Another Canopy Crack on my -8

[PandaCub]

While preflighting my RV-8 yesterday I noticed a brand new 8" crack on the starboard side of my Todd's tinted canopy. No obvious rhyme or reason as to why it occurred.

This makes two major cracks (first one documented here) and I'm wondering what to do now.

I am not the builder but I assume he used SikaFlex; from the photo, however, you can see the crack propagated from the joint up and there seems to be little "flex" to this material.

A new canopy from Van's is $1700 + shipping + the pain of fitting it to my existing frame. I'd prefer a repair, but don't know if that would simply be a band-aid. In the meantime, I've stopped drilled the crack and covered it with both aluminum and duct tape.

Any suggestions appreciated. I've seen the name Dan Kintar referenced as a possible canopy guru, but can't seem to find his contact info.

 

[AN23]

John,

A buddy of mine just had multiple "mystery" cracks show up in his Todd's canopy also. Two of them look very similar to yours. Fortunately, we have Lew Gallagher here (Greenville, SC) and he is an absolute wizard at fixing cracked canopies. Many, many of us RVers have used him with some being shipped to him from all across the country. I'm certain some of his repaired RV friends will join me in recommending him. Lew's retired from a normal job and really does this to make new friends and talk aviation.

PM me if you're interested in calling him. He'll give tons of advice if nothing else.

 

[rv8ch]

John, sorry to hear about that new crack. The photo from the inside looks like some kind of glue, but the line in the glue looks very strange. Is that a shadow or is it a crack in the glue as well?

I'm wondering if there are any rivets or screws also used by the original builder. Some builders used a "belt and suspenders" approach to affixing the canopy. Also, it would be interesting to know what kind of glue was used.

Good luck with the repair or replacement decision and execution.

 

[Daveflier]

Canopy Crack

I second the recommendation for Lew Gallagher. I have a friend who shipped his canopy from Maine to S. Carolina for Lew to repair.

 

[DanH]

The fundamental issue with an RV-8 canopy is a large difference in coefficient of thermal expansion. Plexiglass expands and contracts about 6x that of the steel frame.

Take a simple look. Assume a strip of acrylic is 60 inches long and CTE is 0.000040 inches per inch per degree F, with a temperature change of 100F (say 20F to 120F, roughly the common operating range). The length changes almost a 1/4 inch (0.00004 CTE x 60" x 100F = 0.24)

Luckily we don't have to deal with that much, because we tend to match drill the canopy to the frame at shop temperature (lets say 70F), and although 6X less, the steel also expands and contracts. An approximation might be...

(0.00004 x 30 x 50) - (0.0000065 x 30 x 50) = 0.050"

It's not insignificant if the pop rivets are in tight holes. Even if there are no holes, or the holes are oversize enough for float (as they should), edge finish is a big deal.

Next time you drill a hole in plexiglass, look at it with a 10x magnifier. You may be horrified. A deburring tool doesn't help a lot. Deburring by grinding with a fine pointed stone (below) is much better. Best is grinding followed by chemical deburring with dichloromethane, the result being a smooth melted circumference.

Works on edges too.
.

 

[Dan 57]

Cracked my friend's canopy beginning of the year... the fix was a lot of work, but well worth it as it is almost invisible, and, fingers double crossed, seems to hold

https://vansairforce.net/community/showthread.php?t=216146

 

[ariel_arielly]

Crack in RV8A Canopy

Hi.
I also had a crack in my RV8A canopy.
I put some thouts about and desided to make my 8 to a "combat aircraft". Added a piece of Aluminum strip around and glued. In the end it looks not bad...

 

[AlexPeterson]

Duct tape is a no-no on a canopy. When I was building my canopy, I took some scraps of the acrylic and put duct tape on them. After a few weeks I removed the duct tape - there was all sorts of crazing of the acrylic where the duct tape had been.

I don't know which tapes might be safe on a canopy - perhaps others do.

 

[ronschreck]

Several years ago Jon Thocker experienced a canopy crack at 11,500 feet on a winter day in Ohio. The crack went clear across the canopy just behind his head and he heard the crack over the sound of his engine. Jon told me that the crack was at least 3/8 inch wide. He returned to base and put the airplane in his heated hangar where the crack closed up tightly.

I experienced a similar cracked canopy on a summer day and replaced the canopy, securing it with SikaFlex. I have found that even canopies secured with SikaFlex can crack and the culprit is usually a failure at a point where there is little or no thickness of SikaFlex between the canopy frame and the plexi.

 

[MIKE JG]

How about those with FB canopies? Has anyone had a fastback canopy crack on them??

 

[claycookiemonster]

Yes, what about those FB canopies? I'm sitting on one, hoping for final assembly soon. In my case the canopy is Van's standard. The frame is completely fiberglass and no rivets used at all. Just Sikaflex for adhesive. There were a small number of holes drilled along the lower bonding surface for clecoes to hold things together while the Sika cured. Whether those holes may provide a starting point for a future crack is a question worth asking. The only metal beyond the latch is a piano hinge along one side.

Eagerly awaiting answers to this most recent question. I can't even imagine the heartbreak of a crack. I hope fixes are found for all.

 

[oscaroviedoa]

https://www.flickr.com/photos/188122755@N04/53351094180/in/dateposted-public/

 

[DanH]

Yes, what about those FB canopies?

A much better picture. Compared to a steel frame, the CTE of a epoxy glass frame is much closer to that of an acrylic plastic. Less delta means less stress with temperature change. Composite properties vary based on many factors, but the numbers line up roughly like this:

(in/in/degF)
Epoxy glass 20
acrylic 40
Steel 6

Further, the modulus of an epoxy glass composite is about 1/10 that of steel. It means the composite frame can more easily stretch or compress when the plexiglass expands or contracts due to temperature, again reducing stress on the plexiglass. Think soft spring rather than stiff spring.

Of course it's also shorter, and the highly tapered part has been removed. Both are a big deal, but just for fun, let's consider taper. Every thought about why RV-8 canopies tend to more often crack back toward the rear?

Imagine this strip of material. Let's say it's plexiglass sheet, 1/4 inch thick. Anchor one end, and pull on the other end with a force of 1000 lbs. Now look at stress at two points. On the left, the material is 6 inches wide, and on the right 2 inches wide. The 1000 lb load is uniform along the whole length, but stress per area is three times greater at the small end.

Proportionally widen the sheet, roll it into a half tube, and you have a decent approximation of an RV-8 canopy.