The pilot is then under the necessity of watching three
instruments and manipulating his three controls to keep the
instruments indicating longitudinal, lateral, and directional
stability. That is a feat beyond the capacity of the ordinary
man. If, however, by the simple movement of throttling
down the power and thrust, he can be relieved of looking
after the longitudinal stability, he then has only two instruments
to watch. That is no small job in itself, but it is,
at any rate, fairly practicable.
Aeroplanes are, then, designed, or should be, so that the
centre of gravity is slightly forward of centre of lift. The
aeroplane is then, as a glider, nose-heavy--and the distance
the C.G. is placed in advance of the C.L. should be such as
to ensure a gliding angle producing a velocity the same as
the normal flying speed (for which the strength of construction
has been designed).
In order that this nose-heavy tendency should not exist
when the thrust is working and descent not required, the
centre of thrust is placed a little below the centre of drift
or resistance, and thus tends to pull up the nose of the
aeroplane.
The distance the centre of thrust is placed below the
centre of drift should be such as to produce a force equal
and opposite to that due to the C.G. being forward of the
C.L.
LOOPING AND UPSIDE DOWN FLYING.--If a loop is desired,
it is best to throttle the engine down at point A. The C.G.
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