What Do Flaps Do?

Flaps, conventionally for gliders anyway, are trailing-edge control surfaces occupying the space along the back of the wing between the fuselage and the start of the aileron. Unlike the ailerons they are operated in phase and their main action is to alter the overall profile of the wing. Lowering the flap (known as positive flap) increases the lifting power of the wing by increasing its camber which is a measure of its curvature. Raising the flap has the opposite effect. Increasing the lift comes at a price unfortunately which is that the parasitic drag also increases, and because the effect of parasitic drag rises rapidly with airspeed this additional lift is really only of value at low speeds such as when thermalling or landing.

A glider wing is a compromise of many requirements, but broadly it needs to give the most lift for the least drag over the widest range of speeds and at the same time provide for a reasonable degree of safety in its stalling characteristics.  There are other goals such as maximising performance as the wing surface deteriorates due for instance to bugs or raindrops.  The result of this at any particular wing-loading is the well-known polar curve seen in figure 1 where the aircraft's useful speed range is limited by the degradation in lift/drag. The use of flaps gives in effect a continuously variable wing which can be more optimal for both low- and high-speed flight and so extends this useful speed range. Figure 2 shows how the polar for a flapped wing is made up of a set of polars for each flap position over the optimal speed range for that particular setting. In practice there are fairly severe constraints on what can be achieved simply by modifying the profile of the trailing section of an aerofoil and typically the positive flap settings are useful over decreasingly small ranges of airspeed as can be seen in the figure below.

Comparison of flapped and unflapped polars.
(note - the relative scales should not be compared since self-evidently these are taken from different aircraft)
unflapped glider polar

figure 1
typical single polar of unflapped glider

flapped glider - note separate polars for each flap setting

figure 2:
typical multiple polar of flapped glider

As further reading on the use of flaps on glider wings:
(Note that these files are quite large, typically 500-1000k, so may take some time to download!)
'On the Optimization of Airfoils with Flaps', F.X.Wortmann, published in Soaring, March 1970
'Facts About Flaps', Dick Johnson, published in Soaring, September 1977

Page last updated by on 28/11/2009