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 & 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 
C 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 1s 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, 45, 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 
/ a = ; 
