Figure 1.15 shows how the pressure varies around an aerofoil section. The shaded area represents pressures greater than the general surrounding or ‘ambient’ air pressure, and the unshaded region represents low pressures. It will be seen that the difference in pressures between upper and lower surfaces is greatest over the front portion of the aerofoil, and therefore most of the lift force must come from that region. This effect was quite pronounced on older wing sections, but nowadays the trend is to design aerofoil sections to give a
fairly constant low pressure over a large proportion of the top surface. This produces a more uniform distribution of lift along the section, giving both structural and aerodynamic advantages, as we shall describe at various points later.
Since the relative flow speed reduces to zero at the stagnation position, it follows that the pressure there must have its highest possible value. This maximum value is therefore called the stagnation pressure. Stagnation pressure should not be confused with static pressure defined earlier. Unfortunately, and for obvious reasons, it often is! Static pressure is just the air pressure. Stagnation pressure is the pressure at a stagnation position; a position where there is no relative motion between the air and the surface.
From Fig. 1.15 we see that the pressure falls rapidly as the air accelerates and flows away from the stagnation position, becoming extremely low around the leading edge. This low leading edge pressure is again contrary to expectation, but is linked to the fact that the stagnation position is behind the leading edge on the underside. Thus, the air taking the upper-surface route has to flow forward, and then negotiate a fairly sharp curve. In order for the air to do this, rather than carry on in a straight line, there must be a low pressure on the leading edge, to pull the flow into a curved path: i. e. to provide the necessary centripetal acceleration.