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 degrees. Such decrease applies
to both main surface and stabilizer, since both are fixed rigidly to the
aeroplane.

The main surface, which had 12 degrees angle, has now only 10 degrees,
i.e., a loss of ONE-SIXTH.

The stabilizer, which had 4 degrees angle, has now only 2 degrees, 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 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 f