Torsion.—Torsion is a twisting force, such as an engine propeller shaft receives.
Testing for Strength.—If a wire is an inch square in cross-section and breaks when a load of 150,000 lb. is hung on it, we say that the strength of the wire is 150,000 per square inch. Smaller wires equally strong have a strength of 150,000 lb. per square inch also, but they in themselves will not support a load of 150,000 lb. but only the fraction of that, according to the fraction of a square inch represented by their cross-section.
In the same way, a square inch of wood under a compressive load may break at 5000 lb. If, however, the piece of wood is long in proportion to its thickness, it will bend easily and support much less weight. For example, a perfectly straight walking cane could perhaps have a ton weight put on it without breaking but if the cane were not set squarely or if it started to bend it would immediately break under the load.
These cases illustrate the importance of having struts perfectly straight, not too spindling and evenly bedded in their sockets. Some training machines are built with a factor of safety of 12. That is to say, the breaking strength of any part is twelve times the ordinary load or stress under which the piece is placed. It should be remembered, however, that under any unusual condition in the air, such as banking, etc., extra strains are placed on the parts and the factor of safety is much less than 12. Factor of safety of 12 thus does not mean exactly what it does in other engineering work, where allowances are made for severe conditions. The so-called factor of safety of 12 in airplane work is probably no greater than a factor of safety of 2 or 3 in regular engineering work.
There are three all-important features in the flying machine construction, viz., lightness, strength and extreme rigidity. Spruce is the wood generally used for parts when lightness is desired more than strength, oak, ash, hickory and maple are all stronger, but they are also considerably heavier, and where the saving of weight is essential, the difference is largely in favor of the spruce. This will be seen in the following condensed table of U. S. Government Specifications.
| Wood | Weight per cubic foot, pounds (15% moisture) | Modulus of rupture, pounds per square inch | Compression strength, pounds per square inch |
| Hickory | 50 | 16,300 | 7,300 |
| White Oak | 46 | 12,000 | 5,900 |
| Ash | 40 | 12,700 | 6,000 |
| Walnut | 38 | 11,900 | 6,100 |
| Spruce | 27 | 7,900 | 4,300 |
| White Pine | 29 | 7,600 | 4,800 |
A frequently asked question is: “Why is not aluminum or some similar metal, substituted for wood?” Wood, particularly spruce, is preferred because, weight considered, it is much stronger than aluminum, and this is the lightest of all metals. In this connection the following table will be of interest.
| Material | Weight in cubic feet, pounds | Tensile strength per sq. in pounds | Compression strength per sq. in pounds |
| Spruce | 27 | 7,900 | 4,300 |
| Aluminum | 162 | 15,000 | 12,000 |
| Brass (sheet) | 510 | 20,000 | 12,000 |
| Steel (tool) | 490 | 100,000 | 60,000 |
| Nickel steel | 480 | 100,000[1] | |
| Copper (sheet) | 548 | 30,000 | 40,000 |
| Tobin bronze (Turnbuckles) | ... | 80,000 | |
| Monel metal | 540 | 90,000 | 30,000 |
Wood.—Present practice in airplane construction is to use wood for practically all framing, in other words, for all parts which take pressure or compression. Although wood is not as strong for its size as steel and therefore offers more air resistance for the same strength yet the fact that frame parts must not be too spindling, in other words, that they must have a certain thickness in proportion to their unsupported length, has led to the use of wood in spite of the greater strength of steel. Some airplanes, however, as the Sturtevant, are constructed with practically a steel framing.
It should be borne in mind that any piece or kind of wood will not answer for framing, and more especially for repair parts. There is a tremendous difference in the strength and suitability among different woods for the work. For instance, a piece of wood of cross or irregular grain, one with knots, or even one which has been bored or cut or bruised on the outside, may have only half or less the strength of the original piece. Air drying doubles the strength of green wood, proper oven drying is better yet.