Frequency domain testing of helicopter dynamics using automated input signals

This paper provides a description of a collaborative research project between Westland Helicopters and York University. The main object of the investigation is to provide a characterisation of all significant dynamic phenomena arising from flight test on a helicopter being flown by Westland. Complex multifrequency test signals have been applied to the aircraft actuators during flight testing. Gain and phase data derived from auto and cross spectral estimates are examined to validate mathematical models used in flight simulation and to provide information about significant nonlinearities not accounted for in the simulation model. York University has investigated complex curve-fitting techniques for obtaining estimates of linear system parameters. Magnitude-squared coherency function plots and linear system parameter estimates enable a powerful comparison of the use of different forms of test signal (such as swept sinewave, m-sequence and Schroeder phase). It is concluded that the Schroeder phase signal shows great promise for future flight application; the logarithmic swept sinewave is adequate for flight application; and frequency-domain testing is a powerful method for identifying the physical phenomena of the system under test