﻿DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 7 



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 



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 



