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Fiberglass - Multiple Layer Question

mfleming

Well Known Member
Patron
Scarf%20Extension.jpg


I'm extending the edge of my cowling using the method Dan Horton posted.

Question
  • When laying up multiple layers, do I have to wait for each layer to get tacky before applying the next layer or can I apply multiple layers at once?
  • Can I stack all 5 layers on a plastic sheet, apply epoxy, cover with another sheet and squeegee, then apply all 5 layers at once.

I have a small area to do and it seems like a big waste of epoxy to mix up 5 or 6 batches of epoxy for the 5 or 6 individual layers.
 
When I did the 10 layers to make the fairing at the edge of the windscreen on the RV-10, I just soaked each piece on a piece of clean wood, lifted with a spatula, and applied.

I defer to the materials scientists on this website, but adding layers independently seems like it would leave boundaries / weak spots in the mix. Essentially it would be like a sandwich, where your bread, lettuce, meat, cheese, and bread are all together, but are not physically bonded.

The more the epoxy can saturate in one consistent pour, the more it will be part of the same cured object.

That being said, trying to pour through five layers will be hard. The bottom layers may not saturate. My advice based on my extraordinarily extensive personal experience of doing this a couple of times for my build is to put down a layer or two of cloth, saturate, apply more cloth, saturate, cloth, etc.... so you get one wet, soggy 5 layer mass all at once.
 
All at once is fine.

Depending on the kind of fabric you are using, be aware of the orientation of the strands.
 
Was just about to add the same as the above poster.

If you can see a pattern in the weave of your fabric, trying to turn each layer 90 degrees from the prior layer will make the whole thing stronger.
 
If you can see a pattern in the weave of your fabric, trying to turn each layer 90 degrees from the prior layer will make the whole thing stronger.

This is for UNI, BID should be turned 45*.

There are enough different weaves that it is hard to make a blanket statement about the orientation.
 
Multi layer

Dan will probably correct all of us but here's my input.:D
I've done it both ways and prefer wetting each layer before stacking. It takes forever to wet 5 layers. Plus I used a strip of carbon along the edge. It was wet before laying a wet glass layer over to encapsulate it.
I was told 45 degrees offset yields maximum strength. I haven't tried it.
 
I just did something similar the other day.. I cut one large 12x 12 piece of cloth, lay it on plastic, saturate with resin, lay another piece on top, add a little more resin. The first piece that is saturated will help soak into the second. Now add another plastic sheet and move the resin around with a hotel key card like a squeegee. Any excess resin, you can drag out of the layup into the plastic on the side. Using a straight edge and rotary cutter, you can cut any size pieces and make 2 inch wide “tapes” if that’s what will work for you.
You can also make the layup 5 plies thick and orient the weave in the plastic. You can add layers while wet. It’s actually fun to do when you use the plastic method!
 
BID or UNI, +/- 45 degrees is best for shear strength but not tensile strength. 0/90 is best for that. For UNI, orient the fibers in the direction that you need strength and stiffness, which might be +/- 45 or it might be aligned with a particular direction.

And I saturate the layers one by one as I stack them, and try to do the whole layup all at once.

Dave
Old retired aerospace stress engineer
RV-3B, about to hang the engine
 
There is no practical use for unidirection fabric in an RV shop.

Mostly you want ordinary 9 oz 7500, or 7781 8-harness satin for complex shapes.

Not everyone like 7781 because the tight weave makes it slow to wet out, and it traps air bubbles if you're not careful....but it will wrap around the craziest things. 7500 wets out fast. If you're having trouble wetting out either fabric when stacked, your shop and table are probably too cold. The standard is about 75F.

Ok, see posts 67 and 68 here:
https://vansairforce.net/community/showthread.php?t=22931&page=7

Below, an example of a good use for 7781, an oil cooler inlet diffuser. One ply was wrapped and trimmed, then used as a pattern to cut three more. They were all stacked together on 4 mil plastic and wetted at the same time. Then the entire stack was draped on the foam form and smoothed around it with gloved hands, as one piece. The lap seam is on the small radius. A few strips of tape finished out the flange. A "bandage" of peel ply kept the wet glass tight on the neck.
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For better strength at the joint I would taper the ply drop offs on the backside with first ply the longest and each ply after being 1/4” shorter with last ply still extended past the edge of the existing part. I would not sand the backside flush to make it flush with existing part backside but leave the tapered plies. A bit of sanding to make the ramp smooth would be ok. (If you really want it to be done per the strength book, the last ply would be the longest and run down the ramp made by the previous plies)
I would definitely do this tapered backside if the cowl extension included a fastener or other attachment load introduction point anywhere on the new extension added.
 
Phillip makes a good point. When I drew the original illustration above, I was mostly trying to communicate the need for a scarfed repair. As drawn, the extension is too long. Although the single-sided scarf is entirely suitable for the typical "I cut the cowl too short" extension (meaning less than 1/4"), longer extensions may need a double sided scarf. Obviously a lot depends on the anticipated loading. Good news is we're not splicing wing spars, not in the RV world.

I've posted double-sided methods over the years. Somebody tell me the thread name where I put the original and I'll add the others, re-posted below.
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There is no practical use for unidirection fabric in an RV shop....

There is one: if you're using discrete fasteners for your cowling like Skybolts, then a thin layer of UNI on the inside and the outside of the cowl in that area, with the fibers running parallel to the edge, will help prevent bulging between the fasteners. Best to use carbon for the UNI, since it's several times stiffer than glass, and then use a very thin layer of glass (any kind) over the carbon, especially on the inside, to guard against carbon/aluminum corrosion.

Dave
 
Is a Flox Extension Still Valid

This is a great discussion. My test article failed miserably yesterday but it was probably due to the scarf not being just right.

I have a question about this statement from Dan's post. Is this still considered a valid way to make a small extension of the cowl? "Cured flox is quite tough, which is why, with an appropriate scarf, you can use it alone for a short extension. Same procedure as above."


Here's Dan's original post using the drawing in this threads post #1

As an aside, I'm going to use several different methods (layup as well as flox) on test articles today. I've put together two hot boxes for post curing. One is a airline kennel with a heater and temp controller for the small stuff. The other is a 4'x4' foam/foil box for the large pieces.
 

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Scarf

This is a great discussion. My test article failed miserably yesterday but it was probably due to the scarf not being just right.

I have a question about this statement from Dan's post. Is this still considered a valid way to make a small extension of the cowl? "Cured flox is quite tough, which is why, with an appropriate scarf, you can use it alone for a short extension. Same procedure as above."

...

Michael
What or how did it fail?
I had to add to one side. My scarf on the inside was maybe 1" wide. I knew it would be cut so I only added a couple layers. No reason to layup five layers and cut off a bunch. I used a piece of aluminum covered with tape on the outside as a mold. After final trimming, the added area was reinforced with layers of glass and carbon to the thickness of the rest of the cowl.
I cure a few hours in the shop then move parts inside. Very understanding wife.
Dan is correct. Flox is hard as rock. No idea how far it will span. The trick will be molding. I do use it to contour areas by applying it before the layup. It will squeegee to shape under the layup with peel ply. That way the top layer sands evenly.
 
There is one: if you're using discrete fasteners for your cowling like Skybolts, then a thin layer of UNI on the inside and the outside of the cowl in that area, with the fibers running parallel to the edge, will help prevent bulging between the fasteners. Best to use carbon for the UNI, since it's several times stiffer than glass, and then use a very thin layer of glass (any kind) over the carbon, especially on the inside, to guard against carbon/aluminum corrosion.

Great idea!

As an aside, I'm going to use several different methods (layup as well as flox) on test articles today. I've put together two hot boxes for post curing.

Good man...test articles are always educational.

Hard to imagine a need to post cure a little bitty cowl edge extension. Cold shop, needed to get cure?
 
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Great idea!



Good man...test articles are always educational.

Hard to imagine a need to post cure a little bitty cowl edge extension. Cold shop, needed to get cure?

The hot boxes are just to speed up production as the shop is about 60°F, plus I'm impatient ;)
 
This is a great discussion. My test article failed miserably yesterday but it was probably due to the scarf not being just right.

If memory serves, a 12:1 slope is the desired scarf angle (wood), but glass is more "forgiving" so less of an angle can still be "OK". So "... not being just right" would have to be something along the lines of 30 degrees or something way too shallow like that. If the scarf has plenty of slope, then the failure was probably due to lack of adhesion due to contamination, improper cure or incompatible materials. The scarf/slope has to be CLEAN to insure good adhesion.

PS - 12:1 equates to 1/8" thickness requiring a 1.5" slope. :)
 
There's a great thread about fiberglassing a rudder that really showed me I was not making the scarf correctly.

My scarf is the dinky one and the rudder scarf is from Sean Blair's post.
 

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Angle grinder

There's a great thread about fiberglassing a rudder that really showed me I was not making the scarf correctly.

My scarf is the dinky one and the rudder scarf is from Sean Blair's post.

I use an angle grinder with a paddle sanding disk. One or two passes and scarf is done.
 
The scarf joint should be much wider. AC43.13-1B says 50x the thickness of the material for some structural repairs on composites. It certainly wouldn't hurt to use several orientations of the material (bias cut and straight cuts of the cloth). The strength in the fabric is along the warp and weave, so doing a layer or two on the 45 degree will add more general strength in that direction.

You can certainly lay it all up in one go. Wet out the fiberglass layers between sheets of plastic, then roll or squegee out all of the excess resin. Cut the strips you need (rotary blade cutters work great), peel off the top layer of plastic, then apply to the area, remove the other plastic layer and stipple with a brush to work out any bubbles. If you have to do the layers separately curing each, you must either scuff sand the epoxy or lay down some peel ply to get a good enough surface to bond the next layers. Peel ply on the final layer is usually a good idea to wick up any excess epoxy and smooth the surface for final sanding.

Providing good support to the new layers is important. Gravity will want to move your new extension. Wax up a strip of aluminum for release from the epoxy. Drill some holes in the strip and you can simply cleco it along the edge of the cowling. Filling the holes later is trivial with some flox.

There are some great videos on the EAA website.
 
The hot boxes are just to speed up production as the shop is about 60°F, plus I'm impatient ;)

That's just too cold for glass work. Cure aside, the resin is too viscous to wet out well. Everything works better if the shop, all the tooling, and all the components are about 75F.
 
That's just too cold for glass work. Cure aside, the resin is too viscous to wet out well. Everything works better if the shop, all the tooling, and all the components are about 75F.

I put my resin and harder in a cardboard box. And throw in a droplight ( non led). Makes the epoxy nice and warm!! One day I’ll build a proper epoxy heating station, but probably won’t..
 
I put my resin and harder in a cardboard box. And throw in a droplight ( non led). Makes the epoxy nice and warm!! One day I’ll build a proper epoxy heating station, but probably won’t..

Here's a simple one for larger pieces built out of foil covered foam, a 1500w heater and a temp controller.

I'm using this hotbox today to cure the lower cowl training edge extensions.
 

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Michael, I’m talking about preheating the epoxy before mixing.. although heating the part is nice too.. I’ll probably hit Home Depot tomorrow for that foam to build a box for parts! I have some spare electric heaters that I couldn’t throw away.. now I have a use for them!
 
Cowl heat box

Not to be smart ..., but if you stand the cowls up with a heater inside, they firm a nice chimney and the heat goes out the top. I did it almost every day I worked them.
 
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A Success and Another Failure.

My #2 test article for extending the cowling using a layup technique worked. Having a much wider scarf made all the difference. That's going to work well for the bottom of the cowl where it needs to be extended about a 3/16".

My #1 test article using a flox mixture to extend the cowling failed...badly.
Nice wide scarf, peanut butter constancy epoxy/flox mixture, cured in heatbox at 125°F. The epoxy/flox mixture adhered to the scarf but easily broke at the edge where the scarf ended.... certainly would not hold up for normal service.
Is the epoxy/flox extension just not the way to go or maybe a different technique needed??

The area I want to use the epoxy/flow technique is the front of the cowl where the cowl curves around sharply to the inlet...That is about a ⅛"+ gap.
 
Flox

Snip...

The area I want to use the epoxy/flow technique is the front of the cowl where the cowl curves around sharply to the inlet...That is about a ⅛"+ gap.

Consider flox basically fiberglass with more epoxy. It needs a scarf area to spread the load or it fractures at the edge.
It should work fine for 1/8" gap but make sure the edge is clean and rough. A little scarf are wouldn't hurt. I've plugged quite a few holes for redrilling. They were much bigger than 1/8.
 
The epoxy/flox mixture adhered to the scarf but easily broke at the edge where the scarf ended....Is the epoxy/flox extension just not the way to go or maybe a different technique needed??

Mike, we don't really know what technique you're using. Look close at the failed bond surface with a 10x magnifier. Any shiny surface, or all obviously fractured? As noted, you're working in a cold shop, and cold epoxy doesn't wet cold surfaces well. Confirm or deny by bringing everything up to 70F or better before start.

Or, use the fabric technique which does work for you, and get 'er done.
 
Thanks Mike

Great question and thread for me as I just mis-trimmed my lower cowl side. I might add a light thin, hinge “extension doubler” strip of aluminum to beef up my edge repair, as I worry about long term strength in a heavy vibration area. To start over is $1,200 plus shipping for a new lower cowl. Almost as expensive as a new canopy.
Cal
 
Hope the experimenting is going well.
The scarfe angle should be 30:1 - much steeper that in your picture (50:1 with carbon). Rough up very well with 60 grit 2" beyond where you expect the resin will stop. I use a 60 grit angle grinder disc in a corded electric drill, as it spins more slowly than an angle grinder the dust doesn't get spread so far, but an angle grinder is fine - anything that achieves a consistent scarfe.
Wet out the cloth either on plastic of on the part. I would probably wet out some cloth, cut on the 45, 10x wider than needed, then cut into 5 strips. Paint resin on the scarfe and backing piece. Please each of the 5 strips on the job - staggering the layers will mean less to grind away. Stipple together. Make sure the lay up is 1/2" wider than needed (at least). Add tear strip over the top. Leave to cure - agree the work shop needs to be at 70F to cure in less than 2 days.
Once cured pop off the former and cut to length/width with a diamond or carbide wheel, grind the step off with the same disc used to cut the scarfe. If the layup is not left to cure long enough it will be rather rubbery and will load up the disc very quickly.
Is there anyone locally who can show you the techniques? This is reasonably straight forward stuff, with a little guidance you should be able to do this stuff repeatedly after only 1 or 2 attempts.
Pete
 
Well I finished extending the edges of my cowl. Thanks for everyones input, that really helped.
 
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