Vol. 25 No. 19
Able to fly like a conventional airplane but hover, take off and land like a helicopter, the military version of the tiltrotor aircraft, the V22 Osprey, has already been flight tested. These aircraft have oversized front-facing propellers -- rotors -- when cruising from place to place, which tilt upward to become horizontally oriented, helicopter-like rotors for takeoff and landing.
However, much must still be done before commercial tiltrotor aircraft can take off from downtown urban vertiports and fly the short hops now serviced by commuter airlines.
"With the tiltrotor aircraft, we will no longer need to have commuter planes following 757s on the runway at airports like Washington National," Edward C. Smith, assistant professor of aerospace engineering, said.
Dr. Smith and Anna Howard, a graduate student in aerospace engineering, are working with NASA to develop analysis techniques to design composite rotor blades for commercial tiltrotor aircraft that will help reduce blade stresses, lower hub vibration and improve reliability.
The tiltrotor is seen by many as a replacement for commuter aircraft of 50 or fewer seats. The aircraft will directly connect cities, especially in such densely populated areas as the Northeast corridor, via downtown vertiports. Tiltrotor flights will also replace many short commuter hops that now connect airline hubs.
The conversion from military to commercial passenger aircraft is, however, more complex than a paint job and softer seats. The V22 Osprey is designed to be bulletproof, and the wings and rotors fold for storage on board a ship. Advanced composite materials make up most of the Osprey's fuselage and wings.
While the commercial version need not fold or be bulletproof, it will need to be quieter -- inside, for the comfort of passengers and outside, to gain community acceptance. Like turboprops, the frame and skin of the fuselage will probably be aluminum, but the wings will remain composite. Composites, and especially how they are arranged, are the key to controlling vibration and providing added stability, Dr. Smith said.
"Tiltrotors feature many safety improvements compared to conventional helicopters," Dr. Smith said. "The tiltrotor configuration, with one set of rotor blades on each wing tip, does not require a tail rotor, stabilizing the performance of rotorcraft for all weather, day and night operation."
The helicopter industry has also eliminated many of the hydraulic couplings for the rotors, instead using non-moving, non-fluid substitutions to improve maintenance and reliability, and provide damping to control noise and vibration. The researchers believe that tailored composite materials used at the rotor hub and in the blade could replace damping mechanisms and improve acceptability of the tiltrotor.
Common composite materials are epoxy and graphite, glass or Kevlar. These materials are produced in thin layers that are laminated to create the required flexibility and strength. Composites usually do not possess the same characteristics in all directions. They may be very flexible in one direction but rigid in the perpendicular direction. "By carefully designing the orientations of the thin layers, the bending and twisting motions of the blade can be coupled together, thereby reducing vibration and improving stability of the rotor system," Dr. Smith said.
Dr. Smith and Ms. Howard have designed analytical tools and completed a feasibility study of coupled tailored composites in tiltrotor craft rotors and blades. This includes comprehensive finite element models of rotor blades. The researchers are currently developing a refined structure for flexible beams for bearingless rotors.
By A'ndrea Elyse Messer
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