Sequoia Aircraft Corporation F.8L Falco, Construction Manual

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F.8L Falco Construction Manual 8–1 Revision 4, March 1, 2002
Chapter 8
Aircraft Woodwork, Materials
Spruce
The Falco was originally designed and built with Sitka spruce for all of the spar caps. Poplar was
used for the ribs, fuselage frames and all other places where we now use spruce. We are in the
process of phasing out all mention of poplar in the Falco plans and use only spruce. It is
important that you use only Sitka spruce for the spars. Spruce is the only wood commonly used in
aircraft. It is the standard against which all other woods are judged. Spruce is slightly stronger
than poplar and slightly lighter. If you build your Falco entirely out of spruce, you will have a
lighter and stronger airplane. The entire aircraft wood technology has been built up around
spruce. We suggest that you use only spruce in your Falco.
Aircraft quality spruce is very expensive, largely due to the amount of waste that is created in the
grading process. The wood is kiln dried, then the wood is subjected to a series of inspections.
Only the best wood is used for aircraft, and the waste is enormous. As a result, most mills do not
want to bother with aircraft spruce. We suggest you only purchase spruce from companies that
grade and certify their spruce. It is very important that you use only certified aircraft spruce for
the spar caps. You can use lesser quality spruce for such things as blocking.
Other Woods
The Falco uses a small amount of other woods. There are a couple of blocks of Douglas fir used in
the main wing spar at the landing gear fittings. This may be purchased locally from a lumber yard.
The leading edge cap strip of the ailerons are made of beech, walnut or oak. The reason for this is
that the heavier wood is used as an aileron balance weight. These three woods weigh about the
same. Beech was used on the original production Falcos. You should be careful if you use white
oak with epoxy glue. Special instructions are included with Chem Tech's T-88 epoxy on gluing
white oak.
Birch Plywood
The plans for the Falco specify birch plywood in metric sizes. Some of the sizes are not available
in the U.S., and the builder will have to use the next larger size. In particular 1.2mm birch
plywood seems to be rather hard to obtain in the U.S. First, you might check the thickness of the
plywood you have purchased as “1mm”, and you may find that is actually measures to be 1.2mm.
If you are unable to select such pieces, then you will have to use 1.5mm plywood. Birch plywood
has a hard surface sheen as a result of the manufacturing process, and it must be lightly sanded
before gluing. This is very important as the glue will not penetrate the wood fibers without a light
scuff-sanding and will result in an unsafe joint. Mahogany plywood should not be used on the
Falco as it is weaker than the birch plywood specified.
Birch plywood, by the way, is actually a combination of birch and poplar veneers. Birch is used on
the face plys, while poplar is used for the core.
Moisture Content
The moisture content of wood is dependent upon the humidity and temperature of the
surrounding air. When wood is subjected to a constant temperature and relative humidity it will
in time come to a definite moisture content, which is called the equilibrium moisture content.
This relationship between the moisture content of Sitka spruce and the surrounding atmospheric