The Osprey X-28A was to cost less than five thousand dollars a copy, and that was not a happy accident of procurement. It was the requirement. The United States Navy study that led to it asked for an aircraft that could patrol under visual flight rules, be trailered behind a vehicle, be built in Southeast Asia, work out of canals as narrow as twenty-five feet, and cost under five thousand dollars when bought in quantity. The trailer and the canals are in Tails Through Time, and the other three, the price among them, are in Jenkins, Landis and Miller. Four of those five are not flight requirements at all. This is the twenty-ninth article 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, the X-24, the X-25, the X-26, and the X-27.

It is the exact complement of the article before it. The X-27 was a supersonic fighter for which no money was appropriated and which was never built beyond a mock-up. The X-28 was a homebuilt flying boat that a man in Sacramento designed for the sport market, and the Navy bought the actual aeroplane, put it on the Delaware River, and flew it for five weeks, as Tails Through Time records.

And the two aircraft were bought for the same war. The X-26B, two articles ago, was flown over the Mekong Delta at night, made expensive so that it could be silent. The X-28 was intended, in its maker’s account, for reconnaissance over the same Mekong Delta waters, made cheap so that there could be many. One programme spent money to remove a signature and the other refused to spend money at all, and both were answers to the same question about watching somebody else’s rivers.

The Research Question

The binding constraint is a price, and the requirement states it as a number. Few research aircraft begin that way. Most exist to measure something nobody has measured. This one existed to find out what five thousand dollars buys.

So the keystone is not aerodynamic. It is whether a useful patrol capability can be assembled under a set of constraints that are almost entirely non-aeronautical, and whether the physics permits the answer to be yes.

The Constraint That Set the Aeroplane

Of the five requirements, one turns out to have determined the shape of the aircraft.

The aircraft had to work from canals as little as twenty-five feet wide. Its wingspan is twenty-three feet.

\[\text{clearance each side} = \tfrac{1}{2}\left(25 - 23\right) = 1\ \mathrm{ft} = 12\ \mathrm{in}\]

Twelve inches on each side. On this reading the span was not chosen for lift or for induced drag. It was chosen so that the aeroplane would fit down a ditch, and everything aerodynamic follows from that.

The aspect ratio is the span squared over the area.

\[A = \frac{b^2}{S} = \frac{(23.0\ \mathrm{ft})^2}{97.0\ \mathrm{ft}^2} = 5.454\]

The figure is respectable for a light aircraft and poor for a patrol aircraft, because endurance is what a patrol aircraft sells and endurance wants span. The calculation below prices the canal in that currency.

Why This Is the Right Keystone

The obvious alternative is to make the hull the keystone, since the aircraft is a flying boat and its hydrodynamics usually hold the interesting physics. The hull is treated at length below, and it was never the problem, because an aircraft this light gets over its resistance hump with an enormous margin.

A keystone should be the thing that was actually binding. For this aircraft the water was easy and the ditch was hard.

Programme Origin

The programme has two origins that met by accident, and their meeting made it a strange aeroplane.

The Sport Aircraft

George Pereira designed boats for a living and aeroplanes for pleasure. In 1969, in Sacramento, California, he began the Osprey I, a single-seat flying boat for the growing homebuilt market, as Tails Through Time records. Jenkins, Landis and Miller date its first flight to 12 August 1970, and its initial test flights were over the Sacramento Delta.

It was built of wood, fibreglass and expanded polystyrene, had folding wings and an open cockpit, and weighed nine hundred pounds with full fuel, by the account in Tails Through Time, and its plans included a trailer from which one person could launch and recover it. None of that was a military specification. All of it turned out to match one.

The Navy Study

The Naval Air Development Center at Warminster, Pennsylvania, had been studying patrol requirements for Southeast Asia. In the list Jenkins, Landis and Miller give, the study asked for an aircraft that could be flown under visual flight rules by easily trained pilots, be manufactured locally, and cost under five thousand dollars when bought in quantity. Tails Through Time adds that it had to be movable by trailer at under a thousand pounds and to operate from canals as little as twenty-five feet wide.

Navy officials asked Pereira for a demonstration of the Osprey in 1971 and found their own requirements in it. The Navy bought the aircraft and its trailer on 27 July 1971, as Tails Through Time reports, designated it X-28A, and ran the evaluation under the name Project Air Skimmer, which Jenkins, Landis and Miller call the Air Skimmer program.

What the Names Mean When the Sources Disagree

The aircraft is called several things and the record does not keep them straight. Osprey I is the name the homebuilt took retrospectively, once Pereira’s Osprey II existed. Sea Skimmer and Air Skimmer attach variously to the aircraft and to the Navy project, the encyclopaedia entry using both for the aircraft and Jenkins, Landis and Miller using Air Skimmer for the programme. Some sources, the same entry among them, credit the design of the aircraft itself to Eut Tileston working under contract to Pereira.

This article treats the aeroplane the Navy bought as Pereira’s Osprey I and does not attempt to adjudicate the naming, because the sources that disagree are all secondary and none of them shows its working.

The Vehicle

Quantity Value
Wingspan 23 ft 0 in
Length 17 ft 3 in
Height 5 ft 3 in
Wing area 97 ft²
Aspect ratio 5.454
Empty weight 600 lb
Gross weight 900 lb
Wing loading 9.28 lb/ft²
Engine Continental C90-12, 90 hp
Power loading 10.0 lb/hp
Maximum speed 135 mph
Cruise speed 103 kt
Minimum control speed 48 kt
Rate of climb 2,200 ft/min
Service ceiling 18,000 ft
Construction wood, fibreglass, expanded polystyrene

The two loadings come first, because between them they explain most of what follows, and they are the first two numbers any of the sizing texts asks for, in Raymer, Torenbeek and Gudmundsson.

\[\frac{W}{S} = \frac{900\ \mathrm{lb}}{97\ \mathrm{ft}^2} = 9.28\ \mathrm{lb/ft^2}\] \[\frac{W}{P} = \frac{900\ \mathrm{lb}}{90\ \mathrm{hp}} = 10.0\ \mathrm{lb/hp}\]

The weight breakdown is equally plain, and the fuel fraction is the number every endurance figure below is proportional to.

\[\frac{W_{\mathrm{empty}}}{W_{\mathrm{gross}}} = \frac{600}{900} = 0.667, \qquad \frac{W_{\mathrm{fuel}}}{W_{\mathrm{gross}}} = \frac{90}{900} = 0.10\]

Two numbers in that table are the whole aeroplane.

The wing loading is 9.28 pounds per square foot, which is about a tenth of the X-27’s eighty-one. The power loading is ten pounds per horsepower. An aeroplane this light with this much wing and this much power relative to its weight is capable out of all proportion to its price, and the Navy’s evaluation was a test of that proposition.

Sizing From First Principles

The inventory of record for the whole series, and the source for this aircraft’s place in it, is Jenkins, Landis and Miller.

Nothing about this aircraft’s aerodynamics was published. No drag polar, no maximum lift coefficient, no fuel capacity, no hull beam, no propeller diameter. What was published is a handful of performance numbers, and those are enough to invert.

The Lift Coefficient, Inverted From the Quoted Stall

The lift equation at the stall gives the relation between speed and maximum lift coefficient.

\[L = \tfrac{1}{2}\rho V^2 S\,C_L, \qquad V_{\mathrm{stall}} = \sqrt{\frac{2W}{\rho\,S\,C_{L_{\max}}}}\]

The record gives the speed and not the coefficient, so the relation is inverted.

\[C_{L_{\max}} = \frac{2W}{\rho\,S\,V_{\mathrm{stall}}^2} = \frac{2 \times 4{,}003}{1.225 \times 9.011 \times (24.69)^2} = 1.189\]

The coefficient is ordinary for a light-aircraft wing, although Jenkins, Landis and Miller describe flaps and ailerons along almost the whole trailing edge, and its ordinariness is the point. The inversion returns something plausible, so the quoted minimum speed and the quoted weight and area are mutually consistent.

The Drag Polar, Built Rather Than Inverted

\[C_D = C_{D_0} + K C_L^2, \qquad K = \frac{1}{\pi A e}\]

The dynamic pressure and the drag it produces are used from here to the end of the article and are defined once, following Anderson.

\[q = \tfrac{1}{2}\rho V^2, \qquad D = q\,S\,C_D\]

At the loiter condition computed below, which is twenty percent above the stall,

\[q = \tfrac{1}{2} \times 1.225 \times (29.63)^2 = 537.8\ \mathrm{Pa}, \qquad C_L = \frac{W}{qS} = 0.826, \qquad D = 85.0\ \mathrm{lbf}\]

The sizing relations used here follow Raymer, Torenbeek and Gudmundsson, with the drag data behind them in Hoerner and the underlying flight mechanics in Anderson.

A flying boat is not a landplane with floats. The hull is a planing bottom with a step and a chine in it, and it carries a drag penalty a faired fuselage does not, so the estimate is built with that penalty stated separately rather than buried.

\[C_{D_0} = C_{fe}\,\frac{S_{\mathrm{wet}}}{S_{\mathrm{ref}}} + \Delta C_{D,\,\mathrm{hull}} = 0.0055 \times 4.2 + 0.010 = 0.0331\] \[e = 0.886, \qquad K = \frac{1}{\pi \times 5.454 \times 0.886} = 0.0659\] \[\left(\frac{L}{D}\right)_{\max} = \frac{1}{2}\sqrt{\frac{\pi A e}{C_{D_0}}} = \frac{1}{2}\sqrt{\frac{\pi \times 5.454 \times 0.886}{0.0331}} = 10.71\]

Ten point seven is modest, and the hull accounts for the shortfall. Without the hull increment the same estimate gives twelve point eight.

No Room in the Climb Claim

The quoted rate of climb is 2,200 feet per minute, which for a nine-hundred-pound aeroplane with ninety horsepower deserves checking. Power available divided by weight is an absolute ceiling on rate of climb, reachable only by an aircraft with no drag whatever.

\[\frac{dh}{dt} = \frac{\eta_p P - D V}{W} \qquad \Longrightarrow \qquad \left.\frac{dh}{dt}\right|_{\mathrm{ceiling}} = \frac{\eta_p P}{W}\]
Propeller efficiency Zero-drag ceiling Claim as a fraction of it
0.70 2,310 ft/min 95.2 percent
0.75 2,475 ft/min 88.9 percent
0.80 2,640 ft/min 83.3 percent

The claim is not impossible and it has almost nothing left over. Inverting instead, and asking what propeller efficiency the claim would require against the drag polar built above,

\[\eta_p^{\mathrm{req}} = \min_V \frac{W\,\dot h + D(V)\,V}{P} = 0.823\]

Eighty-two percent is achievable by a well-matched fixed-pitch propeller at climb speed and is toward the top of the range. On balance the figure is reachable under favourable assumptions. Of the published numbers, it alone has no margin in it.

Inverting the other way makes the point sharper. If the propeller is taken at 0.75 and the climb figure is believed, the drag it implies is

\[C_{D_0} = 0.0050\]

which is impossible for a hull with a step in it. The inversion is not wrong. Its premise is. One of the three quantities, the climb rate, the weight or the power, is not what the table says it is, and the most likely candidate is that the climb figure was measured light.

What the Canal Cost

The canal calculation is the centre of the article. If the span was set by a ditch, the question is what that cost in the currency the mission valued.

A patrol aircraft’s endurance is set by the power required at its loiter speed, and the power required is minimised at a speed which follows from the polar.

\[P_{\mathrm{req}} = D V = \tfrac{1}{2}\rho V^3 S\,C_{D_0} + \frac{2 K W^2}{\rho V S}\] \[V_{\mathrm{mp}} = \left(\frac{4 K W^2}{3\rho^2 S^2 C_{D_0}}\right)^{1/4}\] \[E = \frac{W_{\mathrm{fuel}}}{c\,P_{\mathrm{shaft}}}\]

And here the calculation returns something the designer would not have wanted.

\[V_{\mathrm{mp}} = 47.2\ \mathrm{kt}, \qquad V_{\mathrm{stall}} = 48.0\ \mathrm{kt}, \qquad \frac{V_{\mathrm{mp}}}{V_{\mathrm{stall}}} = 0.984\]

The minimum-power speed lies below the stall speed. The endurance optimum is not reachable. The aircraft cannot fly slowly enough to reach the bottom of its power curve, so its loiter speed is set by a stall margin rather than by the power required, and every hour it spends on patrol is spent somewhat off the optimum.

It follows directly from the two constraints acting together. The trailer limit forced the weight down, which forced the wing loading down, which pushed the stall speed down. The canal forced the span down, which pushed the induced drag up and the minimum-power speed down further. The two requirements pushed the same two speeds toward each other until they crossed.

Taking a stall margin of twenty percent, the least a patrol pilot would accept, and ninety pounds of fuel, an unpublished figure stated here as an assumption,

Loiter speed Drag Shaft power Endurance Range
1.0 $V_s$, 48.0 kt 95.6 lbf 18.8 hp 9.59 h 460 nmi
1.2 $V_s$, 57.6 kt 85.0 lbf 20.0 hp 8.98 h 517 nmi
1.3 $V_s$, 62.4 kt 84.0 lbf 21.5 hp 8.39 h 523 nmi
1.5 $V_s$, 72.0 kt 87.7 lbf 25.8 hp 6.97 h 502 nmi

Range follows from endurance and speed, and the loiter speed follows from a stall margin rather than from the power curve, for the reason just given. The propeller-aircraft endurance relations are as given in Torenbeek and Gudmundsson.

\[R = E\,V_{\mathrm{loiter}}, \qquad V_{\mathrm{loiter}} = k\,V_{\mathrm{stall}}, \qquad k \ge 1.2\] \[R = 8.98\ \mathrm{h} \times 57.6\ \mathrm{kt} = 517\ \mathrm{nmi}\]

Nine hours is a great deal of patrol from ninety pounds of fuel, a point to make before the criticism arrives. The aeroplane needs twenty of its ninety installed horsepower to loiter, the same margin the X-26B exploited two articles ago for a different purpose.

Now the counterfactual. Holding the wing area, the weight and the polar fixed and varying only the span,

Span Aspect ratio Endurance at 1.2 $V_s$ Change
23 ft, as built 5.45 8.98 h baseline
26 ft 6.97 10.30 h +14.7 percent
30 ft 9.28 11.08 h +23.3 percent
34 ft 11.92 11.63 h +29.5 percent

A thirty-foot span, which a thirty-two-foot canal would pass, gains a little under a quarter more endurance. The ditch cost that much, a real price rather than a rhetorical one.

But the price was worth paying. A twenty-three foot span fits a twenty-five foot canal, and a thirty-foot span does not. An aircraft with twenty-three percent more endurance that cannot reach the water it is meant to patrol has no endurance at all. The constraint was not stupid. It was expensive, and the requirement made the trade deliberately.

These relations come from the literature on light-aircraft performance, drag estimation and endurance flying.

Search as the Aeroplane’s Product

The mathematics of this section is Koopman’s, developed for this problem during the Second World War, with the modern treatment in Stone.

A patrol aircraft’s product is area examined per hour. Search theory exists to quantify it, and has a well-developed mathematics for the purpose.

\[\dot{A} = W_{\mathrm{sweep}}\,V\]

The sweep width is not the sighting range and the difference matters by a factor of two. Sweep width is the width of a hypothetical perfect sensor that would find as many targets as the real imperfect one. It equals the integral of the lateral-range curve.

\[W_{\mathrm{sweep}} = 2\int_0^{\infty} p(x)\,dx\]

Under the definite-range law, where everything inside a radius is seen and nothing outside it, the integral is exactly twice the radius. Every real curve lies below that, so

\[W_{\mathrm{sweep}} = \varphi \cdot 2R_{\max}, \qquad 0 < \varphi \le 1\]

and the honesty lives in $\varphi$. A sweep width larger than twice the sighting range is therefore impossible, and any coverage figure built on one is inflated.

Maximum sighting range $\varphi$ Sweep width Coverage at 103 kt
0.5 nmi 0.3 0.30 nmi 30.9 nmi²/h
0.5 nmi 0.5 0.50 nmi 51.5 nmi²/h
1.0 nmi 0.5 1.00 nmi 103.0 nmi²/h
1.0 nmi 0.8 1.60 nmi 164.8 nmi²/h

The random-search law and the coverage factor are Koopman’s, with the modern statement in Stone.

A sweep width is only half the answer. What a commander wants is the probability that a target present in the area was found, and for a search whose track spacing is not controlled the random-search law gives it, by treating each element of area as independently exposed.

Detection probability, the lateral-range curve and the random-search law are an operations-research literature contemporary with the programme, and the visual-detection figures a patrol aircraft was planned around are period results.

\[C_{\mathrm{cov}} = \frac{W_{\mathrm{sweep}}\,V\,t}{A_{\mathrm{search}}}, \qquad P_{\mathrm{det}} = 1 - e^{-C_{\mathrm{cov}}}\]

Covering an area once finds about sixty-three percent of what is in it.

\[P_{\mathrm{det}}(C_{\mathrm{cov}} = 1) = 1 - e^{-1} = 0.632\]

That exponential explains why patrol aircraft are bought in numbers rather than singly. It is also the quantitative form of the argument the next section makes in dollars.

The tested altitude was three hundred feet, restricted by airspace around Philadelphia rather than by the aircraft. The geometric horizon there is 20.3 nautical miles, which bounds the sweep width and does not set it, because a sampan is not visible at the horizon.

\[d_{\mathrm{horizon}} \approx 1.17\sqrt{h_{\mathrm{ft}}} = 1.17\sqrt{300} = 20.3\ \mathrm{nmi}\]

Sweep width and the lateral-range curve are products of the search-theory literature, largely operations-research work and much of it contemporary with the programme. Its records form the cluster on search theory, visual detection and sweep width under The Contemporary Literature.

Area Per Dollar as the Only Permitted Comparison

A five-thousand-dollar aeroplane loses every comparison except one. It is slower, it carries less, it flies in worse weather not at all, and it sees only what a pilot can see. The comparison the requirement asks for is area examined per hour per dollar spent.

The quantity being compared is a rate divided by a price, and the requirement was asking about its fleet form.

\[\frac{\dot{A}}{C} = \frac{W_{\mathrm{sweep}}\,V}{C}, \qquad N = \frac{B}{C}, \qquad \dot{A}_{\mathrm{fleet}} = N\,W_{\mathrm{sweep}}\,V = B\,\frac{\dot{A}}{C}\]

The fleet coverage is the budget multiplied by the area per dollar, which is why that ratio is the figure of merit and not the coverage rate alone.

Each aircraft is given its own sweep width, because giving them the same one is not a comparison. A maritime patrol aircraft carries search radar. The X-28A carried a man looking out of an open cockpit.

Aircraft Speed Sweep width Coverage Unit cost Area per $1,000
X-28A, visual at 300 ft 103 kt 1.00 nmi 103 nmi²/h $5,000 20.60 nmi²/h
OV-10A Bronco, visual 223 kt 1.00 nmi 223 nmi²/h $480,000 0.46 nmi²/h
P-3C Orion, search radar 328 kt 20.00 nmi 6,560 nmi²/h $36,000,000 0.18 nmi²/h

Put the same budget through both columns and the comparison becomes concrete.

\[N = \frac{\$36{,}000{,}000}{\$5{,}000} = 7{,}200\] \[\dot{A}_{\mathrm{fleet}} = 7{,}200 \times 103 = 741{,}600\ \mathrm{nmi^2/h} \quad \text{against} \quad 6{,}560\ \mathrm{nmi^2/h}\] \[\frac{741{,}600}{6{,}560} = 113\]

The X-28A is about a hundred and thirteen times better per dollar than the Orion, after giving the Orion a twenty-mile radar sweep against the Osprey’s one-mile eyeball. The Orion covers sixty-four times the area per hour and costs seven thousand two hundred times as much.

The ratio is the entire argument for the programme, so what it does not say matters. It does not say an Orion is bad value, because an Orion flies at night, in weather, for thousands of miles, with a crew and sensors and weapons, over ocean where no canal exists. It says that for a particular mission over particular water in daylight, buying seven thousand small aeroplanes would examine more water than buying one large one. Whether seven thousand pilots existed is a different question, which this calculation leaves open.

This comparison borrows its method from the literature on cost-effectiveness analysis, operating cost and the economics of military aircraft. The Contemporary Literature lists its records in the cluster on cost-effectiveness, affordability and design to cost.

Dependent Systems

Every system below is dimensioned against a constraint that is not aerodynamic, a fact that makes this aircraft unusual to write about.

The Hull Was Never the Problem

The planing hydrodynamics below follows Savitsky and the planing-hull literature.

A flying boat must exceed its water resistance to take off, and that resistance peaks near the planing transition at what the Langley tank literature calls the hump. For most flying boats the hump is the design driver, the reason they carry so much power.

\[R_{\mathrm{hump}} = \left(\frac{R}{\Delta}\right)_{\max}\Delta\]

Static thrust cannot be found from a propulsive efficiency, because that quantity is zero at zero forward speed. Momentum theory gives the scaling.

Actuator-disc theory, disc loading and the induced velocity behind a propeller are the mechanism this section uses, and the literature under those names derives and measures it.

\[T_{\mathrm{static}} = \left(2\rho A_{\mathrm{disc}} P^2\right)^{1/3}\]

The same momentum balance gives the disc loading and the velocity the propeller must add to the air. Without it the static case could not be computed at all.

\[\frac{T}{A_{\mathrm{disc}}} = 21.9\ \mathrm{lb/ft^2}, \qquad w = \sqrt{\frac{T}{2\rho A_{\mathrm{disc}}}} = 20.7\ \mathrm{m/s}\]
Propeller diameter Static thrust
5.0 ft 549 lbf
5.5 ft 585 lbf
6.0 ft 620 lbf
Hump resistance ratio Resistance Thrust over resistance
0.15 135 lbf 4.59
0.20 180 lbf 3.44
0.25 225 lbf 2.75

The margin is between two and a half and four and a half to one across the plausible range. The aeroplane walks over its hump. A flying boat’s characteristic difficulty does not apply at ten pounds per horsepower, and this holds without knowing the hull’s beam, its deadrise or its resistance curve, because the conclusion survives every value they could take.

The beam loading coefficient is the parameter every tank test is indexed by, and it is reported parametrically for the same reason.

\[C_{\Delta_0} = \frac{\Delta}{\gamma_w b^3}\]

The displacement and the Froude scaling below are standard naval architecture, as set out in the planing-hull literature collected in Savitsky.

The displacement follows from Archimedes, and comes to a small volume of water.

\[\nabla = \frac{\Delta}{\rho_w g} = \frac{4{,}003}{1025 \times 9.807} = 0.398\ \mathrm{m^3} = 14.1\ \mathrm{ft^3}\]

The speed at which a hull transitions from displacing to planing is set by the beam Froude number, and the hump occurs at a fixed value of that parameter.

The Froude number, the hump and the transition from displacement to planing are the vocabulary the tank literature indexes this problem by, and the hump resistance ratios used above are taken from the period work.

\[\mathrm{Fr}_b = \frac{V}{\sqrt{g\,b}}\]
Speed Beam Froude number at a 2.5 ft beam
0.3 $V_s$, 14.4 kt 2.71
0.5 $V_s$, 24.0 kt 4.52
0.7 $V_s$, 33.6 kt 6.32

The hump for a hull of this loading falls in the lower part of that range, well below the speed at which the wing carries any useful share of the weight. For that reason the thrust margin computed above settles the question.

Beam $C_{\Delta_0}$
2.0 ft 1.758
2.5 ft 0.900
3.0 ft 0.521

The tank-test literature behind all of this is one of the most completely documented experimental programmes in aeronautics, and almost all of it is period work. The records are collected under The Contemporary Literature, in the cluster on flying-boat hulls, planing and the towing tank.

Sea State as the Real Water Limit

What limits a small flying boat is not resistance but waves. A hull is safe while it is long compared with the waves it meets and dangerous when it begins to span them.

\[\lambda = \frac{g T^2}{2\pi}\]
Wave period Wavelength Hull length over wavelength
2.0 s 20.5 ft 0.842
3.0 s 46.1 ft 0.374
4.0 s 81.9 ft 0.211
5.0 s 128.0 ft 0.135

At a two-second period the hull is spanning most of a wavelength, the condition in which a seaplane hull begins to be thrown rather than to ride. A two-second sea is a modest chop. The aircraft was for canals and rivers, and this table explains why.

The limit is documented in the literature on water impact loads, rough-water operation and hull seaworthiness.

Porpoising and the trim limits of stability are the other half of that subject, and they decided whether a hull could be flown off at all.

Propulsion

A ninety-horsepower Continental C90-12, a four-cylinder horizontally opposed air-cooled engine of the kind fitted to many light aircraft, among them the Piper Super Cub and the Cessna 150. The propeller is characterised by its advance ratio, and by the helical speed of its tip, which limits how fast it may be turned.

Propeller charts, blade-element theory and the tip-speed limit are among the oldest quantitative subjects in the field, and almost all of the work behind the relations below is period work.

\[J = \frac{V}{n D}, \qquad V_{\mathrm{tip}} = \sqrt{(\pi n D)^2 + V^2}, \qquad M_{\mathrm{tip}} = \frac{V_{\mathrm{tip}}}{a}\]

At cruise, with an assumed six-foot propeller at 2,700 revolutions per minute,

\[J = \frac{53.0}{45.0 \times 1.829} = 0.644, \qquad V_{\mathrm{tip}} = 263.9\ \mathrm{m/s}, \qquad M_{\mathrm{tip}} = 0.776\]

A fixed-pitch propeller wants a tip Mach number under 0.8, and that limit fixes the diameter and the engine speed.

\[\eta_p = \frac{T V}{P_{\mathrm{shaft}}}\]

The engine is the most conventional thing about the aeroplane, by design rather than by omission. An aircraft required to be manufacturable in Southeast Asia and maintainable by whoever is there cannot carry an engine that only one factory understands. Its literature appears under The Contemporary Literature as the cluster on propellers, momentum theory and the light-aircraft engine.

Structure

Wood, fibreglass and expanded polystyrene. The requirement for local manufacture appears here as a material list. None of the three needs a forging press, an autoclave or a heat-treatment line, and all three can be worked with hand tools by people who build boats.

The empty weight is six hundred pounds against a trailer limit of a thousand, a margin of four hundred pounds, spent on the folding wings and the launch trailer.

Instrumentation and the Absence of It

There is nothing to describe. The aircraft carried a pilot and whatever he could see. The requirement for visual flight rules is a requirement for no instrumentation, and the cost requirement would not have survived any.

The Flight Test Record

The flight test record is the shortest in the series so far, and unlike the previous article’s it is not empty.

  • Purchased, aircraft and trailer, on 27 July 1971, as Tails Through Time reports.
  • Tested from 16 September to 22 October 1971, five weeks, from the Philadelphia Naval Base on the Delaware River, by the same account.
  • Restricted to 300 feet maximum altitude throughout, by the airspace rather than by the aircraft.
  • First flight of the airframe had been on 12 August 1970, with initial test flights over the Sacramento Delta.
  • Last flight on 22 October 1971, ending the evaluation, as Jenkins, Landis and Miller record.

What the evaluation concluded is recorded in one sentence and no more. The aircraft was found to be easy to fly and easy to master by pilots of limited experience, and its performance was considered exceptionally good for its ninety horsepower, according to Jenkins, Landis and Miller.

That sentence is doing a great deal of work and no test report supports it in the open literature. It is treated here as a reported conclusion, not as a measurement.

Comparison With Ground Prediction

No flight measurements were published, so this section again compares a prediction with a prediction. What can be done is to check the manufacturer’s figures against each other and against first principles.

Claim Independent check Verdict
Minimum control speed 48 kt Implies $C_{L_{\max}} = 1.189$ Ordinary and consistent
Wing loading 9.28 lb/ft² 900 lb over 97 ft² Arithmetically exact
Rate of climb 2,200 ft/min Needs $\eta_p = 0.823$ Reachable, with no margin
Cruise 103 kt on 90 hp Requires about 44 hp at that speed Consistent
Span 23 ft in a 25 ft canal 12 in each side Consistent, and explanatory
Empty 600 lb under a 1,000 lb trailer limit 400 lb margin Consistent

The set hangs together. No more can be said, and the previous article could not say as much. Five of six claims check. The sixth, the climb rate, is reachable only at the top of the plausible range for its propeller, and is reported that way rather than declared false.

What the Data Changed

Almost nothing, because a war ended, not because of a technical failure.

The evaluation ended on 22 October 1971. Project Air Skimmer ended quietly as the Vietnam War wound down, and with it the requirement the aircraft had been bought to meet. No further military examples were produced.

What the programme did change lay outside the military. Pereira developed the design into the Osprey II, a two-seat amphibian sold as plans to homebuilders, and more than five hundred have been built and flown. Of the X-planes this series has treated so far, the X-28A is the only one whose design descendants exist in the hundreds, in private hangars, built by their owners.

The outcome is unusual for this series and deserves stating plainly. The X-15 flew one hundred and ninety-nine times and has no descendants. The X-28A flew for five weeks, changed no military requirement, and plans for its design are still sold to builders half a century later.

A Fourth Consecutive Designation That Did Not Go to a Research Aeroplane

The pattern named in the last three articles is now four deep, which is past coincidence.

  • X-25, a civil autogyro design ordered in modified form to investigate a rescue concept.
  • X-26, a commercially available sailplane bought twice, for training and for quiet observation.
  • X-27, a manufacturer’s private-venture export fighter that was never built at all.
  • X-28, a homebuilt sport flying boat bought off a private individual against a five-thousand-dollar unit cost.

Three of the four were aircraft that already existed and were bought for properties they already had. The fourth did not exist. Not one of the four was a research aeroplane designed and built to answer a research question, the purpose for which the X-series was created.

The X-28A is the clearest case in the series so far, because the Navy did not commission anything. It watched a man demonstrate his own aeroplane, recognised its requirements in it, and wrote him a cheque. The X-designation was applied to a purchase, and the research it marked was an evaluation of something already built.

This article does not claim that is misuse. Evaluating an existing aircraft against a requirement is research, and it produced an answer. The observation is narrower: by 1971 the X-series had become, at least sometimes, a way of labelling procurement evaluations rather than a way of labelling purpose-built research aircraft. The observation joins the evidence the closing article of this series assembles, and it now has four consecutive instances behind it.

The Contemporary Literature

Cluster Records
Flying-boat hulls, planing and the towing tank 499
Propellers, momentum theory and the light-aircraft engine 286
Light sport, ultralight and amateur-built aircraft 245
Light aircraft performance, drag and endurance 244
Water impact, porpoising and rough-water operation 213
Coverage path planning and cooperative search 184
Small unmanned aircraft for patrol and observation 165
Maritime surveillance, search and rescue and domain awareness 138
Cost-effectiveness, affordability and design to cost 128
Low-cost manufacture, composites and materials 104
Search theory, visual detection and sweep width 85
Riverine warfare, patrol and Southeast Asian rescue 20
Total 2,311

The bodies of work above are the ones the argument sits inside. This section reviews what has happened since, and then sets out the literature cluster by cluster in the order of the table.

The short version is that the X-28A’s question has been answered, decisively, and not by an aeroplane with a pilot in it.

The cheap patrol aircraft came back without the pilot

Everything the Naval Air Development Center asked for in 1971 now describes an unmanned aircraft. Visual flight rules, low cost, local manufacture, operation from confined water, and a price that makes the aircraft close to expendable. The requirement did not go away. The pilot did.

And the removal of the pilot dissolves the constraint that shaped the X-28A. A small unmanned aircraft does not need a cockpit, a canopy, a seat or a man’s weight and volume, so the thousand-pound trailer limit and the twenty-five-foot canal stop binding in the way they did. The ditch that set the wingspan is not a constraint on something you can carry.

The last three articles found the same shape of answer, and the repetition is now a pattern worth naming. The X-25 asked how to give a descending crewman a choice and was answered by a device made of fabric. The X-26B tried to make a manned aircraft inaudible and was answered by making the aircraft small enough not to need silencing. In every case the programme’s binding constraint turned out to be the thing that could be removed.

The records behind this subsection are in the cluster on small unmanned aircraft for patrol and observation below.

Search theory became a path-planning problem

Sweep width and the lateral-range curve are still the right mathematics and they are no longer the whole of it. When a human searches, the pattern is a ladder and the analysis is statistical. When a machine searches, the pattern itself is the decision, and coverage path planning has become a subject in its own right.

The X-28A’s coverage figure of a hundred square miles an hour measures a pilot’s eye and a hundred-knot aeroplane together. Contemporary work separates those, because the sensor and the platform no longer have to travel together.

The cluster on coverage path planning and cooperative search below holds the records for this subsection.

Maritime domain awareness is the mission’s modern name

The riverine and coastal surveillance problem did not end with the war. It is now called maritime domain awareness, it is fought over fisheries, smuggling and migration rather than insurgency, and the literature is large.

One thing in it would have surprised the 1971 study. The binding constraint has moved from the cost of the aircraft to the cost of processing what it sees. An aeroplane that examines a hundred square miles an hour is cheap. Looking at the result is not, which inverts the economics the X-28A was designed around.

Its records appear below in the cluster on maritime surveillance, search and rescue and domain awareness.

Planing hydrodynamics is computed now, and the tank tests still stand

The National Advisory Committee for Aeronautics ran a tank-test programme that is one of the few bodies of period aeronautical work that has been neither superseded nor contradicted. Contemporary work simulates planing hulls, wave impact and slamming numerically, and validates against towing-tank and full-scale measurements of the kind cited above.

The explanation is instructive. A planing hull is a free-surface problem with a moving contact line and spray, which is hard to compute and was easy to measure with a towing carriage and a dynamometer. The measurements were right and the difficulty was always on the other side.

The cluster on flying-boat hulls, planing and the towing tank, below, collects the work this subsection draws on.

Cheap manufacture is a design discipline rather than a compromise

Wood, fibreglass and expanded polystyrene can all be worked with hand tools. The modern version of that requirement is composite manufacturing for low volume, additive manufacturing, and the explicit design-for-cost literature.

The X-28A’s material list would still work. What has changed is that choosing it is now a defensible engineering decision with a literature behind it rather than what a man in Sacramento happened to have in his shop.

For the records, see the cluster on low-cost manufacture, composites and materials below.

The light aircraft itself has a new regulatory category

The class of aircraft the Osprey belonged to is now defined in regulation. Light sport and ultralight categories exist, the Osprey I would sit inside the light-sport limits on weight and speed though not inside the ultralight ones, and the literature on their design, performance and safety is substantial.

The cluster on light sport, ultralight and amateur-built aircraft below lists that literature.

Affordability as a stated requirement

The X-28A is an early instance of design to cost with the cost stated first, and design to cost has since become a subject of study.

The modern literature would recognise the programme’s structure immediately and would also flag its weakness. A five-thousand-dollar unit price excludes the cost of operating, maintaining, basing and crewing an aircraft, and those dominate. The study asked the right question in the wrong units, a failure mode the contemporary work names explicitly.

These records sit in the cluster on cost-effectiveness, affordability and design to cost below.

Flying-boat hulls, planing and the towing tank

The towing tank and the hull it measured. Flying-boat and seaplane hulls, floats and planing surfaces, their resistance, spray, trim and stability on the water, the step and the dead rise, and the computation of planing that has followed the tank.

499 records. [A Flying Boat Series 1930] [A German Long‐Range Flying‐Boat 1939] [A German Long‐Range Seaplane 1941] [A Long‐Range Seaplane 1935] [A Seaplane Depot Ship 1933] [A Seaplane for the Atlantic 1937] [Abdulov et al 2018] [Aboozar and Mofid 2025] [Adli et al 2024] [Afshar and Keshvari 2015] [Ai et al 1964] [Akbari and Nikseresht 2025] [Allison 1935] [Allison 1938] [Allison 1936] [Allison 1938] [Allison and Ward 1936] [Amphibious Airplane 1986] [An American Amphibian 1935] [An Italian Flying Boat 1937] [Arabian 1952] [Ardian et al 2025] [Balestrieri et al 2026] [Bao et al 2026] [Begovic et al 2024] [Bell 1935] [Bell 1936] [Bell and Olson 1939] [Bell 1933] [Bell 1946] [Bendarkar et al 2013] [Benoit 1926] [Benson 1943] [Berman 1938] [Blanchard 1953] [Bott 1943] [Brennan 1952] [Brown 1971] [Brown and Klosinski 1977] [Bunyawanichakul et al 2024] [Butman 1915] [Buxton 1983] [C.C. 1926] [Canamar and Smrcek 2011] [Carroll 1928] [Carter 1943] [Carter and Haar 1948] [Carter and Weinstein 1949] [Carter et al 1947] [Casling 1978] [Casling 1980] [Casling and King 1980] [Castelluccio et al 2016] [Castro-Feliciano et al 2016] [Castro-Feliciano et al 2019] [Cedeno-Mendez et al 2026] [Chambliss and Boyd 1953] [Clark and Sparrow 1949] [Clement 1957] [Clement 1962] [Clement 1964] [Clement 1979] [Clement and Daniels 1947] [Clement and Kimon 1957] [Clement and Pope 1961] [Cooke 1936] [Coombes and Perring 1934] [Crowley and Ronan 1925] [Crowley and Ronan 1926] [Crowley and Ronan 1927] [Cucinotta et al 2017] [Cumberbatch 1958] [D’orcy 1915] [Daidola 2021] [Dala 2015] [Dawson 1934] [Dawson 1935] [Dawson 1936] [Dawson and Walter 1946] [Dawson 1933] [Dawson 1935] [Dawson 1937] [Dawson 1938] [Dawson and Truscott 1936] [Dawson et al 1946] [Dawson et al 1946] [Demirel and Alarçin 2020] [Denny and Feller 1979] [Dewey and Bynne 1963] [Diehl 1938] [Diehl 1938] [Diehl 1940] [Doctors 2020] [Doctors 2025] [Dornier “Superwal” commercial seaplane 1927] [Dornier 1929] [Dornier 1930] [Doustdar and Kazemi 2019] [Dowling 2002] [D’Amato et al 2023] [Ebert 1943] [Ebrahimi et al 2024] [Elaghbash 2021] [Epstein and Rogers 2024] [Fathuddiin and Samuel 2021] [Ferguson et al 1949] [Fernandes and Sundaravadivelu 2022] [Ferrando et al 2015] [Fisher and Hoffman 1953] [Fisher and Hoffman 1953] [Flying Boat Construction 1921] [Flying Boat Fallacies 1936] [Flying BOAT Kokutai April-December 2024] [Flying BOAT Kokutai December 2024] [Freihofner 1943] [Freihofner 1944] [Fullmer 1943] [Fullmer 1948] [Gan et al 2024] [Garner 1933] [Garner and Coombes 1930] [Garo et al 2012] [Garrison and Clement 1948] [Garrison and Hacskaylo 1947] [Ghadimi et al 2018] [Ghadimi et al 2025] [Gifford and Wrobel 2023] [Goldenbaum 1945] [Goldenbaum 1945] [González-Cantos et al 2020] [Gough 1937] [Gregory and Beach 1979] [Gruson 1936] [Guo et al 2011] [Guo et al 2016] [Guo et al 2021] [Guo et al 2021] [Gustosa 1930] [Haar 1945] [Haar 1948] [Hajizadeh et al 2017] [Hamilton and Allen 2003] [Hampden 1930] [Handler 1963] [Harper 1937] [Harper 1938] [Hartman 1935] [Havens 1943] [Havens 1943] [Havens 1945] [Havens 1946] [Henry 1960] [Herrman et al 1928] [Herrmann 1927] [Herrmann 1927] [Hieser et al 1956] [Hodges and Woodward 1948] [Hope 1920] [Hosseini et al 2021] [Hosseini et al 2024] [House 1938] [Howe 1986] [Huang et al 2018] [Hugli 1945] [Hunsaker 1927] [Husser and Brizzolara 2021] [Huynh and Tran 2021] [Huynh and Tran 2022] [Ide 1928] [Suneela and Krishnankutty 2025] [Jackson 1943] [Jang et al 2019] [Jangam 2021] [Jangam 2022] [Jenkinson and Marchman 2003] [Jiang and Ding 2021] [Jiang et al 2022] [McGehee 1958] [Jokar et al 2020] [Jones 1935] [Judge 2012] [Judge et al 2020] [Kafali 1960] [Kagawa et al 2007] [Kallio 1978] [Kapryan and Boyd 1953] [Kapryan and Weinstein 1952] [Kapryan and Weinstein 1956] [Kapryan 1949] [Kapryan and Clement 1949] [Kazemi et al 2019] [Keuning and Gerritsma 1982] [Khazaee et al 2019] [Kikuhara 1960] [Kikuhara 1972] [Kikuhara and Tokuda 1966] [Kimon 1957] [King 1943] [King and Hill 1946] [Knowler 1952] [Korvin-Kroukovsky 1950] [Land 1944] [Land 1945] [Land and Fontana 1952] [Land and Pelz 1952] [Land and Posner 1946] [Land and Zeck 1946] [Land and Zeck 1947] [Land et al 1949] [Langley 1928] [Large Flying-Boats in Series 1939] [Latecoere 38-0 flying boat 1931] [Latorre 1982] [Lee 1982] [Leiser 2017] [Lewe 1921] [Li 2024] [Liang Hou and Liang Yun 2015] [Lipscomb 1947] [Locke 1943] [Locke 1943] [Locke 1945] [Locke 1946] [Locke 1946] [Locke 1947] [Locke 1948] [Locke 1944] [Locke 1944] [Long 1993] [Lotfi et al 2015] [Lowry and Riebe 1948] [Lowry and Riebe 1948] [Luoma 1943] [Macchi M. 39 Seaplane 1927] [MacDIARMID 1951] [MacDonald et al 2021] [Malik et al 2021] [Marín and Cuenca 2015] [Mas 1945] [Massy 1913] [Mathias and Holzapfel 1929] [Matveev 2018] [Matveev 2025] [Matveev and Morabito 2020] [Maynard 1943] [Mayo 1945] [McBride 1955] [McBride 1956] [McKann et al 1949] [McKann and Suydam 1948] [McKann et al 1949] [McKann et al 1949] [Meade 1993] [Millward 1978] [Millward 1982] [Morabito 2015] [Morabito and Lorio 2015] [Moran 1965] [Morse et al 1956] [Moss 1969] [Mottard 1956] [Mottard and Ruggles 1948] [Mottard 1959] [Munk 1921] [Naeseth and Macleod 1949] [Nafiri Muhammad Kautsar and Ketut Suastika 2023] [Nasirudin et al 2024] [National advisory committee aeronautics Report no. 328. Water pressure distribution on a twin-float seaplane, 1930] [New concept Centaur amphibian 1999] [Nimmagadda et al 2020] [Notes on Flying Boat Hulls 1920] [Nugnes 2025] [Numerical Hydrodynamic Performance of the Stepped Planing Craft and Its Step Height Effect 2019] [Numerical Modeling of the Stepped Planing Hull in Calm Water 2016] [Nutku 1965] [O’Shea et al 2012] [Olson 1941] [Olson and Land 1943] [Olson and Land 1948] [Olson and Truscott 1942] [Olson and Allison 1939] [Olson 1943] [Olson and Lina 1941] [Onishi 2003] [Onishi 2004] [Oswald 1927] [Pacuraru et al 2022] [Pan et al 2023] [Park et al 2019] [Parkinson 1933] [Parkinson 1934] [Parkinson 1935] [Parkinson 1936] [Parkinson 1943] [Parkinson 1943] [Parkinson 1944] [Parkinson 1947] [Parkinson 1948] [Parkinson and House R O 1938] [Parkinson 1935] [Parkinson 1938] [Parkinson and Robertson 1936] [Parkinson 1946] [Parkinson and Dawson 1937] [Parkinson et al 1939] [Parkinson et al 1943] [Parkinson et al 1947] [Payne 1981] [Payne 1982] [Pearson and Smull 1942] [Peningkatan Unjuk Kerja Seaplane 2025] [Penny 1927] [Pepper and Kaplan 1966] [Pepper and Kaplan 1968] [Perry 1954] [Petynia et al 1958] [Pierson 1944] [Pierson 1951] [Pierson and Burghardt 1945] [Pierson et al 1954] [Platzer 2001] [Pope 1958] [Posner et al 1944] [Pratama and Arif Winarno 2022] [Qin et al 2015] [Qiu and Song 2016] [R.A.F Big Flying Boat 1935] [Rahmansetayesh et al 2023] [Ramsen and Gray 1951] [Ramsen and Gray 1951] [Ramsen et al 1951] [Reif and Guenther 1978] [Ren et al 2025] [Rennie 1928] [Report no. 209, characteristics of a single float seaplane during take-off 1925] [Report No. 290. Water-pressure distribution on a seaplane float 1928] [Report no. 346. Water pressure distribution on a flying boat hull 1930] [Reynolds 1947] [Richard-Penhoet commercial seaplane 1927] [Richardson 1919] [Riebe 1948] [Riebe and Naeseth 1947] [Riebe and Naeseth 1948] [Riebe and Naeseth 1952] [Riebe and Naeseth 1952] [Riebe and Naeseth 1947] [Riebe and Naeseth 1951] [Roberts and Beranek 1952] [Rohrbach 1926] [Rohrbach 1930] [Rosenthal 1949] [Sadati and Zeraatgar 2023] [Sadati et al 2022] [Saeedi Namini and Kohansal 2024] [Sambraus 1938] [Samuel et al 2019] [Samuel et al 2020] [Samuel et al 2026] [Savander et al 2010] [Savitsky 1964] [Savitsky and Gore 1980] [Savitsky et al 1958] [Savoia seaplane S.55 1927] [Schroder 1931] [Schroder 1932] [Schuettel 1931] [Seaplane Design By W 1935] [Seaplane Float and Hull Design By Marcus Langley, M.I.Ae.E., A.M.I.N.A. Published by Sir Isaac Pitman and Sons, Ltd. Price 7/6 1935] [Searight 1912] [Sedov 1940] [Sekulic et al 2025] [Shao et al 2022] [Shen 1970] [Shen et al 2026] [Shi et al 2019] [Shi et al 2022] [Shin Meiwa Industry Co Ltd Japan 1961] [Shoemaker 1933] [Shoemaker 1934] [Shoemaker and Bell 1934] [Shoemaker and Parkinson 1933] [Shoemaker and Parkinson 1934] [Shoemaker 1934] [Shuford 1957] [Simmonds 1925] [Smethers 1976] [Smiley 1950] [Smiley 1952] [Smiley and Haines 1949] [Smith 1943] [Smith and Shaw 1976] [Sokolov 1950] [Sottorf 1933] [Sottorf 1934] [Sottorf 1942] [Sottorf 1944] [Sottorf 1949] [Struble 1957] [Subramanian and Subramanyam 1970] [Sun et al 2021] [Supplement to the Supermarine 1932] [Suydam 1948] [Svaitsky and Breslin 1958] [Syamsuar 2017] [Syamsuar 2017] [Syamsuar et al 2015] [Syamsuar et al 2016] [Tagliafierro et al 2021] [Tavakoli and Dashtimanesh 2019] [Tavakoli et al 2024] [The Benoist Flying Boat 1913] [The C.A.M.S. 80 amphibian 1932] [The Caudron P.V. 200 1933] [The Coming Seaplane 1929] [The Flying Boat 2020] [The Latecoere 521 “Lieutenant 1936] [The Liore Olivier “Le 1930] [The Neglected Seaplane 1930] [The Saunders “Cutty Sark” 1929] [The Seaplane Carrier “Argus” 1919] [The Seaplane Testing Basin 1931] [The short “Empire” commercial 1936] [The Short “Valetta” commercial 1930] [The Sikorsky Twin Engined 1928] [The Supermarine “Southampton” seaplane 1926] [The Supermarine S.6.B. Racing 1931] [The Vickers-Supermarine “Scapa” British 1936] [Thomas 1967] [Thompson 1929] [Thompson 1930] [Thompson 1931] [Thurston 1974] [Ting and Keller 1974] [Ting and Keller 1977] [Toro 1969] [TOWARD A NUCLEAR‐POWERED SEAPLANE 1958] [Trimulyono et al 2025] [Truscott and Dawson 1942] [Truscott and Parkinson 1938] [Truscott 1935] [Truscott et al 1938] [Tuccillo and Ruocco 2025] [Tuck 1982] [Tuck 1982] [Utomo et al 2022] [Uyeda 1961] [Vagianos and Rooney 1964] [Vagianos and Thurston 1970] [Vaughan and Earl 1979] [Verduzio 1932] [Verduzio 1923] [Voloshchenko 2018] [Vorus 2008] [Wadlin 1944] [Wadlin 1953] [Wadlin and McGehee 1949] [Wadlin and McGehee 1950] [Wadlin and Christopher 1958] [Wagner 1948] [Waldin 1943] [Wang 2015] [Wang and Rispin 1971] [Wang et al 2019] [Wang et al 2022] [Ward 1940] [Ward and Larrabee 1977] [Ward 1937] [Weigao 1996] [Weinig 1938] [Whitaker and Bryce 1951] [Willis 1943] [Wind Tunnel and Seaplane Basin at Langley Field 1930] [Woodward et al 1947] [Wornom 1960] [Wright and Luburg 1930] [Wu and Landweber 1963] [Wu and Whitney 1972] [Wu et al 2022] [Xia et al 2024] [Xiao et al 2019] [Xiao et al 2020] [Xiros et al 2019] [Xu et al 2023] [Xu et al 2023] [Yang et al 2019] [Yates and Riebe 1947] [Yates and Riebe 1947] [Yousefifard and Maboodi 2020] [Yu et al 2022] [Yu et al 2024] [Zeck 1945] [Zeck 1943] [Zha et al 2024] [Zhandong et al 2026] [Zhang and Kim 2015] [Zhang et al 2024] [Zheng et al 2019] [Zheng et al 2021] [Zheng et al 2023]

294 of these records date from before 1960, more than in any other cluster, and 226 are on the Technical Reports Server, which is where the National Advisory Committee for Aeronautics tank programme is held. Of the 49 titles that name numerical simulation, computational fluid dynamics or computation, 48 date from 2000 or later, which is the literature’s own measure of the move from measuring a planing hull to computing one.

Propellers, momentum theory and the light-aircraft engine

What turns ninety horsepower into thrust. Propeller theory and charts, blade-element and momentum methods, static thrust, propeller noise, and the piston engines of light aircraft, with the marine propeller literature that shares the same theory.

286 records. [A “New Type Adjustable-Pitch 2017] [Adamson 1968] [Aerodynamics in the Age 1997] [Agung Nurachmat et al 2023] [Asnaghi et al 2021] [Baek et al 2015] [Baek et al 2021] [Baker 1946] [Barshai 1970] [Berenyi and Brouwers 1980] [Berman 2003] [Berton and Nark 2018] [Betz 1922] [Betz 1928] [Betz 1939] [Beveridge 1962] [Beyer et al 1984] [Biber 2005] [Biermann and Conway 1942] [Block 1986] [Bosschers 2018] [Boswell and Miller 1968] [Boës 1970] [Broersma and Tasseron 1967] [Brusse and Kettleborough 1975] [Brusse and Cronk 1965] [Bryant and Walters 2021] [Burkhardt 1921] [Burnham 1942] [Butler et al 1966] [Caldwell 1940] [Cao et al 2025] [Castles and Ducoffe 1948] [Castles et al 1960] [Chen 2012] [Chyla 2024] [Coles 1949] [Composite propeller blade 1972] [Coon 1963] [Coward 1955] [Cronk 1965] [Cui et al 2019] [Cummings 1968] [Dallas and Irvin 1956] [Ingraham and Falck] [Ingraham et al] [Decker 1966] [Deming 1940] [DeStories and Takahashi 2014] [Diehl 1934] [Dinc and Otkur 2020] [Dittmar 1977] [Dittmar 1978] [Dittmar et al 1979] [Dorfling and Rokhsaz 2015] [Duan et al 2023] [Duan et al 2024] [Enos 1937] [Erickson and John C. 1969] [Erickson et al 1965] [Ernsthausen 1937] [Evans and Liner 1948] [Fabri and Siestrunck 1953] [Feng et al 2025] [Fortin and Ahuja 2024] [Fournis et al 2026] [Gao et al 2018] [Gennaretti et al 2006] [Gill and D’Andrea 2017] [Gilman 1944] [Goland et al 1964] [Goodman 1979] [Greenberg and Ordway 1964] [Gropp 1938] [Guangnian et al 2020] [Hall et al 1981] [Handasyde 1931] [Handasyde 1932] [Hanson 1986] [Healy 1969] [Heck et al 2023] [Heinzen 2018] [Higgens et al 2020] [Hodges 1980] [Hoehne and Hoffman 1961] [Hong et al 2022] [Hoover and Shen 2019] [Hoover et al 2018] [Hough 1961] [Howlett and Schoenster 1979] [Hoyos et al 2021] [Hoyos et al 2022] [Hu et al 2019] [Hu et al 2021] [Hu et al 2022] [Hubbard and Regier 1947] [Idzubuchi and Kito 1941] [Improved Propeller Design Will 1937] [Ingraham et al 2019] [Iwasaki et al 1968] [Jeracki and Mitchell 1981] [Jia et al 2022] [Jian et al 2019] [Jiang et al 2016] [Johnson and White 1983] [Jones 1965] [Kallergis 1991] [Kawada 1930] [Kerwin and Leopold 1963] [Kettleborough 1969] [Khalid] [Khasyofi and Hartono 2019] [Kim and Kinnas 2023] [Kim et al 2025] [Kim et al 2026] [Kinoshita 1952] [Korkan et al 1980] [Korvin-Kroukovsky 1954] [Korvin‐Kroukovsky 1951] [Krüger et al 2016] [Laitone 2000] [Lan et al 2020] [Lays and Murray 1979] [Lays and Murray 1979] [Lee et al 2017] [Lee et al 2018] [Lee et al 2018] [Leishman 1966] [Leong Chee Hang et al 2022] [Lerbs 1955] [Li et al 2021] [Li et al 2025] [Lieberman and Hoffman 1964] [Light aircraft propeller 1993] [Ligtenberg 1962] [Lister 1923] [Liu et al 2016] [Loenbaek et al 2021] [Ma and Wang 2021] [Magliozzi 1983] [Mahaki and Sahebi 2026] [Mahendra Indiaryanto Hendra et al 2024] [Margoulis 1922] [McCarthy et al 1961] [McCormick 1956] [McCormick 1987] [McCOY 1939] [McCurdy and Powell 1984] [McLemore and Pegg 1980] [Measurement of the effect 1987] [Mendoza 1977] [Merrill et al 1973] [Meyerhoff and Zvengrowski 1962] [Middleton 1965] [Miley et al 1981] [Mixson et al 1979] [Mohammad Nouri and Mohammadi 2018] [Molland and Turnock 2002] [Moshkov 2026] [Moshkov and Samokhin 2021] [Moshkov and Yakovlev 2023] [Murray 1967] [Möhren et al 2025] [Nakajima 1960] [Narbut 1975] [National advisory committee for aeronautics: Report No. 447, Static Thrust of Airplane Propellers 1933] [Neves et al 2023] [Nguyen and Ngo 2017] [Noosomton and Charoensuk 2015] [Norton 1956] [Nouroozi and Zeraatgar 2019] [Ordway and Hale 1960] [Ormsbee 1976] [Osovskii and Sharatov 2019] [Otsuka et al 1969] [Park and Seong 2017] [Patience and Nahon 2024] [Pegg et al 1977] [Pennings et al 2015] [Pennings et al 2016] [Petrovskii 1965] [Piancastelli 2025] [Plunkett 1949] [Podsędkowski et al 2023] [Pollock. 1925] [Prakash 2018] [Prior and Newman-Sanders 2024] [Propeller Cuffs 1940] [Propeller Performance 2012] [Reissner 1937] [Reissner 1937] [Report No. 436, Tests of Nacelle-Propeller Combinations in Various Positions with Reference to Wings. II. Thick Wing-Various Radial-Engine Cowlings-Tractor Propeller 1932] [Report No. 481, Working charts for the determination of propeller thrust at various air speeds 1934] [Riach 1922] [Richard 1995] [Romig 1912] [Roosenboom and Schröder 2010] [Ruijgrok 1980] [Saari and Wallner 1946] [Sabzehparvar 2005] [Schatz et al 2016] [Schey and Gove 1930] [Schouten 1982] [Schroeder and Dai 2010] [Seeni 2019] [Seeni 2019] [Seeni 2020] [Sellman 1974] [Sezen and Bal 2020] [Sezen et al 2021] [Showers 1952] [Simizu 1944] [Simmonds 1967] [Simmonds 1967] [Simmonds 1967] [Simmons 2022] [Simulation of Isolated Propeller Noise Using Acoustic-Vortex Method 2023] [Sinacori and Lange 1961] [Son and Kim 2023] [Spreemann 1958] [Sticchi et al 2026] [Strack 1983] [Stresses in a rotating propeller blade 1938] [STRIVING FOR 80 PER CENT PROPELLER EFFICIENCY 1925] [Stuart 1943] [Stuhlpfarrer et al 2017] [Stuper 1938] [Swiatek et al 2017] [Taback et al 1983] [Talbot et al 1994] [Taylor 1958] [Teixeira and Cesnik 2019] [Teixeira and Cesnik 2020] [THE MEASUREMENT OF PROPELLER THRUST 1921] [Theodorsen 1960] [Theodorsen 1960] [Tilley 1938] [Togino 1941] [Tomy et al 2024] [Traub 2016] [Traub 2017] [Traub 2021] [Trenka 1967] [Treuren and Wisniewski 2019] [Tung 1950] [Unruh 1988] [Unruh et al 1979] [van Aken and Tasseron 1955] [van Aken and Tasseron 1956] [van Eck 2026] [van Gunsteren 1971] [Vepa 2020] [Wald 1965] [Wang et al 2019] [Wang et al 2020] [Wang et al 2021] [Webb and Willer 1952] [Weick 1932] [Weick 1926] [Weick 1926] [Widoro et al 2026] [Wiegman 1940] [Wilby 1989] [Wilby 1992] [Windsor 1968] [Wood and Woodward 1944] [Worobel 1972] [Worobel and Mayo 1971] [Worobel and Peracchio 1968] [Wróblewski 2021] [Wróblewski and Rogólski 2021] [Wu et al 2024] [Xu et al 2024] [Xue and Zhou 2020] [Xue and Zhou 2020] [Yamada et al 2015] [Yang et al 2013] [Yang et al 2017] [Yazaki et al 1967] [Yazaki et al 1969] [Yilmaz et al 2020] [Zhang et al 2021] [Zhang et al 2025] [Zhiyuan et al 2025] [Zhou 2025] [Zhu et al 2021]

Light sport, ultralight and amateur-built aircraft

The class of aircraft the Osprey belongs to. Light, ultralight, light sport and amateur-built aircraft, their design, structure, noise, handling, crashworthiness and safety record.

245 records. [A German Light Aeroplane 1932] [Abbasy 1983] [Abdollahi et al 2023] [Aguiar et al 2017] [AI-Based Predictive Maintenance for General Aviation Aircraft 2026] [Aircraft maintenance training scheme 1999] [Alfaro-Bou and Vaughan 1977] [Alfaro-Bou et al 1985] [Alonso Castilla et al 2021] [Anderson 1994] [Anderson 2015] [Angelov and Velkova 2020] [ASTM light sport aircraft 2007] [Ballou 1963] [Ballou 1963] [Baynes Bee light airplane 1937] [Beauregard 1964] [Bement 1980] [Bement 1981] [Bender and Lewis 2002] [Beranek et al 1950] [Bienert and Regnet 2025] [Blackwell 1970] [Bortolussi 1997] [Boudreault 1983] [Boyd 2015] [Boyd 2015] [Boyd 2016] [Boyd and Guinn 2024] [Boyd and Hinkelbein 2017] [Boyd and Stolzer 2016] [Breidenthal 2018] [Bruensicke and Cholvin 1964] [Burgess et al 2018] [Burns and Bonaceto 2020] [Bushra et al 2012] [Butler and Salomonsky 1965] [Cannaday and Suit 1977] [Carden 1984] [Carden et al 1985] [Castle and Alfaro-Bou 1978] [Castle and Alfaro-Bou 1979] [Catherines and Jha 1976] [Catherines and Mayes 1975] [Chambers and Yip 1984] [Cho 2015] [Cramer et al 2019] [Crane et al 1973] [Crash Impact Survival in 1994] [Cvetelina Velkova et al 2015] [Czarnecki and Davis 1948] [Diefes and Deering] [Donateo et al 2024] [Donateo et al 2024] [Donely et al 1972] [Driggs 1925] [Driggs 1925] [Dudziak and Guła 2017] [Duerksen 1996] [Duerksen 1997] [Eastland et al 1994] [Eiband et al 1953] [Engine Failure in Light 1980] [Evans] [Fan and Yang 2017] [Farrens et al 1994] [Faruque 2014] [Feil and Hajek 2021] [Grosveld 1984] [Ferrara 1989] [Finger et al 2020] [Finger et al 2020] [Friedrich and Vollrath 2021] [Galloway et al 1984] [Gan et al 2021] [Garbell 1945] [General Aviation Propulsion 1980] [General Aviation Technology Conference 1984] [General aviation technology program 1976] [Gerhardinger et al 2023] [Ghiţescu et al 2021] [Goode et al 1976] [Grbović et al 2019] [Górski et al 2022] [Hague 1978] [Hague 1978] [Hague 1978] [Hana.M. Almagzob et al 2021] [Harris et al 1978] [Herrera et al 1984] [Hirabayashi 2021] [Hoadley 1988] [Holmes 1993] [Hospodka et al 2020] [Huang 2020] [Hubbell 1966] [Hunter and Vensel 1947] [Ilić et al 2017] [Iscold et al 2006] [Ivanković et al 2023] [Jewel 1987] [Johnson et al 1980] [Kalagher et al 2021] [Karuskevich et al 2022] [Khaddage] [Kim 2020] [Klein 1979] [Klesa 2022] [Kohlman 1977] [Kohlman and Brainerd 1974] [Kohlman and Ellis 1974] [Kohlman et al 1980] [Kratochvíl and Valenta 2024] [Krisbudiman et al 2024] [Kubit et al 2022] [Kuk 2025] [Köhler and Jeschke 2021] [la Cour‐Harbo and Schiøler 2019] [Labett 1972] [Lampton and Valasek 2005] [Lampton and Valasek 2007] [Lange 1994] [Lee 2016] [Li and Lu 2019] [Li et al 2026] [Liang et al 2026] [Light aircraft structural design 1982] [Linka et al 2018] [Littell and Stimson 2016] [Liu et al 2025] [Liu et al 2025] [Loschke et al 1974] [Lutz et al 2025] [Lyu et al 2024] [Majumdar et al 2021] [Marchman 1979] [McAvoy et al 1949] [McBride 1995] [McGowan 1961] [McQueen et al 2022] [McQueen et al 2022] [Melosh and Kamat 1977] [Merrill 2003] [Mihalić et al 2024] [Mills and DeJohn 2016] [Mixson et al 1978] [Mixson et al 1977] [Moletsane] [Molnár and Dezső 2018] [Morane-Saulnier 180 light airplane 1929] [Moul and Taylor 1980] [Mujahid 2020] [Murri and Jordan 1987] [Murri et al 1987] [Naca 1933] [Natali 1978] [Nelson and Goldman 2003] [Newsom et al 1982] [North et al 1984] [Novel suspension for light 2000] [Novák et al 2016] [Obryan et al 1980] [Omar 2019] [Pamadi and Taylor 1984] [Papapetrou et al 2017] [Park et al 2016] [Pazmany 1969] [Peele and Steiner 1970] [Pisapia et al 2023] [Poitz 1983] [Portevin and Defleury 1924] [Puranik et al 2017] [Rajagopalan Srilatha] [Reyer and Ogg 1986] [Riboldi et al 2023] [Robb and Platt] [Rogólski and Olejnik 2018] [Roskam and Gomer 1991] [Roskam and Gomer 1991] [Roskam et al 1974] [Rueter et al 1994] [Rummer et al 1982] [Salam et al 2022] [Salam et al 2024] [Satran 1986] [Scheuring 2005] [Schrenk 1930] [Schwarz et al 2019] [Scutaru et al 2024] [Seckel et al 1973] [Sedláčková et al 2023] [Selberg and Cronin 1985] [Smetana et al 1972] [Smetana et al 1973] [Smith 1985] [Stengel 1988] [Stewart 1975] [Stewart 1976] [Stewart 1993] [Stewart and Redd 1977] [Stewart et al 1975] [Stewart et al 1988] [Stickle 1977] [Stickle et al 1974] [Sundstrom 1980] [Sziroczak et al 2020] [The “Junkers-Junior” Light Airplane 1930] [The A.N.E.C. IV “Missel 1926] [The Mono-Spar light airplane 1931] [The Stieger ST. 4 1932] [Theodorsen and Regier 1946] [Tomczyk 1998] [Tracy 1986] [Tyan et al 2019] [Ud-Din and Yoon 2018] [Unruh 1981] [Unruh et al 2002] [Unruh et al 2002] [Vagner et al 2023] [Vaicaitis 1980] [Vaughan and Alfaro-Bou 1979] [Vaughan and Alfaro-Bou 1979] [Velkova and Todorov 2015] [Vlase et al 2021] [Vogeley 1948] [Waters et al 1973] [Weintraub et al 2022] [Wells and Ramachandran 1977] [White 1932] [White 1982] [Wilkinson and Bruce 1973] [Winblade 1975] [Winblade and Westfall 1975] [Winkelmann 1990] [Worobel and Mayo 1973] [Wróblewski et al 2023] [Wu et al 2023] [Xue and Fu 2018] [Yan et al 2024] [Yip and Coy 1982] [Yip et al 1986] [Zenowicz 2021] [Zhang et al 2024] [Zimmerman 1949] [Šimák and Škultéty 2020]

65 of these records are reports-server documents from 1970 to 1989, the period of the agency’s general aviation technology programme, and 31 titles name a crash or an accident.

Light aircraft performance, drag and endurance

The inversions in the sizing argument. Maximum lift and the stall, the drag polar and span efficiency, climb, range and endurance of propeller aircraft, and the flight-test methods that measure them.

244 records. [A Long-Range Light Aeroplane 1932] [A Study on the 2007] [Air Force Test Pilot School Edwards Afb Ca 1969] [Aircraft Range and Endurance 2011] [Aircraft Range and Endurance 2011] [Airplane Spin Testing 2003] [Alley et al 2007] [Ananda and Selig 2016] [Anderson 1978] [Anderson 1979] [Anderson 2004] [Application of Dynamic Programming 1957] [Arshad et al 2026] [Barber et al 1966] [Barufaldi and Morales 2023] [Batra et al 2024] [Batterson 1983] [Bennett 1976] [Bennett et al 1980] [Berenschot 2001] [Bert 1981] [Bert 1999] [Biber 2011] [Biber 2016] [Biber 2017] [Biber 2017] [Bihrle and Bowman 1980] [Bihrle and Hultberg 1979] [Bihrle et al 1978] [Bihrle et al 1978] [Bogdanovic et al 1965] [Boschetti et al 2015] [Bowman 1971] [Bowman and Burk 1973] [Bowman and Burk 1974] [Bowman et al 1978] [Boyles and Barnett 1964] [Bradshaw 1980] [Bravo et al 2021] [Bridges 2021] [Brown 1987] [Bull and Bridges 1980] [Bunge et al 2015] [Burk and Wilson 1975] [Burk et al 1977] [Burton and Hoburg 2018] [Carden 1984] [Carstoiu 1960] [Cavcar 2011] [Cavcar and Cavcar 2005] [Chambers 1980] [Chambers and Stough 1986] [Chang and Yu 2015] [Cheng et al 2024] [Chevalier 1982] [Chevalier and Brusse 1973] [Coe 1976] [Conner et al 1965] [Craig 1976] [Craig and Romani 1993] [Crombie and Moorhouse 1980] [Cross 1975] [Cross 1976] [Cunis et al 2020] [Davis and Sweberg 1948] [Delgado et al 2024] [Dicarlo and Johnson 1979] [Dicarlo et al 1980] [Dicarlo et al 1984] [Dicarlo et al 1986] [Diehl 1924] [Diehl 1925] [Donderi 1994] [Donnelly 1972] [Drag and propulsive efficiency 1990] [Dunham 1986] [Dunham et al 1987] [Exploratory study of the 1979] [Feng et al 2020] [Frost and Rutherford 1963] [Garcia et al 2026] [Gatti et al 2015] [Goetzendorf-Grabowski and Antoniewski 2016] [Goncharenko 2022] [Gousman et al 1991] [Graham and Luskin 1946] [Gray 1964] [Grendysa 2019] [Grillo et al 2024] [Hantrais-Gervois and Destarac 2015] [Harper and Flanigan 1948] [Hasan et al 2025] [Hicks and Moulton 1988] [Holmes et al 1980] [Holzhauser and Bray 1956] [Hunter 1948] [Iliff 1980] [Iscold 2008] [Jo and Majid 2023] [Kaptsov and Rodrigues 2018] [Katz and Feistel 1987] [Katz et al 1980] [Keiter 1982] [Keye and Rohlmann 2014] [Kish et al 2018] [Knaus 1981] [Knaus 1983] [Kohlman and Hammer 1985] [Kohlman 1977] [Kohn 1979] [Konar 2020] [Kramer 1932] [Labonte 2015] [Labonté 2012] [Labonté 2025] [Laine 1982] [Laitone 1981] [Laitone 1983] [Laitone 1984] [Lednicer and Roncz 1989] [Level Flight Performance Flight 2003] [Li et al 2024] [Liu 2023] [Liu et al 1983] [Lockwood 1972] [Loschke et al 1972] [Lowry 1999] [Lutze and Lluch 1996] [Manuel et al 1989] [Marinus and Poppe 2015] [Mastroddi et al 2018] [Matsuo and Hirano 1986] [McCormick 1981] [McDonald 2015] [McDonnell and Ning 2020] [McElroy and Sharp 1971] [Metzger et al 1980] [Milenković-Babić 2018] [Milenković-Babić et al 2015] [Millikan 1934] [Morawski et al 2019] [Moul and Brown 1979] [Mulcay and Rose 1980] [Nguyen et al 2017] [Nial 1972] [Nikolaev 2019] [Norris and Bauer 1993] [Obryan et al 1974] [Obryan et al 1979] [Ormsbee and Chen 1972] [Paek et al 2020] [Pamadi and Taylor 1983] [Pamadi and Taylor 1987] [Parkinson 1983] [Patton 1980] [Patton 1980] [Philipson 1995] [Phillips and Alley 2007] [Press and McClanahan 1951] [Puranik et al 2020] [Pınar and Özer 2026] [Ragheb et al 2013] [Ranaudo 1977] [Rate-of-Climb, Time-to-Climb, and Ceilings 2011] [Report no. 618, Comparative 1938] [Rhode 1941] [Riley 1985] [Rockfeller 1939] [Rodrigues 2008] [Rogers and Fasciano 2016] [Rohlmann and Keye 2015] [Roskam 1975] [Rostami and Bagherzadeh 2016] [Sachs 1980] [Satran 1981] [Schulz and Nygren] [Segall and Gajdjis 1991] [Shimoyama and Kamisori 2017] [Shirbhate et al 2025] [Silver 1976] [Silverstein et al 1938] [Simmons et al 2023] [Singh and Jategaonkar 1996] [Sliwa 1979] [Smith 1977] [Smith 1989] [Somers 2005] [Son and Yee 2018] [Soule and Hootman 1937] [Stalker 1935] [Stewart et al 1982] [Stough 1990] [Stough and Patton 1979] [Stough et al 1985] [Stough et al 1987] [Stough et al 1987] [Stough et al 1987] [Stough et al 1987] [Succi 1981] [Szaruga and Doty 2011] [Tapolcai et al 2012] [Taylor and Klein 1981] [Taylor and Smith 1978] [Tenhave et al 2026] [The First Design Revolution 2003] [Theodorsen 1959] [Tischler and Barlow 1979] [Tischler and Barlow 1980] [Tischler and Barlow 1980] [Tischler and Barlow 1981] [Tischler and Barlow 1982] [Traub 2011] [Traub 2016] [Tseng et al 2006] [Uzun 2025] [Van Dam et al 1980] [Vegh 2025] [Veillette 1993] [Velazquez Salazar et al 2021] [Vicroy 1990] [Vlasov and Ovdienko 2020] [Von karman and Millikan 1935] [Oswald 1932] [Wang et al 2021] [Ward 1968] [Wares and Ferrara 1989] [Weiser and Ossmann 2022] [Wheaton et al 1964] [Wiarso et al 2021] [Wintzer 2010] [Witkowski et al 2022] [Wolowicz and Yancey 1972] [Wolowicz and Yancey 1972] [Wootton 1967] [Xie and Haberland 1999] [Yajnik and Subbaiah 1976] [Young 2008] [Youngblood and Talay 1982] [Yuan et al 2024] [Zhang et al 2020] [Zhang et al 2026] [Zhao and Gao 2019] [Zhu 2024] [李 2023]

74 of these titles name the stall or the spin, and 39 of those are reports-server records, 38 of which date from 1970 to 1989, the period of the National Aeronautics and Space Administration’s general aviation stall and spin research. The cluster is weighted toward the edge of the envelope the Osprey loitered beside rather than toward the loiter itself, as The Source Base sets out under Thin Subjects.

Water impact, porpoising and rough-water operation

What limits a small flying boat on the water. Hydrodynamic impact on landing, porpoising and skipping, rough water and waves, slamming of planing craft, and the water landing of spacecraft, which shares the impact theory.

213 records. [Al-Furjan et al 2022] [Aliffrananda and Sulisetyono 2021] [Aliffrananda et al 2022] [Allaka and Groper 2021] [Arianti et al 2023] [Bahrami and Ghassemi 2022] [Baker and Westine 1966] [Baker and Westine 1967] [Bales 1975] [Bao et al 2020] [Barklie 1944] [Batterson 1947] [Batterson 1951] [Batterson and McArver 1950] [Batterson 1944] [Begovic et al 2016] [Begovic et al 2016] [Benscoter 1947] [Benscoter 1948] [Benson 1944] [Benson 1965] [Benson et al 1952] [Benson 1942] [Bi et al 2019] [Bi et al 2020] [Bossau and Bekker 2022] [Buyukozturk et al 1974] [Cai et al 2023] [Camilleri et al 2018] [Capasso et al 2023] [Cappelli and Furuike 1967] [Caramatescu and Iulian Mocanu 2019] [Carter et al 1956] [Carter 1949] [Castro-Feliciano et al 2018] [Cecchini and Yusif 2019] [Cecchini et al 2017] [Celano 1998] [Chambliss and Blanchard 1950] [Chen et al 2022] [Chen et al 2024] [Chey 1964] [Chi et al 2026] [Clement and Havens 1947] [Cui et al 2026] [Darevsky 1944] [Diez et al 2022] [Dimas Bahtera Eskayudha 2023] [Direct Measure of Rigid Body Accelerations for Wave Impact of a Planing Hull 2016] [Djamaluddin and Ahmad 2025] [Du et al 2024] [Duan et al 2019] [Edge 1958] [Edge 1959] [Edge and Mason 1958] [Edge and Mixson 1957] [Feng et al 2020] [Feng et al 2025] [Fridsma 1969] [Gao and Sang 2023] [Garouge et al 2022] [Garrison 1949] [Gersten et al 1978] [Ghodrat and Nikseresht 2025] [Haines 1949] [Hakim et al 2025] [Halswell et al 2016] [Hamada and Maki 2023] [Hamada et al 2023] [Hirayama et al 1999] [Hoffman 1956] [Horta et al 2010] [Hou et al 2019] [Hu et al 2026] [Huang et al 2022] [Jin et al 2023] [Jones 1970] [Judge and VanDerwerken 2019] [Judge et al 2016] [Judge et al 2020] [Keller 1988] [Kettleborough 1969] [Kettleborough 1970] [Kettleborough 1972] [Khoirunnisa et al 2023] [Kim et al 2017] [Kim et al 2020] [Klein 1943] [Klemin et al 1939] [Land 1943] [Land and Zeck 1947] [Lavroff and Davis 2015] [Li and Sigimura 1967] [Li et al 2021] [Lin and Lin 2019] [Lina 1942] [Littell et al 2007] [Liu and Tian 2023] [Liu et al 2024] [Liu et al 2026] [Liu et al 2026] [Liu et al 2026] [Locke 1943] [Locke 1943] [Locke 1943] [Locke 1943] [Lu et al 2024] [Lu et al 2026] [Lv and Grenestedt 2015] [Mahdi Sajedi et al 2019] [Mansoori and Fernandes 2016] [Markey 1956] [Markey 1959] [Markey and Carpini 1957] [Martin 1976] [Masumi and Nikseresht 2017] [Maynord 2005] [Mayo 1945] [Mayo 1947] [Mayo 1952] [McArver 1951] [McKann et al 1949] [Merten and Beck 1951] [Miki 1933] [Miller 1948] [Miller 1952] [Miller 1954] [Miller 1958] [Miller and Leshnover 1947] [Miller and Merten 1951] [Miller and Merten 1952] [Milwitzky 1947] [Milwitzky 1952] [Mixson 1959] [Mohsin et al 2026] [Morabito 2015] [Mousaviraad et al 2015] [Mu et al 2025] [Naval Academy Annapolis Md 1998] [Nebylov and Nebylov 2011] [Nichols and Hirt 1978] [O’Shea et al 2012] [Ogilvie 1969] [Ogilvie and Shen 1973] [Pabst 1930] [Parkinson 1955] [Payne 1974] [Payne 1984] [Riley et al 2014] [Roper 1969] [Roshan et al 2024] [Rosén et al 2020] [Saeedi Namini and Kohansal 2024] [Sajedi and Ghadimi 2020] [Savitsky 1948] [Savitsky 1968] [Savitsky and Brown 1976] [Schnitzer 1953] [Schnitzer 1955] [Schnitzer and Hathaway 1953] [Schwab et al 1955] [Sellars 1976] [Shabani and Shabani 2022] [Shao et al 2023] [Shao et al 2025] [Sims and Schnitzer 1952] [Smiley 1952] [Song et al 2023] [Steiner and Miller 1946] [Steiner 1948] [Steiner 1949] [Stubbs 1972] [Suarez 1943] [Sun et al 2021] [Sun et al 2024] [Suneela 2026] [Tahmasvand and Zeraatgar 2021] [Tahmasvand et al 2023] [Taub 1931] [Tavakoli et al 2026] [Thompson 1967] [Townsend et al 2020] [Townsend et al 2025] [Townsend Valencia et al 2021] [Townsend Valencia et al 2022] [VanDerwerken and Judge 2017] [Vassilakos 2014] [Vassilakos 2014] [Vassilakos 2015] [Vassilakos and Mark 2018] [Vassilakos et al 2012] [Voloshchenko et al 2020] [Volpi et al 2017] [Von Karman 1929] [Wang et al 2021] [Wang et al 2021] [Wang et al 2021] [Waugh 1968] [Weinig 1936] [Weston and Humpheson 2024] [Whitnah and Howes 1971] [Wibowo et al 2026] [Wu et al 2019] [Wulandari et al 2025] [Xia et al 2024] [Xie et al 2023] [Yang and Qiu 2021] [Yuan and Wang 2018] [Yulianti et al 2023] [Zan et al 2023] [Zan et al 2026] [Zeraatgar et al 2023] [Zheng et al 2023]

36 titles name porpoising, and 20 of them date from 2000 or later, so the instability the tank programme mapped is still studied. The water landing of capsules and other spacecraft is the subject of 13 titles, kept because their impact relations are the seaplane relations.

Search as a planning problem. Coverage path planning, cooperative and swarm search, and persistent surveillance by unmanned vehicles.

184 records. [A Novel Framework to Efficient Path Planning Through Real-time Cost Map Generation Using Neural Networks for Search and Rescue Missions 2024] [Adoni et al 2026] [Ai et al 2021] [Ai et al 2025] [Aksaray et al 2015] [Almadhoun et al 2019] [Bai et al 2026] [Baras and Dasygenis 2023] [Baras and Dasygenis 2023] [Bezas et al 2022] [Bouras et al 2022] [Bucci and Ridolfi 2026] [Cabreira et al 2019] [Cai et al 2022] [Cao and Wu 2026] [Cao et al 2022] [Causa et al 2018] [Chatterjee and Issac 2022] [Chen and Yu 2024] [Chen et al 2022] [Cho et al 2021] [Cho et al 2021] [Choi et al 2025] [Civicioglu 2025] [Conceição et al 2026] [Coombes et al 2018] [Coverage Path Planning Method of Unmanned Aerial Vehicle for Aircraft Surface Detection Task 2023] [David and Harrison 2023] [Deng et al 2025] [Ding 2025] [Dogru and Marques 2022] [Duan et al 2024] [Duan et al 2026] [Fan et al 2024] [Fang et al 2024] [Felsner et al 2021] [Fevgas et al 2022] [Gainer Jr. et al 2018] [Gao et al 2026] [Ghaziani et al 2026] [Gong and Lee 2023] [Gramajo and Shankar 2017] [Guastella et al 2019] [Guastella et al 2019] [Güven and Yanmaz 2024] [Hazra et al 2018] [He and Wang 2024] [He and Wang 2024] [Hu et al 2026] [Hu et al 2026] [Hua and Chen 2025] [Huang et al 2018] [Huo et al 2021] [Hwang et al 2025] [Höffmann et al 2023] [Ibrahim et al 2026] [Jayalakshmi et al 2025] [Jayalakshmi et al 2025] [Ji et al 2022] [Jiang and Han 2026] [Jiang et al 2024] [Jiao et al 2024] [Kannan and Bae 2025] [Karimi and Saghafi 2024] [Karthik and Balasubramanian 2024] [Karthik et al 2025] [Kazemdehbashi and Liu 2025] [Khaneghaei et al 2026] [Kim 2026] [Kumar and Kumar 2023] [Kumari and Srirangarajan 2025] [Kyriakakis et al 2021] [Kyriakakis et al 2022] [Le et al 2022] [Lee and Kim 2017] [Lee and Lee 2025] [Lee et al 2024] [Li and Wang 2026] [Li et al 2016] [Li et al 2025] [Li et al 2025] [Li et al 2025] [Liang et al 2021] [Liu and Hao 2024] [Liu et al 2018] [Liu et al 2019] [Liu et al 2024] [Liu et al 2024] [Liu et al 2026] [Luna et al 2022] [Lv et al 2026] [Majeed and Lee 2019] [Matlekovic and Schneider-Kamp 2023] [Morin et al 2023] [Mu et al 2023] [Muñoz et al 2021] [Nedjati et al 2016] [Nguyen-Ngoc and Giang 2026] [Oh et al 2025] [Ohki et al 2015] [Optimal coverage path planning and object detection for UAV-based surveillance systems 2025] [Pan et al 2025] [Park et al 2020] [Pehlivanoglu and Pehlivanoglu 2021] [Piciarelli and Foresti 2020] [Portugal and Rocha 2015] [Pu et al 2025] [Qi et al 2026] [Ramezani et al 2024] [Reimer et al 2026] [Rios et al 2025] [Sabag et al 2025] [Sandamurthy and Ramanujam 2019] [Scherer and Rinner 2020] [Shah et al 2022] [Shang et al 2020] [Sharifi et al 2015] [Shi and Qin 2016] [Sun et al 2024] [Sun et al 2025] [Tan et al 2023] [Tang 2023] [Tang and Ma 2024] [Tang et al 2025] [Tang et al 2026] [Torres et al 2016] [Tsiakas et al 2024] [Van Pham and Moore 2018] [Vasquez and Merchán-Cruz 2022] [Wang 2023] [Wang et al 2023] [Wang et al 2025] [Wei and Isler 2018] [Wei et al 2024] [Wu 2023] [Wu 2025] [Wu and Marais 2026] [Wu and Zhang 2023] [Wu et al 2024] [Wu et al 2024] [Wu et al 2025] [Xia et al 2023] [Xiao et al 2021] [Xiong et al 2023] [Xiong et al 2025] [Xu and Zhu 2025] [Xu et al 2021] [Xu et al 2021] [Xu et al 2023] [Xu et al 2025] [Xue et al 2026] [Yang et al 2022] [Yang et al 2023] [Yang et al 2025] [Yang et al 2025] [Yang et al 2026] [Yanmaz 2023] [Yao et al 2025] [Ye 2025] [Ye et al 2026] [YongBo et al 2017] [Yu 2023] [Yu and Lee 2023] [Yu et al 2023] [Yu et al 2025] [Yuan et al 2026] [Zabransky et al 2016] [Zeng et al 2025] [Zhan et al 2024] [Zhang and Wan 2025] [Zhang et al 2023] [Zhang et al 2023] [Zhang et al 2024] [Zhang et al 2024] [Zhang et al 2025] [Zhang et al 2025] [Zhao 2024] [Zhao et al 2022] [Zhao et al 2024] [Zheng et al 2025] [Zong and Lin 2026] [Zou and Zhao 2023] [Zuo et al 2020] [徐 2019]

Every one of these 184 records dates from 2015 or later. The subject did not exist in this form when the X-28A flew, which is the point the review above makes.

Small unmanned aircraft for patrol and observation

The pilot removed. Small unmanned aircraft for patrol, observation and inspection, their design, control, propulsion and certification, and the detection of targets in their imagery.

165 records. [Abiko et al 2023] [Abramov et al 2015] [Advanced Control Techniques for Unmanned Aerial Vehicle (UAV) Navigation and Flight Control 2023] [Al-Kaff et al 2020] [Aleisa et al 2023] [Analysis of the relationship between mass, speed and cost of an unmanned aircraft 2024] [Approach to conception and modeling for distributed hierarchical control for autonomous drone swarm 2024] [Arafat et al 2024] [Bai and Zhu 2026] [Bai et al 2025] [Benaouali and Boutemedjet 2024] [Benavides et al 2020] [Benyeogor et al 2020] [Brazinskas et al 2016] [Bryson et al 2016] [Byrne et al 2017] [Cai and Cui 2026] [Chakraborty and Kumar 2019] [Chang and Lai 2025] [Chen and Lin 2025] [Chen and Liu 2025] [Chen et al 2020] [Cheng et al 2024] [Choi 2020] [Chrétien et al 2016] [Cooper et al 2015] [Design and Fluid Flow Analysis of Unmanned Aerial Vehicle (UAV) 2015] [Devasia and Lee 2016] [Dinc 2016] [Donateo and Ficarella 2017] [Dou and Duan 2016] [Duan et al 2024] [Dudziak et al 2020] [Erdman and Mitchum 2013] [Eubank and Atkins 2011] [Eubank et al 2010] [Fajar and Arifianto 2018] [Fire Detection Using Unmanned Aerial Vehicle 2023] [Fukuda and Takimoto 2013] [Fukui et al 2020] [Gavrilovic et al 2018] [Ghasemi et al 2025] [Giernacki et al 2019] [Gopejenko et al 2026] [Han et al 2019] [Harvey and O’Young 2016] [Hasan et al 2018] [Hebbar and Langbort 2021] [Henderson 2023] [Hiba et al 2020] [Hlinková et al 2022] [Hochstetler et al 2016] [Hodgson et al 2017] [Hu and Lanzon 2018] [Hu and Yang 2022] [Huang et al 2023] [Hvala et al 2023] [Internal Model Control Tuned Proportional Integral Derivative for Quadrotor Unmanned Aerial Vehicle Dynamic Model 2020] [Ji et al 2021] [Jiao and Abdullah 2024] [Jimenez et al 2016] [Junaidin et al 2026] [Junos and Khairuddin 2024] [Keilman 2019] [Kim 2018] [Kim and Atkins 2022] [Kim et al 2026] [Kisabo et al 2017] [Konert and Kasprzyk 2024] [Krampf 2024] [Kuantama et al 2023] [Kucharski et al 2026] [Kulanthipiyan et al 2023] [Kuwamura and Hara 2017] [Lai 2024] [Large and Pesyridis 2019] [Lawn et al 2021] [Lei and Liu 2019] [Leslie et al 2024] [Leylek and Costello 2015] [Li et al 2019] [Li et al 2021] [Li et al 2025] [Lijia et al 2019] [Lin et al 2026] [Liu and Zhou 2019] [Liu et al 2019] [Liu et al 2020] [Liu et al 2023] [Liu et al 2026] [Long Nhat Dang 2026] [Luo and Zhu 2025] [Made et al 2026] [Mathur and Atkins 2023] [Matt et al 2025] [McMahon et al 2021] [McNeal 2016] [Melnichuk 2024] [Mihalik and Keane 2022] [Minwalla et al 2016] [Mosov et al 2020] [Mousavi et al 2024] [Mrusek 2021] [Muniraj and Farhood 2019] [Müller and Moormann 2023] [Odzer et al 2022] [Oktay and Eraslan 2024] [Onushkin et al 2022] [Oshin Mittal et al 2024] [Panta et al 2018] [Papadopoulos et al 2021] [Papadopoulos et al 2022] [Papageorgiou et al 2018] [Park et al 2021] [Parshin et al 2019] [Patria et al 2021] [Patterson 1989] [Peterson et al 2019] [Qi et al 2025] [Rajmohan and Ramasubramanian 2023] [Rostami 2024] [Sabatini et al 2015] [Sajith Kumar et al 2019] [Sawake 2026] [Sells and Crossley 2023] [Shah Alam and Oluoch 2021] [Shakir 2017] [Shao et al 2026] [Shetty et al 2022] [Shoro 2021] [Shuang and Zhang 2021] [Simmons et al 2019] [Singh 2021] [Stevenson et al 2020] [Tian et al 2026] [Tijani et al 2015] [Truong et al 2024] [Trusfus and Abdullin 2021] [Veeraperumal Senthil Nathan et al 2025] [Venkataraman and Seiler 2019] [Victor Antonio and Yan 2023] [Wang and Yi 2025] [Wang et al 2021] [Wang et al 2022] [Wang et al 2024] [Wang et al 2025] [Ward et al 2023] [Watts] [Winslow et al 2018] [Wood et al 2019] [Xiao and Zhen 2017] [Yamaura et al 2024] [Yenpiem et al 2025] [Youngblood et al 1984] [Yuan and Duan 2021] [Zaitseva et al 2023] [Zhang 2026] [Zhang et al 2017] [Zhang et al 2025] [Zhao and Chen 2024] [Zhao et al 2021] [Zhao et al 2023] [Zhao et al 2024] [Zhou 2016] [Zhou and Yu 2024]

158 of the 165 records date from 2015 or later, and 44 titles name detection, which is the processing task the review above identifies as the new binding constraint.

Maritime surveillance, search and rescue and domain awareness

The mission under its modern names. Maritime surveillance and domain awareness, search and rescue at sea and on land, coast guard patrol, and the sensors and planning that serve them.

138 records. [Abhinav Potineni 2022] [Adaptive Sea-Clutter Model Based Detection for Coastal Surveillance Radar 2024] [Ahmed and Yilmaz 2026] [Ajith and Jolly 2021] [Alhaqbani et al 2020] [Aman Eddine and Guliyeva 2024] [Ambroziak et al 2021] [Amelin 2022] [Bai et al 2025] [Bal Şahin et al 2026] [Bany Abdelnabi and Rabadi 2024] [Barus et al 2026] [Bell et al 1978] [Bhuva et al 2024] [Bialas et al 2026] [Bjurling et al 2026] [Blackburn and Joslin 2021] [Bone 1988] [Braverman 2021] [Bushuev et al 2017] [Caelli et al 2018] [Cao et al 2022] [Cardona and Calderon 2019] [Chen et al 2024] [Chinnasamy et al 2022] [Chow et al 2019] [Ciccone and Ceruti 2025] [Collignon et al 2025] [Currie 1999] [Dacey et al 2022] [Dahal et al 2021] [Damanik et al 2025] [DePalo 2005] [Department Of The Army Washington Dc 1980] [Dinh et al 2019] [DiVittorio 2010] [Dol 2020] [Dou 2024] [Eney 1981] [Et. al. 2021] [Ewers et al 2023] [Ewers et al 2025] [Feldens Ferrari and Chen 2020] [Forti et al 2022] [Freeman and Pritchett 2025] [Freitas et al 2017] [Garate et al 2021] [Geng et al 2025] [Gopal 2020] [Han et al 2024] [Hansen 2001] [Haq 2024] [Hashemzadeh and Kumbasar 2024] [Hayat et al 2020] [He et al 2026] [Hirschkron et al 1979] [Ho et al 2022] [Horyna et al 2022] [Hou et al 2019] [Hypher 1980] [Ikrami 2018] [Ilcev 2019] [Jayaweera and Hanoun 2021] [Jeong 2025] [Jung et al 2019] [Kapadais et al 2019] [Karst et al 2025] [Kopyt and Czaplińska 2025] [Kundid Vasić and Papić 2022] [Kupriyenko and Zakharchuk 2025] [Lavrenov et al 2016] [Lee and Ha 2023] [Lee et al 1985] [Legarde 2023] [Li et al 2025] [Looney 2009] [Lupanchuk 2022] [Lyu et al 2023] [Macwan et al 2015] [Magalhães et al 2025] [Malyszko et al 2025] [Mazher et al 2018] [Milanović et al 2021] [Mills 1988] [Moe 2007] [Mohta 2021] [Moir et al 2023] [Moon et al 2023] [Mukesh Kumar Ojha et al 2024] [Mullarky 1990] [Nasios and Vogklis 2025] [Nathisiya and Saji 2026] [Nauwynck et al 2019] [Nelson 2004] [Newell and Herrick 1979] [Ng 2013] [Novitasari 2026] [Novotný 2020] [Nozica et al 2023] [Pa et al 2024] [Palsson 1975] [Pargaonkar 2022] [Quero and Martinez-Carranza 2025] [Ramachandran and Sangaiah 2021] [Rashid et al 2025] [Recommended facilities for search 1946] [Rockwood 2020] [Rossmo et al 2019] [Rossmo et al 2023] [Sadati 2025] [Sambolek and Ivasic-Kos 2021] [San Juan et al 2018] [Secretary Of The Air Force Washington Dc 2000] [Setiawan et al 2026] [Shen et al 2018] [Son and Lee 2025] [Stasik and Szandala 2025] [Study on attitude heading reference system for an Unmanned Aerial Vehicle in an aerial surveillance scenario 2022] [Submarine Search and Rescue Study Based on Digital Elevation Model (DEM) and Probabilistic Grids 2024] [Suseno and Wardana 2021] [Thompson et al 2026] [Vincent-Lambert et al 2023] [Vragov 2022] [Watkins 1991] [Watkins and Penrod 1978] [Wei et al 2023] [Weldon and Hupy 2020] [Whitcomb 2006] [Williams 1997] [Yang et al 2020] [Yoo and Jung 2017] [Yukhno et al 2020] [Zell et al 2026] [Zhang and Zhu 2022] [Zhang et al 2023] [Zhang et al 2023] [Zhang et al 2026] [Zhao et al 2025]

78 titles name search and rescue and 7 name maritime domain awareness, so the humanitarian search problem is the larger part of what the patrol requirement has become.

Cost-effectiveness, affordability and design to cost

The requirement stated as a price. Cost-effectiveness analysis, life cycle and operating cost, cost estimating relationships, procurement cost and design to cost.

128 records. [andNa 1979] [Adams and Hinders 1974] [Al-Shamma and Ali 2022] [Al-Shamma and Ali 2024] [Anderson 1978] [Anderson 1986] [Anderson and Dixon 1975] [Ankenbruck 1967] [Ascani and Lackman 1977] [Ascani and Lackman 1979] [Beltramo and Anderson 1977] [Beltramo et al 1977] [Beltramo et al 1979] [Blackwell and Eastlake] [Blumstein 1965] [Bodoh and Widmeyer 1965] [Booz-Allen And Hamilton Inc New York 1957] [Brown 1978] [Brown 1978] [Camin and McKenzie 1980] [Carleton 1979] [Carlyle 1976] [Castagne et al 2008] [Chen 2023] [Chen et al 2019] [Chen et al 2020] [Chen et al 2021] [Chen et al 2023] [Cherry 1975] [Collins and Williamson 2013] [Computer program to perform 1973] [Corlis et al 2023] [Cost Effectiveness and Cost-benefit Analysis: Session Report 1970] [Cost-Benefit and Cost-Effectiveness Analysis 1981] [Cost-Estimating Relationships for Aircraft 1972] [Cost-utility Analysis, Cost-effectiveness Analysis, Budget Impact Analysis 2020] [Cramer et al 1998] [Davis et al 1978] [Della Vecchia et al 2022] [Denz et al 2007] [Department Of The Air Force Washington Dc 1980] [Department Of The Air Force Washington Dc 1990] [Department Of The Air Force Washington Dc 1994] [Department Of The Air Force Washington Dc 1995] [Department Of The Army Washington Dc 1990] [Department Of The Army Washington Dc 1998] [Department Of The Navy Washington Dc 1985] [Department Of The Navy Washington Dc 1991] [Diez and Iemma 2007] [Edwards 1975] [Egorov 1972] [Eijsink and Rice 1978] [Emero and Alvey 1967] [Enthoven 2019] [Etesse 1979] [Etherington 1985] [Evolution and Design Philosophy Performance 1967] [Fioriti and Vercella 2019] [Fioriti et al 2018] [Fox 1965] [Gille et al 1990] [Gluckman and Celmins 1993] [Gordon 1974] [Grand 2026] [Greer 2010] [Greer et al 1984] [Gregory and Wilcox 1970] [Grosse 1965] [Haifeng et al 2014] [Hartley 2015] [Hays et al 1975] [Herd et al 1965] [Herlitz 2025] [Heymont et al 1965] [Hiller Aircraft Corp Palo Alto Ca 1955] [Himpler 1975] [Indriyanto and Hartalita 2016] [Johnson 1989] [Johnson 1990] [Jones 1965] [Keating 1979] [Knorr 1966] [Kulkarni and Rosen 1977] [Kundu et al 2002] [Luchkov 2020] [Ma and Wang 2022] [Markowitz] [Marx et al 1995] [Miller et al 1974] [Navarro and Smith 1970] [Neal 1975] [Neiss and Brown 1967] [Novick 1979] [O’Neill et al 1966] [Olason 1969] [Orlansky and String 1977] [Ozdoeva and Smolyakov 2020] [Ozkar and Genc 2025] [O’Mahony 2020] [Palmer 1993] [Pohya et al 2018] [Laracy and Marlowe 2019] [Roy and Baskey 2022] [Sassone 1979] [Scanlan et al 2002] [Schildhauer 1948] [Scholz et al 2021] [Schuetz 1982] [Sendi 2020] [Shahriar et al 2022] [Sharp 1966] [Shoemaker 1981] [Stahl et al 1984] [Stewart and Campion 1980] [Sundararajan 2013] [Sweet 1982] [Sweet and Curtis 1981] [The Aircraft Life Cycle 2013] [Tierney et al 2021] [Tirole 1984] [Wagner 1977] [Watkins and Tsiao 2020] [Wicke and Pohya 2018] [Wijk et al 2017] [Xu et al] [Yates 1965] [Zhao et al 2011] [Zhao Xiaojia et al 2015]

Low-cost manufacture, composites and materials

The material list as a requirement. Composite and sandwich construction, additive manufacturing, low volume and local production, and manufacturing cost.

104 records. [A Review on Properties of Aerospace Materials Through Additive Manufacturing 2017] [Abc 2023] [Abe et al 2020] [Ackermann and Haase 2023] [Adamski 2015] [Adlin et al 2020] [Akpan et al 2023] [Alexopoulos et al 2016] [Ancient 1968] [Application Requirement Analysis of Additive Manufacturing Technology in Aircraft Structure 2024] [Arbeiter et al 2020] [Bezek and Williams 2023] [Biele and Mönch 2019] [Bonnín Roca et al 2019] [Buffum and Thompson 1978] [Canziani et al 2020] [Cardeal et al 2021] [Carneiro et al 2020] [Centea and Nutt 2015] [Chang et al 2024] [Chen and Smith 2021] [Chen et al 2020] [Chen et al 2022] [Chen et al 2022] [Chen et al 2022] [Chittaranjan . V and Muni Sekhar 2017] [Comparative Study on the Microstructure Evaluation of Al-Mg (Er5356) Aluminum Alloy for Aircraft Wing Stiffener Fabricated By Wire Arc Additive Manufacturing 2023] [Crispo and Kim 2026] [Cubillo et al 2016] [Cui et al 2021] [Dittmann et al 2024] [Dolev et al 2021] [Donelli et al 2025] [Dong et al 2021] [Ellis 1957] [Fałek et al 2017] [FEATURES OF THE COST MANAGEMENT SYSTEM AT AIRCRAFT MANUFACTURING ENTERPRISES 2021] [Fette et al 2016] [Fiber-reinforced plastic unitized boat hull frame 1978] [Griffiths et al 2024] [Gu 2022] [Harkins 1979] [Hassan et al 2017] [Hueber et al 2016] [Hueber et al 2019] [Iryani et al 2019] [Isaac and Ezekwem 2021] [Jain 2022] [Jeon et al 2023] [Kraemer 1938] [Krueger 2017] [Kumar and Supale 2023] [Kumar et al 2020] [Li et al 2020] [Li et al 2023] [Liu et al 2024] [Liu et al 2024] [Lomo et al 2023] [Mahadik and Masel 2018] [Marinelli et al 2020] [McGurk et al 2024] [McKenna et al 2019] [Merrett 2016] [Meyne 1975] [Minetola and Galati 2018] [Morse et al 2022] [Nam et al 2016] [Noyes 1966] [Onuike et al 2018] [Oshima and Takano 2024] [Oyesola et al 2020] [Oyesola et al 2022] [Park 2016] [Patterson and Grenestedt 2018] [Prasad et al 2022] [Rai et al 2017] [Rao et al 2021] [Rao et al 2025] [Ricci et al 2021] [Rui et al 2026] [Sahasrabudhe et al 2015] [Shevtsov et al 2016] [Shuvo 2019] [Sorrentino et al 2016] [Sunny et al 2021] [Tamura et al 2017] [The Influence of the Distribution of Alloying Elements on the Structure and Properties of Aircraft Engine Parts in Additive Manufacturing 2026] [Thompson et al 2019] [Viscusi et al 2020] [Vishnu Prashant Reddy et al 2018] [Vistein et al 2019] [Wang and Song 2025] [Wang et al 2019] [Wang et al 2021] [WOOD AND PLASTICS 1944] [Wu 2017] [Wu et al 2023] [Yi et al 2024] [Yim and Mah 2024] [Yin et al 2025] [Zawaski and Williams 2020] [Zhang et al 2026] [Zhou et al 2019] [Кожевников and Мингалева 2023]

49 titles name additive manufacturing or printing and 4 name wood or balsa, so the contemporary literature of cheap manufacture has moved to processes the Osprey’s builder did not have.

Search theory, visual detection and sweep width

Koopman’s subject. Search theory, detection probability, the lateral-range curve and sweep width, and the visual detection of targets by observers.

85 records. [Arkin 1965] [Arnold and Bram 1962] [Baldwin 1973] [Bechtel and Amos 1975] [Beltrami and McGill 1966] [Bloomfield et al 1978] [Bucklin 1971] [Chiacchia 2020] [Chiacchia and Houlahan 2023] [Chiacchia and Scelza 2023] [Chiacchia et al 2015] [Chiacchia et al 2025] [Dezeeuw 1995] [Dillon 1981] [Dobbie 1968] [Engel 1952] [Erickson 1965] [Erickson and Gordon 1970] [Eriksen 1954] [Finlay 2024] [Finlay 2026] [Forrest 1993] [Franklin and Whittenburg 1965] [Geri 2010] [Geri et al 1999] [Glen et al 2018] [Grace 1992] [Green 1958] [Guter 1964] [Hakimipanah et al 2025] [Hong 2026] [Hopson 1979] [Huntoon et al 1979] [Jacob et al 2018] [Jacobson 1978] [Jewell 1985] [Johnson 1966] [Koester 2020] [Koester et al 2017] [Kurdyla 1963] [Kuznetcov et al 2021] [Levine et al 1969] [Loane et al 1964] [Loffi et al 2016] [Loffi et al 2021] [Luria 1982] [Mallett and Brennan 1963] [Mangel and Cope 1979] [Mangel and Thomas 1979] [Maruyama 1970] [Matsuo and Kuroiwa 1969] [Mehlis 1973] [Meitzler et al 2005] [Mela 1961] [Metz et al 1976] [Nicoll and Hsu 1995] [Oatman 1965] [Peterson and Graham 1974] [Pierce 1978] [Poe and Arthur C. 1974] [Posner and Rumsey 1967] [Richardson 1968] [Richardson and Belkin 1972] [Rivers et al 1980] [Scheiner 1999] [Scinto 1986] [Shnidman 1975] [Shubert 1975] [Snyder et al 1974] [Stone et al 1959] [Stromquist and Stone 1981] [Swerling 1960] [Swerling 1966] [Swets et al 1955] [Task and Verona 1976] [Teichner and Krebs 1970] [Tishchenko and Artyshchenko 2020] [Vallerie and Link 1968] [Vance et al 2017] [Volkmann and Corbin 1965] [Volkmann et al 1964] [Wallace and Vance 2019] [Wallace et al 2018] [Washburn 1978] [Woo et al 2020]

54 of the 85 records date from 1982 or earlier, so this cluster, unlike the coverage planning that succeeded it, is mainly period work.

Riverine warfare, patrol and Southeast Asian rescue

The war the aircraft was bought for. Riverine warfare and patrol, coastal and inland waterway craft, patrol by seaplane, and the Air Force reports on rescue in Southeast Asia.

20 records. [Anderson 1966] [Bassett 2006] [Benbow et al 2006] [Bonk 2023] [Brunskole et al 1997] [Coggins 2002] [Durkee 1968] [Fishery Patrols by Seaplane 1923] [Francis and Nelson 1974] [Lassaline et al 1967] [Mitchell 1971] [Overton 1969] [Radian Inc Alexandria Va 1993] [Sandway 2010] [Seaplane Forest Patrol 1921] [Sloss et al 1969] [Stilwell 2013] [Tilford 1975] [Wenderlin and Cutler 1989] [Wilson 1972]

9 of the 20 titles name Vietnam, the Mekong, Cambodia or Southeast Asia, and 16 of the 20 are report primaries. No title in the survey names Operation Market Time or Operation Game Warden, the coastal and river patrol operations of the war.

Where the Framing Breaks Down

The canal explanation is a clean inference, and clean inferences call for care. Twelve inches of clearance each side is a striking coincidence, and the article treats the canal as having set the span. No document says so. Pereira designed the aircraft for the sport market before the Navy saw it, so the causation cannot run the way the framing implies. What is defensible is that the requirement and the aeroplane matched, and that the Navy chose an aircraft whose span already fitted. Even the span is not settled. The encyclopaedia entry gives twenty-three feet and ninety-seven square feet, while Osprey Aircraft lists twenty-four feet nine inches and a hundred and three square feet, which would leave an inch and a half on each side of a twenty-five-foot canal.

The endurance calculation depends on a fuel weight that was never published. Ninety pounds is an assumption. The percentages comparing spans are robust to it, because it cancels, but the absolute hours are not.

The area-per-dollar comparison is the strongest claim and the softest number. Unit costs for the Bronco and the Orion are era-approximate, sweep widths are assumed, and neither aircraft’s cost includes operation. The ratio would have to be wrong by two orders of magnitude to change the conclusion, so the claim stands, but it remains a ratio between estimates.

The programme was cancelled for reasons outside the aircraft. The war ended. Nothing in the evaluation caused the cancellation, so this article cannot say the X-28A failed, and it also cannot say it would have succeeded.

And the aircraft was tested on the Delaware River in autumn, not on a canal in the Mekong Delta. Tails Through Time places the tests on the Delaware. The five-week evaluation says the aeroplane flew well near Philadelphia. It does not say what it would have done in the place it was bought for.

The Source Base

The vehicle’s own record is thin and the physics record is deep, which is the same shape as the previous article.

No test report exists in the open literature. A search of the National Aeronautics and Space Administration’s Technical Reports Server for X-28A, for Sea Skimmer or for the Osprey flying boat returns nothing, and a search for Air Skimmer returns only unrelated records. No record in this survey carries X-28, Pereira, Sea Skimmer or Air Skimmer in its title. The evaluation’s findings survive as a sentence in secondary sources.

The vehicle facts come from secondary aviation history and they disagree with one another. The aircraft’s designer, its names, and its current museum are each stated differently by different sources, and the article reports the disagreements rather than resolving them.

The physics comes from primary literature about everything except the aircraft. The National Advisory Committee for Aeronautics tank tests, the search-theory reports, the light-aircraft performance work and the cost-analysis literature are all richly documented, and the article is built on those. The evidence for this subject is the physics rather than the vehicle, as it is for the X-27.

Period coverage, with counts alongside fractions because either alone misleads.

  Count Fraction
Report primaries 782 33.8 percent of all records
Period, through 1982 928 40.4 percent of dated records
Contemporary, 2015 onward 1,107 48.2 percent of dated records

The research survey holds 2,311 research records drawn from Crossref, the NASA Technical Reports Server and the Defense Technical Information Center, of which 782 are report primaries, being 33.8 percent, by the identifier rule set out under The Definition of Primary. The median publication year is 2010. Of the 2,297 dated records, 928 were published in 1982 or earlier, being 40.4 percent, and 1,107 were published in 2015 or later, being 48.2 percent. The records fall into 12 clusters, set out under The Contemporary Literature.

Every figure in this article is given as a count and a fraction together, because a fraction moves with its denominator and a count does not, so either alone can suggest a change in coverage that is only a change in the size of the survey.

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 National Advisory Committee for Aeronautics tank reports that carry the hull argument are report primaries under that rule, and so are the agency’s general aviation reports. The rule does not reach the period journal literature. Of the 928 records published in 1982 or earlier, 295 are journal, society and conference papers registered with Crossref, which are primary in the ordinary sense and are not counted, so 33.8 percent is a lower bound.

Thin Subjects

Eight 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.

Subject Title terms counted Records
the span limit of a confined operating site folding wing, folding wings, wing fold, wing folding, span limit, span constraint, trailer, trailerable, canal, canals 0
the beam loading coefficient of a planing hull beam loading, beam loadings, load coefficient, loading coefficient 6
the resistance hump of a hull hump 0
the static thrust of a propeller static thrust 18
the minimum-power speed and the endurance optimum minimum power, maximum endurance, minimum drag speed, minimum power speed 4
sweep width and the lateral-range curve sweep width, sweep widths, lateral range 11
design to cost design to cost 12
homebuilt and amateur-built aircraft homebuilt, home built, amateur built 5

The keystone constraint has no literature. No title in the survey names a folding wing, a span limit, a trailer or a canal, so the argument that a twenty-five-foot ditch set the aeroplane rests on the programme’s requirement and the arithmetic above alone.

The beam loading coefficient appears in titles only as an impact parameter. All 6 titles that name it concern water landing or impact, and none concerns take-off resistance, which is the use this article makes of it.

The hump is named in no title. The hump resistance ratios rest on the general tank tests, which report resistance across the whole speed range without naming the peak in their titles.

Static thrust is a small literature and mostly not about light aircraft. Of the 18 titles, 5 name an aeroplane, an aircraft or an airboat, and the rest treat propellers, rotors and ducted propellers on test stands.

The finding that the minimum-power speed lies below the stall has no published counterpart. The 4 titles that name minimum power or maximum endurance include 1 on propeller aircraft, and none on a speed optimum that the stall makes unreachable.

Sweep width is rarely measured from the air. Of the 11 titles, 1 names air-to-surface search, and 5 name a land search, a dog team, a missing person or mountain terrain.

Design to cost was a large-aircraft discipline. None of the 12 titles that name it concerns a light or small aircraft, so the X-28A’s cost-first requirement is an early instance that no title in this literature names.

The Osprey’s actual descendants have almost no research literature. Only 5 titles name homebuilt or amateur-built aircraft, and none names the Osprey or its designer.

What the Survey Admits

A record is admitted only when a person reading its title finds it on this article’s subject. Many words this subject depends on also name something else, and a record that shares only such a word is excluded. Cost-effectiveness and affordability also name the pricing of drugs, treatments, diets, housing, schooling and public services. Planing also names the surgical planing of skin and teeth and the planing of metal, and amphibian also names a frog, so developmental biology is excluded. Porpoising also names a dolphin’s or a penguin’s swimming, the pitching of racing cars on the road and the braid of a shielded cable. Hydrodynamic also names the physics of bearings, torque converters, blood cells, shock waves and explosives, and the hydrodynamics of displacement ships, tankers, submarines and underwater vehicles shares nothing with a planing hull but the water. Coverage also names the reach of a wireless network, probability of detection also names the nondestructive evaluation of materials, random search also names an optimisation method, and patrol also names ground robots, air-defence combat air patrol and police training. Visual search also names a laboratory task in perception psychology, the Mekong Delta also names a region with its own literature of hydrology, agriculture and society, and the surveillance of aircraft by air traffic control, security cameras and the watch kept on satellites are excluded with them. Autonomous surface vessels share the water with this subject and none of its aeronautics, and are excluded. Wind turbines, wave energy converters, offshore platforms and coastal hydraulics share the fluid mechanics of a propeller or a hull and nothing else, so they are excluded as well. Correction, corrigendum and erratum notices, news items, conference titles, dictionary headwords and the chapters and appendices of manuals are excluded because a survey counts research works, and travel narratives and heritage studies of seaplane bases are excluded because they are not about the aircraft. A review of a book, a glossary of terms, a reference or definition entry of two pages or fewer and a second deposit of a document already in the survey are not admitted either, while an authored encyclopedia article of more than two pages and a textbook chapter are research works and are kept.

Some records of doubtful relevance are kept. Cost-effectiveness methods stated without a medical subject, aerial surveys of wildlife that estimate a probability of detection from an aircraft, marine propeller theory and propeller tip-vortex cavitation, the human factors of visual search at sea and from the air, coverage path planning for ground, indoor, inspection and underwater robots, sweep-width studies of searches on land, the hydrodynamics of amphibious ground vehicles, additive manufacturing process studies with no stated aircraft application, the water landing of spacecraft, high-altitude long-endurance aircraft among the endurance literature, and period trade-journal descriptions of individual seaplanes each share a method, a mechanism or a vehicle 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. The doubtful classes named above are the places where that judgement could reasonably have gone the other way.

Epistemic State

Historical fact, well attested.

  • George Pereira designed the Osprey I in Sacramento from 1969, and it first flew on 12 August 1970.
  • The Navy purchased the aircraft in 1971, designated it X-28A, and evaluated it that autumn at Philadelphia, with a last flight on 22 October 1971.
  • The Naval Air Development Center study required visual flight rules, light weight, local manufacture, and a cost under five thousand dollars in quantity.
  • The programme ended as the Vietnam War wound down, with no further military examples.
  • The design was developed into the Osprey II, of which several hundred have been built.

Reported but not independently corroborated here.

  • That the purchase was made on 27 July 1971, that testing ran from 16 September 1971 from the Philadelphia Naval Base on the Delaware River, and that it was restricted to 300 feet.
  • That the study also set a thousand-pound trailer limit and canals as narrow as twenty-five feet.
  • That the evaluation found the aircraft easy to fly and to master by pilots of limited experience.
  • The published dimensions, weights and performance, none of which is a measured test figure.
  • That more than five hundred Osprey II aircraft have been built and flown.

Contested in the sources and left contested.

  • Who designed the aircraft also called the Sea Skimmer, and whether that name attaches to the aircraft or to the Navy project.
  • Which museum holds the airframe. Two different institutions are named by otherwise careful sources.
  • The span and wing area, given as 23 feet and 97 square feet by one source and as 24 feet 9 inches and 103 square feet by the manufacturer.

Engineering analysis, computed here and reproducible from the stated inputs.

  • The maximum lift coefficient of 1.189 inverted from the quoted minimum speed.
  • The drag polar, the maximum lift-to-drag ratio of 10.71, and the loiter power.
  • That the minimum-power speed lies below the stall speed, at a ratio of 0.984.
  • The endurance against span, and the twenty-three percent the canal cost.
  • The static thrust, the hump margin of between 2.75 and 4.59, and the beam loading coefficients.
  • The sea-state limit from wavelength against hull length.
  • The coverage rates and the area-per-dollar comparison.

Inference, labelled as such.

  • That the twenty-five-foot canal requirement and the twenty-three-foot span are related. The numbers fit and the chronology argues against causation in the obvious direction.
  • That the climb figure was measured at a weight below the quoted gross. It is the explanation that requires the fewest other things to be wrong.

Not established.

  • What fuel the aircraft carried, which every endurance figure here depends on.
  • What the hull’s beam, deadrise or step depth were.
  • Whether the aircraft would have been usable in the theatre it was bought for.
  • Whether any number of these aircraft could have been manned, which the cost argument quietly assumes.

Out of Scope

  • The Osprey II and GP-4, Pereira’s later designs, treated here only as the programme’s consequence.
  • The full history of riverine operations in Vietnam, which the references cover.
  • The V-22 Osprey, an unrelated aircraft that shares only the name.
  • Modern unmanned maritime patrol systems in detail, surveyed here rather than analysed.
  • The economics of homebuilt aviation, a separate subject.

Conclusion

The X-28A was specified by its price, a unit cost under five thousand dollars, and the price was the requirement rather than the result.

The Navy asked what five thousand dollars buys. The answer, computed rather than asserted, is about a hundred square miles of water examined per hour, for about nine hours, from an aeroplane that fits down a twenty-five-foot ditch and can be towed home behind a truck. Per dollar spent that is roughly a hundred and thirteen times what a maritime patrol aircraft delivers, even after giving the larger aircraft a twenty-mile radar against the Osprey’s one-mile eyeball.

The constraints that shaped it were almost all non-aeronautical, and they cost something measurable. The canal, on this reading, set the span, the span set the aspect ratio, and the aspect ratio cost about a quarter of the endurance a slightly wider aeroplane would have had. The trailer limit and the canal together pushed the minimum-power speed below the stall speed, so the aircraft could not reach the bottom of its power curve and loitered at a stall margin instead.

What was never a problem was the water. At ten pounds per horsepower the aircraft cleared its resistance hump by a factor of between two and a half and four and a half, across every hull the record permits it to have had. A flying boat’s characteristic difficulty does not arise at this weight, and the real water limit was a two-second chop rather than a resistance curve.

The programme ended because a war did, not because the aeroplane failed. Its evaluation concluded that it was easy to fly, and then the requirement it answered ceased to exist.

It is also the fourth consecutive designation in this series that did not go to a purpose-built research aeroplane. The Navy did not commission an X-28. It bought one, from a boat designer, off the water.

The next article treats the X-29, the Grumman forward-swept-wing demonstrator, which returns the series to an aircraft built from a clean sheet to answer a question that could not be answered any other way.

References

Books

Reference

Related Post

Research