Jet Flap

Treating the jet flap as a high-velocity sheet of air issuing from the trailing edge of an aerofoil at some downward angle в to the chord line of the aerofoil, as shown in Figure 6.11, an analysis can be made by replacing the jet stream as well as the aerofoil by a vortex distribution [1, 2].

The flap contributes to the lift in the following two ways:

1. The downward deflection of the efflux produces a lifting component of reaction.

2. 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 в can be assumed to be on the ox-axis, shown in Figure 6.11.

Considering both the contributions mentioned above, it can be shown that the lift coefficient can be expressed as:

CL = 4nA0в + 2n(1 + 2B0) a. (6.49)

where A0 and B0 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 of the jet.