Camber

At any distance along the chord from the nose, a point may be marked mid-way between the upper and lower surfaces. The locus of all such points, usually curved, is the median line of the section, usually called the camber line. The maximum height of the camber line above the chord line is denoted by 6 and the quantity 100<5/c% is called the percentage camber of the section. Aerofoil sections have cambers that are usually in the range from zero (a symmetrical section) to 5%, although much larger cambers are used in cascades, e. g. turbine blading.

It is seldom that a camber line can be expressed in simple geometric or algebraic forms, although a few simple curves, such as circular arcs or parabolas, have been used.

Thickness distribution

Having found the median, or camber, line, the distances from this line to the upper and lower surfaces may be measured at any value of. v. These are, by the definition of the camber line, equal. These distances may be measured at all points along the chord and then plotted against x from a straight line. The result is a symmetrical shape, called the thickness distribution or symmetrical fairing.

An important parameter of the thickness distribution is the maximum thickness, or depth, t. This, when expressed as a fraction of the chord, is called the thickness/ chord ratio. It is commonly expressed as a percentage 100//c‘%. Current values in use range from 13% to 18% for subsonic aircraft down to 3% or so for supersonic aircraft.

The position along the chord at which this maximum thickness occurs is another important parameter of the thickness distribution. Values usually lie between 30% and 60% of the chord from the leading edge. Some older sections had the maximum thickness at about 25% chord, whereas some more extreme sections have the max­imum thickness more than 60% of the chord behind the leading edge.

It will be realized that any aerofoil section may be regarded as a thickness distribution plotted round a camber line. American and British conventions differ in the exact method of derivation of an aerofoil section from a given camber line and thickness distribution. In the British convention, the camber line is plotted, and the thickness ordinates are then plotted from this, perpendicular to the chord line. Thus the thickness distribution is, in effect, sheared until its median line, initially straight, has been distorted to coincide with the given camber line. The American convention is that the thickness ordinates are plotted perpendicular to the curved camber line. The thickness distribution is, therefore, regarded as being bent until its median line coincides with the given camber line.

Since the camber-line curvature is generally very small the difference in aerofoil section shape given by these two conventions is very small.