Favourable and unfavourable conditions

As described above, separation tends to occur when air flows from a low pres­sure to a high one. This is therefore known as an adverse pressure gradient. Conversely, flow from a high pressure to a low one is called a favourable pres­sure gradient.

A favourable pressure gradient not only inhibits separation, but slows down the rate of boundary layer growth, and delays transition. In the next chapter, we will show how we can exploit this factor to produce low-drag aerofoil sec­tion shapes.

Leading-edge separation

Flow separation is particularly likely to occur when the air tries to go round a very sharp bend, as on the nose of the thin aerofoil. For air to travel around a curve, the pressure on the outside of the curve must be greater than on the inside, in order to provide the necessary ‘cornering’ (centripetal) force. Thus, the pressure on the leading edge of an aerofoil is often locally very low. On the upper surface, the pressure initially rises again rapidly with distance from the leading edge. A strong adverse pressure gradient (flow from a low pressure to a high one) is therefore produced, and the flow tends to separate at, or very near the leading edge.

When such leading-edge separation occurs, the stall or loss of lift may be both sudden and severe. Aerofoils with a large radius leading edge are less prone to producing leading-edge separation, and therefore tend to have a more progressive and safer stall characteristic. As we shall see later, however, there are various reasons why it is sometimes advantageous to use an aerofoil with a sharp leading edge.

It is a common mistake to confuse separation and transition. Transition is where the boundary layer changes from laminar to turbulent. Separation is where the flow ceases to follow the contours of the surface. The fact that separation is normally accompanied by large-scale turbulence is probably the source of the confusion.