There has been some discussion lately on cracked canopies and some speculation on the cold weather being the culprit.
I thought I would start a new thread based on Danny Kings comment:
Danny has postulated that the difference in thermal expansion between the steel frame and the canopy is the smoking gun. I think he has hit the rivet on the head here. Danny oversized his attachment holes.. and I think he is on the right track. I have attempted to put some numbers to the problem....
Assuming a canopy built to plans we have the following numbers:
A #40 drill through the steel rail. (0.098")
A #27 drill through the canopy. (0.144")
12 " between fasteners. (Lo in the equations)
EDIT: Thanks to Alex' comments below...things have changed A LOT.
The thermal expansion Ct for steel is approximately 0.000008 in/inR
The thermal expansion Ct for acrylic is approximately 0.0000417 in/inR
I'm going to take a lot of liberties with my assumptions from here on (feel free to shoot me down)
Lets say we go flying at -5 degF outside. Lets also say that our cockpit/sun heater is able to bring the interior of the cockpit up to 32 degF for a deltaT of 37 degF as we fly around.
The expansion of a material dl = Ct * deltaT * Lo
The steel will expand by: 0.003552 inches between fasteners.
The acrylic will expand by: 0.01851 inches.
Yikes. But wait, we have some slop in the holes of 0.046 inches (#27 - #40)
So the difference in length the steel and the canopy is going to be 0.01852 - 0.003552 - 0.046 = -0.03104 in.
So, there is enough oversize in the hole to have no strain on the plexi.
If we assume we didnt drill the plexi hole in perfect alignment with the frame, (in other words, the rivet is right against one edge) there will be a maximum strain of
Well strain is deltaL/Lo = .046/12 = .00383
Stress = Youngs Modulus * Strain
Youngs Modulus for Acrylic is about 400000 PSI
So the stress rise in the acrylic is .015 * 400000 = 500 PSI
The tensile strength of acrylic is about 9000 PSI (this has a big range so I just picked one)....so we have some margin.
Now... if we increase our deltaT to 50 (meaning we managed to warm up to 45 degrees in the cockpit) the stress goes to 675 PSI.
But wait! We drilled a hole in this thing.... what did that do? Well, holes create a stress concentration.... and there's a lot of math we could go through to try to figure that out, but we will just take a shortcut, and set our hole stress rise factor to 3. (look it up)
So, NOW with a deltaT of 37 deg we have:
500*3 = 1500 PSI Still OK
Now, I have made a LOT of assumptions and taken a lot of liberty with geometry etc...
There have been a lot of comments wrt installation loading due to bending the frame to fit. But from this calc, maybe the cold differential is enough along with the preload to crack the canopy....
That being said, it looks like if you align those hole VERRY carefully during installation, you wont be introducing load due to thermal expansion.
The take away is Danny's approach looks like a good idea.
For all of us flying around with plans built canopies.... well..... I don't know the answer to that.
Except..... my canopy is still fine... and so are thousands of others..... So I have to conclude that there is some basic error in my calculations.. but, that being said, Danny's idea is still a good one.