Head-resistance is a term often applied to passive
drift, but it is apt to convey a wrong impression,
as the drift is not nearly so much the result of
the head or forward part of struts, wires, etc.,
as it is of the rarefied area behind.
Above is illustrated the flow of air round two
objects moving in the direction of the arrow M.
In the case of A, you will note that the rarefied
area DD is of very considerable extent; whereas
in the case of B, the air flows round it in such a
way as to meet very closely to the rear of the
object, thus DECREASING DD.
The greater the rarefied area DD. then, the less
the density, and, consequently, the less the pressure
of air upon the rear of the object. The less such
pressure, then, the better is head-resistance D
able to get its work in, and the more thrust will
be required to overcome it.
The ``fineness'' of the stream-line shape, i.e.,
the proportion of length to width, is determined
by the velocity--the greater the velocity, the
greater the fineness. The best degree of fineness
for any given velocity is found by means of wind-
tunnel research.
The practical application of all this is, from a
rigging point of view, the importance of adjusting
all stream-line parts to be dead-on in the line of
flight, but more of that later on.
2. Angle of Incidence.--The most efficient angle of
incidence varies with the thrust at the disposal
of the designer, the weight to be carried, and the
climb-velocity ratio desired.
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