The Aeroplane Speaks [33]
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.
Propeller torque affects lateral stability. An aeroplane tends to turn over sideways in the opposite direction to which the propeller revolves. 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:
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.
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 illustrated 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:
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 attitude, 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 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.
Propeller torque affects lateral stability. An aeroplane tends to turn over sideways in the opposite direction to which the propeller revolves. 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:
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.
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 illustrated 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:
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 attitude, 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