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Old Apr 12, 2011, 07:44 PM
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Discussion

Supra tail boom push rods.. friction issue.


Hi Gents
Lots of archived info close to our problem.... but we still have the problem.

We have just installed an elevator push rod in the boom.. preparatory to attaching to the pod. It consists of 0.063 in. Cf rod, in stretched bondable teflon tubing, 0.067 in. ID. Attached to the inside of the boom by 1/2 inch wraps of kevlar at three inch intervals. The pushrod follows a rough 90 degree corkscrew over its entire length, and was positioned using the brilliant previously suggested magnets method. The Cf rod has been lightly sanded to get rid of the surface irregularities, and averages 0.063 in. over the full length.

We have a Vladimir built fuse for comparison and the push rod friction is negligible by finger pressure standards.. The sleeve tubing is a thin walled black plastic, no idea what its technical composition is. (we've actually sent a note to Vladimir to see if he can shed some light)..and so we thought we could duplicate it. Not so..

Powdered graphite on the rod made no appreciable change in slipperyness,
and exterior testing of a few similar bend radii resulted in very little difference to what we have in the boom. Obviously a friction issue ..we believe.

We also wondered if there are sources of nylon tubing available for the same purpose..??

We are trying (and doing quite well overall actually) to mimic Dr.Drella's weight limits on the project, and would like to stay with the CF rods. Someone else has done this successfully somewhere we are sure... so we
are just asking for thoughts and someone else's experience with this.

Thanks for any responses gentlemen...
Stymied-in-the-basement

Ray
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Old Apr 12, 2011, 09:43 PM
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Move South where it's warmer and things will loosen up.
Old Apr 12, 2011, 10:36 PM
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I had the exact same problem with a replacement pushrod I ordered from KC. I broke a rudder pushrod at a contest and had Barry at KC send me a replacement. He sent me two .063 diameter rods. I put one in the plane and the rudder was extremely stiff to operate. I also tried graphite lubricant to no avail. I measured the broken rudder pushrod that came with the plane and it was .050 diameter. I ended up ordering six .050 x 96" pushrods from a cf supplier from the internet. The rudder works great now and I save a little weight. I would suggest that you do something similar.

George
Old Apr 12, 2011, 10:38 PM
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FTL... that cracked me up..
.but I'm ahead of you ... been to the Buzzards Tangerine event last 2 years, only to come back here.. Ticks me off to see all those guys flying every other weekend in shorts and T's. Just need to change a postal code for a zip code...(during the winter)
Old Apr 12, 2011, 10:52 PM
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Hi SaltyOne...
Thanks for the input... we have only teased the thought of doing what you did, but if you got the results you liked.. maybe we should up the ante on your idea

Appreciatively
Ray
Old Apr 12, 2011, 11:26 PM
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My fuse is Vladimir built and it came with .050 pushrods. I think KC just sent me the wrong size. Where did you get your pushrods?

George
Old Apr 13, 2011, 10:03 AM
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Quote:
Originally Posted by Upvector View Post
The pushrod follows a rough 90 degree corkscrew over its entire length
Not sure what this means, but you want CF pushrods as straight as possible over their entire length. Otherwise, you will get binding.
Old Apr 13, 2011, 10:12 AM
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Quote:
Originally Posted by nuevo View Post
Not sure what this means, but you want CF pushrods as straight as possible over their entire length. Otherwise, you will get binding.
Hi Nuevo
The pushrod enters straight at the side of the boom at the back of the fuze pod, (aligned with the servo arm position,it then has to transit the length and exit up on the top of the boom at the rear, to attach to the stab mount. It must therefore do a 1/4 turn (along 40 inches length) from front to back to be positioned correctly. We have attached the rod sleave to the inside of the boom with kevlar windings.

How have your experiences been with this..?
Thank You
Hope that helps
Ray
Old Apr 14, 2011, 10:36 PM
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Gents...
Just an update on our dillema..
I tried an 0.058 cf rod in the existing set up and it has made all the difference
in the world. Can't believe what 0.005 in. difference makes..
..looks like we won't need to tear out the original set up.
Thanks for your hint George, that definitely is a sticky in our build

Cheers
Ray
Old Apr 14, 2011, 11:45 PM
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a binding issue


In the same boat but remedied to a degree by ensuring outer carved a straight path through the boom for both rudder and elevator. Needed a lot of rat-tail filing and fitting/ removing/ fitting /removing.

Sanded the pushrod a little and that helped. It's better but not as good as other gliders; still not as good as internal routing.

IMHO the Supra rear end is awkward compared to conventional gliders.

All that stuff hanging out in the breeze makes me squirm.

I'd like to see a Supra Pro-V with RDS.


Chris.
Old Apr 15, 2011, 07:08 PM
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Hi Chris... thank you for the input.
Q!.. what size diameter pushrods are you using..?? just visually
they look substantially larger than ours.... maybe just the perspective tho.

Agreed, letting it "all hang out" feels like tying a drag chute onto the back,
but no empirical testing to show that.

We are exploring nylon instead of teflon as the inner tube, technically has a lower
C of F... but then there are a variety of nylons to deal with...

Should get a medal for all this when we get it done.. :>

Ray
Old Apr 15, 2011, 07:50 PM
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I wonder if your kevlar wraps compressed the tube enough to cause the binding? How were they done, tow, fabric? I've never tried to put my pushrods inside a boom on my DLGs, so this could be useful info for me.
Thanks,
Dave
Old Apr 16, 2011, 03:08 AM
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Hi Ray,
.08 " or 2mm carbon rods bought locally (Tasmania). Outers from Kennedy Composites circa 2008
Chris.


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