The effect of XU and Tq2 on autonomous in-flight refueling: A Global Hawk RQ-4A approach

The effect of two parameters, Xu and Tq2 , are explored for their influence on autonomous in-flight aerial refueling. It is believed that these two parameters are very influential to longitudinal control. A Global Hawk (RQ-4A, receiver) to Global Hawk (RQ-4A, tanker) configuration is considered, using a probe-and-drogue fueling method. The refueling is simulated via a 6 DOF state space model developed for both the receiver and the tanker. The receiver and tanker model formulations are presented as well as the probability distribution of a successful connection, probe to drogue, with the variation in each predefined parameter. For each parameter the study was performed with two unique control law implementations (linear and nonlinear). The parameter Xu was varied from -0.15 to 0.0, while Tq2 was varied from 11.3 to 0.33. It was found, with the varying of parameter Xu, the probability of a successful connection remained rather unchanged. With the implementation of control law one, the probability of a drogue contact was between 20% to 30%. While for control law two the probability increased to between 70% to 80%. With the variation of Tq2 , it became apparent that the probability of a successful connection quickly dropped veering away from the value for which the control laws were implemented. With respect to control law one, the probability of contact with the drogue dropped from about 25%, for the initial Tq2 design, to less than 10% for all other values. With the use of control law two, the probability increased with respect to control law one; however, veering from the initial design point probability of contact dropped from about 73% to less than 25% for extreme values of Tq2 .