standard
aeroplane has its lifting surface set at the most efficient angle, and
the practical application of all this is in taking the greatest possible
care to rig the surface at the correct angle and to maintain it at such
angle. Any deviation will adversely affect the lift-drift ratio, _i.e._,
the efficiency.

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 obvio