Prediction of Pilot-Induced Oscillations (PIO) Due to Actuator Rate Limiting Using the Open-Loop Onset Point (OLOP) Criterion

Rate limiting has been a causal or contributing factor for Pilot Induced Oscillations (PIO). As part of the joint Air Force Institute of Technology/Test Pilot School (AFIT/TPS) program, an examination of the Open-Loop Onset Point (OLOP) criterion, developed by DLR German Aerospace, was conducted to see if it could predict PIO due to rate limiting and to evaluate its potential as a design tool. The criterion was applied to three previous flight test programs involving rate limiting. Findings from this analysis led to the HAVE OLOP flight test which was flown on the NF-16D Variable Stability In-flight Simulator Test Aircraft (VISTA). HAVE OLOP evaluated four longitudinal configurations with rate limiting elements inside the feedback loop. OLOP was found to over-predict PIO in some cases when using maximum stick amplitude as DLR suggests. When using actual stick amplitudes, correlation between OLOP predictions and PIO ratings was good. A new metric called stick ratio was developed to help explore the full range of stick amplitudes when using OLOP. OLOP could be a useful design tool, but because of the strong influence of stick amplitude, engineering judgement would have to be exercised. Recommendations on its use as a design tool are presented.Air Force Inst of Tech Wright-Patterson AFB OH is the author of 'Prediction of Pilot-Induced Oscillations (PIO) Due to Actuator Rate Limiting Using the Open-Loop Onset Point (OLOP) Criterion', published 2001 under ISBN 9781423530572 and ISBN 1423530578.