4-6 lb balsa, the so-called "punky" stuff, has the advantage of being super soft,
and carving easily. Its use is justified in free flight, and often competitive CL stunt.
However, keep in mind that the very softest balsa is NOT generally the strongest
per weight. For example, a slightly heavier cut of 1/16" balsa can perhaps replace
super punky 1/8" balsa, with the same or better strength. If it is twice as strong
but not twice as heavy, you're ahead.
I usually make my wing ribs from 3/32" and 1/8" balsa, simply because they are far
easier to deal with than 1/16" material. And, 4-6 lb 1/16" sheet is not the optimum
choice for balsa ribs, not just because they are overly fragile, but because the bulk
of the rib gets cut away with interior cutouts. So, the amount of weight saved for
a 1/16" set of ribs over a 3/32" set of ribs, finished, is a small amount, insignificant.
If you really want to save weight on a wing construction, look closely at construction
methods. For example, you can eliminate a leading edge spar completely, with a single
molded planking sheet for it. Virtually all of the important strength is retained, meanwhile
you've eliminated at least a 1/4" square spar, or maybe even a 3/8" square spar.
Sitka spruce is an example of a wood that is considerably stronger than balsa for a
given weight. Piper Cubs use it for wing spars; you can bet that if balsa was stronger,
it would be used. A few clever guys, like Tom Morris, have exploited spruce for spars.
For example, you replace a 1/4" square balsa top/bottom spar with a 1/4" tall x 1/8"
wide spruce spar. The weight of the spars is comparable, but the spruce one has
more useful strength.
That all said, carving this punky balsa material is a pure joy, but if you are like me, it
makes you a bit sick to waste and carve away the hollow inside of a top or bottom
block..
A better solution then is to mold 1/8" balsa sheet for these blocks.. Cheaper, and
actually stronger. And, my own preference is to use two 1/16" balsa sheets in
lamination for molding. They comply easier to the mold, and you can carefully
peel them off the mold, apply the very thinnest coat of epoxy glue, and remold.
The result is an incredibly strong laminated shell.
For fuselage sides, I always use 3/32" instead of 1/8" thick wood. And I use either
1/64" or 1/32" plywood laminate inside doublers for the nose to aft of the flap
hinge line; the nose curvature is molded in with this laminate. The technique is
borrowed from a Bob Hunt construction video tape.. The other thing I do, which
improves rigidity of the fuselage aft section is to CAD-produce formers all tilted
at 60 degrees, the so called "geodesic" geometry of equilateral triangles. AND, I
provide curvature in two axes, the fuselage sides are convex about 1/8" or more,
which "cups" the surface. And the aft fuselage sides are also curved like a banana
when viewed from the top. The result is a slightly stressed skin structure which
is light and rigid, and builds absolutely true without needing a jig.
Now, remember I was talking about eliminating excess material in the ribs? Well,
I do the same in the formers in the aft fuselage, which are cut out, not with a
rectangular shape, but an elliptical one. This is far stronger than any cutout with
sharp corners..
Finally, I regularly make up 1/8" balsa "plywood", with cross grained 1/16" material, which
is ideal for the formers in the aft fuselage. In fact, I did the last ones with 1/32" material
in the vertical axis and 1/16" in the horizontal. These formers are very strong, much
stronger than the equivalent 1/8" soft punky balsa counterparts.
The point of all this is to suggest that you can do quite a bit of optimization of the material
on balsa structures, and use slightly heavier (considerably cheaper and more available)
balsa to benefit of strength and finished weight. Extra design work is required, but there
is payback.
L.
"Tell the truth, and so puzzle and confound your adversaries." -Henry Wotton
Topic: Where to buy Balsa (Read 10309 times)