General Notes
=Stability.=—The principles underlying the design of a successful
flying model aeroplane are almost, if not quite, as complex as those
involved in the planning of full-size machines. In some respects
perhaps this is more so, owing to the fact that models must be
practically automatically stable both longitudinally and laterally,
since they are not under any sort of control after they have left
the hands of the person flying them. The adjustments to produce this
automatic stability must be made before the machine is launched, and
the fact that there are models which are capable of flying distances of
several hundreds of yards, and high up in the air, is evidence that it
is quite possible to make this adjustment accurately.
A useful rule to remember is that to produce a longitudinally stable
effect, the leading plane should make a greater angle (that is, a
positive angle) than the following plane. And that conversely a
condition of instability is set up when the leading plane makes a
negative angle to the trailing one (see Fig. 160).
Referring now to lateral stability, the same principle applies,
although in the case of biplanes stability to a great extent can be
obtained in another way.
Monoplanes and sometimes biplanes are made stable by what is known as a
dihedral angle shown by Fig. 161, which represents the usual method of
shaping the planes. This last-mentioned figure is intended to represent
a front (or back) view of the machine. In aeroplanes of this type the
elevator or tail, whichever is employed, may also be given a dihedral,
though this is not often done.
[Illustration: Fig. 160.—Machine without Stability]
[Illustration: Fig. 161.—Dihedral Angles]
=Lubricant.=—A good lubricant can be made from pure soft soap 4 parts,
pure glycerine 2 parts, water 6 parts, these constituents being boiled
together to the consistency of syrup.
Another excellent lubricant is made from castile soap 1 part, boiled
in water 3 parts. Add black lead or plumbago sufficient to make a thin
paste.
[Illustration: Fig. 162.—Fixing Skids]
[Illustration: Fig. 162A.—Fixing Skids]
[Illustration: Fig. 163.—Fitting Wheels]
=Fitting Skid.=—Steel-wire skids can be fixed to model aeroplanes as in
Figs. 162 and 162A. The wheels can be obtained of any model aeroplane
firm, and can be fixed to the machine as shown in Fig. 163. This would
increase the weight of the machine, and more rubber may have to be used
and the main planes adjusted to suit. Use No. 21 B.W.G. wire (steel).
=Elevator Adjustment.=—Elevators are sometimes fixed to the fuselage as
shown in the accompanying sketch (Fig. 164), which shows it fixed above
the spar. It is claimed that, in this position, the elevator is more
efficient.
[Illustration: Fig. 165.—Fixing Struts to Biplane]
[Illustration: Fig. 164.—Elevator Adjustment]
=Loading per Square Foot.=—Generally speaking, the loading per square
foot of supporting surface for rubber-driven models should not exceed 6
oz., nor be less than 3 oz., while to obtain good stability the ratio
of machine length to span should be somewhere in the neighbourhood of
3: 2.
=Fixing Struts to Biplane.=—One very suitable method of attaching the
inter-struts to wings of model biplanes, that admits of dismantling
the model for packing, is shown at =A= (Fig. 165). A short length of
brass tubing of ³/₃₂-in. bore is bound with fine florist’s tinned iron
wire to the wing spar, the inter-struts being bent to the shape given
at =B=, so that they spring tightly into the sockets. A simpler method
is illustrated by =C=. Here the inter-struts are bent at right angles
on the ends, bound to the wing spar, and soldered. Much will depend
on whether a fuselage is one-or two-membered, but a frame attachment
capable of adaptation to either is given by =D= and =E=. Wire crutches
are bent to take the cross-section of the frame member or members, and
fixed by binding and solder to the central inter-struts. Yet another
method is shown at =F=, which is self-explanatory.
[Illustration: Fig. 166.—Water-Surface Hydroplane]
=Water-surface Hydroplane.=—For a water-surface hydroplane 3 ft. long
try a breadth of 11 in., same beam all the way. Make the depth 2½ in.
at the stem and 3 in. at the step. The step may be ¾ in. deep, and
would be placed about 15 in. from the stern. The writer would suggest
fins as shown by dotted lines in the accompanying illustration (Fig.
166) for steering purposes, one fin on each side at the stem, and one
on the centre line forward. The fins should be made of thin aluminium
and be quite sharp on the edges. Canvas would be too rough a surface,
producing too much skin friction; oiled silk would be better, but the
writer would recommend thin wood, french polished. The power needed to
make these boats “plane” is very great, and considerable difficulty
might be experienced in getting any distance out of it with clockwork.
[Illustration: Farman This Section is inefficient]
[Illustration: Streamline An efficient Section]
[Illustration: Wedge This formation cannot be recommended]
[Illustration: Stepped
Fig. 167.—Sections of Hydroplane Floats]
=Calculating Capacity of Hydroplane Floats.=—The floats of a model
hydroplane must be made sufficiently large to displace about three
times the model’s weight of water, since it is necessary that they
should be only one-third immersed. A cubic foot of water weighs 1,000
oz. approximately. Then
1728 8
---- × --- = 13·8 cub. in.
1000 1
must be displaced to float 8 oz. Multiplying this by three gives 42
cub. in. as the total cubic capacity of the floats. Two front floats,
each 5 in. by 2 in. by 1 in. maximum depth, and a rear float 8 in. by 3
in. by 1 in. would be about the correct size to use. A slightly larger
diameter and pitched propeller would be necessary on a hydroplane to
develop more thrust to overcome the resistance of the floats. Some
well-known hydroplane sections are given in Fig. 167.
[Illustration: Showing Marking of Block]
[Illustration: Finished Screw of Truly Helical Formation]
[Illustration: Setting out the Angles]
[Illustration: Cross-Section]
[Illustration: Showing Halving of Block
Fig. 168.—Four-bladed Screws]
=Waterproofing Silk for Model Aeroplanes.=—A waterproofing solution can
be made of pure coach varnish reduced in consistency with turpentine in
the proportion 2: 1. It is, however, more important to make the fabric
airtight than waterproof. This solution accomplishes both. Rubber
lubricant is made of 1 part of graphite, 6 parts of pure soft soap,
1 part of glycerine, 4 parts of water, and 1 part of salicylic acid,
boiled together and allowed to cool.
=Making Four-bladed Air-screws.=—Fig. 168 on the preceding page shows
how truly helical and also four-bladed screws are carved. Four-bladed
screws are not so efficient for models as two-bladed ones. The drawing
also shows the method of marking out and also of halving the blocks
together at the centre, so that the four blades are at right angles to
one another. A view of an ordinary twin-blade screw of similar design
is appended to give some idea of the finished shape of the blades. The
pitch should not exceed the circumferential measurement of the disc
swept by the propeller; that is to say, the pitch angle, or the angle
made by the propeller tip with the axis, should not exceed 45 degrees.
The angles along the blade are determined in the manner illustrated
by Fig. 168. A line is laid off to any convenient scale equal to the
circumference of the propeller disc = π × diameter. The tip angle (or
pitch angle) may now be produced and the triangle completed by erecting
a line at right angles to the first, or the pitch may be erected
perpendicularly to the circumferential line to the spiral scale and
the pitch angle line drawn in. The circumferential line may now be
divided into a number of equal parts and the points connected up. In
the illustration three points have been taken, which will be enough for
a small propeller. Cardboard templates should be cut to these angles
and the blades checked at the points corresponding. The balance of the
screw must be attended to and is a most cogent factor in such a small
design. Marine-screws are exceedingly inefficient when working in air,
and their use for driving model aeroplanes, etc., is to be strongly
deprecated. The formula for propeller pitch is: p = πd tan =A=, where
p = pitch, π = 3·14, d = diameter of propeller, tan =A= = tangent of
pitch angle.
[Illustration: Fig. 169.—Securing Wooden Planes]
=Fixing Planes.=—Fig. 169 shows a neat and effective method of securing
the planes.
Very little wood is now used for the planes of model aeroplanes; but to
build a plane of veneer, it should first be cut to the shape required
and then a strip of birch pinned and glued on the under-side of the
leading edge for strength. The veneer is then pinned and glued to the
ribs which have previously been bent to the correct camber. The reader
may be reminded that birch is the most suitable wood for bentwood
propellers, the wood being first cut to shape with a fret-saw, then
soaked in hot water and bent over a bunsen burner to the desired pitch.
This requires considerable experience, but can be done quite quickly by
an experienced workman. For propellers up to 12 in. in diameter, use
¹/₁₆-in. wood; this should gradually taper off to ¹/₃₂ in. at the tips.
[Illustration: Fig. 170.—Geared Motor for Model Aeroplane]
[Illustration: Fig. 171.—Fixing Shaft of Carved Screw]
=Making Motor for Model Aeroplane.=—A geared motor suitable for a model
aeroplane is given by Fig. 170. A cage =E= for the gears =A= can be
made from ¼-in. by ¹/₃₂-in. strip iron, and the propeller shafts =B=
can be made from cycle spokes. The illustration also shows a method
of fixing the propeller to the shaft. The piece =C= is soldered to
the shaft =B=, and engages with two holes drilled in the propeller
boss, the propeller being secured by the nut =D=. The gears =A= have
an equal number of teeth. There is no advantage in using a geared-up
motor. =F= is the fuselage of the model.
=Fixing Propeller of Model Aeroplane.=—With a carved propeller, the
motor hook can be secured to the propeller as in Fig. 171. If a
bentwood propeller, the best way is to fasten a strip of tin round the
centre and solder the motor hook to this.