Summary The longitudinal stability and control of a large receiver aircraft was considered during air-to-air refuelling. A simple horseshoe vortex was used to model the tanker wake and approximate expressions were derived for the additional aerodynamic derivatives due to the position and attitude of the receiver aircraft within the downwash field. These derivatives were shown to depend on the mean variation of downwash with vertical displacement at the receiver wing and tailplane. The mean downwash gradients, in turn, depend mainly on the vertical separation between the tanker and receiver aircraft and the ratio of the tanker-to-receiver aircraft wing spans. Solutions of the linearised equations of motion were obtained for a range of values of the downwash gradients. The large receiver aircraft, considered in the paper, typically exhibits two divergent modes which appear to be controlled in flight by frequent alternate movement of the elevators and engine throttle.
SummaryWind 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.
Wind tunnel data from a tanker wing and receiver aircraft model at varying vertical separation have been compared with theoretical results. In the aerodynamic model the tanker wing is represented by a horseshoe vortex while 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. In the longitudinal case data were obtained for low, mid and high tailplane positions and, with the exception of the pitching moment results, fairly good agreement is obtained between theory and experiment. The relatively small differences are due mainly to the wind tunnel boundary interference effect which could not be quantified for the pitching moment measurements. The lateral aerodynamic interference was determined by banking the tanker wing and displacing it sideways and by yawing the receiver model. Fairly good agreement is obtained between the theory and experiment for the most significant terms which are the rolling moments due to bank and sideways displacement. The effect of the sidewash due to the tanker wake on the receiver in yaw is found to be relatively insignificant. Over the range of bank, yaw and sideways displacements tested the results are almost linear.
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