3172. Advanced Tiltrotor Design


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John Lloyd: 3172. Advanced Tiltrotor Design. 2001.



Development of any new aircraft from preliminary design through first flight involves interesting design challenges and solutions. This is especially true if the aircraft represents a new niche or technology relative to government and industry requirements and experience base. The Bell-Agusta BA609 will be the world’s first production civil tilt-rotor. First flight is scheduled for later this year. This paper will address two specific configuration design issues unique to tilt-rotors faced during development and delve into the importance of weight reduction very early on the 609 program.
Determining the proper wing and pylon location, with respect to the fuselage, to achieve the maximum aircraft longitudinal operating envelope involved consideration of both fixed and rotary wing conditions. Ideally, the wing and pylon should be located such that the aircraft c.g. is well within the desired wing % M.A.C. in airplane mode, and nearly centered on the rotor in helicopter mode. The approach taken to achieve optimum location, methods of conducting the early balance estimates, and results are all addressed.
Another configuration design challenge arose from well-publicized turbine burst incidents on airliners during the 1990’s. FAA concerns on this subject required the 609 design comply with turbine burst survivability. The selection of a highly reliable PT6 engine with no recorded turbine bursts in service would not be enough to satisfy the FAA or us regarding aircraft safety. The engine and drive systems were rearranged within the nacelles to reduce the risk of aircraft loss. The cost, weight, and schedule impacts of rearranging the nacelles were weighed against the weight and physical limitations of shielding the engines. Shot-line analysis, system failure analysis, and nacelle to ground clearance were all critical drivers in the final design.
The importance of aggressive weight reduction very early in the 609 program was critical in bringing the product through development. Without many of the criteria reductions and management weight reviews, the aircraft would be too heavy to perform its missions effectively, and would likely have been cancelled before more capital was invested. Weight growth is an inherent part of a development program, and should be planned for and resisted all through the design phase. The emphasis on early weight reduction as on the 609, followed by a weight control program that is integral with design as on the V-22, reflects the best approach for the next new development aircraft.


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