Another Sikaflex question

I just finished bonding my forward tip up canopy bubble to the frame. I'm real happy with the results. I'm wondering if anyone has attached the rear window 100% with sika? Seems most people screw the aft end to the skin and bond the front to the roll bar. Just wondering.

I installed mine (-7 tipup) with just Sika. Not flying yet, so I can't report anything except that it hasn't fallen out over the last couple of years sitting in the hangar.

The tricky area is where the roll bar meets the fuselage. I didn't cut down the width, so the joint is ~1/8" thick at the top & tapers to near nothing where the plexi tucks in between the skin & the roll bar just above the top longeron. The edge next to the skin across the back has a very thin layer & a small fillet to transition to the plexi.

Charlie

Yeah I'm looking at that spot too. I have not riveted it yet however so I can still give it a tad bit of play. How did you hold the glass up into place while the sika cured? I'm thinking possibly just drilling for clecoes and filling the holes with glass resin.

Sika tip up

I did mine complete Silka. Have help on several more. Did you have a question.

I'll do the same soon. What are the primary references to get it right the first time? The RV wiki? Something else?

Yes I used the wiki instructions and it worked well.

Gary, my question was weather anyone has bonded the rear window on the tip up to the aluminum fuselage skin. From what I have seen folks have used Sika everywhere except that location. The more I think about it I think I'll use screws there as well to get the fit tight to the alum skin as the small gap required for the sika bond would be quite obvious in that spot.

+1 all Sika

My tip up and rear canopies are bonded - no fasteners, since 2008. No problems. I made the overlap about 1.5" and sanded the plexiglas a lot to provide a bit more thickness for the Sika.

I will be doing this next weekend. It will be 100% Sikaflex with no fasteners. My roll bar is the standard size. The plexiglas meets the skin at an angle along the top aft edge, meaning that the materials diverge slightly behind this and the gap for the Sikaflex widens out aft-ward if you were to look at a vertical section through the overlap. So I plan on no more than 1/16th of fillet visible at this edge and my overlap will be just over 1 1/4 inch wide so that the fillet should be well over 1/8 thick at the rear and pretty close to 1/8 on average.

While I took special measures to ensure I had a minimum of 1/8inch fillet everywhere on the main forward canopy, I think there are two reasons why the aft window is less prone to any risk of departing the airframe:

1. The plexiglas is INSIDE the skin all along the side/aft edge so there is no question of any force that will tear the joint open. I presume that forces pushing the plexiglas into the aircraft are rare enough that we don't have to worry much about them. The exception to this is the top of the roll bar, where nothing other than the Sikaflex itself mechanically restrains any force that would tear the plexiglas up and away from the joint. If the joint were to fail and the air stream were to get in there in flight, then the window would act as a scoop and would be gone pretty quickly. It obviously makes sense to have at least the minimum recommended depth of Sikaflex along there. If you are still nervous about it then maybe Sikaflex is not for you.

2. The size of the aft window is quite small, particularly fore-aft, so the potential for thermal expansion/contraction causing a shear force on the join is limited, at least relative to the forward canopy. Given that the window is mechanically restrained by the skin around the side/aft edge, I imagine any thermal expansion will mostly have the effect of pushing the side edges downward. The expansion should be cumulative, with the centre staying put and the edges, particularly in this case the side/ forward corners, moving most. For that reason, I will be ensuring that there is at least a quarter inch between these corners and the roll bar mounting angle. Otherwise, the expanding plexiglas could make contact here and push against the angle thereby transferring further expansion into an upward tearing force at the top of the roll-bar.

I am not an engineer so make up your own mind about this, but it makes sense to me.

Loman said:

....The size of the aft window is quite small, particularly fore-aft, so the potential for thermal expansion/contraction causing a shear force on the join is limited....

Click to expand...

It's really not something at all obvious until you get into the math of it, but the forces developed by thermal contraction or expansion are independent of the length of the parts.

One thing worth considering is that nearly always (it depends on the materials), adding bond thickness tends to lower the shearing stresses in the bond. The reason is that the parts being bonded tend to expand and contract as they will, and the bond carries the strain difference. Adding thickness to the bond reduces the strain through the bond thickness. Since the bond has some stiffness, the load imparted on the materials being bonded is therefore lower.

Dave

rv7charlie said:

I installed mine (-7 tipup) with just Sika. Not flying yet, so I can't report anything except that it hasn't fallen out over the last couple of years sitting in the hangar.

The tricky area is where the roll bar meets the fuselage. I didn't cut down the width, so the joint is ~1/8" thick at the top & tapers to near nothing where the plexi tucks in between the skin & the roll bar just above the top longeron. The edge next to the skin across the back has a very thin layer & a small fillet to transition to the plexi.

Charlie

Click to expand...

I think that I just used various sticks wedged into the right places. Another idea is to partially fill the baggage area with sturdy boxes or crates, & use beach balls, etc as flexible wedges to hold the edges in place. You can add z-clips around the perimeter of the plexi to secure it without drilling holes in the plexi.

Someone mentioned forces on the back glass being unable to pull it out. If you look at a pressure diagram of a fuselage, I think you'll find that air pressure is actually pushing *in* at that point on the upper fuselage, not out. The z shaped clips should give some comfort that the inward pressure won't cause 'implosion'. ;-)

FWIW,

Charlie

Since 2006

Mine has been in since 2006 and several more in the hood have also. No screws and no problems. We did the boxes and the beach balls also.

thanks for all the good info. I'll plan to forge ahead without firing up the drill. Skirts and all. Looking at the bond strength of the fwd canopy after it's cured for a couple days, I can't imagine that thing will ever come off. My caulking gun hand is mighty sore though...

This is what we did when we glued our rear screen in.

A piece of 1 x 6 across the fuselage and then thin bendy ply with carpet on to provide the outward pressure required.

Mask off, apply the primer and put the Sika on the pre scuffed plastic.

DON'T FORGET - take the protective plastic off the inside before you fit it 'cos you can't remove it once it is in from above the targa strip. Just ask me how I know.......