﻿DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 9 



It is quite difficult to judge the specific gravity of diffuse porous 

 hardwoods by visual examination except in those pieces patently 

 very low or very high. Actual specific-gravity determinations will 

 have to be used to a greater extent when handling this class of woods. 



In examining a piece of wood of any considerable length to deter- 

 mine its specific gravity, care must be used to examine it throughout. 

 Pieces in which the grain is not perfectly straight may have high 

 specific gravity in one portion and a low density in another, as at- 

 tested by the percentage of summer wood. This is due to the fact 

 that trees may not develop wood of the same or nearly the same spe- 

 cific gravity throughout their life. Such a condition is not at all 

 uncommon in white-ash longerons, and it must be remembered that 

 any given piece of wood is no stronger than its weakest portion. 



As a general rule, airplane timber should be purchased under speci- 

 fications so worded in regard to the ratio of spring wood and summer 

 wood per annual ring and number of annual rings per inch of radius 

 as to reject at the source of supply most of the stock of low specific 

 gravity. 



COMPRESSION WOOD. 



Occasional pieces of wood of unusual growth are encountered. The 

 annual rings are very broad, with an abnormally large proportion 

 of summer wood per annual ring, and there is little contrast between 

 the spring wood and the summer, wood. The specific gravity is very 

 much higher than that of normal material. The abnormal growth is 

 supposed to be due to the fact that the tree or portion of the tree 

 from which the piece came had been under some long-continued un- 

 usual stress or had been in an unusual position. The term " com- 

 pression wood " is usually applied to material of this nature. The 

 writer remembers particularly a spruce wing beam with six annual 

 rings per inch of radius, 75 per cent or more of summer wood per 

 annual ring, and a specific gravity of 0.85. Since the usual specific 

 gravity of spruce used is about 0.40, it can readily be seen that the 

 weight of this wing beam was more than double the normal. Com- 

 pression wood is not confined to spruce, but may be found in other 

 soft woods. This type of wood is not desirable. Its strength proper- 

 ties are uncertain, and its shrinkage does not correspond to that of 

 normal wood, the longitudinal shrinkage being several times as great, 

 while the radial and tangential shrinkage is very much less. The 

 excessive weight is also a factor that must be considered in a deli- 

 cately balanced machine. 



STEAMING AND BENDING. 



Wood may be rendered brittle or otherwise injured by steam bend- 

 ing if this is not properly done. It is necessary to bend certain 

 parts of an airplane frame in this way in order to obviate the ini- 

 tial stresses which would result if these members were simply sprung 

 into place. This should not be attempted on thoroughly air-dry or 

 kiln-dry material, because wood once dried is weaker when brought 

 back to a higher moisture content, and in addition such material has 

 a tendency to spring back after the clamps are removed if it was 

 not thoroughly resoaked. As a rule, wood with less than 18 per cent 

 of moisture based on oven-dry weight should not be steamed and 

 bent. 



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