to the flight speed.

[Illustration]

_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.

The best angles of incidence for these varying factors are found by
means of wind-tunnel research and practical trial and error. Generally
speaking, the greater the velocity the smaller should be the angle of
incidence, in order to preserve a clean, stream-line shape of rarefied
area and freedom from eddies. Should the angle be too great for the
velocity, then the rarefied area over the top of the surface becomes of
irregular shape with attendant turbulent eddies. Such eddies possess no
lift value, and since it has taken power to produce them, they represent
drift and adversely affect the lift-drift ratio. Also, too great an
angle for the velocity will result in the underside of the surface
tending to compress the air against which it is driven rather than
accelerate it _downwards_, and that will tend to produce drift rather
than the _upwards_ reaction, or lift.

From a rigging point of view, one must presume that every