RV-6 Tip-up Canopy *Frame* Crack

I have been trying to localize a problem with my tip-up canopy for a couple of months now... The front corners of the canopy frame "lift" in flight, making my attempts at weatherstripping the join between the canopy and the cockpit sidewall useless. Sitting on the ground, I could see about 1/4" gap between the bottom rail of the canopy and the top of the cockpit sidewall. In flight, that gap grew to somewhere between 1/2" and 5/8" on my last flight. This gap is much worse on the pilot's side, the passenger side the gap is only 1/8 to 3/16" in flight, and none on the ground.

On shutdown after my last flight, when I opened the canopy, I found the culprit. The weld between the hinge arm and the front canopy bow is cracked, almost all the way through on the pilot's side, and more than half-way through on the passenger side. And on top of that, the C-channel that it rivets to is also cracked.

If i've done this right, here's a picture. I've posted more on my Picasa site.


It seems odd to me that the joint is designed in this way. The curved part of the canopy hinge requires cutting out the lower flange from the canopy bow, which seriously compromises the load-bearing capability of that piece... It forces the loads directly onto the weld. It's almost *guaranteed* to fail, due to its design. Even though i'm religiously careful with the canopy when opening/closing, it still flops around a bit so this has probably been building over the last year i've owned the plane.

I'll email Vans on Tuesday and get their input as well, but I just wanted to see if anyone else has seen this or if anyone has any suggestions. I tried both Google and the search here on VAF but found not one article referencing this. Surely i'm not the first?

So far, the best sequence I and my fellow local RV'ers have come up with seems to be:

1. Remove front trim piece (thankfully, screwed in place)
2. Drill out pop-rivets holding plexiglas to the rear canopy bow
3. Remove screws holding plexiglas to the rest of the canopy frame
4. Have the weld Tig welded (the frame is aluminium)
5. Have the cracks in the bow also Tig welded
6. Optionally, add a plate to the front of the bow, bolted in place, to reduce the stress on that weld

Isn't that the channel that is recommended to be filled with foam and glass on the -6s?

Perhaps yours flexed too much without the stiffening?

Stiffeners

You might want to look at the RV7 setup and add a set of stiffeners rather than fill and glass.

Peter

Cracked weld

I just had this happen on my RV-7 with about 750 hrs on it. I drilled out the rivits on the cracked piece, used a dremel grinder to grind out the weld left on the "C" arm. The piece I removed was replaced with a bracket fashioned from 1/8 inch angle material rivited to where the old piece was rivited and then bolted to the "c" arm using AN3 bolt with wide area washers. I figured this would be less susceptible to cracking than getting it rewelded. I rivited the repair angle with about 1/16th of an inch gap between it and the arm so when bolted up, it would pull that side of the canpoy down to the right level. Seems to work fine, and is an easier fix than trying to prepare for re-welding.

Worked for me anyway.

Ed Booth, Trenton, SC

az_gila said:

Isn't that the channel that is recommended to be filled with foam and glass on the -6s? Perhaps yours flexed too much without the stiffening?

Click to expand...

Maybe, and almost certainly. I don't see how filling the channel with anything would help though... There would still be a break in the load path when it reached the hinge plate.

Why they didn't just make the plate go another 3/8" lower to clear the bottom flange of the channel is a mystery. I looked at a slightly older -6 here on the field that has the original straight hinges, and they didn't compromise the channel. His has 1300 hours and no signs of wear in this area.

Pete, I've seen the stiffeners on the -7, that looks like it would make the whole structure stiffer, which would help. I'll be asking Van if retrofitting that reinforcement is an option.

Ed, the angle bracket was suggested to me as well... I like it for the simplicity, but don't like loading bolts in tension and bending at the same time. Although the stresses here really shouldn't be that high... I'll have to think about that.

Thanks for the ideas! Keep them coming!

kiwipete said:

You might want to look at the RV7 setup and add a set of stiffeners rather than fill and glass.

Peter

Click to expand...

I agree the newer angled stiffeners are much better - if you have a RV-6x tip up that doesn't have the canopy fabricated yet then this is the only way to go...

I was referring to a plans update (RVator article?) in which Vans recommended the stiffener.

The foam/glass may still have the load interuption due to the notch, but I did notice a remarkable reduction in the twist/flex of the entire canopy frame when I added mine. Perhaps it's this twisting action that is breaking the frame rather than a direct side load?

Any reports of cracks on frames that have the foam/glass?

Snowflake said:

Ed, the angle bracket was suggested to me as well... I like it for the simplicity, but don't like loading bolts in tension and bending at the same time. Although the stresses here really shouldn't be that high... I'll have to think about that.

Click to expand...

If stronger struts were installed, this would increase the twisting loads on that weld also. As mentioned above, grind out the weld and add an angle bracket on each side of the hinge ear (total of 4) and BOLT IT ON. Rivet it to the channel, and move on...........

Gasman, how would you deal with the crack in the channel itself? Just stop-drill it and leave it?

Snowflake said:

Gasman, how would you deal with the crack in the channel itself? Just stop-drill it and leave it?

Click to expand...

A good place to start is to purchase this................ http://www.vansaircraft.com/cgi-bin...5331136-384-309&browse=videos&product=ac43.13

Normally there is enough space fwd of the channel to make a patch, but I see scuff marks in your photo. I would drill the ends of any crack. Place a spacer between the hinge points to save that dimension. Then grind out the weld. Make a surface patch that would fit on the front of the channel and extend 2 inches past the edge of the angle that you will install to replace the weld. Place an angle on each side of each ear and bolt through the ear. Rivet the patch and angle according to normal rivet practice. See the above mentioned publication.

I've got 43.13 (and the Aeronautical Engineering degree to understand it too) so i'm good in that regard. Just because I *can* engineer something myself, doesn't mean i'm not lazy enough to see if someone else has already done it for me...

I talked to Vans today, they agree that trying to weld probably isn't worth the effort due to the risk to the plexi. I have a local weld shop that comes very highly recommended, I may run it past them to see what they think too, but with an aluminium frame it's hard to see the plexi being safe with that much heat getting applied. Replacing the broken tab with an angle does make a fair bit of sense, and adding a reinforcing strip on the front will help too. I'll look at replacing both tabs on each hinge ear with two angles, but the weld isn't broken on the inside of the ear... Nor is there any real load there I don't think.

As for the scuffing, I expect I can taper the reinforcement to be wider at the bottom of the channel and shorter at the top, as all the load i'm trying to carry is at the bottom anyway. That shoudl take care of most of the problem, with any luck. I may need to make the flange on the subpanel a little narrower if more clearance is required.