, desired
position, and view of pilot, observer, etc.

6. Horizontal Equivalent.--The vertical component of the reaction, i.e.,
lift, varies as the horizontal equivalent (H.E.) of the surface, but
the drift remains the same. Then it follows that if H.E. grows less, the
ratio of lift to drift must do the same.

A, B, and C are front views of three surfaces.

A has its full H.E., and therefore, from the point of view from which
we are at the moment considering efficiency, it has its best lift-drift
ratio.

B and C both possess the same surface as A, but one is inclined upwards
from its centre and the other is straight but tilted. For these reasons
their H.E.'s are, as illustrated, less than in the case of A. That means
less vertical lift, and, the drift remaining the same (for there is
the same amount of surface as in A to produce it), the lift-drift ratio
falls.

THE MARGIN OF POWER is the power available above that necessary to
maintain horizontal flight.

THE MARGIN OF LIFT is the height an aeroplane can gain in a given time
and starting from a given altitude. As an example, thus: 1,000 feet the
first minute, and starting from an altitude of 500 feet above sea-level.

The margin of lift decreases with altitude, owing to the decrease in
the density of the air, which adversely affects the engine. Provided
the engine maintained its impulse with altitude, then, if we ignore the
problem of the propeller, which I will go into later on, the margin of
lift would not disappear. Moreover, greater velocity for a given power
would be secured at a greater altitude, owing to the decreased density
of air to be overcome. After reading that, you may like to light your
pipe and indulge in dreams of the wonderful possibilities which may
become realities if some brilliant genius shows us some day how to
secure a constant power with increasing altitude. I am afraid, however,
that will always remain impossible; but it is probable that some very
interesting steps may be taken in that direction.

THE MINIMUM ANGLE OF INCIDENCE is the smallest angle at which, for
a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

THE MAXIMUM ANGLE OF INCIDENCE is the greatest angle at which, for
a given power, surface (including detrimental surface), and weight,
horizontal flight can be maintained.

THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift
ratio is highest. In modern