Title: (Bulletins on forest pathology : from Bulletin U.S.D.A., Washington, D.C., 1913-1925)
Identifier: bulletinsonfores00unit (find matches)
Year: 1913 (1910s)
Authors: United States. Dept. of Agriculture
Subjects: Trees; Plant diseases
Publisher: (Washington : U. S. Dept. of Agriculture)
Contributing Library: Robarts - University of Toronto
Digitizing Sponsor: University of Toronto

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DECAYS AND DISCOLOEATIONS IN AIRPLANE WOODS. of very old trees. If each annual ring is composed of approximately one-third or more of summer wood the piece possesses the necessary strength. In those pieces with very narrow annual rings, in which the summer wood is indicated by a mere dark line at the outer edge of each annual ring, the wood is very soft and weak, often having a specific gravity as low as 0.34 (Fig. 1). Sometimes the proportion of material of low specific gravity in Douglas fir airplane lumber is exceedingly high. The writer has seen several consecutive carload lots of selected wing-beam stock at one factory in which from 25 to 50 per cent of the pieces in each car were below the minimum specific gravity. The stock was cut from old
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Fig. 1.—Cross sections of wing beams of Douglas fir of average and low specific gravity. The large proportion of summer wood, indicated by the dark bands, in the piece of good specific gravity (on the right) in comparison, with that in the piece with low specific gravity (on the left) is plainly shown. slow-grown trees, which yield the " yellow fir " so much preferred by the trade, but which invariably contain a large percentage of material of low specific gravity not suitable for aircraft or any other type of construction where high strength is requisite. The same general relations hold good in Sitka spruce. Here, again, if the annual rings are too few or too many per inch, they in- dicate wood of low density. The minimum specific gravity for this species is established at 0.36. It is often difficult to approximate the specific gravity by visual examination of the proportion of summer wood per annual ring in the case of those pieces close to the minimum density permitted in softwoods. There is considerable chance for error even with Douglas fir, but with spruce this is increased, owing to the fact

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