﻿DECAYS AND DISCOLOBATIONS IN AIRPLANE WOODS. 13 



Worm holes not only weaken the wood', but the presence of the 

 larvae 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 b}^ 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 (61) 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 

 longeronu 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 



