Musings on balsa vs. coroplast.

The availability and cost of balsa is making coroplast more of a viable alternative. I know coroplast planes are not new, just about everyone’s built and flonw one or more.  I’m just thinking about how to expand on the planes currently out there.  The most recent that I’ve done is the Man-Win Trainer I made a couple of week ago.  I’ve seen lots of coroplast planes over the years, but they are almost uniformly plain looking, if not outright ugly.  The best looking ones I’ve seen are Goozgog’s PT-19 coroplast trainers, three of which I’ve built.   A few Baby Ringmasters looked pretty good as well.

But why, for the most part, are they consistently ugly?  I always thought it was because coroplast is heavier than balsa and it’s not worth the time to make a nice looking plane if it’s going to be a rock.  If it weren’t for the weight, there are so many sheet wing planes out there that look so much better.  Look at all the planes by Dick Sarpolus.  How about the Skyray or Deweybird from SIG?  Or the hundreds of other slab wings designed over time for sheet balsa published in the various magazines?

Well, I got to wondering just how much heavier is Coroplast versus 1/8” balsa sheet that seems to be the norm for 1/2A planes?  So a few minutes ago I pulled out my scale, a sheet of medium 1/8”balsa, and a 20”x24” sheet of white coroplast.  I realize that there are different weights of balsa, but around here you take what you can get, so the medium available to me it is.  Measuring the sheet of 1/8” balsa gave a weight of 0.399 gm/in².  The coroplast came out to 0.392 gm/in².  The coroplast is lighter?!?  I re-measured three times, but this is what my digital kitchen scale shows.

Yes, coroplast doesn’t have the same stiffness as balsa and needs stiffeners through the flutes.  But it doesn’t need the finishing methods that balsa needs.  No grain fillers, no fuel proofing, no paint or Monokote.  Yes, colors are limited.  Vinyl sheet or decals can solve this if more decoration than base colors are desired.  Also, coroplast should be more prang resistant.

But what about cost?  The last 3”x36” sheet of 1/8” balsa cost me $4.99, and the last sheet of 24”x20” coroplast cost me $7.99.  That’s 4.6 cents/in² for the balsa and 1.7 cents/in² for the coroplast.  The coroplast is much cheaper when not just the base price is considered, but also including the price of finishing materials for the balsa.  I haven’t checked Amazon for coroplast prices.

Even though I really need to not build any more planes right now, I’m very tempted to build some comparison planes.  I’m thinking a Sarpolus Flip and a Netezband/Testors Ambush.  I have these on the shelf that I can use to compare weight and flight characteristics between balsa versions and coroplast versions.  And maybe a WingMaster to experiment with flaps.

Also, my normal method of hinging coroplast is the simple method of cutting off one side of a flute to leave one surface as the hinge.  I’ve also seen the tail surfaces cut so that the stab and elevator interconnect with a rod through both to make a hinge.  But I’m wondering how well a thread hinge would work…

Yes, balsa flies better.  But why not see how far we can take the cheaper and more available coroplast for Cox engines?  Maybe I should also take a look at a couple of the S.P.A.D. websites also.  Who knows what we can learn from them for, say, .35 sized planes?


The Reanimator Mark