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New Fiberglass-Faced Foam Boards and Aircraft Use

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jonnycowboy

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Hi all,
I've been researching building a Wainfan Facetmobile copy - NASA PAV version with the flat foam/honeycomb panels. In order to reduce costs, I have been looking at facing blue DOW foam with fiberglass (hand-laid).

However the research I've done identified two new products that could be interesting for use, namely construction-grade fiberglass-faced 1/2 foam panels. The two I've found so far are:
  • IKO IKOTherm Covershield: 1/2" poly-iso foam faced on both sides with fiberglass, 90 psi compressive strength.
  • Firestone ISOGARD HD: 1/2" poly-iso foam faced on only one side with fiberglass, 120 psi compressive strength.
Both of these boards come in 4'x8' sheets and weight around 11 lbs each. Cost is around 15-20$/sheet.

Links:

As I said I want to build a Facetmobile with foam-core or honeycomb construction as detailed in Wainfan's PAV report. I think using these panels, doubled-up (epoxy them together as a 1" nominal thickness) as the structural walls, along with a single-layer outer skin (top & bottom) could be a lot cheaper than using the aircraft grade sheets of honeycomb found at TEKLAM (~600-1000$/sheet).

Poly-iso foam in particular cannot be hot-wired, which is a disadvantage but it can be routed (which I would do with a CNC panel router for this project).

Any comments on these boards? Good idea or bad? I'd use a build-up at the intersecting joints with foam and fiberglass in a L-flange shape.

thanks
Jonathan
 

Hot Wings

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Any comments on these boards? Good idea or bad?
I'm kind of surprised this hasn't been used more in homebuilt aircraft. It is used in the fiberglass industry around here to build all kinds of structures and has proven to be very durable. The skin can be removed from one side very easily (peel failure is cohesive, not adhesive) letting the board be curved and then glassed to hold the shape. The skin cuts cleanly with a utility knife. It also carves very nicely if you need compound curves.

I have a shop door made with this type of insulation and covered with a thin layer of chopper gun polyester glass and it shows no sign of delamination after nearly 20 years of hard service. I also have made a set of ribs for a U-2 using this and they were as strong as the wood ribs, significantly stiffer, and weighed less.

It just might be a very good material to use for a Facetmobile type project. If I ever build another Quickie fuselage I'll probably use this type of foam.
 

WonderousMountain

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"Neutral" fibers is a polite way of saying random oriented strand. Lose 1/2? of strength compared to double bias.

Not being able to hot wire is not a problem, fuel degradation can be serious problem, do a search if you're not aware.

Wainfain's structure is very resilient whereas many other planes have less depth, more tenuous load paths.

There doesn't seam to be an accepted prefabricated panel of moderate price popular in homebuilding. Perhaps simply because glassing a flat foam panel is not all that labor intensive compared to most build time requirements.

LuPi
 

Apollo

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Not much data on the strengh/stiffness of these materials. How thick are the facesheets? What is the peel strength? You're gonna have to do a fair amount of testing to quantify the material properties of these boards. Compressive strength of the foam is not enough.

I once did some calculations to reverse-engineer the weight assumptions used in the PAV report and came up with 1/4" thick core with .015" thick facesheets. Skins of that thickness are pretty easily damaged by a variety of things (like airshows). Don't write on a piece of paper on that surface, your pen or pencil will poke right through.

The Teklam boards I was looking at were about 0.50 lbs per square foot. At 11 lbs per 4 X 8 ft sheet, these new boards are about 0.35 lbs per square foot. Certainly light enough, but do they have adequate material properties (and durability) for aircraft applications? Take a look at the Product Data Sheet for Teklam panels to see the kind of information an engineer might want before designing an aircraft with these panels. I'm already concerned about the facesheet peel strength based on Hot Wings' comment.

These panels look interesting but more data and some testing is required before their suitability for aircraft can be determined.
 

Hot Wings

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I'm already concerned about the facesheet peel strength based on Hot Wings' comment.
I've not done any formal tests but just judging by feel the peel strength of the raw board I've used is greater than the urethane foam in my Quickie fuselage,* and it seems to get noticeably better with a second layer of epoxy resin and cloth. Peel strength may be less than with PVC or blue foam: but then shouldn't we be designing parts so peel strength isn't a real factor?

I also didn't look at the numbers for the specific board referenced but the foam I have used in the past has comparable compressive strength. The face sheet is a single layer of what looks like chopper gun glass, but with significantly longer strands, and is held with something more like a binder than a resin coat. The U-2 ribs I built had an extra coating of epoxy resin applied (no additional cloth) that noticeably toughened the surface with very little weight gain.

Product specific testing would be required for a structural part. Based on my informal testing/use I'd make a small wager that it will prove to be quite adequate for a lot of homebuilt applications. Unfortunately this foam is not normally stocked by most building supply stores and needs to be ordered.

*and less than the tapes that held together a Q-2 I dismantled. Shiny glass under the secondary "bonds" :speechles
 
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autoreply

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Using unproven materials (even if they're in wide use in boats or so) is a good way to Darwinate oneself.

Divinycell is the standard. A 32"x48" panel is 30 US$. If you can't afford that, building an airplane is probably prohibitively expensive.

Have a good look at carbon. Strength of a foam-cored structure is ruled by stiffness, not strength. While glass is not thát far away from carbon in strength, carbon is way cheaper, the dollar/stiffness ratio isn't much worse than glass and it'll be a lot lighter.
 

FritzW

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...Both of these boards come in 4'x8' sheets and weight around 11 lbs each. Cost is around 15-20$/sheet.
At that price it would be worth getting a few sheets to play with. Where can a guy buy a few sheets and not have to buy a whole pallet load?
 

Hot Wings

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A 32"x48" panel is 30 US$. If you can't afford that, building an airplane is probably prohibitively expensive.
It's not the cost that makes this material attractive. In spite of my comment that it generally has to be ordered it is an item that is stocked by some building supply companies and getting a small order shipped in with another load often makes shipping free if one is willing to wait. For many of us the shipping costs are often as high or higher than the material being shipped.

The skin makes it very easy to handle and resists damage, unlike conventional aircraft use foams.

It carves/sands as easily as urethane foams. Thinner sections can be made by cutting a sheet in half with a string bow saw and some guide boards. Both skins can be removed leaving a clean surface ready for the resin/cloth system of choice.

I have yet to find a resin system that can't be used with it so even locally obtained polyester can be used to make non structural parts/forms.

There are some negatives associates with these isocyanate foams. When burned or heated they release some very noxious fumes, but then other foams do as well. Some people are allergic to isocyanates. I'm one. I can't even open a can of some paint anymore with out using a respirator and need a GOOD supplied air system to spray it. So far the foam, or sanding dust, hasn't bothered me.
 

jonnycowboy

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Using unproven materials (even if they're in wide use in boats or so) is a good way to Darwinate oneself.

Divinycell is the standard. A 32"x48" panel is 30 US$. If you can't afford that, building an airplane is probably prohibitively expensive.

Have a good look at carbon. Strength of a foam-cored structure is ruled by stiffness, not strength. While glass is not thát far away from carbon in strength, carbon is way cheaper, the dollar/stiffness ratio isn't much worse than glass and it'll be a lot lighter.
Divinycell is around 150$ for an equipvalent sized piece (4x8, 1/2" thick), so about 7 times more expensive...

Carbon is great and all but really, you don't need carbon for this utilisation and it's quite a bit more expensive as well.
 

jonnycowboy

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At that price it would be worth getting a few sheets to play with. Where can a guy buy a few sheets and not have to buy a whole pallet load?
You can go to any good roofing supply store and get one sheet or a pallet. That said, it's easier to get them to deal with you if you're a contractor. (You must know one in your circle of friends)
 

fredoyster

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I would be concerned about bonding to the face ply, given who knows what mold release in their process. That would be an important qualification to run before knowing what could be done with it. A toughened (polysulfide+apoxy) adhesive might be the answer.
 

Hot Wings

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I would be concerned about bonding to the face ply, given who knows what mold release in their process. That would be an important qualification to run before knowing what could be done with it. A toughened (polysulfide+apoxy) adhesive might be the answer.
Without knowing the exact manufacturing process one may want to test a sample from each batch, "just to make sure". I've never seen a bond problem with any resin system with my own projects or heard of any problems from anyone else I know that has used this material for non aviation uses. As far as I've been able to tell, just from use - no testing, the limiting factor is the shear strength of the foam which is about the same as the urethane foams, and maybe a bit better.

If you are still worried about the bond to the factory face ply, just remove it and laminate directly on the foam. You will still need to do some testing to establish numbers to use but you have one less variable to track.
 

jonnycowboy

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Without knowing the exact manufacturing process one may want to test a sample from each batch, "just to make sure". I've never seen a bond problem with any resin system with my own projects or heard of any problems from anyone else I know that has used this material for non aviation uses. As far as I've been able to tell, just from use - no testing, the limiting factor is the shear strength of the foam which is about the same as the urethane foams, and maybe a bit better.

If you are still worried about the bond to the factory face ply, just remove it and laminate directly on the foam. You will still need to do some testing to establish numbers to use but you have one less variable to track.
Yes I plan to do some tests this winter, in single-ply, double-ply, and then both configurations with an additional epoxy coat like you say (Hot Wings) to improve the bonding, and see what the stiffness numbers are like. I'll put an 8-foot section between sawhorses and measure the deflection with some standard weights. After measuring I'll try to go to destruction and see what the ultimate load and, critically, what the failure mode is (peeling/shearing or a more bening mode).

Thanks
 

autoreply

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Divinycell is around 150$ for an equipvalent sized piece (4x8, 1/2" thick), so about 7 times more expensive...
AS is considerably below that.
Carbon is great and all but really, you don't need carbon for this utilisation and it's quite a bit more expensive as well.
It's way stiffer than glass, notably unwoven glass. If we look at dollar per stiffness (which defines panel strength), they're much closer.



As for testing, how does one test for aerospace use? Some issues (like PU being vibration-sensitive) don't show up until years have passed.
 

jonnycowboy

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AS is considerably below that.

It's way stiffer than glass, notably unwoven glass. If we look at dollar per stiffness (which defines panel strength), they're much closer.



As for testing, how does one test for aerospace use? Some issues (like PU being vibration-sensitive) don't show up until years have passed.
Could you send me the AS link?

If you were to use 100% carbon, yes it would be stronger, but for an equivalent strenght/weight/cost, your carbon panel would end up so thin (think <1/8" that it would likely fail in buckling before the foam panel would.

As for long-term effects, you mitigate as best you can, by defining routine inspections (eg every 50FH or similar) for evidence of upcoming panel failure. This is the same as they do in real (commercial or military) aerospace use for critical aircraft structures.
 
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