-8A - Another Cracked Canopy

pvalovich · #1 03-25-2019, 10:55 AM

Cruising along at 4500 ft. Saturday from Rostraver, PA (FWQ) to Bolivar, MO (M17) fat, dumb and happy. OAT 23 degrees; 163 kts TAS; no turbulence. Hear a loud bang, but no change in engine parameters or flying qualities. Landed at I23 (Fayette County, OH) to check things out. Discovered a 15 inch crack in the right side of the canopy about ten inches forward of the cross brace. Sikoflex install with not a single rivet in the canopy. Previously had and stopped drilled two 6-inch cracks on either side just forward of the cross brace so I felt further cracking was not going to occur. Have no idea what the specific causes were unless I inadvertently created stress points when clamping the canopy during initial fitting.

Want to attempt a repair before punting to a new install. Can anyone point me to the previous forum threads that offer the best repair advice?

And yes - that dull roar you may have heard early Sat morning was me repeatedly going through my curse word inventory.

Capt · #2 03-25-2019, 02:12 PM

Ouch! I feel for you. I only have a small 2" crack near the Cross brace that just appeared one day but it continues to annoy me, can't imagine what your going thru! Hope you get it sorted

RV7 To Go · #3 03-25-2019, 06:33 PM

I know a 7A slider owner who had his canopy crack when doing his initial engine run-in. Very disheartening! It had not even been in the air yet.

He contacted the canopy manufacturer of the new replacement and was told to de-stress the canopy after installation to the frame. He built a box and placed the new canopy attached to the frame inside. Using a ceramic heater controlled by a temp sensor he heated it to 180 F (not entirely sure, but I think this was the temp) for 24 hours. He then decreased the temp 10 degrees every hour until back to room temp. Then it was painted and installed. So far so good.

It was the first time I had heard about doing this. Sure hope I don't experience this issue with my Sika installed 7 tipper. So far so good, going on 2 years and 240 hrs.

Gusmax · #4 03-25-2019, 07:22 PM

I found this; http://www.plasticsmag.com/ta.asp?aid=2514
It talks about supporting the plastic so it does not sag. That does not sound good?

YvesCH · #5 03-26-2019, 01:50 AM

Quote:

Originally Posted by Gusmax

I found this; http://www.plasticsmag.com/ta.asp?aid=2514
It talks about supporting the plastic so it does not sag. That does not sound good?

Maybe they talk about very htin large sheet?

But it makes somehow sense.. Maybe an autoclave would work as well?

DanH · #6 03-26-2019, 05:45 AM

Quote:

Originally Posted by pvalovich

Discovered a 15 inch crack in the right side of the canopy about ten inches forward of the cross brace. Sikoflex install with not a single rivet in the canopy. Previously had and stopped drilled two 6-inch cracks on either side just forward of the cross brace....

These cracks extend from an edge?

pvalovich · #7 03-26-2019, 09:21 AM

Cracks extend from the edge.

I was an early Sikaflex guy. Probably made the Sikaflex bead way too thick and subsequently reduced flexibility.

It's been nearly nine years so memory isn't 100% but I may have introduced residual stress when clamping the canopy to the frame in a few places.

Will be forever mystified as to why crack occurred when it did - straight and level, cruise speed, 23 degree OAT and zero turbulence - when I regularly fly through Eastern Sierra turbulence and do 3-4g acro. However, on the trip into PA east of the Mississippi the OAT dropped from mid-20's to 7 then 2 above zero for about three hours. Perhaps the extended cold permanently affected plexi properties. Plane was built with a drafty cockpit that helped in normal 110 summer degree days in the Mojave desert - not for severe winter excursions.

DanH · #8 03-26-2019, 10:14 AM

High probability that the issue is edge finishing. The edges must be both rounded and polished.

RhinoDrvr · #9 03-26-2019, 09:10 PM

There used to be a post here called ?Unhappy Cracky Club? where a gentleman walked through how he fixed almost exactly the same crack in his canopy. By my recollection, it was the same gentleman who did a cross country trip writeuo over the summers. His airplane was called ?The Dove?, and had a red nose and 3 blade catto prop. I can?t find the post anymore for some reason, but maybe someone else can post it? Might be helpful!

BillL · #10 03-27-2019, 06:37 PM

Quote:

Originally Posted by DanH

High probability that the issue is edge finishing. The edges must be both rounded and polished.

Dan, it has to be more than just the edge issues, aren't all the cracks right at the lower brace like this post from Ron?

If so that is the base of the the cantilever of the back frame. IF it is always there, why not just put a strain gage on there and measure the strain under expected conditions? Is is just thermal or internal pressure during flight bending that rear frame? Data should help formulate a solution.