Vortex generators

Many devices are used by the designer to control the separation or breakaway of the airflow from the surface of the wing – all these devices, in one way or another, over one part of the wing or another, have this in common, that they are intended to prevent or delay this breakaway. How? Well, that depends to some extent on the device, and we will consider vortex generators first (Fig. 11D).

The fundamental reason for the breakaway is that the boundary layer becomes sluggish over the rear part of the wing section, flowing as it is against the pressure gradient. The formation of a shock wave makes matters worse; the speed in the boundary layer is still subsonic which means that pressure can be transmitted up stream, causing the boundary layer to thicken and, if the pressure rise is too steep, to break away from the surface. Now vortex gener­ators are small plates or wedges, projecting an inch or so from the top surface of the wing, i. e. three or four times the thickness of the boundary layer. Their purpose is to put new life into a sluggish boundary layer; this they do by shed­ding small lively vortices which act as scavengers, making the boundary layer turbulent and causing it to mix with and acquire extra energy from the sur­rounding faster air, thus helping it to go farther along the surface before being

Fig 11.20 Area rule – effect of wings and tail on plan view of fuselage slowed up and separating from the surface. In this way the small drag which they create is far more than compensated by the considerable boundary layer drag which they save, and in fact they may also weaken the shock waves and so reduce shock drag also; and the vorticity which they generate can actually serve to prevent buffeting of the aircraft as a whole – a clever idea indeed, and so simple. The net effect is very much the same as blowing or sucking the boundary layer, but the device is so much lighter in weight and simpler. The greater the value of the thickness/chord ratio the more necessary does some such device become.

There are various types of vortex generator; Fig. 11.21 illustrates the bent – tin type, which may be co-rotating or contra-rotating. The plates are inclined at about 15° to the airflow, and on a wing are usually situated on the upper surface fairly near the leading edge.