General Notes on Model Designing
=Calculations in Designing a Plant for a Model Aeroplane.=—The correct
method to adopt in designing a model is to build the machine exclusive
of the motor, weigh it, and design the plant to suit; or to build the
plant first, determine the thrust it develops, and vary the dimensions
of the machine (and hence the weight) to suit.
It can now be taken as a general rule that a plant will fly a machine
three times the weight of its thrust. Hence a plant developing 3-oz.
thrust would fly a machine weighing 9 oz. or 10 oz. But, since
the thrust of a compressed-air engine is not constant, gradually
diminishing as the pressure in the container grows less, for a machine
weighing 9 oz. (assuming the machine to be built first) the plant will
require to develop about 5 oz. initial thrust. The diameter of the
propeller is dependent on the most efficient speed of the particular
motor employed.
Assuming that the model does not weigh more than 17 oz., a
four-cylinder engine constructed would answer admirably. It would
require a container 24 in. long and 3 in. in diameter, constructed
of copper or hard-drawn brass foil ·002 in. thick. Should, however,
the reader particularly desire to fit a rotary motor, doubtless the
five-cylinder rotary previously described will suit.
=Building Scale Models.=—The great difficulty in building scale models
to fly with rubber motors is to get the centre of gravity in the same
relative position that it holds in the prototype. This is due to the
long length of rubber motor required to give the requisite power, and
also to ensure a reasonable time-length of flight.
It is in connection with the fixing or arranging of the rubber motor
that the most radical departures in the design of the prototype will
have to be made, although the writer has evolved an arrangement
whereby even this need not entail much departure from the lines of the
original. This arrangement consists in providing a separate strut or
frame to take up all strain from the rubber, it only being necessary
to arrange suitable fastenings for the strut, which may take the form
of clips, so making it possible either to remove the motor for the
purpose of changing or repairing the rubber, or substituting a motor
of different power or length. This is a great advantage on tractor
monoplanes (with the main plane in front), where the rubber is more
or less inaccessible by reason of the closed-in frame or fuselage of
the machine. Another important advantage to be gained from the use of
a detachable motor is that its position fore and aft on the machine
can be varied, in order to bring the centre of gravity into the proper
position to obtain correct balance, or, speaking with more technical
accuracy, to make the centres of pressure and gravity coincide.
With regard to the type of motor to adopt, this depends very largely on
the machine which is being modelled. Whenever possible it should be of
the simplest possible kind, consisting of the main strut to take the
tensile strain, compressive, of course, so far as it affects the strut,
and the torsional strain put on by the twisting of the rubber. At one
end of this strut a hook of wire or other form of metal is formed
to hold the rubber skein, whilst at the other end is fixed a plain
bearing. Through this the propeller spindle is passed, having a hook at
its end, over which the other end of the rubber is placed.
On certain types such a simple motor is not possible. In order to
concentrate the weight more at one point, the rubber and its struts
have to be shortened; and to get the necessary number of revolutions of
the propeller a gearing of two to one, three to one, or four to one,
as the case may be, must be used. By this means the small number of
turns which can be got on a short thick skein of rubber of great power
will still give the number of propeller revolutions required to make a
good flight, just the same as with a motor of ordinary thickness and of
great length. Of course, some power is lost in the gearing.
To resist fuselage distortion the spar must be suitably braced in a
lateral direction, the outrigger carrying the bracing wires being
situated just forward of the centre of the spar. No. 35 S.W.G. is quite
strong enough for fuselage bracing. Silk fishing-line or Japanese silk
gut is admirably suited for wing bracing, and is not so liable to
stretch as the tinned iron or brass wire sometimes used. Piano wire
is generally used for elevators, tail planes, chassis, and propeller
shafts, of a gauge ranging from No. 17 S.W.G. to No. 22 S.W.G. A
clock-spring or piano-wire protector fitted to the nose of a model
aeroplane will also prevent a broken spar should it strike some object
such as a tree or wall during flight.
The Kite and Model Aeroplane Association, which is the paramount body
to observe and control model flying in England, and which is recognised
by the Royal Aero Club, stipulate that protectors must be fitted to all
machines competing in their contests.