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RV-8 canopy bubble to frame fit

Lars

Well Known Member
Sponsor
Hi all-

Helping a friend replace the canopy and windshield on his 2005 RV-8 due to unfortunate circumstances. He is not the original builder; the actual bubble on this aircraft was probably manufactured a few years before the airworthiness date.

Fortunately the old frame, roll bar, canopy skirt and even glass windshield surround all survived unbent and unscathed, so we are reusing those pieces. The replacement bubble is a stock canopy, received recently from Van's.

We have the frame installed on the aircraft and have been trimming/fussing/fitting the new bubble to the old frame, roll bar, fuselage etc.

The new bubble fits well enough around the forward fuselage skin and over the roll bar and forward part of the frame. But at the rear, it seems like a completely different shape. The new bubble is far wider than the aft foot or so of the frame; it takes a lot of pressure to force it against the frame, which in turn distorts the canopy so it doesn't fit elsewhere.

Since the rear 2/3 of the original canopy survived, we pop-riveted it to the frame, then dropped the new bubble over the old to compare fit. The old bubble is much more severely tapered than the new one when looking in plan view (top down). And interestingly, when looking in elevation view (from the side) the old plastic is at least 1-1/2" lower over the passengers head, and presumably the pilot's as well (that part was shattered so can only speculate).

So, some questions:

1. Could the old bubble be a Todd's canopy? I found an old thread here that implied that the Todd's shape was lower: http://www.vansairforce.com/community/showthread.php?t=32660

2. (more importantly) has the shape of the stock canopy frame changed over the years? I will be calling Van's on Monday to ask about this. Or perhaps, did Todd's or anyone else sell an alternative canopy frame? This one appears to be stock based on construction, but you never know...

3. Is it typical to need a tremendous amount of pressure to force the bubble against the frame, especially at the rear?

Thanks for insights!
 
I think what you are describing is fairly common. The important thing to try to NOT do, is put undo stress on that aft portion of the bubble. To achieve this, After you have made your initial trim of the bubble, start your drilling/clecoeing of the bubble to frame at the aft end and work your way forward a clecoe or two at a time on each side of the bubble/frame. When you get to the front of the frame, you?ll find that the bubble seems to have ?grown? a little down near the bottom of the forward frame tube on each side. You?ll then re-trim the bubble to where you want it and finish to a smooth, rounded edge. On mine I made the holes in the plexi a little over size for the 1/8 pulled rivets and made sure the holes were very smooth and deburred. You can squirt a little clear RTV in the plexi holes when installing the rivets to provide a little cushion for plexiglass expansion/contraction.
 
Hi all-

Helping a friend replace the canopy and windshield on his 2005 RV-8 due to unfortunate circumstances. He is not the original builder; the actual bubble on this aircraft was probably manufactured a few years before the airworthiness date.

Fortunately the old frame, roll bar, canopy skirt and even glass windshield surround all survived unbent and unscathed, so we are reusing those pieces. The replacement bubble is a stock canopy, received recently from Van's.

We have the frame installed on the aircraft and have been trimming/fussing/fitting the new bubble to the old frame, roll bar, fuselage etc.

The new bubble fits well enough around the forward fuselage skin and over the roll bar and forward part of the frame. But at the rear, it seems like a completely different shape. The new bubble is far wider than the aft foot or so of the frame; it takes a lot of pressure to force it against the frame, which in turn distorts the canopy so it doesn't fit elsewhere.

Since the rear 2/3 of the original canopy survived, we pop-riveted it to the frame, then dropped the new bubble over the old to compare fit. The old bubble is much more severely tapered than the new one when looking in plan view (top down). And interestingly, when looking in elevation view (from the side) the old plastic is at least 1-1/2" lower over the passengers head, and presumably the pilot's as well (that part was shattered so can only speculate).

So, some questions:

1. Could the old bubble be a Todd's canopy? I found an old thread here that implied that the Todd's shape was lower: http://www.vansairforce.com/community/showthread.php?t=32660

2. (more importantly) has the shape of the stock canopy frame changed over the years? I will be calling Van's on Monday to ask about this. Or perhaps, did Todd's or anyone else sell an alternative canopy frame? This one appears to be stock based on construction, but you never know...

3. Is it typical to need a tremendous amount of pressure to force the bubble against the frame, especially at the rear?

Thanks for insights!

It sounds to me like the new bubble needs to be re-trimmed so that the rear sits lower on the frame or you need to look carefully at the fore-aft positioning of the bubble in relation to the frame. These variables all affect the canopy to frame fit. You can also check the height of the very aft part of the frame above the turtle deck and "adjust" it slightly. This will change the clearance to the sides at the rear portions because of the way the canopy cross section is tapered near the back.

Skylor

Skylor
 
Last edited:
bend the frame

I'd recommend two things: 1) bend the frame to match the canopy; 2) use glue instead of rivets.

Any forcing of the canopy to a particular shape could end up in a crack down the road. Pretty much no matter how you shape the frame, and install the canopy on it, you can make it look great with the skirts. They handle the smooth transition from the plexi to the fuselage, as you know. That will take a lot more time than fitting the canopy to the frame.

About the Todd's canopy - many people wanted them because they were a bit thicker and tinted. I'd say if the old one is about the same thickness and color, it's probably from Van's.

If you don't have the canopy installation instructions, they are available on the USB stick - section 9 in the "construction manual". Lots of good hints in there. Might also try to find a local RV-8 builder, I'm sure he or she will be happy to share their experiences. Nothing better than an experienced 2nd or 3rd pair of eyes on it.
 
I think what you are describing is fairly common. The important thing to try to NOT do, is put undo stress on that aft portion of the bubble. To achieve this, After you have made your initial trim of the bubble, start your drilling/clecoeing of the bubble to frame at the aft end and work your way forward a clecoe or two at a time on each side of the bubble/frame. When you get to the front of the frame, you?ll find that the bubble seems to have ?grown? a little down near the bottom of the forward frame tube on each side. You?ll then re-trim the bubble to where you want it and finish to a smooth, rounded edge. On mine I made the holes in the plexi a little over size for the 1/8 pulled rivets and made sure the holes were very smooth and deburred. You can squirt a little clear RTV in the plexi holes when installing the rivets to provide a little cushion for plexiglass expansion/contraction.

We trimmed this thing minimally. Because of the severe taper at the back, small cuts shorten the canopy substantially. A little bit of trimming with the canopy in its natural (unclamped) state got the front to fit quite well, over the forward skin, the roll bar and the forward frame bow. Back was awful. Trimmed it just enough that the canopy didn't hit the aft deck skin when pushed in. But even when allowed to float freely the amount of force to push it in to contact the frame was scary, and at that point the forward part was badly distorted. I totally follow your process, but I don't see how we can do that without building in a huge amount of stress.

It sounds to me like the new bubble needs to be re-trimmed so that the rear sits lower on the frame or you need to look carefully at the fore-aft positioning of the bubble in relation to the frame. These variables all affect the canopy to frame fit. You can also check the height of the very aft part of the frame above the turtle deck and "adjust" it slightly. This will change the clearance to the sides at the rear portions because of the way the canopy cross section is tapered near the back.

Skylor

Skylor

Thanks for the input. The problem with trimming the back to fit the frame better would mean that the canopy would be way too short. Cue my comment above about trimming at the back. As I understand it you are suggesting that we bend the aft-most part of the frame up? Only problem with that is that the frame would be exposed above the back part of the existing skirt, which we hope to re-use.

I'd recommend two things: 1) bend the frame to match the canopy; 2) use glue instead of rivets.

Any forcing of the canopy to a particular shape could end up in a crack down the road. Pretty much no matter how you shape the frame, and install the canopy on it, you can make it look great with the skirts. They handle the smooth transition from the plexi to the fuselage, as you know. That will take a lot more time than fitting the canopy to the frame.

About the Todd's canopy - many people wanted them because they were a bit thicker and tinted. I'd say if the old one is about the same thickness and color, it's probably from Van's.

If you don't have the canopy installation instructions, they are available on the USB stick - section 9 in the "construction manual". Lots of good hints in there. Might also try to find a local RV-8 builder, I'm sure he or she will be happy to share their experiences. Nothing better than an experienced 2nd or 3rd pair of eyes on it.

Thanks for input about the Todd's canopy. I seem to remember reading about that myself somewhere, but wasn't paying much attention because I was building side by side RVs.

I was an early adopter of SikaFlex on my own RV-7, having done my canopy (twice, sadly) back in 2010 and I've since assisted with several more. Have never done a tandem canopy with any process though, and while it would ordinarily be my preference, we were hoping to use rivets here simply because we are re-using existing bits. Thinking being it would be more expeditious.

As it is, no way am I going to apply that much strain (and resulting stress) to the canopy as it currently fits. Seems like a guarantee for cracks, and quickly. Will go in search of local builders and plans.
 
another thought

Lars,
This might help you as it did me. I noticed when test-fitting my canopy that if I laid the canopy on the frame and started clamping from the front and worked my way to the back, the fit at the back was dramatically different than when I did the clamping in reverse order (from back to front). So, If I laid the canopy on the frame and started clamping from the rear - the point on the aft tail, then worked forward with the clamps, the canopy fit differently and with much less force at the rear and with no noticeable difference at the front (same good fit). I am not sure if you tried that or how you are "clamping" your canopy to the frame while you fit it - but I thought it would be worth a try if it worked for you. Good luck.
 
another approach

I built a frame with straps lined with very soft fabric, old towels and such. I put the canopy upside down in the frame and lowered the canopy frame into it. this greatly helped with adjustments. working from back to front attach clamping to frame. Also did not do any drilling on the canopy installed using sika flex only. been in service for 7 years with no issues.
 
To close this out...

Lars,
This might help you as it did me. I noticed when test-fitting my canopy that if I laid the canopy on the frame and started clamping from the front and worked my way to the back, the fit at the back was dramatically different than when I did the clamping in reverse order (from back to front). So, If I laid the canopy on the frame and started clamping from the rear - the point on the aft tail, then worked forward with the clamps, the canopy fit differently and with much less force at the rear and with no noticeable difference at the front (same good fit). I am not sure if you tried that or how you are "clamping" your canopy to the frame while you fit it - but I thought it would be worth a try if it worked for you. Good luck.

Great advice, thank you. It turned out to be the solution. We had been working front to back. When we went the other way this morning we quickly realized we had a lot more trimming do do, which in turn made the canopy more flexible (lots of sun didn?t hurt either) and eventually we got it to lay down nicely. We have since done the big cut and the new canopy is drilled and clecoed to the old frame.

I built a frame with straps lined with very soft fabric, old towels and such. I put the canopy upside down in the frame and lowered the canopy frame into it. this greatly helped with adjustments. working from back to front attach clamping to frame. Also did not do any drilling on the canopy installed using sika flex only. been in service for 7 years with no issues.

We did something similar and it was super handy, especially since we were working in a remote location. Draped an old piece of carpet between the long sides of the canopy shipping crate, then filled it in with the paper packing material and a couple of moving blankets. We did all the trimming with the canopy upside down in the crate.

Were it new, and my airplane, I?d have used Sika 295UV for sure. My RV-7 canopy is now 10 years old and has lived through a lot of fairly severe thermal cycles and almost 1000 hours of fun flying. Holding up just fine. As are the canopies of several friends. I would do it again in a heartbeat.
 
This has been a helpful thread for me as I'm finally getting around to fitting my canopy. Got some good advice and some follow-up calls from PMs I sent out (Thanks Scott) and have experienced builders coming out Thursday to help get me through the trimming and Big Cut. Thanks to all; more to follow here on VAF as it happens...
 
I did my canopy with pop rivets per Vans plans. No stress on the canopy is a must. Making the canopy frame fit the canopy rather than vis/versa is the way to go. I think the best tip I got and followed was to put a light coat of oil on the frame when fitting the canopy. This showed exactly where the tangent/contact line between the canopy and frame was and where to drill without putting stress on the plexi.
 
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