Application of a wake roll-up method coupled with the vortex lattice method and approximate expressions for the receiver fuselage effect have been used to determine the induced loads on a Hercules receiver aircraft behind a KC10 tanker. The induced loads depend strongly on the vertical position of the receiver wing and fin 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. This is due mainly to the combined effects of the yawing moments due to bank, yaw and side displacements. 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 linearized equations of motion gives three lateral characteristic oscillations for the air-to-air refuelling case. These include the usual Dutch roll oscillation, a highly damped rolling oscillation and a divergent oscillation involving mainly bank and side displacements.
The aerodynamic forces and moments on the Hercules receiver aircraft, due to its horizontal and vertical position and bank, yaw, and pitch attitudes in the wake of the KC10 tanker aircraft, are assessed relative to the receiver's aerodynamic characteristics in free air. Large changes in lift, drag, and pitching moment are predicted near the tanker wake centerline. As the receiver is displaced sideways towards the tanker wingtip 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. b C » t C L C. r 'V.
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