Design and Flight Testing of Nonlinear Autoflight Control Laws Incorporating Direct Lift Control

This publication describes how direct lift control has been incorporated in a nonlinear autoflight control algorithm. Direct lift control demonstrated its use in earlier aircraft designs. In two recent internal DLR projects, accurate flight path tracking in atmospheric disturbances was an important research goal, where direct lift control could potentially provide an important contribution. In these projects, use has been made of nonlinear control techniques. Objective of this research publication is to incorporate direct lift control in these nonlinear control laws. Simulations as well as flight tests have shown that more accurate flight path changes are achieved by this addition. Direct lift control can be particularly useful for path tracking as well as in turbulent air, since it facilitates lift changes without pitching. More precisely, the non-minimum phase behaviour of the aircraft caused by the elevators is eliminated in this way.