aeroplanes it is that angle of incidence
possessed by the surface when the axis of the propeller is horizontal.

THE BEST CLIMBING ANGLE is approximately half-way between the maximum
and the optimum angles.

All present-day aeroplanes are a compromise between Climb and horizontal
Velocity. We will compare the essentials for two aeroplanes, one
designed for maximum climb, and the other for maximum velocity.

ESSENTIALS FOR MAXIMUM CLIMB:

1. Low velocity, in order to secure the best lift-drift ratio.

2. Having a low velocity, a large surface will be necessary in order to
engage the necessary mass of air to secure the requisite lift.

3. Since (1) such a climbing machine will move along an upward sloping
path, and (2) will climb with its propeller thrust horizontal, then a
large angle relative to the direction of the thrust will be necessary in
order to secure the requisite angle relative to the direction of motion.

The propeller thrust should be always horizontal, because the most
efficient flying-machine (having regard to climb OR velocity) has, so
far, been found to be an arrangement of an inclined surface driven by
a HORIZONTAL thrust--the surface lifting the weight, and the thrust
overcoming the drift. This is, in practice, a far more efficient
arrangement than the helicopter, i.e., the air-screw revolving about
a vertical axis and producing a thrust opposed to gravity. If, when
climbing, the propeller thrust is at such an angle as to tend to haul
the aeroplane upwards, then it is, in a measure, acting as a helicopter,
and that means inefficiency. The reason of a helicopter being
inefficient in practice is due to the fact that, owing to mechanical
difficulties, it is impossible to construct within a reasonable weight
an air-screw of the requisite dimensions. That being so, it would be
necessary, in order to absorb the power of the engine, to revolve the
comparatively small-surfaced air screw at an immensely greater velocity
than that of the aeroplane's surface. As already explained, the
lift-drift ratio falls with velocity on account of the increase in
passive drift. This applies to a blade of a propeller or air-screw,
which is nothing but a revolving surface set at angle of incidence, and
which it is impossible to construct without a good deal of detrimental
surface near the central boss.

4. The velocity being low, then it follows that for that reason also the
angle of incidence should be comparatively