3. Camber.—(Refer to the second illustration in this chapter.) The lifting surfaces are cambered, i.e., curved, in order to decrease the horizontal component of the reaction, i.e., the drift.
The bottom camber: If the bottom of the surface was flat, every particle of air meeting it would do so with a shock, and such shock would produce a very considerable horizontal reaction or drift. By curving it such shock is diminished, and the curve should be such as to produce a uniform (not necessarily constant) acceleration and compression of the air from the leading edge to the trailing edge. Any unevenness in the acceleration and compression of the air produces drift.
The top camber: If this was flat it would produce a rarefied area of irregular shape. I have already explained the bad effect this has upon the lift-drift ratio. The top surface is then curved to produce a rarefied area the shape of which shall be as stream-line and free from attendant eddies as possible.
The camber varies with the angle of incidence, the velocity, and the thickness of the surface. Generally speaking, the greater the velocity, the less the camber and angle of incidence. With infinite velocity the surface would be set at no angle of incidence (the neutral lift line coincident with the direction of motion relative to the air), and would be, top and bottom, of pure stream-line form—i.e., of infinite fineness. This is, of course, carrying theory to absurdity as the surface would then cease to exist.
The best cambers for varying velocities, angles of incidence, and thickness of surface, are found by means of wind-tunnel research. The practical application of all this is in taking the greatest care to prevent the surface from becoming distorted and thus spoiling the camber and consequently the lift-drift ratio.
4. Aspect Ratio.—This is the proportion of span to chord. Thus, if the span is, for instance, 50 feet and the chord 5 feet, the surface would be said to have an aspect ratio of 10 to 1.
For a given velocity and a given area of surface, the higher the aspect ratio, the greater the reaction. It is obvious, I think, that the greater the span, the greater the mass of undisturbed air engaged, and, as already explained, the reaction is partly the result of the mass of air engaged. I say "undisturbed" advisedly, for otherwise it might be argued that, whatever the shape of the surface, the same mass of air would be engaged. The word "undisturbed" makes all the difference, for it must be remembered that the rear part of the underside of the surface engages air most of which has been deflected downwards by the surface in front of it. That being so, the rear part of the surface has not the same opportunity of forcing; the air downwards (since it is already flowing downwards) and securing there from an upwards, reaction as has the surface in front of it. It is therefore of less value for its area than the front part of the surface, since it does less work and secures less reaction—i.e., lift. Again, the rarefied area over the top of the surface is most rare towards the front of it, as, owing to eddies, the rear of such area tends to become denser.
