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Old Jul 07, 2006, 09:00 PM
Registered User
Sydney, Australia
Joined Jun 2005
323 Posts
Discussion
Rigid Polyurethane Foam wing cores?

There's currently two major ways of building dlg wings today.
1. Bagged foam.
2. hollow moulded.

Bagged wings are much more robust than moulded wings, but the moulded wings are significantly more accurate.

My problem is that I'm a ordinary flyer and take way too many risks so my wings tend to be rather beat up. So I have no love for hollow moulded wings!


Question: What would the problem be with using moulds to create wing skins (with bagged wing type thickness) and then clamping the moulds together and filling the wing with a rigid polyurethane foam mix?

The numbers for the foams seem to be comparable to hiload-60: Around 35 Kg/m^3 density and around 450 kilopascals of strength.

Would this get something closer to the best of both worlds? Strength and weight of bagged wings, with the accuracy of moulded wings?

Obvious problems:

1. Mixing and pouring the foam is tricky. Seems that you've only got about 20 seconds to get it in.
2. Produces some fairly nasty fumes.
3. Not normally UV stable.
4. May get brittle over time? (I read mixed comments on this).

It's also got no tensile strength to speak of, but that shouldn't matter.

Comments?


(For example: http://www.lympro.com/lympol.htm )
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Last edited by moreil; Jul 08, 2006 at 01:45 AM.
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Old Jul 08, 2006, 12:02 AM
PWP!
ajroahkni's Avatar
USA, HI, Makawao
Joined Oct 2002
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I think it'd be great - where does the mould come from?
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Old Jul 08, 2006, 12:34 AM
H.A.L. 9000
checkenbach's Avatar
Benicia Ca. USA
Joined Jan 2001
730 Posts
I have used P.U. glue to repair hollow molded wings, slit or drill hole, shoot some water in the hole, shoot or pour glue in, tape up, presto, instant foam core!! I did this to my Risk which had wing saddle damage to the front of the wing, very thin balsa, even thinner glass. It came out very solid, did have to add some weight to balance wing. I figured if it didn't work, the plane was junk, lucky it worked.
I know you're talking about building rather than repairing a wing, the biggest problem is foam-up, next biggest one is repair afterwards. Weight may play a major factor as well. Chris
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Old Jul 08, 2006, 01:49 AM
Registered User
Sydney, Australia
Joined Jun 2005
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Quote:
Originally Posted by ajroahkni
I think it'd be great - where does the mould come from?
Make a wing mould as normal for a moulded wing. But rather than laying it up with glass/balsa/glass sandwitch, just lay it up with glass/kevlar.

Then before the epoxy is fully cured, trim and clamp the two moulds together. Then pour in PU foam mix from the root end. Swish it around and wait for it to foam up (and probably over, knowing me ) Put aside to set. Trim root and voila!
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Old Jul 10, 2006, 10:27 PM
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Australia, NSW, Sydney
Joined Feb 2003
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Moreil,
There's a similar discussion here: http://www.rcgroups.com/forums/showthread.php?p=3894124

Graham.
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Old Jul 10, 2006, 11:07 PM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
1,457 Posts
I used to do this with rally cars to stiffen up the chassis rails drill holes pour the foam in & cover any holes where the foam would come out so it was blocked in you could hear the metal creak as it expanded.

My concern is if you block up the holes it will expand so much it could distort the moulds !

You may be better off using high load 60 as the core in side of the bagged skins & gluing it in place with pu glue as the high load it a lot stiffer than just the pu glue


Rick
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Old Jul 10, 2006, 11:23 PM
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checkenbach's Avatar
Benicia Ca. USA
Joined Jan 2001
730 Posts
On my wing repair it added a considerable amount of weight as well. Chris
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Old Jul 11, 2006, 12:40 AM
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Australia, NSW, Sydney
Joined Feb 2003
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Quote:
Originally Posted by BMtech
I used to do this with rally cars to stiffen up the chassis rails drill holes pour the foam in & cover any holes where the foam would come out so it was blocked in you could hear the metal creak as it expanded....
Ha!!! I see we share a common past! Mine were Datsun 1600s in the 1980s.

I also once carefully poured polyurethane foam through a hole in the bow bulkhead of a dinghy I'd just made, to find 5 minutes later that the deck had separated from the hull by 15mm.

I think this is one of the main reasons that this technique would be impractical for a model wing. I also see issues with the relaxation that the foam undergoes after it's initial expansion.

Graham.
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Old Jul 11, 2006, 03:29 AM
Scorpian 60'' 2012
BMtech's Avatar
Kurrajong Australia
Joined Jul 2005
1,457 Posts
Quote:
Originally Posted by Salto
Ha!!! I see we share a common past! Mine were Datsun 1600s in the 1980s.
Graham.
Datsun,Escorts,Renaults & lots of cars I cant remember must be getting old ( Its all BM's now)

I will have to come over & see you in the next week or so as I'm on holidays to talk about some Hand launch bits


Rick
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Old Jul 11, 2006, 04:01 AM
Charlie
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Canada, BC, Victoria
Joined Dec 2004
419 Posts
I think the guys who make the MINIJ and PRODIJ mold the top surface(in a mold) with the wing core on top of it - bagged, and then bag the bottom surface on. The results are really nice, and they have a reputation for strength. i seem to have an idea that the top surface is more critical for accuracy.
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