DYNAMICS CONSIDERATIONS FOR HIGH SPEED FLIGHT OF COMPOUND HELICOPTERS

The advantages of augmenting the lift and propulsive thrust by the addition of a wing and an auxiliary propeller on a helicopter with a gross weight of 20,000 lb are studied. A compound helicopter analysis model is developed using the comprehensive rotor analysis code UMARC. The influence of rotor flexibility, airfoil tables, and wake modeling are included. Parametric studies of the influence of the wing lift and propeller thrust on the fuselage attitude, power required, vibration levels and the control settings are presented for three rotor RPMs (100%, 90%, and 80%) for a range of speeds from 100 to 220 knots. Lift compounding results in a more nose-down attitude of the helicopter while thrust compounding leads to a more nose-up attitude. Use of modest levels of lift and thrust compounding is beneficial even at modest high speeds as 130 knots and can lead to power reductions of the order of 20% and to vibration reductions of about 40%. At speeds above 175 knots, it is necessary to use both lift and thrust compounding to maintain a level fuselage attitude. From the combinations studied, a flight profile is selected for speeds up to 220 knots that results in level fuselage attitude, low power, and low vibrations.