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 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.--The greater the velocity the greater the proportion of
drift to lift, and consequently the less the efficiency. Considering
the lifting surfaces alone, both the lift and the (active) drift, being
component parts of the reaction, increase as the square of the velocity,
and the efficiency remains the same at all speeds. But, considering the
whole aeroplane, we must remember the passive drift. It also increases
as the square of the velocity (with no attendant lift), and, adding
itself to the active drift, results in increasing the proportion of
total drift (active + passive) to lift.

But for the increase in passive drift the efficiency of the aeroplane
would not fall with increasing velocity, and it would be possible, by
doubling the thrust, to approximately double the speed or lift--a happy
state of affairs which can never be, but which we may, in a measure,
approach by doing everything possible to diminish the passive dr