A Nonlinear Pre-Filter to Prevent Departure and/or Pilot-Induced Oscillations (PIO) Due to Actuator Rate Limiting

Closed loop instability caused by excess phase lag induced by actuator rate limiting has been suspected in many aircraft departures from controlled flight and pilot-induced oscillations (PIO). As part of the joint Air Force Institute of Technology Test Pilot School (AFITITPS) program, a nonlinear rate limiter pre-filter (RLPF) was developed to minimize the phase lag induced by rate limiting. RLFF performance was evaluated inside the feedback path, but primary emphasis was on the pilot command path. Closed loop computer and motion-based flight simulation were conducted to prepare for the flight test. The HAVE FILTER flight test project was flown using the NF-16D Variable Stability in-flight Simulator Test Aircraft (VISTA) aircraft and evaluated using a software rate limit (SWRL) with and without an RLPF on the pilot command path. A programmable heads-up-display (HUD) was used to generate a fighter tracking task. Flight test results showed the SWRL was useful in preventing departure and/or PIO. However, with low SWRL settings (\u3c40 deg/sec) handling qualities deficiencies were uncovered due to sluggish initial pitch response. The RLPF plus SWRL combination resulted in more departure and/or PIO protection than the SWRL alone. But with low SWRL settings (\u3c40 deg/sec) significant handling qualities deficiencies were sometimes found