Pressure Distribution around an Aerofoil
When air flows past an aerofoil, there are changes in velocity on the upper and lower surfaces and associated pressure changes.
Although most low speed aerofoils are similar in shape, each section is intended to give certain specific aerodynamic characteristics. Therefore, there can be no such thing as a typical aerofoil section or a typical aerofoil pressure distribution and it is only possible to discuss pressure distributions around aerofoils in the broadest of general terms. So, in general, at conventional angles of attack, compared with the free stream static pressure, there is a pressure decrease over much of the upper surface, a lesser decrease over much of the lower surface so that the greatest contribution to overall lift comes from the upper surface.
The aerofoil profile presented to the airflow determines the distribution of velocity and hence the distribution of pressure over the surface. This profile is determined by the aerofoil geometry, i.e. thickness distribution and camber, and by the angle of attack. The greatest positive pressures occur at stagnation points where the flow is brought to rest, at the trailing edge, and somewhere near the leading edge, depending on the angle of attack. At the front stagnation point the flow divides to pass over and under the section. At this point there must be some initial acceleration of the flow at the surface otherwise there could be no real velocity anywhere at the aerofoil surface, therefore there must be some initial reduction of pressure below the stagnation value. If the profile is such as to produce a continuous acceleration there will be a continuous pressure reduction and vice versa. Some parts of the contour will produce the first effect, other parts the latter, bearing in mind always that a smooth contour will produce a smoothly changing pressure distribution which must finish with the stagnation value at the trailing edge.
Fig below shows the pressure distribution around a particular aerofoil section at varying angles of attack. The flow over the section accelerates rapidly around the nose and over the leading portion of the surface, the rate of acceleration increasing with increase in angle of attack. The pressure reduces continuously from the stagnation value through the free stream value to a position when a peak negative value is reached. From there onwards the flow is continuously retarded, increasing the pressure through the free stream value to a small positive value towards the trailing edge. The flow under the section is accelerated much less rapidly than that over the section, reducing the pressure much more slowly through the free stream value to some small negative value, with subsequent deceleration and increase in pressure through free stream value to a small positive value toward the trailing edge. If the slight concavity on the lower surface towards the trailing edge was carried a little further forward, it might be possible to sustain a positive pressure over the whole of the lower surface at the higher angles of attack (reflexed cambered aerofoil). However, although this would increase the lifting properties of the section, it might also produce undesirable changes in the drag and pitching characteristics. Therefore, it can be seen that any pressure distribution around an aerofoil must clearly take account of the particular aerofoil contour.
Pressure Distribution Around Aerofoil
From the typical pressure distribution around an aerofoil, the following points can be noted:
- At a small negative angle of attack (-40), for this aerofoil, the decrease in pressure above and below the section would be equal and the section would give no lift.
- At higher angles of attack (+80), lift is partly due to the decreased pressure on the upper surface and partly due to increased pressure on the lower surface.
- At small angles of attack (+40), the lift arises from a difference between the pressure reduction on the upper and the lower surfaces.
- At positive angles of attack (+140), the increase in pressure over the lower surface remains almost constant. The greatest contribution to lift comes from the increasing reduction of pressure on the upper surface as the AOA is increased.
- Beyond the stalling AOA (+16°), there is a sudden flattening of the pressure envelope above the upper surface. This causes a drastic loss of lift and is referred to as stall. Whatever lift remains, does so primarily due to the increased pressure on the lower surface.
- The zero lift angle of attack for a positively cambered aerofoil is at about –4o. For a symmetrical aerofoil, the zero lift AOA would be zero degrees and for a reflexed camber aerofoil it will be some positive value.