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Jun 08, 2019, 07:54 AM
Registered User
Thread OP
Discussion

Multi-core wings - core joining methodology


I'm in the process of making F5j wings - a little simplified version of GT's SynerJ wings. I simplified the tips to better suit my modest capabilities, making the wing from 6 panels and not the original 8. I'm gradually working through the CNC kerf size, cut speeds, temps and No. of points to get an acceptable result. With the making of a 400mm bow tonight I got the tips to close to where I want them to be.
The question I have is in the strategy in joining all of the panels to make a coherent whole.
I assume that many use 3M77 to glue the panels/shucks together, but do people cut a little oversize and then trim down? Or do you block out with a hot wire to accurate size and join them as cut?
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Jun 08, 2019, 08:27 AM
The Junk Man
Personally, I cut to exact size, then use spray adhesive (a la the Phil Barnes method) to assemble the cores.

And I learned a long time ago to use short panels. Long dropwire cut wing cores show excessive wire drag that results in cupping and other nasty stuff happening to the cores. If you are using CNC cut cores where the wire never actually touches the foam, then cut them as wide as you like.

Tom
Jun 09, 2019, 12:07 AM
Registered User
Thread OP
Thanks Tom. I have Phil's videos and they are great.
I can currently cut 1m cores accurately, but none of the current projects are more than 600mm. None of the bagged panels will be longer than 1m for this plane.
Jun 09, 2019, 12:23 PM
Registered User
I used to use 3M77 as Phil suggests. However, at $23/can for the classic 3M77 (cyclohexane based) only available from ULine with a 2 can minimum, I’ve now gone over to aliphatic glue.

Woodworkers glue takes longer to dry so plan accordingly. However, there is no incompatibility with the foam (acetone based 3M77 eats foam when sprayed directly on the foam) and it is much neater as the 3M77 will have overspray.

I still use 3M77 for other things such as applying the kevlar leading edge material; but that works because the acetone has flashed off by the time I apply the kevlar.

Oh - I cut the cores exactly, but verify they are aligned before gluing. Sometimes there will need to be a bit of sanding.

-Keith
Jun 09, 2019, 02:58 PM
It's time for me to fly
JimZinVT's Avatar
Are we talking about joining the cores end-to-end? I tape the beds together with packing tape and join the cores in the beds with 5 min. epoxy.

If the cores have a lot of taper the small end may end up slightly undersized. The wire is moving slower at that end so a bit more foam is melted.
Jun 09, 2019, 03:30 PM
The Junk Man
Quote:
Originally Posted by kablair
I used to use 3M77 as Phil suggests. However, at $23/can for the classic 3M77 (cyclohexane based) only available from ULine with a 2 can minimum, I’ve now gone over to aliphatic glue.
There is no need at all for the high priced 3M stuff. I use the 3 buck a can spray adhesive (Duro) from Wally World. What melts the foam is not the adhesive, but the spray propellant. I use Phil's method, but keep the spray back from the foam so that the propellant evaporates and never puddles on the foam itself.

Put on a few light coats, keeping the can well back, and Bob's yer Uncle.

Tom
Jun 09, 2019, 03:54 PM
Registered User
Quote:
Originally Posted by T_om
Put on a few light coats, keeping the can well back, and Bob's yer Uncle.
Yup - that works, and I did that many times. Holding it further back gives the acetone time to flash off.

However, at least for me, I find that the further I hold it back, the more that gets in the air and on other things. When I use spray glues, I prefer to hold it as close to the surface as possible to avoid all that mess.

You're also right that the acetone (or cyclohexane) are not the glue and I did not mean to imply that they are; they are the solvents. I'd be surprised if they are propellants though, butane is commonly used as the propellant due to it's vapor pressure.

As with everything, there are many, many ways to do the same thing - just with varying 'side effects' to which each of have a different willingness to accept.
-Keith
Jun 09, 2019, 04:39 PM
The Junk Man
Yeah, I think you are correct on the propellant vs. solvents... I do know that keeping the surface from getting a "wet" appearance keeps the foam from melting.

And I always spray outside because you are also correct about the stuff getting on everything. In Phil's videos I was astounded to see him nonchalantly spraying the stuff everywhere.

Tom
Jun 12, 2019, 05:33 PM
Registered User
I do not use 5-minute epoxy because it crates a hard lin at the joint that does not sand well. If yr is really careful and wipes the surfaces clean after joining, it can work, but if you want to do any blending of the foam joint, it does not sand out s nice as when using a wood glue or 3M77 black can.

Tom f.
Jun 12, 2019, 06:28 PM
It's time for me to fly
JimZinVT's Avatar
You're right Tom, you do have to be careful about wiping off any squeeze-out when using epoxy ( or PU )....sanding either is a bich. I use it very sparingly so usually no squeeze-out. It's not a structural joint, the composite skin/spar is where the strength is, so no need to go crazy with the glue. If there is squeeze-out, get rid of it before it sets hard.
I wouldn't use PU for a bagged wing, but it's great on EPP/EPO where the foam is providing part of the strength
Jun 13, 2019, 04:42 AM
Registered User
Thread OP
some of the tip cores - the 'sharpish LE on the small core is due to the X-cut used at the LE. This means the wire does not go around the LE in one smooth movement. I find that the LE is too small with the tip taper.
The bow is 400mm.
600 points top/bottom surface
.25mm NiChrome wire
8.4v
1.3amp
cut 1mm/sec
Jun 13, 2019, 05:06 AM
Themadartist

Thanks ...


Thanks for all the input guys, I've found this thread very interesting.

I've never thought about using aliphatic glue but I am going to give it a go. You only have to experience a core separating (once you've wet out the hinge and the leading edge) to know that one time is too many times. I typically use a spray adhesive but really get frustrated when they don't 'grab' as they should.

Thanks again, and happy building, Steve.
Jun 13, 2019, 09:03 AM
Registered User
Quote:
Originally Posted by Guy97
Thanks for all the input guys, I've found this thread very interesting.

I've never thought about using aliphatic glue but I am going to give it a go. You only have to experience a core separating (once you've wet out the hinge and the leading edge) to know that one time is too many times. I typically use a spray adhesive but really get frustrated when they don't 'grab' as they should.

Thanks again, and happy building, Steve.
3M77 is a contact cement. Both sides get a coat then It has to dry for 10 minutes before putting the two sides together. However, when I use it to hold down fabric on the wing core, I let it dry for not longer than a minute so I can reposition the cloth as needed. This is especially helpful when putting a bias strip of Kevlar on the LE so that you do not have any wrinkles.

Tom f.
Jun 13, 2019, 10:15 PM
Registered User
RaceMag those cores look excellent. The leading edge is easily sanded to the finished profile. I zoomed in on the surface and they a very smooth.
Jul 12, 2019, 08:00 AM
Barney Fife, Vigilante
tom43004's Avatar
My methodology is always to cut cores a little oversize, even with CNC... trim them to be exact matches, and use standard 3m77 sprayed from about 1 foot away. I used to use titebond to connect the cores together but sometimes you have to lightly sand a hardened ridge down. I have also used PU with a similar issue, thought PU is a little harder to sand because it generally gets a little more proud of the foam surface.

There are lots of ways to skin the wing.


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