The Ground Effect on the Jet Flap in Two Dimensions

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Abstract
Summary The effect of the ground on a jet-flap aerofoil in two-dimensional flow is investigated, with particular reference to the conditions under which the jet flow hits the ground. Experimental results are included for the change of lift and pitching moment for 58·1° and 31·4° jet-deflection angles, with a limited investigation into the downwash changes behind the model with a 58·1° jet-deflection angle. It is concluded that there is a definite limit to the jet coefficient, which may be used at any ground position, if the problems of a sudden change of lift and pitching moment are to be avoided. There remains the unresolved difficulty of the downwash variation at the tail arising from the presence of the ground. A provisional assessment of the performance of a jet-flap aircraft near the ground is made, and shows that for a 58·1° jet-deflection angle there is a definite limit to the minimum flying speed for any given distance of the wing from the ground. The fact that there appears to be a maximum pressure lift coefficient, independent of the jet-deflection angle, which may be obtained at any ground position, is investigated. An attempt is made to provide a theoretical explanation of this on the basis of the critical condition of blockage underneath the aerofoil.

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