Thickness Distribution

The thickness distribution and the maximum thickness strongly influence the aerodynamics characteristics of the aerofoil section. The maximum local velocity to which a fluid element accelerates as it flows around an aerofoil increases as the maximum thickness increases, in accordance with the area-velocity relation for subsonic flow. Thus the value of the minimum pressure is the smallest for the thickest aerofoil. As a result, the adverse pressure gradient associated with the deceleration of the flow, from the location of this pressure minimum to the trailing edge, is greatest for the thick aerofoil. As the adverse pressure gradient become larger, the boundary layer becomes thicker. This thickening of boundary layer is likely to cause flow separation, leading to large increase of form drag. Thus, the beneficial effects of increasing the maximum thickness are limited.

For a thin aerofoil section, with relatively small leading-edge radius, boundary layer separation occurs early, not far from the leading edge of the upper (leeward) surface. Because of this, the maximum section lift coefficient for a thin aerofoil section is relatively small. The maximum section lift coefficient increases as the thickness ratio increases.

The thickness distribution for an aerofoil affects the pressure distribution and the character of the boundary layer. As the location of the maximum thickness moves aft, the velocity gradient in the mid­chord region decreases. The favorable pressure gradient associated with this decrease of velocity gradient in the mid-chord region promotes the boundary layer stability and increases the possibility of boundary layer remaining laminar. As we know, the skin friction drag associated with laminar boundary layer is less than that caused by turbulent boundary layer. Further thicker aerofoils benefit more from the use of high-lift devices but have a lower critical Mach number.

4.16.3 Trailing-Edge Angle

The trailing-edge angle influences the location of the aerodynamic center, the point about which the section moment coefficient is independent of angle of angle of attack, a. The aerodynamic center of this aerofoil section in a subsonic flow is theoretically located at the quarter-chord point.