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Old Feb 21, 2013, 10:36 PM
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Joined May 2008
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Need Help Designing an EPP Wing

I want to design a two meter wing for a scratch built RC airplane. I want to make it out of expanded polypropylene. So I calculated the weight of a 200 cm wingspan, 20 cm wing chord NACA 2412 airfoil made of solid EPP wing (not including any internal structure) with no wing taper and calculated a weight of 8.6 kg (18.9 lbs). I never realized EPP was this heavy.

Has anyone else designed and built wings out of EPP? Is there any way to get the weight down?

Thanks for the help.
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Old Feb 22, 2013, 12:19 AM
Do it Right, the first time!
CoolerByTheLake's Avatar
United States, MN, Hermantown
Joined Dec 2008
5,033 Posts
I think you maybe looking at the wrong foam for a wing. I weighed a piece of Extruded Polystyrene (owens corning pink foam) 2.25" x2.0' x 21.5" = 97.88 cuin@ 1.385oz Figuring 1" thick airfoil, your 632cu/sq in, should weigh 6.55oz.
BTW, lots of people have/do use this for there wings.

You are correct on the weight, darn heavy!

wiki.answers.com

The density of polypropylene depends on whether it is crystalline or amorphous. I'll give both numbers:
Amorphous: 0.85 g/cm3
Crystalline: 0.95 g/cm3
1 cm3 = 3.53147 x 10-5 cubic feet
or
1 foot3 = 28316.847 cm3
So 1 ft3 of polypropylene weighs 24,069.32 g and 26,901.00 g for amorphous and crystalline respectively.
That's also equal to 24.069 kg or 26.901 kg
1 kg = 2.204622 pounds
So 1 ft3 of amorphous polypropylene weighs: 53.064 lbs
and 1 ft3 of crystalline polypropylene weighs: 59.307 lbs

From this site: http://www.builderswebsource.com/_discBT/00000d75.htm
Insulation ...
...Extruded Polystyrene 1.8 pcf
...Expanded Polystyrene (EPS) 1.0 - 4.0 pcf, depending on density (0.9 to 2.19 pcf typ range)
...Polyurethane 1.5 pcf

Weight per Unit
pcf = pounds per cubic foot
pcy = pounds per cubic yard
plf = pounds per lineal foot
psf = pounds per square foot
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Last edited by CoolerByTheLake; Feb 22, 2013 at 12:36 AM.
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Old Feb 22, 2013, 09:57 AM
An itch?. Scratch build.
eflightray's Avatar
South Wales U.K.
Joined Mar 2003
12,900 Posts
Just had a quick look in the Foamies(Scratchbuilt) forum for any big EPP models, found a C-130, (long narrow 1968mm span wing) in EPP.

C-130 Hercules EPP

Never tried EPP, my preference is for balsa or Depron foam.
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Old Feb 22, 2013, 10:40 AM
treefinder
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Joined Oct 2004
9,467 Posts
You can get different densities of EPP, but generally as the density goes down, the strength/stiffness goes down faster. It's good for smaller planes and the 3D profiles, but for a general use plane I'd go with Ray and Cooler and stay with a styrene based foam.

Are you intending to build a flying wing, or a wing for more conventional plane? If a wing, then there are a half dozen designs in the scratchbuilt foamies forum already designed, validated and built by dozens of folk. you can typically download the PDF files of the designs and build from that. Or, just browse the threads and get ideas for your own!

Which foam you can get will vary with where you are located. It's good to add that to your profile, as answers will vary, depending on where the answerer is located versus where you are. You will also get folk in your country who will give most appropriate answers about availability.
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Old Feb 22, 2013, 02:26 PM
I think I'm inverted. Maybe.
acetech09's Avatar
United States, CA, Pacifica
Joined Apr 2012
1,505 Posts
+1 to all of what above has been mentioned... I think you'll find more help in the Scratchbuilt Foamies subforum, it's the go-to place for foamy building.
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Old Feb 22, 2013, 03:54 PM
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Joined Jan 2009
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EPP only works with stiffening. But when it's done right, it's phenomenal. A 46" EPP foamie 3D plane -- airframe only -- is probably well under a pound.

Stiffening can come from CF rods or strips arranged in I-beam fashion from root to tip (of a wing.) Or you can build hollow structures using thin EPP as the skin, and some other material (or thick EPP) for the ribs and possibly spars. Putting a curve into the skin (as in a wing) contributes to the stiffening.

Solid EPP, without any internal or external support, isn't going to get you very far.
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Old Feb 22, 2013, 07:16 PM
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The 'Wack, BC, Canada
Joined Oct 2002
11,246 Posts
You're looking at the density of the polypropylene itself. Not the density of the foamed version. That's why its up over 50 pcf.

The EXPANDED PolyPropylene foam it a lot less depending on the density of the foaming process. One maker lists the product as ranging from 1.3 to 1.9 pcf.

Either that or you badly slipped a decimal point in your wing volume calculations.
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Old Feb 23, 2013, 08:29 PM
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Joined May 2008
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A lot of good information here, but I think BMatthews has identified the problem. I was calculating the weight based on the amorphous Polypropelene density, which is nearly a gram per cubic cm, and because my wing was estimated to have a volume of 10,000 cubic centimeters (50 cm airfoil area x 200 cm wingspan), I was getting an enormous weight. Looks like a need to find good density numbers for EPP.

Thanks for the help.
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Old Feb 23, 2013, 08:38 PM
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Here's a thread on RCG discussing EPP density. Hobby stuff is anywhere from 0.9 to 1.9 lbs. per cubic foot.

http://www.rcgroups.com/forums/showthread.php?t=8080

And here's a page of stats for various types and thicknesses of foam sheets

http://www.rcfoam.com/stpg.php?page_...65a7d29b96d045
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Old Feb 24, 2013, 10:03 AM
Proud to eat Kraut ;-)
Julez's Avatar
Germany
Joined Dec 2003
5,214 Posts
Take this design, scale it up a little:

http://www.extremflug.de/seite100.htm

2.5° wing twist works well.

Google PW106 Airfoil for maximum lift, works well with the rough surface.

If you cover the surface and want more speed, the PW75 is great.
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