Unreliability of Tabulated Tests.
The various tables which exist indicating the strength, weight, and characteristics of various woods are of very doubtful utility, in some cases fallacious, and in nearly all cases far too specific. The foregoing enumeration of some of the variations existing with wood will indicate the enormous difficulty of obtaining with any exactitude a result representative of the species of wood tested, and which could be regarded as reliable data for the calculation of stresses, or for general design. The moisture content of timber, an extremely variable quantity, greatly affects the figures relating to the strength and weight of timber, so that tables indicating the properties of woods should include the percentage of moisture contained in the examples tested. Again, certain woods possessing relatively high strength values, are frequently short-grained and brittle, and therefore not so suitable as other woods of lower strength values, but of greater elasticity and resiliency.
Woods in Use.
Silver Spruce.
The wood most extensively used for the main items of construction is silver spruce, or Sitka spruce, found in great quantities in British Columbia. Experience has proved this wood pre-eminently suitable for aeroplane construction, its strength-weight ratio is particularly good, it can be (at least until recently) obtained in long lengths up to 30 ft., and, moreover, is particularly straight grained and free from knots and other defects. There are other woods possessing higher strength qualities, but in most cases their value is greatly diminished by reason of the greater weight, and that only a limited portion straight of grain and free from knots is obtainable. The weight of Sitka spruce varies from 26 to 33 lbs. per cubic foot, and although it is difficult to give a precise figure, a good average specimen fairly dry would weigh about 28 lbs. per cubic foot. Some impression of the extent to which it enters into the construction of the aeroplane will be gathered if the components usually of spruce are detailed. For the main spars of the planes spruce is almost universally used, as here great strength for the least weight is of extreme importance, while a consideration almost as important is the necessity of a good average length, straight grained and free from defects. It is also used for the webs and flanges of the wing ribs, the leading and trailing edges and wing structure generally. The longerons or rails of the fuselage of many machines are spruce, although in this instance ash and hickory are used to a moderate extent. The growing practice is to make the front portion of the fuselage of ash, as this is subject to the greater stress, while the tail portion is of spruce; but in a number of cases the latter material is used throughout. The cross struts of the fuselage are invariably of spruce, as well as such items as interplane and undercarriage struts and streamline fairings.