THE STEERING OF THE MODEL.
§ 1. Of all the various sections of model aeroplaning that which is
the least satisfactory is the above.
The torque of the propeller naturally exerts a twisting or tilting
effect upon the model as a whole, the effect of which is to cause it
to fly in (roughly speaking) a circular course, the direction
depending on whether the pitch of the screw be a right or left handed
one. There are various devices by which the torque may be
(approximately) got rid of.
§ 2. In the case of a monoplane, by not placing the rod carrying the
rubber motor in the exact centre of the main aerofoil, but slightly to
one side, the exact position to be determined by experiment.
In a biplane the same result is obtained by keeping the rod in the
centre, but placing the bracket carrying the bearing in which the
propeller shaft runs at right angles horizontally to the rod to obtain
the same effect.
§ 3. The most obvious solution of the problem is to use _two_ equal
propellers (as in the Wright biplane) of equal and opposite pitch,
driven by two rubber motors of equal strength.
Theoretically this idea is perfect. In practice it is not so. It is
quite possible, of course, to use two rubber motors of an equal number
of strands (equality should be first tested by _weighing_). It should
be possible to obtain two propellers of equal and opposite pitch,
etc., and it is also possible to give the rubber motors the same
number of turns. In practice one is always wound up before the other.
This is the first mistake. They should be wound up _at the same time_,
using a double winder made for the purpose.
The fact that this is _not_ done is quite sufficient to give an
unequal torsion. The friction in both cases must also be exactly
equal. Both propellers must be released at exactly the same instant.
Supposing _all_ these conditions fulfilled (in practice they never
are), supposing also the propellers connected by gearing (prohibitive
on account of the weight), and the air quite calm (which it never is),
then the machine should and undoubtedly would _fly straight_.
For steering purposes by winding up one propeller _many more times_
than the other, the aeroplane can generally speaking be steered to the
right or left; but from what I have both seen and tried twin-screw
model aeroplanes are _not_ the success they are often made out to be,
and they are much more troublesome to deal with, in spite of what some
say to the contrary.
The solution of the problem of steering by the use of two propellers
is only partially satisfactory and reliable, in fact, it is no
solution at all.[45] The torque of the propeller and consequent
tilting of the aeroplane is not the only cause at work diverting the
machine from its course.
§ 4. As it progresses through the air it is constantly meeting air
currents of varying velocity and direction, all tending to make the
model deviate more or less from its course; the best way, in fact, the
only way, to successfully overcome such is by means of _speed_, by
giving the aeroplane a high velocity, not of ten or twelve to fifteen
miles an hour, as is usual in built up fabric-covered aerofoils, but a
velocity of twenty to thirty miles an hour, attainable only in models
(petrol or steam driven) or by means of wooden or metal aerofoils.
§ 5. Amongst devices used for horizontal steering are vertical "FINS."
These should be placed in the rear above the centre of gravity. They
should not be large, and can be made of fabric tightly stretched over
a wire frame, or of a piece of sheet magnalium or aluminium, turning
on a pivot at the front edge, adjustment being made by simply twisting
the fin round to the desired angle. As to the size, think of rudder
and the size of a boat, but allow for the difference of medium. The
frame carrying the pivot and fin should be made to slide along the rod
or backbone of the model in order to find the most efficient position.
§ 6. Steering may also be attempted by means of little balancing tips,
or ailerons, fixed to or near the main aerofoil, and pivoted (either
centrally or otherwise) in such a manner that they can be rotated one
in one direction (tilted) and the other in the other (dipped), so as
to raise one side and depress the other.
§ 7. The model can also be steered by giving it a cant to one side by
weighting the tip of the aerofoil on that side on which it is desired
it should turn, but this method is both clumsy and "weighty."
§ 8. Another way is by means of the elevator; and this method, since
it entails no additional surfaces entailing extra resistance and
weight, is perhaps the most satisfactory of all.
It is necessary that the elevator be mounted on some kind of universal
joint, in order that it may not only be "tipped" or "dipped," but also
canted sideways for horizontal steering.
§ 9. A vertical fin in the rear, or something in the nature of a
"keel," i.e. a vertical fin running down the backbone of the machine,
greatly assists this movement.
If the model be of the tractor screw and tail (Blériot) type, then the
above remarks _re_ elevator apply _mutatis mutandis_ to the tail.
§ 10. It is of the most vital importance that the propeller torque
should be, as far as possible, correctly balanced. This can be tested
by balancing the model transversely on a knife edge, winding up the
propeller, and allowing it to run down, and adjusting matters until
the torque and compensatory apparatus balance. As the torque varies
the mean should be used.
In the case of twin propellers, suspend the model by its centre of
gravity, wind up the propellers, and when running down if the model is
drawn forward without rotation the thrust is equal; if not adjustment
must be made till it does. The easiest way to do this _may_ be by
placing one propeller, the one giving the greater thrust, slightly
nearer the centre.
In the case of two propellers rotating in opposite directions (by
suitable gearing) on the common centre of two axes, one of the axes
being, of course, hollow, and turning on the other--the rear propeller
working in air already driven back by the other will require a coarser
pitch or larger diameter to be equally efficient.
FOOTNOTE:
[45] These remarks apply to rubber driven motors. In the case of
two-power driven propellers in which the power was automatically
adjusted, say, by a gyroscope as in the case of a torpedo--and the
_speed_ of each propeller varied accordingly--the machine could, of
course, be easily steered by such means; but the model to carry such
power and appliances would certainly weigh from 40 lb. to 60 lb.