Slowed rotor idea takes flight

March 2, 2011
The concept is envisioned as a versatile military support aircraft

by  Ron Ossi, Contributing Editor

A recent ‘AHS International’-managed powered-lift (vtol) conference in Philadelphia was the scene for discussions about a multitude of new and evolved ideas on future vertical lift aircraft.  One curious development is the SR/C (slowed rotor/compound) proposed by AAI Corp.

The concept is envisioned as a versatile military support aircraft in such roles as unmanned cargo delivery, “flying jeep,” or UCAV.  The AAI  SR/C , basicallly a hybrid of an airplane and an autogyro, attempts to exploit the advantages of both basic machines in one aircraft for specific, particular segments of flight operations.

 The conventional autogyro derives all of its lift from a main, helicopter-type rotor powered by a combination of propeller wash and forward flight.  The successes of autogyro inventor Juan de la Cierva proved the concept 90 years ago. Nevertheless, it is its highly successful derivative, the helicopter, that has really set the mark for vertical lift.

The SR/C differs in that it is categorically a conventional, winged airplane that utilizes an adjunct main rotor as essentially only a take-off and landing device.  In forward flight the main rotor auto-rotates, as on the autogyro, but is controlled to provide only about only 10% of total lift.  The major lift component\complement comes from the sizable main wing.  Two additional features - a clutch and ballast weights - individualize the SR/C.  Although not totally exclusive to the SR/C, the rotor-drive clutch, when engaged (through a low-capacity transmission) to the engine at flight start-up, lets the rotor be revved up to its maximum ~250 RPM while the aircraft remains stationary on the ground.  The ballast weights are not installed for balance purposes but (with 75 lb on each blade tip) to provide for the substantial flywheel effect of a high inertia rotor.

So, with the rotor at full RPM and the clutch now disengaged, pulling collective pitch causes the machine to leap into the air.  Lift can last 15 seconds; plenty of time airborne for the directly-engine-driven propeller to accelerate the aircraft to a consistent airborne sustaining-lift flight speed.  On return to base, the opposite control is easily performed.  As the aircraft slows to negligible flight speed, the high inertia rotor is employed for a vertical set-down just like a normal autorotating helicopter or autogiro.

Why this solution?  As an unmanned system, unlike many others of such types, this aircraft requires neither ground-based launch nor recovery machinery.  As for its typical and prime-intended potential mission, cargo field delivery, the SR/C promises delivery cost reductions of 85 to 90% with respect to expensive and complicated currently employed aircraft (CH53 \ V22).  For such a typical mission, the SR/C could carry 1,600 lb. 650 miles at 285 mph; under an installed 650 hp.  The technology, says AAI, may be applied to similar prospective systems covering a vast scaling range of 10 to 10,000 lb payload.

 Basically, it is to be a relatively inexpensive aircraft with an accompanying low operating and support cost.

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