Variable camber

Many attempts have been made to provide aerofoils with some kind of vari­able camber so that the pilot might be able to alter his aerofoil from a high-lift type to a high-speed type at will. Owing to the tremendous advantage to be gained by such a device, it is not surprising to find that much ingenuity has been expended, many patents have been taken out, and it is not easy to compare the rival merits of the various slots, flaps, slotted flaps, and so on. Figure 3.32 (overleaf) shows some of the devices with the increase in maximum lift claimed for each, but we must not take these figures as the only guide to the usefulness or otherwise of each device, because there are other points to be considered besides maximum lift. For instance, we may want a good ratio of maximum CL to minimum CD (which indicates a good speed range), or an increase in drag as well as lift, the flaps acting as an air brake, which may be useful in increasing the gliding angle (explained later). Another important consideration is the simplicity of the device; anything which needs complicated operating mechanism will probably mean more weight, more controls for the pilot to work, something more to go wrong.

Flaps and slots

Although there is a large variety of high-lift devices nearly all of them can be classed as either slots or flaps – or a combination of the two (Fig. 3.32).

Slots may be subdivided into –

(a) Fixed slots.

(b) Controlled slots.

(c) Automatic slots.

(d) Blown slots.

Flaps may be subdivided into –

(a) Camber flaps.

(b) Split flaps.

(c) Slotted flaps.

(d) Lift flaps.

(e) Blown flaps.

(/) Jet flaps.

(g) Nose flaps.

(b) Spoilers.

(/) Lift dumpers.

(/) Air brakes.

We can also classify the effects of both slots and flaps on the characteristics of an aerofoil by saying that their use may cause one or more of the following –

(a) Increase of Lift.

(b) Increase of Drag.

(c) Change of Stalling Angle.

(d) Decrease of Lift.

(e) Change of Trim.