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.
The direction of the reaction is, of course, the resultant
of the forces Lift and Drift.
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