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Old Nov 20, 2011, 04:43 PM
Just call me crash for short
Quick61's Avatar
United States, OH, The Plains
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Question
Roll Your Own Carbon Fiber Booms With Torsional Strength?

If this has been answered somewhere else, please point me to it.

What I am looking at is doing a build of an Eclipse Sailplane (October 1974 issue of American Aircraft Modeler) and the original build used a fiberglass fishing poll blank with 1 1/4" big end on the taper. 50" long. OK, Sounds a bit heavy and I was wondering what technique one would use to 'roll your own' boom that would have good torsional strength to keep the big V tail from flopping around like an old rag doll? I was thinking something like a uni wrap, then spiral wrap with ~12K tow then top off with a braided sleeve? (carbon/glass comes in colors or carbon/Kevlar) Would the middle layer of tow be needed or would the sleeve give enough torsional strength for the application? Or, am I thinking completely wrong and there is some other way it would be done?

Mark
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Old Nov 20, 2011, 06:09 PM
Just fly it!
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Cody, WY
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Mark, the layup for many hobby based structures are figured out by trial and error.

I always start by doing the first one on the weak side. Then I test it, squeeze it, bend it, and breaking. Once I figure out where it wants to fail I start adding material it key areas to get the properties that I desire. In my experience with tappered structures the layup typically gets a staggared layup from the front to the back. If the back end has the same layup and the front end then the structure is typically much heavier than it needs to be. As the diameter decreases, it's hoop strength increases if the same layup is used. For this reason I opt to diminish the amount of +/-45* and 90* as the boom tappers.
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Old Nov 20, 2011, 06:51 PM
Just call me crash for short
Quick61's Avatar
United States, OH, The Plains
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Quote:
Originally Posted by wyowindworks View Post
Mark, the layup for many hobby based structures are figured out by trial and error.

I always start by doing the first one on the weak side. Then I test it, squeeze it, bend it, and breaking. Once I figure out where it wants to fail I start adding material it key areas to get the properties that I desire. In my experience with tappered structures the layup typically gets a staggared layup from the front to the back. If the back end has the same layup and the front end then the structure is typically much heavier than it needs to be. As the diameter decreases, it's hoop strength increases if the same layup is used. For this reason I opt to diminish the amount of +/-45* and 90* as the boom tappers.
Thanks for the tips on the tapered boom layup. After I made the post, I got to thinking that I should have more material at the big end, and maybe more than 1 wrap of uni. I was just pricing material and it looks like I could get enough to build 3 (resin and all) for the price of a suitable CF fishing rod blank. Figure I should get at least that much to cover the learning curve...

Mark
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Old Nov 20, 2011, 07:24 PM
Just fly it!
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Cody, WY
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BTW, I personally think that winding 12K carbon tow will end up being pretty heavy unless you can spread it to a wider dimension. It would be very stout though due to a thicker wall.

Adam
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Old Nov 20, 2011, 10:04 PM
Just call me crash for short
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Originally Posted by wyowindworks View Post
BTW, I personally think that winding 12K carbon tow will end up being pretty heavy unless you can spread it to a wider dimension. It would be very stout though due to a thicker wall.

Adam
I was planing on spreading it out as I rolled it on. After your last post about where one wants the material and where one does not, I'v been giving thought as how best to do it. Think I have it worked out in my head, we'll see once I get the material.

Mark
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Old Nov 22, 2011, 06:11 PM
Balsa breaks better
Thermaler's Avatar
Buchanan Mi
Joined Apr 2005
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Well Mark, it looks like I have started something here looking for these plans.

http://www.rcgroups.com/forums/showthread.php?t=1543756

There is also a responce on the RCSE, could that be you or 1967250s?

Maybe a group build log?

Joe
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Old Nov 23, 2011, 02:36 AM
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Liempde, Netherlands
Joined Jul 2007
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Boom layup suggestion

Hi Mark,

I've documented the roll of my tailboom for an F3B model on the dutch forum:
http://www.modelbouwforum.nl/forums/...b-model-5.html
See post #64. Text is dutch but comments on pictures are english.
The shown layup delivered a 50gram boom with good stiffness but insufficient hoop strength.
I later reinforced it with a layer of 0-90 C93 on the first half and another piece of 0-90 C93 on the first 20cm of the boom. It is now strong (OK for hard landings with ballast) and very stiff. I have the Kevlar (A38) on the outside to limit the damage in case of a crash, and as a flexible outer tissue to easiliy roll all the other - heavier- tissues around the boom mandrell, and for torsional stiffness.

You could replace the A38 with a light glass fabric (25grams) and add a layer of 45-45 C93 for tosional stiffness, if you think you need it.

Good luck
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Old Nov 23, 2011, 03:11 AM
Just call me crash for short
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Thank you, great info in that post. I have had my own thoughts on the construction and have posted them here - http://www.rcgroups.com/forums/showthread.php?t=1543931 in post #3. I will study your post carefully and take it all in given that you have already done it. Great post, thanks again!

Mark
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Old Dec 07, 2011, 01:18 PM
Entropy is happening!
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Bellingen NSW Australia
Joined Aug 2008
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Carbon sock.

Quote:
Originally Posted by wyowindworks View Post
Mark, the layup for many hobby based structures are figured out by trial and error.

I always start by doing the first one on the weak side. Then I test it, squeeze it, bend it, and breaking. ...............
I follow a similar approach as does Adam here. Then if its too weak, I "sock it" with some carbon sock obtained from Soller Composites. Glass sock could be used equally as well if you need to avoid the carbon for 2.4 ghz reasons or such.
My more recent ones started with some spiral wound 100gsm glass strip over spiral wound drop sheet plastic or video tape. I like the drop sheet plastic best. The glass fabric can be wet out before or after winding. Then spiral wind with either isulation tape inside out, or more strips of drop sheet plastic. It could be vacuum bagged, although I have never tried this. When cured and released etc., sand lightly and sock with the carbon sock to required stiffness/hoop strength etc.

Jim.

edit: For a really good finish, you can use the special shrink tube over the sock. Soller have a good description of making shafts on their site.
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Last edited by Jim.Thompson; Dec 07, 2011 at 01:33 PM. Reason: see edit: extra information
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Old Dec 08, 2011, 02:20 PM
Just call me crash for short
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Originally Posted by Jimbello View Post
I follow a similar approach as does Adam here. Then if its too weak, I "sock it" with some carbon sock obtained from Soller Composites. Glass sock could be used equally as well if you need to avoid the carbon for 2.4 ghz reasons or such.
My more recent ones started with some spiral wound 100gsm glass strip over spiral wound drop sheet plastic or video tape. I like the drop sheet plastic best. The glass fabric can be wet out before or after winding. Then spiral wind with either isulation tape inside out, or more strips of drop sheet plastic. It could be vacuum bagged, although I have never tried this. When cured and released etc., sand lightly and sock with the carbon sock to required stiffness/hoop strength etc.

Jim.

edit: For a really good finish, you can use the special shrink tube over the sock. Soller have a good description of making shafts on their site.
Thanks for the info. Given that this is a rather long boom, and needing stiffness, torsional force resistance, as well as hoop strength, I am planing on several layers of uni-carbon, 2 @ 100% length, one at 50% and one at 25% from big end, wrapped, then spiral wrap (wind) spread CF tow over top of that, followed by some strips of tow or uni (top, bottom, and sides), then socked with some of that CF/glass braided sleeve. Most everything that I have been reading and learning about is telling me this should work out, or at the very least, give me a good starting point from where to work.

I've attached a graphic to show my idea of construction. (minus the sleeve). Maybe you all can let me know what you think?

Mark
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Old Dec 08, 2011, 02:36 PM
Entropy is happening!
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Bellingen NSW Australia
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Spread Tow?

Mark,

My first thoughts are that the spread tow carbon is a bit of an overkill here. The sock will provide diagonal fibre orientation and with it the torsional rigidity required. And the spread tow is fairly expensive isn't it? I have never bought it, but have spread my own tow by pulling it through a stainless oven rack, wound in and out the rails and pulled back and forth.
The sock can be one or more layers as required. This way you can start light, (which is always a good plan), test the product for a subjective strength assessment, and add more layers if and as needed.
The spiral wound tow will add ridges along the boom which will make finishing difficult and so on.
I would make a light glass shell, Add your longitudinally oriented uni-carbon if you think it is necessay, then add one or more sock layers. For a good finish use the special heat shrink. It removes excess epoxy and makes for a good finish. Less work, less excess weight, better all round.

Just my thoughts.

Jim.
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Old Dec 08, 2011, 03:42 PM
Just call me crash for short
Quick61's Avatar
United States, OH, The Plains
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Originally Posted by Jimbello View Post
Mark,

My first thoughts are that the spread tow carbon is a bit of an overkill here. The sock will provide diagonal fibre orientation and with it the torsional rigidity required. And the spread tow is fairly expensive isn't it? I have never bought it, but have spread my own tow by pulling it through a stainless oven rack, wound in and out the rails and pulled back and forth.
The sock can be one or more layers as required. This way you can start light, (which is always a good plan), test the product for a subjective strength assessment, and add more layers if and as needed.
The spiral wound tow will add ridges along the boom which will make finishing difficult and so on.
I would make a light glass shell, Add your longitudinally oriented uni-carbon if you think it is necessay, then add one or more sock layers. For a good finish use the special heat shrink. It removes excess epoxy and makes for a good finish. Less work, less excess weight, better all round.

Just my thoughts.

Jim.
Again, thinks Jim. My main concern is that this boom is to be 50" long, with a 1" dia. at the big end. tapering down to 1/2" at the other end. At the small end will be a ~58" span, ~4.8 sq. ft. V tail. I really don't want that big a tail wagging around up there in the breeze as I have seen other V tails do. However, with that being said, you are the second person to relay that the sock / sleeve will add loads of torsional stiffness to the build. So maybe I should concentrate on the stiffness of the boom and let the sleeve take care of the torsion.

As for the hear shrink, I really like that idea, until I come to the part of the size that the 1.1"
shrink tube will not shrink all the way down to the 1/2 inch on the small end. Which leaves me with doing the Video tape wrap. Now if I could find some real assurances that the shrink tube would work (shrink tight down on to 1/2") I might give it a go, even at $1.89 / ft.

You have given me some more things to think about.

Mark
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Old Dec 08, 2011, 03:57 PM
Entropy is happening!
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Bellingen NSW Australia
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Big expectation!

Quote:
Originally Posted by Quick61 View Post
Again, thinks Jim. My main concern is that this boom is to be 50" long, with a 1" dia. at the big end. tapering down to 1/2" at the other end. At the small end will be a ~58" span, ~4.8 sq. ft. V tail. I really don't want that big a tail wagging around up there in the breeze as I have seen other V tails do.............................Mark
Mark,

I am no structural engineer, most of the things I have built (ocean going yachts, model planes, houses et.al) have been designed by intuitive and empirical judgement. (read here: "guesswork"!).
But I have major doubts about what you are expecting from such a skinny boom supporting flying surfaces of the dimensions you quote above!
For what its worth, I think you need to go back to the drawing board and re-engineer the basic dimensions.
The torsional and longitudinal stiffness of a round tube increases at an exponential rate to the increase in diameter. Sure the hoop strength will decrease for a given layup thickness, but this is not your major concern.
And furthermore, the appearance of a skinny stick with such big surfaces on the end of it does not sound appealing to me. It will not "look right".

Jim.
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Old Dec 08, 2011, 04:13 PM
Just call me crash for short
Quick61's Avatar
United States, OH, The Plains
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Quote:
Originally Posted by Jimbello View Post
Mark,

I am no structural engineer, most of the things I have built (ocean going yachts, model planes, houses et.al) have been designed by intuitive and empirical judgement. (read here: "guesswork"!).
But I have major doubts about what you are expecting from such a skinny boom supporting flying surfaces of the dimensions you quote above!
For what its worth, I think you need to go back to the drawing board and re-engineer the basic dimensions.
The torsional and longitudinal stiffness of a round tube increases at an exponential rate to the increase in diameter. Sure the hoop strength will decrease for a given layup thickness, but this is not your major concern.
And furthermore, the appearance of a skinny stick with such big surfaces on the end of it does not sound appealing to me. It will not "look right".

Jim.
I don't know if you have looked over the plans for the Eclipse, - http://www.outerzone.co.uk/plan_details.asp?ID=1832 but the original one was built with a fiberglass boom (piece of a fishing poll) and it worked out well. I am wanting to replace that original boom which would be rather heavy, with a lighter CF boom, reducing the need for excessive nose ballast. ( I would like to avoid having to dump up to 12 oz of led into the front of my pod) The text of the build can be found in scans on this post - http://www.rcgroups.com/forums/showp...75&postcount=8 if that would help any?
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Old Dec 08, 2011, 06:09 PM
Balsa breaks better
Thermaler's Avatar
Buchanan Mi
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I am learning a thing or two, thanks guys.

Joe

The other Eclipse builder. ;<)
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