Identification of bearingless main rotor dynamic characteristics from frequency-response wind-tunnel test data

The control response dynamics of the Sikorsky Bearingless Main Rotor (SBMR) were determined from frequency-response test data in the NASA 40x80 foot wind tunnel, The CIFER(R) (Comprehensive Identification from FrEquency Responses) tool was used to extract the rotor's physical characteristics based on a linearized 9 degree-of-freedom analytical formulation of the SBMR dynamics. The paper reviews identification methods and results for two flight conditions (40 kts,mu = 0.093; and 100 kts, mu = 0.233), with particular emphasis placed on off-axis modeling. The identified model responses track the wind tunnel data closely, and the extracted physical parameters show excellent consistency across the flight conditions. There is also very good agreement between the identified parameters and the key GenHel simulation parameters. An empirical modeling parameter "aerodynamic phase lag" (psi(a),) is included in the identification structure that corrects the SBMR off-axis dynamic response modeling discrepancies for the wind tunnel case, and is applicable to free-flight modeling. The results indicate that the primary physical sources for the total aerodynamic phase lag are dynamic wake distortion and 2-D compressible unsteady aerodynamics, There is good agreement with theoretical predictions of these effects. A proposed modification to the 3-state dynamic inflow equations provides an alternate explicit correction for the wake distortion effect that is based on theoretical analyses. The SBMR results support the validity of rotor models based on: effective hinge-offset, dynamic flow, and the aerodynamic phase lag correction to simulate the flight mechanics responses of bearingless main rotor helicopters.