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Pilot Induced Oscillations are still a serious safety problem in aviation. Especially in regard of the
continuous evolution of modern fly-by-wire flight control systems, PIOs seem to occur more
frequently. Although test pilots, flight test engineers and handling qualities specialists have dealt with
this phenomenon over the past three decades, it still is difficult to apprehend and all too often it
catches pilots as well as engineers by surprise. This report gives a brief overview of the mechanisms
and the contributing factors in pilot behaviour, in aircraft dynamics and in the environment that lead to
a PIO-condition.
A great effort has been made over the years to develop reliable tools, analytically as well as
experimentally, which are capable of identifying PIO-prone and PIO-resistant configurations. Five of
the most acknowledged, state-of-the-art frequency and time domain criteria for evaluating PIO-susceptibility, based on linear aircraft dynamics, are introduced and compared. These are the Neal and
Smith Criterion (original definition), the Bandwidth/Pitch-Rate Overshoot Criterion, the Smith-Geddes Criterion, the Gibson Phase Rate Criterion and the Gibson Dropback Criterion. These Criteria
are applied to two selected flight conditions of a linearised, small perturbations model of the F-4C (Phantom II) aircraft, based on the longitudinal equations of motion. The responses of the
mathematical aircraft model, which is developed for this purpose using the state space method, are
examined and verified with the MATLAB software package and the applicability/suitability of the
criteria for this configuration is assessed. Finally, similarities and differences in the application of the
criteria, the utilised criterion parameters and the obtained results are discussed. The objective of this
exercise is to provide a consolidated review of current criteria for longitudinal PIO-evaluation. |
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