[Ill.u.s.tration: R, Direction of reaction of wing indicated.

R R, Resultant direction of reaction of both wings.

M, Horizontal (sideway) component of reaction.

L, Vertical component of reaction (lift).]

In the case of A, the resultant direction of the reaction of both wings is opposed to the direction of gravity or weight. The two forces R R and gravity are then evenly balanced, and the surface is in a state of equilibrium.

In the case of B, you will note that the R R is not directly opposed to gravity. This results in the appearance of M, and so the resultant direction of motion of the aeroplane is no longer directly forward, but is along a line the resultant of the Thrust and M. In other words, it is, while flying forward, at the same time moving sideways in the direction M.

In moving sideways, the keel-surface receives, of course, a pressure from the air equal and opposite to M. Since such surface is greatest in effect towards the tail, then the latter must be pushed sideways.

That causes the aeroplane to turn; and, the highest wing being on the outside of the turn, it has a greater velocity than the lower wing. That produces greater lift, and tends to tilt the aeroplane over still more.

Such tilting tendency is, however, opposed by the difference in the H.E.'s of the two wings.

It then follows that, for the lateral dihedral angle to be effective, such angle must be large enough to produce, when the aeroplane tilts, a difference in the H.E.'s of the two wings, which difference must be sufficient to not only oppose the tilting tendency due to the aeroplane turning, but sufficient to also force the aeroplane back to its original position of equilibrium.

It is now, I hope, clear to the reader that the lateral dihedral is not quite so effective as would appear at first sight. Some designers, indeed, prefer not to use it, since its effect is not very great, and since it must be paid for in loss of H.E. and consequently loss of lift, thus decreasing the lift-drift ratio, _i.e._, the efficiency. Also, it is sometimes advanced that the lateral dihedral increases the "spill" of air from the wing-tips and that this adversely affects the lift-drift ratio.

_The disposition of the keel-surface_ affects the lateral stability. It should be, in effect, equally divided by the longitudinal turning axis of the aeroplane. If there is an excess of keel-surface above or below such axis, then a side gust striking it will tend to turn the aeroplane over sideways.

_The position of the centre of gravity_ affects lateral stability. If too low, it produces a pendulum effect and causes the aeroplane to roll sideways.

If too high, it acts as a stick balanced vertically would act. If disturbed, it tends to travel to a position as far as possible from its original position. It would then tend, when moved, to turn the aeroplane over sideways and into an upside-down position.

From the point of view of lateral stability, the best position for the centre of gravity is one a little below the centre of drift. This produces a little lateral stability without any marked pendulum effect.

_Propeller torque_ affects lateral stability. An aeroplane tends to turn over sideways in the opposite direction to which the propeller revolves.

[Ill.u.s.tration]

This tendency is offset by increasing the angle of incidence (and consequently the lift) of the side tending to fall; and it is always advisable, if practical considerations allow it, to also decrease the angle upon the other side. In that way it is not necessary to depart so far from the normal angle of incidence at which the lift-drift ratio is highest.

_Wash-in_ is the term applied to the increased angle.

_Wash-out_ is the term applied to the decreased angle.

Both lateral and directional stability may be improved by washing out the angle of incidence on both sides of the surface, thus:

[Ill.u.s.tration]

The decreased angle decreases the drift and therefore the effect of gusts upon the wing-tips, which is just where they have the most effect upon the aeroplane, owing to the distance from the turning axis.

The wash-out also renders the ailerons (lateral controlling services) more effective, as, in order to operate them, it is not then necessary to give them such a large angle of incidence as would otherwise be required.

[Ill.u.s.tration: Note: Observe that the inclination of the ailerons to the surface is the same in each case.]

The less the angle of incidence of the ailerons, the better their lift-drift ratio, i.e., their efficiency. You will note that, while the aileron attached to the surface with washed-out angle is operated to the same extent as the aileron ill.u.s.trated above it, its angle of incidence is considerably less. Its efficiency is therefore greater.

The advantages of the wash-in must, of course, be paid for in some loss of lift, as the lift decreases with the decreased angle.

In order to secure all the above described advantages, a combination is sometimes effected, thus:

[Ill.u.s.tration: "Wash Out" on both sides relative to the Centre.]

BANKING.--An aeroplane turned off its course to right or left does not at once proceed along its new course. Its momentum in the direction of its first course causes it to travel along a line the resultant of such momentum and the thrust. In other words, it more or less skids sideways and away from the centre of the turn. Its lifting surfaces do not then meet the air in their correct att.i.tude, and the lift may fall to such an extent as to become less than the weight, in which case the aeroplane must fall. This bad effect is minimized by "banking," _i.e._, tilting the aeroplane sideways. The bottom of the lifting surface is in that way opposed to the air through which it is moving in the direction of the momentum and receives an opposite air pressure. The rarefied area over the top of the surface is rendered still more rare, and this, of course, a.s.sists the air pressure in opposing the momentum.

The velocity of the "skid," or sideways movement, is then only such as is necessary to secure an air pressure equal and opposite to the centrifugal force of the turn.

The sharper the turn, the greater the effect of the centrifugal force, and therefore the steeper should be the "bank." _Experientia docet_.

_The position of the centre of gravity_ affects banking. A low C.G. will tend to swing outward from the centre of the turn, and will cause the aeroplane to bank--perhaps too much, in which case the pilot must remedy matters by operating the ailerons.

A high C.G. also tends to swing outward from the centre of the turn. It will tend to make the aeroplane bank the wrong way, and such effect must be remedied by means of the ailerons.

The pleasantest machine from a banking point of view is one in which the C.G. is a little below the centre of drift. It tends to bank the aeroplane the right way for the turn, and the pilot can, if necessary, perfect the bank by means of the ailerons.

_The disposition of the keel-surface_ affects banking. It should be, in effect, evenly divided by the longitudinal axis. An excess of keel-surface above the longitudinal axis will, when banking, receive an air pressure causing the aeroplane to bank, perhaps too much. An excess of keel-surface below the axis has the reverse effect.

SIDE-SLIPPING.--This usually occurs as a result of over-banking. It is always the result of the aeroplane tilting sideways and thus decreasing the horizontal equivalent, and therefore the lift, of the surface. An excessive "bank," or sideways tilt, results in the H.E., and therefore the lift, becoming less than the weight, when, of course, the aeroplane must fall, _i.e._, side-slip.

[Ill.u.s.tration]

When making a very sharp turn it is necessary to bank very steeply indeed. If, at the same time, the longitudinal axis of the aeroplane remains approximately horizontal, then there must be a fall, and the direction of motion will be the resultant of the thrust and the fall as ill.u.s.trated above in sketch A. The lifting surfaces and the controlling surfaces are not then meeting the air in the correct att.i.tude, with the result that, in addition to falling, the aeroplane will probably become quite unmanageable.

The pilot, however, prevents such a state of affairs from happening by "nosing-down," _i.e._, by operating the rudder to turn the nose of the aeroplane downward and towards the direction of motion as ill.u.s.trated in sketch B. This results in the higher wing, which is on the outside of the turn, travelling with greater velocity, and therefore securing a greater reaction than the lower wing, thus tending to tilt the aeroplane over still more. The aeroplane is now almost upside-down, _but_ its att.i.tude relative to the direction of motion is correct and the controlling surfaces are all of them working efficiently. The recovery of a normal att.i.tude relative to the Earth is then made as ill.u.s.trated in sketch C.

The pilot must then learn to know just the angle of bank at which the margin of lift is lost, and, if a sharp turn necessitates banking beyond that angle, he must "nose-down."

In this matter of banking and nosing-down, and, indeed, regarding stability and control generally, the golden rule for all but very experienced pilots should be: _Keep the aeroplane in such an att.i.tude that the air pressure is always directly in the pilot's face._ The aeroplane is then always engaging the air as designed to do so, and both lifting and controlling surfaces are acting efficiently. The only exception to this rule is a vertical dive, and I think that is obviously not an att.i.tude for any but very experienced pilots to hanker after.

SPINNING.--This is the worst of all predicaments the pilot can find himself in. Fortunately it rarely happens.

It is due to the combination of (1) a very steep spiral descent of small radius, and (2) insufficiency of keel-surface behind the vertical axis, or the jamming of the rudder and/or elevator into a position by which the aeroplane is forced into an increasingly steep and small spiral.

Owing to the small radius of such a spiral, the ma.s.s of the aeroplane may gain a rotary momentum greater, in effect, than the air pressure of the keel-surface or controlling surfaces opposed to it; and, when once such a condition occurs, it is difficult to see what can be done by the pilot to remedy it. The sensible pilot will not go beyond reasonable limits of steepness and radius when executing spiral descents.

[Ill.u.s.tration: Nose Dive Spin.]

In this connection every pilot of an aeroplane fitted with a rotary engine should bear in mind the gyroscopic effect of such engine. In the case of such an engine fitted to a "pusher" aeroplane, its effect when a left-hand turn is made is to depress the nose of the machine. If fitted to a "tractor" it is reversed, so the effect is to depress the nose if a right-hand turn is made. The sharper the turn, the greater such effect--an effect which may render the aeroplane unmanageable if the spiral is one of very small radius and the engine is revolving with sufficient speed to produce a material gyroscopic effect. Such gyroscopic effect should, however, slightly _a.s.sist_ the pilot to navigate a small spiral if he will remember to (1) make _right-hand_ spirals in the case of a "pusher," (2) make _left-hand_ spirals in the case of a "tractor." The effect will then be to keep the nose up and prevent a nose-dive. I say "slightly" a.s.sist because the engine is, of course, throttled down for a spiral descent, and its lesser revolutions will produce a lesser gyroscopic effect.

On the other hand, it might be argued that if the aeroplane gets into a "spin," anything tending to depress the nose of the machine is of value, since it is often claimed that the best way to get out of a spin is to put the machine into a nose-dive--the great velocity of the dive rendering the controls more efficient and better enabling the pilot to regain control. It is, however, a very contentious point, and few are able to express opinions based on practice, since pilots indulging in nose-dive spins are either not heard of again or have usually but a hazy recollection of exactly what happened to them.

GLIDING DESCENT WITHOUT PROPELLER THRUST.--All aeroplanes are, or should be, designed to a.s.sume their correct gliding angle when the power and thrust is cut off. This relieves the pilot of work, worry, and danger should he find himself in a fog or cloud. The pilot, although he may not realize it, maintains the correct att.i.tude of the aeroplane by observing its position relative to the horizon. Flying into a fog or cloud the horizon is lost to view, and he must then rely upon his instruments--(1) the compa.s.s for direction; (2) an inclinometer (arched spirit-level) mounted transversely to the longitudinal axis, for lateral stability; and (3) an inclinometer mounted parallel to the longitudinal axis, or the airspeed indicator, which will indicate a nose-down position by increase in air speed, and a tail-down position by decrease in air speed.

The pilot is then under the necessity of watching three instruments and manipulating his three controls to keep the instruments indicating longitudinal, lateral, and directional stability. That is a feat beyond the capacity of the ordinary man. If, however, by the simple movement of throttling down the power and thrust, he can be relieved of looking after the longitudinal stability, he then has only two instruments to watch. That is no small job in itself, but it is, at any rate, fairly practicable.

[Ill.u.s.tration]

Aeroplanes are, then, designed, or should be, so that the centre of gravity is slightly forward of centre of lift. The aeroplane is then, as a glider, nose-heavy--and the distance the C.G. is placed in advance of the C.L. should be such as to ensure a gliding angle producing a velocity the same as the normal flying speed (for which the strength of construction has been designed).