ws:

_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.

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 drift.

Every effort is then made to decrease it by "stream-lining," _i.e._, by
giving all "detrimental" parts of the aeroplane a form by which they
will pass through the air with the least possible drift. Even the wires
bracing the aeroplane together are, in many cases, stream-lined, and
with a markedly good effect upon the lift-drift ratio. In the case of a
certain well-known type of aeroplane the replacing of the ordinary wires
by stream-lined wires added over five miles an hour