s 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.

[Illustration]

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:

[Illustration]

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:

[Illustration]

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 surface has been given an angle of
incidence, and therefore has a lift and C.P. In practice the stabilizer
is often set at no angle of incidence. In such case the proposition
remains the same, but it is, perhaps, a little easier to illustrate
it as above.

Now, we will suppose that a gust or eddy throws the machine into the
lower position. It no longer travels in the direction of T, since the
momentum in the old direction pulls it off that course. M is now the
resultant of the Thrust and the Momentum, and you will note that this
results in a decrease in the angle our old friend the neutral lift line
makes with M, _i.e._, a decrease in the angle of incidence and therefore
a decrease in lift.

We will suppose that this decrease is 2 deg.. Such decrease applies to
both main surface and stabilizer, since both are fixed rigidly to the
aeroplane.

The main surface, which had 12 deg. angle, has now only 10 deg., _i.e._, a loss
of _one-sixth_.

The stabilizer, which had 4 deg.