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 V2.
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, at efficient angles of incidence, approximately at right-angles to the neutral lift line 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.
The Drift is the villain of the piece, and must be overcome by the Thrust in order to secure the necessary velocity to produce the requisite lift for flight.