As with the fuselage frames, I elected to purchase pre-cut
spruce from Western Aircraft and build the spars from plans.
It was first necessary to build a 28 foot x 2 foot table. I found
that the easiest way was to use prefabricated floor joists. They
are remarkably straight and are available in the necessary lengths.
I used short lengths of joists to build a rigid box assembly
and then used 4x4 timbers as legs. Each leg had a bolt tapped
into the bottom to permit leveling. A few quick passes with a
jointer plane was sufficient to remove any irregularities on
the top of the joists and yield a flat surface onto which 0.75"
particle board was screwed. The final leveling was accomplished
with a water level, and it was cross-checked with a laser to
yield a surface flat to within ±0.5mm.
The next step was laying out the WL and BL coordinates. This
is where a laser proved its worth. Once the coordinates were
laid out, then the dihedral could be marked and the clamping
cleats located.
I did notice after the entire glue-up process was complete
that there was a tendency for the wing spar to take a softer
turn at the point where the dihedral starts. Part of this problem
was traced to the flexing of the table top surface and part to
the movement of the cleat, even though it was securely screwed
in place. If I were repeating this process, I would have doubled
up the particle board or used a stronger material. Also, I would
have built stronger cleats around the bend points.
Glue-up was done with resorcinol and required every clamp
that I, my neighbors, and a woodworker friend of mine owned.
This photo shows a detail of the glue-up. The spar top and bottom
were glued up separately, moving the cleats as necessary. The
spar was protected during the glue-up by use of particle board
cauls on both sides.
|