AH-56A VEHICLE DEVELOPMENT

The paper covers the development of the rotor stability and handling qualities of the AH-56A compound helicopter. The AH-56A uses a rigid main rotor with a direct-linked control gyro and with flapping feedback. A wing and pusher propeller permit the rotor to be substantially unloaded at high speed. The solution to early problems with rotor stability is described. The first Involved a producted 2-per-rev in-plane motion times 1-per-rev flapping, called 1P x 2P which was potentially troublesome on the ground at about 80% rpm. The solution was a reduction of in-plane frequency and an increase in the flapping feedback. The second involved a dynamic Instability at high speed, involving rotor plane oscillations at near 1/2-per-rev. The solution involved an increase in collective stiffness and reduction to zero of an adverse pitch-flap coupling, or delta-three effect. The net effect of these changes has been a highly stable rotor system with nearly dead-beat response to control excitations at high speed, and with low gust sensitivity. Nap-of-the-earth flight dictated very high roll control power requirements relative to the pitch control. This was developed with the exercise of some mechanical ingenuity to permit the control gyro to "see" equal impedance in the two control axes. For hover, the roll sensitivity about trim required some reduction for best handling feel. The anti-torque rotor development showed that a pusher configuration with the forward blade going up opposite to the direction of the main rotor downwash was decidedly superior to other configuration alternatives in thrust capability, thrust/horsepower ratio, and in handling qualities in sideward and rearward flight. The development tools used included whirl-tower, flight and wind-tunnel tests, as well as analytical efforts. The paper includes the correlations between test and analysis. The development of a new aerial vehicle inevitably brings with it a considerable increase in knowledge. This knowledge should be shared so that there can result the maximum, improvement, in state of the art. In the case of the Army-Lockheed AH-56A compound rigid-rotor helicopter, advances were made in a number of areas, some particularly applicable to the gyro-controlled rigid-rotor concept of the AH-56A, and some generally applicable to all helicopters. This paper is devoted to two areas of rotor/airframe development in which new problems were encountered and solved (1P x 2P blade flapping/inplane interaction during ground operation, and subharmonic rotor instability in high-speed flight), and two areas in which new data are added – anti-torque rotor effectiveness and pilot/vehicle matching for maximum roll rates in nap-of-the-earth flight. The AH-56A, Figure 1, is a compound helicopter in which the pusher propeller and the wing permit the rotor to be substantially unloaded at high speed. The rigid rotor provides pitch and roll control at all speeds. The rotor blade feathering is controlled by the control gyro tilt. The control gyro tilt is determined by the balance of moments among those caused by pilot's control input, precession rates, and blade feathering moments, or feedback.