Walnut.
This wood is almost entirely devoted to the making of air-screws, although the dwindling supplies and the very short lengths obtainable has practically enforced the employment of other woods for this purpose.
Mahogany.
The term “mahogany” covers an infinite variety of woods, possessing widely different characteristics, many of the species being quite unsuitable for the requirements of aircraft work. That known as Honduras mahogany possesses the best strength values, is of medium weight, about 35 lbs. per cubic foot, and is in general use for airs-crews and seaplane floats. It has been used on some German machines for such parts as rib webs, but is not really suitable for parts of comparatively small section, such as longerons, as it is inclined to brittleness. It is of particular value for seaplane floats and the hulls of the flying-boat type of machine, as it is not affected by water. A defect peculiar to Honduras mahogany is the occurrence of irregular fractures across the grain known as thunder shakes. Although other so-called mahoganies are similar in appearance to the Honduras variety, a species quite distinct in appearance is that known as Cuban or Spanish mahogany, which is of darker colour, and much heavier in weight, averaging about 50 lbs. per cubic foot, which latter factor almost precludes its use for aeroplane construction.
Birch.
One finds very few instances of the use of this wood for aeroplane details, although it is used fairly extensively in America for air-screw construction, for which it is only moderately suited. It possesses a high value of compressive strength across the grain, but is much affected by climatic changes, and does not take glue well. It is useful for bent work, and might conceivably be used instead of ash for small bent work details. Its weight is about 44 lbs. per cubic foot.
Poplar.
Under this name is included such woods as American whitewood, cotton wood, bass wood, etc. The wood sold under one or other of these names is generally very soft and brittle, and although of a light nature, weighing about 30 lbs. per cubic foot and less, it is of very little utility for the work under discussion. It has been used for minor parts such as rib webs, and fairings to tubular struts.
Oregon Pine.
The scarcity of silver spruce has led to the adoption of the wood known as Oregon pine for most of the components for which the former wood has hitherto been used. The term “Oregon pine” is applied to the Douglas fir, one of the largest of the fir species, a length of 200 ft. being an average. It is altogether heavier than silver spruce, weighing about 34 lbs. per cubic foot, and also differs greatly in appearance, possessing a reddish-brown grain, with very distinct annual rings. Its strength to weight ratios are practically equal to those of silver spruce, although in the writer’s experience it has a tendency towards brittleness, and is not so suitable as Sitka spruce for components of small scantling. With some specimens of this wood it is noticeable that the effect of drying on freshly sawn lengths for longerons, etc., is the appearance of “shakes” or cracks, not previously discernible. Its appearance generally is reminiscent of pitch pine, for which wood it is sometimes substituted in connection with building.