• Version
• 178 Downloads
• 2.70 MB File Size
• 1 File Count
• November 2, 2016 Create Date
• November 2, 2016 Last Updated

National Advisory Committee for Aeronautics, Report - Application of Several Methods for Determining Transfer Functions and Frequency Response of Aircraft from Flight Data

In the process of analyzing the longitudinal frequency-
response characteristics of aircraft, information on some of the
methods of analysis has been obtained by the Langley Laboratory
of the National Advisory Committee for Aeronautics. In the
investigation of these methods, the practical applications and
limitations were stressed.

In general, the methods considered may be classed as: (1)
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 solu-
tions of the coeflicients of an assumed equation for either the
transient time response or frequency response (sometimes
referred to as curve-fitting methods).

The investigation has led to the following observations: The
curve-fitting methods (Donega/n-Pearson and exponential—
approrimation 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 ofiers
comparable results but requires lengthly calculations. Special
machines for performing the Fourier analysis, such as the
Coradi harmonic analyzer and the Fourier synthesizer, reduce
the time required for the solution but do not offer particular
improvement in accuracy over the usual manual methods.
The use of punchcard calculating machines for the evaluation
of the Fourier integrals appears to offer possibilities of more
accurate results with a large reduction in time over the usual
manual methods.