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A Theory for Longitudinal Short-Period Pilot Induced Oscillations

Report Number: AFFDL-TR-77-57
Author(s): Smith, Ralph H.
Corporate Author: Systems Research Laboratories, Inc.
Laboratory: Air Force Flight Dynamics Laboratory
Date of Publication: 1977-06
Pages: 141
Contract: F33615-77-C-3011
Project: 8219
Task: 821904
AD Number: ADA056982

Abstract:
This report presents a theory for longitudinal, short-period, pilot-induced oscillations (PIO). The theory explains how the airplane's pitch attitude and normal acceleration modes can couple with pilot dynamics to produce large-amplitude, uncontrollable oscillations. The effects of control system dynamics and feel system nonlinearities are encompassed by this theory. It is concluded that an airplane's potential for PIO can be determined based entirely on linear systems analysis; the prediction of fully-developed PIO frequency and amplitude requires that all significant nonlinearities be considered. The theory postulates that PIO can develop either as a result of closed loop control of pitch attitude or from abrupt control or atmospheric inputs of size sufficient to excite a lightly damped, dominant, stick-free airplane mode. A number of PIO case histories are examined; it is shown that these confirm the proposed theory. The implications of the theory to flight test and to simulation are discussed. A generalized definition for PIO is given which permits distinctions to be made between pilot-vehicle system oscillations due only to attitude control and those due to attitude plus path control modes. It is suggested that the theory can be easily extended to the study of lateral-directional PIO. A bibliography of PIO source material is included with this report.

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