rgin of lift when the propeller thrust is horizontal. By this means,
when the altitude is reached where the margin of lift disappears
(on account of loss of engine power), and which is, consequently, the
altitude where it is just possible to maintain horizontal flight, the
aeroplane is flying with its thrust horizontal and with maximum
efficiency (as distinct from engine and propeller efficiency).

The margin of lift at low altitude, and when the thrust is horizontal,
should then be such that the higher altitude at which the margin of lift
is lost is that altitude at which most of the aeroplane's horizontal
flight work is done. That ensures maximum velocity when most required.

Unfortunately, where aeroplanes designed for fighting are concerned, the
altitude where most of the work is done is that at which both maximum
velocity and maximum margin of lift for power are required.

Perhaps some day a brilliant inventor will design an aeroplane of
reasonable weight and drift of which it will be possible for the pilot
to vary at will the above-mentioned opposing essentials. Then we shall
get maximum velocity, or maximum margin of lift, for power as required.
Until then the design of the aeroplane must remain a compromise between
Velocity and Climb.

[Footnote 14: See Newton's laws in the Glossary at the end of the book.]

[Footnote 15: See "Aerofoil" in the Glossary.]

CHAPTER II

STABILITY AND CONTROL

STABILITY is a condition whereby an object disturbed has a natural
tendency to return to its first and normal position. Example: a weight
suspended by a cord.

INSTABILITY is a condition whereby an object disturbed has a natural
tendency to move as far as possible away from its first position, with
no tendency to return. Example: a stick balanced vertically upon your
finger.

NEUTRAL INSTABILITY is a condition whereby an object disturbed has no
tendency to move farther than displaced by the force of the disturbance,
and no tendency to return to its first position.

In order that an aeroplane may be reasonably controllable, it is
necessary for it to possess some degree of stability longitudinally,
laterally, and directionally.

LONGITUDINAL STABILITY in an aeroplane is its stability about an axis
transverse to the direction of normal horizontal flight, and without
which it would pitch and toss.

LATERAL STABILITY is its stability about its longitudinal axis, and
without which it would roll sideways.

D