MINIMUM ANGLE.

This gives the greatest velocity during horizontal flight at a low
altitude. Greater velocity would be secured if the surface, angle, and
camber were smaller and designed to just maintain horizontal flight with
a horizontal thrust. Also, in such case, the propeller would not be
thrusting downwards, but along a horizontal line which is obviously a
more efficient arrangement if we regard the aeroplane merely from one
point of view, _i.e._, either with reference to velocity or climb.

OPTIMUM ANGLE. (Thrust horizontal).

The velocity is less than at the smaller minimum angle, and, as
aeroplanes are designed to-day, the area and angle of incidence of the
surface is such as to secure a slight ascent at a low altitude. The
camber of the surface is designed for this angle of incidence and
velocity. The lift-drift ratio is best at this angle.

BEST CLIMBING ANGLE.

The velocity is now still less by reason of the increased angle
producing increase of drift. Less velocity at a given angle produces
less lift, but the increased angle more or less offsets the loss of lift
due to the decreased velocity; and, in addition, the thrust is now
hauling the aeroplane upwards.

MAXIMUM ANGLE.

The greater angle has now produced so much drift as to lessen the
velocity to a point where the combined lifts from the surface and from
the thrust are only just able to maintain horizontal flight. Any greater
angle will result in a still lower lift-drift ratio. The lift will then
become less than the weight and the aeroplane will consequently fall.
Such a fall is known as "stalling" or "pancaking."

=NOTE.--The golden rule for beginners: Never exceed the Best Climbing
Angle. Always maintain the flying speed of the aeroplane.=

SUMMARY.

_Essentials for Maximum Climb._

1. Low velocity.
2. Large surface.
3. Large angle relative to propeller thrust.
4. Large angle relative to direction of motion.
5. Large camber.

_Essentials for Maximum Velocity._

1. High velocity.
2. Small surface.
3. Small angle relative to propeller thrust.
4. Small angle relative to direction of motion.
5. Small camber.

It is mechanically impossible to construct an aeroplane of reasonable
weight of which it would be possible to vary the above opposing
essentials. Therefore, all aeroplanes are designed as a compromise
between Climb and Velocity.

As a rule aeroplanes are designed to have at low altitude a slight
ma