DECAYS AND DISCOLORATIONS IN AIRPLANE WOODS. 9 
It is quite difficult to judge the specific gravity of diffuse porous 
hardwoods by visual examination except in those pieces patently 
very low or very high. Actual specific-gravity determinations will 
have to be used to a greater extent when handling this class of woods. 
In examining a piece of wood of any considerable length to deter- 
mine its specific gravity, care must be used to examine it throughout. 
Pieces in which the grain is not perfectly straight may have high 
specific gravity in one portion and a low density in another, as at- 
tested by the percentage of summer wood. This is due to the fact 
that trees may not develop wood of the same or nearly the same spe- 
cific gravity throughout their life. Such a condition is not at all 
uncommon in white-ash longerons, and it must be remembered that 
any given piece of wood is no stronger than its weakest portion. 
As a general rule, airplane timber should be purchased under speci- 
fications so worded in regard to the ratio of spring wood and summer 
wood per annual ring and number of annual rings per inch of radius 
as to reject at the source of supply most of the stock of low specific 
gravity. 
COMPRESSION WOOD. 
Occasional pieces of wood of unusual growth are encountered. The 
annual rings are very broad, with an abnormally large proportion 
of summer wood per annual ring, and there is little contrast between 
the spring wood and the summer wood. The specific gravity is very 
much higher than that of normal material. The abnormal growth is 
supposed to be due to the fact that the tree or portion of the tree 
from which the piece came had been under some long-continued un- 
usual stress or had been in an unusual position. The term “ com- 
pression wood” is usually applied to material of this nature. The 
writer remembers particularly a spruce wing beam with six annual 
rings per inch of radius, 75 per cent or more of summer wood per 
annual ring, and a specific gravity of 0.85. Since the usual specific 
gravity of spruce used is about 0.40, it can readily be seen that the 
weight of this wing beam was more than double the normal. Com- 
pression wood is not confined to spruce, but may be found in other 
soft woods. “his type of wood is not desirable. Its strength proper- 
ties are uncertain, and its shrinkage does not correspond to that of 
normal wood, the longitudinal shrinkage being several times as great, 
while the radial and tangential shrinkage is very much less. The 
excessive weight is also a factor that must be considered in a deli- 
cately balanced machine. 
STEAMING AND BENDING. 
Wood may be rendered brittle or otherwise injured by steam bend- 
ing if this is not properly done. It is necessary to bend certain 
parts of an airplane frame in this way in order to obviate the ini- 
tial stresses which would result if these members were simply sprung 
into place. This should not be attempted on thoroughly air-dry or 
kiln-dry material, because wood once dried is weaker when brought 
back to a higher moisture content, and in addition such material has 
a tendency to spring back after the clamps are removed if it was 
not thoroughly resoaked. Asa rule, wood with less than 18 per cent 
a meieure based on oven-dry weight should not be steamed and 
ent. : 
9997—22——_2 
