nce the centre line of
pressure (C.P.) moves forward.

The C.P. is a line taken across the surface, transverse to the direction
of motion, and about which all the air forces may be said to balance, or
through which they may be said to act.

[Illustration]

Imagine A to be a flat surface, attitude vertical, travelling through
the air in the direction of motion M. Its C.P. is then obviously along
the exact centre line of the surface as illustrated. In B, C, and D the
surfaces are shown with angles of incidence decreasing to nothing, and
you will note that the C.P. moves forward with the decreasing angle.[17]

Now, should some gust or eddy tend to make the surface decrease the
angle, _i.e._, dive, then the C.P. moves forward and pushes the front
of the surface up. 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 deg..

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 deg., the C.P. moves
forward as in the case of flat surfaces (see B); but angles above 30 deg. do
not interest us, since they produce a very low ratio of lift to drift.

[Illustration]

Below angles of about 30 deg. (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. Should the surface tend to
assume too large an angle, then the reverse happens; the pressure D
decreases, with the result that C.P. moves forward and pushes up the
front of the surface, thu