Any unevenness in the acceleration and
compression of the air produces drift.
The top camber: If this was flat it would produce
a rarefied area of irregular shape. I have already
explained the bad effect this has upon the lift-
drift ratio. The top surface is then curved to
produce a rarefied area the shape of which shall
be as stream-line and free from attendant eddies
as possible.
The camber varies with the angle of incidence,
the velocity, and the thickness of the surface.
Generally speaking, the greater the velocity, the
less the camber and angle of incidence. With
infinite velocity the surface would be set at no
angle of incidence (the neutral lift line coincident
with the direction of motion relative to the air),
and would be, top and bottom, of pure streamline
form--i.e., of infinite fineness. This is, of
course, carrying theory to absurdity as the surface
would then cease to exist.
The best cambers for varying velocities, angles
of incidence, and thicknesses of surface, are found
by means of wind-tunnel research. The practical
application of all this is in taking the greatest
care to prevent the surface from becoming distorted
and thus spoiling the camber and consequently
the lift-drift ratio.
4. Aspect Ratio.--This is the proportion of span to
chord. Thus, if the span is, for instance, 50 feet
and the chord 5 feet, the surface would be said to
have an aspect ratio of 10 to 1.
For A GIVEN VELOCITY and A GIVEN AREA of surface,
the greater the aspect ratio, the greater the reaction.
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