Figure 3.14. Aerofoil characteristics

3.9 Generation of Lift

Figure 3.12 is a qualitative description of the flow field and its resultant forces on the aerofoil. The result of skin friction is the drag force, shown in Figure 3.12b. The lift is normal to the flow.

Section 3.5 explains that a typical aerofoil has an upper surface more curved than the lower surface, which is represented by the camber of the aerofoil. Even for a symmetrical aerofoil, the increase in the angle of attack increases the velocity at the upper surface and the aerofoil approaches stall, a phenomenon described in Section 3.10.

Figure 3.13a shows the pressure field around the aerofoil. The pressure at every point is given as the pressure coefficient distribution, as shown in Figure 3.13b. The upper surface has lower pressure, which can be seen as a negative distribution. In addition, cambered aerofoils have moments that are not shown in the figure.

Figure 3.14a shows the typical test results of an aerofoil as plotted against a variation of the angle of attack, α. Initially, the variation is linear; then, at about 10 deg α, it starts to deviate and reaches maximum Cl (Clmax at αmax). Past αmax, the Cl drops rapidly – if not drastically – when stall is reached. Stalling starts at reaching αmax. These graphs show aerofoil characteristics. Figure 3.14b depicts the corresponding distribution of the pressure coefficient Cp at an angle of attack of 15 deg.

Deflection of either the control surface or a change in the angle of attack will alter the pressure distribution. The positive Y-direction has negative pressure on the upper surface. The area between the graphs of the upper and lower surface Cp distribution is the lift generated for the unit span of this aerofoil.

Figure 3.15 shows flow physics around the aerofoil. At the LE, the streamlines move apart: One side negotiates the higher camber of the upper surface and the other side negotiates the lower surface. The higher curvature at the upper surface generates a faster flow than the lower surface. They have different velocities when they meet at the trailing edge, creating a vortex sheet along the span. The phenomenon can be decomposed into a set of straight streamlines representing the