The analysis of wake-induced unsteady aerodynamics as related to higher harmonic control
Abourahma, Ahmed Ali Mohammed Hassan
Wood, E. Roberts
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Performance data from the NASA-Army OH-6A higher harmonic control (HHC) flight test program showed significant reductions in main rotor shaft torque and engine power in hover and forward flight 1. The unsteady aerodynamics with the higher harmonic control system application, including wake effects. were considered to study whether such power reductions are feasible. An airfoil oscillating in pure plunge can achieve propulsive force ( Katzmayr effect ), as in the case of birds' wings flapping in flight. Here it will be shown that this effect can be enhanced in the presence of layers of shed vorticity with the proper phasing. In addition, while it is known that an airfoil oscillating in pitch, will typically produce drag at most values of reduced frequency, it is found that the presence of another layer of shed vorticity of the proper phase, can reduce the drag on the pitching airfoil depending upon wake spacing reduced frequency, and phase. Under some conditions the added layer or layers of shed vorticity will even result in propulsive force acting on the pitching airfoil similar to the Katzmayr effect for the plunging case. It was found, for the OH- 6A helicopter,, that the measured reductions in main rotor shaft torque and engine power are feasible when evaluated with respect to the Katzmayr effect and the additional drag reduction or propulsive force obtained due to pitch and plunge oscillations with the effect of adjacent wake layers of shed vorticity
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