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.
The direction of the reaction is, of course, the resultant of the forces Lift and Drift.
The Lift is the useful part of the reaction, for it lifts the weight of the aeroplane.