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The Polar Vortex got me !

P

petersb

Well Known Member
The Polar Vortex got me, cracked canopy !

After 2 1/2 years building and then only 1 month flying I went out to the hangar and found large cracks in my slider canopy.

This occured after several -25 C evenings.

The first crack originates from a rivit just behind the canopy handle and goes in both directions about 6 inches, the second is four rivits back in both directions about 4 inches each way.

Have stop drilled and will reinforce so I can keep flying for now.

From what I have read in the Forums it seems that most slider cracks mostly originate from the top center bow.

So my thoughts re: replacement is to rivit the foreward and aft bows and use the existing screws in the skirt, however, use glue (sika) for the centre bow. This should allow a good fit with the existing windscreen fairing etc.

Use the clecos in the exisiting rivit holes in the top center bow to hold the plexi in place untill the glue sets. Some have suggested still installing the top rivits, they will also hold the trim strip in place, the glue will stop the plexi expanding with temperature changes and putting pressure on the rivits
 
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Ouch! Sorry to hear about your canopy, I just finished mine last summer. I did lots of reading here from folks a lot smarter than I. And what I discovered and completely comprehend is the fact that the coefficient of thermal expansion is significantly different in expansion rates between plexi and steel. This is the reason I went with Silkaflex. Here in the sierras we experience -20 to over 100 F. Hopping the rubbery stuff is nice to me :D
 
petersb said:
Use the clecos in the exisiting rivit holes in the top center bow to hold the plexi in place untill the glue sets. Some have suggested still installing the top rivits, they will also hold the trim strip in place, the glue will stop the plexi expanding with temperature changes and putting pressure on the rivits
Click to expand...

No, adhesive will not stop the expansion or contraction of plexiglass. The adhesive's modulus of elasticity is surely too low for that.

Even if you could fasten plexiglass to an absolutely rigid frame with an absolutely rigid adhesive, the result would merely be very high stress in the plexiglass. In truth, the perfect installation would allow the plexiglass to expand and contract freely, without constraint by the frame.

Canopies crack because of a bad combination. We (1) introduce weak spots, and then (2) guarantee high stress by attaching them to a relatively rigid steel frame with an inelastic fastening system (rivets and screws).

The weak spots are the holes, as well as various nicks/gouges on panel edges and the edges of the holes. The stress multiplication factor of a clean hole is 3. The stress multiplication factor of a nick varies from small to infinity, depending on the sharpness. The really bad news? If you look at one of your drilled holes with a magnifier you'll learn that a drill rarely leaves a clean hole in plexiglass. The edges are full of nicks unless you carefully grind them to smoothness.

Steel and plexiglass both change dimension with temperature change, but their coefficient of thermal expansion is different by a factor of 6 or more. For each degree of temperature change the plexiglass expands or contracts 6X more than the steel.

OK, so look at the slider canopy with its centerline bow. The plexiglass is fastened at the front, rear, and lower sides. It tries to contract when the temperature drops, but the centerline bow serves as a compression strut, resisting the contraction in the fore and aft axis. The result is very high tensile stress in the plexi, along an axis with a bunch of holes in it...and it pulls apart at one of the holes.

You improve your odds by (best to less) eliminating the strut (not practical), eliminating the stress concentrations (meaning no holes), or making the edges of the holes perfectly smooth (stress concentration reduced to only 3x).

Here's a flat panel model. From left to right, normal temperature (no stress), reduced temperature, and reduced temperature with centerline strut.

n3t6jk.jpg
 
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Sorry to hear Peter and thanks for sharing. I fly all year around all aver the place and I am watching my plexi closely.
Dan great lecture on thermal expansion thanks.
 
Dan did his usual great job of explaining the stresses and the cause of the problem - can't get much better than that!

I will add, however, that since you are a "holeguy" and not a "Sikaflex Guy" (at this point - it is tough to change over from one to another once the plane is finished, because the fit is different), a trick that we are trying with holes is to use oversized holes and make washers out of surgical rubber tubing. You can buy the soft tubing with 1/4" OD and 1/8" ID. Cut it in to washers as thick as the canopy. Drill 1/4" holes in the canopy, insert the washer, then the rivet or screw goes in the middle. This allows the canopy to expand and contract a bit without it coming up against the hard fastener.

This method seems to be working for a couple of us that have used it on our second (or, in some cases, third) RV-8 canopies....
 
Dan, Paul, Vlad, thanks for the replies.

So how about my thoughts of using rivits around the perimeter (so it will fit existing structures ) and gluing the center bow where most of the tension will concentrate ?

I thought that holes would still have to be drilled in the center bow plex area to hold it down during drying ?

The original holes where drilled using a unibit, thought this would give a smooth hole surface, and then debured with no skip deburer

On to my temporary crack repairs, looking at Plastex from ACS. Some cracks are touching and a thin Weldon 3 type product may work but one has a 1/32 gap, the two sides of this crack are also at different heights so the glue would also need to work vertically to hold in alignment.
 
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holes, grommets etc.

we've done exactly like Paul, and I think it's a good idea, just an observation;
with something as big and complex as the slider or tipup, it is pretty likely that the final construction fit will be 'x', and the hot or cold temp will be 'y' and 'z'....meaning, that at some conditions, or the point of worst fit, the surgical tubing or other grommet is mashed up against one side of the oversize hole, and eventually splits etc. ( we tried surgical tubing, fuel line, rubber, nylon tubing etc. etc.)

We did several flat wrap windshields, and found it really hard to get the fit exact enough so each bolt was centred in it's hole.
Still worth doing oversize holes, just saying they are still subject to a lot of variables.
good luck!
 
Ironflight said:
Dan did his usual great job of explaining the stresses and the cause of the problem - can't get much better than that!

I will add, however, that since you are a "holeguy" and not a "Sikaflex Guy" (at this point - it is tough to change over from one to another once the plane is finished, because the fit is different), a trick that we are trying with holes is to use oversized holes and make washers out of surgical rubber tubing. You can buy the soft tubing with 1/4" OD and 1/8" ID. Cut it in to washers as thick as the canopy. Drill 1/4" holes in the canopy, insert the washer, then the rivet or screw goes in the middle. This allows the canopy to expand and contract a bit without it coming up against the hard fastener.

This method seems to be working for a couple of us that have used it on our second (or, in some cases, third) RV-8 canopies....
Click to expand...

Hi Paul, I just did some calculations for what stresses would be induced if the length of the plexi was constrained along it's length with a steel backing and it was only 2000 psi at a 100 F temp delta. Also assumed is zero shear displacement at the interface.

The conclusion was that while it is thermal diff that is the root cause, the real failure point is the resulting force concentrated on a few holes. Here is why it seems this way: For 100 F temp delta from a nominal zero stress installation, there is a .065" contraction ( at each end) over 30 " length, assuming the center remains in the center. (2*.065" total) This strongly supports your soft centering solution approach. The only detail that may improve your design (if I remember it correctly) it would be for the tubing sleeve to extend through the frame and use a larger washer under the nut. That would allow twice the shear displacement between the plexiglass and the frame than the standard hole for the fastener. At least for the fasteners at the ends of long pieces.

This also means I will have to allow an adequate gap for hot day expansion on my 7 tip up :).
 
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It's been so cold in Minnesota this winter that I'm near deciding just to take all the screws out of the tip up for the winter. This -25 stuff is pretty serious business.
 
After many years of watching threads about this, I am still not convinced there is anything but circumstantial evidence of what causes some canopies to crack and others in seemingly identical conditions not too.
I love Paul's approach and would do that next time.
I took great care to insure there where no stresses induced by a poorly fitting canopy by adjusting the canopy frame where I could and using nylon spacers where I couldn't. Oversized the holes one size above factory recommendation at that time and I have been lucky.
However, I have not experienced anything below zero.

I took a piece of canopy scrap, the long piece cut from the lower edge, and a few of the bow pieces, put them in the freezer (0 deg), drilled them with split points, beat them with a hammer, bent them in a vice, whacked the unfinished edge, and abused them in many ways. I could not get them to crack. I know that isn't very scientific, but you would have thought....

My canopy is going to crack tomorrow now. I just feel it!
I wish we could figure this out. I feel,so badly for folks when they have done all the right things and it still happens.
 
RV7 four hangars down had his canopy crack just after phase one last year. Also a very cold day. Two large cracks starting at about the 5th rivet back from the handle straight to the sides, and another one about 6 rivets behind that one, also straight to the sides. Stopped drilled the ends and placed clear shipping tape over them.

I offered to help him repair/replace the canopy but he is having too much fun and doesn't want to stop flying. Need to twist him arm and get him to fix it soon.
 
flyboy1963 said:
we've done exactly like Paul, and I think it's a good idea, just an observation;
with something as big and complex as the slider or tipup, it is pretty likely that the final construction fit will be 'x', and the hot or cold temp will be 'y' and 'z'....meaning, that at some conditions, or the point of worst fit, the surgical tubing or other grommet is mashed up against one side of the oversize hole, and eventually splits etc. ( we tried surgical tubing, fuel line, rubber, nylon tubing etc. etc.)

We did several flat wrap windshields, and found it really hard to get the fit exact enough so each bolt was centred in it's hole.
Still worth doing oversize holes, just saying they are still subject to a lot of variables.
good luck!
Click to expand...

I see the advantage of using rubber grommets, however, wont the dimple in the trim strip still be in direct contact with the plexi and this would then be the point of failure.

I remember with my RV6 I did all the drilling and fitting at higher temperature and when refitting at the hangar at normal temperature it had shrunk and would not fit, had to reheat to install the rivits. This means at normal temperatures there would always be stress on the plexi , other than being careful with the drilling, it would seem to be better to do the fabrication close to normal temperature. ( The RV6 tip up canopy flew 12 years and 1600 hours without cracking )
 
petersb said:
I see the advantage of using rubber grommets, however, wont the dimple in the trim strip still be in direct contact with the plexi and this would then be the point of failure.

I remember with my RV6 I did all the drilling and fitting at higher temperature and when refitting at the hangar at normal temperature it had shrunk and would not fit, had to reheat to install the rivits. This means at normal temperatures there would always be stress on the plexi , other than being careful with the drilling, it would seem to be better to do the fabrication close to normal temperature. ( The RV6 tip up canopy flew 12 years and 1600 hours without cracking )
Click to expand...

In the -8's, the rivets are going through the relatively thick skirt, which acts like a big washer plate. Effectively, the plex is sandwiched between the skirt and teh frame, and with the oversize hole and soft grommets it can float. This doesn't work for the front bow rivets of course, ands just polish the hole well and cross our fingers for those.
 
NOOOOOOOOOOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!

Paul K said:
...... Stopped drilled the ends and placed clear shipping tape over them.
Click to expand...
:eek:

there's gotta be something better than clear packing tape....it has to be the WORST to get off!!!

Friends don't let friends use clear tape...except on boxes! ;)
 
Ironflight said:
In the -8's, the rivets are going through the relatively thick skirt, which acts like a big washer plate. Effectively, the plex is sandwiched between the skirt and teh frame, and with the oversize hole and soft grommets it can float.
Click to expand...

In order for it to float you also need to not sandwich it down tight with the rivets or it will just bind up between the frame and the skirt and not be able to move. Do you also not rivet it down so tight?
 
plehrke said:
In order for it to float you also need to not sandwich it down tight with the rivets or it will just bind up between the frame and the skirt and not be able to move. Do you also not rivet it down so tight?
Click to expand...


Aluminum rivits are used to reduce the pressure, however, you have no control on how tight the fit.

It was suggested to use screws in the centre bow but as my bow is already drilled the existing holes are too large to tap for 6 - 32 screws.
 
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Paul K said:
flyboy1963

The cracks are 6 to 8 inches long! The canopy is junk.
Click to expand...

I noticed on the forum that another builder used Plastifix to repair 6 inch cracks with success, will try that as a temporary measure until canopy can be replaced
 
Slider Canopy Repair

petersb said:
I noticed on the forum that another builder used Plastifix to repair 6 inch cracks with success, will try that as a temporary measure until canopy can be replaced
Click to expand...

Sorry to read about your canopy cracking, I know how you feel.:mad:

I repaired my slider with Platifix, that may have been the post you are referring to. I can assure you the cracks were more than 6" long. The repair is holding up well, a bit unsightly from the inside but barely visible from the ground.
I did open the holes in the canopy where the screws attach it to the center bow of the canopy frame. I also used slices of soft tubing as a spacer to act as a spacer or washer. I believe this also is a good idea as it allows the canopy to move a bit as it expands and contracts.

I will replace it eventually but as it is holding up just fine I will put it off a while, OK so I'm a bit cheap.:D
 
Skid said:
Sorry to read about your canopy cracking, I know how you feel.:mad:

I repaired my slider with Platifix, that may have been the post you are referring to. I can assure you the cracks were more than 6" long. The repair is holding up well, a bit unsightly from the inside but barely visible from the ground.
I did open the holes in the canopy where the screws attach it to the center bow of the canopy frame. I also used slices of soft tubing as a spacer to act as a spacer or washer. I believe this also is a good idea as it allows the canopy to move a bit as it expands and contracts.

I will replace it eventually but as it is holding up just fine I will put it off a while, OK so I'm a bit cheap.:D
Click to expand...

Thanks for the reply. I sent you a PM in regards to applying the Plastiflex, would like to use it this weekend if possible

Peter
 
slider canopy

I saw a 6A slider recently that had no rivets on top or cover strip. I do not know how long it had been flying. There were a couple of spots that had some wear on the plexi from the center frame member. Builder said it was done to save weight. Great idea if it is safe and holds. After all, tip ups don't have reinforcement there. Randy
 
rstisser said:
I saw a 6A slider recently that had no rivets on top or cover strip. I do not know how long it had been flying. There were a couple of spots that had some wear on the plexi from the center frame member. Builder said it was done to save weight. Great idea if it is safe and holds. After all, tip ups don't have reinforcement there. Randy
Click to expand...

Interesting, so no glue or rivets ?
 
I used a process similar to Paul's too on my slider...

No cracks so far, but the Las Vegas temp swings aren"t too bad except when I go really high.

I used no rubber spacers, just drilled the plexi holes along the top bow larger than the 1/8 soft rivet. I ran a strip of teflon tape between the center top bow and the inside of the plexi. To deal with the dimples on the trim strip, I just countersunk the plexi deeper than normal, allowing the plexi to move (theoretically at least) without the dimple in the trim strip being a problem. A one hole countersink (from Cleavland) worked well in the microstop to countersink the plexi.

I also made trim strips for the front edge of the slider canopy and used the same hole method to allow some movement. Those two trim strips took many tries 'till I got the curve right. If you do this, start extra wide and trim to fit after the holes and dimples are in place. Once everything fits you can trim the trim strip to the proper width.
 
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