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.
Fig. 47a.—"Aeroplane Alma."
A very effective French Toy Monoplane.
§ 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.