Adaptation Of A Modern Medium Helicopter (Sikorsky S-76) To Higher Harmonic Control

Sikorsky Aircraft has performed analytical studies, design analyses, and risk reduction tests for Higher Harmonic Control (HHC) on the S-76. The S-76 is an 8-10,000 lb helicopter which cruises at 145 kts. Flight test hardware has been assembled, main servo frequency response tested and upgraded, aircraft con- trol system shake tested and verified, open loop controllers designed and fabricated, closed loop controllers defined and evaluated, and rotors turning ground and flight tests planned for the near future. Open loop analysis shows that about 2° of higher harmonic feathering at the blade 75% radius will be required to eliminate 4P vibration in the cock- pit. Analytical computer simulations of a closed loop controller have been evaluated, relative to the theses of reducing vibration to low levels while maintaining good ride quality and aircraft structural stress attributes. The analytical results, design concepts, program approach, and risk reduction tests are reviewed herein, providing a status report on HHC for the S-76.