Sunday, December 1, 2013

Karem Tiltrotor A Contender For Army Utility Role

By Graham Warwick

Credit: Karem Aircraft Concept

When the Pentagon set out its Future Vertical Lift (FVL) strategy to develop a family of advanced rotorcraft to replace its fleets of helicopters originally designed in the 1960s and '70s, one goal was to engage non-traditional suppliers to bring more innovation into the sector.

With the inclusion of two startup companies among the four teams awarded contracts for the U.S. Army's Joint Multi-Role technology demonstration (JMR TD), the Defense Department has taken a step toward that goal. Bell Helicopter and a Sikorsky/Boeing team may still be the favorites to fly two high-speed rotorcraft demonstrators in 2017, but they face real competition from two relative unknowns.

AVX Aircraft had already declared its hand, previously unveiling the 230-kt. coaxial-rotor, ducted-fan compound helicopter it is designing for JMR. But Karem Aircraft was not confirmed as a contender until Oct. 2, when the Army announced the four cost-sharing technology investment agreements for the $217 million JMR TD Phase 1 flight demonstration.

Karem Aircraft was formed in 2000 by Abe Karem, designer of the Predator unmanned aircraft and A160 Hummingbird unmanned helicopter, to develop his optimum-speed tiltrotor (OSTR) concept. Along with the other teams, Karem now has $6.5 million and nine months to complete preliminary design of its JMR demonstrator, the TR36TD, after which the Army will select two designs to be built and flown.

Karem is saying little about the design, except that the demonstrator will have two 36-ft.-dia. variable-speed rotors, powered by existing turboshaft engines, and that a production version would be capable of 360 kt. in level flight—faster than Bell's 280-kt. V-280 “third-generation” tiltrotor and Sikorsky/Boeing's 230-kt. coaxial rigid-rotor, pusher-propulsor design.

But some insight into the OSTR design is available from Karem's private-venture work on the 90-seat AeroCommuter and 180-seat AeroTrain commercial tiltrotors, and concept studies of a large cargo rotorcraft, the TR75, performed from 2005-10 under the Army-led Joint Heavy Lift project. JMR is a smaller rotorcraft, the precursor to a planned replacement for the Sikorsky UH-60 Black Hawk (see page 77).

The OSTR has long, light and stiff blades rigidly attached to the hub, which in turn is rigidly mounted to the nacelle—a lighter and less complex design than the articulated and gimballed hub on the Bell Boeing V-22. Instead of a swashplate and pitch links, the blades are individually controlled by electric actuators in the hub, saving weight and increasing reliability.

“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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