Practical Construction: Model Aeroplane Fuselages
In no other portion of a model aeroplane has standardisation become
more marked than in the design and construction of the fuselage or main
frame, both with regard to general details, methods, and materials.
This fact is singular, because in other components contributing in a
greater degree to the success of the model great diversity of opinion
exists. It is difficult to ascribe this lack of uniformity to any
particular reason, unless it is the failure on the part of zealous
amateurs to appreciate the meaning of the term “efficiency.” Very few,
it is thought, endeavour to extract the maximum amount of work for a
minimum expenditure of power from the propellers, surfaces, and so
forth, and the writer, in judging and tabulating some of the model
aeroplane competitions held in different parts of the country, has
found models giving excellent spectacular results which, judged on an
efficiency basis, such as
(distance flown × duration of flight)
-------------------------------------
(weight of rubber)
show a very poor result. The model should be made to fly the longest
possible distance, and to remain in the air the longest possible time
with the smallest possible amount of elastic.
This chapter is devoted to the various types of fuselage for flying
models (as distinct from “scale” models of full-size prototypes) and
methods of constructing them, and the list is as representative of
best practice as it has been possible for the writer, in his extensive
connection with this subject, to make it.
The first shown is the =A= frame (Fig. 23), brought into prominence by
Mr. R. F. Mann. It should have birch longitudinals and spruce cross
members. Quite the best section wood to employ is that shown at =B=,
which forms a convenient seating for the cross members, the latter
being pinned and glued into position. The middle bay of such a frame
requires to be braced, to counteract the torque or distortion caused
by the elastic skein when the latter is in torsion. Diagram =A= shows
the joint at the juncture of the longerons or longitudinals. The hooks
which embrace the elastic skeins are formed from one continuous length
of wire following round the nose of the machine. The bearings may be of
brass, with a lug to follow round the end of the longeron to which it
is bound.
Fig. 24 shows the =T= or cantilever frame, so named because of its
resemblance to that letter. It is usual to make the spar of this
hollow, by channelling out two pieces of wood, and gluing and cramping
them together under pressure. Where the bracing kingpost passes through
the channel should be packed, previous to gluing the two half spars
together, with a piece of hard wood, so that the assembled spar is not
weakened by the piercing necessary for the insertion of the kingpost.
Such a spar should not exceed 4 ft. in length. =C= is a section of the
spar.
[Illustration: Fig. 23.—=A= Frame]
[Illustration: Fig. 26.—=T= Frame]
[Illustration: Fig. 25.—=A= and =T= Frame]
[Illustration: Fig. 24.—=T= Frame]
A much stronger twin-screw fuselage, which is a combination of the =A=
and =T= frames, is shown by Fig. 25. Here, again, a single-channelled
longeron should be used, although the propeller bar and supports should
be solid. The channelled spar can be of silver spruce or birch, and the
bar and supports of mahogany. A section of the spar is given at =D=.
Fig. 26 shows a =T= frame made from a hollow spar, having the ends
splayed out to give the required support to the elastic skein. No
propeller bar is used but there is a tension wire from bearing to
bearing to prevent the splayed section of the spar from spreading when
the rubber is wound up. =E= shows a section of the spar. It should
be pointed out that the greatest width of a spar should be placed
vertically. Never use a square-sectioned spar. Moreover, the bearing
centres should only exceed the propeller diameter by ½ in., since,
apart from consideration of weight, greater rigidity is obtainable from
short spars than from long ones. It is in details such as this that not
only is a considerable saving in weight effected, but also a material
increase in strength and efficiency.
The =T= frame adapted to a “rise-off-ground” fuselage is shown in Fig.
27. A hollow spar should be used for preference, but the spar cut from
the solid is shown, as most amateurs will not be in possession of a
joiner’s plough, which is the tool required for this job. =F= is a
section on the vertical line of the spar.
[Illustration: Fig. 27]
[Illustration: Fig. 28]
[Illustration: Fig. 29]
[Illustration: Fig. 30
Figs. 27 to 30.—Various Forms of Fuselage]
In bracing such frames as those dealt with, fine No. 35 S.W.G.
(Standard Wire Gauge) should be used, fixed to small hooks bound in
suitable places on the spar; the hooks should be made from No. 22 S.W.G.
Fig. 27 shows at =G= a perspective view of a =T=-frame propeller bar
and support. As there shown, the propeller bar fits into a slot cut in
the spar end. If the spar is hollow, the channel should be filled with
hard wood, such as birch, before the slot is cut, to strengthen the
spar at this point. So much for twin-screw fuselages.
Fig. 28 is a perspective view of a boat-shaped tractor fuselage, it
being understood that a tractor machine is one with the air-screw in
front. A model built on such lines is extremely neat in appearance and
has a pleasing aspect in the air. The three longerons are attached to a
three-way brass bearing at the front end, and are simply bound together
at the rear, the hook for the elastic being inserted between the two
top members and turned round the end of one of them for security, as
shown at =H=. The bottom member should be cut 1 in. longer than the
two top ones, to compensate for the shortening due to the curve, which
is effected by compressing the bottom member to the same length as the
top ones. The curved cross members are of bamboo, bent to the required
shape over a lamp flame; or they could be made from piano wire. Their
shape should be drawn full-size to use as a template during the
bending operation. As will be seen, a skid is used to protect the
tractor screw from damage. This should extend for 2 in. beyond the
bearing, and must be attached to the bottom longitudinal directly
beneath the first cross member, so that the latter absorbs the shock of
landing. At the point of intersection between the skid and the axle,
the former should be bound to the latter with fine florist’s wire and
neatly soldered.
A two-membered fuselage can be adapted from this design by omitting the
bottom member and skid. Such a fuselage would be suitable for a light
machine.
It is an essential point with tractor models to fit a chassis; the
purpose thus being twofold. First, it protects the propeller, and
secondly, it obviates the characteristic tendency of tractor machines
to ascend “nose first,” by keeping the weight low (in technical
language, providing a low centre of gravity). Hand-launched tractor
machines that are unprovided with a landing gear are seldom successful
and notoriously troublesome. Furthermore, the centre of thrust
(literally the axis centre, or centre of rotation of the bearing)
should always be above the centre of resistance. The centre of
resistance can usually be taken (although not quite accurate) as being
on a level with the planes.
An exceedingly strong two-membered tractor fuselage of the fusiform
or cigar-shaped type is that shown by Fig. 29, the bearing, which
is bracketed and cut from brass, being shown in detail at =I=. In
this instance the greatest width of the top spar should be disposed
horizontally, the bottom member, with the two cross members, providing
rigidity and a girder-like form of construction. The bottom member need
be only one-half the weight of the top one, as it will be in tension
and so acting as a tie. Silver spruce should be used throughout.
A simple single-spar chassis, consisting of a hollow spar, is shown
by Fig. 30. A kingpost and bracing is fitted underneath the spar, to
counteract the tendency of the twisted skein to bow it. The chassis
should be (and this applies to all models) of piano wire of from No. 17
S.W.G. to No. 18 S.W.G.
A twin-screw propeller-behind fuselage of the cantilever type that
is exceedingly strong, although more difficult than it appears to
construct, is shown by Fig. 31. This can be made exceedingly light from
a hollow spar (packed solid at the point where the kingposts are let
through), and braced with No. 35 S.W.G. piano wire. The bracing is the
difficult operation, as the tension on each wire requires to be very
delicately adjusted to maintain the truth of the spar.
The box-girder type of fuselage shown by Fig. 32 is more suited to
models which aim at an accurate representation of some prototype. It is
intricate in construction yet of neat appearance, the difficulty being
in the adjustment of the large number of bracing wires necessary. The
illustration gives a view of a Blériot type of fuselage, =J= being a
detail of the cross member and compression-strut joint.
Fig. 33 gives some spar sections which are in common use by some of
the crack aero-modellists. All spars should taper in a fore-and-aft
direction, so that it virtually becomes a cantilever. The greatest
cross-section should be one-third of the total length from the front
end of the spar.
[Illustration: Fig. 31]
[Illustration: Fig. 32]
[Illustration: Fig. 33]
[Illustration: Fig. 34
Figs. 31 to 34.—Various Forms of Fuselage]
Another form of spar construction is that given by Fig. 34. =Q= shows a
spar fretted out, the sides being covered with a thin veneer glued and
cramped into place, and =R= the method of making a slotted spar.
Fig. 35 is the simplest possible form of model aeroplane fuselage, if
such it can be called.
Choice of materials and the method of utilising them to the best
advantage, so that the machine is strong without being unduly heavy,
is a phase of model aeroplaning that calls for some care and judgment.
There is a very erroneous impression prevalent among novices that
packing-case wood or similar material is suited to the requirements
peculiar to model aeroplanes. Nothing could be farther from the
truth; and the fact that 50 per cent. of the total marks awarded in
competition are for design and construction should show that this
matter is of primary importance. The true test of any model is the way
it “stands up” to a nose dive, for then the care and forethought of
the builder in providing for anticipated eventualities will manifest
itself. It is to be feared that those who had lavished much care and
infinite pains in the scientific construction of models were woefully
handicapped in competition, the flimsy freak that could flutter aloft
for a minute or so, with three strands of rubber wound to nearly
breaking point, gaining priority over the properly built machine.
There are three salient points to be borne in mind on which the
durability of the machine largely depends: (1) its capacity for
resisting the torque of the rubber motor; (2) of absorbing the shocks
of rough landing; and (3) the provision that has been made for the
rigid attachment of the various parts. If the machine is at fault
with regard to point one, fuselage distortion is likely to occur, and
resulting from this there will be lack of alignment of the surfaces and
attendant troubles. Point two calls for suitable bracing of the spar or
spars, and careful choice of timber. It is inadvisable to use wood of
square cross-section, an oblong section with the greatest measurement
placed vertically being preferable. If no arrangement has been made
to fix rigidly the wings, chassis, etc. (point 3), these parts are
likely to rock or sway when the machine is in the air, and so occasion
bad stability, apart from which a couple of landings would shake the
machine out of truth.
[Illustration: Fig. 35.—Simple Fuselage]
To obviate these difficulties a knowledge of the strength of the
various timbers will be found useful, and there is appended a table of
the weights of various timbers. The writer prefers birch for fuselage
members over 3 ft. 6 in. in length. Although on the heavy side compared
with spruce, it will stand a great amount of rough usage. Spruce is
also suitable for fuselages up to this length, while maple is more
suited for main planes. Bamboo can be used more efficaciously for cross
members, struts, etc. Some model-makers use bamboo for planes, the
joint of the rib to the spar being by means of glue and cross-binding.
Although planes so built are exceedingly strong, it is not possible to
make quite so neat a job of them as with spruce or maple.
Another method of building main planes is to use spruce or birch spars
with piano-wire ribs, these latter being bound to the former.
For single-spar models the main spar should be tapered fore and aft
from a point one-third of the length from the front of the machine.
Where it is necessary to pierce the spar of a model aeroplane for the
reception of a kingpost or other member, silk tape binding should be
used, the joint being soaked with clean, weak glue.
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 any object
during flight.
[Illustration: Fig. 36.—Built-up Plane]
[Illustration: Fig. 37.—Wing Plans]
[Illustration: Fig. 38.—Types of Bearings]
[Illustration: Fig. 38A.—Twin-screw Bearings]
WEIGHT OF WOODS CHIEFLY USED
Mahogany 35 lb. per cubic foot
Birch 45 ” ” ”
Maple 46 ” ” ”
Spruce 31 ” ” ”
Bamboo 25 ” ” ”
=Building Scale Models.=—Models of well-known machines should be built
to correct proportions, if as perfect a resemblance as possible is
aimed at. The best way to do this is, of course, to adopt a definite
scale. The particular scale will depend principally on the size the
builder requires his model; but the size of the prototype must, of
course, be considered, because the large machines differ so much in
point of size.
Taking the span or width across the planes as the base from which to
start, it is assumed that the width of the model is desired to be from
25 in. to 35 in., which is perhaps the best all-round minimum and
maximum to adopt. Then having decided on the prototype, multiply the
span of the real machine by a fraction, which brings the model span
somewhere between the two figures. For instance, suppose it is desired
to model an Antoinette monoplane, the span of which is about 46 ft.,
and multiplying by ¾ the model span becomes 34½; therefore the scale is
¾ in. to the foot.
If the model is to be a Blériot, then as the original has a span of 28
ft., the model may be built to a scale of 1 in. to the foot. The Wright
machine has a span of 41 ft., so a model to ¾ in. to the foot would
have a span of 30¾ in. In this case, perhaps, 1-in. scale would not
be considered too large. Odd scales such as ⅞ in. to the foot can, of
course, be adopted; but whatever the scale is to be, the model should
be set out full-size on a sheet of cartridge paper, and the scale drawn
accurately at the foot. The ribs should be built up as in Fig. 36.
In designing a rubber-driven model, absolute scale must of necessity be
departed from, except in the principal measurements and in the distance
of centres apart of spars and other important members, which if not
reproduced in their proper form and position would mar the otherwise
correct appearance of the machine. Many of the spars will, of course,
need to be increased in cross-sectional dimension in order to make them
of sufficient strength. Some efficient wing plans are given by Fig. 37.
Stated briefly, there are essentially three kinds of model aeroplanes.
First, the scale model, which is a reproduction to scale of a real
machine; second, a modified copy of a large machine, which is so
designed as to resemble in general form some well-known prototype,
while retaining by means of a suitable motor, generally twisted rubber,
some ability to fly; third, a machine which does not in any way follow
the lines of full-size machines, and is built for flight only.
The first of these is essentially an exhibition model; it is more
often built either to illustrate points in the design and construction
of large machines, or to demonstrate the functions of the various parts
to technical classes, etc.
Scale models, as a rule, are unsatisfactory flyers, and if they fly
at all the flight is so short that little can be learned from their
performance.
Some serviceable types of bearings are given by Figs. 38 and 38A, on p.
31.