Sika Tipup Should Rollbar be Narrower than Plans?

[Noah]

Just did the big cut a week ago I had casually read and bookmarked many threads about how to do a Sika (bonded) canopy but hadn't really looked at them again until the last few days. It now occurs to me that the rollbar (cabin frame in VanSpeak) should be at least 1/4" narrower than plans in order to accomodate the sika bond thickness of 1/8 or more per side. Ditto for the canopy frame. Can anybody confirm or deny this? I spent 12 weeks getting the canopy frame to fit like I wanted before touching the bubble and beginning all the fitting and trimming, now it seems that because of that extra bond thickness, especially at the skirts where it is 2X, that the canopy frame needs to be quite a lot narrower. I'm thinking that bonding the canopy instead of following the plans requires a lot more advance planning and parts mods than I had realized? Or am I missing something?

 

[Wicked Stick]

Have you ordered any Sika yet ? I'm going to be doing my canopy and windscreen in Dec/Jan. Perhaps we can split the cost of the primers, etc.
(yes I have a heated garage)

 

[flyvans.com]

hi,

we did our tipup with sikaflex.

we did build the rollbar according to plans, i only remember that it's actually impossible to get it just right as the assembly tends to live quite a bit during riveting.
there is enough space to make a nice corner bead from the inside to the back rear canopy.
the tipup frame however was modified

- "rollover bar" style frame slightly narrower and lower
- siderails offset towards the inside by .125" achieved by inserting two .063" spacers where the forward tipup frame meets the siderails, the position of the "ear" from the tipup frame is now just on the other side of the siderail as per plans. beveled the ear to fit.
- attached 2 .063" strips on the outside of each siderail (ordered some extra strips, the same as used on the inside of the rollbar), spacing the outside skirt out and flush with the fuselage, creating a gap for a nice bead on the inside.

turned out great. only thing we forgot was to slightly angle the rollover bar on the bottom, so it sits maybe 10/32" or so higher than per plans.

http://www.flyvans.com/log/2008_01_30_canopy_frame.html
and following.

rgds,
bernie

 

[flion]

For another perspective, I built both the frame and rollover bar to plans. I fitted my canopy so I had the correct gap over most of the forward deck and the top of the rollbar/frame area. Since the canopy had to fit in the side frame and there was no way to put the necessary gap there, I had to allow the gap to lessen as it approached the sides. However, I don't think it will be an issue. The sides are held with screws (under the outside strip) and sealed with a thin layer of Sika and the finished canopy seems firm. Also, for the aft portion, only the rollbar was glued; the aft skin was riveted over a thin layer of Sika. So the thin areas are held with fasteners other than Sika except for the transitions along the sides of the rollbar and frame; I'm not worried about it.

However, I did have one frustration with my fit: before you glue, make sure the forward deck and skin are riveted on and all the frame is complete, including struts and latching mechanism. I added my struts and forward skin later and it shifted the fit of the canopy.

 

[Noah]

Staring into the Abyss

Perhaps we can split the cost of the primers, etc.

Dave, If I end up going this way, yeah, lets definitely split the cost.

there is enough space to make a nice corner bead from the inside to the back rear canopy.

Not sure what you mean

- siderails offset towards the inside by .125" achieved by inserting two .063" spacers where the forward tipup frame meets the siderails, the position of the "ear" from the tipup frame is now just on the other side of the siderail as per plans. beveled the ear to fit.
- attached 2 .063" strips on the outside of each siderail (ordered some extra strips, the same as used on the inside of the rollbar), spacing the outside skirt out and flush with the fuselage, creating a gap for a nice bead on the inside.

OK, I get that. I could probably do those mods without too much trouble, need to think about it some more.

turned out great. only thing we forgot was to slightly angle the rollover bar on the bottom, so it sits maybe 10/32" or so higher than per plans.

I would think that might be a good thing, especially for taller pilots!

For another perspective, I built both the frame and rollover bar to plans. I fitted my canopy so I had the correct gap over most of the forward deck and the top of the rollbar/frame area. Since the canopy had to fit in the side frame and there was no way to put the necessary gap there, I had to allow the gap to lessen as it approached the sides.

Makes me worry that you would have high stresses in these areas due to differential thermal expansion, but since you also screwed this area...

However, I did have one frustration with my fit: before you glue, make sure the forward deck and skin are riveted on and all the frame is complete, including struts and latching mechanism. I added my struts and forward skin later and it shifted the fit of the canopy.

Good advice, thank you.

It now occurs to me that to do this "right" meaning consistent gap everywhere, the rollbar needs to be 1/2" narrower than plans, not the 1/4 I had mentioned, because the side skirts require 2 bondlines each of sika. And the sideframes need to be 1/4" inboard of the plans location, as indicated by Bernie.

I just can't believe that in all the reading I had done about using Sika, that this wasn't more clear - that there are significant mods required in order for this to work properly. It really makes me wonder whether people are building these with their canopies overlapping the fuse by a quarter inch, or whether they are screwing where the bond thickness gets too small? I don't really want to put any holes in the canopy if I can avoid it (this is the second canopy for this project - the first builder cracked the canopy at 2 drill holes, lost faith, and sold the project). Well, at least now I think I know what my options are:

1. Modify the rollbar to be 1/2" narrower than it already is (not even sure this can be done, need to look at it), same goes for the canopy frame (somewhat easier). (Bernie's approach)

2. Live with the reduced bond thickness and drill and screw the canopy in those areas. (Flion's approach)

3. Build it per plans (I think Chad Jensen came to this conclusion after staring into the abyss as I am doing).

4. Build it per plans except stick some Sitka in too (not sure what this accomplishes except adding weight and cost)

Thanks again for the input gents. Any other thoughts?

 

[Radomir]

Just say no to screws (on the front part of the tip up.. baggage compartment window does not seem to have the same stress)

 

[rv7charlie]

Are you guys interested in a Sika kit at a slight discount? I ordered 3 tubes of sealant, & 1 can each of cleaner & primer back in Oct thinking I was ready to go, then realized that I won't be ready to do the back glass for a looong time. Expiration dates are Feb/Mar; products are unopened. The stuff would probably be fine for years if refrigerated, but if you're ready to go I'll knock some off the price & just reorder for myself when I'm actually ready to do the whole thing.


Email me off-list at [email protected] if you're interested; I forget to check this forum for days at a time.

Charlie

 

[DGlaeser]

Just did the big cut a week ago I had casually read and bookmarked many threads about how to do a Sika (bonded) canopy but hadn't really looked at them again until the last few days. It now occurs to me that the rollbar (cabin frame in VanSpeak) should be at least 1/4" narrower than plans in order to accomodate the sika bond thickness of 1/8 or more per side. Ditto for the canopy frame. Can anybody confirm or deny this? I spent 12 weeks getting the canopy frame to fit like I wanted before touching the bubble and beginning all the fitting and trimming, now it seems that because of that extra bond thickness, especially at the skirts where it is 2X, that the canopy frame needs to be quite a lot narrower. I'm thinking that bonding the canopy instead of following the plans requires a lot more advance planning and parts mods than I had realized? Or am I missing something?

I did make the frame narrower, and also offset the side rails inboard by 1/8".
See pics at: http://www.wideopenwest.com/~glaesers/BondedCanopy.htm

 

[dhall_polo]

I think most people doing sika have built the canopy per plan, with some folks seeing around the corner and making the mods so you can have thick sika on the sides.

Along the roll bar and front, a good bond of sika is the only thing keeping you from flying a convertible, so meeting the spec'd thickness is critical. Along the sides, the clamping power of the rail and side skirt on the plexi is pretty strong, and paired with a thin layer on each side of the plexi along the rails, it's set.

If you make the mod to allow thick sika on the sides, you're just choosing for all the holding strength to come from the sika, since the rail and canopy will not provide any clamping or friction force.

I don't think there's a wrong answer. Whether you build per-plan or make some slight mods to allow for thicker application of sika along the sides, you'll end up with canopy that aint going anywhere.

 

[Janekom]

Having built both a slider and a tip-up and used Sika on both, I would recommend to build as per plans and make sure you have the correct gap on the roll bar and also LOTS of Sika on the front bottom. We ended up with about 60 mm wide in front where the plexi sits on the frame. Oh - remember to let the plexi rest on pieces of cut tubing to form the gap.
The only part where you will have less than ideal gaps, will be as said before on the side skirts. As the plexi is sort of clamped in between I cannot see a problem there. Do a small test section and see if you can pull a piece of plexi out of "between to pieces of alu" when fixed with Sika.