Windtunnel tests on a two-dimensional model of a three element high lift system in a takeoff mode indicate that airjet vortex generators can produce a significant increase in lift at a given angle of incidence and a substantial increase in CLmax
. An associated reduction in profile drag is inferred from measurements of momentum defect on the upper surface.
The improvements are much greater than those typically achieved with vane vortex generators and cannot simply be associated with the suppression of boundary layer separation. Instead, the airjets and the vortices which they generate promote enhanced mixing and momentum transfer across the complex shear layers above the main wing, from the external flow right through to the surface. Detailed surveys reveal, for example, that the slat wake is dispersed — or absorbed into a region of greater shear — and that the growth of the main wing boundary layer is significantly reduced.
It is suggested that neither the relatively low Reynolds numbers . and Mach numbers of the tests, nor the effects of windtunnel interference, detract from the general relevance of the novel principle involved. Attention is drawn, however, to the need for further work to elucidate and quantify the flow mechanisms and to establish the balance between the benefits from the improved aerodynamics and the performance costs of installing and supplying the jets.