er thrust. It is a pity, however,
that these practical considerations have resulted in a bad definition
of the angle of incidence becoming prevalent, a consequence of which has
been the widespread fallacy that flight may be secured with a negative
inclination of the surface. Flight may conceivably be secured with a
negative angle of chord, but never with a negative inclination of the
surface. All this is only applicable to cambered surfaces. In the case
of flat surfaces the neutral lift line coincides with the chord and the
definition I have criticised adversely is then applicable. Flat lifting
surfaces are, however, never used.

The surface acts upon the air in the following manner:

As the bottom of the surface meets the air, it compresses it and
accelerates it DOWNWARDS. As a result of this definite action there is,
of course, an equal and opposite reaction UPWARDS.

The top surface, in moving forward, tends to leave the air behind
it, thus creating a semi-vacuum or rarefied area over the top of the
surface. Consequently the pressure of air on the top of the surface
is decreased, thus assisting the reaction below to lift the surface
UPWARDS.

The reaction increases approximately as the square of the velocity. It
is the result of (1) the mass of air engaged, and (2) the velocity and
consequent force with which the surface engages the air. If the reaction
was produced by only one of those factors it would increase in direct
proportion to the velocity, but, since it is the product of both
factors, it increases as V(2S).

Approximately three-fifths of the reaction is due to the decrease of
density (and consequent decrease of downward pressure) on the top of the
surface; and only some two-fifths is due to the upward reaction secured
by the action of the bottom surface upon the air. A practical point in
respect of this is that, in the event of the fabric covering the surface
getting into bad condition, it is more likely to strip off the top than
off the bottom.

The direction of the reaction is approximately at right-angles to the
chord of the surface, as illustrated above; and it is, in considering
flight, convenient to divide it into two component parts or values,
thus:

1. The vertical component of the reaction, i.e., Lift, which is opposed
to Gravity, i.e., the weight of the aeroplane.

2. The horizontal component, i.e., Drift (sometimes called Resistance),
to which is opposed the thrust of the propeller.