HELICOPTER PERFORMANCE METHODOLOGY AT BELL HELICOPTER TEXTRON

Forward flight performance methodology currently in use at Bell Helicopter Textron is reviewed with emphasis on the two key aspects of the power required calculation, pitch attitude for trim and rotor aerodynamics. The modeling approach to trim is described, and its importance is shown by a pitch attitude sensitivity study. Contrasts among airframe component and rotor forces and moments for several cases highlight the relative influences of airframe aerodynamic characteristics and rotor/airframe interference. Rotor aerodynamics are also discussed from the point of sensitivity to the modeling approach. It is shown that improved correlation results from a more physical treatment of the blade tip loss model used in combination with unsteady, three-dimensional blade element aerodynamics. This tip model delays the onset of blade stall and reduces advancing blade negative lift and profile drag. The sensitivity of the performance calculation to airfoil data and blade elastic motion are shown, and the accuracy of simple momentum theory for prediction of power required in low speed flight is addressed. The paper is concluded by statistically evaluating the accuracy of BHT's methodology for over 70 direct theory to test comparisons drawn from 18 cases presented in the Appendix of this paper.