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
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.
From a rigging point of view, one must presume
that every standard aeroplane has its lifting
surface set at the most efficient angle, and the
practical application of all this is in taking the
greatest possible care to rig the surface at the
correct angle and to maintain it at such angle.
Any deviation will adversely affect the lift-drift
ratio, i.e., the efficiency.
3. Camber.--(Refer to the second illustration in this
chapter.) The lifting surfaces are cambered, i.e.,
curved, in order to decrease the horizontal component
of the reaction, i.e., the drift.
The bottom camber: If the bottom of the surface
was flat, every particle of air meeting it would do
so with a shock, and such shock would produce a
very considerable horizontal reaction or drift. By
curving it such shock is diminished, and the curve
should be such as to produce a uniform (not
necessarily constant) acceleration and compression
of the air from the leading edge to the trailing
edge.
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