SubscribeThe Air Force Research Laboratory is investigating automated aerial refueling for Unmanned Air Vehicles (UAV) in order to extend the utility of these configurations.
Because the primary operational limitation of an unmanned vehicle is the volume of onboard fuel, and the most critical flight phases for a UAV are the launch and recovery, this capability is highly desirable for UAV operations. Such a capability will make these vehicles more effective by extending UAV’s ferry ranges, and significantly improving vehicle mission range and time-on-station. However, the operation of an unmanned vehicle in a precision flight control mode at such close proximity to a manned tanker is a considerable technical challenge.
Further, many of the current and evolving UAV vehicles have limited maneuverability because of restricted operational requirements and for stealth reasons, further complicating vehicle control in the turbulent tanker wake. In order to assess a vehicle’s control capabilities and develop robust automated refueling controls and strategies, a high fidelity model of the vehicle and it’s aerodynamic characteristics in the wake of a tanker aircraft are needed. These models must be assessed in a simulation environment that allows the close coupling of the parent and receiver models with the mutual aerodynamic interactions.
In order to develop the UAV refueling capability, the development of a number of technologies were required. The acquisition of the vehicle interactions was critical in order to establish the remaining technical goals. Bihrle Applied Research (BAR), under SBIR Phase II funding from the Air Force, recently developed a new testing capability using the Langley Full Scale Tunnel to collect wind tunnel test data on multiple models concurrently while accurately positioning the models in the x, y, and z axes.
The development of this test technique resulted in the successful acquisition of multi-vehicle test data as a function of position. While no tanker UAV combinations were examined in the initial effort, the scaling of some of the UAV test data was proposed for the demonstration of the simulation capability and control algorithm development.
For the initial effort, a preliminary close-formation control logic for the UAV was designed and implemented on an engineering F-16 flight model hosted in BAR’s D-Six simulation environment. Formation-induced aerodynamics effects acquired from previous wind tunnel tests for F-18’s and USAF Innovative Control Effector (ICE) UAV configuration were scaled to the baseline F-16 aerodynamics model.
This evaluation provided the opportunity to examine the projected UAV’s aerodynamic control authority required to maintain position and to maneuver in close-formation near the tanker. Successful station keeping and around-the- tanker maneuverability was demonstrated using the refueling control algorithm, although the control power available in this preliminary assessment was not necessarily representative of a current UAV.
In a follow-on, Bihrle Applied Research was tasked by AFRL to perform a formation wind tunnel test using a KC-135 wind tunnel model in the lead and the USAF ICE configuration in trail. This arrangement accurately models the aerodynamic interaction between the tanker and the UAV in its wake flow field. A comprehensive relative positional matrix was examined and the test results were added to ICE simulation flight model.
The simulation was used to examine the effects of the tanker wake on stability and control characteristics of a UAV during the refueling sequence and refueling approach strategies were developed and evaluated. The effect of adverse atmospheric conditions on the stability of the control algorithm was also evaluated. Finally, collision avoidance logic for the UAV refueling scenario was also assessed as part of this very successful program.
