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Old Feb 27, 2012, 10:17 PM
Detail Freak
target's Avatar
Harbor City, CA
Joined Oct 2003
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Question
Pre bent Supra spar caps?

I'm near to embark on a Supra wing build....
I have read somewhere that you can do awway with all the small pieces of .007" pre-cured CF laminates on the bend of the stock spar, if you layup your own, pre-bent.

My question is:
What material does one use, and how do you calculate the layup schedule? Do you go by thickness alone (made to specs) or does the actual material have any bearing on it?? I assume that I won't get the resin to fiber ratio ideal....

Also, should I use peel ply to get a textured surface on the spar caps? I haven't used peel ply before, but I have some in my shop, actually.

I'd REALLY like to hear from someone that has done this already with success.

Thanks.

R,
Target
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Old Feb 28, 2012, 12:17 PM
Just fly it!
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Cody, WY
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Target, I've never built a Supra but I can make a few comments.

1) The fiber to resin:resin ratio will effect the thickness if the fiber are fully saturated. I you don't add enough resin to fully saturate the the fiber the thickness will not change much but you will have an inferior product. If you are attaining full saturation then the only way to really improve the fiber:resin ratio is to press the fibers closer together leaving less space for resin between them. This WILL effect the thickness. Laminate thickness for a given amount of fiber won't change the tensile strength. It can impact the compression functionality since a thinner laminate is more prone to buckling. If the bond to the shear web is good and the spar (caps + shear web) are wrapped with bias cut glass then the buckling strength is less of an issue.

2) If the caps are going to require secondary bonding then peel-ply I a good idea. I now use straight polyester based peel-ply if secondary bonding is required. I've quite using the teflon coated blue stuff because it will yield a lower bond strength.

Adam
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Old Feb 28, 2012, 12:26 PM
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I posted a how to on this a few years back under my 'Tony D' ident. but for some reason I can not access it now.

What you want to use is uni that has the light adhesive webbing on it, looks like very small strings of glue holding the uni in train. (CST or ACP have this). Stay away from the stuf that has the heavier glue lines or any type of woven cloth when you make your caps.

On a flat surface build a simple dihedral jig using plywood or mdf that is sligtly longer than your caps, make sure it is braced well and will not sag under load.

On this surface screw on two pieces of 10mm MDF the width of the cap you want to achieve (these are the side guides for the cap layup), where they meet at the centre bend make it perfect with light weight filler or bondo. Make sure these side guides are laying perfectly flat when srewed down against the gig.

Now make make a clamp piece out of another piece of MDF 15mm (or so) thick, this piece will lay between the side guides over the uni and when pressure is applied will squeeze the uni and create a perfectly centre bent cap with the correct width for what ever spar caps you want to use.

Don't be shy with the epoxy in the initial layup on the cap strips you have cut from the uni, make sure it is well wet out and then roll the wet out strip with paper towel to remove the excess epoxy...it's better to have a small amount too much than have too little and a dry layup.

Inside the side guides you can use wax/PVA or a single layer of packaging tape but make sure you treat the sides/tops of the guides as well...same with the clamp piece.

When you have laid up your cap place a piece of either Peel Ply or plain ole roofing plastic with zillions of holes poked in it (I use a 'WoodPecker' to make these holes) and then lay a few layers of paper towel over the PP or plastic.

If you are going to bag the cap use a piece of Bagging Felt over teh whole layup area, an extra long bag and dog ear it well.

I prefer to clamp the whole thing and over the course of a half hour or so continue increasing the clamp pressure. If you have a hot box them even better, go for max clamp pressure, put it in the box at 60C for an hour or two and then 12 hours at 80C.

There are some more tips and tricks but I have to go back to work now, let me know if you want them and I'll post them later.

This is how I do it and I've never had one fail.


T.D.
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Old Feb 28, 2012, 02:56 PM
Detail Freak
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Harbor City, CA
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Sweet!
Thank you both.

R,
Target
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Old Feb 28, 2012, 05:18 PM
Composites guy
North OC, Ca.
Joined Jun 2005
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T-
Add a dry light glass(.5 oz) to the tool side to bleed the part and reduce internal porosity if you have the room.
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Old Feb 28, 2012, 05:30 PM
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USA, CA, San Diego
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Target - I'm at a similar place to you, and am trying to figure out a way to make a jig that will allow me to lay up the entire spar in a single go. I haven't gotten very far, but a jig similar to what T.D. talks about, sized to hold the whole spar instead of just the cap seems like it would work (see the picture).

If it would work, you'd get a perfectly sized Supra spar right out of the mold, and wouldn't have to do any additional fabrication of the spar. So no secondary bonding required.

Any reason why that would be a bad idea?

Chuck
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Old Feb 28, 2012, 06:04 PM
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Chuck, if you do that I would advise only using it to bring the spar to the stage where you need to add the 'glass wrap, then triple check the height of the spar in the cores and then bag or tape wrap the 'glass wrap. The wrap is very important to the performance of the spar as a 'sub system' in the total design package of the Supra.

For instance I attach my carbon spar caps to the balsa or foam core using 3M-77 spray and the getting 'glass wrap right is the only reason I can get away with doing this. It only adds one step and is no big deal and I highly recommend doing it as a seperate step in the build process.

Badger (Warren Man-Son Hing) makes his spars all in one go with the 'glass wrap included, he bags them against a metal 'L' form, I tried his method but feel I have more control over potential voids, carbon/resin ratio etc.


T.D.
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Old Feb 28, 2012, 06:09 PM
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Cody, WY
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Quote:
Originally Posted by cn0rris View Post
Target - I'm at a similar place to you, and am trying to figure out a way to make a jig that will allow me to lay up the entire spar in a single go. I haven't gotten very far, but a jig similar to what T.D. talks about, sized to hold the whole spar instead of just the cap seems like it would work (see the picture).

If it would work, you'd get a perfectly sized Supra spar right out of the mold, and wouldn't have to do any additional fabrication of the spar. So no secondary bonding required.

Any reason why that would be a bad idea?

Chuck
The hardest part will be controlling the force applied to the press. It would be very easy to crush the foam shear web.
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Old Feb 28, 2012, 06:14 PM
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Good points about the glass wrap, T.D. I imagine I could set up the mold so the dimensions are exactly right for the wing - that is part of the appeal of this approach to me.. But getting good compression on the glass wrap might be hard that way I've described.

Maybe if I made the foam core slightly oversized and compressed it to the correct dimensions. That would put some compression on the fiberglass wrap... It would make the spar mold a bit more complicated to make, but I think it could work. It's starting to look simpler to do it the way Dr. Drela specified it, though.

If I were making a bunch of these, I'd definitely figure out the mold, though.
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Old Feb 28, 2012, 06:18 PM
Detail Freak
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Harbor City, CA
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Chuck-

I mostly interested in not having to deal with bending the spar caps and adding the thin CF shims that spread the load to the bend. It just seems easier and better to me not to build in stress to the part, not to mention the builder!
I just planned to deal with the rest as stock.

R,
Target
If the foam core is 60psi, then at least by vacuum you will never have to worry about crushing it. The press is a different matter.
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Old Feb 28, 2012, 06:25 PM
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I, for one, would enjoy watching your build, Target. If you're interested in sharing it here on RC Groups, that is. I've just finished cutting my own Supra wing cores, and they've turned out pretty nicely I think. So I'm starting to get the next steps worked out. I'm a slow builder, though, and you'll probably be well ahead of me. Watching your build would help me plan mine.
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Old Feb 28, 2012, 06:40 PM
Detail Freak
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Harbor City, CA
Joined Oct 2003
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Thanks, Chuck-

I will try to remember to post the spar stuff here when I start....
I can post the rest on my Blog.

But, I am the proud owner of an "Oculus RES" kit. I won 50% off the kit at the Visalia Fall Fest last year, and see Merrill regularly, so it was an easy decision to get one. And it looks to be a real easy build (where are my covering irons again??!).
So that will be before the Supra. Supra after that one.

R,
Target
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Old Feb 28, 2012, 06:49 PM
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Sounds good - considering I started my Supra over a year ago, learning how to cut my own cores, I may still be behind you though. With two very active kids, I have to fit in my building time when I can.

I have finally figured out how to cut good-looking long-span wing cores, though. The 32" center panels forced me to re-think my wire cutting strategies because I was getting unacceptable results due to wire lag. It's easy to waste a lot of foam when you're screwing up the big spans like that. The answer for me ended up being Rene wire (thin and not a lot of stretch under heat) cinched much tighter than I had been doing, and replacing my wire cutter power supply with something with more power.
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Old Feb 28, 2012, 06:51 PM
Just fly it!
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Cody, WY
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Quote:
Originally Posted by target View Post
So that will be before the Supra. Supra after that one.
So, will that be around 2015?
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Old Feb 28, 2012, 08:13 PM
Detail Freak
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Harbor City, CA
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Adam, I'm really disappointed in your lack of faith in me.
Just because your two Stinger 45 kits still haven't seen the light of day yet.....

I'm thinking that 2013 will see Supra flight. I actually also want to make an additional V-tail version, much to the chagrin of the Supra fan purists.

R,
Target
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