Effect of flaps on trim

The lowering or raising of flaps affects the airflow not only over the lower surface of the wing, but also over the upper surface – probably the more important effect – and in front of the wing and behind the wing (Fig. 6.10). The airflow in turn affects the pressure distribution and the forces and moments on the wing and on the tail plane. It is hardly surprising, therefore, that the trim may be affected, but it may seem curious that the lowering of the flap sometimes tends towards nose-heaviness, sometimes tail-heaviness.

Consider the top surface of the wing. When the flap is lowered, the air flows faster over the top, especially near the leading edge. There will be greater suction here and the chances are that the centre of pressure on the top surface will move forward, thus tending towards tail-heaviness.

The downwash behind the wing will be large; and if the tail plane is so situ­ated as to receive the full benefit of this downwash, there will be a downward force on the tail plane, tending towards tail-heaviness.

In a low-wing aircraft the low position of the drag on the flap, especially when fully lowered, will tend towards nose-heaviness. On a high wing aircraft the drag, being high, may tend towards tail-heaviness.

The net effect on the pitching moment depends entirely on the type of flap or slots used, on how much they are lowered, and on the situation of the tail plane. Slotted flaps, and flaps that move backwards so increasing the rear portion of the wing area, will nearly always cause a nose-down moment which sometimes has to be counteracted by leading edge slots and flaps.

Sometimes, too, the change of trim is in one direction for the first part of the lowering of the flap, usually tail-heavy; and in the other direction, nose – heavy, when full flap is lowered. In some aircraft the effects, whether by design or good luck, so cancel each other that there is little or no change of trim, and no one is more pleased than the pilot.

It should be noted that the technique of landing a modern high performance aircraft, be it military or civilian, is nothing like the straightforward seat-of-

the-pants procedure described earlier for the case of a light private aircraft. For such modern aircraft, the speed, height, angle of descent and a host of other factors such as the flap and power setting must be correct within very close limits as the aircraft crosses the airfield threshold. If not, the pilot must abort the approach and try again. Landing a modern airliner manually requires a great deal of skill and concentration, and the majority of landings are nowadays made under automatic control with the pilot merely keeping a watchful eye, and being ready to take over at any instant in the event of a system malfunction. In order to maintain their skill (and their licence) pilots are, however, required to make a certain proportion of landings under manual control. It would be virtually impossible to land machines such as the Space Shuttle without some form of computer assistance.

Can you answer these?

1. If, when an aeroplane is gliding at its minimum angle of glide, the pilot attempts to glide farther by holding the nose of the aeroplane up, what will be the result, and why?

2. Discuss the effect of flaps on the gliding angle.

3. How does the load carried in an aeroplane affect the gliding angle and gliding speed?

4. Does the flattest glide give the longest time in the air? If not, why not?

5. Does (a) the stalling speed, (b) the stalling angle, change with height?

6. What are the advantages of the engine-assisted approach?

7. Why may the lowering of flaps affect the trim of an aeroplane?

8. You are flying an aeroplane well out to sea when the engine fails; there is a good airfield just on the coast and it is touch and go whether you can reach it; you have disposable load on board, luggage, bombs, and fuel; should you jettison your load, and if so when, and what should be your tactics in an endeavour to reach the airfield? (Note. There is more in this question than one might at first think, e. g. your tactics should be different for different wind conditions, so consider conditions of no wind, head winds, and tail winds; if you have surplus speed, what can you do with it?; can you reduce your drag?; should you use flaps?; at what speed should you fly?; should you jettison your load and, if so, when?)

For solutions see Appendix 5.

Numerical examples on gliding and landing will be found in Appendix 3.