Part 10 (1/2)

[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 was.h.i.+ng 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.