DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 13 
Worm holes not only weaken the wood, but the presence of the 
larvee of powder-post beetles in the wood may render it unsafe. 
Ambrosia beetles or borers already in the wood can be killed by the 
ordinary dry-kiln process, but certain types of powder-post beetles 
require higher temperatures. It is much simpler to prevent attack, 
and this can be done by slight modifications in business management. 
Full information on these and other insect defects can be obtained 
from the Bureau of Entomology, United States Department of 
Agriculture. ° 
CHARACTERISTICS OF SPECIES. 
Different woods have certain inherent qualities which must be 
recognized. Douglas fir has a decided tendency to splinter. The 
separation usually occurs along the annual rings in the spring wood 
adjacent to the summer wood. It is quite probable that this char- 
acteristic can be accentuated by excessive steaming with high tem- 
peratures during kiln drying, since it has been shown (6) that 
certain softwoods in which the spring wood is sharply differ- 
entiated from the summer wood, in which category Douglas fir 
belongs, have the spring wood weakened more easily than the sum- 
mer wood by prolonged boiling. On account of this tendency of 
Douglas fir to splinter, aside from other reasons, Sitka spruce and 
Port Orford cedar are more desirable. 
White elm can be readily steamed and bent, but it usually warps 
and twists badly in drying. Douglas fir is very subject to splitting 
when nailed, while basswood is one of the least troublesome species 
in this respect. Black ash is low in stiffness. Other examples might 
be cited, but these are sufficient to show that the failure of a wood to 
meet certain requirements may be unavoidable. 
FAULTY DESIGN AND ASSEMBLY. 
As an example of faulty design the following instance may be 
cited. In one of the types of combat planes constructed in the air- 
craft factories of this country two horizontal bolts were placed 
directly through the neutral plane in each upper front longeron 
of Douglas fir. A fitting was hung on the head of these bolts and 
two opposed high-tension wires, pulling at right angles to the direc- 
tion of the grain in the longeron, were attached to the fitting. 
While the ship was subjected to shocks and jars which occur par- 
ticularly during landing, these wires were in very unequal and 
irregular tension, varying from loose to very tight, and the strain 
on the longeron was exactly the same as if a chisel blade had been 
inserted through the neutral plane and the handle was being jerked 
sharply up and down. On the test flight of the first ships built the 
longerons did the obvious thing; they split in both directions from 
the bolts for a distance of a few inches to 2 feet. The failure was 
promptly blamed on the wood, which was assumed to_be either 
| weakened by decay or faulty kiln drying, instead of on the faulty 
design, where it belonged. Such mistakes arise from the lack of 
knowledge of the mechanical properties of wood on the part of the 
designer. 
Incorrect assembly also plays a part in the unjust condemnation 
_ of perfectly good wood. One of the most common occurrences was 
to have interplane struts of Douglas fir or spruce, especially the 
