Centre of pressure

The second thing that we learn from the pressure distribution diagram – namely, that both decreases and increases of pressure are greatest near the leading edge of the aerofoil – means that if all the distributed forces due to pressure were replaced by a single resultant force, this single force would act less than halfway back along the chord. The position on the chord at which this resultant force acts is called the centre of pressure (Fig. 3.9). The idea of a centre of pressure is very similar to that of a centre of gravity of a body whose weight is unevenly distributed, and it should therefore present no diffi­culty to the student who understands ordinary mechanics.

To sum up, we may say that we have a decreased pressure above the aero­foil and an increased pressure below, that the decrease of pressure above is greater than the increase below, and that in both cases the effect is greatest near the leading edge (Fig. 3.8).

All this is important when we consider the structure of the wing; for instance, we shall realise that the top surface or ‘skin’ must be held down on to the ribs, while the bottom skin will simply be pressed up against them.

Fig 3.9 Centre of pressure