Sequoia Aircraft Corporation F.8L Falco, Construction Manual

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F.8L Falco Construction Manual 13–1 Revision 4, March 1, 2002
Chapter 13
Making the Spars
This chapter is devoted entirely to making the spars. If you are purchasing the spars, you may skip
this chapter. As always, it is best to start with the simplest parts and work your way up to the
most difficult pieces.
Spar Jigs
Spars are made on jig boards just like ribs and fuselage frames. Use wood blocks to hold the parts
in position while gluing. The simplest spars are those which do not have any taper to the
thickness of the spar caps (or “booms”, as they are sometimes called).
The rudder beam is probably the first part that you should make. When this is complete, you can
build the forward fin beam. The laminated portion is made in the same way as the fuselage frame
laminations. Because of the tight bend radius, you will find that you will have to soak and pre-
bend the spruce laminating strips.
The elevator beam and forward stabilizer beam should be the next to be built. These are simple
and like the others you have made.
At some point you are going to have to contend with the tapered spars. These are more difficult.
In most cases, you will find that the spar caps are tapered both ways. Most builders are able to saw
the spruce to the required taper on a bandsaw and then sand the spruce smooth. It is ideal if you
can make the spars so that the tapering occurs on the inner face of the caps. This way, you have
the grain of the wood next to the skin parallel to the skin.
You may also taper the spars by making the spars with non-tapered stock and then taper the
finished spar. The risk of this method is that you might end up ruining the spar on the final
tapering operation.
Plywood Grain Direction
Note that all of the spars specify that the plywood is at 45°. The plywood that you should use will
be normal plywood with each layer at 90° to each other. The face grain of this 90° plywood is
installed so that it is at 45° to the centerline of the spar.
A debate can always be started on the subject of the direction of the face grain of the plywood on
spars—should it be “up and in” or “up and out”? For the wing spars, the grain should be “up and
in” according to Mr. Frati and ANC-18. The theory of this method is that the plywood resists
buckling better this way, but others insist that it is equally acceptable to have it run the other way
where you have the greater strength of the wood running in the direction of the greater load in
tension. The manuals of Fairchild and other aircraft companies frequently had wing spars with
one method on the front and the other method on the aft face. Do it any way that you like, but
the airplane was engineered with “up and in” in the calculations, and that is the way that the
production aircraft were built.
For the horizontal stabilizer, the spars take a download, so the grain direction of the spars should
be “up and out”.