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F.8L Falco Construction Manual
8–3
Revision 4, March 1, 2002
widely separated cities in the United States at different times of the year. Similar seasonal
variations occur in other parts of the world. In tropical and subtropical areas, where long rainy
seasons are followed by long dry spells, the spread of equilibrium moisture content between
seasons may be considerable. Low equilibrium moisture content conditions may be expected in
desert areas, while in Europe generally the average equilibrium moisture content would be as high
as, or higher than, that along the northeastern coast of the United States.
The approximate equilibrium moisture content for wood can be estimated for any section of the
country and for any season by noting the relative humidity given in the table above and reading
the corresponding moisture content from Figure 1 at the particular temperature under
consideration.
0
10
20
30
40
50
60
70
80
90
100
0
5
10
15
20
25
30
35
Relative Humidity in Atmosphere (%)
Moisture Content of Wood (%)
70°F
141°F
212°F
Figure 2
The relationship between relative humidity, temperature and the moisture content of wood is
shown in Figure 2 in a form that may be easier to understand than Figure 1.
Most Falco builders pay little attention to the moisture content of the wood that goes into their
aircraft. Since wood expands as it picks up moisture and contracts as it dries out the moisture
content of the wood will have an effect on the appearance of your aircraft. In general, there are
two things to watch out for.
First, the moisture content of spruce changes more quickly than that of birch plywood. The
reason for this is that plywood has layers of waterproof glue, and these layers slow down the rate of
change in plywood. Since aircraft plywood is manufactured in a hot press (the phenolic glue
requires about 300°F to cure), it is very low in moisture content when first made. For most parts
of the country, spruce and plywood will reach a moisture content of about 12% in time, regardless
of the finish applied to the wood—for more on the subject of finishes, see Chapter 9 “Moisture
Protection of Wood”.
When you purchase birch plywood, it is quite likely that the plywood will have a low moisture
Summary of Contents for F.8L Falco
Page 1: ...F 8L Falco Construction Manual...
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