The jet flap
Considering the jet flap (see also Section 8.4.2) as a high-velocity sheet of air issuing from the trailing edge of an aerofoil at some downward angle т to the chord line of the aerofoil, an analysis can be made by replacing the jet stream as well as the aerofoil by a vortex distribution.*
The flap contributes to the lift on two accounts. Firstly, the downward deflection of the jet efflux produces a lifting component of reaction and secondly, the jet affects the pressure distribution on the aerofoil in a similar manner to that obtained by an addition to the circulation round the aerofoil.
The jet is shown to be equivalent to a band of spanwise vortex filaments which for small deflection angles r can be assumed to lie along the O. v axis (Fig. 4.17). In the analysis, which is not proceeded with here, both components of lift are considered in order to arrive at the expression for Cl’.
Cl — 4ттАот + 2тт( l + 250)n
where A о and Bo are the initial coefficients in the Fourier series associated with the deflection of the jet and the incidence of the aerofoil respectively and which can be obtained in terms of the momentum (coefficient) of the jet.
It is interesting to notice in the experimental work on jet flaps at National Gas Turbine Establishment, Pyestock, good agreement was obtained with the theoretical Cl even at large values of r.