Leading-Edge Radius and Chord Line

The chord line is defined as the shortest (straight) line connecting the leading and trailing edges. The leading edge of aerofoils used in subsonic applications is rounded, with a radius of about 1% of the chord length. The leading edge of an aerofoil is the radius of a circle with its center on a line tangential to the leading-edge camber connecting tangency points of the upper and lower surfaces with the leading edge. The magnitude of the leading-edge radius has significant effect on the stall characteristics of the aerofoil section.

The geometrical angle of attack a is the angle between the chord line and the direction of the undisturbed freestream.

4.16.2 Mean Camber Line

Mean camber line is the locus of the points midway between the upper and lower surfaces of the aerofoil. In other words, mean camber line is the bisector of the aerofoil thickness. The shape of the mean camber line plays an important role in the determination of the aerodynamic characteristics of the aerofoil section. One of the primary effects of camber is to change the zero-lift angle of attack, a0l. For symmetrical aerofoils, zero lift is at a = 0 and for cambered aerofoils, zero lift is at negative a for positive camber and vice versa.

The camber has a beneficial effect on the maximum value of the section lift coefficient. If the maximum lift coefficient is high, the stall speed will be low, all other factors being the same. However, it is essential to note that the high thickness and camber necessary for high maximum value of section lift coefficient produce low critical Mach numbers at high twisting moments at high speeds.