The theoretical evaluation of the downwash behind jet-flapped wings

Abstract

The dovwnwash behind jet-flapped wings of infinite and finite span has been evaluated, using the solutions given by Spence and Maskell for the flow in two and three dimensions respectively. The wing incidence and jet deflection are assumed to be small, and the displacement of the jet is taken into account by displacing the downwash field so that the relative distance between the jet and tailplane is correct. It is also assumed that the spanwise loading on the finite wing is elliptic, and that the trailing vorticity is generated at one particular chordwise position. The wake is considered to be a flat sheet, and so the effect of the rolling-up of the wake has been neglected. Charts are given for the functions required in the calculation of the downwash for four specific values of jet momentum coefficient. Results have been obtained for the infinite wing and for a wing of aspect ratio 6.0, for various tailplane positions, and varying wing incidence, jet deflection and jet momentum coefficient. A comparison is also made with some American experimental results for a wing of aspect ratio 8.4, and the agreement is good, especially when the experimental lift coefficient is used in the calculations (since it differs considerably from the theoretical lift value, owing to the fact that the experimental wing has a blown flap).