From the Wright Flyer to fly-by-wire, the phenomenon of pilot-induced oscillations (PIO) has been observed on prototype, experimental, and operational military and commercial aircraft. The introduction of irreversible control systems with surfaces driven by powered actuators brought many benefits along with increased system complexity and the introduction of additional nonlinearities. Chief among these nonlinearities are the hardware and software rate limits associated with the control surface actuators. Basic sizing tradeoffs conducted in the design process set the maximum rate of an actuator, whereas software rate limits are introduced to prevent overdriving the control surface when loads or structural limitations exist. It is demonstrated that, when operating as intended, there are usually no ill effects associated with rate limits; however, certain conditions can lead to a highly saturated condition. This results in the sudden introduction of significant added phase lags to the pilot-vehicle system. In many cases, the end result is often PIO or other related loss of control events. One of the earliest well-documented PIO events involving rate limiting occurred on the first flight of the X-15. This event is significant in that common linear systems analysis techniques do not reveal a susceptibility to PIO. The analysis of X-15 flight 1-1-5 and the results of a more recent flight research program are presented in detail.
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