Assessment of Rotor Blade Angle of Attack from Experimental Inflow Data

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A series of experiments was conducted by NASA Langley Research Center and the U.S. Army, over a 10-year period beginning in the mid-1980s, to provide some insight into the nature of helicopter rotor-system-induced velocity and to provide calibration data for many promising computational methods. Rotor-induced velocities were measured above the rotor system for several forward e ight-test conditions in the NASA Langley Research Center 14 by 22 Foot Subsonic Tunnel using a 15%-scale, fully articulated, stiff-in-torsion rotor and in a joint government/industry partnershipwith Bell HelicopterTextron,Inc.,foralarger,aeroelastically scaled, bearingless rotor. Atwo-componentlaservelocimeterwasused to makethesemeasurements in thefacility.Thedata fromthese tests have been published previously as NASA quick release reports and conference papers, and they are available electronically. Further analyses of the data to assess the local angle of attack of the rotor blades as a function of azimuth, span, and test condition aredocumented. The results indicatethat assessing ine owusing thetime average ofa rotorrevolutiondoesnotalwayscaptureadequately thelocaleffectsofbladepassageontheine owdistribution. The ine ow distribution should be assessed using the average of the blade azimuth-dependent ine ow. At advance ratioslessthan0.30, theeffectsof theindividual trailing vorticeswereevidentintheine owmeasurements, andthey have a signie cant impact on the local blade e ow angle. For higher advance ratios, the blade azimuth-dependent ine ow velocities differed very little from the time-averaged ine ow characteristics. The analyses demonstrated that dynamic twist is signie cant for aeroelastically scaled rotors and must be measured or modeled to assess the rotor performance accurately.