I was wondering what the advantages and disadvantages of balsa vs very thin plywood wing sheeting were? I am working on the design of a scale model and was wondering if I should maybe use the thin plywood instead of balsa. Does any one have density figures on the plywood so I can compare to the balsa? The real plane is all aluminum so I want to sheet everything to give it a smooth surface like the real thing.
-- Chris W
"They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania
The very thin (1/64") plywood is much harder and stronger than 1/16" balsa. It's also much more expensive. If you don't need the extra strength of the plywood, you might use balsa covered with tissue to make it smooth.
Since you can easily attain a slick and smooth finish for painting with balsa, I'd stay with that. Balsa is easier to work with and (I think) is lighter than thin ply even after you use the Dan Parsons type cloth to glass it (if you do it correctly). which will give you the paintable surface you're looking for.
A number of reasons to select balsa sheeting vice thin ply . . .
Sheet balsa is less expensive than thin plywood and more readily obtained.
Balsa is easier to permanently bend around a curve without splitting than thin plywood. Make the curve permanent by spraying the balsa sheet with a 30-70 mix of household ammonia and water; you can use ammonia right out of the bottle if you can deal with the fumes - I can't.
The only way I know to permanently bend plywood is steam; by that I mean the un-glued curved skin could be removed from the structure without losing it's shape.
It is easier to bond the balsa wing skin to the balsa structure than to bond plywood to the balsa structure, although the latter isn't all that difficult.
There are two easy ways to get a 'glass smooth' finish on balsa (or plywood, for that matter).
One is to apply a coat of thinned epoxy, let it cure, sand lightly, and apply a second coat.
If you thin the epoxy 50% with denatured alcohol (not isopropyl from the drugstore) the cure time will be extended four-fold or more, so be patient.
You can spray it, brush it, or just pour it on. Spraying is messy due to over spray, but you don't need to do anything to remove the excess. Brushing or pouring it on requires sopping up the excess with a roll of paper towels.
An unseen advantage is that the thinned epoxy penetrates the balsa up to half the thickness of the wood, making it _very much_ harder and resistant to hangar rash. Being a good bit thicker than wing skin-type plywood it is much more puncture resistant with the thinned epoxy coatings.
The other easy way to a decidedly smooth finish is to make wing skins with epoxy and fiberglass, and bond the fiberglass skins to the balsa sheet.
How-to at
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under 'tech tips'. Cheers, Fred McClellan the dash plumber at mindspring dot com
No density figures, however I have used 1/64 ply on several airplanes with foam wings. One was an older pattern plane and then some scale models.
The ply is heavier and therefore I would not recommend it on less than 600 sq. in. wing for a scale model. The larger wing areas handle added weight per 100 sq. inches much better than smaller models.
You can obtain an excellent finish base over ply with one thin coat of polyester or epoxy finishing-resin and a couple coats of primer. I find it much less time-consuming than balsa, even with a glass cloth and resin base. The ply will not ding as easily as glass-coated balsa. There will be no seam lines to worry about, like balsa sheets. If you don't mind a bit of waste, you can even fold the ply around a leading edge. Ply is by far my choice over foam. More expensive, and a little heavier, but very strong and takes a finish much easier.
I would NOT try to sheet a built-up wing with 1/64 ply unless you use a lot more ribs than I care to cut out. (;-))<
Does anyone remember the honeycomb/fiberglass coated wings that were available in the early seventies? The company's advertisement had a picture of the wing being supported at the wingtips with a man standing in the middle of the wing. They were expensive and heavy, but I really wanted one of those for my Kaos.
In exchange for an ultralight (Weedhopper) I did a scratch 1/4 scale Cessna 140 for a fellow a few years back and used 1/64" ply over blue foam cores that I cut. Like you I was aiming at a good base for the metallic finish which was on his full scale. I vacuum bagged the ply onto the cores using Pacer Zpoxy after pie cutting the tips to fit where they taper. I don't think they were any heavier after finishing than balsa, the ply was much easier to fill and sand, and I don't think that the surface soaked up as much paint/sealer. There is a picture of it in the front of the January 1999 ussue of M.A.N. The fuse is skinned with 1/32 ply and the control surfaces are corregated litho plate. The plane flew very well and I wished that I had kept it for myself altho I had a lot of fun in the ultralight, till my wife put the kabosh on it after a few of the locals had severe accidents.
Finishing issues aside, you might want to watch the fact that all plywood is multiple plies at 90 degrees to each other, and the ones in the direction 90 degrees to the wingspan do bugger all for bending strength of the wing.
1/64" is two plies sanded to thickness, and if the one in the "right" direction ends up thinner than the other in the sanding process, you may have a whole lot less strength than you were hoping for. Balsa has excellent tensile strength in the spanwise direction and will I believe make a stronger wing for equivalent weight buildup. Cover it with lightweight finishing cloth applied with the right amount of resin (versus too much) and I think you will have a good surface with the structural benefits. An alternative, less popular here than in Europe, but that gives you the harder surface of plywood and unidirectional grain is obechi.
Balsa is the wood to beat when considering the strength to weight ratio. Due to it's light weight, you need more thickness, but that is an advantage, because it provides more material to sand out building anomalities or get that perfect final shape. It glues well, can be shaped well with moisture and heat, and is quite resilient. With good reason it is used in space- and aircraft floors design. Plywood (beech and birch) is only beneficial, when multi-directional strength or hardness is required, and even then, it is not without drawbacks. It is bad on a strength to weight ratio. The worst imho is lite ply. It is perfect tho for unstressed milled formers, that is why kitters love it. Obechi is brittle, but can be used well for landing gear blocks where a bit more hardness is required. However, hard balsa will do just as well or better, if you can get it in straight pieces. For smaller pieces, cross laminating hard balsa will provide the thickness you need and straighten the warps often encountered in the very hard qualities. (at 45 degree grain angles)
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