The weight is, of course, balanced about a C.P., the resultant
of the C.P. of the main surface and the C.P. of the stabilizing
surface. For the sake of illustration, the stabilizing 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.
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