R

roee

Well Known Member
Question for the composites experts.

I'm working on the fairing around the front of the tip-up canopy. So far I've created a contoured fillet (flox), and now I'm getting ready to lay up some glass.

Now, I've read other builders' accounts of how they've done this, and there's quite a few variations, but I'm a bit puzzled about something. Seems like most folks have done this using anywhere from 3 to 7 layers of fiberglass. Even 3 layers sounds excessive to me in this application. Why are so many layers needed? Or are they?

Seems to me that this entire fairing is not structural, and should not experience any significant stress since the canopy is already held securely in place with fasteners along the side rails. The fiberglass also does not have to support its own shape. Along its entire surface the fiberglass is bonded to a rigid substructure: the plexiglass canopy, the aluminum skin, and the flox fillet between them.

I want to do what's really necessary, no more no less. So I'm thinking that an adequate fiberglass layup could be done here in either of the following methods:

Method A: Two layers. Each layer consists of several individual strips 8" to 12" long to go across the canopy. The seams of the top layer are staggered from the seams of the bottom layer, so that each layer reinforces the other's seams.

Method B: One layer only. A single strip to go all the way across, no seams. Done. I especially like this one!

Any reason why either of these approaches, i.e. just 1 or 2 layers, wouldn't suffice?

Thanks,
-Roee
 
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3 Ply BID on T/U fairing

I'm not a composites engineer, but based on having built a Long-EZ, I used 3 plies of RAF BID glass tapes. You want it ridged enough that it won't flex and debond. When you put the glass down be sure to apply it on a new layer of flox. Don't put glass over top micro.
You can find my pics here: http://picasaweb.google.com/mikerv9a/Canopy#
 
I'm still confused...

Mike,

Thanks for the reply, but I still don't quite get it. Where exactly is the concern for de-bonding? I assume that the weakest bond in this whole fairing is between the fiberglass and the plexi, correct? Is that the bond that we're worried about de-bonding? If so, how would building up additional layers of fiberglass help that? We still have the same total bond area to the plexi. I understand that additional plies would make the fiberglass more rigid, but then wouldn't that actually make the potential de-bonding problem worse? Seems like if the fiberglass is more rigid, it would resist more against flexing with the rest of the aluminum + plexiglass structure, and would therefore have a greater tendency to separate from the rest of the structure. No? Or did I misunderstand something? Please explain.

Thanks!
-Roee
 
The weak bond in my opinion is to aluminum!

I ripped my original fiberglass off the fuse when I changed the plexi. The fiberglass popped right of the aluminum. If I was making a perminent bond to aluminum I would lay up two layers then countersink screws to the fuse. Then lay up a final layer of glass. I made my fiberglass removable so I have countersunk stainless screws holding it down.
 
Roee, there is no "right" answer for this one....way, way too many variables.

Just one of those many variables would be glass fabric weight and weave, and you didn't mention what you planned to use. Three plies of Rutan BID sounds right to me; fabric weight in the 8 or 9 oz range. Another builder might have chosen to use 9 plies of 3 oz.

A thicker patch laminate should reduce concentration of peel stress at the edge of the substrate.

My Showplanes canopy got two plies of 9 oz on the outside, but the plexi is also bonded to a glass flange on the inside (with Hysol), and the outer tape bond was to glass, not aluminum.

You're welcome to use what you wish. It's your experiment. In the absence of data or the presence of unknowns, it is usually best to do what the guys before you did if it didn't break <g>.
 
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DanH said:
Roee, there is no "right" answer for this one....way, way too many variables.

Just one of those many variables would be glass fabric weight and weave, and you didn't mention what you planned to use. Three plies of Rutan BID sounds right to me; fabric weight in the 8 or 9 oz range. Another builder might have chosen to use 9 plies of 3 oz.

A thicker patch laminate should reduce concentration of peel stress at the edge of the substrate.

My Showplanes canopy got two plies of 9 oz on the outside, but the plexi is also bonded to a glass flange on the inside (with Hysol), and the outer tape bond was to glass, not aluminum.

You're welcome to use what you wish. It's your experiment. In the absence of data or the presence of unknowns, it is usually best to do what the guys before you did if it didn't break <g>.
Click to expand...

I was perplexed with the same question. I set my window on the frame and there was many gaps with a wide one on the pilot side. If I pushed the window down, there was a good amount of stress on the window side front corner. I let it sit like that for a couple weeks, than thought, ok if there is stress there, it will crack eventually. So I went and got some Windo-Weld, super fast urethane and calked it in the cracks all around the front. I used a tongue depressor and smoothed it out. After it dried it was nice and stable, I tried to remove some from both the window and the aluminum, nope going nowhere. Than I filled in the top to the shape I wanted, several layers, with what, West system and 410. Did I do right, don't know, that's just what I did.
 
DanH said:
Roee, there is no "right" answer for this one....way, way too many variables.

Just one of those many variables would be glass fabric weight and weave, and you didn't mention what you planned to use. Three plies of Rutan BID sounds right to me; fabric weight in the 8 or 9 oz range. Another builder might have chosen to use 9 plies of 3 oz.

A thicker patch laminate should reduce concentration of peel stress at the edge of the substrate.

My Showplanes canopy got two plies of 9 oz on the outside, but the plexi is also bonded to a glass flange on the inside (with Hysol), and the outer tape bond was to glass, not aluminum.

You're welcome to use what you wish. It's your experiment. In the absence of data or the presence of unknowns, it is usually best to do what the guys before you did if it didn't break <g>.
Click to expand...

Dan, I hear what you're saying. And I agree, in the absence of being able to arrive at a "correct" solution by analysis, doing what has yielded satisfactory results in the past is a relatively safe bet.

But I have two different objectives here:

1. Get satisfactory results for my canopy fairing.

2. Gain knowledge that applies to a broader spectrum of future situations.

Simply doing what others have done on the canopy in the past indeed accomplishes #1, but not #2. So maybe to attack #2 I'll take a different approach. Let's forget about the canopy fairing. I'll pose a question in abstract terms. Here it goes:

Hypothesis: It seems to me (from deduction, not experience) that making a thicker layup (thicker cloth and/or more plies) would make the layup itself stiffer/stronger, but would not make its bond to the underlying substrate any stronger. And furthermore, a stiffer layup may have a greater tendency to detach from the underlying substructure as the underlying substructure flexes. So if the weak link in a given structure is the strength of the fiberglass layup itself, then making it thicker (thicker cloth and/or more plies) will help. On the other hand, if the weak link in a given structure is the strength of the bond between the fiberglass layup and the underlying substrate, then making the layup thicker (thicker cloth and/or more plies) will not help, and may even increase the potential for separation from the underlying substructure.

Is this hypothesis, in general, correct? Or did I take a wrong turn somewhere?

Thanks,
-Roee
 
I like your method A. Thats the way we did it. Flew it about 5 hours and it cracked in two places so patched and blended it with 3 staggared layers in those areas only. Now has 40 hours and still going. We used Vans recomendations in the instructions
 
hudgin said:
I like your method A. Thats the way we did it. Flew it about 5 hours and it cracked in two places so patched and blended it with 3 staggared layers in those areas only. Now has 40 hours and still going. We used Vans recomendations in the instructions
Click to expand...

Interesting! Where did the cracks form?
 
We made the canopy so that when it is closed the edges go over the side of the fuselage covering the gap that would of been left there. The side skin butts up against the deck cover. It cracked at that junction and followed the base of the windshield about 1/4 the way up toward the center on both sides. I hope this makes sense to you. If not send me a PM and I will call you. DH
 
hudgin said:
We made the canopy so that when it is closed the edges go over the side of the fuselage covering the gap that would of been left there. The side skin butts up against the deck cover. It cracked at that junction and followed the base of the windshield about 1/4 the way up toward the center on both sides. I hope this makes sense to you. If not send me a PM and I will call you. DH
Click to expand...

I think I understand what you're describing, that you used the fiberglass to extend the skirt below the canopy deck. Right? In that case, I'm not at all surprised that it cracked, since that extended skirt portion would be unsupported by any other structure. And, it might take a pretty good beating by the airflow, and/or possibly from vibration against the side of the fuselage if it actually makes contact there. I would think that the fiberglass would need to be heavily reinforced in such a situation. So if I understood you correctly, your scenario is interesting, but actually very different from what I'm targeting.
 
We made the skirting out of metal and made the windshield fairing out of fiberglass. The side skirt and the upper deck butt together with no way to reinforce it so the windshield fairing was brought down to inforce the butt joint. It was three layers so it cracked at the joint. The crack also turned 90 deg to the joint and followed between the base of the glass and the deck. We put on that skirting to keep out the airflow and noise in the cabin. Works pretty good. We have no air leaks except at the canopy hinges, it whistles pretty good there.
 
roee said:
Dan, I hear what you're saying. And I agree, in the absence of being able to arrive at a "correct" solution by analysis, doing what has yielded satisfactory results in the past is a relatively safe bet.

But I have two different objectives here:

1. Get satisfactory results for my canopy fairing.

2. Gain knowledge that applies to a broader spectrum of future situations.

Simply doing what others have done on the canopy in the past indeed accomplishes #1, but not #2. So maybe to attack #2 I'll take a different approach. Let's forget about the canopy fairing. I'll pose a question in abstract terms. Here it goes:

Hypothesis: It seems to me (from deduction, not experience) that making a thicker layup (thicker cloth and/or more plies) would make the layup itself stiffer/stronger, but would not make its bond to the underlying substrate any stronger. And furthermore, a stiffer layup may have a greater tendency to detach from the underlying substructure as the underlying substructure flexes. So if the weak link in a given structure is the strength of the fiberglass layup itself, then making it thicker (thicker cloth and/or more plies) will help. On the other hand, if the weak link in a given structure is the strength of the bond between the fiberglass layup and the underlying substrate, then making the layup thicker (thicker cloth and/or more plies) will not help, and may even increase the potential for separation from the underlying substructure.

Is this hypothesis, in general, correct? Or did I take a wrong turn somewhere?

Thanks,
-Roee
Click to expand...


Roee,

Your hypothesis seems sound in principal, assuming the elastic modulus of the materials in question are really that different. 3 layers of Rutan BID is hardly "stiff". One layer is NOT enough. Just hold the cloth up to a window or light and you will see the gaps between strands of glass. These gaps will fill up with epoxy and epoxy alone is not adequate for proper bonding, even for non-structural parts. One layer of this type of glass can be torn easily almost by hand as there is very little surface area bonding the fibers together.

Glassing around the -7 canopy can be done time similar to the way it is described at:

http://www.maddyhome.com/canardpages/pages/waynehicks/chapter_18_11_3.htm

I have done this to a couple of -8s (ok..easier than a 7) and they turn out great. Make sure you paint a layer of epoxy on the al/plexi before applying the glass.

My 2c

Ryan
 
Methods

Dan hit most of the points, too many variables. Personally, I think 3-4 is minimum. Here's what I did.

Got the canopy fixed to the fuselage initially with micro balloons and epoxy. Then a fillet with flox mix. More epoxy and micro to get the contour desired.

This process continued until the final contour was established. Next a 2 inch layer of 6 oz fiberglass tape, followed by a 3 inch piece. These were done at the same time. Next a layer of biased 4" was overlayed, followed by a 4" piece of standard glass tape.

Sanded, filled, sanded filled until finished. No cracks.

All tapes were purchased from CSTsales. These are nice and don't have a large woven edge like many do.
 
Thanks! Follow up questions -->

Thanks everybody for the insights!

So let's say 3 layers then. For best results, what are the preferred techniques?

Q1: Should each layer consist of:
a) one full-length strip of cloth (will a straight "tape" follow the curve ok?)
b) several partial-length strips of cloth overlapping each other
c) several partial-length strips of cloth butting up against each other
d) other?

Q2: What is the preferred layup technique:
a) wet layup
b) pre-preg (i.e. "plastic sandwich")
c) other?

Thanks again!
 
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Ordinary tape with a selvage edge probably won't lay. Have not seen the tape Darwin mentioned, but it sure sounds good. Cut crowfoot or 8-harness is guaranteed to conform.

Avoid partial lengths and overlaps if possible. No structural downside, but you will have lumps and joggles to fair and fill later.

I'd probably wet out all the plies in one stack between 4 mil plastic. Wetting out multiple plies of narrow strips, in place, one ply at a time can be a pain. The plies tend to slide around and/or push up bubbles between the plies. You wind up with a lot of excess resin too, because you can't squeegee or roll it out without shifting the plies. Stacking and wetting between plastic lets you fully saturate the cloth, then roll out all the excess resin, then cut through the plastic with a roller knife leaving a strip with plastic on both sides. Carry it across the shop, carefully peel plastic off one side, and place the stack where you want it. You just want to place the strip. You can't make it conform until you peel the top plastic layer and stipple with a brush.

Brush some neat epoxy on your filet before placing the plies, wipe off the excess. You're prewetting the surface (better bond) and giving yourself a bit of "lubricant" to make final positioning of the stack a little easier; you can slide it around intact.
 
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