On Developing and Flight Testing a Higher Harmonic Control System

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Abstract
This paper presents results of a flight test program, using a modified U.S. Army OH‐6A helicopter, in order to evaluate higher harmonic rotor blade pitch control as a means for reducing helicopter vibrations. The higher harmonic control (HHC) system was developed by Hughes Helicopters, lnc. under contract with the U.S. Army and the National Aeronautics and Space Administration. Substantial supporl for flighl testing was also provided by Hughes Helicopters, Inc. Feasibility of the concept was first proven in wind tunnel model tests at the NASA 1angley Research Center. For the modified OH‐6A, higher harmonic bladc pitch control is achieved by superimposing 4/rev swashplate motion upon basic collective and cyclic control inputs. The aircraft was flown from zero airspeed to 100 knots with the HHC system operated both open loop (manually) and closed loop (computer‐controlled). Flighl test results presented show that the system provides a significant reduction in helicopter vibrations without undue penalties in blade loads or aircraft performance.

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