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01-08-2018, 10:19 AM
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Join Date: Jan 2005
Location: Houston, TX
Posts: 868
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Quote:
Originally Posted by jthocker
I'm in the same boat as Scott. 1500 hrs. +. Lots of G, 10 years and it finally let go at 11,000 ft and -1 F.
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Uh oh... sounds like my canopy will be cracking any day now; I?m at 11 years and 1475 hours, no cracks yet. Yet...
Last month I was droning across northern Arizona and New Mexico at 9500? and 18F OAT; I kept waiting for the crack to happen but it didn?t. Lucky me.
__________________
John Bixby
RV-8 QB sn 82030 - 1750 hrs
O-360-A1D/CS/Pmags
Houston, TX
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01-08-2018, 11:21 AM
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Join Date: Aug 2006
Location: Taylor Texas
Posts: 811
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Plan B
The problem discussed here is very real, and steps to avoid the problem (plexi vs metal expansion rates) are nothing new.
I inserted a few steps in the F1 assy plans to help reduce the cracking problem: drill the w/s holes at ~3/8" before final assy and use a 'washer' cut from auto vacuum line as a buffer between the plexi and the fasteners. The plexi is retained by a tinnerman washer/screw, and it is not to be tightened down firmly - the idea is to allow the plexi to move around as necessary in changing temperatures - AND fuselage flexing. The fiberglass fairing is then laid up over the washer/screw setup. No fasteners are evident in a proper installation.
The slider canopy is sandwiched in the same manner as the RV-6 slider, but the holes are again oversized, and the canopy is sealed with paintable caulk - allowing some relative movement.
So far, no cracks from temp extremes in the fleet.
I don't see why such a setup could not be used on almost any aircraft that we are discussing here, but the -8 install would likely change to screws, not rivets. The screws could be hidden - overlaid with fiberglass, as is done with the w/s plexi.
YMMV, as usual.
__________________
Best,
Mark
"Not everyone needs a Rocket. Some folks, however, shouldn't live life without one.
You know who you are."
Budd Davisson, 1997
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01-08-2018, 01:31 PM
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Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,333
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Quote:
Originally Posted by Scott Chastain
I agree. Without a doubt, I remember fitting the canopy over 11 years ago and anguishing over the pre-loads required to fit the canopy to the frame. Even the Todd's canopy I installed would not fit the frame without those 1" pre-loads you reference here. And after over 1400 airframe hours, plenty of g-loads, cold Wx, turbulence, and blasting sunlight, she gave way on a tarmac on December 1st while I slept in a pilots lounge. Not even a Sikaflex installation could save her.
On a side note, I have not yet heard anybody mention shrinkage. Unless the material is MIL-P-8184 or some other pre-shrunk acrylic, I think we can always expect the canopies to shrink over time. Whether the installation can compensate for that shrinkage is the key question. It is clear, however, that Sikaflex is not the anti-crack panacea I bought into over a decade ago.
The repair post is forthcoming later this week . . .
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The reflective discontinuity across the crack seems to indicate (as you note, Scott) that more than thermal stresses are at work. Looks like either built-in curvature problems, or curvature which doesn't match the canopy frame.
__________________
Alex Peterson
RV6A N66AP 1700+ hours
KADC, Wadena, MN
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01-08-2018, 02:12 PM
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Join Date: Dec 2012
Location: chattanooga,tn
Posts: 231
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Quote:
Originally Posted by rvbuilder2002
There is nothing wrong with the RV-8 canopy mold, because there is no mold.
The canopies for the RV-3, 4 and 8 are free blown bubbles.
When making the canopy, the heated material is restrained within a frame that establishes what the perimeter shape will be around the base. Differential air pressure is used to push the bubble to the specified height. The overall shape is then whatever the bubble naturally settles at (all of the bubbles are snowflakes in a sense.... no two are exactly the same)
The molding frame shape is what establishes the free blown shape.
The profile of the molding frame could be changed to produce canopies that more closely fit the canopy frame but it would greatly influence the canopy shape so that head room would be reduced an undesirable amount (I.E., the molded width has a direct correlation to what the free blown height will be).
While I don't disagree that having a pre-load on the canopy as installed is a less than ideal situation, I have seen no indication that RV-8's have a higher tenancy towards cracking in service than any of the other RV models.
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Well, then there must be a lot of them cracking because mine cracked yesterday as well. That makes 5 ,that I’ve herd of, in one day. I’m beginning to think the only fix is to let it crack and live with it.
__________________
Tracy Willingham
RV-8
Powered Paraglider 
Pitts S2B- sold
Chattanooga, Tn
Dues Paid
Last edited by tracy : 01-08-2018 at 02:15 PM.
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01-08-2018, 03:35 PM
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Join Date: Apr 2006
Location: Ottawa , Canada
Posts: 224
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A lot of insightful comments so far..now...
I'm glad (although that really isn't the correct word here) there has been a lot of comments on possible causes for cracking of a canopy. I'm sure if we perused some soaring web boards, we would find quite a few as well... I know my old 1-34 had a lot of "stitching" on the canopy because we were too cheap to replace it.
Now the next obvious question:
For those of us with canopies that are just fine, what does the collective brain think should be done to mitigate cracking risk on installed canopies?
Based on the assumption that there IS preexisting stress on the canopy, either from installation loading or from thermal stress, what can we do ?
Obvious statement #1: Try to minimize environmental thermal shock so if winter ops, and you are in a heated hangar, cover the canopy with an insulating blanket when moving it outside.
Obvious statement #2: If you need to preheat your electronics in a COLD hangar, just preheat the panel.... NOT the whole cockpit.
There are other possibilities... like don't fly when OAT is below XXX Degrees..... but that's probably not too useful....
Any other ideas? There has to be a few....
Of course, the obvious one might be
Don't worry about it. Just fly the airplane and enjoy it. If it cracks at some point , deal with it..
__________________
Chris Hepburn
Ottawa, ON
RV-8 C-GOGO FLYING
Renew 12/20
Last edited by chepburn : 01-08-2018 at 04:03 PM.
Reason: Added a comment
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01-08-2018, 06:36 PM
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Join Date: Jul 2007
Location: Madison, Wisconsin
Posts: 778
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Quote:
Originally Posted by chepburn
There has been some discussion lately on cracked canopies and some speculation on the cold weather being the culprit....
...So, NOW with a deltaT of 37 deg we have:
3454*3 = 10360 PSI === SNAP
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Chris,
Interesting calculation!
Any idea what the shear strength of the aluminum rivets might be? Perhaps they would fail before the canopy? I did an experiment on this long ago while I was building, by riveting together two ~3 cm squares of acrylic with their edges offset by ~5 mm (I think I used two rivets). I then squeezed the assembly edgewise in a vise, which caused shear failure of the rivets but no cracking of the acrylic. Repeated the experiment with an assembly that had been chilled in the freezer and got the same result. I'm not sure how relevant this is to the problem at hand but it did make me wonder about how much deformation the rivets experience in actual service.
Like some of the other posters I've been flying for over 1000 hrs in temps down to -20C and no cracks - yet.
__________________
Alan Carroll
RV-8 N12AC
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01-09-2018, 08:33 PM
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Join Date: Apr 2016
Location: Bucking ham, Virginia
Posts: 208
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Your rivet shearing experiment used the acrylic plastic primarily in compression,I believe the cracks result from tension occurring in or to the canopy from its relative quick reaction to shrinkage from exposure rapid cooling. Also repeated "G" loading can account for the warm weather cracked canopies such as Ron Shreck's. As stated earlier in this thread accurate alinememt of mounting holes
( canopy to frame) and impeccable workmanship of the holes in the acrylic, and an isolating rubber/ silicone bushing with washers will most likely give the best service.
Sikaflex maybe better but I don't trust the bond of the powder coat to the frame.
__________________
Maule MXT-7(daily flyer)
Lancer 235(partner)
Raidial RV-8R Ephanage Done, Wings Done, Fuselage on it's tall Grove gear & engine hung
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01-10-2018, 06:08 AM
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Join Date: Aug 2010
Location: McAlpin, FL
Posts: 253
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Good Morning All,
I have a couple of ideas concerning the canopies cracking when sikaflex is used. I wonder how thick the filet is on the canopies that failed. If my guess is correct, it's as thin as possible. Personally, I always try to keep weight to a minimum, and I expect most others do too. I don't think sikaflex has enough flex in it if the gap it's filling is too tight. That and it gets quite a bit stiffer when it's cold. I did the refrigerator/freezer test back when I used it on the RV8A. After seeing how stiff it became I opted to use a fairly thick spacers to set the gap.
Can those of you with cracked canopies post the thickness of the filet used. Maybe we can figure out if there's a minimum thickness that will keep this from happening, or if I have an over active imagination.
Thanks, Lance
__________________
Lance Logan
McAlpin, FL
Plane at FL10
Scratchbuilt Biplane
Preceptor N3 - Sold
Zenith Zodiac 601HD - Sold (good riddance)
Kitfox IV - Sold my share, but loved that plane
RV8A Sold! Loved the plane, but not the nosewheel!
RVX (RV6/4) Sold
Cherokee 180 pickup truck
RV8 Fastback
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01-10-2018, 07:00 AM
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Join Date: Jul 2007
Location: Madison, Wisconsin
Posts: 778
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Quote:
Originally Posted by VA Maule
Your rivet shearing experiment used the acrylic plastic primarily in compression,I believe the cracks result from tension occurring in or to the canopy from its relative quick reaction to shrinkage from exposure rapid cooling.
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Interesting point. It doesn't seem like this should make a difference in terms of material strength though? It probably would affect the direction of crack propagation.
I do know there has been some deformation/movement of the aluminum rivets on my canopy, based on paint cracks that have formed around the rivet heads. This is most noticeable at the aft end of the canopy skirt.
__________________
Alan Carroll
RV-8 N12AC
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01-10-2018, 07:06 AM
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Join Date: Sep 2005
Location: Maple Grove, MN
Posts: 2,333
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Quote:
Originally Posted by chepburn
SNIP
The thermal expansion Ct for steel is approximately 0.00008 in/inR
The thermal expansion Ct for acrylic is approximately 0.000417 in/inR
SNIP
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I believe there is one zero missing from each coefficient.
4130 Steel: about 7 x 10E-6
Acrylic: about 40 x 10E-6
Both with in/in/R units.
__________________
Alex Peterson
RV6A N66AP 1700+ hours
KADC, Wadena, MN
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