Building a Model Airship
It has not for obvious reasons been possible, in the design here
presented, to rigidly adhere to the lines of the prototype, as in
the adaptation of the design to rubber-driven model form several
modifications have necessarily been introduced. It has been the aim of
the writer to bring the model within the constructional capabilities
of the amateur; indeed, it is hardly possible to have simplified the
construction further. Now, the success of a dirigible, whether full
size or model, depends primarily on the observance of the fact that
an airship is lighter than air, and thus, unlike the aeroplane, does
not rely on speed to obtain lift. Secondarily, the lifting power of
hydrogen must be remembered, and although this varies according to
temperature and the purity of it, it may be taken as a general rule
that hydrogen will lift 80 lb. per 1,000 cubic feet. It is a good rule
to adopt a lower figure, say 70 lb. lift per 1,000 cubic feet, to allow
for discrepancies. In the design here submitted, aluminium (or what is
equally as good, magnalium) tube forms the framework of the body or
envelope. The general arrangement will be apparent from Figs. 178 and
179, which show the model in plan and side elevation respectively. The
framework is of hexagonal cross section, the longitudinal members of
which terminate at each end in a brass cap, to which they are riveted
with soft brass pins. It will be necessary to anneal the tubes before
bending them to impart the conical shape to each end of the frame, and
this can best be effected in a weak spirit flame, care being taken
to keep them on the move in it, to obviate fusing them. Where it is
necessary in the construction to rivet the tubes, solder should be run
over the pin to take up any play. Fig. 180 shows the method of securing
the longitudinal members to the brass end-caps. The tubes are first
flattened out, as at =B=, and then riveted to the caps. This figure
also shows the method of adjusting the angle on the rudder. A piece
of brass tube is soldered to the end-cap; and it should be of such a
bore that the No. 18 gauge wire of which the rudder is constructed
makes a bare fit through it. Two similar pieces of tube, ⅛ in. long,
are soldered to the rudder, to maintain the position of it. Thirteen
hexagonal cross members will be required, and each is formed from ³/₁₆
in. aluminium tube. In order that they may not become out of truth,
they are cross-braced with No. 35 S.W.G. piano wire, the ends of each
wire being made off in a small hole drilled through the tube. Fig.
180 will make the detail clear; each cross member is riveted to the
longitudinal, and the latter is flattened out at those points where the
cross member is attached.
[Illustration: Fig. 178.—Side Elevation]
[Illustration: Fig. 179.—Plan]
The model is driven by four elastic motors, transmitting their power
through equal gearing to the twin propellers. The four motor rods may,
for preference, be hollow spars, of the same cross-section as the
solid ones indicated. If such are used, they may be made by ploughing
a groove in a length of wood of suitable cross-section so that it
represents the letter =U=. Two will be required for each spar, so glued
together that a hollow tube is formed. At the point where it will be
necessary to pierce the spar to admit the bracing outrigger, small
packing pieces of birch should be placed in the grooves previous to
assembling the two halves of the spar. The spars, or, more correctly,
motor rods, are suspended from the envelope framework by aluminium
tube outriggers of ¼-in. diameter. Fig. 181, which shows the machine
in end elevation, and Fig. 182, showing the central cross-section,
indicate the form they are to take. Eight of them will be required. The
angles must be cleanly and accurately formed, so that the two centres
of thrust lie in the same plane. It will be found good practice, when
forming both the outriggers and the cross-sectional members, to make
a full-size drawing of them to use as a template. More especially is
this needed in the construction of the cross-sectional members, for the
purpose of ensuring that, in the operation of embracing them, they are
not strained in any way so as to become out of truth.
Fig. 183 shows the joint of the motor-rod outrigger to the motor rod
itself. As there shown, the tube is flattened out partly to engirdle
the spar, to which it is attached by pinning and clinching.
[Illustration: Fig. 180.—Joint of Tube to End-Caps]
[Illustration: Fig. 181.—End Elevation]
[Illustration: Fig. 182.—Central Section]
[Illustration: Fig. 183.—Joint of Outrigger to Motor Rod]
The gearing (see Fig. 184) consists of a brass framework (bound to the
ends of the motor rods) which provides bearings for the gears. The
use of gears is obvious; they eliminate torque on the spars, or the
tendency which a single skein would have to twist the spar. Pieces
of tube are passed over the shafts to bear between the gears and the
gear bearing. In order to counteract the tendency of the rubber hooks
to pull out straight when the rubber is in tension, the hook ends
are secured as shown at =A=. When it becomes necessary to detach the
skeins, it is only necessary to slide the tubes along the shaft to open
the hook.
In Fig. 185 the kingpost attachment is shown. The spar is mortised, the
kingpost forced through, glue having previously been brushed into the
slot, and a pin tapped through from the side to secure it. Birch should
be used for the kingposts, and their widest cross-section should be ⅜
in. by ³/₃₂ in., tapering off towards the extremities to ⅛ in. by ¹/₁₆
in.
To the ends of each motor rod are attached small No. 22 S.W.G. piano
wire hooks, to which the spar bracing is made fast. A suitable length
of wire is passed through the spar, and the hooks then formed. The
bracing is fastened to the kingpost by a couple of turns being taken
round it.
Next, four twin hooks should be made, of the form shown in Fig. 186.
Sixteen gauge wire is to be used for them, bent tightly to clip the
spar ends, to which they are bound. All the hooks should be covered
with valve tubing to prevent them from cutting through the rubber when
this latter is in tension.
The joint of the cross members of the envelope to the longeron is given
by Fig. 187, from which the bend in the longitudinal, hitherto referred
to, will be clear.
To the tail of the machine a pair of superposed surfaces or elevators
are fixed. These are fastened at their foremost extremities to the
motor-rod outriggers, and at the rear they are supported by the two
cross ribs of the rudder. Small slits are to be cut in the fabric with
which the rudder framework is covered to enable any slight alteration
in the angle of incidence of the elevators to be effected.
[Illustration: Fig. 184.—Arrangement of Gearing]
[Illustration: Fig. 185.—Kingpost Attachment]
[Illustration: Fig. 186.—Detail of Twin Hooks]
[Illustration: Fig. 187.—Cross Member Joint]
[Illustration: Fig. 188.—End View of Screw]
Attention may now be given to the propellers. As twin screws are used,
they may be made of fairly long pitch, since their torques will be
opposite and consequently balanced. Fig. 189 gives a perspective view
of the propeller block marked out ready for carving, and Fig. 188 shows
the screw in end elevation. The screw is first carved as a true helix,
and then shaped up to the form shown in the sectional view (Fig. 182).
American whitewood should be used for the blocks, or, failing this,
poplar would do. Circular tin discs are pinned to each side of the
propeller boss, to which the shaft is soldered. Great care is essential
to ensure that both propellers are of the same weight, and that each
is poised; that is, assumes an angle of 180° when balanced on a shaft.
They may be finished with a coat of gold size and one of varnish. Each
gear is driven by six strands of ¼-in. strip elastic, well lubricated
with soft soap and glycerine.
[Illustration: Fig. 189.—Propeller Block before Carving]
[Illustration: Fig. 190.—A Model Airship]
The covering is to be yellow Japanese silk, proofed with varnish
diluted with 20 per cent. of linseed oil and 10 per cent. of turpentine;
two coats should be given before the fabric is applied, and two
afterwards. A covering strip of fabric should be glued over all
seams to make the envelope as impervious as possible. And where the
outriggers pass through further pieces should be glued over, and
flanged on to the tube itself, being afterwards well doped. It is well
to impress here the importance of making the envelope as gasproof
as possible, and although a fabric entirely impervious has yet to
be invented, it is possible, with care, to reduce loss of gas by
percolation to a very low figure indeed.
A Lucas cycle valve is soldered into the brass end-cap constituting the
rear of the envelope, for the purpose of inflation, which is effected
in the following manner: A =T=-piece is fixed to a cycle pump, and
a pipe from the gas container to the =T=-piece. A further tube is
connected from the remaining arm of the =T=-piece to the valve. To
inflate the envelope, release the pressure from the container until
the pump handle is forced out to its full extent, and then shut off
pressure and force the pump down, thereby causing ingress of gas to the
envelope. Continue thus until the envelope “swells.” Of course, this
method could be adopted if hydrogen from the gas jet is used, although
this does not possess the lifting capacity of hydrogen procured from
the balloon manufacturers.
In conclusion, it may be pointed out that the weight of the complete
model inflated should not exceed 2 lb. A view of the appearance of the
model in flight is given by Fig. 190.
For those who wish to design model airships of their own it may be
stated that coal-gas lifts about 35 lb. per 1,000 cub. ft., or half
the weight lifted by an equal volume of hydrogen. A small model would
not be very successful, as the volume, and hence the lifting power,
decreases as the cube of the diameter. Thus assuming a model to be
built of half the dimensions given in this chapter, the weight of it
could only be ½ × ½ × ½ = ⅛ of the original model, and it would be
found difficult to work to this limit. The writer would point out
that little success can be expected from an airship of such small
dimensions, as the following elementary calculation will show. It
takes 35,000 cub. ft. of hydrogen to support 1 ton. Then 1 cub. ft. of
hydrogen supports ²²⁴⁰/₃₅₀₀₀ lb. Now, the cubic contents of a model
dirigible, we will assume, is
22 5 5 36 1
---- × --- × --- × ---- × ------ cub. ft.,
7 2 2 1 1728
and therefore the total weight it is capable of supporting is
22 5 5 36 1 2240
---- × --- × --- × ---- × ------ × ------ = .026 lb., or .4 oz.
7 2 2 1 1728 35000
From this it will be seen that it is extremely improbable that a model
can be built to this weight.
No more suitable covering than gold-beater’s skin exists. If the model
is of the rigid type, then the covering should be stretched over the
framework that imparts the ichthyoid shape to the envelope. Strips of
the fabric must be attached with mucilage over all seams to make the
envelope as impervious as possible. It is worthy of note that full-size
airships have as many as three or four coverings to eliminate loss of
hydrogen by escape through the pores of the fabric. If, however, the
reader contemplates building a model of the nonrigid type, a wooden
hull should be cut and the fabric fitted up to this. The hull should
represent the shape of the inflated envelope.
Compressed air also lends itself to model airship propulsion.
In conclusion, a word of warning: do not place the model near any fire,
gas, match, etc., as any small leaks may cause an explosion.
In building model airships, it is advisable to remember that by
_doubling_ the diameter of the envelope we get _four_ times the
capacity and hence four times the lift for only double the weight. This
will be seen from the following calculation:
Lift of airship 3 ft. long 2½ in. diameter equals, as we have just
seen, ·026 lb.
By doubling the diameter the calculation becomes:
22 5 5 36 1 2240
---- × --- × --- × ---- × ------ × ------ = ·052 lb.
7 1 1 1 1728 35000
From this it will be seen that if it is desired to lift a certain
weight with a lighter-than-air craft, the minimum capacity should first
be calculated and the diameter (which should always be as large as
possible) then varied to obtain the required capacity.
Although it has been stated that the lift of hydrogen is 70 lb.
per 1,000 cub. ft., it will be in order to explain two important
considerations. It will be clear that, firstly, a full-size balloon
must be inflated to a much higher pressure than a model, owing to
the heavier mass of material to be forced out to form. As a direct
adjunct to this fact it will be seen that percolation will be high.
Consequently the lift weight ratio 70: 1,000 is somewhat a low estimate
for a model, since, firstly, it will only require to be inflated to
less than half the pressure of a full-size balloon; and, secondly,
percolation will be considerably less. The writer intentionally gave
the full-size limit in order that, should the builder’s model fail to
come within the prescribed limit of 2 lb., its flying capability would
not be appreciably impaired.
Further, the envelope is kept to its shape by a framework inside, and
is consequently full of air. Now if we start pumping hydrogen in it
will make the contrivance heavier instead of lighter, because no outlet
is provided for the escape of air. This could easily be remedied by
standing the model on its end with the cycle valve at the top, with a
valve or tap on the bottom brass cap. Open this bottom tap to allow
the air to escape, and allow the hydrogen to fizz in through the cycle
valve. Hydrogen will not escape through the bottom tap until all the
air has been displaced. If the hydrogen cannot be had at a high enough
pressure to operate the valve itself a valve should not be used.
It may in such a case be necessary to fit an induction valve to the
pump. The envelope should be inflated to as low a pressure as possible,
otherwise the lift is correspondingly reduced. In all cases it is
exceedingly difficult with model airships to prescribe any definite
formula, and although the lift coefficient for models exceeds the
figure stated, the conjectural nature of the purity (and hence of lift)
of hydrogen in various parts of the country renders it a safe one to
use for all practical purposes.
INDEX
Action and reaction, 5
Aerofoil, flow of air round, 10
——, lift of, 10
Aeroplane, speed of, 4
——, types of, 12-16
——: Why does it fly? 1-11
=A= frame, 20, 22, 23
Air, density of, 35, 36
—— flow, direction of, 5
—— pressure, 2
Air-screw, action of, 35
—— balancing, 38
——, calculations for, 36
——, efficiency, 35
——, four-bladed, making, 133
——, laminated, 39
——, suitable woods for, 36, 38
Bamboo planes, 30
Bending air-screws, 42-46
Biplane, fitting struts to, 128
——, sample twin-screw, 61-68
Blade width, maximum, 45
Blériot type monoplane, 18
Box kite, 138, 139, 140
Bragg-Smith, 15
Building airship, 141, 154
Calculations in designing aeroplane plant, 120
Camm air-screw, proportions of, 46
Canard, meaning of, 8
Carving air-screws, 35-41
Chauvière type air-screw, 38
Clarke, T. W. K., 14
Compressed-air engine, 94-102
—— ——, container for, 99
—— ——, crank for, 97, 98
—— ——, inlet and exhaust, etc., of, 97
—— ——, pistons for, 98
Compressed-air engine, testing the container, 100
—— ——, three-cylinder, 94-103
—— ——, valve and tap for, 100
Compressed-air-driven biplane, 104-114
—— ——, bracing planes of, 115
—— ——, calculations for, 104-109
—— ——, flying, 118, 119
—— ——, inter-struts for, 112
—— ——, materials for, 106
—— ——, planes for, 109
—— ——, tail for, 111
Designing, general notes on, 120, 123
Dihedral angle, 7
Double-winder, 70-72
Drift and lift, 3, 6
Efficiency air-screw, 35
—— formula, 19
Elastic motor, disadvantages of, 94
Elevator adjustment, 127
Equilibrium, 2
Fairey, 12
Force of gravity, 2
Forces acting on kite, 2, 3
Formula, efficiency, 19
——, type, 18
Four-bladed air-screws, making, 133
Fuselages, 19-34
——, boat-shaped, 24
——, box-girder, 26
——, distortion, 30
——, simple, 28, 29
——, single-spar, 26
Fuselages, twin-screw, 26
——, two-membered, 25
Geared motor for model aeroplane, 134
General notes, 124-135
Gravity and pressure, 6, 7, 8
Gravity, force of, 2
Hargreaves kite, 132
Hollow spar, 22
Houlberg, 14
Hydro-biplane, 17
Hydro-monoplane, 87-93
——, bearings for, 88
——, elevator for, 90
——, floats for, 90
——, fuselage for, 87
——, mainplane for, 88
——, weight of, 93
Hydroplane floats, calculating capacity of, 130
——, water surface, 128
Kite, box, 138, 139, 140
——, forces acting on, 2-3
——, Hargreaves’, 132
——, speed of, 4
——, tailless, 136-141
Lateral stability, 7
Lift and drift, 3, 6
—— of aerofoil, 10
Loading, 44
—— for aeroplanes, 127
Longitudinal stability, 8
Lubricant, 125
Mann, R. F., 20
Materials, choice of, 28
Monoplane, Blériot type, 18
——, collapsible, 73-79
——, ——, chassis for, 78
——, ——, mainplane for, 75
——, ——, propellers for, 79
——, ——, tail for, 74
——, hydro, 87-93
——, Ridley, 12
——, ——, twin-screw, 54-60
——, ——, mainspar for, 54
——, ——, propellers for, 58
——, tractor, 80-86 (_see_ Tractor monoplane)
Penaud, Alphonse, 94
Piano wire, uses of, 30
Pitch, 35, 36, 39, 42, 44, 45
—— angles, setting out, 40
“Plane” and “aeroplane,” meaning of terms, 8
Planes, bamboo, 30
——, cane, 50
——, constructing, 47-53
——, fixing, 133
——, swept-back, 52
——, types of, 47
——, umbrella ribbing, 50
——, wire, making, 50, 51
——, wooden, 48
Plant, calculations for designing, 120
Pressure and gravity, centres of, 6, 7, 8
Resistance, air, 3
Ridley monoplane, 12
Scale models, building, 32, 34, 121, 122, 123
Silk, waterproof, 130
Skid, fitting, 126
Slip, 35
Small biplane, driving, 102-103
Spar sections, 26, 27
Speed of aeroplane, 4
Spindles, attaching, 46
Stability, 29, 124, 125
——, lateral, 7, 42
——, longitudinal, 8
Tailless kites, easily made, 136-141
=T= frame, 20, 22, 23
Thrust, 42
—— required for aeroplane, 11
Timber, strength of, 29, 30, 32
Torque, 29, 36, 42
Tractor hydro-biplane, 17
——, monoplane, 80-86
——, ——, chassis for, 82
——, ——, mainplane for, 84
——, ——, mainspar for, 80
——, ——, twin-gears for, 84
Twining, E. W., 14, 17
—— air-screw, 46
Twin-screw biplane, simple, 61-68
—— —— ——, elevator for, 62
—— —— ——, flying the, 66-68
—— —— ——, mainspar for, 61
Type formula, 18
—— of aeroplanes, 12-18
Waterproof silk, 130
Water-surface hydroplane, 128
Winders for elastic motors, 69-72
——, double, 70-72
——, push-in-the-ground, 69-70
Wing bracing, 30
PRINTED BY CASSELL & COMPANY, LIMITED,
LA BELLE SAUVAGE, LONDON, E.C.4
50.220