ICocurek TanglerThe application of lifting surface theory to the caiculation of rotor hover performance, and the development of an improved prescribed wake representation resulting from schlieren flow visualization studies of model rotor wakes are described. Qualitative comparisons between lifting line and lifting surface methods show the tendency of the lifting line to react excessively when a vortex passes close to a blade. The flow visualization studies reveal wake sensitivity to thrust coefficient, number of blades, and twist not identified in previous Investigations. The need to include a recirculation mechanism in the analytical model t o provide inflow in addition to that available from the prescribed wake structure is established. A possible source of the recirculation is demonstrated to be the result of vortex interaction and wake expansion immediately below the welldefined near wake. NOTATIONAR = Blade aspect ratio, R/c b = Number of blades c = Blade chord CI = Section lift coefficient CQ = Rotor torque coefficient, torque/pnR3-( a m 2 CT = Rotor thrust coefficient, t h r~s t / p n R ' ( n R )~ D = Aerodynamic influence coefficient k,,k2 =Axial slope of tip vortex trajectory before and after passage of the following blade n = surface panel unit normal vector r = Radial distance from axis of rotation Presented at the 32nd Annual National Forum of the American Helicopter Society, May 1976. r = Collocation point position vector R = Rotor blade tip radius V = Free stream velocity x,y,z = Rectangular coordinates zt = Axial coordinate of wake relative to tip path plane, positive up a, = Blade section effective angle of attack r = Tip vortex circulation strength 8 , = Blade linear twist, washout negative 8 ,, = Blade collective pitch a t 0.75R K = Strength of circulation distribution A =Wake contraction rate parameter p = Air density o = Rotor solidity, bc/nR I) b = Azimuth angle between blades $ w = Wake azimuth angle relative t o blade = Rotor rotational speed Subscripts i = Blade collocation point index j = Vortex box index te = Trailing edge
Constant speed/pitch rotor operation lacks adequate theory for predicting peak and post-peak power. The objective of this study was to identify and quantify how measured blade element performance characteristics from the Phase VI NASA Ames 24m×36m80ft×120ft wind tunnel test of a two-bladed, tapered, twisted rotor relate to the prediction of peak and post-peak rotor power. The performance prediction code, NREL’s Lifting Surface Prescribed Wake code (LSWT), was used to study the flow physics along the blade. Airfoil lift and drag coefficients along the blade were derived using the predicted angle of attack distribution from LSWT and Phase VI measured normal and tangential force coefficients. Through successive iterations, the local lift and drag coefficients were modified until agreement was achieved between the predicted and Phase VI measured normal and tangential force coefficients along the blade. This agreement corresponded to an LSWT angle of attack distribution and modified airfoil data table that reflected the measured three-dimensional aerodynamics. This effort identified five aerodynamic events important to the prediction of peak and post-peak power. The most intriguing event was a rapid increase in drag that corresponds with the occurrence of peak power. This is not currently modeled in engineering performance prediction methods.
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