VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.


Go Back   VAF Forums > Main > RV General Discussion/News
Register FAQ Members List Calendar Search Today's Posts Mark Forums Read

View Poll Results: Did you Sika your canopy
Yes, and I am happy with the result. 43 30.28%
Yes, but I am not happy with the result. 0 0%
No. Satisfied with what I have. 18 12.68%
No, but wishing I had 4 2.82%
I am planning to when I get to that stage. 40 28.17%
I am still on the fence. 20 14.08%
I might, depends on what this poll reveals. 7 4.93%
No way, Never!! 7 4.93%
King method- Rivets and Sikaflex 3 2.11%
Voters: 142. You may not vote on this poll

Closed Thread
 
Thread Tools Search this Thread Display Modes
  #81  
Old 05-14-2008, 03:48 PM
lorne green lorne green is offline
 
Join Date: Dec 2005
Location: Oliver, B.C. Canada (Okanagan valley)
Posts: 786
Default Sika flex

Quote:
Originally Posted by Captain Avgas View Post
In answer to your question, having the canopy frame made of epoxy or aluminium would lessen that stress because both of these materials have a CLTE that is closer to acrylic.
My tipup frame is aluminum, thank goodness. I believe that if we follow Sika's specified instuctions for this application (1/8" minimum fillet etc.), there should be few problems. As posted previously, with the front fairing fiber glassed/sika'ed and the side aprons rivetted, the only area on the canopy which is not "reinforced" is the trailing edge which has no other support, other than Sika acrylic adhesive. Captain Avgas. I'm enjoying sitting on the sidelines reading these different posts. Thanks for bringing up some important safety issues with regard to our projects.
__________________
Lorne
RV 7a tip-up
Pre-cover MD-RA Inspected.
Canopy completed. Bonded with Sika-Flex.
Up on her mains, Firewall Fwd and wiring on going.
  #82  
Old 05-14-2008, 08:01 PM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,929
Default The boy with his finger in the dike.

Quote:
Originally Posted by rvtach View Post
Now before you start banging your head on the keyboard... I spoke with Sika USA about how to apply the adhesive between 2 perpendicular surfaces. My specific concern was how to apply the glue between the canopy and the side bows on my -7A slider canopy. They said that a fillet type joint was structurally as strong as a 1/8" lap-type joint. The thing here is that a fillet of glue applied into the corner between 2 surfaces has a lot of material and at the thickest point is 3/8-1/2" thick.
Dear Jim, please don't be offended (I truly don't mean to offend) but I think this might be a case of a small amount of knowledge on your part being dangerous.

I am imagining that you are referring to a "fillet" bond as a bond between two perpendicular surfaces (ie the bond edges are at 90 degrees to each other and touch at the root of the fillet and the "fillet" itself is triangular in cross section).

If that is the case then that is virtually the worst case scenario for an elastomeric bond between two parts with differential movement because it places INFINITELY high stresses at the root of the fillet. The sealant bead will fail almost immediately at the root (ie the point at which the 2 bonding surfaces converge) regardless of the side dimensions of the bead.

As a sole means elastomeric bond that configuation is useless. To make that configuration work you would need to run a closed cell polyethylene foam backer rod along the root before applying the sealant. This will prevent the sealant bonding to the root (and it will not bond to the polyethylene). I have attached a drawing to explain this better.



I would suggest that when you asked for advice from Sika one of two things (or possibly both) occurred.

1. You did not give them a drawing to fully detail exactly what it was you were proposing to do. You simply rang them up and everyone became confused trying to discuss a very technical matter over the phone.

2. You were talking (as many builders seem to do) with the absolute bottom of the food chain in scientific knowledge at Sika and received very poor information. The bottom of the food chain at Sika is the pimply faced kid just out of college who has absolutely no science background but goes by the title of "technical representative" when in reality he is nothing more than a salesman trying to increase his quota by flogging off a few more tubes of goo to whoever.

*****************************************

To all and sundry. I have to give this away now and leave you all to your own devices because it is never ending and I'm starting to feel like the boy with his finger in the dike.

In the final analysis I have come to the conclusion that the enthusiasm with which builders embrace Sikaflex as a SOLE means of canopy restraint is inversely proportional to their scientific understanding of the subject.

My best advice is to not use Sikaflex as the only means of restraining your canopy. Back it up with mechanical fastening of some type as insurance. That is the conservative thing to do.

In my opinion there WILL be Sikaflex canopy failures.... eventually. I do not know if the first one will be in a day or a month or a year or several years...but I am 100% certain that there WILL be failures given sufficient time and exposure.

I say that because it is quite obvious that most builders are not technically up to the very demanding task of designing and fabricating a fully bonded canopy with materials exhibiting such vastly different coefficients of linear thermal expansion. In particular the understanding of optimised sealant joint geometries is an absolute science in itself and RV builders who use "intuitive" principles are destined for disappointment. Those who blindly follow others who have no expertise in the area will obviously fare no better.

It is also not possible to predict how the impending failures of canopies without any mechanical fastening might manifest themselves. At the one extreme they might be local and lend themselves to repair if they can be detected early. At the other extreme they might result in something much more catastrophic.

Thank you to those people who emailed me personal messages thanking me for my technical input to the subject.

Cheers
__________________
You’re only as good as your last landing
Bob Barrow
RV7A

Last edited by Captain Avgas : 05-14-2008 at 10:01 PM.
  #83  
Old 05-14-2008, 09:38 PM
apatti apatti is offline
 
Join Date: Dec 2005
Location: Middle Georgia
Posts: 654
Default How about this...

First, thanks to Capn Avgas and the others who have contributed to this debate. I certainly understand the issues here better. It would be nice if Capn can continue to contribute... But, alas, it does take a lot of time to type well-thought out posts.

How about this... Would it be helpful to design in a few early indicators of pending failure. By that I mean, could we select a few spots where we would purposefully use a smaller fillet for about an eigth of an inch or so. These weaker bonds will hopefully show the signs of stress well before the rest of the joints fail, thus alerting us to the need for proactive measures. Kind of like wear bars on tires. Is this practical?
__________________
Tony
RV8A
N97AP
Warner Robins, GA
Phase I complete
  #84  
Old 05-15-2008, 09:28 PM
DGlaeser DGlaeser is offline
 
Join Date: May 2005
Location: Rochester Hills, MI
Posts: 898
Default It will be obvious...

Quote:
Originally Posted by apatti View Post
First, thanks to Capn Avgas and the others who have contributed to this debate. I certainly understand the issues here better. It would be nice if Capn can continue to contribute... But, alas, it does take a lot of time to type well-thought out posts.

How about this... Would it be helpful to design in a few early indicators of pending failure. By that I mean, could we select a few spots where we would purposefully use a smaller fillet for about an eigth of an inch or so. These weaker bonds will hopefully show the signs of stress well before the rest of the joints fail, thus alerting us to the need for proactive measures. Kind of like wear bars on tires. Is this practical?
Pending failure will be easy to spot with regular inspection. The 'weaker' spots in your concept will only show signs of stress if the 'stronger' ones do as well. The softness of the Sikaflex means you will see evidence of movement well before the material is close to failure.
I hate to be contrary, but Capn Avgas's explanation is way off base. I think the Sikaflex folks have been doing this for a while now and do have a clue about how their product works.
A proper bond would have not only a complete fillet, but bonding agent between the two parts as well, in a decent gap. As the two parts move, due to thermal expansion (or any other force), the Sikaflex 'stretches' to accomodate the movement. A nice fillet reduces the peel stresses that would tend to pull the Sikaflex away from the surface at the edges.
If in doubt, bond a couple of scrap pieces of plexi together with Sikaflex and judge for yourself how well it works.
__________________
Dennis Glaeser CFII
Rochester Hills, MI
RV-7A - Eggenfellner H6, GRT Sport ES, EIS4000, 300XL, SL30, TT Gemini, PMA6000, AK950L, GT320,
uAvionixEcho ADSB in/out with GRT Safe Fly GPS
  #85  
Old 05-15-2008, 09:48 PM
szicree szicree is offline
 
Join Date: Jan 2005
Location: SoCal
Posts: 2,061
Default

Cap'n Avgas,

I feel your pain. If I understand your position, it is not that Sika is an unsuitable product for this application, only that there has been little true engineering put into the Sikaflex use. Your point that the shear stresses go to infinity as the gap goes to zero is absolutely lost on many. It's like folks saying that you can't tear a phonebook in half, but we've all seen it done by concentrating the stress to a small area.
__________________
Steve Zicree
Fullerton, Ca. w/beautiful 2.5 year old son
RV-4 99% built and sold
Rag and tube project well under way

paid =VAF= dues through June 2013
  #86  
Old 05-16-2008, 01:44 AM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,929
Default

Quote:
Originally Posted by DGlaeser View Post
I hate to be contrary, but Capn Avgas's explanation is way off base. I think the Sikaflex folks have been doing this for a while now and do have a clue about how their product works.
A proper bond would have not only a complete fillet, but bonding agent between the two parts as well, in a decent gap. As the two parts move, due to thermal expansion (or any other force), the Sikaflex 'stretches' to accomodate the movement. A nice fillet reduces the peel stresses that would tend to pull the Sikaflex away from the surface at the edges.
If in doubt, bond a couple of scrap pieces of plexi together with Sikaflex and judge for yourself how well it works.
There's nothing like accusing a man of being a bullsh*tter to draw him back into the fray.

So here we go again.

The problem with this discussion is that many people are totally confused about the difference between an adhesive and a sealant.

For everybody's information.....a sealant is designed to allow movement at a joint....whereas an adhesive is designed to restrict movement at a joint. And it is important to know which is which because the optimum geometry of a joint bead will differ depending on whether it is acting primarily as a sealant or primarily as an adhesive.

Sikaflex is an elastomeric SEALANT formulated with the specific intent of allowing movement at a joint. In the case of an RV canopy the Sikaflex acts as a "structural" sealant in that it is also expected to transmit loads. That the Sikaflex "adheres" to the surfaces does not make it primarily an adhesive. Virtually all sealants are "adhesive" by nature but they are not PRIMARILY adhesives. Epoxy is an adhesive (and it may act also as a sealant...but it is not PRIMARILY a sealant).

It is relevant that in his post #78 Jim McChesney claims he asked Sika "how to apply the ADHESIVE between 2 perpendicular surfaces" and was advised to use a full depth fillet. And that would be correct advice for an ADHESIVE....but incorrect advise for a SEALANT.

I have done a few quick (and rough) drawings to indicate a cross section of the canopy to side bow joint on the RV7 slider with 3 possible joint geometries utilising Sikaflex. You will need to left click on the thumbnail to bring it up to a reasonable viewing size.

Drawing A shows the detail that I proposed in my post #82 using a polyethylene bead. I recommended this because it's probably the simplest solution to implement.

Drawing B shows an alternative joint geometry using a polyethylene bead (between the bow and the acrylic) which would work just as well but would require moving the acrylic away from the bow by up to 1/4" which might prove difficult without serious mods to Vans canopy design.

Drawing C shows a Sikaflex joint geometry that is destined to fail. It's a good geometry for an adhesive but a very poor one for a sealant. Infinitely large stresses will arise at the root of the fillet as the side bow and the acrylic move differentially.



The fact that I make these points should not be construed by anyone as an indication that I recommend using Sikaflex to bond RV canopies without an additional system of mechanical attachment.
__________________
You’re only as good as your last landing
Bob Barrow
RV7A

Last edited by Captain Avgas : 05-16-2008 at 08:33 AM.
  #87  
Old 05-16-2008, 02:08 AM
Captain Avgas Captain Avgas is offline
 
Join Date: Oct 2005
Location: Melbourne, Australia
Posts: 1,929
Default

Quote:
Originally Posted by szicree View Post
Cap'n Avgas,

I feel your pain. If I understand your position, it is not that Sika is an unsuitable product for this application, only that there has been little true engineering put into the Sikaflex use. Your point that the shear stresses go to infinity as the gap goes to zero is absolutely lost on many. It's like folks saying that you can't tear a phonebook in half, but we've all seen it done by concentrating the stress to a small area.
Hi Steve, basically you're correct but I'll clarify my position. I believe that Sikaflex is unsuitable to use as a sole means system of RV canopy attachment. The large expansion and contraction of acrylic makes it an absolute bugger to work with in bonded glazing systems. And it becomes more problematic as the area of the acrylic panel becomes larger (and an RV canopy is a VERY large piece of acrylic). That's why professional designers use mechanical restraint on anything other than small acrylic panels used in non critical applications. Look at formed acrylic skylights for domestic and industrial use....they're ALL mechanically restrained using the picture frame method. Its the same for Pipers and Cessnas and any other certificated aircraft. And I'll bet dollars to donuts that Dick VanG's new RV12 is not about to appear with a Sikaflex bonded canopy.

Using Sikaflex alone to secure an RV canopy is REALLY pushing the product to the edge of its envelope and to have any reasonable chance of long term success would require that all joint geometries be truly optimised. And it is obvious from this thread alone that very few RV builders have the technical expertise to achieve that goal.
__________________
You’re only as good as your last landing
Bob Barrow
RV7A

Last edited by Captain Avgas : 05-16-2008 at 07:00 PM.
  #88  
Old 05-16-2008, 02:35 AM
lostpilot28's Avatar
lostpilot28 lostpilot28 is offline
 
Join Date: Nov 2006
Location: Boise, ID
Posts: 1,095
Default Flip-floppin' views?

Quote:
Originally Posted by szicree View Post
Cap'n Avgas,

I feel your pain. If I understand your position, it is not that Sika is an unsuitable product for this application,....
Huh? Did you miss the first half of this thread?
__________________
Sonny W
Boise, Idaho
RV-7A Flying!
  #89  
Old 05-16-2008, 09:45 AM
DGlaeser DGlaeser is offline
 
Join Date: May 2005
Location: Rochester Hills, MI
Posts: 898
Default Doing double duty...

Quote:
Originally Posted by Captain Avgas View Post
The problem with this discussion is that many people are totally confused about the difference between an adhesive and a sealant.
There is no reason the same material can't be BOTH.
From the Sika website:
"Sikaflex? 295UV is a moisture curing polyurethane sealant/adhesive that exhibits outstanding UV resistance. This high strength elastic adhesive is specifically formulated for use with plastic windows. Sikaflex? 209 is a black primer that develops tenacious adhesion to acrylic and polycarbonate. Used as a system, Sikaflex? 295UV and SikaPrimer? 209 provides very durable adhesion to plastic windows." (I added the bold)
The ability to flex is a built-in stress reliever. As long as the load is within the strength capability of the material, nothing happens (rigid or flexible). If the movement is within the elastic range of the material, it returns to it's original shape when the load is removed. Your wing does this every time you pull a few G's.
A joint made with 'super glue', which is very rigid/inelastic, will hold with virtually no movement right up to its load limit, then fail totally. A sharp edge driven between 2 surfaces super-glued together can pop it right off.
Use 'Goop' (another adhesive/sealant - but not for canopies ) for the same job and the joint will flex a lot before it fails, it will fail much more gradually, even if you pry it off. (assuming good adhesion in both cases of course)
__________________
Dennis Glaeser CFII
Rochester Hills, MI
RV-7A - Eggenfellner H6, GRT Sport ES, EIS4000, 300XL, SL30, TT Gemini, PMA6000, AK950L, GT320,
uAvionixEcho ADSB in/out with GRT Safe Fly GPS
  #90  
Old 05-16-2008, 10:54 AM
Don Jones's Avatar
Don Jones Don Jones is offline
 
Join Date: Nov 2005
Location: Snohomish, Washington
Posts: 700
Default

Quote:
Originally Posted by DGlaeser View Post
There is no reason the same material can't be BOTH.
I agree with that, automotive windshields have been using the same type of stuff for years to both glue in as well as seal the windshield to the structure of the body.
__________________
Don Jones
Technical Support Manager
Dynon Avionics
CFI-IA, AGI, IGI
RV9-A
Closed Thread


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 09:45 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.