angle, has now only 2 deg., _i.e._, a loss of
_one-half_.

The latter has therefore lost a greater _proportion_ of its angle of
incidence, and consequently its lift, than has the main surface. It must
then fall relative to the main surface. The tail falling, the aeroplane
then assumes its first position, though at a slightly less altitude.

Should a gust throw the nose of the aeroplane up, then the reverse
happens. Both main surface and stabilizer increase their angles of
incidence in the same amount, but the angle, and therefore the lift, of
the stabilizer increases in greater proportion than does the angle and
lift of the main surface, with the result that it lifts the tail. The
aeroplane then assumes its first position, though at a slightly greater
altitude.

Do not fall into the widespread error that the angle of incidence varies
as the angle of the aeroplane to the horizontal. It varies with such
angle, but not as anything approaching it. Remember that the stabilizing
effect of the longitudinal dihedral lasts only as long as there is
momentum in the direction of the first course.

These stabilizing movements are taking place all the time, even though
imperceptible to the pilot.

Aeroplanes have, in the past, been built with a stabilizing surface
in front of the main surface instead of at the rear of it. In such
design the main surface (which is then the tail surface as well as the
principal lifting surface) must be set at a less angle than the forward
stabilizing surface, in order to secure a longitudinal dihedral. The
defect of such design lies in the fact that the main surface must have
a certain angle to lift the weight--say 5 deg.. Then, in order to secure
a sufficiency of longitudinal stability, it is necessary to set the
forward stabilizer at about 15 deg.. Such a large angle of incidence results
in a very poor lift-drift ratio (and consequently great loss of
efficiency), except at very low velocities compared with the speed of
modern aeroplanes. At the time such aeroplanes were built velocities
were comparatively low, and this defect was, for that reason, not
sufficiently appreciated. In the end it killed the "canard" or
"tail-first" design.

Aeroplanes of the Dunne and similar types possess no stabilizing surface
distinct from the main surface, but they have a longitudinal dihedral
which renders them stable.

The main surface towards the wing-tips is given a decreasing angle of
incidence and corre