Other Woods.
The foregoing constitute woods which are in fairly general use for one purpose or another, there being, of course, very many other varieties, some of which may be called into use with the progress of the industry. Of the conifer species, silver spruce is easily the most suitable timber for aeroplane construction, and one realizes this more as the various substitutes are tried. As an instance, cypress is straight of grain with no very great increase over the weight of spruce, being also well up the table of strengths. It is, however, much too brittle for the various members of small section of which an aeroplane is composed, and does not seem to have any extensive future for aircraft work. Another, at one time much-advertised wood, is Parang, a species of mahogany. It has been reputed to bend well, but it certainly does not enter into the construction of modern aeroplanes. A consignment handled by the writer some years ago and intended for bending, was found to be exceedingly brittle, and although standing a good load, fractured almost square across the grain, in a manner known colloquially in the workshop as “carrot-like.” The latter term is indicative of a characteristic which precludes the use of many woods possessing other physical properties especially suitable for aircraft work.
Multi-ply Wood.
This term is applied to the sheets of wood composed of a number of thin layers glued together with the grain reversed. As the layers are obtained by rotating the tree against cutters in such a manner that a continuous cut is taken from the outside almost to the centre, it is possible to get very great widths, which makes it particularly suitable for aircraft work. It is made in varying widths up to 4 ft., and in thickness from 1/20 in. up to ½ in., consisting of three, five, and seven layers, although the three-ply variety in thicknesses up to 3/16 in. is more commonly used. It is made up in nearly all woods, but those mostly utilized in the aeroplane industry are birch, ash, poplar, and satin-walnut, birch being superior by reason of its closeness of grain. Ash ply-wood in some instances tends towards brittleness, while poplar, although exceptionally light, is very soft and only used for minor parts. Satin-walnut is very even in quality but is apt to warp.
Defects in Timber.
Fig. 1.—Heart shake.
Fig. 2.—Star shake.
