The aerodynamic interference between a flapped tanker aircraft and a receiver aircraft during air-to-air refuelling

Summary

Wind tunnel data have been obtained from a flapped tanker wing and receiver aircraft model at varying vertical separation and the results compared with theory. In the aerodynamic model, the tanker wing is represented by a pair of horseshoe vortices from the wing and flap tips and an allowance is made for the self-induced vertical displacement of the trailing vortices at the receiver aircraft position. The aerodynamic loads on the receiver are determined by the vortex-lattice method and lifting-line theory, although an approximate method is used to determine the side force on the fin. Data were obtained from open and closed test sections in order to estimate the wind tunnel boundary interference effect. In the longitudinal case, significant differences were obtained between theory and experiment. Two reasons for the differences are the assumption that the trailing vortices are fully rolled up and the neglect of viscous decay of the vortices. The lateral aerodynamic interference was determined by banking the tanker wing and displacing it sideways and by yawing the receiver aircraft model. In the case of the rolling moment due to sideways displacement, which is the most significant lateral aerodynamic interference term, the wind tunnel boundary interference is highly significant and the difference between theory and experiment is large due to incomplete roll up of the trailing vortices. The theoretical and experimental trends are similar although the theory overpredicts the rolling moment due to bank and sideways displacement while the corresponding side force and yawing moment are underpredicted. As in previous tests using an unflapped tanker wing, the effect of the sidewash due to the tanker wing on the receiver in yaw is found to be relatively insignificant.