14.8.2 Knob Twisting

Informal and rather elementary stability and control derivative extraction took place starting in the early 1950s, when the first electronic analog computers, such as the Reeves Instrument Company’s REAC, were used to get time histories of airplane motions. Numerical values of individual dimensionless stability derivatives, such as C„e, are represented by potentiomenter settings on analog computers. Computed airplane motions appear on pen-type recorder records. The experimenter can try to match an actual flight record for a given control input by resetting potentiometers and rerunning cases over and

over. Since potentiometer settings are controlled by knobs on the face of the analog computer cabinet, this trial-and-error process is known familiarly as knob twisting.

Knob twisting is not altogether a random process, since an experimenter is guided by approximations to the modes of airplane motion. We know, for example, that the period of the Dutch roll oscillation is controlled by the directional stiffness derivative Cn. The amplitude of the roll oscillation relative to that of sideslip or yawing velocity is controlled by the dihedral effect derivative Clp, and so on.

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