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Old 11-15-2018, 09:23 AM   #61
aviator79
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Default Re: #Northernfox Build

I am also not arguing. I think it's a good discussion to have. I do want to encourage you, and anyone else reading this thread, to make a decision informed by data and/or analysis, and not anecdote and/or speculation.

I do think it's possible, probably even likely, that the glue alone will do just fine. Kitfoxes were built that way with Poly-Fiber for quite some time. Of course, that was before people were putting 140 HP engines and STi wings on them.

I don't know that the mass of the material will matter much. Whatever attachment you use must resist the weight of the airplane at 3.8G regardless of the fabric density. The fabric density would come into play for dynamic loading, but I would not expect the effect to be dramatic. In fact, it may be that the fabric with higher density resists dynamic or transient loads better because it has more inertia than the lighter fabric. (Quasi-informed speculation only).

As a low-fidelity test, bond a strip to a piece of varnished wood, and try peeling it off. You could even attach a fish scale, pull it at 180 degrees at a rate of 6"/min and calculate the ASTM peel strength. My back-of-the-envelope calculation gives a peel strength requirement of about 4.5 lbs/in assuming a uniform spanwise lift distribution, which is not realistic. Unless you're ready to get into some hard-core modeling, you could assume a triangular lift distribution, which would double that and should be somewhat conservative. Then add some safety margin.

To be complete, you'd want to repeat the test at 5, 10, 15, and 20 years, but I assume you plan to have your plane built long before then. You could make up the samples, and perform the tests at intervals, which would let you know over time whether you need to worry about the bond.
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Old 11-15-2018, 09:32 AM   #62
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Default Re: #Northernfox Build

I agree, in hand evidence and numbers is what I want to base the decision off of. I'm waiting on the spec sheets/tests from Oratex to at least have informed numbers that they tested with.

Then like you said, some of my own testing will be required.
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Old 11-15-2018, 10:42 AM   #63
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Default Re: #Northernfox Build

Also keep in mind that you are not gluing the fabric to the wood. It is glued to the varnish coating the wood. I doubt there are any specs or STI's spelling out the adhesion strength of the varnish to bare wood, or your method of applying it.

I wish someone could find that video of a Kitfox with the top wing fabric pulled loose and ballooning up while the camera shot it from above.
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Old 11-15-2018, 11:40 AM   #64
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Default Re: #Northernfox Build

As per Oratex instructions surface must be properly prepped for proper adhesion. You want to sand rib caps prior to glueing so I think it’s non issue. I understand the interest in the video, but I am assuming that’s poly fiber covered wing? Speaking in length to Lars about this topic and various others, I am not quite sure what my thoughts are. Lars spoke of various STCs in Europe they have replacing legacy covering systems with Oratex and lack of rib lacing/stiching specified. He spoke of testing they have done to obtain these STCs and inflating the wing (with water for better data) to failure point and how the ribs failed and not attachment points. With that being said he also spoke of various aircraft that Oratex states you “must rib stich” because the attachment point of the ribs are too small or surface areas and or irregular shaped ribs. The just of the conversion I got is they are not going to say it’s not needed for obvious reason. Although he did mention aircraft with large cap strips and proper installation the fabric will not separate. He also mentioned the adhesion strength in there testing is 5-8 times greater than other legacy systems and fabric or rib damage results. They list these test numbers in the data sheet provided but they are just numbers to me.

I am no engineer, but the little I have “played” with Oratex I think you would brake the rib before you remove fabric. I can see both sides of the argument. Are we doing unnecessary steps and what some would say not as clean look when no reason other than it was done in the legacy systems? Or are we cutting corners and putting safety at risk? One reason I chose to build and experimental aircraft, ability to choose😬!

In picking brains of multiple builders that have used Oratex they have state they installed covering in ocordance with poly fiber instructions. Lars stressed to me and in the instructions he sent not to following instructions of other legacy systems it will be done incorrectly if you do. One point comes to mind on reinforcing tapes and finishing tapes. Looking at Steve Henry’s plane it was interesting to note that he had no reinforcing tapes over attachment points like I often see covering frame tubes and ribs. When asked he said “everybody gets crazy about reinforcing tapes, not needed” From what I have read you want the surface glued down and no need for tapes. I don’t think there is an issue of adding then but they seem to just add weight and take time. I think this also must be a carry over?

Great discussion👍

Last edited by Shadowrider; 11-15-2018 at 12:06 PM.
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Old 11-15-2018, 11:56 AM   #65
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Default Re: #Northernfox Build

Quote:
Originally Posted by aviator79 View Post
This seems to imply the glue is mandatory and the lacing optional. I believe the inverse is true - you need to lace regardless of whether you glue or not. The Poly Fiber manual actually says "You won't cement fabric to the ribs themselves, since later you'll use rib lacing or some other mechanical means to hold the fabric to the ribs..."

The Kitfox manual agrees with John in that the instructions explicitly tell you to bond to the capstrips, but also say that "Poly-Fiber and Kitfox recommend that you secure the fabric to the rib capstrips by rib stitching". Acutally, Kitfox might recommend it, but Poly-Fiber requires it, and uses 3 paragraphs in the manual to emphasize that it is required.

edit... I assume the Oratex instructions require gluing. So you should glue. But unless there is strong evidence that Oratex adhesives resist peel loads in a vastly superior sense than Poly-Tak, even after 20 years, then you should follow Poly-Fiber's advice, even if you're using Oratex.
A couple of comments - first of all, Oratex’s instructions, according to Lars, are to definitely glue to the capstrips but also follow the aircraft designer’s recommendations in regard to rib lacing, and regardless of that they do recommend it. Since Kitfox recommends it also, it’s a no-brainer to me. Also, the same argument that most all of us are making should logically go both ways... why not glue as well as lace?

Next, Oratex has tested and published comparisons of their glue strength to Polyfiber’s and others, and claim it to be quite a bit stronger in all ways, peel strength included. Like most things, it has to be applied correctly with the proper amount of pressure and heat. There is not the experience history of Polyfiber of course, but it has been being used in Europe for close to 2 decades now, I believe.

Edit... after seeing Shadowrider’s latest post below, it’s possible that Oratex has revised their recommendations, but I was definitely told to follow the designer’s recommendations.
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Old 11-15-2018, 12:10 PM   #66
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Default Re: #Northernfox Build

Here is another thought that hasnt been mentioned yet in this interesting discussion. Oratex claims that the rib may fail before the fabric separates. Ok, the kitfox cap strip is only held to the rib by a very tjin glue line. It may very well fail at this point, but that is just as bad as a fabric separation. Rib stitching goes all the way around the rib and cap, helping to hold the whole structure together. I personally would want this extra safety factor for such little effort and cost.
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Old 11-15-2018, 12:17 PM   #67
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Default Re: #Northernfox Build

Quote:
Originally Posted by aviator79 View Post
This seems to imply the glue is mandatory and the lacing optional. I believe the inverse is true - you need to lace regardless of whether you glue or not. The Poly Fiber manual actually says "You won't cement fabric to the ribs themselves, since later you'll use rib lacing or some other mechanical means to hold the fabric to the ribs..."

The Kitfox manual agrees with John in that the instructions explicitly tell you to bond to the capstrips, but also say that "Poly-Fiber and Kitfox recommend that you secure the fabric to the rib capstrips by rib stitching". Acutally, Kitfox might recommend it, but Poly-Fiber requires it, and uses 3 paragraphs in the manual to emphasize that it is required.

edit... I assume the Oratex instructions require gluing. So you should glue. But unless there is strong evidence that Oratex adhesives resist peel loads in a vastly superior sense than Poly-Tak, even after 20 years, then you should follow Poly-Fiber's advice, even if you're using Oratex.
I had typed these sentiments not recalling that the thread was specific to Oratex (hence the edit). If the glue is as strong as they claim, and remains so as it ages, then it's likely the glue bond will be sufficient. It's up to the builder to decide if the available evidence is convincing enough.
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Old 11-15-2018, 01:14 PM   #68
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Default Re: #Northernfox Build

Quote:
Originally Posted by jiott View Post
Here is another thought that hasnt been mentioned yet in this interesting discussion. Oratex claims that the rib may fail before the fabric separates. Ok, the kitfox cap strip is only held to the rib by a very tjin glue line. It may very well fail at this point, but that is just as bad as a fabric separation. Rib stitching goes all the way around the rib and cap, helping to hold the whole structure together. I personally would want this extra safety factor for such little effort and cost.
This is something that I've thought about for this situation. Rib failure itself, not the fabric. That's something that's probably never been tested on a kitfox. I'd also want to know how much strength does the rib lacing add to the structure, not the fabric in such a case. And is that failure of the rib above the maximum and then some stresses applied in flight?
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Old 11-15-2018, 01:21 PM   #69
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Default Re: #Northernfox Build

Ribs are glued and stapled down. Rib caps are very strong. I tried to remove the end of mine because the cap was not installed perpendicular. Rib cap broke before separating. 😬
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Old 11-17-2018, 09:46 PM   #70
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Default Re: #Northernfox Build

If it were my plane I would do the rib stitching for peace of mind.

It seems the rib stitching could be completed in less time than has been spent on this thread.
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