Another Sikaflex question

[dwilson]

Is there a minimum thickness below which sikaflex is compromised?

I am build an RV-10 and want to glue the rear windows in using Sika. The gap between the plexiglass and the supporting fiberglass flange of the cabin cover is very thin in some areas.
I vaguely remember reading somewhere that there is a recommended thickness for Sikaflex, but can't remember the details.

Is this an issue?

Thanks

[bruceh]

The manufacturer has this information online. There is a chart that shows the bonding line thickness to largest pane dimension. The minimum they show on the chart is 4mm. The issue is that for thermal expansion there needs to be a certain amount of bedding under and around flush mounted window panes. The usual use for this is large windows on boats. For something small, bubble shaped and curved like our canopies, it is probably less of an issue to have a thick amount of sikaflex under the canopy bonds. Thermal expansion has a place to go outward on the canopy. The rule of thumb on these forums has been 1/8" thickness, which is slightly less than 4mm.

[dwilson]

Thanks Bruce,
I will see if I can get 1/8" gap without making the flange too thin.

[Bill.Peyton]

You mentioned getting the flange too thin. Remember, if the window protrudes a little proud it will be hidden when you run the fiberglass strips around the perimeter and fare it into the canopy cover. That is exactly what I did and was able to maintain the roughly .125" thickness of the Sikaflex bounding material.

[Lars]

Sikaflex 295UV is a polyurethane rubber adhesive. It is useful in applications where there are large differences in thermal expansion between the substrate (the part you are bonding to, in this case a canopy frame or the airframe) and the part being bonded (the window or canopy).

295UV will tolerate 300% shear or more without failure. That being defined as the ratio of the lateral deflection to the thickness of the joint. It's not as strong as other adhesives such as epoxy, but it's a whole lot stretchier.

Epoxy (for example) will create a stronger joint, but because it doesn't give, it means that if the bonded part (windshield) expands or contracts a lot more than the substrate, the epoxy is going to stay put- then the window is going to get highly stressed, and possibly (probably) crack.

The downside to the urethane is that, if the joint is thin and the expansion of the window is large, the shear in the joint can easily exceed the maximum that the urethane can tolerate, and it will fail in shear (rip). Thus, the bigger the linear dimension of the window, the thicker the joint needs to be.

It's late and I'm tired. Hope that makes sense to someone. Apologies if it needs more clarification.

[pierre smith]

The caveat is that most all of the -10's haven't used Sika to hold the glass in.

There is a tremendous amount of "lift" on the -10's curved fuselage and I'd highly recommend asking Van's about using Sika.

Doors have already been sucked off in flight! It'd be terrible to lose windows in flight!

Best,

[dwilson]

Thanks for the excellent explanation of the underlying concept Lars.

That makes sense!

It would be interesting to see a graph or table of the elasticity/bond strength curve of the other adhesives that get talked about for bonding the windows in!

[aerhed]

Sometimes I think the 10 lid & doors were built inside-out. I keep seeing a vision of flanges on the outside and flush windows with a stepped edge inside. Then maybe seal the cabin & add 3psi. Well maybe not the last part, but inside windows would be nice.

[Lars]

Originally Posted by aerhed

Sometimes I think the 10 lid & doors were built inside-out. I keep seeing a vision of flanges on the outside and flush windows with a stepped edge inside. Then maybe seal the cabin & add 3psi. Well maybe not the last part, but inside windows would be nice.

Outside lip = good as far as I'm concerned. Even though I used Sika 295UV to bond my canopy to the frame, the riveted-on (to the frame) skirt acts like a strap across the back of the canopy as a mechanical attachment. Also means the sides can't blow out in tension. And then there is the latch on top, etc. Belt and suspenders and all that.