Measurement of Pilot Describing Functions from Flight Test Data with an Example from Gemini X

It is well known that there is an error in identifying the pilot describing function from routine flight test records because the pilot's output noise is correlated with the input error signal. This paper shows that this identification error can be reduced in the computer processing by shifting the input signal an amount equivalent to the pilot's time delay. This technique for reducing the identification error is analyzed with theory and is demonstrated with the identification of a simulated pilot model. This technique is also applied to flight test records obtained from the retrofire phase of the Gemini X mission.