Feb 282012

The gallery above reproduces “The XP-77 Airplane,” a report by Bell Aircraft Corporation dating from around June 1944 found in US Army Air Force (USAAF) files held in National Archives II in College Park, Maryland. The report is both a review of the development work done on the Bell XP-77 wooden light weight fighter and a proposal for an improved, all-metal version with turbosupercharging. The document was written by Robert J. Woods, Chief Design Engineer, who would go on to design the legendary X-1 and other aircraft. The following paragraphs summarize the opening pages of the report and the concluding proposal section. Written from the perspective of the contractor, the report is a defense of a troubled project that would never achieve its intended objective.

The Bell XP-77 (also designated as MX-272 by the USAAF) was an attempt to produce a small size, light weight, single place, single engine fighter airplane that would be generally equivalent to contemporary fighter types in performance and military utility. Bell believed it desirable to reiterate the fundamentals behind the conception of the XP-77, hoping that some of the apparent misunderstandings regarding this airplane and its place in the fighter picture could be cleared up.

The basic consideration in the development of the XP-77 was to produce a properly developed fighter type conforming to the minimum requirements of the USAAF for minimum size and weight. The single place fighter had grown in the last ten years from the 3,000 odd lbs of the P-12 to types running between 8,000 and 20,000 pounds. This had been a logical and reasonable growth based generally on expanding power, fuel and armament requirements. A net increase in performance and fire power had resulted in a considerable increase in size and weight. Simplicity for manufacture, ease of service and maintenance, simple flight operation and “hot” acrobatics had been considerably reduced. Fuel consumption, initial cost, maintenance and storage space per operating unit had increased considerably.

Much discussion had taken place during this time as to whether an attempt should not be made to maintain the gross weight as near the 3,000 lb original value as possible because of the inherent possibility of such a smaller plane for simplified manufacture and operation, simplified maintenance and repair, simplified problems of shipment and the possible use of so-called substitute materials.

During the preceding ten years many people in responsible positions had ventured the idea that a serious effort should be made to produce a light weight, small size, single place fighter prototype. No less a person than General H. H. Arnold had at one time proposed development of a 4,000 lb, 400 mph fighter. Unfortunately, most advocacy for such an aircraft was limited to talk, and although several design studies were initiated by the Army Air Forces, none materialized.

Lack of an engine of the type required having the necessary small size, high output and production possibilities had generally prevented such a development from taking place. The development of the Ranger inverted V-12 engine to power outputs that would permit the approach of a 6 lb/hp power loading and provide critical altitude operation at 25,000 ft or more made such a small plane possible. It was, of course, necessary that the engine project be somewhat in advance of the airplane development program so that the airplane would be ready with the engine. Delays of considerable magnitude occurred to both the airplane and engine projects because of higher priority work, but by mid-1944 it appeared that the airplane and engine project could be completed fairly near together. Both the engine and airplane projects were in the final development stage, both showed reasonable promise of meeting their anticipated performance and both required the final intense push that was always required at the end of a project to properly complete development. Bell sincerely hoped that enough importance could be attached to this work to permit completion without further delay, completion being development of the tactical requirements needed for war into the airplane and engine as production prototypes.

To understand the reasoning behind the XP-77 airplane it was first necessary to understand the so-called “vicious circle” of weight and size increase that was a part of the compromise that is an airplane. As the power, range or speed of an airplane is increased and equipment or requirements added, weight is added which requires more airplane to carry it around, which further adds more weight and, therefore, generally more size. It had long been a point for discussion whether the smaller size airplane with less power would provide the utility required. If the question of using less power alone were used as a criteria, it appeared that it could not be done. If, however, the vicious circle could unwind as effectively for a reduction in power and size as it does for an increase in power and size, it would appear that comparative performance could be provided by careful design. It would be necessary, in addition, to obtain comparative utility by the use of ingenuity and invention. How successfully the XP-77 represented such an effort could be determined from the following detail description of the airplane project as it stood circa mid-1944.

XP-77 studies were made in preliminary form in the fall of 1941. These studies indicated that with careful design and 500 BHP at 27,000 ft altitude, the minimum performance, structural and tactical requirements of the Air Forces could be met in an airplane grossing about 3550 lbs, maximum speed 410 mph at 27,000 ft altitude, maximum range about 1,000 miles, and two .50 cal guns and a 20 mm cannon. A proposal was made to the Air Forces in April 1942 in which such an airplane was presented, the fuselage and tail surfaces to be made of metal, the wing of wood, which was accepted. Some modification was made to the design to use all-wood construction throughout for the airframe and to use the existing low altitude Navy model Ranger engine in the first airplanes, pending completion of the development on the higher powered, higher altitude Ranger engines.

A quantity of twenty-five airplanes was authorized which subsequently had to be reduced to six airplanes because of the limited availability of the Navy model engines. The conversion to all-wood was made on a basis that, at that time, a serious shortage in aluminum alloys appeared imminent. Promotion of wooden airframes was at its peak at this period. It was felt at the time, and circumstances have justified the opinion, that the low altitude engine would provide satisfactory operation to permit initial evaluation of the light weight type airplane without the complication of “nursing” an experimental engine. Cooling, at the beginning a most serious problem, could be studied in the low altitude engine just as well as on the high altitude unit; performance and flight operation of the airplane could be checked at altitudes up to 12,000 ft with the expectancy of being equivalent.

The change to all-wooden construction was a somewhat large bite for Bell Aircraft since the company was strictly a metal airplane constructor. The accomplishment of it required some considerable trial and error design work in the initial stages and considerable missionary work with sub-contractors adding a large increase to the original estimated costs and causing many delays in construction. In retrospect, this decision to use all-wood construction appeared to have been wrong. It is interesting to note, however, that its inclusion as a liability had not prevented satisfactory attainment of the objective in the XP-77 airplane.

The report goes on to discuss the developmental history of the XP-77 in great detail, thoroughly covering such topics as weight, structural tests, aerodynamics, test flights, performance, engine, armament and firepower, service features, construction methods and materials, facilities for shipping and handling, performance (again). To read these sections in depth, please click through the gallery above. At the end of the report, Wood proposes an improved version of the XP-77, an all-metal airplane incorporating an improved Ranger engine with either a mechanical 27,000 ft supercharger or a 36,000 ft turbosupercharger. It would have carried 106 gals total fuel, including 36 gals in an external tank for 1,500 miles ferrying range and 500 miles combat radius, with four synchronized .50 caliber machine guns in the nose and the pilot moved forward to behind the engine with the gas tank disposed aft of him; a bubble canopy for improved vision was also planned. Gross weight was estimated at 4,100 lbs. Wood claimed that such an aircraft could be built at Bell in quantities of 3 or 6 within 6 months time. The handling and flying characteristics of the XP-77 would be maintained and in some instances somewhat improved without any difficulty. The retention of a tricycle landing gear was up to the USAAF, though pilots who had flown the plane were generally positive about it. While no three-view of the improved Bell XP-77 has been found, a simplified inboard profile of the proposal was included at the end of the report. The aircraft appears to retain the wing and tail surfaces of the original XP-77, mating them to a substantially redesigned fuselage; based on these facts, a reasonably accurate three-view of the proposal could be reconstructed.

The USAAF was not persuaded by the Bell report and passed on the proposal. At the time, the XP-77 project had suffered numerous delays and cost overruns, with the AAF continuing the project solely as an experiment to evaluate the use of wooden construction and materials in combat aircraft. The first XP-77 had flown April 1, 1944 at Wright Field, with the flight tests revealing vibration problems due to the direct mounting of the engine to the airframe without vibration isolation. The long nose and aft location of the cockpit also limited visibility relative to contemporary fighters. The USAAF found the XP-77 to be seriously underpowered and difficult to fly, even without guns or armor. Additional trials were conducted at the USAAF Proving Ground at Eglin Field with the second aircraft, which was destroyed after entering an inverted spin while attempting an Immelmann, with the pilot bailing out. Development of the Bell XP-77 was terminated in December 1944.

Despite being an obscure prototype aircraft, a large number of XP-77 photos have survived and been posted online. For example, NASA has posted two impressive photo sets of full-scale XP-77 wind tunnel tests performed at Langley here and here. The Flickr Photostream of the San Diego Air & Space Museum also has some rare XP-77 images, though they are generally of low resolution. Finally, the excellent Secret Projects Forum has a thread devoted to the fighter with additional images from various sources.

Those interested in NACA’s involvement in improving the XP-77 configuration should visit the NASA Technical Reports Server, which has several interesting reports on the aircraft, including this one testing various tail configurations. Image quality varies, though the drawings usually reproduce reasonably well.

All images from NARA Archives II, College Park, MD, RG 18

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