [McElreath et al 2020] [Nepal et al 2023] [Nik Mohd 2017] [Olmstead and Hector 1968] [Petrović et al 2016] [Rahal and Dutta 2024] [Lumba et al] [San Martin et al 2017] [Serré et al 2019] [Shrestha et al 2016] [Shukla and Komerath 2019] [Stephenson and Whitfield 1970] [Schmitz et al] [tang et al 2026] [Tran 2025] [Veismann et al 2021] [Wang et al 2024] [Koning et al] [Yamaguchi and Anyoji 2023] [Zhang and Yang 2021] [Zhao et al 2019] [Zhao et al 2025]

Rotary decelerators and rotor recovery systems

The X-25’s own concept. A rotor carried to slow the descent of a person, a payload, a capsule or a probe. 9 of these records date from 1956 to 1970, 21 from 2003 or later and 2 from the three decades between, so the concept lapsed after the period the X-25 belongs to and returned for small probes, sensors and payloads.

32 records. [Aksaray and Mavris 2013] [Bartz and Miklosovic 2003] [Barzda 1970] [Barzda and Schultz 1963] [Beyer and Costello 2008] [Beyer and Costello 2009] [Brindejonc et al 2005] [Brindejonc et al 2007] [Costello et al 2010] [Datta and Sirohi 2009] [Erb 1966] [High Drag Rotating Parachute Developed and Tested 1985] [Hla Win et al 2019] [Howard and Elger 2009] [Iversen 1968] [Jain et al 2022] [Kim et al 2022] [Kroo 1995] [Levin 1968] [Levin 1968] [Liu et al 2025] [Liu et al 2026] [Macneal and Schuerch 1963] [Martiarena et al 2015] [Nadal Mora et al 2005] [Nadal Mora et al 2007] [Patnaik and Kanishk 2021] [Rivadeneira et al 2023] [Robinson 1956] [Schuerch 1965] [Ventura et al 2022] [Young et al 2005]

Bluff-body drag and the drag coefficient

What the canopy comparison rests on. The drag of bluff bodies, spheres, plates, cylinders and bodies of revolution, and of falling and tumbling bodies. 8 of these 28 records date from 1968 or earlier.

28 records. [Abraham 1970] [Air Drag on a Falling Plastic Ball 1966] [Brogna and Hawks 1978] [Cantwell 1993] [Dyke 1971] [Fackrell 1975] [Falkner 1943] [Gorlin and Khudyakov 1969] [Griffin 1975] [Griffin and Ramberg 1975] [Hopke and Slattery 1970] [Ko and Graf 1972] [Maull and Bearman 1964] [McLaren et al 1969] [Menduni 1988] [Norling 1961] [Richardson 1986] [Roshko 1955] [Sakamoto et al 1975] [Smith 1979] [Squire 1968] [Stengel 1968] [Stilley 1967] [Szewczyk 1992] [Walsh 1977] [Williamson 1995] [Wortman 1969] [Zarin 1970]

The final descent and the touchdown

The end of every descent. The control of the glide path and the descent rate before touchdown, vertical landing, and the landing of unpowered orbiters, capsules and reentry vehicles.

20 records. [Akturk and Camci 2022] [Broglio 1961] [Campbell 1960] [Compilation of Papers Presented 1962] [Cui et al 2019] [Durand and Wasicko 1967] [Fisher 1960] [Franklin and Innis 1977] [Glideslope descent-rate cuing to 1986] [Greene 1977] [Greene 2020] [Guy 1976] [Huang 2022] [Newby et al] [P Greenhalgh et al] [Park 2020] [Qin 2023] [Romere et al 1979] [Su and Wang 2015] [Vorobуev et al 2020]

Wings, airfoils and the lift-to-drag ratio

What makes the rotor a wing. Low-speed wing and airfoil aerodynamics, induced drag and the lift-to-drag polar, on which this article’s glide and range figures depend.

19 records. [Aerodynamic Design Wing 1969] [Brenckmann 1958] [Chi 1985] [Friedman et al 2016] [Gilman and Burdges 1967] [Hariyadi 2017] [Jarvinen 1973] [Maksymiuk and Watson Viken 1986] [McInville and Moore 1994] [Olson and Selberg 1976] [Reissner 1951] [Rosti et al 2017] [Son et al 2023] [Tetervin 1943] [Wentz and Nagati 1975] [Wu 1986] [Wu 1989] [Yegorov 1945] [Zhao et al 2018]

Gliding flight and the sink rate

The range argument. Gliding and soaring flight, glide performance, the sinking speed of aircraft and endurance trajectories.

16 records. [Amalia et al 2019] [Baudinette and Schmidt-Nielsen 1974] [Bowers et al 2007] [Bowers et al 2011] [Collins 1934] [Entwistle 1931] [Gliding Flight in Germany 1935] [Grossman et al 1988] [Jin 2024] [Lundberg 1937] [Metzger and Hedrick 1975] [Mueller and Heuermann 1985] [Powers et al 2020] [Reeder 1959] [Welch 1961] [Williamson 1979]

The standard atmosphere and the flight condition

The medium, named rather than assumed. The United States and reference standard atmospheres, from which the density ratios in this article’s altitude section are taken.

17 records. [B-34. U. S. Standard Atmosphere, 1962 1963] [Dubin et al 1962] [Dubin et al 1966] [Johnson et al 1991] [Johnson et al 1993] [Johnson et al 2002] [Krueger and Minzner 1976] [Minzner 1967] [Minzner 1977] [Minzner et al 1973] [Minzner et al 1976] [Peters 1977] [Snell and Heiser 1968] [U. S. Standard Atmosphere 1963] [U.S. Standard Atmosphere Supplements 1966] [U.S. Standard Atmosphere, 1962] [U.S. Standard Atmosphere, 1976]

V/STOL and other vertical-lift aircraft

The other ways to rise and descend vertically. Lift fans, ducted fans, deflected slipstream, ground-effect machines and short and vertical takeoff aircraft that are not rotorcraft.

10 records. [Blick and Homer 1971] [Brogan et al 1961] [Butsko et al 1963] [Deckert and Hodgson 1962] [Goldberger 1966] [Hirsh 1965] [Irvin and Swan 1956] [Moser 1959] [Shindo and Rae 1978] [Smith and Moushegian 2021]

Other aeronautical and foundational literature

The residual, reported and not hidden. Flight measurement and performance method, the descent, spinning and recovery of aircraft and other bodies, high-altitude aerodynamics and general aerodynamic texts that bear on this article’s descent, altitude and range arguments and belong to no cluster above. It holds 14 records, because fixed-wing work with no tie to a rotor, a descent or an escape is not admitted.

14 records. [Aerodynamic Design and Performance 1965] [Brunk 1963] [Cole 1996] [Drysdale 2013] [Dunlap and Porter 1971] [Hall et al 2021] [Harris 1980] [Kerr 1954] [Kusumoto et al 2022] [Miller and Koch 1985] [Muntz 1977] [Pearson and Garvin 1941] [Richardson 1976] [Thompson 1937]

Where the Framing Breaks Down

The X-25 is not a research aircraft in the sense the rest of this series uses. It is a civil production design, sold as plans to amateur builders, ordered in modified form and given a designation. Little was developed for the two aircraft that flew, and the one new element the record names, the automatically deploying rotor of the unpowered gyro-chute, is not known to have flown with a pilot, in Jenkins, Landis and Miller. No aerodynamic result is recorded. Treating it through a keystone derivation implies a design process that did not take place, and this article’s derivations describe the concept rather than an engineering effort that anyone performed on this airframe.

The keystone was never in doubt. Autorotation was understood in the 1920s, and Cierva’s C.4 made the first documented autogyro flight in January 1923, in the autogyro entry. The X-25 did not test whether a rotor autorotates, and an article that dimensions the rotor as though it were being designed misrepresents what the money bought.

The designation is the anomaly, not the aircraft. The X-series designations went to aircraft meant to explore new flight regimes or untried technologies, in Jenkins, Landis and Miller. The X-25s that flew were modified catalogue items with serial numbers applied to them. This is a genuine finding about how the designation system drifted, and it sits with the other evidence the closing article of this series will assemble.

Finally, the comparison at the centre of this article may be unfair in one direction. A parachute is compared against a rotor at equal weight, but the parachute is also the reserve. If the rotor follows the parachute in the sequence and does not replace it, the crewman carries both and the mass comparison should arguably be against the marginal mass of the rotor rather than against the canopy that is carried regardless. On that accounting the rotor is worse than this article makes it look, not better.

The Source Base

The record for the aircraft itself is the thinnest in the series so far, and the record for the physics around it is not. That asymmetry shapes the whole article and is worth stating plainly, so that a reader does not have to infer it from the reference list.

The vehicle itself has essentially no technical literature. One research record cited in this article carries Bensen in its title, which is the chapter on Igor Bensen and the gyrocopter in a history of the type, Chapter 11 IGOR Bensen 2003, and none carries the X-25 or the Discretionary Descent Vehicle. Three concern a flying rotor seat, which are Barzda 1970 and Hollrock and Barzda 1972 on the SAVER and Gross and Mawhinney 1970 on the AERCAB flying ejection seat, rather than the X-25. There is no flight test report, no aerodynamic data report and no programme summary in the open literature reached here. The specifications in this article come from museum fact sheets and reference compilations, not from primary documents, and where those disagree the article says so rather than choosing.

The supporting physics is abundant and that asymmetry is the point. Autorotation, the autogiro, momentum theory, blade element theory, parachutes and escape systems are all well documented from the 1920s onward. The article can derive everything about the concept and can verify almost nothing about the aircraft.

The research records are listed cluster by cluster under The Contemporary Literature, including the period reports, the work of the intervening decades and the standard atmosphere on which every number in this article rests.

The research survey holds 2,707 research records drawn from Crossref, the NASA Technical Reports Server and the Defense Technical Information Center, of which 828 are report primaries, being 30.6 percent, by the identifier rule set out under The Definition of Primary. The median publication year is 2005. Of the 2,672 dated records, 430 were published in 1968 or earlier, the year the X-25 first flew, being 16.1 percent, and 1,151 were published in 2015 or later, being 43.1 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 NACA autogiro and autorotation reports that carry this article’s keystone are report primaries under that rule. The rule does not reach the period journal papers, such as Glauert 1927 and de la Cierva 1930 on the theory of the autogiro, so 30.6 percent is a lower bound on how much of the survey is first-hand.

The keystone is thin on primary sources. Autorotation carries 166 research records and 27 of them are report primaries, which is 16.3 percent against 30.6 percent for the survey as a whole. A record is an autorotation record when its title contains a word beginning autorotat, the word auto-rotative, a windmill or windmilling brake or rotor, or a power-off descent or rotor.

The likely reason is a vocabulary that spans both eras. Autorotation is a word the 1930s used and the 2020s still use, and on that shared word the larger and better-indexed modern computational literature simply outnumbers the period work. That is an inference about the literature and not a measurement of it.

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, and where a row adds terms after the words or else, a title carrying one of those terms is counted as well.

Subject	Title terms counted	Records
rotor spin-up and prerotation	prerotation, pre-rotation, prerotator, spin-up, rotor starting, starting torque, rotor run-up	2
stored rotor energy	rotor, blade together with inertia, kinetic energy, stored energy, or else autorotative index, flywheel	4
the flying rotor seat	SAVER, flying ejection seat, rescue autogyro, rotor seat	3
gyroplane safety and accidents	gyroplane, autogyro, autogiro, gyrocopter together with safety, accident, crash, airworthiness	3
the glide of an autorotating rotor	autorotating, autorotation, autorotative, gyroplane, autogyro, autogiro together with glide, gliding	2
cutting away a parachute	parachute, canopy together with cutaway, cut away, jettison, disconnect	0
gyroplane and autorotation training	gyroplane, autogyro, autogiro, autorotation together with training, trainer, instructor, test standards	7

Rotor spin-up and prerotation stands at two records, and neither is the case this article needs. One concerns the engine loads of prerotating an autogyro rotor, Czyż et al 2016, and one the control of the blades while a helicopter rotor is spun up in wind, Kargaev et al 2025. No title in the survey names the spin-up of a rotor deployed in descent, although the deployable rotors of the SAVER and of the rotary decelerators under The Contemporary Literature must perform one. The word prerotation also names the prerotation of landing gear wheels and of the human trunk. That gap is the reason the spin-up section of this article reasons from energy rather than from measurement.

Stored rotor energy appears to stand at four records and does not really. The relevant work exists and is cited, but it sits inside the autorotation and blade-motion literature rather than beside it, because a paper on autorotative landing is a paper about spending exactly that energy. A dedicated heading for it is nearly empty while the subject is well covered. A thin heading is not the same thing as a thin subject, and the distinction is easy to get wrong in the direction of claiming a gap that is not there.

One further caution belongs here. The word flywheel also names flywheel energy storage for spacecraft and power grids, meaning containment rings and composite burst testing, which is a different discipline that happens to store energy in something that spins. That literature is a homonym of this subject rather than a part of it, and no record of it is counted here.

The flying rotor seat, the class to which the SAVER belongs, stands at three records, given above, and cutting away a parachute stands at no record, although it would be the irreversible step in any deployment sequence that included it.

Gyroplane safety stands at three records. They are Doberentz et al 2020, Houston et al 1998 and Practice for Continued Airworthiness, and no title among them compares trained with untrained pilots, which is why the training inference under Epistemic State is labelled as an inference.

The glide of an autorotating rotor stands at two records, the flight measurement of an autogiro’s gliding performance in Wheatley 1933 and a controller for steep glide slopes of an unmanned gyroplane in Wu 2015, so the lift-to-drag ratio of 4 on which the range figures rest is assumed rather than taken from a measured X-25 polar.

Gyroplane and autorotation training stands at seven records, three of them the practical test standards for rotorcraft pilots and instructors and the rest on autorotation training in helicopters, so the subject the programme’s testing addressed is among the thinnest here.

What the Survey Admits

A record is admitted only when a person reading its title finds it on this article’s subject. Its 2,707 records come from NASA’s Technical Reports Server, the Defense Technical Information Center and the journal literature registered with Crossref. Many words this subject depends on also name something else, and a record that shares only such a word is excluded. Occupant, crash and vehicle also name road traffic safety, landing also names knee and ankle loading in sport, and Parachute is also the name of a cardiac device, a heart valve, a band and a form of executive compensation. Vortex ring also names the quantised vortex rings of superfluid helium, samara also names the Samara Region, and canopy also names the crown of a tree. Autorotation also names the rotation of a telephone screen and of the jaw in surgery, height velocity also names the growth rate of children, and descent also names human gait. Prerotation also names the prerotation of landing gear wheels and tyres, of the human trunk, of pump impellers and of signal constellations in wireless networks, and rotor also names the rotors of electric machines, wave energy turbines, gyroscopes and industrial machinery. Drag coefficient also names the drag of sediment and process particles, of the sea surface in meteorology and of the ether in relativity. Electrical power systems, spaceflight outside the atmosphere, the testing and design of ordnance and the engineering of airfield landing mats are excluded as well. Correction, erratum, clarification, retraction and withdrawal notices, figure, table and supplementary-material records, peer-review reports and journal front matter are excluded too, because a survey counts research works and those are parts of works or editorial events rather than works. A signed comment on a paper, a discussion of it and its author’s closure or reply are research works, since each argues about a result, and they are admitted when their subject is, while an unsigned discussion and a referee’s report on a manuscript are not. Dictionary entries, advertisements and fiction are excluded for the same reason. The residual cluster admits only flight measurement and performance method, the descent, spinning and recovery of aircraft and other bodies, high-altitude aerodynamics and general aerodynamic texts, and fixed-wing work on supersonic and hypersonic flight, missiles and stores, engines, avionics, structures or operations, with no tie to a rotor, a descent or an escape, is not admitted to it or to any other cluster. The same rule excludes, from every cluster, fixed-wing electric and hybrid-electric aircraft, electric machines, the avionics and the operations and markets of unmanned aircraft and air mobility, and the supersonic drag of bodies and projectiles. It also excludes the landing autonomy, guidance, sensing and path planning of unmanned aircraft even when the vehicle is a rotorcraft, the structures, loads, controls, generators and floating platforms of wind turbines and every vertical-axis turbine, the drive trains, gears, lubrication, engines, ice protection, landing gear and cargo equipment of rotorcraft, the stresses and construction of propeller blades, open rotors and propfans, fixed-wing high lift, landing gear, arresting gear and runway operations, the noise of fixed-wing aircraft and engines and occupational noise exposure, simulator engineering and the administration of training, powered rocket landing, glide weapons, the imaging payloads of parafoils, the network coverage of landing sites, hearing protection and speech reception in aircraft, the quality assurance of manufacture, and airships and balloons. Conference proceedings, symposium volumes, appendices, patent notices and book reviews are excluded as parts of works or notices rather than works. Reference and definition entries such as glossaries, news items and announcements, and a second deposit of a document already counted are not admitted either, because none of them is a separate research work.

Some records of doubtful relevance are kept. Injury biomechanics under blunt impact, parachute physiology and equipment, helmet-mounted display optics, bird and insect flight aerodynamics, classical vortex rings in fluid mechanics and combustion, kites and tethered autogyros for airborne wind energy, autorotation used to harvest energy and the drag of projectiles at low speed each share either the physics of a rotor, a descent or a body in flight, or the human side of an escape, with this subject, so each is kept and a reader may weigh it accordingly.

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.

Epistemic State

Historical fact, from the public record. Three X-25 type aircraft were ordered in early 1968 under the Discretionary Descent Vehicle programme, in the X-25A fact sheet, and they were the basic X-25 gyro-chute, the X-25A and the X-25B. The X-25A, serial 68-10770, first flew on 5 June 1968. The X-25B, serial 68-10771, first flew on 5 June 1968 by Guinness World Records, the date the inventory gives for the type, and on 23 January 1968 in the Bensen B-8 entry, which cites no source for it. The Guinness maximum takeoff weight of 547 pounds is the X-25A figure in the fact sheet, so Guinness may not be independent of the inventory, and the adopted date rests in the end on that one monograph. Both aircraft derive from Bensen B-8 family designs. Testing addressed piloting technique and training requirements, and the fact sheet reports that pilots could be trained quickly and easily. No full-scale operational test was performed. Funding ended with the American withdrawal from Vietnam, and the fact sheet adds doubts about operational feasibility. The X-25A is held by the National Museum of the United States Air Force and the X-25B by the Air Force Flight Test Museum at Edwards, in Jenkins, Landis and Miller. The Kaman SAVER flew towed on 28 December 1971 and with a telescoping rotor on 10 January 1972, and was cancelled, in the Kaman KSA-100 SAVER entry.

Engineering analysis, derived here and checkable. The equivalence between the autorotation ratio of 1.8 and a disc-referred drag coefficient of 1.23 is algebra containing no fitted quantity. The finding that an autorotating rotor descends faster than a personnel canopy at equal weight follows from disc loading alone. The glide range figures follow from an assumed lift-to-drag ratio of 4. The spin-up energy of about 57 feet of ideal height loss follows from an assumed blade mass and tip speed. The SAVER descent rate of 56 feet per second follows from documented weight and rotor diameter and is the least assumption-dependent number in the article. The statement that a rotor at Martian density descends about ten times faster is the same square-root relation evaluated at one percent of sea-level density and assumes nothing else, though it is an illustration of the relation and not a claim about any particular Mars vehicle, which would fly a rotor sized for that atmosphere.

Assumed, and marked as such throughout. The escape-configuration gross weight of 350 pounds. The lift-to-drag ratio of 4. The best-glide speed of 45 miles per hour. The blade mass of 12 pounds each and tip speed of 400 feet per second. The canopy comparison figures of 28 foot nominal diameter, 250 pound suspended weight and a drag coefficient of 0.75. The blade chord of 7 inches. The rotor and parachute assembly masses of 38 and 28 pounds. None of these is a measurement of an X-25.

Inference, and labelled as inference. That the programme’s testing concerned training rather than aerodynamics is an inference from the record’s statement that the aircraft were used to evaluate piloting technique and training requirements, combined with the absence of any aerodynamic finding, and the one training result the record reports is favourable. That the doubts about operational feasibility concerned the deployment sequence, the rotor’s mass alongside a parachute or the stowage geometry is an inference from this article’s analysis, since the fact sheet names the doubts without describing them. That the ram-air parachute is the historical answer to the Discretionary Descent Vehicle question is an inference from capability comparison, not from any documented decision connecting the two.

Two further inferences come from the contemporary survey and are labelled here because they are the article’s strongest claims and its least documented. That a multirotor cannot autorotate follows from the stored-energy and disc-loading relations this article derives, applied to a configuration those relations describe accurately, but no source consulted here states it as a finding and the airworthiness literature treats redundancy as a design choice rather than as a concession. And that the gyroplane’s safety reputation bears on a crewman with no rotorcraft training is an inference from the type’s general reputation and from the contemporary safety literature, not from any study that compares trained and untrained gyroplane pilots, which would be the evidence the claim needs.

The vehicle-by-vehicle programme record for the X-series generally is Jenkins, Landis and Miller on the American X-vehicles.

What the record does not settle. Whether the X-25 rotors were the 20 foot rotor of the standard B-8M. Whether the engine was 72 or 90 horsepower. Which of the X-25A and X-25B flew first. What became of the third aircraft, the gyro-chute. What the doubts about operational feasibility were. Whether the escape sequence included a parachute cutaway at all, and whether any part of it was demonstrated below full scale. Whether any quantitative descent or glide measurement was taken and survives in a closed report.

Written from current knowledge. The contemporary survey postdates the editorial date of this article.

Out of Scope

The detailed aeroelastic behaviour of wooden rotor blades. Ejection seat dynamics and the injury mechanisms of the ejection itself. The combat search and rescue apparatus the programme was meant to supplement. The amateur-built rotorcraft accident record, which is substantial and is a different subject. The full history of the Cierva autogiro. Powered-lift and compound rotorcraft. The detailed design of ram-air canopies. Wind turbine engineering beyond the identity of the flow state.

Conclusion

The Bensen X-25 showed that pilots could learn the machine, and it never reached the escape it was for.

The programme was framed as giving a descending crewman a choice, in Jenkins, Landis and Miller. The arithmetic shows that an autorotating rotor is a worse decelerator than the parachute it was to replace, by nine percent at equal weight and twenty-nine percent as flown, and that everything it offers comes from the fact that it is a wing rather than a brake. Seven and a half miles from ten thousand feet is the whole product.

The obvious objection, that a rotor must be spun up while a canopy merely inflates, does not survive being written down. Fifty-seven feet of ideal height loss is cheap. The difficulty was never energetic, and on the record it was not training either, since the one test result reported is that pilots learned the X-25 with ease. The programme ended because the air war that motivated it was winding down and because of doubts about its operational feasibility, and the gyro-chute that embodied the escape has no recorded piloted flight. By this article’s inference the doubts had ground. The rotor buys range and not a softer landing, weighs about as much as the parachute it accompanies, would need much heavier blades or an engine to fold into a seat, and in any sequence through an open canopy would require cutting a working parachute away.

The question was good and it was eventually answered by fabric. A ram-air canopy reaches nearly the same glide ratio with no rigid structure, no spin-up and, by inference, less training, and guided airdrop has since removed the pilot from the loop as well. The rotor’s physics did not die. It went into wind energy, where the windmill brake state is the design condition rather than an emergency.

The next article in this series takes up the Schweizer X-26 Frigate.

References

Books

• Brooks on the Cierva autogiros
• Charnov’s account of the type
• Gessow and Myers
• Hoerner’s drag compendium
• Jenkins, Landis and Miller on the American X-vehicles
• Johnson’s helicopter theory
• Knacke’s recovery systems design manual
• Leishman’s helicopter aerodynamics
• Prouty on helicopter performance, stability and control

Reference

• autogyro
• autorotation
• Bensen Aircraft
• Bensen gyroglider
• Edwards
• ejection seat
• FAI record list
• Guinness World Records
• Igor Bensen
• Ingenuity helicopter
• Kaman KSA-100 SAVER
• ram-air parachute
• Rogallo wing
• X-25A
• X-25B

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-22
• X-23
• X-24
• X-3
• X-4
• X-5
• X-6
• X-7
• X-8
• X-9
• X-Planes series

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