Assessment of rotor blade angle of attack from experimental inflow data

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 flight-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 partnership with Bell Helicopter Textron, Inc., for a larger, aeroelastically scaled, bearingless rotor. A two-component laser velocimeter was used to make these measurements in the facility. The data from these 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 are documented. The results indicate that assessing inflow using the time average of a rotor revolution does not always capture adequately the local effects of blade passage on the inflow distribution. The inflow distribution should be assessed using the average of the blade azimuth-dependent inflow. At advance ratios less than 0.30, the effects of the individual trailing vortices were evident in the inflow measurements, and they have a significant impact on the local blade flow angle. For higher advance ratios, the blade azimuth-dependent inflow velocities differed very little from the time-averaged inflow characteristics. The analyses demonstrated that dynamic twist is significant for aeroelastically-scaled rotors and must be measured or modeled to assess the rotor performance accurately.