Model aeroplanes / The building of model monoplanes, biplanes, etc., together with a chapter on building a model airship

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.

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.

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.

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.

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.

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

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.

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.

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.

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.