Frequency‐Response Method for Rotorcraft System Identification: Flight Applications to BO 105 Coupled Rotor/Fuselage Dynamics

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 and significantly improves the dynamic range and accuracy of the identified frequency‐responses relative to single window methods. 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 9‐DOF hybrid model of the DLR BO 105 dynamics from flight test data at 80 knots. The identified model includes coupled body/rotor‐flapping and lead‐lag dynamics, and is accurate to 30 rad/sec. The 9‐DOF results are compared with 6‐DOF (quasi‐steady) identification results. An application of the model to flight control design shows that the maximum roll rale gain is limited by the destabilization of the lead‐lag dynamics.