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Mar 21, 2017, 08:00 PM
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Supra Wing Build


I'm building a Supra wing using this method. Sorta....

https://www.rcgroups.com/forums/show...m-bagged-wings

I'm trying my own twist to the method since this will be a ALES plane. I'm not using a tube spar and instead built a carbon capped spar with a foam core balsa shear web and the standard joiners. I'm hoping this will save a little weight.

My question is does the bond between the spar and core cutout matter structurally. My inner engineer doesn't think it matters​ as long as there are no voids. The spar only has to transfer the load to the skins, not the foam core. I am thinking of bonding the spar to the core using gorilla glue since it expands instead of epoxy (more weight savings).

Any thoughts?

Thanks

Brian
Last edited by bennetbr; Mar 25, 2017 at 03:12 AM.
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Mar 22, 2017, 08:09 AM
Barney Fife, Vigilante
tom43004's Avatar
I think you're on the right track Brian.

Assuming you have a good skin to spar interface, you should have what you need, esp in a F5J environment.
Mar 22, 2017, 11:42 AM
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Hello,

I think the build log linked below is very informative. If you haven't looked at it yet I think it is worth your time to look at it, especially the back and forth discussion on stressed skin.
Regards,
Eusebio

https://www.rcgroups.com/forums/show...-Merle-ALES%29
Mar 22, 2017, 02:26 PM
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Thanks for the link. Actually I already built a wing using Curtis's method. I bought his video too. I decided to build another wing because I felt the stressed skin is needlessly heavy and I prefer a spar. His design is fast and easy to build. I'm certainly not discrediting his design it works great. I felt like challenging myself to build a lighter and stronger​ wing using a little different approach. Maybe I'm a glutton for punishment.

With that said I started building spars. I used a 3/4" aluminum channel and unidirectional CF to build them. This was much more cost effective than precured laminate. Since I'm using 25 psi foam I made the shear web out of foam with 1/16 vertical grain balsa on each side. This was because the foam doesnt have enough compressive strength. So far the center spar with joiners is only 5oz. After wrapping it should be close to Drela's weights.



Brian
Last edited by bennetbr; Mar 29, 2017 at 12:08 AM.
Mar 23, 2017, 10:58 AM
Themadartist
Brian,

My experience is that when I join panels with 3M77 or the like, those joints fill with resin under vacuum if there's any space at all for it to get in.

I suggest that similarly, your spar arrangement will do the same, so the initial bond to the core doesn't need to be "bullet-proof".

Just my 2C.

Cheers, Steve.
Mar 23, 2017, 11:47 AM
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Thread OP
Thanks. I've noticed that too. I think I'm going to make a few trial run pieces. Drela says the bond between the front and back sections of core are not critical to the spar assembly. I interpret that to mean the bond between my cutout and spar isn't critical either.

My idea is to make test three sections using gorilla glue, epoxy, and low expanding window spray foam. The goal is to see what is easiest and lightest. I'm interested in how the spray foam works since it seems lite and easy to trim after it cures.

Here's a couple of progress pictures.

Brian


Quote:
Originally Posted by Guy97
Brian,

My experience is that when I join panels with 3M77 or the like, those joints fill with resin under vacuum if there's any space at all for it to get in.

I suggest that similarly, your spar arrangement will do the same, so the initial bond to the core doesn't need to be "bullet-proof".

Just my 2C.

Cheers, Steve.
Mar 24, 2017, 02:39 AM
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Thread OP

Update


I guess this thread is turning into a mini build thread (anyone know how to change the thread name?). Going back a few months I previously built a stressed skin Supra wing. I was not very happy with the final product and felt I could do better. It came out heavier than I was expecting and seemed like it was unnecessarily heavy with all the unidirectional cloth. Nothing wrong with the design it flew fine. Given my personality that I can't leave anything alone (I'm an engineer in real life, it's a disease ) I figured I would try a different route. So here we go.....

A few days ago I made top and bottom spar caps from 9oz unidirectional cloth from Soller. Five layers of cloth were laminated together to get 0.070" at the root and .014 at the joiner. I used aluminum channels and square steel tubing as a press to layup the wet cloth. Both spars we heat cured for strength.

The spar layup was calculated using this spreadsheet.

https://www.rcgroups.com/forums/show...up-spreadsheet

Since this is a ALES/F5J plane the spar can be lighter (less strong) than the original Supra spar. The wing will not be winch launched. It's likely the spar core will fail before the carbon does. Either way if it breaks on a ALES plane we probably have bigger problems.

After the spars were finished I built another jig using aluminum angle and square steel tubing. I waxed everything up so epoxy doesn't stick. To build the shear blocking I made a "balsa foam plywood". 1/16th balsa vertical grain was attached to the foam core on both sides with 3M77. I made a few 12" x 2" sheets. The foam core was precut on my table saw to the correct width (3/4") to match the spar caps. I carefully measured the slot I cut in my cores and measured the cap thickness, after some mathematical wizardry I figured out how tall the shear web should be at any given point along the spar length. All the shear webs were cut on my table saw using a sliding table. This worked out really well and it was easy to achieve square accurate cuts. The spar core and joiners basically follow the Supra plans. Since I'm using 25 psi foam for the core I had to add the balsa lamination on both sides to resist compression. It may not be needed, but it's light and gives me piece of mind. Prior to bonding everything together the spar weighed 120 g. Hopefully the fiberglass wrap doesn't add a bunch of weight.

I've included a few pictures of the jigs and spar layup. My goal here is to build a spar that is relatively easy to build, lite, and strong. And can be dropped into the cores via the slot I cut without disturbing the top surface of the airfoil. I'm not real good with reshaping the top surface, if I had cut the cores apart like the Supra plans it would be hard for me to reshape the profile. This hobby is all about experimentation right?

The spar is curing tonight, tomorrow it comes out of the press and will need some sanding before getting a fiberglass wrap. I will wait a few days before bonding it into the core slot. I want to try a few different methods of gluing it in first. The bond between the wing core and the spar is not critical. I don't think epoxy is required, plus it's heavy. I'm going to try using Gorilla Glue and Great Stuff low expansion insulating foam (the sticky messy stuff that comes out of a pressurized can). I have not idea how well these will work. I hope it works well because the foams are easy to trim to flush to the bottom of the wing and can be sanded. Plus if it fills in any voids the spar is basically part of the wing not just bonded to it.

Brian
Mar 25, 2017, 03:12 AM
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Took the spar out of the bag this afternoon. After a little sanding to clean it up and round the edges of the carbon it was ready for the fiberglass wrap. As per the plans the spar got 2 layers of 1.7 oz fiberglass and 3 over the joiners. Kevlar tow was wrapped over the joiners and center joint for extra security. The weight before epoxy was 150g, on track to meet the specs. The spar got a good coat of epoxy and I used toilet paper to remove all the excess resin. It went bag in the bag for another night to cure.
Last edited by bennetbr; Mar 25, 2017 at 03:18 AM.
Mar 25, 2017, 05:57 PM
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heliwill10's Avatar
I've built about a dozen wings with Anker's design you showed in the first post. They work great.
Mar 25, 2017, 06:04 PM
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https://www.rcgroups.com/forums/show...r-J-Foam-Build

Some ideas in there. You can see how I had some problems when I went to the IM7 uni. I use tow now, but I might just go back to the T700.
Mar 25, 2017, 06:49 PM
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I did read your build thread. That was sorta my inspiration to build a new wing like this. It seemed like your spar issues were from the binder causing a stress point. I don't plan to winch launch so I don't think I will come close to breaking the spar.

I noticed you used foaming epoxy to glue the spar into the core. What's your thoughts on using gorilla glue instead?

Thanks

Brian




Quote:
Originally Posted by heliwill10
https://www.rcgroups.com/forums/show...r-J-Foam-Build

Some ideas in there. You can see how I had some problems when I went to the IM7 uni. I use tow now, but I might just go back to the T700.
Mar 25, 2017, 07:01 PM
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heliwill10's Avatar
I think that would work fine!
Mar 25, 2017, 07:33 PM
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I'm a little worried about controlling the amount of foaming with gorilla glue. I've used it before and it seems like it can get out of control real fast.


Quote:
Originally Posted by heliwill10
I think that would work fine!
Mar 25, 2017, 07:35 PM
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heliwill10's Avatar
Yup, I agree. Just glue it in with expoxy then. It's not a super critical step.
Mar 25, 2017, 09:46 PM
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Spar Finished


Finished the spar today. Weight without joiners is 176g. Should be right at spec with using brass tubes instead of kevlar tubes.

I did some test section using gorilla glue. From right to left. Glue with no water. Middle; glue with just wiping the foam with a wet paper towel. Left; mixing glue and water in a cup and applying it to the slot. The picture was only taken 20 minutes after gluing them. I haven't decided yet if I'm going to use Gorilla Glue. It might be a "gorilla" to deal with if it gets out of control.

Has anyone used gorilla glue inside a vacuum bag? Does it cure inside the bag?

I want to put the whole wing inside the bag when gluing the spar in to maintain the airfoil shape. This is to the keep glue from expanding and creating bumps on top surface of the foam cutout.


Brian


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