Aerodynamic interference and lateral stability and control during air-to-air refuellin

An investigation of the aerodynamic interference and the lateral stability and control of a Hercules receiver aircraft refuelling in flight from a KC10 tanker has been carried out. Theoretical models have been developed in order to determine the aerodynamic forces and moments acting on the receiver due to its position and attitude within the tanker wake. The tanker aircraft is represented by its main wing with the wake modelled by a three-dimensional roll-up method. The vortex lattice method and approximate expressions for the receiver fuselage effect are used to determine the tanker induced loads on the receiver. The theoretical models are validated by comparing with existing experimental data obtained in a low speed wind tunnel. The tanker induced loads on the receiver are found to depend strongly on the vertical position of the receiver relative to the tanker wing wake. In the case of steady sideslip there is a large decrease in the directional stability of the receiver as quantified by the gradient of the rudder angle versus sideslip. Minimum directional stability corresponds to the tip of the receiver fin intersecting the tanker wing wake. The associated aileron angle is two to three times the value in free air in agreement with flight test data. Solution of the linearised equations of motion of the Hercules receiver reveals a divergent oscillation involving mainly bank and side displacements. Typical aerodynamic data of the interference between the KC10 tanker and Hercules receiver are obtained over an envelope of the receiver positions and attitudes required for flight simulation. The tanker induced forces and moments on the receiver are assessed relative to the receiver's aerodynamic characteristics in free air. Large changes in lift, drag and pitching moment occur near the tanker wake centre line. As the receiver is displaced sideways towards the tanker wing tip vortices it experiences large side force and yawing moment and particularly high rolling moment. The most significant term due to the receiver attitude is the rolling moment due to bank.