APPLICATION OF SEVERAL METHODS FOR DETERMINING TRANSFER FUNCTIONS AND FREQUENCY RESPONSE OF AIRCRAFT FROM FLIGHT DATA

摘要

In the process of analyzing the longitudinal frequency-response characteristics of aircraft, information on some of the methods of analysis has been obtainedjby the Langley Laboratory of the National Advisory Committee for Aeronautics. In the investigation of these methods, the practical applications and limitations were stressed.nIn general, the methods considered may be classed as:(l) analysis of sinusoidal response, (2) analysis of transient response as to harmonic content through determination of the Fourier integral by manual or machine methods, and (3) analysis of the transient through the use of least-squares solutions of the coefficients of an assumed equation for either the transient time response or frequency response (sometimes referred to as curve-fitting methods).nThe investigation has led to the following observations:The curve-fitting methods (Donegan-Pearson and exponential-approximation methods) appear to be less critical to inputs having regions of low harmonic content than Fourier methods and present the frequency response as analytical expressions (transfer functions). Fourier methods indicate characteristics of frequency response that may be missed in curve-fitting methods because of the limitations on the assumed form of the equations used in the curve-fitting methods. For manual calculations, the Donegan-Pearson method appears to be best suited for highly damped systems in response to arbitrary control inputs, the exponential-approximation method appears to be best suited for lightly damped systems in response to step or short-pulse control inputs, and the Fourier method offers comparable results but requires lengthy calculations. Special machines for performing the Fourier analysis, such as the Coradi harmonic analyzer and the Fourier sythesizer, reduce the time required for the solution but do not offer particular improvement in accuracy over the usual manual methods. The use of punch-card calculating machines for the evaluation of the Fourier integrals appears to offer possibilities of more accurate results with a large reduction in timeover usual manual methods.