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