Vertical Strip Foam

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Markproa

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Has anybody tried vertical strip foam as a method for building a one-off compound curved fuselage? It is described further down in this link- http://www.f-boat.com/pages/construction/methods_sb.html
I have plenty of experience with strip planking in cedar and end grain balsa cores but didn't use this method. I always thought it would be difficult to get a good seal for bagging or infusion and am amazed the vacuum doesn't pull the foam away from the mould, but plenty of boat builders have used this method. The problem for an aircraft would be the thin foam as it can't be held against the mould with screws from the back. Maybe some sort of temporary adhesive such as double sided tape. Once the fuselage inside has been glassed the section moulds could be carefully removed from the mould strips, then the strips peeled away.

Has anyone tried Corecell foam?

Mark
 

pictsidhe

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I've been wondering what boat technique you might bring in. This one looks interesting. A vacuum film glued to the mould, then holding the foam to that with weak glue (post-it type?) might be worth a look?
 

Aerowerx

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.... Once the fuselage inside has been glassed the section moulds could be carefully removed from the mould strips, then the strips peeled away.
I don't quite understand.

Are you leaving the foam in place and glassing both the inside and outside? Or using the foam just as a mould to form a fiberglass shell?
 

wsimpso1

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I do not get it either. Bigger picture Mark, tell us more of what you are talking about...

When making a compound curved sandwich structure, like the fuselage, we usually do something like this sequence:
Make an airtight mold by any of a bunch of means that extends beyond the part;
Cut the cores to shape. Where the curvature of the core is small, we stop there, but if it has a lot of curvature, we heat and form the cores to reflect the shape it will end up. It does not have to be perfect, we will vacuum bag it in place;
Cut peel ply, perf ply, glass cloth, batting, etc;
Then by either wet layup or dry layup and infusion, we get the first facing of structural cloth wetted out and intimately formed to the mold;

The art comes in several ways. We have to decide if we are using a male mold or a female mold, if we are putting in the entire sandwich in one bag session, two, or more sessions. For some parts, I have put in the cloth against the mold and the cores, peel plied it and vacuumed there, then followed with the other facing in a separate session. For others with small curvature (like wing skins), I have done the whole sandwich. Some folks do the mold side facing in one session, the core in the next, and the other facing in a third session.

Further variations exist in if you "hard shell" the foam before applying the second facing, which is applying dry micro to the foam, letting, it cure, sanding it, then doing the second facing. The alternative is to just wet the foam with neat epoxy, and accept the slightly higher weight...

Me? I have done male and female molds, all air tight, and have never screwed cores to the mold. You can not get a vacuum that way, and the mold is ruined on the first try. I have done wing skins in simple female molds and do the entire sandwich in one shot. For the fuselage, I have used male molds for most of it, with small curvature done in two steps, first with mold side facing and foam cores, then offside facing. For my gullwing doors and roof, it was multiple layups in afemaile mold. For spars, I have installed precured spar caps while other folks have done thick spar caps in stages. It is all a matter of what you can manage in the mold in the time you have.

Tell us more about your methods, and we can explain our methods some more. You can look up my stuff on here. I have done a bunch of big parts and shared what worked and did not work too.

Billski
 

wsimpso1

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Oh, Farrier Yacht techniques.

In airplanes, the facings are thin. Glass at 21-22 oz/yd cloth, graphite at 10-12 oz/yd. Do the foam verticle without a vacuum bag, and it will have a lot of resin in it.

Maybe you can seal the foam and wet laminate and bag or infuse, there is a well known website that covers that, but I have always been concerned that it still ends up heavy because there is no class A surface. The outside will have significant shape errors that are both weaker and require a lot more shaping and fairing to get right, then is heavier because of it. The inside is a tight fit to the foam, but all irregularities due to the discontinuities are weak spots.

One other point about the vertical foam method in a wooden lattice mold... When you finish putting the requisite (0.010" in graphite or 0.022" in glass) amount of glass or graphite on one side of the foam, then try to pop the foam and internal facings from the lattice mold, you may find it is way too soft and floppy to hold a good shape by itself. You will likely need a bunch of internal temporary structure that can be removed later just to get the whole thing off the lattice mold and prepped for facings on the other side of the foam. Beginning to see why we leave our parts on the mold until they have all of the facings on them?

The site you gave us a pick to goes to vacuum bagging from molds at the end. My preference for light parts. Say it with me... These are airplanes, WEIGHT IS THE ENEMY. You really do have to aim light at strength in airplanes or they become too heavy.

Billski
 

Markproa

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Thanks everyone for the thoughts.
The Farrier vertical strip foam method relies on the thick (15 -20mm) foam held to the mould strips with screws from the back. Of course we can't do that with such thin foam cores so I was mulling over a way to hold the foam to the strips. I was thinking of some sort of temporary adhesive but it would be difficult to remove from the mould without damaging the foam. Then I thought if the individual strips could be peeled away........To do this the strips would have to be skew screwed to each frame. They could then be unscrewed from one end allowing each strip to spring away from the fuselage peeling as it went. All sounds a bit tedious I know but aircraft fuselages are very small so not a lot of work. I'm used to big catamarans so fuselages are very small to me.
As you say Billski, the fuselage may well be too floppy to hold shape so may need a set of male frames to hold shape. Ian Farrier fits out the interior of his boats while they are still in the mould which holds them in shape. BTW, I have done a lot of wet layup vacuum bagging, infusion etc with all sorts of cores in proper moulds to understand their benefits; I have also made enough moulds to know I don't want to make two proper moulds to make one fuselage, hence my investigation into something quicker.

My inquiry was to find out if anyone had tried this method as I can see many pitfalls. I'm not convinced the foam will hold its shape when under vacuum. Unless the foam is held perfectly fair to the mould it will not turn out fair and I'm not sure that is possible. If Ian hadn't made a lot of boats this way I wouldn't have thought it possible, but then boatbuilders don't mind pouring heavy layers of fairing compound onto there pride and joy. These are airplanes, WEIGHT IS THE ENEMY, These are airplanes, WEIGHT IS THE ENEMY, These are airplanes, WEIGHT IS THE ENEMY................
 

Aerowerx

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No, vacuuming to the inside laminate of a foam core, removing from the mould then vacuuming to the out side.
I asked two questions. Which are you saying 'no' to?

Rephrase into one question: Do the foam strips stay in the final structure?
 

wsimpso1

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Mark,

I see your reasoning. let me tell you how I did my one-off fuselage and why I did it that way.

I know that even with really good female molds, airplane wings, tails, and control surfaces have to get a coat of dry micro for fairing and making them perfect. And fuselages? They have joints and are almost always covered with a layer of dry micro and faired. Since making a perfect female mold is still going to take fairing, why go through the steps of making a perfect plug, then perfect female molds, then parts, then assembly, then fairing? Then I thought about how almost all of the fuselage is going to be a certain thickness sandwich everywhere. Why not do it with a male mold?

I decided to make a simple male mold for each of the top and bottom of my fuselage. Bought two big blocks of white styrene foam, made big templates from masonite for profile and plan view, made a 54" hotwire saw, and cut the profile and plan view. Shaped the foam using a big disc sander, sanding sticks, and a bunch of templates to get the left and right symmetric. When I liked each mold, applied micro and two BID, then faired it to 80 grit, sealed the pinholes and sanded it to 200 grit. Is it perfect? Nope, but its shape is on.

The bottom half was built in three pieces, left side, right side, and belly. The sides have subtle curvature until we get to the last three feet, where the fuselage is narrowing towards the rudder, so they are made full length and I had to heat curve the cores aft. The belly was gentle curvature and runs out three feet from the rudder post. So I applied foam strips where the breaks will be, shaved angles on them and shaved the thickness on them for each part made to give joggles for later assembly. I did each of the three parts in one session, but if I were to do it again, I would have done the sides in two sessions each, mold side facing and foam in the first vacuum bag layup session, then do a second session to put the outer facing on. I even assembled my 16 foot tub on that male mold. For the top half mold, I did the turtle deck in two sessions, the forward deck in one, and heat curved the cores ahead of layup.

Now all of this saves having to do a plug and then pull a mold from it. Just build the plug small in radius by the thickness of your intended sandwich. Mine is 3/8 PVC foam and 18 oz BIAX inside and 22 oz TRIAX outside, or about 0.415" thick. By they way, this stuff, vacuum bagged, is 0.68 lb/ft^2. I could have saved some more weight if I had the courage to build in graphite fiber.

Does all of this sound like a lot? Yeah, but so does making the wooden lattice mold 16 feet long, figuring out how to attach and then detach the foam cores with only a thin facing of composite on one side of the foam, then install half of the firewall and bulkheads at the instrument panel, seat back(s), back of the baggage bay and near the tail, and hope it is not too flexible to work for assembly. If you do decide to go this way, I would think about if you want to build left and right halves or top and bottom. When you build a tub, you have the whole tub sitting there, and you have access and can build and attach and make work all of the systems and structures including the wings and tail (assuming low- or mid-wing design)- the decks and canopy system are relatively simple to add after all of that other stuff is done. If it is to be a high wing, a bunch more thinking is required - I would probably still build in top and bottom and think really carefully about how I connect the systems across the join.

How about the roof and gullwing doors? There is so much build up and thickness to the doors and then the roof, which is a rollover structure that I did not see any way around doing a female mold for that. How? Made a plug on the top half fuselage mold. Waxed it again, vacuum bagged two plies BID and hotwire cut 0.41" thick white foam over the entire canopy area plus a bit. Then faired it all with curing type drywall compound (the type with plaster in it), painted it, waxed it, and pulled a female mold. That female mold then got attached to a platform, and made two gullwing doors, then the roof over the doors. There was a lot of work in the doors and roof, so a sturdy female mold was the right way to do it.

For my wing skins, I made female molds directly. Since the wings are simple prismatic shapes, they can be hotwired. I broke each skin into three sections, came up with hotwire templates for the eds and the break lines, hotwired the blocks from blue flotation foam, hotwired the sections, and bonded them up on a big flat table. To make a good surface, I vacuum bag bonded coated aluminum roof flashing into the molds and applied wet fiberglass tapes around the edges. Other folks use plastic countertop laminates. You have to go around and spoon feed the joints wet micro to seal them all, but they have worked great and only take a dozen hours or so apiece to build.

I hope that this helps with your overall thinking. I you still go with the vertical foam strip method, you can probably use five minute epoxy or hot glue to hold the foam to the lattice. Yeah, it will tear foam out when you pull the piece from the mold, but you can go through with dry micro to fill those divots and maybe to get a faired outside hard shell before doing the outside facing layer. It will still need fairing afterwards, but it will be close on shape and have little in the way of discontinuities due to faceting. Now you know what I know about the tradeoffs involved. More thinking and discussion? We are right here...

Billski
 

Tiger Tim

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So if I understand correctly, 'vertical strip foam' is just planking a core with really wide foam panels that go from the upper centre line of the piece to the lower? If that's the case, how about wrapping foam panels around a male mold, glass the outside, then shore it up with temporary female 'formers,' pull from mold, glass inside, install bulkheads, mate halves? It would be similar to the aft fuselage of the AR-5 but with wrapped foam cores instead of hot wired.
 

cluttonfred

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With all the effort going in to making the wooden frame even before the foam and glass and finishing, I'd be tempted to just build a geodetic wooden airframe, cover it in fabric and go flying.

23vx56w.jpg Thalman.jpg thalman-t4.jpg

Thalman T-4 [N53389]
All-wood geodetic construction. Later converted to T-tail with 170hp O-340.
T-4 1953 = 4pChwM rg (manual)
135hp Lycoming O-290
span: 40'0"
load: 1050#
v: 175/155/45
range: 700
Source: http://aerofiles.com/_ta.html

Only half-kidding, actually....
 

wsimpso1

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So if I understand correctly, 'vertical strip foam' is just planking a core with really wide foam panels that go from the upper centre line of the piece to the lower? If that's the case, how about wrapping foam panels around a male mold, glass the outside, then shore it up with temporary female 'formers,' pull from mold, glass inside, install bulkheads, mate halves? It would be similar to the aft fuselage of the AR-5 but with wrapped foam cores instead of hot wired.
That idea is the inverse of what Mark is talking about, but with some significant disavantages. The big disadvantage is the external support structure you mention - You end up with much of the lattice mold anyway plus the work of the male mold. If you do the female lattice mold approach, you can populate the inside with lots of structure and systems before you even pull it from the mold.

The nice things about making the part on a vacuum capable male mold is you have smooth curves (no discontinuities or faceting), minimum epoxy in the parts, can build it all - facings and core - and get glass-to-glass bonds between inner and outer facings before it comes off the mold. Shape is fixed or at least pretty darned close, fairing is minimized, discontinuities are eliminated, it just works better.

Billski
 

Himat

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At model boat size hot melt glue work to fix the core to the formers. The direction of the "planking" does not matter. As already said, at light plane size to turn the half finished part is the difficult part.
 

Riggerrob

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........ For spars, I have installed precured spar caps while other folks have done thick spar caps in stages. It is all a matter of what you can manage in the mold in the time you have.
.........

Billski[/QUOTE]
---------------------------------------------------

How many times can you vacuum-infuse the same block of foam?

Wandering off the original topic ......

I have been sketching a variation on Burt Rutan's mold-less composite method. I will copy Burt's methods for hot-wiring (or CNC carving) foam blocks. The foam blocks start as male molds then fly inside the final product.

The process starts with hot-wire cutting a long, D-shaped block of foam. Spar caps and webs are vacuum-infused onto the rear face of the block. This might require two stages to vacuum-infuse all the different spar layers. Most vacuum-bagging is done with the mold fastened to a large, perfectly-flat table (ala. American Vision airplanes or Kelsail boats).

The second step involves vacuum-infusing a composite skin around all of the D-spar.

The third step requires gluing foam blocks to the rear face of the D-spar and carving them to shape. Vacuum-infuse the outer wing skin and rear spar.

If foam blocks are hot-wired, can you use the "scrap" foam as jigs/cradles to stabilize "flying" components as they cure?

If foam blocks are CNC routed, can the machine carve enough divots and grooves to accept all the reinforcements, inspection hatches, etc.?
 
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pictsidhe

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Flat wrap appeals to me, as flat, fair moulds can be had off a roll... Something that has also occurred to me, is 'round wrap'.
The idea being to composite, premould foam or other sheet etc over a simple spherical or other simple 3D shape, then wrap it. You'd need a few different premoulds for differing radii, but I suspect this would be easier than a full 3D fuselage. If planking, it would reduce the amount of fairing and/or planks needed. What I haven't looked at yet, is how to determine the optimum premould shape. Need to seriously brush up on my geometry...
 

wsimpso1

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How many times can you vacuum-infuse the same block of foam?
as many times as you want. Vacuum bagging a part with the atmosphere around it is ambient pressure outside, limiting case is full vacuum inside. At sea level, that is about 14.7 psi. Strength of the foams we use is much higher, on the order of 70 to 150 psi, so the foam is not changed by vacuum bagging it. Infusion? Doesn't change that part of the physics. Most of the foam never sees any resin. Fill the broken cells at the surface, and you have done everything that can be done to the foam. Happens on the first cycle. Repeats do nothing to it. Last possible is that the foam elastically deforms under the vacuum, divide the delta P between inside and outside of the bag by Young's Modulus and multiply by thickness of the foam, and that is how much you compressed the foam. Puny, and it cycles over and over if you repeatedly bag the part.

I have been sketching a variation on Burt Rutan's mold-less composite method... The process starts with hot-wire cutting a long, D-shaped block of foam. Spar caps and webs are vacuum-infused onto the rear face of the block... The second step involves vacuum-infusing a composite skin around all of the D-spar... The third step requires gluing foam blocks to the rear face of the D-spar and carving them to shape. Vacuum-infuse the outer wing skin and rear spar.
Been done before. It works. I have written about doing my horizontal and vertical stabilizers and my control surfaces this way. I wet vacuum bag, but you can vacuum infuse and get the same results. Big caution though on spars. Where the spar caps and/or shear webs get kind of thick, they can exotherm, make a bunch of heat and collapse your foam. That is why some folks do the thick parts in stages and others do some precured sections of the caps for most of the build ups.

The other thing I like to point out is that if you are not really careful, you can distort the part under vacuum and get a really stiff strong part that is not the intended shape. I make very carefully rectilinear foam block, hotwire the surface from the blocks and support the foam being bagged on the cutouts. Look up my stuff, I have descriptions and photos.

If foam blocks are CNC routed, can the machine carve enough divots and grooves to accept all the reinforcements, inspection hatches, etc.?
Sure. The big trick is not pulling the part out of shape when the vacuum comes up. I support the part on the cutout, and debulk/cure only the free upper surface. That way the part shape is unconditionally preserved while under vacuum. My flaps and ailerons include the twist and sweep of the wing, and it shows up in the finished part.

You are only limited by your imagination and what you can make work on the table. But I can almost guarantee you won't come up with something truly new. Too many smart people have already been doing this for too long. I know, I have patents and served on a couple major companies' patent committees.

Billski
 
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