Inside Karem's Optimum-Speed Tilt Rotor for JMR

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It was rumored, but never confirmed. Now we know - Karem Aircraft is one those in the running to build a demonstrator for the US Army's future utility rotorcraft. Karem is designing the TR36TD for the Joint Multi Role (JMR) demonstration, using its optimum-speed tiltrotor (OSTR) concept. JMR is the precursor to the planned Future Vertical Lift Medium replacement for the UH-60 Black Hawk.

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TR36TD (Concept: Karem Aircraft)

Those of you familiar with Abe Karem, the inventor of Predator and A160 Hummingbird fame, will recognise his approach to aerodynamic and structural design. The TR36TD has a long-span, high aspect-ratio wing and small tail for low drag in aeroplane mode. The outer section of the wing is attached to and tilts with the nacelle to reduce downwash from the large wing in the hover.

Not so obvious from the illustration, but at the heart of the OSTR concept is the variable-speed rotor design. The light and very stiff rotor blades are rigidly attached to the hub, which in turn is rigidly attached to the mast - there are no hinges and gimbals as in other tiltrotor designs. The blades are controlled individually and electronically - there is no swashplate or pitch links.

This design ensures simplicity, reduces weight and provides high control power (hence the small tail). But the rigid blades put very high loads into the hingeless hub, which is large, and from the hub into the nacelle, which is attached to the wing by a large component called the spinnion, which allows the nacelle to pivot while carrying loads from the nacelle and outer wing into the inner wing and fuselage.

On the V-22 the tilting nacelles are at the tips of the wing and wing span is limited to avoid an aeroelastic instability called whirl flutter, induced by oscillation of the nacelles. Karem's patents says the light. stiff rotors of the OSTR substantially delay whirl flutter, and allow a longer, more slender inboard wing for lower drag. The TR36TD has 36ft-diameter rotors.

Optimum-speed means the speed of the rotor is adjusted between vertical and forward flight to maintain the optimum loading on the blades to maximize propulsive efficiency. Karem's patents say the speed of the rotor can be reduced by at least 25% and as much as 40% between hover and forward flight. Another patent details a multi-speed gearbox to allow the engine to run at its optimum (high) speed while rotor speed is varied.

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TR75 Joint Heavy Lift (Concept: Karem Aircraft)

Other patents show aspects of the OSTR design that may or may not be in the TR36TD if and when it flies. The stiff blades and slender wing require lightweight composites that can withstand high compressive loads, and Karem has patented a method of producing carbon-fiber structures by pultrusion, curing them under high tensile stress to increase their ability to carry compressive loads.

Patents include a mechanism to disconnect the cross-shaft connecting the rotor systems so that, in forward flight, a failure in one rotor will not impact the other, allowing the aircraft to safely continue flight. Another is a combination of leading-edge slats and upward-deflected trailing-edge flaps to eliminate stall and buffeting on the outboard wing extensions when the nacelles are tilted upwards.

Karem and the other three teams in the JMR technology demonstration program - AVX Aircraft, Bell Helicopter and Sikorsky/Boeing - have nine months to complete preliminary design of their demonstrators, after which the Army will pick two to be built and flown in 2017. AVX has a 230kt coaxial-rotor, ducted-fan; Bell a 280kt tiltrotor; and Sikorsky/Boeing at 230kt coaxial rigid-rotor, pusher-propeller design. A production version of the TR36TD could reach 360kt, says Karem.

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