DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 3 
and, ultimately, decay. Then, too, it is almost impossible for the 
stock in the center of ‘the pile ever to become properly dry. 
At best, however, air drying is a matter of months, even with soft- 
woods, while proper kiln ‘drying can be accomplished within one to 
three weeks or so, depending on the thickness of the stock. Asa 
rule, hardwoods both kiln- dry and air-dry more slowly. 
Air-dried stock should be shipped in the same manner as kiln- 
dried and handled in the same way at the factory, except that it 
must be kiln-dried to the proper moisture content before it is condi- 
tioned in the shop. 
The principles given briefly in the foregoing paragraphs, together 
with their application and underlying reasons, are brought out in 
detail in the following pages. 
WOODS USED FOR AIRPLANE CONSTRUCTION. 
The most important wood for aircraft construction is spruce, 
including red, white, and Sitka spruce (Picea rubens Sarg., P. cana- 
densis (Mill.) B. S. P., and P. sttchensis (Bong.) Trauty. and 
Mayer), but of these Sitka spruce, on account of its much larger 
size and the consequently greater quantity of clear lumber that can 
be obtained, is paramount. By far the greatest proportion of the 
‘lumber entering into the construction of most present-day airplanes 
is spruce or one of its substitutes. The combination of strength 
properties with light weight found in spruce is not duplicated in any 
other wood. Most of the beams in the directing surfaces are prefer- 
ably of spruce or a soft wood, as are many of “the struts, and these 
parts account for the bulk of the timber in an airplane. 
An excellent substitute for spruce is Port Orford cedar (Cham- 
aecyparis lawsoniana (Murr.) Parl.), which is slightly heavier. 
Unfortunately the supply of this splendid wood is decidedly lim- 
ited. Douglas fir (Pseudotsuga taxifolia (Lam.) Br.), though much 
heavier than spruce, is an extensively used substitute. Other woods 
which can play some part in this way or may be used for special 
purposes where a softwood is needed are -western white pine (P2znus 
monticola Dougl.), sugar pine (P. lambertiana Dougl.), western 
hemlock (Tsuga heterophylla (Raf.) Sarg.), white fir (Abies con- 
color (Gord.) Parry), amabilis fir (4. amabilis (Loud.) Forbes). 
noble fir (A. nobilis Lindl.), yellow or tulip poplar (Liriodendron 
tulipifera Linn.), basswood (Tilia americana Linn.), incense cedar 
(Libocedrus decurrens Torr.), and western red cedar (huja plicata 
Don.). Certain parts of an airplane frame as a rule are made from 
hardwoods. In such parts great strength and toughness are re- 
quisite. Here, commercial white ash? stands supreme. For ex- 
ample, it is unsurpassed for longerons in those fuselages not con- 
structed wholly or mostly of veneer. Black ash (/vaxinus nigra 
Marsh), which does not possess sufficient stiffness 10 use in highly 
stressed parts, can be distinguished from white ash (2, 30, p. 47; 68, 
p. 62).2, White oak,? hard. maple (Acer saccharum Marsh), and 
1 Commercial white ash includes white ash (Frarinus americana Linn.), green ash (F. 
Ae Borkh.), blue ash (F. quadrangulata Michx.), and Biltmore ash ( F. biltmoreana 
eadle 
: ce numbers (italic) in parentheses refer to ‘*“‘ Literature cited’ at the end of this 
ulletin 
* White oak as used here includes white oak (Quercus alba Linn.), bur oak (Q. macro- 
earpa Michx.), cow oak (Q. michauzii Nutt.), and post oak (Q. minor (Marsh) Sarg.). 
