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 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 degrees. Then, in order to secure a
sufficiency of longitudinal stability, it is necessary to set the
forward stabilizer at about 15 degrees.
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