﻿42 BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 



mercuric chloric!, is unfortunately a deadly poison, corrodes metal, 

 and is very difficult to handle. The subject of preservative treat- 

 ment is one about which little is known as applied to airplanes. 



Little information is available as to what fungi actually cause 

 decay in finished airplanes or as to the types of decay found. Un- 

 doubted^ the fungi most concerned are those commonly attacking 

 the manufactured product, such as the dry-rot fungus, the brown 

 Lenzites, or the rose-colored Fomes. Fungi decaying the heartwood 

 of living trees are not commonly found. When they do appear, this 

 is practically proof positive that the manufacturer used wood with 

 incipient decay in the fabrication of the wooden parts. 



SUMMARY. 



Among the softwoods or conifers the most valuable for airplane 

 construction are red, white, and Sitka spruce, the last being most 

 important on account of its large size and the consequently greater 

 proportion of clear lumber that can be obtained. A. splendid substi- 

 tute for spruce, and its superior where durability must be consid- 

 ered, is Port Orford cedar. However, the supply of this wood is 

 limited. Douglas fir, which is much heavier than spruce and there- 

 fore not so desirable, is also extensively used. In those parts of an 

 airplane frame requiring great strength and toughness, hardwoods 

 are used. White ash is best, but white oak, hard maple, and rock 

 elm may be substituted. Hickory is principally used for tail skids. 

 Black walnut and true mahogany are unsurpassed for propellers, 

 but yellow birch, sweet birch, African mahogany, black cherry, hard 

 maple, and white oak are acceptable substitutes. As the supply of 

 timber diminishes in the future, a wider variety of woods will be 

 acceptable for airplane construction. 



All wood is subject to defects, of which one of the most serious is 

 decay; but other defects which reduce the strength of timber must 

 be recognized. Among these can be mentioned spiral and diagonal 

 grain, specific gravity that is too low or too high, brashness caused 

 by excessive temperatures during steaming or kiln drying, com- 

 pression failures, shakes, pitch pockets, and insect. galleries. 



Decay in its incipient stage is often not readily recognized; but 

 wood with incipient decay must not be used in airplane construction, 

 since infected wood may be reduced in strength. Furthermore, the 

 decay may continue if suitable conditions arise. The first indi- 

 cation of decay is usually a discoloration of the infected wood, but 

 not all discolorations result from decay. Marked discoloration of 

 the wood, particularly the sapwood, usually accompanies pith-ray 

 flecks and wounds made by lightning and sapsuckers. Conditions 

 favorable for decay also promote sap stains. These discolorations of 

 the green sapwood of various softwoods and hardwoods occur in two 

 ways: (1) By an oxidation of the organic compounds in the cells 

 of the sapwood when exposed to the air and (2) by the attack of 

 sap-staining fungi, the hyphae of which feed on the organic com- 

 pounds in the cells of the sapwood without attacking the cell walls 

 except to a negligible extent. The discolorations are confined to the 

 sapwood as a rule, but occasionally the sap-staining fungi, may dis- 

 color the heartwood slightl}\ For practical purposes wood so dis- 

 colored is not reduced in strength. 



