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BULLETIN 1128, U. S. DEPARTMENT OF AGRICULTURE. 



latter, reported as being made from decayed or " dead " wood. This 

 was characterized by the fact that the struts, after assembly, if 

 struck sharply on the side near the middle, would "bow" and re- 

 main in such position. If struck then on the opposite side the 

 "bow" might reverse, but the strut rarely straightened up. This 

 condition results from such an excessive tightening of the drift 

 and flying wires that the struts acting as posts between the wings 

 are placed under such a heavy compression load 

 that they are just on the point of failure, hence the 

 inability to straighten up after bowing. Needless 

 to state, such a condition is dangerous. 



Figure 3 shows a compression failure in the head 

 of a vertical fuselage strut due to severe tightening 

 of the tension wires in the fuselage. This failure 

 occurred before the machine had left the factory 

 floor. 



In the foregoing pages no attempt has been made 

 to specify all the defects in airplane timber aside 

 from decays or discolorations, or to describe fully 

 those mentioned. The writer desires merely to call 

 the attention of the reader to defects that can re- 

 ceive only passing mention or must be omitted 

 entirely, so that, if interested, he can become con- 

 versant with these through the references cited. 



COLOR COMPARISONS. 



Color is a natural characteristic of wood while 

 still in the living tree. For the first few years after 

 formation wood is white or nearly so, and finally 

 when the sapwood is transformed to heartwood a 

 decided color change takes place in most woods, 

 while in some the change is negligible. In such, 

 species as redwood {Sequoia sempervirens (Lam.) 

 Endl.), incense cedar, Douglas fir, juniper (Juni- 

 perus), white ash, true mahogany, and white oak 

 there is a decided contrast between the light-colored 

 sapwood and the dark heartwood, while in spruce, 

 fir. western hemlock, and yellow buckeye (Aesculus octandra Marsh) 

 the heartwood more nearly approaches the sapwood in color, and in 

 some cases it is difficult to distinguish between the two. Color is not 

 always uniform in the heartwood. It is necessary to be thoroughly 

 acquainted with woods to be able to recognize normal color variations 

 at a glance. 



Color should always be observed on a freshly cut surface and the 

 surface (whether radial, tangential, or cross) recorded when making 

 permanent observations. All woods change color on exposure to 

 light and air (54) , the most noticeable change occurring in the lighter 

 colored woods, particularly of the conifers. The first change is a 

 yellowing, then a graying, and finally in some conifers a decided 

 browning. These color changes have no weakening effect on the me- 

 chanical properties of wood, since the discolored portion is a very 

 thin surface layer and microorganisms play no role in this change 



Fig. 3. — Head of a 

 fuselage strut, 

 showing a com- 

 pression failure 

 in Sitka spruce 

 caused by an ex- 

 cessive tighten- 

 ing of the ten- 

 sion wires in the 

 fuselage. 



