1991-04-01

Composite Prototype Aircraft Development A Method For Design, Fabrication and Test Training 911015

The faculty and staff of the Raspet Flight Research Laboratory (RFRL) have concluded a project with an industrial customer to lead a group of engineers and technicians in a study of the “art” of developing a prototype composite aircraft. The objective was to produce a turbine powered, composite, pressurized, single-engine aircraft, which would introduce the team to the many aspects of a complex aircraft in a environment.
The prototype Allison Soloy turbine conversion of the Beechcraft A-36 was chosen as the study aircraft. It was proposed that the A-36 structure be replaced with composite structures in three steps of increasing complexity. The sequence was to first design-build-test-fly the tail, then the wings, and finally the fuselage. To limit the difficulty of the project development and to allow a meaningful comparison between aluminum and composite structures, the configuration modifications were to be minor and to utilize existing RFRL Marvel II composite technology. The dimensions of the tail structure were maintained, and the structure converted to Kevlar and graphite. The Marvel II wing configuration was used with an all graphite structure, integral fuel tanks, and single slotted flaps. The fuselage design followed the Mael BA-42 configuration which had a RFRL ancestry. The fuselage was not constructed when the project was stopped at the completion of the wing flight test program.
The sandwich structures were fabricated using a 250 F., vacuum bag cure. All spars were autoclave cured at 250 F. 50 psi pressure. Room temperature cure adhesives were used for assembly. The prototype composite structural weights were very close to the original aluminum structures. The integration of the A-36 landing gear system into the Marvel II wing proved to be a very difficult task. The critical composite structural designs were verified with sub-assembly test before the critical structures were fabricated. A fuselage “test barrel section” was developed to confirm fuselage fabrication methods and to verify composite pressurized structures design methods.
The flight characteristics of the A-36 with the composite tail structures were identical, as expected, to that of the original configuration. The new composite wing produced a 10% reduction in power required over the A-36 speed range due to reduced surface friction and the increased wing aspect ratio. The most striking improvement was the reduction in stall speed.
This project has shown that a small team can learn much of the “art” of the aircraft development process in a short time in an intense one-on-one R & D environment with much support. It was found that planning, reporting, and 8 to 10 hours of overtime each week kept the project moving and all participants focussed. Many interesting concepts could be explored and developed at a modest cost and risk through this “hands on” study of the “art” of composite aircraft development.

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