﻿DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 33 



again become suitable. Laminations with incipient decay should 

 not be used in propellers. In this place the reduction in strength 

 need not be so carefully considered as the variation from the normal 

 shrinking and swelling that would result. Unequal and particularly 

 unusual strains and stresses must be avoided above all things in 

 propellers. 



Incipient decay usually appears as a discoloration, in some cases 

 pronounced, in others so faint as to be practically invisible. Most 

 of this decay in airplane lumber was actually in the tree when it was 

 cut or in the logs when they left the woods. It is rare that any 

 serious effort is made in the woods or at the mills to cut out incipient 

 decay. When the logs are bucked and sawed the typical decay is 

 usually trimmed off, leaving the less apparent incipient decay in the 

 lumber. After sawing, the upper grades of lumber, which include 

 airplane stock, are usually handled carefully enough at the larger 

 mills to prevent further damage. 



When decay commences in a living tree, it spreads upward in the 

 heartwood if the infection entered at the butt, or in both directions 

 if it occurred higher on the trunk. Very rarely do the decays in the 

 heartwood of living trees attack the sapwood. Beyond the typical 

 decay, that is, where the wood is decidedly rotted, extend the incipi- 

 ent stages of decay, which become less and less apparent as the dis- 

 tance from the typical decay increases. Finally, the incipient decay 

 ends entirely. The wood beyond is then sound. The incipient decay 

 rarely ends abruptly or evenly, but usually fades out in one or more 

 irregular streaks, which may be short or long. It usually extends 

 only 3 or 4 feet longitudinally beyond the typical decay, but with 

 certain wood-destroying fungi on some hosts the incipient decay 

 may extend 15 feet or more in advance of the typical decay. Fur- 

 thermore, the latter is alwa}^ bounded radially by incipient decay, 

 and this boundary is often irregular. Boards sawed from diseased 

 trees may contain all stages of decay or incipient decay, occupying 

 part or all of the board. The fact that the fungi causing decay in 

 standing trees may continue their work of destruction in logs in the 

 woods, or even in sawed lumber if conditions are favorable, indicates 

 the necessity for having logs removed from the woods, sawed, and 

 the lumber dried with reasonable promptness. 



When lumber is green the discolorations indicating incipient decay 

 are more intense than when the wood has seasoned for some time. 

 During the drying process the discolorations fade in varying de- 

 grees. Furthermore, if a new discoloration appears within one or 

 two weeks after the lumber comes from the saw it is practically 

 certain that it is not caused by one of the wood-destroying fungi 

 attacking the piled lumber, since the latter work more slowly. A 

 sap -staining fungus or a chemical reaction is the most likely agent 

 in such a case. 



Incipient decay should be detected and eliminated before the 

 lumber is worked into individual parts. If the entire piece is not 

 defective the sound portion can be sawed out and utilized. In 

 marking a piece for cutting, however, it must be remembered that 

 decay extends more rapidly with the grain in a tree or piece of wood 

 than it does across the grain; thus, to be perfectly safe, an allow- 

 ance of 2 feet should be made in the direction of the grain beyond 



