Making space for Sika bead over Tip-up Roll-bar

[Loman]

I see a small number of references, here and on some builders' websites, to providing space for a minimum 1/8 inch dept of Sika between plexi and metal on the tip-up canopy by moving the bottom frames inboard and making the aft frame narrower and lower.

However I can't find any reference to doing the same on the roll-bar.

Is it necessary and if so, how is it done?

[DGlaeser]

If by roll bar you mean the fixed frame for the aft part of the canopy, I did NOT make any changes to that so that the forward canopy rests normally in it. I figured a thinner glue line was OK on that surface was OK because I used a generous fillet of Sika on the inside.
I allowed a generous overlap inside the aft skin, and sanded the canopy a full 1/16" thinner for most of the ovelap to provide some room for the adhesive.
I also glued the canopy to the fore-aft channel between the frame and the aft fuselage (don't have the part number handy).

[Noah]

The problem is that the forward part of the roll bar needs to be in the "normal" location for the plexi to sit on it and the aft part needs to be brought inwards and downwards to provide for the sika gap that you mentioned for the window. I solved this by having a less than optimal sika bond thickness for the window, and in my case I am screwing the window in place as well as using the thin Sika bond. The canopy has no holes, but the window does. If the window cracks they are cheap to replace compared to the canopy, and since the window is much smaller the relative expansion is much lower due to coefficient of thermal expansion (CTE) mismatch between the plexi and the aluminum.

I think the alternative is that some people also build up the Sika over the rollbar on the front half so that the canopy rests on the Sika, 1/8" above the metal of the rollbar.

Hope this helps.

[Loman]

I see your point Noah but I am thinking of either using a 'crown profile' rubber sealing strip over the forward part of the roll bar or else 'moulding' a bed of cured Sika on which the aft edge of the canopy will rest when closed. It should be possible to get this to exactly match the depth of the Sika that is bonding the plexi to the aft channel frame of the canopy, so that the edges of the canopy and window match up on either side of the 'big cut'. Therefore, it should not be a problem to make both the roll bar and the canopy aft frame 1/8 lower than standard, giving a really good bond to the plexi along the top.

It seems a shame to use screws anywhere on a Sika-bonded canopy or window, so Denis's ideas of sanding to make the plexi thinner in the overlap areas and using a wide overlap with the top skin look attractive.

Because the window plexi meets the top skin at a slight angle (on the top of the fuselage at least), a useful gap between the two materials will open out towards the aft edge of the overlap, which should help, particularly if the overlap is generous.

The problem area is down on the side of the fuselage close to the roll-bar and the sides of the roll bar itself. If needs be, I will use Noah's approach and install just one or two screws here to help the bond strength.

Thanks guys. I feel better now.

[Radomir]

There's no need for any screws anywhere.... it'll work fine even with slightly smaller gap.. typically this is overdone (and gap made even bigger than necessary).. However, in some areas you virtually have no choice but use the gap that's there.. won't be as much as you'd think it should be but it works great under all temps/conditions/G's...

[jaybell]

Thanks all for for the helpful discussion; I built the canopy aluminum structure according to plans, then read everthing I could find about canopies prior to making the first cut on the plexi. Reading the various threads helped make this a less stressful event.

I made the big cut today, and started fine tuning the canopy and rear windows, I need to paint the canopy frame before I can finish the install. So far, everthing looks good. Here is how I created room for the Sika adhesive:

I covered the three F631 channels and F725 canopy frame joggle with electrician's tape, followed with a layer of 1/8 by 1/2 inch closed cell foam self adhesive weather stripping from the local hardware store. (I used electrical tape first because all is temporary until I work up the ambition to paint, and the weatherstripping is difficult to remove from aluminum)

The 1/8th layer of self adhesive weather stripping gives the same desired thickness as the Sika bond. I used a light duty, 1" wide ratchet strap connected to the rear wing spar bolt holes on either side of the fusalage to suck the canopy down to mark the big cut, and also to fine tune the canopy after the big cut. I had to use 1" styrofoam spacers to push in the bottom of the canopy frame since the strap was at such an acute angle to the canopy.

After the frame is painted, the forward and aft F 631 channels will have 1/8 spacers and Sika adhesive as described earlier in this thread, and the middle F631 channel (rivetted to the aft channel) will have a layer of 1/8 self adhesive weatherstripping to help seal the tip-up canopy against air leaks. I will check the usual mail-order supply houses for an aircraft grade of self adhesive foam weather stripping, but I don't mind using the hardware store variety since it is a non-structural component.

Other than laying the weatherstripping on the parts mentioned above, the only extra work was to sand down the bottom inside lip of the canopy where it nests into the F725 joggle. I didn't remove the full 1/8 thickness along the length of the F725, less than 1/16 allowed things to line up nicely.

I plan on installing the windows with no screws, except for the F725 side rails; I will use screws as per plans while the adhesive is curing, and remove them afterwards.

FYI, I am Canadian, it is cold outside, and cool inside my shop. I used a 500 watt hot air blower to warm up the canopy for an hour prior to, and during cutting, and had great results. The cold scrap plexi cut-offs were quite brittle; I could get them to crack and break by flexing unsanded pieces.

Jay

RV7A tip up
Big chunks done, working on finishing kit