The Aeroplane Speaks. Fifth Edition

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. DRIFT.--The drift of the whole aeroplane (we have considered only the lifting surface heretofore) may be conveniently divided into three parts, as follows: Active Drift, which, is the drift produced by the lifting surfaces. Passive Drift, which is the drift produced by all the rest of the aeroplane--the struts, wires, fuselage, under-carriage, etc., all of which is known as "detrimental surface." Skin Friction, which is the drift produced by the friction of the air with roughness of surface. The latter is practically negligible having regard to the smooth surface of the modern aeroplane, and its comparatively slow velocity compared with, for instance, the velocity of a propeller blade. LIFT-DRIFT RATIO.--The proportion of lift to drift is known as the lift-drift ratio, and is of paramount importance, for it expresses the efficiency of the aeroplane (as distinct from engine and propeller). A knowledge of the factors governing the lift-drift ratio is, as will be seen later, an absolute necessity to anyone responsible for the rigging of an aeroplane, and the maintenance of it in an efficient and safe condition.