Performance and Loads Study of a High-Speed Compound Helicopter

An aeromechanics analysis was conducted of a large-winged, single main rotor, compound helicopter modified from the AH-56 Cheyenne, in cruise and high-speed flight (250 knots) at sea level and high altitude (20,000 ft.) conditions. Performance and representative loads were evaluated with the comprehensive code RCAS to show the effect of compound configuration decisions. Suitability of the analysis for high advance ratio predictions was demonstrated through comparison to the UH-60A slowed rotor test data, and validation of compound performance prediction was shown with AH-56 Cheyenne test data. An assessment of the role of compound configuration, collective setting, wing pitch, rotor speed, altitude and trim control strategy on performance and loads was made. The study shows how reducing collective, for a constant wing pitch, is beneficial for peak L=De and reducing loads. Increasing wing pitch, at a constant collective, improves peak L=De, but can reduce L=De at high airspeeds and limit maximum airspeed. A 30% reduction in rotor speed achieved significant performance increases, while larger reductions limited high airspeed and high altitude trim due to stall. High altitude flight improves L=De at all airspeeds if stall on the prop rotor can be prevented. Ailerons and horizontal stabilizer trim control does not adversely affect L=De, but free rotor cyclics to trim for reduced flap bending loads. Interference effects from the rotor on the wing reduced peak L=De, but the effect is marginal above 170 knots. The final compound configuration achieved a peak L=De = 7.6 around 140 knots and L=De > 5.0 at the maximum airspeed considered of 250 knots. The static control derivatives for the compound showed less authority than a conventional helicopter, however increasing control authority is shown across the flight envelope.