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  #1  
Old 05-06-2007, 10:48 AM
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Default Foam Question: Fuselage

I have a question about the foam used for the fuselage sides,bottom & top.
Where the plans call out for Polyurethane Foam ...... could Polystyrene Large Cell be used instead? About the only reasons I can think of that urethane would be used is because it is fuel resistant and cheap.

Would substituting Polystyrene Large Cell cost me anything more than a lighter wallet?
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Old 05-06-2007, 07:47 PM
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Quote:
Originally Posted by TMann View Post
I have a question about the foam used for the fuselage sides,bottom & top.
Where the plans call out for Polyurethane Foam ...... could Polystyrene Large Cell be used instead? About the only reasons I can think of that urethane would be used is because it is fuel resistant and cheap.

Would substituting Polystyrene Large Cell cost me anything more than a lighter wallet?
I wouldn't... in fact I went the other way and substituted H45 for the last-a-foam.

Especially Large Cell - isn't that expanded rather than extruded?

Dow states that expanded polystyrene will, over time, absorb water from the atmosphere, unlike extruded. I wouldn't use expanded polystyrene anywhere on the plane. additionally, large cell polystyrene is weaker than extruded and far, far weaker than the last-a-foam (doublers) and H45 (outer sheet called out) physical properties.

even assuming extruded polystyrene - I wouldn't:

for 4.5 lb last a foam (FR-6700) (data from General Plastics):
Compressive Strength: ~90psi (perpendicular) / ~2850 psi stiffness
Shear Strength: ~85psi / ~1180 psi shear modulus
http://www.generalplastics.com/uploa...ackage2004.pdf?

For expanded polystyrene, compressive strength is about 16 psi at 1.6 pcf density. I couldn't find modulus or shear data - it's simply not intended as a engineered core material.

for Styrofoam Pipe Billet (extruded polystyrene) (data from Dow Chemical):
Compressive strength of 20 psi (1.6 pcf)
http://www.dow.com/PublishedLiteratu...romPage=GetDoc

So, very inferior substitute. The fuselage forms basically a box beam between the canard and main wing lifting surfaces and the major structural, payload and engine weights.

It needs it's strength. My opinion: Substiturion of polystyrene (expanded or extruded) would be critically unsafe and dangerous.
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Last edited by chasingmars : 05-06-2007 at 10:04 PM.
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Old 05-06-2007, 09:31 PM
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hmmmmmmmmmmmm.........Glad I asked!
Thanks Craig.
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Old 05-07-2007, 03:40 AM
chasingmars chasingmars is offline
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I keep forgetting that you're building a Long rather than a Cozy... does the Long call out the same thing for the fuselage sides as the Cozy? i.e. 3/8" H45 with 3/4" 4.5# last-a-foam spacers?
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Old 05-07-2007, 10:42 AM
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The sides of the fuse are in the tanks amd i believe the density is too low
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Old 05-07-2007, 12:09 PM
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Craig, it's suposed to be H45. I'm busting that box open tonight to check the condition of the foam etc.

Yeah, like you I had questions as to what was the higher quality foam to use for this area. I know the original plans were based around ease of build as well as controlling costs.....and if I were building back when I first bought my plans, I'd be very interested in saving as much money as possible.(I was poh back then .)
Now my perspective is a little different. I don't mind throwing in a few extra bucks to end up with a high-end product. The labor is the most expensive part of my build. I have the money..........it's the time I'm having trouble with .
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