§ 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.