Blade Stall—Half Fact, Half Fiction

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Abstract
Blade stall is examined through experimental force results from model rotor tests and measured blade airload pressure distributions from full‐scale flight tests. These new or unpublished results are related to various model and full scale rotor test data available in the literature. Rotor power, blade flapping motion, blade bending and vibratory hub shears are compared to illustrate the practical symptoms of blade stall. For interpretative purposes aerodynamic blade stall is considered as occurring separately in lift, drag and pitching moment. As expected due to scale effects, model rotor data show drag and moment stall at significantly lower thrust coefficients than full scale rotor data, but unexpectedly these data show inconsistent evidence of lift stall. Analysis of the flight test airloads data shows that full scale lift stall is highly coupled with the blade torsional dynamic response. Comparisons between similar flight test points show that the blade section leading edge separation associated with lift stall does not occur unless the blade elastic twisting reaches a significant amplitude. Thus, moment stall which is associated with trailing edge separation must occur first to provide an excitation to the blade twisting and thus precipitate lift stall. Some rotor models tend to be stiff in torsion and have relatively rigid control systems. These model blades do not twist with sufficient amplitude to cause lift stall. There appears to be no significant reason for not providing an equivalent torsional stiffness in full scale rotors. Once this is done the significant influences of blade stall in the flapwise or thrust direction will be generally fiction. Stall influences on the blade drag and pitching moments will continue to be fact. Airfoil section development should provide dynamic stall characteristics tailored to avoid elastic responses of the blade. Incorporating this airfoil development with proper blade twist and planform will yield a major growth in rotor capability.

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