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 roughnesses 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.
Those factors are as follows:
1. Velocity.
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