Should the surface
tend to assume too large an angle, then the reverse
happens--the C.P. moves back and pushes the rear of the
surface up.
Flat surfaces are, then, theoretically stable longitudinally.
They are not, however, used, on account of their poor
lift-drift ratio.
As already explained, cambered surfaces are used, and
these are longitudinally unstable at those angles of incidence
producing a reasonable lift-drift ratio, i.e., at angles below:
about 12 degrees.
A is a cambered surface, attitude approximately vertical,
moving through the air in the direction M. Obviously the C. P.
coincides with the transverse centre line of the surface.
With decreasing angles, down to angles of about 30 degrees,
the C.P. moves forward as in the case of flat surfaces (see B),
but angles above 30 degrees do not interest us, since they produce
a very low ratio of lift to drift.
Below angles of about 30 degrees (see C) the dipping front part
of the surface assumes a negative angle of incidence resulting
in the DOWNWARD air pressure D, and the more the angle of
incidence is decreased, the greater such negative angle and its
resultant pressure D. Since the C.P. is the resultant of all
the air forces, its position is naturally affected by D, which
causes it to move backwards. Now, should some gust or
eddy tend to make the surface decrease its angle of incidence,
i.e., dive, then the C.P. moves backwards, and, pushing up
the rear of the surface, causes it to dive the more.
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