urface 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 large_.

5. _Camber_.--Since such an aeroplane would be of low velocity, and
therefore possess a large angle of incidence, a _large camber_ would be
necessary.

Let us now consider the essentials for an aeroplane of maximum velocity
for its power, and possessing merely enough lift to get off the ground,
but no margin of lift.

1. Comparatively _high velocity_.

2. A comparatively _small surface_, because, being of greater velocity
than the maximum climber, a greater mass of air will be engaged for
a given surface and time, and therefore a smaller surface will be
sufficient to secure the requisite lift.

3. _A small angle relative to the propeller thrust_, since the latter
coincides with the direction of motion.

4. A comparatively _small angle of incidence_ by reason of the high
velocity.

5. A comparatively _small camber_ follows as a result of the small
angle of incidence.

[Illustration: ANGLES OF INCIDENCE (INDICATED APPROXIMATELY) OF AN
AEROPLANE DESIGNED AS A COMPROMISE BETWEEN VELOCITY AND CLIMB, AND
POSSESSING A SLIGHT MARGIN OF LIFT AT A LOW ALTITUDE AND WHEN THE
THRUST IS HORIZONTAL.]