--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 to the flight
speed.
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