Karem Tiltrotor A Contender For Army Utility Role

By Graham Warwick
Source: Aviation Week & Space Technology

“Most designs let the rotor shake the aircraft then try to damp it. That is not a good concept,” says Karem. “We take the loads at the source—the blade—and do not make it flexible. And we do things with the blades as they go round so as not to create those loads. We need individual blade control, and also higher harmonic control.”

To optimize blade-loading and maximize propulsive efficiency in vertical and forward flight, rotor speed is reduced by at least 25%, and as much as 40% in some OSTR designs, between hover and airplane mode. Rather than redesign the power turbine to operate over such a wide speed range, Karem uses a multispeed gearbox to vary prop rpm while letting the engine run at its most efficient high speed.

Where the V-22 has a relatively short, thick wing to support the tilting rotors and avoid an aeroelastic instability known as whirl flutter, caused by oscillation of the nacelles, Karem notes the OSTR's light and stiff rotors delay whirl flutter and allow a longer-span, higher aspect-ratio wing for increased lift-to-drag ratio (3-4 times that of the V-22) and cruise efficiency in airplane mode. To reduce download from rotor downwash on the longer wing, the outboard wing extensions tilt with the nacelles.

Hingeless rotors provide high control authority, allowing Karem to shrink the size of the tail, which is V-shaped on the TR36TD, and reduce drag. The design uses the very high mast moments generated by the rigid rotors for pitch and directional stabilization and control, and in some OSTR designs, the tail area is just 18% of the wing area, compared with the V-22's 105%.

The blades, nacelles, wing and fuselage are made from lightweight, high-strength composite, which would be produced in large integrated pieces using out-of-autoclave processing, and Karem is projecting an empty weight 20-40% lower than the V-22's. He has patented a method of curing composites under tensile stress to increase the compressive strength of the blades and upper wing skins. Aircraft systems on a production OSTR would be all-electric.

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