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Silent_Flyer

Member
We had single digit temperatures back in February here in Maryland. When we went to the hangar in March, this is what was awaiting.

IMG_3191_zpsgbeml5z1.jpg



The crack started from the edge and not a bolt location and when up to nearly 80% of the top of the canopy. Our guess is there is not viable temporary repair while we start on a new canopy install. This canopy was the original since the aircraft first flew in 1991 with a few minor cracks in the aft locations that had been stopped drilled and no issues for the last 3 years of ownership. However this crack is clearly a non-starter.

I have a new Todd's canopy in the mail, but the real question is can I salvage the existing canopy frame and skirts. The canopy is attached by flush screws and 3/8 nuts on the 7/16" tube. I talked to tech support at Vans and they discussed drilling new holes in between existing holes so that alignment would on the new canopy would not have any stress risers, however in the end they said that I should start with a new canopy frame.

Clearly, I need to remove all of the flush rivets from the skirt and will likely have to build new skirts o the existing frame. This would allow me to use the existing frame that is fitted to the airplane and either attach the new canopy with Sikaflex and not drill new holes in the canopy. The other option would be to use the existing mounting tube holes and drill the new canopy, but oversize the holes so there is no chance of having a stress riser with alignment of the flush screw in the new skirt through the plexi to mounting tube.

Anyone that has gone through this headache please weigh in on difficulty and how successful it was to reuse existing frame. I've read quite a bit on Sika so that could be an option with existing frame and just build new skirts.

This puts a bit dent in spring flying and a new unplanned project to stack on top of planned events this spring ;-(
 
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Replace Canopy

I bought a flying -4 10 years ago with a cracked canopy and I replaced it with a Todd's canopy using the old frame and skirt. I carefully drilled the old rivets and reused the old holes. It was a pain and I had a small ripple in the skirt when I was done, but this was the first bit of work I ever did on an airplane so I didn't feel too badly about it. Someone with some building experience would probably have done a much better job. My point is that it is doable. If I did it now I would use Sika.
Hope this helps.
 
Not a 4 but.......

We are replacing the canopy on an 8 now. Original was riveted. We are reusing the frame and using Sika on the new one. NO holes !
All cutting will be made with an pneumatic die grinder with a "thin" abrasive cut-off wheel. Vans construction manual Highly recommends sanding and polishing after all cuts before handling and trial fitting the canopy. Seems like a lot of extra work however, were willing to go to extremes to reduce the chance of encountering stress risers that lead to cracks. Seems imperative to polish, polish, polish ALL edges prior to final assembly. Possibly lack of edge polishing caused yours to crack where it did instead from an attach hole ?

We found the Sika products here:
http://www.merrittsupply.com/search.aspx?searchterms=sika
At substantial savings.

Enjoy
 
There is an excellent account by Paul Dye somewhere on here about replacing a (riveted) Canopy, retaining skirts / frame etc.

Far from starting again, his post shows by matching holes etc. it is a quick and (relatively) easy job.

I am not there is any evidence about Sika v Riveted being better or worse for cracks - certainly one RV-8 owner over here has had 2 Sika canopies crack. The RV-4 in this pic does not have the crack come from a rivet hole?

If you do need to replace a canopy as per Paul's post, not sure it is an option with Sika?
 
Data point, but no conclusion.

I was a little concerned about temperature related cracks at the fastener points do I did a little test. Since the thermal expansion (contraction) coefficient is largest for the canopy, less for aluminum and still less for steel - - I fastened a piece of canopy to a piece of steel angle. No particular effort on the polishing of the holes. No sanding polishing of the acrylic at all - as cut with cutoff disk.

The #6 screws (counter sunk) were assembled snug, but not torqued. No aluminum was on the outside and the steel thickness was 0.125" . The distance between the three screws was 3" and 4". The part was made inside at 73F and left outside for a year. Not directly in the sun, but last winter saw -15F and this winter was about the same. Summer was up 100F in the afternoon sun. There were no cracks.

The actual loading case is much more complex than just the expansion rates. Loading (twisting, bending, etc due to latches) on the frame, potential loosening of fasteners and initial assembly temperatures would all play into the loading.

Like Paul did, the addition of small pieces of tubing around the screw shank would help the fractures at a fastener. If crack initiation is between fasteners, then that would not seem to be a factor.

Good luck in your replacement process.
 
RV-4 Cracked Canopy

Silent Flyer

Well do I know this feeling...:eek:

I'm on my # 3 canopy on my -4 and the first one cracked exactly at the same spot yours cracked..

#2 was riveted back using the old frame and all the side skirts..It lasted about 5 years and cracked on th opposite side of this one..

# 3 was Sikaflexed in place but used 4 small screws as safety but guess what...it cracked where one of the screws was installed...the hole was stop drilled and welded with Weldon 3 and so far it is still holding...( Used the original skirt again...

#4 will be entirely Sikaflex...no more screws or rivets for me...

I will use a new frame on the next one and will go with a thicker Todd canopy..( I have two spare frame and aluminum skirts )

they're not so bad to installed specially if you use the old frame but go Sika and be done with it...The Sika will expand with the canopy and I haven't heard of a rivet expanding yet ...

Good luck

Bruno
[email protected]
 
Silent_Flyer said:
Our guess is there is not viable temporary repair while we start on a new canopy install. (
Click to expand...

It depends how much you want to fly now while you gather all the parts required to fix. Stitching the crack with safety wire can be successful, slightly more permanent is gluing the crack with 'Tensol' cement - requires a V being cut and then filled with cement. Or stick a doubler on the back with Tensol.

Pete
 
Stitching canopy

My canopy was broken nearly in two, but I stitched it to fly back home. Once you get your canopy from Todd it'll only take a couple of days to do the job. Toughest part is just getting started!
 
RV-4 Canopy

Ditto here but it was just a series of small stop drilled cracks at the rivets that I got tired of looking at and was worried that some day I would walk in the hangar and see what you saw. Ordered a Todd's canopy and it is going in this week with NO rivets, NO screws and no holes in the canopy, straight Sikaflex.
 
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