DESIGNING A NEW PROFILE

Knowing the mean line of any aerofoil, it is possible to experiment with a family of profiles based on it Different thickness forms may be fitted to it, and new aerofoils created in this way. Alternatively, a preferred thickness form may be fitted to various differing mean lines to try the effects of increasing or decreasing camber, moving the point of maximum camber forward or aft, and so on. Methods of doing these things are outlined in Appendix 3.

7.3 ERRORS AMONG MODELLERS

Modellers sometimes have mistaken ideas about camber. For example, aerofoils such as the well-known Clark Y, with flat undersides may be more cambered than some thinner sections which have concave undersides. In the same way, changing the thickness form of a profile does not change its camber – the NACA 4415 and 4409 are cambered both by 4%, but while one appears ‘undercambered’ the other is convex on both surfaces. For these reasons, the widespread habit of classifying aerofoil sections as ‘undercambered’, ‘flat bottomed’ and even ‘semi-symmetrical’, is very misleading and should be abandoned. The so-called semi-symmetrical profile is a cambered section and the camber may vary greatly from one such aerofoil to another, depending on the shape of the camber line itself in combination with the thickness form. Even perfectly symmmetrical sections differ considerably in flight because of their various kinds of thickness distribution. From the table of ordinates used to plot an aerofoil, the camber can be found by arithmetic, or from an accurate drawing it may be found graphically. (See Appendix 1). It is seldom possible to judge it by eye. It is also undesirable to modify camber arbitrarily. Some modellers, in the hope of obtaining more lift without increasing drag, ‘droop’ the trailing edge of their aerofoils. This introduces a kink in the mean line with effects usually bad. It would be better to choose a new properly designed mean line with an increased camber. In a similar fashion, either by design or by various tricks and dodges on the building board, modellers sometimes ‘reflex’ the trailing edge of a wing near the tips, intending to give a’ desirable ‘washout’. The effect in many cases is the opposite: a reflexed profile tends to stall sooner, rather than later, than an ordinary one of similar leading edge shape. The purpose of the reflex camber line is to reduce the pitching moment of the aerofoil, not to delay stalling (Fig. 7.3b). Another common term, referring to the leading edge of the

Fig. 7.2 Mean line ordinates.

To obtain a mean line of a desired maximum camber, muliply each YU figure in the table by an appropriate factor.

EXAMPLE The A = 1.0 camber line reaches its maximum 50% chord, where the YU ordinate is 5.515 (5.515%) To reduce this to a 2% camber line, multiply all the YU figures by 2 – 0 3626

5.515

(Use an electronic calculator). Thus the new ordinates will read 0.0,0.0907,0.1269,0.1940 etc.

aerofoil, is ‘Phillips entry’. A profile with Phillips entry is one which has a modified camber line over the front 20-30% of the section, reducing the camber in this region. The camber of the profile should be considered as a whole and it is not good practice to modify a part of the aerofoil without considering the shape of the mean line from leading edge to trailing edge.

As will become apparent in what follows, to vary the camber of a wing towards the tips is an extremely usefiil design technique, enabling tip stalling to be prevented without any bad effects on performance. The technique, however, requires care.