Preventing a canopy crack in a RV-8

[Webb]

It may be perception but it seems like a lot of 8’s with canopy cracks get them in the proximity of the canopy brace that bridges the 2 sides. I remember Mr Horton‘s explanation of the increasing force as the canopy narrows.

i may be overthinking this but if I go ahead and cut the canopy and use proseal to fill the cut and attach a cover strip will alleviate a lot of the stress that causes the crack.

It would be easy to do while fitting the canopy.

Good idea or bad idea?

 

[AN23]

I see RV-8s with that crack way too much. I own a 2004 RV-8 with a non-cracked canopy (knock on wood) and I discussed the methodology used with the builder ( mine is #4 of 14 builds!). He felt the difference was in very careful prep work on all edges (sanding) and especially on each drilled hole. He slightly oversized and countersunk each hole, ensured the inside of each hole was smoothed and and then placed a piece of surgical tube in the hole before using pull rivets to fasten it to the frame. This gives the canopy some room for movement, growth, contraction, etc. All I can say is that it has worked to-date. Who knows, tomorrow it may crack!

 

[rv8ch]

I think if you bend the frame slightly to closely match the "natural" shape of the canopy, polish the edges, only use liquids that are plexiglass-safe, not drill any holes, and glue on the canopy with adequate distance between the frame and the plexiglass, the chances of a crack will be greatly diminished. I personally would not do the mod you are suggesting, but could consider it if I got a crack in that area.

 

[Majorpayne317641]

Mine was done by a first time builder with drilled holes and pop rivets. It hasn’t cracked after 12 years and 1500hrs.

 

[2theskyy]

Maybe it’s this little detail that makes the difference as well. Who knows.

 

[DanH]

Hmmm. That brings an idea.

What if the slip joint was not welded? What if the rear bow was simply inserted some distance into the larger tube, so it remained a slip joint?

To be practical, the larger tube would need to be 0.058" wall. Anyone ever cut one?

 

[Paul 5r4]

Interesting thread. I have a 7A tip up and canopy cracking is one of my biggest fears... especially in the cold! I have a son in MA and will not
fly there in the winter to visit just for this reason. I've often thought of the built in stress that must be present in such a large piece of the
material the canopies are made from. I've also had the thought that smaller pieces would reduce/eliminate these stresses. So, I like your idea.

 

[Ironflight]

Hmmm. That brings an idea.

What if the slip joint was not welded? What if the rear bow was simply inserted some distance into the larger tube, so it remained a slip joint?

To be practical, the larger tube would need to be 0.058" wall. Anyone ever cut one?

After developing a crack in my second canopy in the same spot as the first (right at that joint), I am sorta convinced that its a high stress point, if for no other reason that the weld creates a little bump…so maybe grinding that area into a really nice taper? AND not welding it? On our RV-3, it was roughly the same design, except that the rear bow pieces didn;t meet in teh back - and we just cut them off, using the fiberglass (and a little carbon) in the skirt to support the rear part of the canopy. It has worked great - but its a tipper, not a slider.

 

[chrispratt]

Another crack-related suggestion although not directly to your point: I never latch my canopy when it is parked in my hangar. I simply roll the canopy to the lip of the roll-over bar and leave it without latching. With all the temp changes that can occur in the hangar, especially in winter when I'm not flying as much, I figure this might relieve some potential stress. I can't prove that it makes a difference, but so far (knock on wood), I haven't had a crack.

Chris

 

[rvbuilder2002]

Maybe it’s this little detail that makes the difference as well. Who knows.

Not little.
It’s actually a very important detail, and is likely a factor to some degree in many of the cracks that have occurred in this area.
The change in tube diameter at this location can cause a focused stress concentration if fasteners are located to close to the transition point.

 

[Webb]

Dan, I put a caliper on the tubes. There are some open ends so I checked the wall thickness. Granted there is powder coat but this is what I found. Large tube 0.635”, wall 0.06“. Small tube 0.509”, wall 0.04”.

Paul, there is some weld bump which should be filed down. Note in the picture I found is the crack is well forward of the weld.

Scott, I will definitely pay attention to the joint area when attaching the canopy.

Other pictures I have seen are in the proximity of the joint. I may be pontificating but I think that area of the canopy is under more stress as exhibited by the cracks and similarity of the crack’s line of progression.

 

[Laird]

I've been thinking about cutting the canopy short, adding a rear support, trimming the aft frame and making a new fairing to replace the removed portion of the canopy.

Thought is to shorten the length of the canopy and reduce CTE differential.

Thoughts?

 

[WingnutWick]

I see RV-8s with that crack way too much. I own a 2004 RV-8 with a non-cracked canopy (knock on wood) and I discussed the methodology used with the builder ( mine is #4 of 14 builds!). He felt the difference was in very careful prep work on all edges (sanding) and especially on each drilled hole. He slightly oversized and countersunk each hole, ensured the inside of each hole was smoothed and and then placed a piece of surgical tube in the hole before using pull rivets to fasten it to the frame. This gives the canopy some room for movement, growth, contraction, etc. All I can say is that it has worked to-date. Who knows, tomorrow it may crack!

I recently had my redone after its second crack. I had it redone with this method and so far so good. Prepping edges and some method to account for the expansion/contraction. Oversized holes with surgical tubing (or model airplane fuel tubing) hopefully does the trick. Allows for movement spanning the length of the canopy as well as some shock absorption and no screw on glass contact. Was going to go Sika but have seen canopies cracking from that as well. It's the 8's really only significant flaw IMO.

 

[tracy]

Mine cracked several years ago. Lew Gallagher, at GMU, fixed mine and it has been fine for ever since. People don’t even notice it’s there.

 

[spatsch]

Another crack-related suggestion although not directly to your point: I never latch my canopy when it is parked in my hangar. I simply roll the canopy to the lip of the roll-over bar and leave it without latching. With all the temp changes that can occur in the hangar, especially in winter when I'm not flying as much, I figure this might relieve some potential stress. I can't prove that it makes a difference, but so far (knock on wood), I haven't had a crack.

Chris

I never latch mine either. Mine cracked at the usual spot overnight (not even in flight...). I fixed it (as it's really outside the scope of view of even the passenger) and it has been holding up since. I think there is another thread somewhere where people tried to work out a common pattern (glued vs. riveted, acro vs. not etc..) and the data wasn't really showing any pattern .. .

Oliver

 

[DanH]

Not little.
It’s actually a very important detail...

Scott, happen to recall if the 5/8" tube is .058 wall? Seems likely, given the joint design.

 

[rvbuilder2002]

Scott, happen to recall if the 5/8" tube is .058 wall? Seems likely, given the joint design.

Sorry, I don’t
The powder coating thickness should probably be pretty consistent in that area… perhaps someone could measure the OD of both tubes at the transition point.

 

[Webb]

Sorry, I don’t
The powder coating thickness should probably be pretty consistent in that area… perhaps someone could measure the OD of both tubes at the transition point.

I measured the OD near the weld.

 

[Kyle Boatright]

How's a P-51 canopy installed? That's a huge sliding bubble and I don't hear of those cracking frequently.

 

[Webb]

I’ve decided that to move forward with splitting the canopy. I plan to make the cut at the joint transition and the targa strip will provide a smooth transition and I won’t be concerned about the canopy attempting to conform at the joint.

 

[Webb]

How's a P-51 canopy installed? That's a huge sliding bubble and I don't hear of those cracking frequently.

It appears to be shorter than the RV-8 canopy which may be a factor. I suspect it’s thicker which probably helps. How it’s installed is definitely an interesting question. It looks like screws and nuts. Probably had to be so it could be changed out quickly and pit back in service

 

[AN23]

P-51 canopy structure

 

[Capt]

My 8 was crack free when I bought it, 16 years old, came from a warm humid climate hangared. Within a week it cracked near the skirt at the rear, I suspect a change of climate did it, cold and damp although still hangared!

 

[AN23]

It appears to be shorter than the RV-8 canopy which may be a factor. I suspect it’s thicker which probably helps. How it’s installed is definitely an interesting question. It looks like screws and nuts. Probably had to be so it could be changed out quickly and pit back in service View attachment 54711

TF-51 has a long canopy

 

[MIKE JG]

Has anyone seen an 8 with the FB mod have a cracked canopy??

 

[Laird]

My 8 was crack free when I bought it, 16 years old, came from a warm humid climate hangared. Within a week it cracked near the skirt at the rear, I suspect a change of climate did it, cold and damp although still hangared!

I'm sure there are many contributing factors that could cause cracks in the canopy, but one I don't think that has been mentioned was shop temperature when it was installed.

My guess is that the colder it is when drilling (or bonded with Silkaflex) could give it more resistance to cracking.

Let me explain......as we all know, acrylic is more brittle when cold. If the canopy is installed when warm, the canopy is nice and pliable (good for cutting and handling), and the frame is at the high end of the CTE range.

When it gets cold, the canopy has contracted the most, but it's more brittle than it's warm state, but the steel frame has changed little from it's warm state, so the stress in the acrylic are higher, and is closer to cracking.

Let's say the OAT operating range is 0 deg F to 130 deg F. Average range is 65 deg F. (I'd calculate the CTE range but my mother told me never to do math in public)

If when mounting the canopy, you could bias the neutral point of the CTE range, to the colder side (or even neutral), I think that would help.

Seems to me that if you could mount/drill/bond in a colder state (not crazy cold obviously, say 60-65 deg F) the canopy and the frame would closer to the average CTE range. Then it gets hotter and moves to the other range of the CTE delta, but the canopy would be warmer (more pliable) and more crack tolerant at that end of the CTE range........

Of course you run the risk of cracking the canopy in the colder conditions when handlin/installing it. Interesting problem.

Thoughts?

Laird

 

[Tag]

Hmmm. That brings an idea.

What if the slip joint was not welded? What if the rear bow was simply inserted some distance into the larger tube, so it remained a slip joint?

To be practical, the larger tube would need to be 0.058" wall. Anyone ever cut one?

Hey Dan, this is the idea I have been contemplating for quite a while now. I went as far as asking Vans if there was a way to get the canopy frame delivered with that existing “slip joint” left unwelded, but they said that option wasn’t available. I have yet to persue the idea, being unsure of how allowing the slip joint to slide would affect the structural integrity…? Sure seems to me that would relieve any stresses due to the different rates of expansion/contraction of the frame and plexi…

 

[ksdflying]

Hmmm. That brings an idea.

What if the slip joint was not welded? What if the rear bow was simply inserted some distance into the larger tube, so it remained a slip joint?

To be practical, the larger tube would need to be 0.058" wall. Anyone ever cut one?

I did. I cut all that off on mine.

 

[DanH]

P-51 canopy structure

Aluminum or steel?

 

[2theskyy]

Has anyone created a channel for the canopy to sit in that is attached to the canopy frame. Similar to a window frame as windows expand/contract within the frame without cracking.

 

[cfiidon]

Mine cracked after 11 years of flying. Moderate temp, just a little additional stress as canopy closed on soft object on one side introducing twist.

I repaired as shown in the pic. Picture is large so you can see the crack detail on the far side. Fortunately the crack was conducive to a parallel/orthogonal band.

 

[Webb]

Right above a rib.

 

[N82VM]

I think you’ll spend every first conversation with a new person asking you how long before it cracked and you did the repair. If you sell it the same will be asked. Probably lots of head scratching about what exactly a canopy preventative crack mod is.

I think careful attention to detail while installing is all the crack prevention that is necessary.