Matt,

If you are after stiffness, whether in bending or in torsion, you have greatly handicapped yourself by using 3/8" core material. That means you only have 3/16" from the neutral axis to the outer fiber in bending, in the terminology of a structural analyst. For a 60-sized plane?

That means that to regain the stiffness that you would have had just from 1/2" balsa you're adding weight. And for a 60-sized plane, most guys complain that the fuse isn't really as stiff as they'd like it.

You're going to need more than polyspan to get that back. You may find that even carbon fiber strip will buckle on you if you get a lateral shock load. Think hanger rash, or....

Cutting away core material in a plywood skinned construction may reduce the weight some, since the plywood is going to carry all of the lateral bending loads due to higher modulus and location, but the issue then becomes how much can be cut away without allowing the plywood skin to buckle and separate. And, you have more than just the lateral bending load case. For torsion you need a closed tube to be efficient. A tube with a horrible section aspect ratio of 4 or 5 inches tall and 3/8" wide....is just not efficient. To carry the side skin stresses from one side to the other, the top and bottom balsa sections will need to be pretty deep.

If you want an example of a profile fuse on a 60-sized plane, go look at Frank William's Bearcat. Plans were in Model Aviation. I don't think the core cutouts were completely optimal, but they worked.

Dave

A cheap source for the carbon tube can be found at your local archery club. Ask for discarded women's arrows :-).
Regards,

Wolfgang

There's also the carbon tube fuselage option. 

Consider that a 14x13x1000mm tube only weighs around 32g(1.14oz), and a larger diameter 16x15x1000mm tube only weighs 38g(1.34oz), and is 51% stronger.

Matt,  that plane was built by Bob Branch.  He used a Brodak P-40 ARF wing/tail.  560 sq, 53.5" span.  Fuselage measurements are stretched to around 17.5 hinge line, 10.5"-ish nose.

https://stunthanger.com/smf/gettin-all-amp'ed-up!/c-14-carbon-fuse-electric-profile/

I built a Pathfinder sized test fuselage using 12x10mm tubes and it is astonishingly stiff.  My next carbon tube fuselage will utilize even larger diameter, thinner walled tubing.   Either 14x13mm tubes or 16mmx15mm.  16x15 is 30% lighter and 40% stronger than the 12x10, which is already crazy stiff.
(Possibly even using up to 18mmx17mm, as these large diameter, thin wall carbon tubes are quite light.)

https://design215.com/dcal/toolbox/tubing-calculator

Matt,
Continue making your fuselage out of 3/8 balsa but skip the ply and use 1/16 balsa put on vertically and at 30 or 40 degree angle. You put it on right to left on one side and reverse it on the other. cover with 1/2 oz cloth with water based urethane and  prep for finish.
I've done all of my 60 sized and other also with this method and they are as rigid as a ball bat with absolutely no flex.
 Example put the one fuselage side sheeting on with pieces angled this way /////// and then put it on the opposite way on the other side.

Dennis