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. 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, thus increasing the angle still further, the final result being
a "tail-slide."

It is therefore necessary to find a means of stabilizing the naturally
unstable cambered surface. This is usually secured by means of a
stabilizing surface fixed some distance in the rear of the main surface,
and it is a necessary condition that the neutral lift lines of the two
surfaces, when projected to meet each other, make a dihedral angle. In
other words, the rear stabilizing surface must have a lesser angle of
incidence than the main surface--certainly not more than one-third of
that of the main surface. This is known as the longitudinal dihedral.

I may add that the tail-plane is sometimes mounted upon the aeroplane at
the same angle as the main surface, but, in such cases, it attacks air
which has received a downward deflection from the main surface, thus:

The angle at which the tail surface attacks the air (the angle of
incidence) is therefore less than the angle of incidence of the main
surface.

I will now, by means of the following illustration, try to explain how
the longitudinal dihedral secures stability:

First, imagine the aeroplane travelling in the direction of motion,
which coincides with the direction of thrust T. The weight is, of
course, balanced about a C.P., the resultant of the C.P. of the main
surface and the C.P. of the stabilizing surface. For the sake of
illustration, the stabilizing