﻿6 BULLETIN 1128, U. S. DEPARTMENT OE AGRICULTURE. 



tested by bending fails with a short break instead of a splintering 

 fracture. This is one indication of decay, but not all wood show- 

 ing such defect is decayed. Too often when wood appears to be 

 somewhat brash and develops less than the normal strength, instead 

 of making a serious attempt to determine the real source of the diffi- 

 culty the cabalistic term u dry-rot" is uttered, and the case is set- 

 tled, often resulting in the loss of good material, while the trouble 

 goes on unchecked. Even if the wood be decayed, it most probably 

 is not dry-rot, which term to the pathologist embraces a definite type 

 of decay caused by a certain fungus. Let us consider a few of the 

 more important causes of brashness, aside from decay, in aircraft 

 woods. 



The primary requisite of wood for use in airplanes is that it 

 must be of specific gravity high enough to give the necessary 

 strength. It has long been known that an increase in strength of 

 any species of wood goes with an increase in specific gravity, and 

 it has finally become possible accurately to express this relation for 

 the various strength properties, so that if the specific gravity of a 

 given piece of wood is known it is possible quite accurately to derive 

 its strength under various stresses {41). No matter how perfect a 

 piece may be in other respects and free from all other defects, if it 

 is below the minimum specific gravity it should not be used. These 

 minimum figures have been carefully worked out for the more im- 

 portant airplane woods (68, p. 21; 69, p. 26). Wood of low specific 

 gravity is naturally somewhat brittle, and for this reason is often 

 erroneously considered as slightly decaj^ed. While the actual spe- 

 cific gravity of the wood substance in various species is practically 

 the same {13), having a value of 1.54, the porous nature of the wood 

 is such that most commercial species range from 0.3 to 0.6. In other 

 words, only one-fifth to three-fifths of a unit volume of wood is oc- 

 cupied by wood substance"; the remainder is air. 



It is self-evident that a density or specific gravity determination of 

 every individual piece of wood to be used for a primary member in an 

 airplane is out of the question. Neither is it necessary. The most 

 reliable index of specific gravity, without making an actual test, is 

 the ratio of spring wood to summer wood per annual ring. This is 

 best seen on the cross or end section after it has been smoothed off 

 with a sharp knife or a high-speed miter saw. In the softwoods the 

 summer wood is the darker of the two bands composing each annual 

 ring, as is shown in Figure 1, which illustrates cross sections from 

 two wing beams of Douglas fir, one of average and the other of low 

 specific gravity. In the ring-porous hardwoods (ash, for example) 

 the summer wood appears more solid and very much less porous than 

 the spring wood, but in the diffuse-porous hardwoods (such as 

 birch) this is often very difficult to determine. For Douglas fir a 

 minimum specific gravity of 0.47 has been established for high- 

 stressed members, but this can probably be reduced to 0.45 with per- 

 fect safety when used as a substitute for spruce. As a rule, wood of 

 this species with less than 6 or more than 30 annual rings per inch, 

 measured radially on the cross section, falls below the minimum 

 specific gravity. The former usually comes from the center of the 

 tree, where the wood is rapid growing and brash, while the latter is 

 the slow-grown soft " yellow fir " so characteristic of the outer layers 



