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  #51  
Old 01-18-2018, 02:44 PM
whittfic's Avatar
whittfic whittfic is offline
 
Join Date: Apr 2007
Location: Auckland, New Zealand
Posts: 131
Default Acrylic thickness

A couple more complicating factors:

The thickness of acrylic sheet stock as it comes from the manufacturer can vary slightly across it's surface. Part of our business is making acrylic aircraft windows (including RV12 rear windows for the local builders btw) and for say a typical 3mm thickness we have seen occasional variations from about 2.8mm up to 3.2 in the one sheet. Generally not a problem because many of the windows we make are relatively small so we can avoid any issues. But for the bigger canopies who knows, this might be contributing to the cracking problem.

Something else to consider is that when a canopy is blown, especially the longer narrow types such as the RV8, there is a lot of stretch and thinning of the acrylic happening at the highest point. Starting off with say 3mm acrylic could easily see it reduced to around half that thickness in some cases (think of inflating a balloon). Add in slightly thinner than spec acrylic to start with and this could go some way toward explaining why some installations fair better than others.

I always cringe a little when I think of the compression forces being applied when rivets are used to directly secure acrylic. Not disputing that many get away with it for years on end but it's not ideal. With bolts many hundreds of psi can easily be applied when over-torquing nuts and bolts. That's why good practice is to tighten to a firm fit and then back off a full turn of the nut. It only has to be lightly tightened. Also the hole in the acrylic should be half as big again as the bolt diameter with each bolt centred in the hole i.e. 4mm bolt - 6mm hole. This is another disadvantage of rivets because they try to expand to fill the hole as they are pulled.

Personally I have gone with Sikaflex for my canopy and screen. Trying to give myself the best shot at a trouble free installation. Time while of course tell!

Clive Whittfield
Auckland
New Zealand
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  #52  
Old 01-19-2018, 05:25 AM
wilddog wilddog is offline
 
Join Date: Feb 2008
Location: va.
Posts: 674
Default Fast Back Canopy

Has anyone with the fastback mod had a cracked canopy? This mod would have a shorter canopy and get rid of the point where most cracks seem to occur.
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  #53  
Old 01-21-2018, 10:34 AM
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AlexPeterson AlexPeterson is offline
 
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Location: Ottertail, MN
Posts: 2,459
Default

I re-read this thread, and have some thoughts.

It seems we've been discussing stress concentration factors, which are typically used when, for example, one places a strip of material containing holes under tension. The stress is "concentrated" around the edges of the holes in the manner depicted here:



However, this does not fit our situation entirely. The above depiction assumes the tension is not being applied directly on the edges of the hole. In the case of canopies attached entirely or partially by rivets, once the acrylic has contracted enough to contact the rivets at the extreme ends, the force is applied at the hole. We now have a local contact stress which may be extremely high.

Bill's interesting test of attaching a strip of acrylic to a steel angle might have a different outcome if the strip were wider, yet still attached in the same way. The resulting force on the fastener will be proportional to how wide the strip is. Our canopies are really wide, as analogized to this strip test.

I guess for me, what remains a mystery, is why they don't all crack... When I take mine from a 45F hangar out into 0F or so weather, the clicking it makes is horrifying, once the engine is running and the rapid cooling begins. The clicking is presumably the acrylic shifting relative to the frame, as the mounting scheme was designed to allow. If it stops clicking I need to worry.

It has been doing this every winter for 1600 hours/16 years, and hopefully it will continue to behave. The -8 is obviously worse case, given its length and the apparent mis-match to the curvature near the aft end.
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  #54  
Old 01-21-2018, 10:41 AM
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rv6ejguy rv6ejguy is offline
 
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My 6A canopy also had a serious mismatch to the frame at the rear, like .625 inches at the middle support. I just put plastic standoffs there and longer bolts thinking it would be a bad idea to cinch it down to the frame.

The "clicking" sound worries you initially but 15 years later with thousands of heating/ cooling cycles, mine is still crack free, rivets, bolts and all.
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  #55  
Old 01-21-2018, 11:19 AM
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chepburn chepburn is offline
 
Join Date: Apr 2006
Location: Ottawa , Canada
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Default

Quote:
Originally Posted by AlexPeterson View Post
I re-read this thread, and have some thoughts.
I guess for me, what remains a mystery, is why they don't all crack... When I take mine from a 45F hangar out into 0F or so weather, the clicking it makes is horrifying, once the engine is running and the rapid cooling begins. The clicking is presumably the acrylic shifting relative to the frame, as the mounting scheme was designed to allow. If it stops clicking I need to worry.

It has been doing this every winter for 1600 hours/16 years, and hopefully it will continue to behave. The -8 is obviously worse case, given its length and the apparent mis-match to the curvature near the aft end.
Interesting... I have never heard my canopy click in frigid temperatures..I am assuming that is because the hangar is unheated, so things happen at a much slower rate. (or I'm not in the hangar when it's adjusting)

If we look at the current number of flying RV 8s at 1450 and assume 33% are enduring cold winters (I just yanked that number out of nowhere) so about 500. Lets yank another wild number out of the air and say that we have 20 or so cracked canopies (based on the 7 or so reported on this thread).... that gives us 4% failure rate. Not bad. (AND THIS IS ALL PURE CONJECTURE HERE)

If it really was due to thermal / attachment stresses I'm thinking there would be a lot more failures. Like other posters have said, its probably an accumulation of factors... flight loads... RATE of thermal changes, How well the canopy fit in the first place... Hole treatment on install.. etc etc.

I think if we install with care, the odds are with most of us that we wont experience the unhappy event.
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  #56  
Old 01-21-2018, 01:17 PM
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ronschreck ronschreck is offline
 
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Location: Gilbert, AZ
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Quote:
Originally Posted by chepburn View Post
Lets yank another wild number out of the air and say that we have 20 or so cracked canopies (based on the 7 or so reported on this thread).... that gives us 4% failure rate. Not bad. .
Not bad? If you are one of the 4% it is freaking terrible! And I speak as one of the 4%.

Canopy Crack 2 by Ron Schreck, on Flickr
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  #57  
Old 01-21-2018, 02:05 PM
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chepburn chepburn is offline
 
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Quote:
Originally Posted by ronschreck View Post
Not bad? If you are one of the 4% it is freaking terrible! And I speak as one of the 4%.

Canopy Crack 2 by Ron Schreck, on Flickr
Sorry Ron....

Its not my intent to minimize how awful it is. I -think- I was trying to vocalize that we don't know what the root cause is.

I wish we did so we could prevent it from happening to others. (My self included)
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  #58  
Old 01-23-2018, 12:33 PM
cmmguy cmmguy is offline
 
Join Date: Aug 2012
Location: North Carolina
Posts: 25
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Quote:
Originally Posted by AlexPeterson View Post
I re-read this thread, and have some thoughts.

It seems we've been discussing stress concentration factors, which are typically used when, for example, one places a strip of material containing holes under tension. The stress is "concentrated" around the edges of the holes in the manner depicted here:



However, this does not fit our situation entirely. The above depiction assumes the tension is not being applied directly on the edges of the hole. In the case of canopies attached entirely or partially by rivets, once the acrylic has contracted enough to contact the rivets at the extreme ends, the force is applied at the hole. We now have a local contact stress which may be extremely high.

Bill's interesting test of attaching a strip of acrylic to a steel angle might have a different outcome if the strip were wider, yet still attached in the same way. The resulting force on the fastener will be proportional to how wide the strip is. Our canopies are really wide, as analogized to this strip test.

I guess for me, what remains a mystery, is why they don't all crack... When I take mine from a 45F hangar out into 0F or so weather, the clicking it makes is horrifying, once the engine is running and the rapid cooling begins. The clicking is presumably the acrylic shifting relative to the frame, as the mounting scheme was designed to allow. If it stops clicking I need to worry.

It has been doing this every winter for 1600 hours/16 years, and hopefully it will continue to behave. The -8 is obviously worse case, given its length and the apparent mis-match to the curvature near the aft end.
Thermal expansion is in all directions and not just linear. The holes do get larger but not enough to account for the lack(in comparison to plastic) of expansion of the aluminum. The canopy would not have stress from simply getting warm and expanding but when it pushes, in a point fashion, against the fastener.

It seems that the canopy needs to float within the mounting system. Maybe using small slots instead of holes. Just commenting from a mechanical point of view and not from an experienced builder.
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  #59  
Old 01-23-2018, 07:48 PM
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fl-mike fl-mike is offline
 
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Default

So, how do planes like the P-51 manage the big bubble? Do they float in the frame? I can?t find details on how they are put together. One photo of a NOS canopy shows holes in the front bow area, but I don?t see any along the lower sides.
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  #60  
Old 01-23-2018, 09:05 PM
Scott Hersha Scott Hersha is offline
 
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Location: Cincinnati, OH
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The P-51 - first thing is, it's a 400+Knot airplane and has mush thicker plexiglass, and it might have a different composition for protection. I imagine it's not a 'blown' canopy. It's probably formed in a mold, which ensures a fairly uniform thickness throughout. This also ensures an accurate fit to its canopy frame. I think our problem with RV canopies, particularly RV8 canopies, is twofold. We have a method of attachment that is not ideal. Pulled rivets on plexiglass doesn't allow tempering the stress on the fastener. Secondly, the blown plexiglass canopy is almost never ideal in size and shape, relative to the engineered frame it sits on. There is almost always some stress during attachment to the steel frame, even if you are using sikaflex. This stress seems to be usually concentrated at the rear of the frame on the RV8. Starting the assembly at the rear v.s. the front may eliminate some of this stress. But stresses still exist in this type of assembly. Oversizing the holes will take care of some of this. Using screws that you can gently apply clamping pressure will help too. But the bottom line is, unless you have a plexiglass canopy that is fairly precisely formed in a mold to a frame, steel or otherwise, there's no way can can eliminate stress points, and possible failure due to varying homebuilt construction techniques with this type of structure.
If we were all willing to spend an extra $3K on our engineered canopy, we may have more favorable results. But given that the failure rate is 4%(numbers are suspect....) we are doing pretty good with an affordable canopy on our RV's.
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