﻿DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 5 



Table 1, which is an adaptation of specification 15037-B of the 

 Bureau of Aircraft Production, shows where the various woods 

 may be used in an airplane and the quality desired. The symbol A 

 indicates a grade of wood of the very highest quality and free from 

 all injurious defects; grade B demands a quality of wood similar to 

 grade A in all respects except that a little tolerance is allowed in 

 regard to straightness of grain and specific gravity; wood of grade 

 G is used in parts where little strength is needed and may contain 

 various defects, provided the piece is strong enough for the purpose 

 intended. 



These woods are not the only ones used for airplanes, but they are 

 the most important. Others are mentioned here and there in this 

 bulletin. It can be predicted that, with a growing scarcity of the 

 more desirable species and an increase in our knowledge of the prop- 

 erties of other species, woods little or not at all used at present will 

 become of importance. For a full discussion of this entire subject, 

 the reader is referred to other sources (60; 69, p. 34-40). 



GENERAL DEFECTS OF AIRPLANE WOODS. 



It is impossible to thoroughly understand wood without a work- 

 ing knowledge of its structure and mechanical properties. This is 

 more difficult to attain than with most other materials of construc- 

 tion, for wood, instead of being a relatively simple and more or less 

 homogeneous compound, is a highly complex organic structure whose 

 chemical, composition is even now none too well understood. The 

 discussion in the following pages will be much clearer to the reader 

 provided he has such knowledge. There are a number of valuable 

 publications which may be referred to in this connection (30, 4.5, 47, 

 48,68,69). 



Besides decay, there are other defects which reduce the strength 

 of timber, and these must be given due consideration. Wood may 

 be inherently weak because of its structure, it may be injured by 

 some process of manufacture, or the trouble may be due to faulty 

 design or assembly. Such defects in relation to airplane woods have 

 been discussed in various publications (42; 46> 68, P- 15-20; 69, p. 

 11-22), but a review of the more important of these is essential 

 here, since by the uninitiated some of them are confused with decay. 



GRAIN. 



One of the most common defects in airplane woods is an exces- 

 sive slope of diagonal or spiral grain. Since any deviation from 

 straight grain is accompanied by a reduction in strength, the re- 

 quirements in this respect are very exacting, a deviation from 

 straight grain of more than 1 inch in 20 inches rarely being allowed 

 for any highly stressed portion of an airplane, although this may 

 be reduced to 1 inch in 12 in portions of less severe stress. A dis- 

 cussion of the methods to be employed in detecting this defect, to- 

 gether with its effect on strength, may be found in several publica- 

 tions (31; 42, p. 8-14; 68, p. 15-16; 69, p. 11-20). 



SPECIFIC GRAVITY. 



Brashness or brittleness in wood is another common defect. These 

 synonymous terms denote a lack of toughness in wood to which they 

 are applied. Brash wood is usually low in strength, and when 



