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Jul 23, 2019, 05:30 AM
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JF1980's Avatar
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Glass skinning glider wing with pre-cut live hinged ailerons?


Hi,

I'm currently constructing a Blejzyk Mefisto (http://www.blejzyk.pl/en/model-mefisto-2.html) and would like to glass skin the wings and tail. This would be my first attempt at glassing a glider - I have been lurking in this forum, watching lots of videos on YouTube and reading up on vacuum bagging wings but yet to try it.

For this model the tail is a basic pre-shaped sheet balsa job. It's pre-hinged (seems like mylar). I guess for this I'd just cut the hinges and then use a tape hinge after skinning?

My main issue is the wing. It is a foam core with balsa skin and I believe glass with some carbon/kevlar reinforcement epoxied between the foam/balsa. The ailerons come live-hinged and pre-cut (aileron also shaped to slide under the lower skin).

Is it actually possible for me to vacuum bag this wing given that the aileron is already freed up, foam pushed back in to wing etc? I imagine the live hinge is not a problem as I'd just have to grind/sand through the new wing skin to free it up (thinking a straight edge and Dremel grinding disc by hand). The main issue in my mind is how to prevent the voids in and around the aileron from being filled and clogged up with resin.

Perhaps this is a non-issue and I'm over thinking it. Would love to hear peoples thoughts and advice on this.


JF.
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Jul 23, 2019, 06:31 AM
Themadartist
JF,

I don't know what others think, but if it was me I would do the glassing 'by hand' - no need to vacuum it, and then you eliminate most of the issues you are anticipating.

Most Blejzyk models have obeechi or poplar skins, not balsa, and the core is expanded polystyrene white foam. Too much vacuum will see that foam crush - so if you go that route, be very careful.

I think with a nice tight weave cloth, somewhere around the 50 to 80 gsm mark would lay down extremely well if you use a nice thin epoxy. Screeding the epoxy with a credit card or the like should see a pretty uniform result. Anyway, just my two cents worth.

Cheers, Steve.
Jul 23, 2019, 06:40 AM
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JF1980's Avatar
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Thanks Steve. I have considered this as an alternative, the issue is, to me the nice thing about a glassed wing is the perfectly smooth surface with paint applied via the mylar when bagging. I'm not sure it's possible to get this finish when glassing by hand? At least not without a lot of additional effort and perhaps weight too?

I stand corrected on the skin material

Wonder whether I could tape over the already cut aileron lines to stop the void filling with resin? Only issue that the epoxy won't bond directly to the skin in that area so the skin bonding would only be as good as the tape adhesive?

Another thought is filling the voids with expanding foam before bagging.
Last edited by JF1980; Jul 23, 2019 at 11:57 AM.
Jul 23, 2019, 04:45 PM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by JF1980
Hi,

For this model the tail is a basic pre-shaped sheet balsa job. It's pre-hinged (seems like mylar). I guess for this I'd just cut the hinges and then use a tape hinge after skinning?...............
That would work, but would not look good on a composite flying surface. Silicone hinges would be much better.

Quote:
............................. foam pushed back in to wing etc? .........................
What do you mean by this?

Quote:
The main issue in my mind is how to prevent the voids in and around the aileron from being filled and clogged up with resin.
Perhaps this is a non-issue and I'm over thinking it. ...................
It definitely is an issue! You are not over thinking it.


Quote:
Originally Posted by JF1980
.................... perhaps weight too?.................
Fibreglassing one of these model will add a surprising amount of weight, even if the lightest fabric is used. If it is intended as a slope model, this is not a problem. However, for thermal flying.........................forget it would be my suggestion.............

Quote:
Wonder whether I could tape over the already cut aileron lines to stop the void filling with resin? ................
Another thought is filling the voids with expanding foam before bagging.
No and no.

If you absolutely want to fibreglass this model, either follow Steves suggestion above or separate all control surfaces from the wings and tail.
If there are no drag spar and LE spar fitted, fit these using light timber or hard balsa.
Then vac bag all surfaces at very low vac (1/4 bar.).
Bag with 50 gsm glass fabric on the bias. (45/45), using a low viscosity aircraft grade laminating resin.
Paint the waxed mylars first.

After bagging and (preferably) post curing, run silicone hinges on all control surfaces.
Be prepared for weight increase proportional to the fabric weight used and inversely proportional to your laminating skills.

Jim.
Jul 24, 2019, 03:55 AM
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JF1980's Avatar
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Quote:
Originally Posted by Jim.Thompson
That would work, but would not look good on a composite flying surface. Silicone hinges would be much better.
Will investigate but I hate working with silicon sealant.

Quote:
Originally Posted by Jim.Thompson
What do you mean by this?
The instructions are behind developments in production (pre-cut ailerons with live hinges) - they call for cutting out the ailerons, pushing a few mm of foam back in to the wing core and the cut out aileron piece; filling this void with an epoxy/microbaloon filler, sanding these flush and finally re-attaching the aileron with clear tape. Now the ailerons come pre-cut with a live hinge. The foam does appear to be pushed in a bit but no filler has been applied. I queried this with the manufacturer due to the lack of updated instructions and he said that most people don't bother to fill now that it's pre-cut etc but personally he still likes to seal his so as to protect the foam core from moisture etc.

This is part of the void I'm concerned about filling with resin, not just the small gap between the aileron and wing.

I was thinking I could fill this completely with a microbaloon mix. Skin the wing, then just cut out the ailerons again and the cores would also be sealed too (mark mylars with original position for reference?). That's when I thought perhaps using expanding foam to fill/seal would be even easier. That would be after a test t ensure it doesn't eat the white foam.

Why would this be a bad idea?

Quote:
Originally Posted by Jim.Thompson


It definitely is an issue! You are not over thinking it.
Good, I hate that.

Quote:
Originally Posted by Jim.Thompson

Fibreglassing one of these model will add a surprising amount of weight, even if the lightest fabric is used. If it is intended as a slope model, this is not a problem. However, for thermal flying.........................forget it would be my suggestion.............

Yes this is a slope model (slope wing profile). I have plenty of thermal models (F5J, DLG, 100s etc) but find myself with nothing in my fleet that I can fly on a slope with over 15mph wind. I'll be very happy if I can fly this model in the 10-30mph wind range. I have just rolled a ballast tube which will take slugs made from 15mm copper pipe.

The lighter the better for the purposes of flying in lighter winds, but I want to make sure it's fit for stronger winds too and can take a bit of abuse slope-side.

I guessed that with 25-50g cloth I'd be looking at 120-150g weight for the covering - this a guesstimate based on the size of the wing and a little reading but no experience. I have no idea how much the suggested wood varnish finish adds for comparison.

Initially I was thinking of cheap 200g carbon (12/m2) on the top and 50g glass on the bottom but I don't think that's necessary. There is already carbon/kevlar reinforcement between the wood skin and foam core and I think the weak point will be the little carbon wing joiner. I could update that too but I think it's overkill.

With this in mind I was thinking 25-50g glass with some kevlar tape around the LE and carbon tape around the TE. I can easily source fresh 25g cloth and already have enough 48g cloth in the workshop if I want to go with that (although it's cheap hobbyking stuff with lots of creases).

I'm sure this model would be fine just varnished. I want to use it to gain some experience with composite techniques, no expectation of perfection or it being as light as possible. If it goes badly wrong I can always buy a new wing or just another complete kit and have a spare fuse - they're fairly cheap which is why I selected it as a base to experiment with.
Jul 24, 2019, 06:28 AM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by JF1980
Will investigate but I hate working with silicon sealant............
My video here:
(8 min 19 sec)


Quote:
Why would this be a bad idea?.....................
It is sticky and uncontrollable. Foams at unpredictable rates. Clean up is difficult.
Then of course you have to remove it after................good luck there!
If you don't believe, do a small test on some scrap. It will take you 2 minutes to see what I mean.
Jul 25, 2019, 06:13 AM
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JF1980's Avatar
Thread OP
Quote:
Originally Posted by Jim.Thompson

It is sticky and uncontrollable. Foams at unpredictable rates. Clean up is difficult.
Then of course you have to remove it after................good luck there!
If you don't believe, do a small test on some scrap. It will take you 2 minutes to see what I mean.

I'm not sure why it would be any tougher to remove than any other foam.
Jul 25, 2019, 07:28 AM
Entropy is happening!
Jim.Thompson's Avatar
Quote:
Originally Posted by JF1980
I'm not sure why it would be any tougher to remove than any other foam.
Uuhh, ok!
Jul 25, 2019, 07:31 AM
Themadartist
Quote:
Originally Posted by JF1980
I'm not sure why it would be any tougher to remove than any other foam.
I could be wrong, but I think that expanding foam in a can is polyurethane based. That's why it can be sticky and difficult to remove. It doesn't just expand, it adheres too.
Jul 25, 2019, 11:20 AM
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JF1980's Avatar
Thread OP
I see. I was thinking more of removing whatever I need to after it has set.

Perhaps it'll be fine if I just fill the small gaps (where the aileron has been cut out) with lightweight model filler and then bag it? Once it set I should be able to see the previous cut-out line and just cut along it to re-free the aileron.

I know it's not good form but wouldn't that work? Where would it break down exactly? I have not worked with a vacuum before so genuinely looking to understand the issue and possible hacks to get around it when in a pinch.
Jul 27, 2019, 07:45 AM
supreme being of leisure
ZAGNUT's Avatar
if you've never bagged wings before then don't start with these! a regular lamination with fill and paint will be much safer if you're new to all this. with the appropriate cloth and resin you can get very near what you would with bagging although the fill, prime and paint will always be heavier than painted mylars.

the big thing is the appropriate cloth! weight is only a small part of this. many cloths use 2 yarns twisted togeher to form a thicker yarn. this makes the cloth thicker and the weave more "open"...for a given cloth weight this means more resin uptake and a thicker, heavier laminate. even with vacuum these cloths will not flatten out. very common in surfboards as it gives a stiffer skin and allows a bit more sanding before ruining the integrity of the cloth.

an example of a good cloth: https://www.thayercraft.com/2.47-oz-2313-silane.html notice that the yarns are listed as 1/0, this means a single yarn is used. another is https://www.thayercraft.com/1.6-oz-1280.html

yarns defined as 1/2 means two yarns twisted together.

another good direction is satin weaves. usually very tightly woven and hard to wet out with anything but very thin resin but give a far superior glass to resin ratio. less crimp than regular weaves also adds strength....and the finer finish requires less filling. https://www.thayercraft.com/volan-finish.html

oh, to convert oz. per yard to metric just multiply by 34.
Jul 27, 2019, 10:52 AM
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JF1980's Avatar
Thread OP
Thank you. That's interesting info on the cloth. On the sites I'm browsing for cloths I see 2/2 twill and plain weave as the main fabrics. I can easily and cheaply get 25g and 48g plain weave cloth. 100g 2/2 twill is very cheap but it sounds like that will require a lot of sanding and hold a lot more resin.

So if laying it up by hand:

Cut lay over the cloth
Wet out with resin and squeegie/roll it out to saturate the fabric
Remove excess resin using kitchen tissue and a roller
Let it dry, trim, flip over and do the other side
Primer, sand, paint.

Does that sound correct? Should I add carbon ribbon to the TE and kevlar to the LE and let it dry before I skin it or lay that up at the same time as the skin?

It will be disappointing not to bag it but perhaps that will have to wait until I have my 4-axis CNC setup at which point I can cut and bag my own wings and practise with cheap 100g cloth.
Jul 27, 2019, 11:42 AM
supreme being of leisure
ZAGNUT's Avatar
the "2/2" defining a twill weave has nothing to do with the yarn definitions i mentioned...twill is great and usually uses flat yarns. look at the cloth closely, twisted yarns with big gaps=bad, flat yarns with little to no gaps between them=good.

anytime you let the resin cure you will need to sand if you want another layer to bond to it...so i would put the TE reinforcement in place and let that stiffen up a bit but not fully cure and then do the LE reinforcement and main glass all in one shot. i would wrap the wing in one piece from the TE top around the LE and back to the TE bottom...one less overlap to deal with. i wouldn't try to wrap around the TE at all, PITA, but would run both ends "wild" and if needed (like if the TE is really thick)add a bit of thickened resin and maybe some carbon tow in the gap. after cure trim back the glass.

that's my way, every other guy will have a different way....and every way is the right way. with a bit of experience you will see what works for you and what doesn't. that pretty much sums up composites IMO.
Jul 27, 2019, 12:05 PM
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JF1980's Avatar
Thread OP
Ok that's good to know too. I'll be buying cloth online so can only go by the description and photos so I guess some of it will be trial and error.

One thing that made me think that bagging would be the only way to get a nice finish is that with such a thin cloth I'm not sure it could be sanded smooth before painting without going through the cloth, so it would take a lot of primer to fill out and then very careful sanding to get an acceptable finish?

Is it possible to lay on and roll out some sort of release film that is left in place while drying? That might leave a smooth enough surface to avoid the need for filling/sanding too much before paint or even remove the need for paint if I go with the natural look or dye in the resin?

I think you are right regards experience. I think perhaps I should play around with skinning a few pieces of scrap balsa before deciding exactly how I'm going to finish the wing.
Jul 29, 2019, 04:19 AM
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T.D.'s Avatar
Jim gave you good advice on the expanding foam, you are asking for trouble if you use it for your application. I have laid glass up over poplar, obechi and okoume (the poplar wings had a live hinge) and what worked best for me was to cut the cloth (I used 47.5 gsm = 1.4oz sq yd.) on a + - 45 degree bias in two pieces slightly bigger than the wing surface. Wrap a bias piece of cloth around the leading edge (you do not need to use Kevlar) using a very light mist of 3M77 to attach the LE wrap then wet the LE cloth out and cut the wing surface cloth while the LE greens up.

The Mefisto has a two piece wing so you can insert the wing rod and lightly clamp it in a bench vise which allows you to rotate the wing panel while glassing it which makes things a lot easier. So you have your green LE, the wing clamped in the vise and now rotate the panel so the LE is up, carefully lay the leading edge (the front) of the main wing wing surface glass cloth onto the wet out green LE and you want the front of the main wing surface cloth to be stepped back a few milimetres from the point^ of the wet out LE across the first taper of the semi span...be careful as when the main glass contacts the wet out LE it will stick, it helps to have an extra set of hands for this but on a two metre wing like you have it should be no problem. The reason you cut the main surface glass over size is because you want to lay it in a straight line across the semi span so you will have excess glass hanging over the outer tapers of the semi span that you will trim off later.

Use your hand to smooth the main wing surface glass back to the trailing edge, fold the glass over the TE and use a few pieces of masking tape to tape the cloth to the bottom surface of the panel. To keep epoxy out of the live hinge you can use a grease pencil and run it over the glass a few times at the hinge line, this will prevent epoxy from entering the glass and mucking up the live hinge. You are working on a sloper so weight is not a major concern but it is important to keep the control surfaces as light as possible so do not go crazy with the epoxy or extra reinforcements back there. BTW, if you are using a thick epoxy you can add denatured alcohol to a total of one percent of the epoxy and this will make it flow with no ill effects on the lay up.

I forgot to mention that you want to do each upper-lower surface of each panel separately so after the epoxy is cured you can take a sanding block with 200 grit and run it lightly front to back on the trailing edge while holding the block at a 45 degree angle, this will make the glass that you taped to the opposite surface fall off very nicely and this will give you near perfect trailing edges with no exposed wood.
Last edited by T.D.; Jul 29, 2019 at 01:58 PM. Reason: Clarification


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