HIGHER HARMONIC BLADE PITCH CONTROL FOR HELICOPTER VIBRATION REDUCTION: A FEASIBILITY STUDY.
MASSACHUSETTS INST OF TECH CAMBRIDGE AEROELASTIC AND STRUCTURES RESEARCH LAB
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The concept of eliminating most helicopter vibration by changing certain rotor harmonic airloads and blade motions with harmonic pitch control is discussed in detail. A simple method of obtaining the required pitch harmonics by periodic tilting of the swashplate is presented. Attention is given to the origin and characteristics of four problems expected in applying the pitch control concept shed wake reduction of harmonic pitch effectiveness, high blade bending moments, high blade bending moments, high loads in the pitch control linkages, and the need for adjustment of the pitch control as flight condition changes. Numerical results are given to show the effect of third harmonic pitch on the cockpit vibration caused by a three-bladed articulated rotor at moderate forward speed. Several examples are given to illustrate the strong influence of shed wake on the effectiveness of harmonic pitch variations. Finally, the smooth single-minimum character of vibration dependence on pitch harmonic magnitude and phase suggests that inflight adjustment by steepest-descent type search is feasible. Author