Fluid Power and Control Requirements for the U. S. Army Heavy Lift Helicopter

This paper is a summary of the characteristics and development program for the U.S. Army heavy lift helicopter (HLH) fly-by-wire flight control system. The evolved control system consists of a triple channel direct electrical linkage which replaces the mechanical controls between the cockpit and the rotor control actuators and a triplex digital automatic flight control system. The direct electrical linkage has been successfully tested in a demonstration helicopter.

The test helicopter utilized triple hydraulic driver actuators to control existing power actuators. In the HLH, the driver and power functions will be integrated into a single rotor control actuator consisting of a triple control stage and dual power stage. Transmission-driven dual hydraulic systems at each rotor head supply the two power stages and two of the control stages. A separate electrically driven hydraulic pump supplies the third section of the triplex control stage. Hydraulic power for flight control operation when rotors are not turning is supplied by two pumps at each rotor head driven by an auxiliary power unit and the same aforementioned electrical pump. Hydraulic system redundancy is enhanced by interconnections between forward and aft rotor heads. The interconnecting lines are normally closed by shutoff valves which open in case of a failure at either rotor head.