FREQUENCY-RESPONSE METHOD FOR ROTORCRAFT SYSTEM IDENTIFICATION WITH APPLICATIONS TO THE BO-105 HELICOPTER

A comprehensive frequency-response method for rotorcraft system identification is presented. The overall concept is to (1) extract a complete set of non-parametric input-to-output frequency responses that fully characterizes the coupled helicopter dynamics, and (2) conduct a nonlinear search for a state-space model that matches the frequency-response data set. Each major element of the procedure is reviewed. A new method for combining the results of multi-input frequency-response analyses obtained from a range of spectral windows into a single optimized response is presented. This method eliminates the need for manual optimization of windows. An integrated user-oriented software package for the frequency-response method is described: Comprehensive Identification from FrEquency Responses (CIFER). CIFER is used to identify a 6-DOF (quasi-steady) model of the DLR BO-105 dynamics from flight test data at 80 knots. This model is acceptable for frequencies of up to 13 rad/sec. A 9-DOF hybrid model is identified that includes coupled body/rotor-flapping and lead-lag dynamics, and is accurate to 30 rad/sec. An application of the models to flight control design shows that the maximum roll rate gain is limited by the destabilization of the lead-lag dynamics.