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Foam cored wings (another way)

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mopman365
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Foam cored wings (another way)

Postby mopman365 » Fri Mar 09, 2018 9:58 am

First off, thanks to everyone posting their tips and processes on this forum. I’m learning a lot.

Here’s one way to build foam cored wings that works for me. I’ve only built about 5 of these (2 backs and 3 fronts), so definitely no expert. This is what works for me with my limited skills. What I tried to achieve is a relatively quick process with minimal waste to produce strong wings with a fairly accurate profile.

It’s essentially a high density foam core (Corecell or crossed-linked PVC) that’s shaped by hand or CNC, one-shot vacuum bagged with epoxy carbon skins and then finished with either a balloon mix or 4oz glass gloss coat.

The one disadvantage of this method, that I believe I’ve managed to overcome, is the flush-out (is this the correct term?) on the leading edge where the top and bottom skins join. It messes up your LE profile and you end up having to reshape it with lots of filler and sanding afterwards. Can’t sand the joint down too much without weakening the LE either.

Advantages are as mentioned:
Relatively quick and only one trip to the vacuum bag – less costly consumables.
No moulds required – at the moment, moulds are a waste of time and material for my one-off DIY wings. I don’t need the fancy anhedral curves or mirror carbon finish right now. Maybe later.
Light strong wings. Except or the finishing/sanding layer, the majority of the layup is bagged so less resin and straighter fibre matrix.

Process:
Bagged.png
Core shaped by hand or CNC. I’ve done two by hand, a plywood and Corecell core, and even though I achieve a surprisingly accurate profile, I was concerned about the effect of flat spots in hand shaping. I managed to find a guy that charges around $50 to CNC a core for me – usually from off-cut foams he has lying around. It doesn’t save me much time, because now I have to make the effort in Fusion360. The profile is imported (using RG8 - http://airfoiltools.com/airfoil/details?airfoil=rg8-il) and offset for the thickness of the skin.
DSC_2219.JPG
I prefer the higher density Corecell or cross-linked PVC foams. Had a bad experience with one wing when using lower density Corecell that crushed under the pressure of the fuselage connection and caused the wing to break. Don’t know how you guys build Styrofoam wings that don’t break…

I prep the centre section of the core where it will connect to the fuselage as follows. Drill/cut 30mm holes where the bolts will go. Close the bottom of the holes with masking tape, then fill with a few layers of carbon. This makes a strong layer to countersink for the bolt heads at the end (6). Fill the rest with balloon & microfiber resin mix. I let this lot dry, sand the top flat and then drill the hole for one (centre line) bolt. I put a little self-tapper screw in that hole - this makes it easy to find the hole after the whole wing’s been laminated. (I use this one whole to position and drill the other holes at the end.)

The top and bottom laminate on the front wings are both 2 x 300gsm UD plus 2 x 300gsm BiAx plus 4oz e-glass as the outer layer. For back wings 1 layer of each is probably enough?
DSC_1513.JPG
The trick with the LE flush is as follows: sand a 3-4mm flat strip into the top part of the core’s leading edge. When bagging the laminates onto the core and while the vacuum is increasing, pull and pinch the bag to ensure that the top and bottom laminates join at the lower (sharper) edge of this strip. Then once I have full vacuum, bend the flush-out upwards. This creates a channel as shown in the pic (3). I later fill this channel with 1 or 2 strands from the UD carbon to make it even stronger (5). This is optional if you’re using balloon mix as a finish layer as it will fill the channel anyway, but preferred when doing a gloss coat finish.

After bagging I use a metal saw and 40grit to trim back the flush-out. The leading edge I leave about 5mm (3) and the TE about 10mm (4). I also rough sand the top and bottom to get a better key to the finishing layer.
DSC_0030.JPG
For the finishing I’ve tried two different methods. Both work quite well. One is the normal thick balloon mix that is then sanded back, the other is a wet layup of 4 oz with a gloss coat. The balloon mix is a bit softer and probably best for the top of the wing that should not get scratched too much. It is lighter and better for smoothing out the surface of the laminate, but requires more sanding than the glass & hotcoat layer. Both look good though.
For the gloss-coat option, I mask the other side first (7) and try to wrap the glass around the leading edge as far a possible (8). For even stronger LE, wrapping glass on both sides is probably best.
DSC_2288.JPG
DSC_0113.JPG
DSC_0108.JPG

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Re: Foam cored wings (another way)

Postby opie » Fri Mar 09, 2018 11:59 pm

Nice write up and impressive results.

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Re: Foam cored wings (another way)

Postby TomW » Sat Mar 10, 2018 9:55 am

Really good write up. Thanks for sharing. I'm still in thinking stage of trying to do what you do: make a wing quick and easy that doesn't have to be beautiful, but work and allow some experimentation.

My thinking has been more along the lines of making " soft tooling" directly in HD foam and then using the tooling to make 1-3 wings. If your guy will mill a wing core, how about getting him to mill the mold halves...

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Re: Foam cored wings (another way)

Postby tahoedirk » Sun Mar 11, 2018 4:40 pm

looks great! nice write up too.

Now the fun part, Just add water!!!

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Re: Foam cored wings (another way)

Postby mopman365 » Mon Mar 12, 2018 6:34 am

TomW wrote:
Sat Mar 10, 2018 9:55 am
If your guy will mill a wing core, how about getting him to mill the mold halves...
I might do this to start experimenting with molds. Can't figure out how a mold process will not result in either heavy (solid core) and/or the same flush-out issue. Still trying to visualize the "easter egg" process discussed in another thread as that sounds like it would solve the LE join issue. Also, to get a light wing out of a mold, I'd still need to shape a foam core as well - a less accurate one, but using more resin/fillers. Weight is probably not an issue on smaller wings, but I'm still experimenting and making big beginner/surf wings so weight and wastage is a concern for me.
tahoedirk wrote:
Sun Mar 11, 2018 4:40 pm
Now the fun part, Just add water!!!
I did! Took out the new 1400 wing for a spin over the weekend. Strapless on a surfboard with 12m kite in 6-10 knots. I'm still learning but managed long runs up on the foil. And the wing seems to handle more speed than I feel comfortable with at the moment. And the upwind angle is ridiculous! It did take me about 2 years of building and crashing to get here, so it's incredibly satisfying to finally fly over water...

Now I need to start tuning this monster. Requires quite a bit of back foot pressure at the moment, so might need a bigger rear wing with more AOA and possibly move the rig slightly forward as well.
DSC_0106.JPG

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Re: Foam cored wings (another way)

Postby kostantin » Mon Mar 12, 2018 7:58 am

Greetings,

I would like to suggest some things you might want to make different next time.

The tube you use is a garden type with a soft inlay. If you have a regular vakuum the tube will be squeezed together an your vakuum dies. I use 6mm hydraulic tubes on a ex fridge motor.

You should cover the end of you tube with somthing, toilet paper is ok to prevent to vakuum pump to suck the resin into the gear. Should you use hydraulic tubes, wax the end of the tube or your paper will stick on the tube.

You have way to much holes in your "hole foil" This is the reason your wing looks sort of dry. 3 to 4 holes on each side made with a needle is more then enough. The holes are made to get a vakuum, but not to suck the resin out. I do not recomend to buy industial made hole foil. This is made for mold injection ( tons of holes ) but not for bag vakuum.
Any foil should work and you can use any paper made needle. Don't make the holes on wood, use foam underneath.

tks

Kosta

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Re: Foam cored wings (another way)

Postby mopman365 » Mon Mar 12, 2018 8:46 am

kostantin wrote:
Mon Mar 12, 2018 7:58 am
The tube you use is a garden type with a soft inlay. If you have a regular vakuum the tube will be squeezed together an your vakuum dies. I use 6mm hydraulic tubes on a ex fridge motor.
Thanks for the suggestions Kosta. The hose I use is very thick and stiff - not the normal garden hose. It handles the 25-30 inHg just fine. I also use an old fridge compressor hooked up to a gas cylinder that is supposed to be the resin trap. But I'm not doing infusion so never get resin near the hose - could just hook up the hose straight to the pump and save some work bench area.
kostantin wrote:
Mon Mar 12, 2018 7:58 am
You have way to much holes in your "hole foil" This is the reason your wing looks sort of dry. 3 to 4 holes on each side made with a needle is more then enough. The holes are made to get a vakuum, but not to suck the resin out. I do not recomend to buy industial made hole foil. This is made for mold injection ( tons of holes ) but not for bag vakuum.
Any foil should work and you can use any paper made needle. Don't make the holes on wood, use foam underneath.
I've used the bread-type foil with large holes many years ago when still building twintips. That caused very dry laminates. But the film I use now has very small pinholes and is made for wet layups (medium bleed A2200 release film P3). Cheap too. Only the excess resin gets sucked out and I get between 50:50 and 40:60 resin:fibre ratio by weight - thinks that's fine right? There's always some pinholes in the laminate, but that gets filled up when I do the filler- or gloss coat.
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Re: Foam cored wings (another way)

Postby kostantin » Mon Mar 12, 2018 9:00 am

Greetings,

I have been working for Schempp Hirth beginning of the 80's.and most of my life I have never made anything else, then carbon and glass. Never a hole foil with holes had been used. Have a look on the video ~ 2.17min. You see very few holes on a very large surface. All are hand made after the layup.

[youtube]https://youtu.be/I_atJwXxcrU[/youtube]

On the the tube squeeze problem. It will happen to you sooner or later. Murphies law.

tks

Kosta

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Re: Foam cored wings (another way)

Postby mopman365 » Mon Mar 12, 2018 12:06 pm

kostantin wrote:
Mon Mar 12, 2018 9:00 am
Greetings,

I have been working for Schempp Hirth beginning of the 80's.and most of my life I have never made anything else, then carbon and glass. Never a hole foil with holes had been used. Have a look on the video ~ 2.17min. You see very few holes on a very large surface. All are hand made after the layup.

[youtube]https://youtu.be/I_atJwXxcrU[/youtube]

On the the tube squeeze problem. It will happen to you sooner or later. Murphies law.

tks

Kosta
Thanks Kosta, I can see you guys are pros and probably getting perfect ratios at the layup stage already. I have considered using less/smaller holes before, but was worried about trapping air pockets between layup and film. Do you roll these out before vacuum or does the vacuum take care of trapped air under the film?

Murphie's been kind to me on the last wing. After the last vacuum top-up before going to bed I forgot to turn off the pump. Next morning the pump was still running fine, sucking massive vacuum and it didn't crush the wing. Resin must have set before I left it on.

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Re: Foam cored wings (another way)

Postby Kevin Brooker » Mon Mar 12, 2018 12:42 pm

I run the pump full on with no regulator on foam core R/C wings and have never had a crush problem. Much simpler. Most times it pulls .8 ATM.

I'll try to find my older molds and make up a pert to show the Easter egg method. There are simple ways to make up the leading edge and I'll try to find the rest of the Mylar sets for a small wing and make up a set.


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