THE OPTIMUM ANGLE OF INCIDENCE is the angle at which the lift-drift ratio is highest. In modern 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 ma.s.s of air to secure the requisite lift.

[Ill.u.s.tration]

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 pa.s.sive 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 ma.s.s 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.

[Ill.u.s.tration: 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 ALt.i.tUDE AND WHEN THE THRUST IS HORIZONTAL.]

MINIMUM ANGLE.

This gives the greatest velocity during horizontal flight at a low alt.i.tude. 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 alt.i.tude. 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 alt.i.tude a slight margin of lift when the propeller thrust is horizontal. By this means, when the alt.i.tude is reached where the margin of lift disappears (on account of loss of engine power), and which is, consequently, the alt.i.tude 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 alt.i.tude, and when the thrust is horizontal, should then be such that the higher alt.i.tude at which the margin of lift is lost is that alt.i.tude 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 alt.i.tude 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