Experimental Investigation on Precision Tracking by Use of a Digital Controller and Manual Thrust Control

Abstract

Air-to-air refueling is a possibility to increase the range of an aircraft. As this is also a very demanding maneuver for the pilots, automating this process could be advantageous in simplifying the task. In this thesis, a digital controller is designed and evaluated to regulate the position of an aircraft during in-flight refueling with the probe-and-drogue system. For the design of the controller, a time-continuous linear-quadratic regulator is first developed under consideration of a time delay for the signal transport and processing as well as the actuator dynamics of the control surfaces. The resulting regulator is then discretized by using the Tustin’s method for a given sampling time. For the evaluation of the controller performance, flight simulator tests are carried out with test pilots from the German Air Force. The analysis of the results shows that the control system can establish successful contacts for overtake velocities in a range from 2 to 4 kts and certain minimum distances between the probe and drogue. At higher overtake speeds of 4 kts the probe misses the drogue because the closed-loop system cannot adequately follow the movement of the drogue caused by the bow wave effect. Nevertheless, the success rate is increased and the workload of the pilots is reduced compared to a manual drogue approach. Furthermore, the refueling process took less time when the position controller was activated. This is advantageous in time-limited situations.