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  #11  
Old 01-07-2018, 07:31 PM
ambrosesquadronca ambrosesquadronca is offline
 
Join Date: Sep 2005
Location: Quartz Hill, CA
Posts: 16
Default Canopy Cracking

My personal opinion is there's a fundamental problem with the RV-8 canopy mold that results in a sprung-out piece of plastic resulting in too much preload in the attachment to the frame. I believe this is the primary contributor (or certainly significant) to RV-8 canopies cracking in the back. The design of the canopy weldmet tooling should be very closely matched to the canopy mold and their resulting shapes. Not and easy problem to solve and expensive to get right.

After spending literally 1 month of tweaking my frame and canopy position, the best I could do was a 1" sprung-out condition in the back third area as others have reported. This preload really bothered me and meant more work on the canopy skirt. I put an extra thick Sikaflex bond line in that area to help minimize its effect. It will be a very sad day if I ever hear it pop back there and my heart goes out to those who have. Don't remember if I measured the force required to spring the canopy into position on the frame but this should be added to the stress analysis above.

Having designed composite and plastic parts for modern fighters for over 20 years now there are two approaches we take to getting the final shape correct - analytical and trial and error. Analytical doesn't work well regardless of the number of white papers on the topic. There's a computer program that claims it can take your part shape, cure cycle, part material and tooling materials and tell you the shape of the tool required to build a perfect part. Hopefully this program will work well some day. We plan on 3 sets of tooling for a molded part which is a very expensive approach. Two sets won't do it as the problem is non-linear. If the part is sprung-out 1" and you compensate the tool 1" in, your part will still be sprung out .5" or so. By the third attempt the part is pretty close and you've ran out of money to make it any better.

Having spent a fair amount of time on redesigning plastic parts that had cracking problems in the field due to loaded fasteners/holes I've concluded and preach this to young engineers, don't design holes in plastic parts - ever! The solution is always to remove the holes and design a frame with proper gaps to trap the plastic part. We've played extensively with hole tolerance, "super isolating grommets/inserts", silicons and the holes still crack out even when the stress analysis says they shouldn't. Some suppliers are starting to claim they have invented a "super plastic" that won't crack at the fastener holes under load, we'll see.

I believe Vans told me along time ago the -8 canopy mold/tool was built off the prototype canopy. I would be curious to know if they attempted to make any spring-back or shrinkage adjustments to the tool. I would enjoy helping Vans adjust their canopy frame tooling and canopy tooling to eliminate preload. This and making Sikaflex the standard attachment method could significantly eliminate cracks.

Dave
RV-8A
921E
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  #12  
Old 01-08-2018, 12:15 AM
Jim Frisbie Jim Frisbie is offline
 
Join Date: Jan 2016
Location: Canby
Posts: 21
Default Canopy

This discussion makes me glad I used Sikafllex not rivets or screws to install my windscreen. I wish I had also used it on the canopy.
Has anyone done cold weather tests on canopies glued on va screws?
After 700 hrs I have had no hints of cracks or stress on it.
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  #13  
Old 01-08-2018, 04:24 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
Join Date: Apr 2010
Location: Clinton, Indiana
Posts: 648
Default +1 for ambrose.... comments.

Working with frame and canopy components from Vans is a disadvantage. I understand most working guys with families are not excited about turning the canopy into a 3 month project. But, if you reference the points made above , below is the path I have taken twice:
1-Form carbon fiber inner frame that Replaces the
metal. This should include a shelf for the canopy to sit on and 1/8” clearance to the canopy for Sikaflex.
2-form up your canopy in wooden/angle iron fixture and have Jeff ( airplane plastic) stress relieve it. That removes initial stresses just to fit in the frame.
3- laminate divinicell foam to the CF for a spacer to the outer canopy skirt skin.
4- form outer skin with glass bonded to foam, not plexi, so they expand differentially.
Finish the cosmetic work and put the canopy in the slot with Sika on the CF side only. I believe it is well known that long canopy tails leak air badly due to expansion; even my little -3 does, so a fast back has advantages. Cold shocking is apparently a hazard that only you can deal with.
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  #14  
Old 01-08-2018, 06:14 AM
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Jaypratt Jaypratt is offline
 
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Location: Hicks Airfield, Fort Worth,Texas
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Default Too wide

The canopy’s are too wide in the rear part. We have to preload them.
I think if the things fit better in the initial fitting, we would have fewer cracks
My $.20. Includes. .18 for inflatition -;
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Last edited by Jaypratt : 01-08-2018 at 06:24 AM.
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  #15  
Old 01-08-2018, 06:19 AM
rv8ch rv8ch is offline
 
Join Date: Feb 2005
Location: LSGG
Posts: 2,331
Default crack even with sika - wow

Quote:
Originally Posted by Scott Chastain View Post
... It is clear, however, that Sikaflex is not the anti-crack panacea I bought into over a decade ago. ...
Hi Scott, really sorry to hear about that crack. Not that I can do anything about it on my canopy which is already complete, but it could be interesting for new builders to find out where it started, how thick your sikaflex layer was, temperatures, etc.
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  #16  
Old 01-08-2018, 06:27 AM
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jthocker jthocker is offline
 
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Location: Cincinnati, Ohio
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Default

Quote:
Originally Posted by Scott Chastain View Post
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 . . .
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. My process for fitting the canopy has changed since I did this one.
After initial fitting and the "big cut", I know start clamping at the rear and work my way forward. The canopy stays in place at the top middle but grows forward at the bottom forward windscreen split. I drill it this way and cut off a wedge shaped sliver from each side of about 3/4 in to nothing and maybe 18 in long. Way less stress at the rear this way and the skins fit almost perfectly.

Alas though, mine cracked 8 inches aft of the windscreen split.
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  #17  
Old 01-08-2018, 06:44 AM
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DanH DanH is offline
 
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Location: 08A
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Default

Quote:
Originally Posted by Larry DeCamp View Post
Working with frame and canopy components from Vans is a disadvantage. I understand most working guys with families are not excited about turning the canopy into a 3 month project. But, if you reference the points made above , below is the path I have taken twice:
1-Form carbon fiber inner frame that Replaces the metal. This should include a shelf for the canopy to sit on and 1/8” clearance to the canopy for Sikaflex.
The CTE of carbon/epoxy laminate is near zero, which makes it a very poor choice for this application, certainly far worse than steel. The problem is compounded by its high modulus. It is working as above only because (1) the design floats the canopy on highly deformable foam and sikaflex, and (2) apparently does not bend the plexi to make it conform to the frame.

One could argue that the CF frame doesn't change dimension with temperature change, so it always fits the fuselage, but that would be true only if the fuselage was also a carbon/epoxy structure. It would have a big CTE delta with an aluminum fuselage.

If a composite canopy frame is desired, glass fiber/epoxy is the superior choice, as its CTE is about half that of acrylic plastic. Compare with aluminum at roughly 3x less, steel at 5-6x less, or carbon/epoxy at about 70x less.

BTW, if you really want a metal frame, plutonium would result in the least CTE delta with acrylic.

Assembled glass/epoxy frame from a Showplanes fastback kit. Doing fine so far. Something similar could be fabricated for a standard turtledeck, although there is an advantage to the shorter acrylic bubble.



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Last edited by DanH : 01-08-2018 at 06:56 AM.
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  #18  
Old 01-08-2018, 08:50 AM
Larry DeCamp's Avatar
Larry DeCamp Larry DeCamp is offline
 
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Location: Clinton, Indiana
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Default Good points Dan

Yes Dan, I am a little casual about CTE. Assuming fuse clearances fore and aft are generous enough. I may be mistaken but it feels to me like CF achieves dimensional stability better than glass for the structure. That said glass may be a better choice for the whole task.Just trying to complete the dialogue that may help someone.
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  #19  
Old 01-08-2018, 08:55 AM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Default

Quote:
Originally Posted by ambrosesquadronca View Post
My personal opinion is there's a fundamental problem with the RV-8 canopy mold that results in a sprung-out piece of plastic resulting in too much preload in the attachment to the frame.
Quote:
Originally Posted by Jaypratt View Post
The canopy’s are too wide in the rear part. We have to preload them.
I think if the things fit better in the initial fitting, we would have fewer cracks
My $.20. Includes. .18 for inflatition -;
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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  #20  
Old 01-08-2018, 09:57 AM
Canadian_JOY Canadian_JOY is offline
 
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 1,748
Default

As a counterpoint I'll comment on the Glastar / Sportsman windshield installation technique, which is quite similar to the Sikaflex method of bonding the windshield with an adhesive much akin to RTV silicone.

While clearly not as demanding an installation as the RV8 (not a full, tight bubble) the Glastar and Sportsman windshields span a large area with compound curves and are exposed to significant air pressure loading in flight. Yet breakage of windshields is very rare indeed, and almost always as a result of an "initiating event" as Paul Dye suggested.

What's the difference? The windshields are bonded with what effectively is an elastomeric, are bonded to a composite frame, and are free of pre-load when bonded in place.

Ultimately I believe it's the combination of pre-load, through-hole fasteners and age/thermal cycles that are the nemesis of RV canopies.
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