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Old Jun 25, 2012, 04:37 PM
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Foam versus balsa?

I've wanted to build a foamie for some backyard flying for a while. Something in the 20" wingspan range. I picked up some Dollar store foam board. Today, after cutting out the wing, I weighed the foam and a piece of 1/16" balsa with the same area. I was surprised to see the foam weighed more than twice what the balsa weighed. Probably close to the weight of 3/32" balsa.

So, what kind of wing loading figures do you guys get on the smaller foamies? Building a 20" wingspan foamie sounds like you would be starting out with an almost certain failure. Is there something I am missing?
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Old Jun 25, 2012, 06:38 PM
treefinder
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All depends on what you want it to look like, a single surface plane from 1/16" balsa will look like the Guillows dime store planes. When you start adding thickness, glue, some type of covering, etc, etc, it comes out a lot closer. Click on my username and Check out the OSR, OSP, OSA for foamies down in the 20"s range. Each from a half sheet of DS foam (20x15") and they all fly fine. Plus, don't forget when you crash your balsa plane you get lots of little sticks, when you crash your foamie, you get big easily repairable chunks.
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Old Jun 25, 2012, 08:20 PM
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Didya take the paper off?
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Old Jun 25, 2012, 09:57 PM
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Like Vtidy says. Dollar store foam typically comes with paper backing. Some people call them foam board. The paper is around half the total weight of the foam.

The foam I get around here weighs nowhere near as heavy as balsa. Then again nobody sells contest grade balsa here. Some of the balsa at my local arts & craft store weigh more than plywood.
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Old Jun 25, 2012, 10:19 PM
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Excellent craftsmanship on your planes, Springer. The ones I am interested in, such as the Taylorcraft, Aeronca, Decathlon and so on are mostly simple curves, except the cowling. I tend to avoid compound curves. I have a plan of a rubber powered Aeronca that looks promising.

Didn't take the paper off, Vtdiy.

I have some contest balsa and hate to work with that pithy wood, Slebetman. Also, it's just too fragile to fly outdoors. I do like hard balsa for spars, though.

I'm going to give it some more thought.
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Old Jun 25, 2012, 10:29 PM
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1.) Try weighing again with the paper peeled off.

2.) Glue the paper to a balsa sample and weigh.

3.) A 20" plane is small. If you like it white, Dollar Tree foam needs no additional finish.

4.) Also If you like white, fill a balsa sample with filler, sand smooth, and add a finish coat of white. Re-weigh.

5.) If you like color, repeat 3 and 4 above, with an additional coat of color on.

(No prejudices here for or against balsa or foam. I use both for different things)
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Old Jun 25, 2012, 10:53 PM
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Great point about the paper V! I forgot to even mention it.

GW: In addition to the weight, the paper on DT foam causes a kink in the foam when hit (i.e. in a crash) that makes it harder to fix, and the paper will eventually start to delaminate anyway, faster if it ever gets wet. That's why most of us take it off before building. It's only heat bonded on, so easy to get off. simply get the board wet (like in the shower) and let it dry. Should peel off in one piece. Some boards will peel clean without wetting, so check first.
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Old Jun 25, 2012, 11:30 PM
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Heh, then I turn around and glue paper back on.....

Can't seem to make up my mind.

http://www.rcgroups.com/forums/showt...1171221&page=7
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Old Jun 26, 2012, 12:31 AM
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A balsa build will be far more stiff/rigid than a foam buld of equal weight.
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Old Jun 26, 2012, 05:23 AM
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....and a carbon composite build would be even stiffer.....but more complicated/time comsuming to make. On a given impact the carbon model might literally shatter, the balsa break and split in several places and the foam would bounce, if the right type of foam was used, otherwise it would crack in large chunks as mentioned.

The way I see it, it's not about one of the materials being "the right thing to use for everything" - it's about choosing the right material for the job in case. Foam is by no means as rigid as balsa, but in many cases it cuts, bends and shapes an awful lot easier, making it useful for fast, cheap and simple builds, making/covering large surfaces and similar.
Usually carbon, glass, balsa, ply, metal etc. is added to "foam models" to help where the foam doesn't have the characteristics to do the job without adding too much weight, like wing spars, tail stiffeners, fuse side stiffeners, landing gear, motor mount etc.
Off course, each constructor/builder will have preferences, dependant on model, personal flying style and prior experience, which naturally will influence the plans they make.

With regard to weight it's important to remember that foam comes in many varieties with different stiffness, brittleness and not the least density - same as wood ranging from competition grade balsa to Azobé (Red ironwood) with all kinds of variety in characteristics in between.

For instance Depron (much used over here, originally thought for same kind of wall isolation use as FFF - we don't have FFF, Dollar tree etc.) comes in several densities, Aero depron is a special product that is lighter in the middle with the same density at the surface, compared to the stuff thought for the construction industry, for both types 3mm has higher density than 6mm etc. etc. ; Other types of foam we have over here (typically stuff for laying under wooden floors for isolation/ noise damping) can be both lighter or heavier, are often softer and not as rigid as depron, but where depron will crack on impact, these softer versions are more likely to bounce and/or buckle in stead......and then there's EPP........and so on.....

I assume the same thing goes for the different types of plate foam you've got over there.
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Old Jun 26, 2012, 10:35 AM
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Quote:
Originally Posted by djacobox372 View Post
A balsa build will be far more stiff/rigid than a foam buld of equal weight.
Well of course, everyone knows you can't build light stiff foamies. It's like trying to make Jello fly.

Uhhhhhhhh...no. Those lightweight stiff balsa planes probably depend on flexible covering materials -- tissue, silk, monokote, etc, over an open structure to kinda keep that weight within reasonable limits, balsa itself being used only for supporting skeletons.

Sheeted balsa planes don't seem very light, as models go.

As I said, no prejudices here one way or another, I use balsa and foam, both, for different purposes. It ain't a war.
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Old Jun 27, 2012, 09:04 AM
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Thanks for the replies to all of you. The biggest problem I see is the wing. I'm am going to check out the wing core guys and see if someone cuts small cores. Stepping up in size, say maybe 30", may also be an answer. I did try some Titebond professional glue and it adhered well to foam. Dries clear also.

I also puled the paper off of a small piece of foam, and was surprised to see the thickness of the paper. No doubt it adds some weight.
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Old Jun 27, 2012, 09:19 AM
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A 20" wing can be single skin, either bent cambered, flat, kfm, or hand sanded to a foil shape. Doesn't take much sanding because its so small. Or if you wanted a thicker shaped foil, laminate two layers with Elmer's rubber cement and sand back to foil shape. While it can be done with a wire cutter, this is awfully small to go to that extent when a wing could be hand sanded to shape.
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Old Jun 27, 2012, 10:52 AM
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Anyone mention the expense of Balsa ... I know , I’ve depleted a good deal of the Ecuadorian rain forrest and my life savings in the past , buying Balsa .... Now with foam , I haven’t bought a stick of balsa in over 10 years ... Don’t miss it at all !
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Old Jun 27, 2012, 11:15 AM
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In fact, the companies who build wings for wind generators have damn near vaccumed the market for balsa......I'll bet there's more balsa in one set of those "flappers" for a 3MW generator than all US model builders use in a year.....

GWilliams20: Maybe you would like to take a peek in the BluBaby thread : http://www.rcgroups.com/forums/showthread.php?t=681556

First time Blu Baby 24" RC Airplane (1 min 23 sec)
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