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Old Aug 31, 2012, 12:09 PM
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Ward Hagaman's Avatar
San Diego
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If you are going to be over-filling and cutting anyway, it is pretty easy to use either beaded foam or extruded foam. I just band saw the planform, and then hot wire it a little thicker than the thickest part of the mold, and bag it into each mold half onto the cured skin.
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Old Aug 31, 2012, 05:58 PM
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josh18's Avatar
Australia, QLD, Toowoomba
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Thanks for the tips guys. I left the wing overnight, half expecting some shrink back or something but its just as hard and tough as yesterday, probably better actually.
Andrew that glass seems about as bulky as chopped strand matt but its woven from tows. It seems very tough but is pretty hard to wet out. I dont know how id go doing a whole wing with it- Only one way to find out though!
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Old Aug 31, 2012, 06:07 PM
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Cody, WY
Joined Nov 2007
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Did you bag the skins? Did you measure the skin thickness? I'd love to calculate the Vf% of a fabric like that.
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Old Aug 31, 2012, 07:01 PM
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Australia, QLD, Toowoomba
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Hey adam, I bagged the skins but didnt measure the thickness after bagging. I did do a test layup, wet out and then rollered. It came out to be about 1 to 1.2 mm thick. I imagine it would be nowhere near as efficient as a sandwitch construction but for pure dint resistance, especially backed by hard foam it will be great I reckon. I bought it for a couple of reasons, one, to try to do away with the cores in normal lay up which I find a PITA, and also because Kahnx has some similar fabric but in carbon! We know this is great stuff as from his experience but I wanted to do some practice with the mich ceaper glass first.
As far as Vf% goes, I dont think it would be too much worse than say 4 layers of 6oz cloth. If you wet it out from both sides it seems to soak in well, and then bleeds a fair bit of resin back out into the paper towel. There's no getting past this stuff is heavy though! The place where I got it also has about 500gsm quad cloth,which I think would be perfect but I have to order that.
Cheers
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Old Aug 31, 2012, 07:47 PM
AvB
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Hey Ward, thanks for posting ... I met you at Weaselfest this year. Fond memories of the people and the day. The the method you've described is what I previously did to get a light but very very tough Scratcho wing. It was also mentioned by someone else in this thread (dang I thought I dreamed up a new idea by myself! haha). I hot wire cut accurate half-cores, painted thin resin on the cured skins and vac bagged the half-cores down, then hot-wire cut the excess off and joined them. If you're prepared to spend a few extra nights, it's got to be the gold standard method because blue foam is so much more durable than the PU (doesn't crumble or degrade) and it cuts like cheddar cheese so you can make very accurate trenches for socked spars and stiffeners. Also no hassles with pushing hot rods thru to route servo wires. I really like it - the end result is superbly strong even with very thin skins. My aim with the PU experiment was heading in a different direction ... to see if it could be used as a super fast method, minimal materials and minimal steps ... no internal spars etc, not worried too much about a few glitches ... a moldie-like alternative to foamies at your local "rough" slope. But what I've ended up with is better than I expected in terms of stiffness and general quality.

Josh one point I'd make about foam cored wings (which is the only thing I've done! I've never made a hollow wing and the thought scares me!) ... is that it's a different paradigm ... you don't need a thick skin when you have a good tightly bonded foam core. Mate you would be amazed how rigid the PU wing is, even after cutting the hingelines - and that's a thin HN1038 foil, with skins of only one layer of 100gsm glass and one layer of 120gsm carbon! The foam core is kind of like a 100% surface area shear web. If the foam core has good compressive strength and rigidity it makes the overall box structure more rigid. That's why I've always used the RTM-X high load blue foam. But from the experience with this wing I'd have to say that straight up, PU gives more rigidity than blue foam.

In the discussion about closed expanding vs free rise then cutting, I'm in agreement that it's easier to manage and consistent, and the cutting process is not hard to do. One question I have however relates to Troy's info. Remembering that I used low density (very light) foam, and the liquid was quickly spread over the entire inside surface of both skins, and the mould was closed when the foam had begun to rise. Would it be correct to say that having the foam expand in the wing mould creates some back pressure which will result in some skinning at the outer edges (against the composite skins) which is going to help with its compressive resistance, adhesion and durability?

I'm going to lay up a very light fuse and make my lightest Scratcho yet, with this wing and the PU tails. So only time will tell, but I wouldn't reject the closed expansion idea yet ... this wing has exceeded my expectations and the method is worthy of further investigation!

PS we need to research whether slower reacting PU foams are available ... the ultra-short working time is currently the one thing that makes the method tricky.
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Old Aug 31, 2012, 08:45 PM
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Australia, NSW, Bellingen
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Good method!

[QUOTE=AvB;22606828]................... I hot wire cut accurate half-cores, painted thin resin on the cured skins and vac bagged the half-cores down, then hot-wire cut the excess off and joined them. If you're prepared to spend a few extra nights, it's got to be the gold standard method ...............QUOTE]

This method does read really well Andrew! Thanks for posting the details, I'll be trying it out sometime soon.
I suppose you can choose wether or not to add an additional shear web for high stress (DS) flying, or to simply rely on the foam cores to act as the "full chord" shear web as you mention for a lighter duty front side plane.
Is it better to use uni-carbon strips as the spar caps with this method over carbon tow? I'm thinking it would lay flatter and so not interfere with the foam/skin fit so much.

Jim.
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Old Aug 31, 2012, 08:57 PM
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Hey Andrew, I will try a lighter layup next as I too want a tough, light alround sloper and already have enough heavy planes that never get flown! I also like the idea of a closed mould foam fill but the fill both halves then cut it off way seems like a good compromise- I never expected it to come out so good. One thing I must say is that Ampreg wings must be unbelievable if the PU does such a good job!
I couldnt find an expansion ratio for my foam either so to do a closed mould job would take a lot of experimentation I reckon.
I think for my first full wing lay up, I'll do a simple single layer 6oz job but with a very wide spar cap, because the foam is the shear web.
Also I was thinking that with the 2 halves method you could sandwitch the servo wires between the halves when joining and just have some blocks of blue foam stuck in for the servo holes.
Thanks again for all your experimentation- Im still kicking myself I didnt get on the bandwagon long ago!
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Old Aug 31, 2012, 09:52 PM
AvB
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Nah Josh I admire you for developing your own brews so fast with the hollow molded methods! Looks hard to me.

The Ampreg is very very very tough, but if you rely on it for the shear web, it ends up with more flex than the PU I think. But to be sure we'd have to build 2 wings with the same skins.

Jim yes we use flat ribbon style uni carbon mat for the spar caps and just cut out whatever length and width you need. I've never got into tow - I just cut strips from the mat. Unicarbon mat is the cheapest way to buy unicarbon. For mid to heavy weight wings (and fuses) I use 400gm mat. The first one I got was a pain because it really fluffed up when you wet it out and handled it. But now the one I get from CG is UTC-C400/500gsm Unitex which is shinier and flatter and nicer easier to handle - about $42/sqm. More recently I picked up a much thinner one called IM UD 146gsm which is only 300mm wide and sold by the lineal meter. It's very shiny and flat, and is apparently the fibres are about as flat and straight as you can get. I've been using it for the recent experiments making lighter parts and I used it in the PU wing. You can cut the spar caps in a "diamond" shape, full span but broad at the centre and thin at the tips. I cut one per side and overlap them at the centre. Plus another straight strip bridging the centre too.

Josh that's a great idea for the wiring and foam blocks. If it's a DS wing I would recommend cutting a trench and laying a wetted up socked spar in there. You can cut it out of blue foam and pull the sock over it. Also you should put aileron stiffeners in ... I use really thin square section rods that I cut from the RTM foam, and slide 1/4" sock over those. I've only done a couple that way so far but the results are spectacular.

P.S. another update ... I finished the hingeing, which involved digging out some of the foam to allow the control surfaces to fold into the wing ... and I have to say that the foam is much better consistency than I expected. Actually it's exactly even consistency right along. Similar density to the lighter blue foams.
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Old Aug 31, 2012, 10:42 PM
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Joined Feb 1999
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With regard to density in a closed mold situation, the back pressure will not only crush bubbles along the skin but it will condense the bubbles keeping them smaller. This increases the density. So your trailing edges are indeed much denser than the rest of the wing because it is so narrow.

The best method may actually be mixing up the batch quickly, pouring the liquid across the deeper half of the mold span-wise, and be ready to close it, clamp it, and stand it up so the liquid settles in the leading edge and rises from there towards the TE. The tips of the wing should be closed off to prevent foam from running out (if the mold was built with open tips). This usually takes one or two extra sets of hands on deck and an easy to close mold (like the hinged mold in the video).
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Old Aug 31, 2012, 10:56 PM
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emufingers's Avatar
Australia, SA, Normanville
Joined May 2009
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Quote:
Originally Posted by josh18 View Post
I couldnt find an expansion ratio for my foam either so to do a closed mould job would take a lot of experimentation I reckon.
Hi Josh if you got the PU foam that I shoiwed in my prototype mould from AMC suppies the expansion is 30 to 1 on a warm day with the right humidity. I would calculate for a 20 to 1 rise and leave the mould spaces with a feeler gauge like wyowindworks does for about 10 miniutes this way any extra foam can escape and when you clamp the mold down there will still be enough flex to compress the surfaces slightly giving more peripheral density.
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Old Aug 31, 2012, 11:32 PM
AvB
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My mold isn't airtight. If I use Ampreg, it bleeds out into the flanges. So I think the foam expanding in the wing is able to push out any air.
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Old Sep 01, 2012, 06:24 AM
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Thanks Jerry, Thats the stuff.

I prepped the moulds and layed up one half today, just with 2oz and 1x6oz glass with a very wide unicarbon spar cap (400gsm). Now I know how you make wings so quickly Andrew- This is so much easier! If it works well (I cant see why it wont) Im not going back to hollow moulded!
I wont bother with the sock shear web or stiffeners this time because I just want to see what the foam does. I reckon the wide spar caps will transfer the load really well. I bought 3 metres of it last time so I could do the spar cap in a single piece. I thought of doing it diamond shaped but I dont really care about light tips and Im lazy!
It'll be interesting to see how much foam it takes to fill it. I made a light hollow scorcher wing previously (light for me anyway) and it came to about 1kg. I'll be interested to see how close this one comes to that.
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Old Sep 01, 2012, 04:34 PM
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United States, OH, Dayton
Joined Apr 2004
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This is my favorite thread of the week! Keep up the great work guys!
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Old Sep 01, 2012, 04:50 PM
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United States, OH, Dayton
Joined Apr 2004
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So here are my Electrolyte wing molds. Each panel is 9" in span (yes its small). The molded wings are a lot of work and way overbuilt. I definitely would like to try the expanding foam technique!!! Would these make for a good practice molds since the mold is open at the roots...but could be blocked off if needed?
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Old Sep 01, 2012, 05:03 PM
AvB
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Pdawg, Troy would be the guy to advise, but from watching his video I think you'd pour then stand the mould with the tip down, and let it fill to the root. So you'd have to do it in 2 goes, since you've combined 2 panels opposite ways in one mould ... but that's not much of an issue when you're dealing with a foam that's pretty much solid in half an hour.
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