MATERIALS FOR AEROPLANE
CONSTRUCTION.

§ 1. The choice of materials for model aeroplane construction is more or less limited, if the best results are to be obtained. The lightness absolutely essential to success necessitates—in addition to skilful building and best disposition of the materials—materials of no undue weight relative to their strength, of great elasticity, and especially of great resilience (capacity to absorb shock without injury).

§ 2. Bamboo.—Bamboo has per pound weight a greater resilience than any other suitable substance (silk and rubber are obviously useless as parts of the framework of an aeroplane). On full-sized machines the difficulty of making sufficiently strong connections and a liability to split, in the larger sizes, are sufficient reasons for its not being made more use of; but it makes an almost ideal material for model construction. The best part to use (split out from the centrepiece) is the strip of tough wood immediately below the hard glazed surface. For struts, spars, and ribs it can be used in this manner, and for the long strut supporting the rubber motor an entire tube piece should be used of the requisite strength required; for an ordinary rubber motor (one yard long), 30 to 50 strands, this should be a piece 3/8 in. in diameter, and weight about 5/8 oz. per ft. Bamboo may be bent by either the "dry" heat from a spirit lamp or stove, or it may be steamed, the latter for preference, as there is no danger of "scorching" the fibres on the inside of the bend. When bent (as in the case of other woods) it should be bound on to a "former" having a somewhat greater curvature than the curve required, because when cool and dry it will be sure to "go back" slightly. It must be left on the former till quite dry. When bending the "tube" entire, and not split portions thereof, it should be immersed in very hot, or even boiling, water for some time before steaming. The really successful bending of the tube en bloc requires considerable patience and care.

Bamboo is inclined to split at the ends, and some care is required in making "joints." The ribs can be attached to the spars by lashing them to thin T strips of light metal, such as aluminium. Thin thread, or silk, is preferable to very thin wire for lashing purpose, as the latter "gives" too much, and cuts into the fibres of the wood as well.

§ 3. Ash, Spruce, Whitewood are woods that are also much used by model makers. Many prefer the last named owing to its uniform freedom from knots and ease with which it can be worked. It is stated 15 per cent. additional strength can be imparted by using hot size and allowing it to soak into the wood at an increase only of 3·7 per cent. of weight. It is less than half the weight of bamboo, but has a transverse rupture of only 7,900 lb. per sq. in. compared to 22,500 in the case of bamboo tubing (thickness one-eighth diameter) and a resilience per lb. weight of slightly more than one half. Some model makers advocate the use of poplar owing to its extreme lightness (about the same as whitewood), but its strength is less in the ratio of about 4:3; its resilience is very slightly more. It must be remembered that wood of the same kind can differ much as to its strength, etc., owing to what part of the tree it may have been cut from, the manner in which it may have been seasoned, etc. For model aeroplanes all wood used should have been at least a year in seasoning, and should be so treated when in the structure that it cannot absorb moisture.

If we take the resilience of ash as 1, then (according to Haswell) relative resilience of beech is 0·86, and spruce 0·64.

The strongest of woods has a weight when well seasoned of about 40 lb. per cub. ft. and a tenacity of about 10,000 lb. per sq. in.

§ 4. Steel.—Ash has a transverse rupture of 14,300 lb. per sq. in., steel tubing (thickness = 1/30 its diameter) 100,000 lb. per sq. in. Ash weighs per cub. ft. 47 lb., steel 490. Steel being more than ten times as heavy as ash—but a transverse rupture stress seven times as high.

Bamboo in tube form, thickness one-third of diameter, has a transverse rupture of 22,500 lb. per sq. in., and a weight of 55 lb. per cub. ft.

Steel then is nine times as heavy as bamboo—and has a transverse rupture stress 4·4 times as great. In comparing these three substances it must be carefully borne in mind that lightness and strength are not the only things that have to be provided for in model aeroplane building; there is the question of resistance—we must offer as small a cross-section to moving through the air as possible.

Now while ash or bamboo and certain other timbers may carry a higher load per unit of weight than steel, they will present about three to three and a half times the cross-section, and this produces a serious obstacle, while otherwise meeting certain requirements that are most desirable. Steel tubing of sufficiently small bore is not, so far as the writer knows, yet on the market in England. In France very thin steel tubes are made of round, oval, hexagon, etc., shape, and of accurate thickness throughout, the price being about 18s. a lb.

Although suitable steel tubing is not yet procurable under ordinary circumstances, umbrella steel is.

§ 5. Umbrella Section Steel is a section 5/32 in. by 1/8 in. deep, 6 ft. long weighing 2·1 oz., and a section 3/32 in. across the base by 1/8 in. deep, 6 ft. long weighing 1·95 oz.

It is often stated that umbrella ribs are too heavy—but this entirely depends on the length you make use of, in lengths of 25 in. for small aerofoils made from such lengths it is so; but in lengths of 48 in. (two such lengths joined together) the writer has used it with great success; often making use of it now in his larger models; the particular size used by him weighs 13½ grammes, to a length of 25 in. He has never had one of these aerofoils break or become kinked—thin piano wire is used to stay them and also for spars when employed—the front and ends of the aerofoil are of umbrella steel, the trailing edge of steel wire, comparatively thin, kept taut by steel wire stays.

§ 6. Steel Wire.—Tensile strength about 300,000 lb. per sq. in. For the aerofoil framework of small models and for all purposes of staying, or where a very strong and light tension is required, this substance is invaluable. Also for framework of light fabric covered propellers as well as for skids and shock absorber—also for hooks to hold the rubber motor strands, etc. No model is complete without it in some form or another.

§ 7. Silk.—This again is a sine qua non. Silk is the strongest of all organic substances for certain parts of aeroplane construction. It has, in its best form, a specific gravity of 1·3, and is three times as strong as linen, and twice as strong in the thread as hemp. Its finest fibres have a section of from 0·0010 to 0·0015 in diameter. It will sustain about 35,000 lb. per sq. in. of its cross section; and its suspended fibre should carry about 150,000 ft. of its own material. This is six times the same figure for aluminium, and equals about 75,000 lb. steel tenacity, and 50 more than is obtained with steel in the form of watch springs or wire. For aerofoil surface no substance can compare with it. But it must be used in the form of an "oiled" or specially treated silk. Several such are on the market. Hart's "fabric" and "radium" silk are perhaps the best known. Silk weighs 62 lb. per cub. ft., steel has, we have seen, 490 lb., thus paying due regard to this and to its very high tensile strength it is superior to even steel wire stays.

§ 8. Aluminium and Magnalium.—Two substances about which a great deal has been heard in connection with model aeroplaning; but the writer does not recommend their use save in the case of fittings for scale models, not actual flyers, unless especially light ones meant to fly with the wind. Neither can compare with steel. Steel, it is true, is three times as heavy as aluminium, but it has four or five times its strength; and whereas aluminium and magnalium may with safety be given a permissible breaking strength of 60 per cent. and 80 per cent. respectively, steel can easily be given 80 per cent. Being also less in section, resistance to air travel is again less as in the case of wood. In fact, steel scores all round. Weight of magnalium : weight of aluminium :: 8:9.

§ 9. Alloys.—During recent years scores, hundreds, possibly thousands of different alloys have been tried and experimented on, but steel still easily holds its own. It is no use a substance being lighter than another volume for volume, it must be lighter and stronger weight for weight, to be superior for aeronautical purpose, and if the difference be but slight, question of bulk may decide it as offering less resistance.

§ 10. Sheet Ebonite.—This substance is sometimes useful for experiments with small propellers, for it can be bent and moulded in hot water, and when cold sets and keeps its shape. Vulcanized fibre can be used for same purpose. Sheet celluloid can be used in the same way, but in time it becomes brittle and shrinks. Mica should be avoided. Jointless cane in various sizes is a very useful material—the main aerofoil can be built of it, and it is useful for skids, and might be made more use of than it is.[38] Three ply wood, from 1/50 in. in thickness, is now on the market. Four or five ply wood can also be obtained. To those desiring to build models having wooden aerofoils such woods offer the advantage of great strength and extreme lightness.

Referring to Table V. (Timber) at the end of the book, apparently the most suitable wood is Lombardy poplar; but its light weight means increased bulk, i.e. additional air resistance. Honduras mahogany is really a better all-round wood, and beech is not far behind.

Resilience is an important factor. Ash heads the list; but mahogany's factor is also good, and in other respects superior.

Lombardy poplar ought to be a very good wood for propellers, owing to its lightness and the ease with which it can be worked.

Hollow reeds, and even porcupine quills, have been pressed into the service of the model maker, and owing to their great strength and extreme lightness, more especially the latter, are not without their uses.