WIND TUNNEL TESTS AND FURTHER ANALYSIS OF THE FLOATING WING FUEL TANKS FOR HELICOPTER RANGE EXTENSION. VOLUME 2. GROUND AND AIR MECHANICAL Instability Analysis
BOEING CO MORTON PA VERTOL DIV
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Ground instability characteristics of the H-21, HUP-2H-25, and H-34 with floating wing fuel tanks were investigated through a simulated takeoff with full tanks, and a landing with empty tanks. Four modes were calculated representing over dominant motions respectively in aircraft lateral motion, wing rigid body flap, aircraft roll motion, and wing first pin-free bending. The aircraft lateral, wing flap and aircraft roll instability ranges were generally under the normal rotor speed excitation band, and the wing first bending instability range was well above it. The aircraft roll instability range and the rigid wing flap instability range were nearest the rotor speed excitation, and entered it during some portion of the takeoff or landing sequence. Two types of mechanical instability were shown to be possible in the air, one due to reference natural frequencies of the fuselagewing system on the wing aerodynamic spring, and the second type was shown to occur in helicopters without floating wings as well, but to be controlled by the normal blade lag dampers. The winged configuration of the three helicopters were shown to be stable in the normal operating rotor speed range, but the tanks empty condition was nearest to being critical.