ORIGINAL: knowhereman
can I simply epoxy CF tow to the top and bottom of each spar to increase the stiffness or do I have to do the whole composite spar thing?
Yes you can, BUT the wing will still fail. If the limiting factor was the strength of the spars your wing will be stronger, but if the limiting factor was your shear web to spar bond it won't matter.
The reason they do the whole composite spar thing is the carbon makes the spar so much stronger that the spar-web bond fails, so they wrap the spar with tow to keep it from buckling outward, then the shear webs fail so they wrap the whole thing in biased glass or kevlar to take the shear stresses.
If you just want a stronger spar try:
Make your shear webs as wide as your spars.
Making the best possible fit between parts.
Make the best glue joints you can.
Add CF on the spars, twice as thick on the top as the bottom.
Put a few wraps of kevlar tow at each end of the spar to stop it from peeling off.
Put a few wraps of kevlar to at both ends of any wing tubes you use to keep them from buckling outward.
or
Just don't launch so hard, you use the super spar for 4 seconds a flight, but the weight is in the wing for the whole flight, light wings fly better.