A Brief Study of Conditionally Enabled Phase Compensated Rate Limiters 2008-01-2256
This paper describes how airplane control surface rate limiting can enable a ‘cliff-like’ onset of Pilot-Induced Oscillation, (P.I.O.) and how the danger can be erased by implementation of Conditionally Enabled Phase Compensated Rate Limiters, (PCRLs), in the design of the airplane's flight control system.
The application is particularly important for large airplanes where control surface actuator sizing and the associated hydraulic system volumetric flow rate capability cannot be generously over-sized without large weight and cost penalties.
It is shown that the PCRL can remain inactive during normal airplane operations where RMS control commands are relatively small thus avoiding adverse control surface response effects that have hindered earlier PCRL acceptance. Then, when RMS control commands are significantly increased due to P.I.O. onset, the onset of increased RMS control displacements can be used to enable the PCRL and remove the large control system phase loss that is causing the P.I.O. onset.
The ‘Enable/Disable’ function is transparent to the pilot and performs to significantly reduce susceptibility to P.I.O. due to control surface rate limiting.
Theoretical analyses, simulations and flight test data are presented to support the ideas being proposed and reference is made to methods other than RMS control commands for enabling and disabling the PCRL.
Even if P.I.O. is not expected to occur in a new airplane design, implementation of disarmed ‘Conditionally Enabled’ PCRLs prior to the commencement of flight testing could provide insurance against large schedule delays due to an unexpected P.I.O. encounter during the flight test acceptance program.