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:
{illust.}
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.
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