# Foam of choice for sandwich airplane construction

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#### birdus

##### Well-Known Member
I'm just getting into composite construction (and having lots o' fun learning) and don't know what the foam of choice is for fuselage and wing panels. This is for my F4U (AKA "Carbon Corsair") project. There's blue, yellow, PVC, Divinycell, Last-a-foam, 3lb, 4lb, 5bl, etc., etc., etc. I'm a long way of from building the plane, but am doing research, planning, design, etc. In the near future, however, I want to do some sample panel layups for 1) experience, 2) to get an idea how many layers I want on the outside for resistance to screwdrivers, airshow dummies, bird strikes, etc., and 3) to get some numbers for initial weight estimates.

What foam should I buy? I'll probably get 1/4", 3/8", and 1/2" sheets for testing. Also, I'm going to be building the spar(s) according to Jim Marske's instructions. Will I use the same type of foam for the spar shear webs?

Thanks,
Jay

#### BoKu

##### Pundit
HBA Supporter
Divynicell H60 or its generic equivalent has been our go-to since we started. It's probably the single most common material and density in the industry. We use:

* 1/4" or 6mm for wing skins, wing spar shear webs, and vertical fin sandwich

* 3/16" for horizontal tail (though we might switch this part to 1/4")

* 1/8" for all control surfaces

#### birdus

##### Well-Known Member
Divynicell H60 or its generic equivalent has been our go-to since we started. It's probably the single most common material and density in the industry. We use:

* 1/4" or 6mm for wing skins, wing spar shear webs, and vertical fin sandwich

* 3/16" for horizontal tail (though we might switch this part to 1/4")

* 1/8" for all control surfaces
Great! So, I see that's the 4lb yellow foam. Interestingly, it looks like Aircraft Spruce sells only the 3lb and 6lb. However, they sell something called Last-a-foam in 4.5lb. Is that the same thing? Looks like it may be.

Thanks,
Jay

Log Member

#### BoKu

##### Pundit
HBA Supporter
...Last-a-foam in 4.5lb. Is that the same thing? Looks like it may be...
No, definitely not. Last-a-foam is some sort of urethane, and does not have the strength or toughness of PVC.

For many years, H45 was blue, H60 was green, H80 was pink, and H100 was yellow. However, it appears that DIAB has dialed back on the colors, and now it's pretty much all yellow.

You can get H60 directly from DIAB or through any of a variety of suppliers like Composites One.

Edit add: I don't know why ACS doesn't carry H60. It does seem kind of an odd omission.

--Bob K.

#### wsimpso1

##### Super Moderator
Staff member
Log Member
First, listen to Bob, he definitely has the background in successful sailplanes built of this sort of thing.

For graphite fiber facesheets, 1/4" PVC in 6 pcf is the way to go. For my glass fiber faced stuff, I used 3/8" PVC foam to get my bending stiffness of the whole sandwich up.

Billski

#### birdus

##### Well-Known Member
First, listen to Bob, he definitely has the background in successful sailplanes built of this sort of thing.

For graphite fiber facesheets, 1/4" PVC in 6 pcf is the way to go. For my glass fiber faced stuff, I used 3/8" PVC foam to get my bending stiffness of the whole sandwich up.

Billski
What's a facesheet? The outer surface I'm guessing. I thought the thickness would vary based on whether I'm doing a wing panel or the fuselage, not just on graphite vs. glass. Just trying to understand.

Jay

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Big topic. This is composite beam theory and then goes into plate and shell theory. See my stickies on beams and composite beams for basics. Want to calculate stuff, then you have to get out a book on mechanics of materials, look for beams and composite beams.

If you are applying axial load to a structure, Sum of EA is the axial stiffness, E being Young's Modulus and A being cross sectional area. For each material in the element, you have both terms, multiply E time A for each material in the assembly. A little of what we do uses EA. The amount of foam present has only a little to do with EA. For a rectangle, A is just w*h.

A lot of what we do needs enough bending stiffness, like skin panels with airloads across them, and sum of EI applies. E is Young's Modulus and I is second area moment of inertia about the neutral axis. Foam has one E and I, the composite facings spread apart by the thickness of the foam have another. The distance between facings matters a lot to EI. For a simple rectangle, I is b*h^3/12, where b is width and h is thickness. Now if you make the rectangles really thin, like graphite cloth laminate, h is tiny and I becomes really small, but if you laminate them on both sides of foam t thickness, I becomes b*h^3/12 plus b*h*(t/2)^2. It does not take much foam thickness to make something 10, 100, even 1000 times as stiff.

Last is torsional stiffness. There the figure of merit is GJ. G is torsional modulus, J is 2nd area moment about the centroid. Similar to EI, but pertinent to torsion.

Other thing I like to point out. These equations leap directly form math and theory and have been found to very accurately describe behavior (stresses, strengths, deflections, etc). No empirical stuff at this level of Mechanics of Materials.

My 3/8" foam and 18 oz glass BIAX/ 22 oz glass TRIAX skin laminates are stiff enough and strong enough for my 268 knot Vdive and the size of my panels between supports. The high end glider community has found that one ply of 6 oz graphite fiber cloth on each side of 1/4" foam is stiff enough and strong enough for everything they do. Figure out how to run Sum of EI for each of these sandwiches, and you might find them to be similar... I do know that 2 plies 6 oz graphite is way stiffer than my 3/8" glass sandwich.

Billski

#### birdus

##### Well-Known Member
More good info for me to mull over. Thanks!

#### BoKu

##### Pundit
HBA Supporter
...The high end glider community has found that one ply of 6 oz graphite fiber cloth on each side of 1/4" foam is stiff enough and strong enough for everything they do...
It works for the wings and stabilizer for our 15/18m gliders, and judging by the repair manuals I've seen it's not an uncommon lamination schedule for other glider wings of our general size and shape.

However, I'd qualify that a bit, noting that we have generous reinforcements around all the edges and at the inboard foot of span (where it's basically two-core-two plus four 2" uni tapes over the root rib). But judging by their wing panel weights I'm pretty sure that a lot of sailplanes use two-core-one, or perhaps 9oz or 12oz biax in the outer ply. My suspicion is that it is at least partly due to increasing risk aversion on the part of EASA. But the one-core-one is working out just fine for us so far.

One of the reasons we get away with so little is that we have a high aspect ratio and consequently low span loading. Both of which mean that no particular piece of wing is very far from the wing spar, so it doesn't take much structural meat to transfer lift loads into the spar.

Also, our fuselage is, except for the vertical tail, an uncored shell. For the most part, it's 36oz worth of carbon, which is too much in a lot of places, but the extra weight is trivial. The lamination schedule is primarily optimized for simplicity and ease of layup by minimally-trained crews under relatively primitive conditions. Getting such a big, awkwardly-shaped part laid up, bagged, and vacuumed down is hard enough even when the layup is relatively homogenous.

--Bob K.

##### Well-Known Member

Soric and related types:
Only suitable for tiny panels (low buckling resistance). Potentially lightest, cheapest and simplest. Temperature limitation is almost entirely resin-dependent. Core is about 10 US$/m2. Divinycell H60: The most common one in GA. Affordable, proven, bit heavy on the resin intake, adding substantial weight. Low temp resistance, so white, very light colors, or emigration to Norway. About 20 US$/m2 for typical cores.

Rohacell IG-F:
High-end stuff. Best mechanical properties for lowest weight. Much higher temperature allowance, about 120C, so paint it black. Lowest resin intake. About 80 US\$/m2 for typical cores.

#### BoKu

##### Pundit
HBA Supporter
...Divinycell H60:
The most common one in GA. Affordable, proven, bit heavy on the resin intake, adding substantial weight. Low temp resistance...
That contradicts my own experience, especially with regards temperature resistance. I've gotten parts nearly smoking hot with no apparent deterioration or change to the core.

My suspicion is that what DIAB is now selling as H60 is actually the same formulation as the higher temp rated H60T. It could be that reducing the number of formulations has greater overall utility than maintaining a range of potentially lower-cost formulae.

--Bob K.

##### Well-Known Member
My suspicion is that what DIAB is now selling as H60 is actually the same formulation as the higher temp rated H60T. It could be that reducing the number of formulations has greater overall utility than maintaining a range of potentially lower-cost formulae.

--Bob K.
That would make sense from my own experience. When heat treating the 6mm H100 sheets for my wing ribs they hardly shrunk at all, I had the temperature up to 100 Celsius with maybe 2-3mm change over a meter length. I was expecting to get at least 12mm.

Shannon.

#### KeithO

##### Well-Known Member
BoKu, Composites One do not offer online pricing or ordering. Do you know what they charge per sheet of 1/4" PVC foam and are the sheets 4x8' ?

So far in my research, it seems that http://www.carbon-core.com/store/product/pvc-structural-foam-sheets/ has the best pricing on crosslinked PVC foam.

Keith

You can get H60 directly from DIAB or through any of a variety of suppliers like Composites One.
--Bob K.

#### RSD

##### Well-Known Member
Resurrecting an old thread - are there any new PVC foams on the market? Unfortunately Divinycell is hideously expensive down here in Australia and availability is quite limited.

#### Vigilant1

##### Well-Known Member
Resurrecting an old thread - are there any new PVC foams on the market? Unfortunately Divinycell is hideously expensive down here in Australia and availability is quite limited.
I don't know of any new PVCs.

Not PVC foam, but just to mention it: There's been some discussion here of alternatives to PVC foam for core material, and one thing that as been mentioned is XPS foam that has very high compressive and flexural strength. This isn't your typical pink/blue foam (typical compressive strength 15-25 PSI) that is found in the home center, but it is up to 100 PSI and used in construction (under roads), in commercial freezer floors, etc. One tradename is Dow Highload, it comes in PSI ratings of 40, 60, and 100 PSI ("Highload 100" etc). It is sold in 2" thick sheets of 4' x 8', you could use a hot wire to make sheets as thick as you want. Another brand is Owens Corning "Foamular 1000" etc. Comments:
1) It's still polystyrene. It won't like fuel at all.
2) It will probably be hard to find, but you won't need much of it.
3) The PSI rating is defined as the load at which the foam has compressed 5% or yields, whichever is first.
4) If you search this forum fro "Highload", you'll find other discussions. I don't know if anyone has built with it yet.
5) Spec sheets:
http://commercial.owenscorning.com/assets/0/321/333/bc10573a-e100-4009-a0ce-cdc6e76a6003.pdf

Last edited:

#### RSD

##### Well-Known Member
I don't know of any new PVCs.

Not PVC foam, but just to mention it: There's been some discussion here of alternatives to PVC foam for core material, and one thing that as been mentioned is XPS foam that has very high compressive and flexural strength. This isn't your typical pink/blue foam (typical compressive strength 15-25 PSI) that is found in the home center, but it is up to 100 PSI and used in construction (under roads), in commercial freezer floors, etc. One tradename is Dow Highload, it comes in PSI ratings of 40, 60, and 100 PSI ("Highload 100" etc). It is sold in 2" thick sheets of 4' x 8', you could use a hot wire to make sheets as thick as you want. Another brand is Owens Corning "Foamular 1000" etc. Comments:
1) It's still polystyrene. It won't like fuel at all.
2) It will probably be hard to find, but you won't need much of it.
3) The PSI rating is defined as the load at which the foam has compressed 5% or yields, whichever is first.
4) If you search this forum fro "Highload", you'll find other discussions. I don't know if anyone has built with it yet.
5) Spec sheets: