May 182011

Aeronautical history is littered with many unsuccessful VTOL aircraft proposals and prototypes.  Developing a practical military aircraft that takes off and lands vertically while remaining competitive with conventional types in terms of armament, handling, speed, range, payload, etc. has proven to be an enormous challenge, with only a handful of designs achieving production status.  In surveying early VTOL proposals concepts, the tailsitter projects of engineer Lloyd Hugo Leonard, employed at the NACA Laboratory at Langley Field, Virginia, must rank among the most unconventional.  Submitted both to the US Army Air Force and the Navy in 1939, Leonard’s “helicopter-type aircraft” was an interesting combination of the innovative and the impractical that prevented it from leaving the drawing board and making a lasting mark on VTOL aircraft development.

On paper, the specifications and performance of Leonard’s primary helicopter-type aircraft submission were impressive.  A fighter design, it weighed a modest 5,000 lbs.  It was powered by a twin row Pratt & Whitney radial delivering 1,000 hp.  At sea level, the estimated maximum speed of the proposal was 428 mph with a cruising speed of 371 mph.  At 20,000 ft, Leonard calculated the maximum speed to be 504 mph with a cruising speed of 436 mph.  These figures were far in excess of contemporary fighters and are quite remarkable considering the relatively low horsepower of the engine.  Its estimated rate of climb at sea level was an incredible 3,750 ft/min; at 20,000 ft, it was 3,060 ft/min.  Assuming the engine to be supercharged to sea level horsepower at all altitudes, the fighter’s maximum ceiling was estimated to be an improbable 50,000 ft.  Leonard noted that the practical ceiling would depend upon the design of the supercharger, the actual design of which was beyond the scope of his submission.  The engine drove a pair of two-bladed tandem rotors with a diameter of 40 ft.  The fuselage was only 25 ft long, giving the fighter a rather bizarre appearance.  A ducted spinner was employed to reduce drag.  The slender wing had a span of 20 ft and an area of 40 ft2, while the aircraft’s cruciform all-moving tail had a tiny area of 12.5 ft2.  Wing loading was 200 lb/ft2, exceptionally high for the period.  It is interesting to note that in calculating the tail area required to stabilize the fuselage, Leonard utilized the pitching moment data of the Akron airship hull, since the difference in fineness ratio between the two was negligible.  Transition time from vertical to horizontal flight was estimated to be an amazing 3.27 seconds.

The aircraft’s landing gear system was quite novel.  During descent, a rod would project from the rear of the fuselage that, upon contact with the ground, triggered the landing gear to snap open via pneumatic shock struts, the resulting configuration resembling a strange mechanical flower in bloom.  The landing gear was comprised of four units with small castoring wheels that offered excellent stability on the ground.  The pilot was provided with large side windows, a ventral transparency identical to the dorsal teardrop canopy and a seat capable of pivoting 90º in order to make take-offs and landings less demanding tasks.

Leonard expected criticism of his performance analysis on the grounds that the assumed gross weight of 5,000 lbs was too low to include the armament and fuel supply generally required for military service.  He therefore provided an alternate analysis of an 8,000 lb version of his fighter aircraft with the same overall configuration and engine.  At sea level, this heavier version had an estimated maximum speed of 399 mph and a cruising speed of 349 mph.  At 20,000 ft, it had a maximum speed of 485 mph and a cruising speed of 419 mph.  Though the reduction in performance was especially acute at sea level, the 8,000 lb version of Leonard’s fighter was still superior to most of its contemporaries.

A second major project called the “Helicopter Aircraft—Design B” was included with the primary submission.  Of similar concept, it differed in several important respects.  The cockpit was moved to the nose of the aircraft and the pilot was provided with a large bulbous transparency offering excellent all-around vision.  The 1,000 hp Pratt & Whitney radial and two-bladed rotors were relocated to the middle of the fuselage behind the cockpit.  The arrangement was not without precedent for a Navy aircraft, as the experimental Gallaudet D-4 of 1917 had also employed it, with mixed results.  Design B featured a nose intake to provide air to the buried radial engine.  Based on the track record of aircraft with such engine arrangements, cooling problems would have likely been significant.  The overall dimensions of Design B were smaller than the primary fighter submission, with a rotor diameter of 28 ft, a fuselage length of 25 ft 6 in and a span of 24 ft.  The aircraft appears to have had only vertical tail surfaces, calling into question its directional stability.  No detailed performance analysis accompanied Design B.  In any case, it was hampered by the same major engineering and conceptual deficiencies that afflicted its larger cousin, offering little improvement apart from the improved visibility for the pilot.

Finally, two additional minor projects were illustrated in Leonard’s submission that utilized the configuration of the basic proposal.  One is an improbable 60,000 lb, 5,000 hp transport aircraft with a pair of spherical, pivoting passenger cabins.  Each sphere (which would have been ideal for pressurization) had two floors connected by an elevator.  Multiple engines would have been required to achieve the necessary specified 5,000 hp.  No information is given regarding the proposed transport’s performance or dimensions, but judging by Leonard’s comparative sketch, it would have been about the size of a Curtiss-Wright CW-20 with rotors of enormous diameter.  The engine-coupling and rotor problems inherent with this large design made it the least realistic of Leonard’s proposals.  The other project is illustrated in a sketch depicting the operation of Leonard’s unique landing gear.  It resembles the basic fighter proposal but has a pointed spinner, suggesting a liquid-cooled engine.  The tail surfaces were put onto the landing gear doors and swung upward with them during landings.  The wing was all-moving, rotating on a cylindrical spar.  No canopy or cockpit is suggested in the sketches.  This aircraft may have been a pilotless proof-of-concept model or a primitive missile.  However, it is difficult to be certain given the lack of information in the surviving documents.

Though no official correspondence between L.H. Leonard and the Navy has been located, notes found in Bureau of Aeronautics files shed some light on the Navy’s opinion.  One Navy analyst noted that Leonard overestimated the benefit of reducing profile wing drag and increasing wing loading.  Leonard also failed to allow for protuberances, roughness, guns, radio, etc. in determining the drag coefficient.  These errors resulted in an overly optimistic figure for maximum speed.  The analyst wrote that the adverse effects of increased weight, structural and mechanical complications and the stability and control problems of Leonard’s invention appeared out of proportion to the maximum possible gain in speed over fighters of more conventional configuration.  In another analyst’s opinion, “…the propellers and pitch-changing device would prove almost impossible to handle.  This is like trying to run before we can walk.”

Army Air Force correspondence indicates that Leonard submitted data pertaining to a high-speed rotating wing aircraft in June of 1939 to the Materiel Division at Wright Field.  An analysis of the data by Paul H. Kemmer, Captain and Chief of the Aircraft Laboratory, focused on a number of deficiencies with the basic concept.  Kemmer acknowledged that while the type of aircraft envisaged by Leonard was in the realm of possibility, its realization would have presented a multitude of problems before a successful aircraft could have been developed.

  1. The problem of the helicopter as such was, by itself, difficult to solve.  The lack of mechanical details furnished by Leonard and the Army’s insufficient experience with satisfactory helicopter control prevented Kemmer from determining whether Leonard’s proposals offered the solution.
  2. Considering Leonard’s aircraft as a high speed airplane flying horizontally, the question of propeller efficiency entered into consideration.  Kemmer noted that the diameter of the propellers sufficient to operate the aircraft as a helicopter at low air stream velocities would have been too large for efficient operation at high speed.
  3. The gyroscopic effect of such large propellers would have interfered with maneuverability, which should be exceptionally good for a pursuit plane.  The effect of slipstream on the wings would have been unsymmetrical, necessitating large aileron correction and possible unsymmetrical angle of incidence of the wings.
  4. The difficulty during the transition period from the helicopter to airplane stage would have been considerable and required the development of a special control for the transition.
  5. The engine installation would have required special attention, requiring that its functioning not be adversely affected by the transition from vertical to horizontal flight.
  6. The proposed combination of landing gear with tail surface mounting did not appear desirable.

Overall, the Army’s evaluation of Leonard’s proposals was not encouraging.  However, the data submitted by Mr. Leonard was retained for future reference.  Though innovative and bold in concept, his helicopter-type aircraft were simply too complicated and impractical to get off the ground.  The Navy wouldn’t attempt to build a tailsitter until the 1950’s, with the ill-fated Convair XFY-1 Pogo and Lockheed XFV-1 Salmon turboprop convoy fighters, while the Air Force went the jet route with the Ryan X-13 Vertijet, which also never went past the experimental stage.  One cannot fault Leonard, however, for thinking outside the conventions of his time and attempting to create a new class of aircraft—it certainly would have made for quite a sight in the early 1940’s, as the accompanying drawings suggest.

Further Developments

L.H. Leonard did not give up on his Helicopter-Type Aircraft, patenting refinements of the concept for the next decade and a half; these include US patent nos. 2,387,7622,444,7812,446,4802,479,125, and 2,866,608. He also filed a patent in Great Britain, no. 600,374. US patent no. 2,866,608, filed May 18, 1955, lists him as residing in Inglewood, California; perhaps he had left NACA by this point to work for North American Aviation, which was located in the same city. He was obviously a creative and intelligent individual who undoubtedly produced other interesting concepts that have yet to be uncovered; if someone has more information on his life and the final disposition of his papers (assuming they survived), please contact us, as we’d like to find out more about the man and his work. Some additional images and discussion of Leonard’s designs (and similar tailsitter projects) can be found in threads on the always interesting “Secret Projects Forum” herehere and here.

All images from NARA Archives II, College Park, MD, RG 72 & 342


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