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.

[Illustration]

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. (see illustration above).

LOOPING AND UPSIDE-DOWN FLYING.--If a loop is desired, it is best to
throttle the engine down at point A. The C.G. being forward of the C.P.,
then causes the aeroplane to nose down, and assists the pilot in making
a reasonably small loop along the course C and in securing a quick
recovery. If the engine is not throttled down, then the aeroplane may be
expected to follow the course D, which results in a longer nose dive
than in the case of the course C.

[Illustration: Position A. Path B. Path C. Path D.]

A steady, gentle movement of the elevator is necessary. A jerky movement
may change the direction of motion so suddenly as to produce dangerous
air stresses upon the surfaces, in which case there is a possibility of
collapse.

If an upside-down flight is desired, the engine may, or may not, be
throttled down at point A. If not throttled down, then the elevator must
be operated to secure a course approximately in the direction B. If it
is throttled down, then the course must be one of a steeper angle than
B, or there will be danger of stalling.

[Footnote 16: "In effect" because, although there may be actually the
greatest proportion of keel-surface in front of the vertical axis, such
surface may be much nearer to the axis than is the keel-surface towards
the tail. The latter may then be actually less than the surface in
front, but, being farther from the axis, it has a greater leverage,
and consequently is greater in