# Not-so-solid massive core wings: Lightening the core foam

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#### stanislavz

##### Well-Known Member
HBA Supporter
Some live photos. I do really like laminating between two foils approach, for thin skin it saves you a lot of epoxy - which would be lost in peelply and flow mesh.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Carve out depressions on the underside of a solid foam wing, and have the fiberglass skin conform to those depressions instead of enclosing them. The solid foam fore and aft is the "spars" and you can generally get away with a lot of shenanigans on the underside of a wing.

Curve the edges smoothly and those sections are like a regular wing with a lot of under-camber.
Biggest problem is that it will roughly halve the depth that you can put a spar in. Halving the depth of a spar means it needs to be four times as heavy to be as strong in bending. Bending strength is critical in wings - this will add weight to the wing, not lighten it.

Then there is the substantial increase in wing camber... We normally assume that the original designer picked the airfoil for good reasons. The effective camber of this wing will be between that from the deepest part of the depressions and the original. If the original was pretty good, this will have excess camber.

Billski

#### Hephaestus

##### Well-Known Member
Everyone is different, but I would find hotwiring a foam core and doing a layup on top of it, then removing the hot-wired unneeded channels as more achievable than building two female molds, fabricating the wing skins in them, procuring EPS kernals, and injecting steam into the void to get them to puff up exactly right. I don't have a large quantity of available process steam at home, and making that happen would be some high adventure.
There is EPS that is half the density of XPS, but it is much less than 1/2 the compressive strength of the "normal" XPS foam used for solid core wings. ASTM C578 Type V111 EPS has a minimum density 1.15 pcf, but it has a compressive strength of 13 psi (with 10% compression). It is very soft. You can get 60 psi EPS (it would be ASTM C578 Type XV), but it has a density of 3 pcf (about 50% more weight than comparable XPS).
The steam is easily solved by rental places, most have some kinda steam (what is it a boiler?) generator for the hydrovac, daylighting and construction industries around here. Big ass towable trailer system like the HUGE compressors - but, not terribly expensive.

The Rohr way's been stuck in my head a while and I've been wondering about it for a long ass time... But it's almost deprecated technology now.

Cutting a pair of molds, layup, then assemble/steam sounds good in some ways As I sit ordering more xps blocks because I can't seem to get decent cuts lately to save my life.

#### stanislavz

##### Well-Known Member
HBA Supporter
As I sit ordering more xps blocks because I can't seem to get decent cuts lately to save my life.

#### Hephaestus

##### Well-Known Member
Most are PEBKAC variety. Though we've had 3 different CNC failures (motor driver, then a bearing, then a stepper got wierd), today's failure was me not adequately securing the block... Was good to about 70% done then I decided all was smooth, went to make coffee... Came back and had to dump a fire extinguisher on the problem

You know, normal workshop issues.

#### stanislavz

##### Well-Known Member
HBA Supporter
Was good to about 70% done then I decided all was smooth, went to make coffee...
Ha ha. Designer soul must be presented while working on cnc. If you have to mount foam block - you are cutting too agressivelly. You wire do not touch foam at any time. Just radiant heat. You may mount gauge on you tensioner spring - it may move onle while power is turned on/off. While cutting - rocket solid..

#### Hephaestus

##### Well-Known Member
Ha ha. Designer soul must be presented while working on cnc. If you have to mount foam block - you are cutting too agressivelly. You wire do not touch foam at any time. Just radiant heat. You may mount gauge on you tensioner spring - it may move onle while power is turned on/off. While cutting - rocket solid..
Think it's more the cnc path that's causing the problem, it's rotational - happens in a rather odd taper cut (elliptical) when the 2 Y axis start traveling in opposite directions.

Or there's just something else way off that I haven't found yet. But its same Gcode as the last good cut...

Shhhh, coffee runs during cuts is SOP now that I'm not oilfield May just have to add a smoke detector a little closer to the work surface, have some at the printers that kill power if tripped - probably should smarten up and do the same at the hotwire...

#### Vigilant1

##### Well-Known Member
Cutting a pair of molds, layup, then assemble/steam sounds good in some ways As I sit ordering more xps blocks because I can't seem to get decent cuts lately to save my life.
Ah, I went through this. I've fallen in love with the idea of EFI ever since the Nikki carb on my daughter's riding mower kicked my butt. I never did get it to run, much less run well.

#### Sockmonkey

##### Well-Known Member
Biggest problem is that it will roughly halve the depth that you can put a spar in. Halving the depth of a spar means it needs to be four times as heavy to be as strong in bending. Bending strength is critical in wings - this will add weight to the wing, not lighten it.

Then there is the substantial increase in wing camber... We normally assume that the original designer picked the airfoil for good reasons. The effective camber of this wing will be between that from the deepest part of the depressions and the original. If the original was pretty good, this will have excess camber.

Billski
The camber issue would be solved by factoring it in when choosing your airfoil.
I missed the spar depth issue. Dangit.

#### pictsidhe

##### Well-Known Member
Most are PEBKAC variety. Though we've had 3 different CNC failures (motor driver, then a bearing, then a stepper got wierd), today's failure was me not adequately securing the block... Was good to about 70% done then I decided all was smooth, went to make coffee... Came back and had to dump a fire extinguisher on the problem

You know, normal workshop issues.
I'm going to sheetrock my basement ceiling for fire reasons...

#### Hephaestus

##### Well-Known Member
I'm going to sheetrock my basement ceiling for fire reasons...
ceiling is a LONG way away. Might need some gas to really cause issues in the hangar.

Ah, I went through this. I've fallen in love with the idea of EFI ever since the Nikki carb on my daughter's riding mower kicked my butt. I never did get it to run, much less run well.
GM TBI's man! I swear to gosh we've put them on almost everything carbed... Eye up the old geo metro's

But in some ways the EPS appeals, the labor savings could be pretty decent if you could do it within reason at home... Feel like I should look at the primary gliders wing engineering again and see what that would look like.

#### Sockmonkey

##### Well-Known Member
Is it workable to bag the wing first, and core it with a hot tube after everything sets? You would have to leave the tips bare. would that mess things up?

#### Vigilant1

##### Well-Known Member
Is it workable to bag the wing first, and core it with a hot tube after everything sets? You would have to leave the tips bare. would that mess things up?
The removal of lightening channels could be done after bagging. Doing it before bagging has some advantages, especially being able to do it one section of foam at a time rather than an entire wing half-span. The hot tube idea-- I dunno. After spending all the time and material cost to cover the core, you don't want any high risk drama. Trying to plunge red hot tubing into a 10+ foot long core and keep it aligned so it comes out at the right spot...think we've already hit on some approaches that will work well..

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#### stanislavz

##### Well-Known Member
HBA Supporter
filling the epoxy pump, labeling, organizing, and the like. But when my crew shows up, we just want to go!
The question. I was thinking to make warm cabinet for epoxy and buy dosing pump.

But - it will force you to measure them using scales while on hot time. To be precise.

So - i think i will take 5 or 10 big cups, same smaller for hardenere, and fill both them with required epoxy/hardner amounts. Store in warm closet before use. And just mix. Opinions ?

#### wsimpso1

##### Super Moderator
Staff member
Log Member
The question. I was thinking to make warm cabinet for epoxy and buy dosing pump.

But - it will force you to measure them using scales while on hot time. To be precise.

So - i think i will take 5 or 10 big cups, same smaller for hardenere, and fill both them with required epoxy/hardner amounts. Store in warm closet before use. And just mix. Opinions ?
Your method works, I have assisted on builds done that way. Makes for a lot of cups about on laminating day.

I have a Micheal Engineering Adjustable pump, and have used it on all of my big layups. Proportions of two components are always on. Works great as long as you do not run a tank dry. Unless you are using it every day, some systems will have salts fall out if it sits in the pump. Clogs the check valves. I return material to their cans if not using it right away.

Billski

#### Vigilant1

##### Well-Known Member
As I look at the pressure coefficients to be expected around the airfoil (so we can estimate the "pull" on the wing skin at various lift coefficients), I've found there's a big difference at some locations (near the leading edge) between measured wind tunnel values and the values predicted by JavaFoil (JavaFoil is up to 80% higher in localized areas). The wind tunnel measurements were done in 1938, but even at the time they noted the discrepancy between the values they got and the higher values predicted by theory. I know there are more sophisticated analytical tools available than JavaFoil and its kind, the high  tools surely fix this. For now, I'll probably use the wind tunnel values where I can.

An aside: That early NACA work is really something. A treasure that benefitted aeronautical advancement around the world, and still handy 80 years later. Everything was analog, all the calculations were by hand, and they were thinking well ahead of fielded aircraft technology of the time.

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#### Vigilant1

##### Well-Known Member
This is what I have been knocking around.
This probably deserves its own thread for best consideration of the idea.
How do the loads get to the spar? Passing concentrated loads through the foam is usually best avoided, unless the loads are small and the foam can take them.
Bonding AL to foam?
Is the rear side of the D-cell glassed in, the ribs glued to the back?
That already-built D-cell is a great place for some pultrusion spar caps and a web layed right on the back of the cell. Or, just lots of uni layers instead of the pultrusions (heavier, but still lighter than an AL tube, and easier to ship. Also avoids the AL bonding challenges).
For those who want fabric, this is an interesting idea.

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#### wsimpso1

##### Super Moderator
Staff member
Log Member
Can I ask about the "other" way to lighten a monolithic block wing...

Remember that little section on the rohr 2-175 project on the wing being made using green fiberglass skins then EPS added, and steamed to create the final composite structure?

EPS can be done in down to like 1/2lb density, if you cut an female mold in XPS/other, layed your shells on those and followed a similar process...Compress the halfs together, add EPS then steam.

I haven't really looked into what it takes to mold EPS, but curious if this isn't something a little more achievable for a homebuilder.
Expanded Polystyrene foam, coffee cup foam? I have used it for tooling. The beads barely are attached to each other. MAYBE if it is a nonstructural assembly with it fully contained with composites - Interior panels, inspection covers, glovebox doors...

We are talking about wing structures here, and the OP does not want any sealing layer of cloth and resin on the inside. Regular blue foam exposed gives me the willies, but white coffee cup foam?

Why does exposed polystyrene foam bother me? If nothing else, it can get wet with solvents or invaded by rodents or birds. Both cases mean the skin can lose its support and collapse. Seen it plenty in floating docks made of blue foam. Seen it in stored foam or partially completed airplane parts (exposed foam). I have seen a lot of stored airplane parts, and never yet seen one where a rodent has hollowed out a nest from a foam cored part fully wrapped in glass-epoxy or even closed out with dry micro.

Billski

#### wsimpso1

##### Super Moderator
Staff member
Log Member
Billski. Thank you. No, i am not lonely on this, one or two chaps available on request + while doing some test samples it looks like tailoring temperature in shop is way to go. But you have to start with warm epoxy. And now getting 40/60 glue to fiber ratio due to this. 2 layers of 450gsm weights 1.6kg per m2. But - this was one fifth or sixts test. Foil, spread warm epoxy on it, lay you fibers and other foil. With some help of hair dryer and roller all extra epoxy is mowing where you want it be / new area. With heat off - it is quite thick at 12-15 celsius degree. Gel time was extended from 60minutes to three hours.

For me it is ok for stabilator. For wing will want one extra chap or slower hardener..
We use Gougeon ProSet 125 Resin and 229 Slow Hardener (yeah, the high priced spread) for anything that might take an hour and a half to get under vacuum. For small simple parts I use 226 Medium Hardener. Most systems have slow, medium and fast combos.

The idea of having the room cool, and then warming it up while the resin is wet scares me. Dragons be here - well, not dragons, but air in the laminate comes from this. If everything is wet, and the vacuum is applied, air present in the foam, joints, etc, expands as the pressure drops and then finds its way out to the pump. I may find places where a void was formed by glass that did not lay down, but I do not find air in the laminate. But if your foam is warming as the resin begins to gel, air is still expanding out of its hiding spots and through the laminate as the resin firms up and keeps the bubbles from escaping. This is why even in open layups, you want EVERYTHING, including the foam, fully up to the intended shop temperature well before you start laminating... Back when we used to have incandescent trouble lights in some shops, some folks figured out that holding the trouble light above the freshly squeegeed surface (for inspection purposes) made air come out of the foam and get trapped in the laminate. The smart builders went to fluorescent trouble lights for inspection duty. Once the resin has hardened, you can post-cure, shake-and-bake, whatever benefits the structure.

Billski

#### stanislavz

##### Well-Known Member
HBA Supporter
We use Gougeon ProSet 125 Resin and 229 Slow Hardener (yeah, the high priced spread) for anything that might take an hour and a half to get under vacuum. For small simple parts I use 226 Medium Hardener. Most systems have slow, medium and fast combos.

Billski
My resins come from this shop : Buy Resins online at R&G I am using L hardener now only 40 or 60 min. Have tried to use some 210 min hardener, which is designed for infusion, it needs post curing, and if parts are not lay-flat position - you get thinner edges, and more resin in the bottom. Regardless of vacuum bagging or infusion. I did wait till colder times in our climate to test mine twin foil laminating - all looks like to be working as it should.

And - resin will get to its end of gelling time at 12 degree too. After 6-8 hours. So i may to use it with foam too. But this is for this resin only, have tried same trick with some other - 18 degree is a minimum.