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 w ind tunnel, The CIFER(R) (Comprehensive Identification from FrEquency Respo nses) tool was used to extract the rotor's physical characteristics based o n a linearized 9 degree-of-freedom analytical formulation of the SBMR dynam ics. The paper reviews identification methods and results for two flight co nditions (40 kts,mu = 0.093; and 100 kts, mu = 0.233), with particular emph asis placed on off-axis modeling. The identified model responses track the wind tunnel data closely, and the extracted physical parameters show excell ent consistency across the flight conditions. There is also very good agree ment between the identified parameters and the key GenHel simulation parame ters. An empirical modeling parameter "aerodynamic phase lag" (psi(a),) is included in the identification structure that corrects the SBMR off-axis dy namic response modeling discrepancies for the wind tunnel case, and is appl icable to free-flight modeling. The results indicate that the primary physi cal sources for the total aerodynamic phase lag are dynamic wake distortion and 2-D compressible unsteady aerodynamics, There is good agreement with t heoretical predictions of these effects. A proposed modification to the 3-s tate 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-off set, dynamic flow, and the aerodynamic phase lag correction to simulate the flight mechanics responses of bearingless main rotor helicopters.