AEROFOIL GEOMETRY AND FAMILIES

In designing aerofoils it is usual to consider the effects of camber and thickness form separately. This is justifiable only up to a point. The detailed airflow over the wing is affected equally by both camber and thickness, so both need to be considered simultaneously. It is useful, however, to begin with camber as an introduction to aerofoil theory and practice. In later chapters the complicating effects of the thickness form of the profile will be considered.

Any aerofoil may be considered as a basic ‘thickness form’ which has been bent round or fitted to a curved camber or mean line (Fig. 7.1). Profiles may be classed in families. A given basic thickness form may be fitted to a whole series of different camber lines, some curved more than others, some with the curvature concentrated towards the leading edge, some with it mostly towards the trailing edge, and so on. For example, a simple flat plate has a small thickness, has a leading edge shape – perhaps square, pointed or rounded, and a trailing edge form. It may be cambered in any way to create a family of aerofoils. The mean line might be a simple arc of a circle, or it might be a more complex form derived mathematically. When aerofoil ordinates are published the camber is sometimes stated in terms of percentage of the wing chord, and possibly the position of the maximum camber point is also given. Thus, aerofoils of the N. A.C. A.[1] series contain this information in their designations. When four digits appear after the letters NACA, the first digit refers to the camber amount, the second gives the chordwise location of die point of maximum camber. The last two figures give the thickness of the profile. All these are expressed in percentages of the chord. Thus, the NACA 6409 profile has 6% camber with die highest point of the mean line at 40% of the chord, measured from the leading edge, and the thickness is 09%. The 4412 profile has 4% camber at 40%, and is 12% thick. In the more modem NACA aerofoils of the ‘six digit’ series, the information about camber is given in a different form. The fourth figure in this series gives the lift coefficient, ci, for which the profile has been designed. The larger this figure, in general, the more cambered the profile, so for example the NACA 633615 and 633215 have ‘design lift coefficients’ of.6 and.2 respectively.[2] The last two figures give the thickness percentage as before. In some cases, following the six digits, a further statement appears, such as a = 0.5. This means that a certain type of cambered mean line (see Figure 7.2) has been used. Where this statement does not appear, the NAC A a = 1 mean line has been used. Other aerofoil systems adopt other methods of nomenclature which may include details of camber (see Appendix 3). The precise form of the mean line may differ from profile family to profile family. It is very rare to find a simple circular arc. Usually the curve is designed to serve a particular purpose. For example the NACA four digit profiles have mean lines which are made up of two parabolic curve segments joining tangentially at the point of maximum camber. The ‘five digit’ series (e. g. NACA 23012) have mean lines with the high point unusually far forward, designed to yield high maximum lift coefficients (Fig. 7.3a). It is more usual now to design mean lines to give a desired choidwise load distribution. The most commonly employed of these is the NACA a = 1 mean line (see Fig. 7.2a), which gives an even chord load distribution. The advantage of this is that each part of the profile is contributing its appropriate share of the lift, and hence, for any given value of ci, the least possible camber is required. This means less profile drag, other things being equal. However, there may be good reasons sometimes for using other forms of mean line, to reduce wing-twisting and pitching loads, for example, or, when aerofoils are designed for laminar flow, to help control the detailed pressure distribution.