Nonlinear attitude control laws for the bell 412 helicopter

Helicopters generally exhibit a ratelike response type in pitch and roll axes, and when feedback control is used to increase the level of augmentation to provide attitude command and attitude hold, there is generally a reduction in performance. Use of nonlinear elements in the control system can lead to recovery of some of this performance. The paper investigates such use of nonlinearities in the pitch control loop of a helicopter with a full-authority digital fly-by-wire control system. The nonlinear elements are used to specify the rate of response and thus the attitude quickness. Describing function analysis was used to test compliance with the relative stability requirements of MILF-9490D. The control laws were successfully flight-tested on a Bell 412 modified for fly-by-wire research and results from those tests are presented. Control laws of the type presented here can potentially be optimized to maximize agility within the available actuator limits. The first control law presented was intended to test the concept; modest pitch-axis performance was therefore specified. The second control law was designed to provide ADS-33E-PRF level-1 handling qualities for noncombat-mission task elements. Both controllers gave a stable closed loop and provided the required response type. Closed-loop bandwidth predictions based on analysis of linear models were close to the bandwidths achieved in flight. Likewise, the attitude quickness achieved in flight was very close to that specified via the nonlinear element.