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cattflight

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This is a great grid. But I feel I may be ignorant to the true implications of a negative value. Does anyone have any recommendations on text books that can put all this into perspective. I bought Zeke Smith's Understanding Composite Aircraft Construction but it's really high level and a bit theoretical. Looking for more "practical applications" perspective.
 

cattflight

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Great details. I also just spent an hour or so on the Aircraft Spruce website. Looking at Divinycel, I noticed they do not have a 1/2" thickness, wherein Last-a-Foam does (and both 1/4" and 1/2" are necessitated in the Vision plans). Spruce website also seems to tout LaF as being an excellent, versatile product. So, that combined with the thread mentioning LaF seems to be acceptane so long as the edges are closed out properly, am I being too critical of Last-a-Foam?
 

Jan Carlsson

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Have you read all about Last a Foam? if you sat it under vibrations it become to a puder, and same in a impact, what make it easy to bend and work with also make it a poor choise.
Aircraft Spruce don't sell any poor material, how could they? "This is a poor material but cheep, buy it by click here!"
There are better material out there.

Jan
 

NickH

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Divinycell is made in 1/2" thickness, I got mine from fiberglass supply in Seattle, and I think it was a little cheaper. They also carry H80 as listed in the chart. It's the 5lb foam which is closest to Last-A-Foam's 4.5lb specified for the Vision. I'd rather go stronger and take the extra weight, even if H45 still has better numbers than the Last-A-Foam. There is also an H60, which may be the best choice, but to be honest the foam makes up a very small part of the weight of the structure.
 

cattflight

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Divinycell is made in 1/2" thickness, I got mine from fiberglass supply in Seattle, and I think it was a little cheaper. They also carry H80 as listed in the chart. It's the 5lb foam which is closest to Last-A-Foam's 4.5lb specified for the Vision. I'd rather go stronger and take the extra weight, even if H45 still has better numbers than the Last-A-Foam. There is also an H60, which may be the best choice, but to be honest the foam makes up a very small part of the weight of the structure.
From Fiberglass Supply's website, I only found Divinycell H80, H100 and H200. For the H80, I found the 1/2" listed with these P/Ns and descriptions. Is this what you used?

[FONT=Comic Sans MS,Chicago,Sans-serif,cursive][SIZE=-1]L18-1090 [/SIZE][/FONT][FONT=Comic Sans MS,Chicago,Sans-serif,cursive][SIZE=-1]1/2", H-80 Divinycell Plain, Sheet 48" X 85.6" (28.5 sq. ft.) [/SIZE][/FONT][FONT=Comic Sans MS,Chicago,Sans-serif,cursive][SIZE=-1]$102.26[/SIZE][/FONT]
[FONT=Comic Sans MS,Chicago,Sans-serif,cursive][SIZE=-1]L18-1094 [/SIZE][/FONT][FONT=Comic Sans MS,Chicago,Sans-serif,cursive][SIZE=-1]Quarter Sheet 1/2” H-80 Divinycell Plain, 42.8” x 24” [/SIZE][/FONT][FONT=Comic Sans MS,Chicago,Sans-serif,cursive][SIZE=-1]$42.33[/SIZE][/FONT]

Aircraft Spruce only seems to cell 1/4" and 5/8" Divinycell. No 1/2". This stuff gets confusing!!
 

NickH

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Yes, that's the stuff. I bought some 1/4", 1/2" and 3/8" full sheets to get started, hopefully through the HS. Freight shipping was $120ish, and was very quick. I was impressed.
 

cattflight

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Sorry to belabor this (let me knowif I should PM you) but are you using any of the scored material or are you heat-forming it for any of the curved areas? Is Divinycell up to 1/2" really that hard to form that I would need to be worried?
 

NickH

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I haven't formed any yet, so this is based on what I've read from the builders archive. Divinycell is supposed to be easier to form than last-a-foam with some heat. The scored sheets don't make curves that are as neat and swallow micro to fill the scored gaps. From feeling the material, it seems to bend a little quite easily, but I can see that it will need something to form to the larger curves needed for the vision. The curved parts are made with 1/4" foam which should be a lot easier to bend. Hopefully I'll know more in a week!
 

wsimpso1

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For wing skins and most of the fuselage, I just used flat stock and let the vacuum bagging process do the bending for me.

When the bend radius got to 20" and less, I formed 3/8" foam with a very gently applied heat gun. It is not hard to do, but it does take some patience. For big pieces, I left one long edge attached, slit the rest (with knife cuts) into strips, and then heat bent each strip. Foam is a good insulator, so it takes up heat really slowly, and takes a while to warm enough to bend. Go slow, and it will work. An oven big enough to gently warm big pieces is a little hard to come by. For smaller more intricate pieces a used kitchen oven (out in the shop) might work - anyone done this?

Billski
 

autoreply

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An oven big enough to gently warm big pieces is a little hard to come by. For smaller more intricate pieces a used kitchen oven (out in the shop) might work - anyone done this?
I've heard about doing layups, bagging them and then put a load of (hot) water over the bags. Might also work for Divinycell, H-grade requires 70-80 C to form (170F or so), HP requires much higher temperatures. A water-cooker and a small water-pump might be required to keep/make it warm (enough)
 

NickH

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OK, now I've had a go. The Divinycell bends relatively easily to the curves on the Vision, without any danger of cracking or breaking. It does take quite a bit of force to move it and to hold it there, and it was easily bending my stringers out of line before I reinforced them. When released it wants to spring back all the way to its original position. I've made it work by bending it, and then heating it, then bending it some more. It is easy to scorch with the heat gun and takes a lot of patience, especially working in a cold shop. It's taking me about three hours per piece for the curved sections, but may get a little faster as I get more practice.
 

Will Aldridge

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You know, I have been thinking about using Divinycell the same way and I had an idea. I have seen on the KRSuper2 site a small oven that's basically a box made from insulation foam with a heat gun stuck trhough the side and a fan inside to circulate the air.

My idea goes like this;

1.Get the foam rough cut to shape then attach a strip of wood to the longitudinal edges of the foam.
2.Put in the oven and let heat.
3.Remove from oven and hold in place on form.
4.The wood strips would make it easy to hold even pressure against the entire length of the foam part being formed.
5.Hopefully by the time it cools off it will hold it's shape.

I guess since the foam is conforming without breaking you might not need this at all but one thing RC builder do when trying to bend foam around a tight radius is to put tape on the outside of the piece of foam and it can then be bent without breaking.

That would be an awful lot of tape but maybe there is another similar method? Maybe use 3m adhesive and laminate a sheet of thin plastic like a painters dropcloth to the foam and peel it off when done?

Just some ideas.
 

Vigilant1

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That would be an awful lot of tape but maybe there is another similar method?
The most straightforward approach is to glass the outside of the panel before bending it. As one builder said "I could bend a potato chip in half without breaking it if the outside was epoxied and glassed." This is the basis of Steve Rahm's "foldaplane" method for making curves, though folks had been doing it long before that.
 

Will Aldridge

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The downside to this is that you pre stress the skin. It won't have near the strength that it would otherwise. I was pretty excited when I first heard about it but Orion's comments have made me somewhat leery of using it.
 

orion

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There are several other approaches to this problem, both in material and technique. First of all, Divinycell foam is a cross linked PVC and thus rigid in its structure. For ease of bending, you can substitute Airex foam, which has a non-cross linked structure and form curves with relative ease without heating. Airex has very similar properties for the same density and has been used in the marine industry for years, but has also seen quite a bit of use in aerospace and light planes.

Additionally, you can use a thinner material if the structure would allow or, for the same thickness, use two thinner layers.

You can also kerf the material using something like the RotoZip and the carbide circular blade. If you cut about half way through the thickness the material will bend with ease. Make sure to bend into the kerfs. If the cuts are on the outside of the bend, the bending action along with the stress concentration of the kerfs may cause the foam to break and by making the cuts wider, the layup would absorb additional resin. Some of these materials are also available already kerfed, mounted to a thin scrim fabric that holds all the pieces together, sort of like the pre-patterned squares used in bathroom tiles.

And for maximum bending you can also look at another core material called CoreMat. This material drapes like fabric but when infused with resin it forms sort of a honeycomb cell structure that is structurally quite effective.
 
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