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Old Jun 20, 2012, 04:59 AM
Peewee Paraparaumu New Zealand
New Zealand
Joined Sep 2004
104 Posts
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Molded DLG wing TE

Hi Listers
I have been molding DLG wings with some success for some time now but I have a question.
I have attached a couple of photos of commercially molded wings and it appears that these have the inner skins sanded at the trailing edge to give perfect clearance before joining. ( the give away here is that the carbon disser stops short of the trailing edge).

I have been pre sanding the skins and or using a roller to crush the rohacell at the trailing edge but this can have variable results in that the thickness, if too great, means we dont get perfect mold closure and hence thicker trailing edges.

I have tried a sanding jig but this can result in a black stained trailing edge as the carbon disser is sanded and distributed along the trailing edge. ( Also this hand finishing has other drawbacks, inadvertant mold sanding being the main one!)

Does anyone know how the "commercial builders" achieve this.
best regards
Peter Williams
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Old Jun 20, 2012, 06:17 AM
Father of Fr3aK, DLG Pilot
tom43004's Avatar
USA, OH, Worthington
Joined May 2002
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I've been sanding to a bevel both the top and bottom rohacell and then 3m77ing the rohacell to the disser cloth, wetting out in the mold, then rolling it while the half is being bagged. This leaves a hard surface just below the "centerline" so a bit of bog gives clean closure when the mold halves come together.

I'm not totally sure what the others are doing.
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Old Jun 20, 2012, 04:06 PM
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Germantown, Maryland
Joined Apr 2004
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I was taught how to mold DLG wings by Tom Siler so my methods are similar to his, but they are still a bit different. It would be really nice if some of those other builders would share their methods, but at least, Tom and I have no secrets:

It is still a mystery to me why other builders put the disser cloth on the inside skin rather than the outsdie skin. My disser cloth is used for the outside skin, with the disser threads on the inside of that glass fabric. The disser fabric is prepared in advance by adding the 1K disser threads to the 25gsm glass fabric.

The layup begins (in terms of the rear part of the wing) by rolling epoxy into the mold with a foam roller. Then the disser glass is dropped in place and rolled out with the foam roller. Hence, the disser threads go all the way to the mold edge.

The Rohacell is prepared by cutting it to size and shape, thinning of tip and trailing edge areas by sanding with a sanding block, cutting out areas of the Rohacell for the hinge line areas and servo areas, bevelling the edges of the cutouts, attaching the inner 25gsm glass cloth with 3M77, and finally, painting the part of the Rohacell that is aft of the carbon D-Box area of the wing, which also paints the 25gsm glass that is aft of the Rohacell, or in the cutout areas of the Rohacell.

During the layups, the inner glass which is attached to the Rohacell is wet out with a foam roller and then the Rohacell/glass assembly is dropped into the mold. Aftyer vacuum is drawn, some hard roller work is used to crush the Rohacell as Tom does, but in my case, it only does a little crushing because I sand most of the excess thickness away long before the Rohacell gets into the mold.

I would expect that mold closing pressure would crush the Rohacell even if no thinning or earlier crushing were done. The problem there would be that this would likely lead to a lumpy area of the wing where the Rohacell was crushed because you would be crushing the Rohacell after the epoxy for the skins was cured and so the Rohacell would want to rebound a bit after it came out of the mold. Tom and I call this lumpyness; "cellulitis" because it looks like the lumpy fat depositis on fat ladies' thighs that goes by the same name.

You can avoid this cellulitis by either sanding all the excess Rohacell off before it goes in the mold, or by crushing the Rohacell with a hard roller while the skins are in the vacuum bag and the epoxy is still wet, or by using some combination of the two methods.
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Last edited by Phil Barnes; Jun 20, 2012 at 05:58 PM.
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Old Jun 20, 2012, 04:16 PM
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Joined Apr 2004
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I should also point that the Rohacell is sized such that it stops about 6mm ahead of the wing trailing edge. Only four layers of 25gsm glass (two layers with their disser threads) and a bit of splooge are in that last 6mm or so of trailing edge.

If you were having trouble with mold closure, perhaps you were letting the Rohacell get too close to the trailing edge. I wouldn't expect mold pressure or hard rolling to crush the Rohacell down to zero thckness, but it should get it really thin, especially if you give it a head start with sanding before the Rohacell gets in the mold.

In your pictures of the commercially available wings, it looks like they also stop their Rohacell ahead of the wing trailing edge. For some reason, they appear to have atatched the disser cloth to the Rohacell and then trimmed it flush to the edge of the Rohacell before putting that into the mold. On my wings, I attach the inner glass to the Rohacell but trim the glass off maybe 10mm beyond the Rohacell at the trailing edge. I think it is better to have all four layers of 25gsm glass in the trailing edge for a stronger trailing edge.
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Last edited by Phil Barnes; Jun 20, 2012 at 05:46 PM.
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Old Jun 20, 2012, 05:18 PM
Peewee Paraparaumu New Zealand
New Zealand
Joined Sep 2004
104 Posts
Molded DLG TE

Hi Phil and Tom
Thanks for your replies. I had always cut my Rohacell to go to the TE so I will look at leaving it short of the TE to allow more room.

I have attached a photo of a flapperon and it appears as if they have sanded/cut the inner disser+skin and some of the core material after the skin has been cured in the mold.
The TE past the edge of the inner glass+diseer skin is certainly the core material and not just splooge.

I imagine they are using some sort of mechanical setup with a sanding drum or plane to trim the inner skin and core material to below the parting plane and then joining as per usual.
Just hoped someone had seen how this was done as I imagine that it may be a "why didn't I think of that" kind of idea.

Certainly all the European production planes are built like this so it must be fast and easy I imagine.

Love the term cellulitis BTW.

best regards
Peewee
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Old Jun 20, 2012, 05:32 PM
Father of Fr3aK, DLG Pilot
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USA, OH, Worthington
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One method I'm aware of is that the material is bagged into the molds, then a router on wheels is "rolled" across the parting plane to remove any material proud of the parting plane. Jaro Muller does this on his ships, including the spars, etc. Then the two halves are "mated" with a very small bead of splooge. I have a part of an Espada wing that I was given to inspect a couple of years ago and it's an example of PERFECT construction technique IMHO. I'm just not sure it's practical on a DLG.

I headed down the path of doing it that way and decided that it was quite messy, and didn't do any cleaner of a job of joining than I was already getting. My parting plane isn't a plane either due to non-linear washout, so I had some mating issues due to that.

Phil certainly coined the term and that has been one of the major concerns of the first few months of production.
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Old Jun 20, 2012, 05:56 PM
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If it's a faster, easier, "why didn't I think of that" method that you seek, perhaps you should widen the search beyond looking for a clever method to sand Rohacell and glass while it is in the mold. Perhaps thinking more about the total process and looking on the other side of the Atlantic ocean could provide the enlightement you seek.

I have a crashed Stobel wing in my shop. I frequently pick it up and dissect it bit by bit as I come up with questions about how it was built, just as you apparently are doing. Your posts have made me pick up and dissect that wing one more time. You are correct, the Rohacell goes all the way to the TE, the disser (applied to the inside of the inner glass layer) stops short of the trailing edge. I think that either they are machining the cured Rogacell/glass sandwich after bagging but before closing the molds, or, the disser glass that is attached to the Rohacell inside surface is attached with it's aft edge a few mm ahead of the edge of the Rohacell.

I should also mention that every time I pick up that Stobel wing, I see cellulitis in the trailing edge area.
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Old Jun 21, 2012, 02:28 AM
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Madrid, Spain
Joined Sep 2004
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I cut my rohacell 5mm shorter chord wise so it stops aprox 5mm away from TE and when I do mu layup, since it's made with disser, I also cut it shorter so I get the thinnest TE. If I have the inner layer with disser up to the TE, it's much thicker with the disser tows placed aprox at the same spots. It also is weaker, since the bumps displace the splooge and the adhesion is not great.

I put the disser on the inner side because this way I get the best adhesion between the rohacell and the skins, without bumps or voids.
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Old Jun 21, 2012, 03:01 PM
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Dbox's Avatar
Planet Earth
Joined Jun 2005
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Quote:
Originally Posted by flyboypeter View Post


it appears as if they have sanded/cut the inner disser+skin and some of the core material after the skin has been cured in the mold.
.
Absolutely correct and there is not much trick or special equipment needed.
Also +1 with Chetos on putting disser inside,best adhesion without bumps.
Yuri.
Yuricomposites.com
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Old Jun 22, 2012, 08:19 PM
Peewee Paraparaumu New Zealand
New Zealand
Joined Sep 2004
104 Posts
TE Prep

Hi Yuri
Thanks for the reply.

Is it possible to get a picture of the way this is done??

Are they using a router and some sort of attachment or doing this by hand

best regards
Peewee
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Old Jun 26, 2012, 09:35 PM
Aurora Builder
United States, MD, Lusby
Joined Nov 2003
3,378 Posts
I'm willing to share and help whenever possible, but for the most part Tom and Phil are much ahead of me!

Putting disser on the inside still doesn't make sense to me. Outside is best structurally. The only advantage I see is you might be able to reduce a step, as you could bound the disser to the glass and the glass to the rohacell all in one process. The other potential advantage is a cleaner wing, as the disser strands won't go everywhere. But the trade off will be the potential extra weight used when wetting out the inner skin. In a perfect world the weight difference is 0, but since I pre-bag the very outside skin (skin on the mold surface), I pull any excess resin out. As I wrote the above I am seeing advantages to both methods. Enough so that I will have to experiment by building a wing each way.

I stop my rohacell 5mm before the TE. The inner glass stops here. The outer glass goes past the TE. I've only been hard rolling the rohacell but will be sanding to reduce cellulitise in the future. One trick I've found is to hard roll the rohacell next to the flaperons window, works fine although sanding may have better adhesive properties between the inner and outer skins, I can't tell.

-Sam
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Old Jun 27, 2012, 02:28 AM
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Australia, SA, Normanville
Joined May 2009
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If the mould is designed the right way you could use an inverted flush cutting router bit with the bearing riding on the flat mould surface. The router is mounted in a router table. very quick and very neat with no damage to the mould.
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Old Jul 10, 2012, 12:11 AM
Peewee Paraparaumu New Zealand
New Zealand
Joined Sep 2004
104 Posts
Hi Listers
I made a handheld router to trim the TEs of the wing before mold closure. I will let you know how it goes.

If you hear screaming then the answer will be "not well"

best
Peewee
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Old Jul 23, 2012, 09:06 PM
Peewee Paraparaumu New Zealand
New Zealand
Joined Sep 2004
104 Posts
Success

Hi Listers
Ok so the handheld router works well, I had to add a couple of plastic standoffs to clear the wing ( the mold has the wing molded with reflex).

I will see what the result looks like after I joing the wing and post some photos then.

best
Peewee
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