Impact of Controller Delays on the Nonlinear Dynamics of Remotely Piloted Aircraft

Abstract

Time delays arise in most feedback systems and have specific relevance for remotely piloted vehicles with ground-based pilots and controllers. NASA’s test facility for flight dynamics and control research using subscale vehicles, the Airborne Subscale Transport Aircraft Research facility, has developed the remotely piloted generic transport model. Analysis of a numerical model of this has previously provided insight into open-loop upset dynamics and the impact of flight controllers. However, to date, studies have not considered the effect of time delay on the system’s stability. Current developments at the Airborne Subscale Transport Aircraft Research facility are aimed at testing a subscale generic airliner model during loss-of-control conditions over extended distances and altitudes compared to the generic transport model. In developing controllers for such a remotely piloted vehicle, it is helpful to understand the effect of delay in the communication links and protocols. This paper uses bifurcation analysis to evaluate the effect of delay on the closed-loop stability for the generic transport model numerical model with a linear quadratic regulator controller with proportional and integral components. The impact of time delays in both fixed-gain and gain-scheduled versions of the controller are presented in terms of the stability of nominal and offnominal solutions. This is followed by a discussion of stability maps, again generated by bifurcation analysis, which can be used to assess, over a wide flight envelope, the maximum acceptable delay before instability arises.