Tip-up canopy fairing -- minimum number of layers of fiberglass?

roee · #1 03-16-2009, 04:00 AM

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

rv9av8tr · #2 03-16-2009, 07:24 AM

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#

roee · #3 03-17-2009, 01:49 AM

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

RFazio · #4 03-17-2009, 07:32 AM

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.

DanH · #5 03-17-2009, 08:10 AM

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>.

allbee · #6 03-17-2009, 08:48 AM

Quote:

Originally Posted by DanH

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>.

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.

roee · #7 03-17-2009, 07:03 PM

Quote:

Originally Posted by DanH

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>.

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

hudgin · #8 03-17-2009, 09:31 PM

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

roee · #9 03-18-2009, 01:48 AM

Quote:

Originally Posted by hudgin

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

Interesting! Where did the cracks form?

hudgin · #10 03-18-2009, 07:58 AM

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