Optimum Two-Per-Revolution Inputs for Improved Rotor Performance

Abstract

The results of a study to determine the optimum two-per-revolution blade control inputs required to minimize rotor power, maximize thrust, or minimize rotor speed, using formal numerical optimization techniques, are presented. The impact of improved rotor modeling on the prediction of the effects of two-per-revolution input on rotor power is also addressed, in particular, the effect of using a free wake inflow model, and a flexible rotor blade model. The main conclusions of the present study are as follows: The optimization for optimum two-per-revolution input is best carried out by solving the trim problem for every value of the inputs proposed by the optimizer, as opposed to integrating the trim calculations as equality constraints in the optimization. An appropriate two-per-revolution input can increase the rotor maximum thrust at high speed by about 11%. The additional thrust is generated at the front and the rear of the rotor disk. The two-per-revolution input can also further reduce the rotor speed compared with a case without such input. The physical mechanism is the same as in the optimization for maximum thrust, and the maximum value of C T /σ is also the same. Although the power predictions obtained using a free wake model are different from those obtained with a simpler linear inflow model, the accuracy of the optimum two-per-revolution input predicted using the simpler model is adequate. The best practical strategy to optimize the two-per-revolution input is to conduct an initial study with a simple linear inflow model and then refine it with the more sophisticated one. The same conclusions hold for more sophisticated flexible blade models, compared with simpler rigid blade ones.