§ 3. But the aerofoil surface is not flat, owing to the increased "lift" given by arched surfaces, and a much more complicated set of phenomena then takes place, the centre of pressure moving forward until a certain critical angle of incidence is reached, and after this a reversal takes place, the centre of pressure then actually moving backwards.
The problem then consists in ascertaining the most efficient aerocurve to give the greatest "lift" with the least "drift," and, having found it, to investigate again experimentally the movements of the centre of pressure at varying angles, and especially to determine at what angle (about) this "reversal" takes place.
Fig. 43.
§ 4. Natural automatic stability (the only one possible so far as models are concerned) necessitates permanent or a permanently recurring coincidence (to coin a phrase) of the centre of gravity and the centre of pressure: the former is, of course, totally unaffected by the vagaries of the latter, any shifting of which produces a couple tending to destroy equilibrium.
§ 5. As to the best form of camber (for full sized machine) possibly more is known on this point than on any other in the whole of aeronautics.
In Figs. 44 and 45 are given two very efficient forms of cambered surfaces for models.
Fig. 44.—An Efficient Form of Camber.
B D Maximum Altitude. A C Chord. Ratio of B D: A C :: 1:17. A D 1/3 of A C.
