VIBRATION REDUCTION WITH HIGHER HARMONIC BLADE FEATHERING FOR HELICOPTERS WITH TWO-BLADED TEETERING AND FOUR-BLADED HINGELESS ROTORS

Theoretical investigations have been conducted to compare higher harmonic blade pitch requirements for vibration reduction on helicopters with two-bladed teetering and four-bladed hingeless rotors in trimmed level flight. For the two-bladed teetering rotor, elimination of oscillatory two-per-rev hub vertical shear was studied. For the four-bladed hingeless rotor, higher-harmonic control (HHC) was used to minimize four-per-rev hub vertical shear, rolling and pitching moments. Certain penalties in pitch-link loads and performance were predicted using higher-harmonic blade feathering. The penalties seem to be more serious for a two-bladed teetering rotor than those for a four-bladed hingeless rotor. Analyses were also performed to demonstrate that optimal HHC inputs for the multiple-input case are not simply a linear combination of optimal inputs for the respective single-input cases due to interharmonic coupling. Analytical results indicate that both amplitude and phase of HHC vary significantly with airspeed for the four-bladed hingeless rotor. For the two-bladed teetering rotor, the required pitch amplitude increases as airspeed increases with nearly invariant phase angle.