The method which was developed in an earlier paper for calculating the lift on a thin jet-flapped aerofoil has been applied to the case in which the jet enters the main stream tangentially from a deflected flap. The lift derivative with respect to the (small) angle of deflection β is of the form

where , E being the ratio of flap length to aerofoil chord, and D0 a function both of E and of the jet momentum coefficient CJ. In the special case CJ = 0, Do vanishes and the classical results for attached flow on flapped aerofoils without super-circulation are obtained. More generally, Do has been evaluated over the range 0<CJ<5 for six ratios E between 0·02 and 0·50, using an electronic computer. The lift derivatives so found lie between those in the limiting cases of “deflection” (E = 0) and “incidence” (E=1) which were calculated in Reference 1. They show that the effectiveness of the jet-flap scheme might be greatly increased by blowing over flaps of the order of 20 or 30 per cent of the chord, rather than over the small “shrouds” which have hitherto been proposed.