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stiff pushrods - help!

Old 01-20-2011, 10:08 PM
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joescottsdale
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Default stiff pushrods - help!

I am building a Sig LT-40 and have been following RC Ken's Pay It Forward Build. Everything has been going great. I am about to hook up my pushrods to the servos. After getting the pushrods to the rudder and elevator installed, I noticed that they were very hard to move back and forth. Much harder than I would have expected. They are installed correctly (I have double/triple checked) and look to be following more or less a straight path through the fuselage.

Is there such a thing as lubricating plastic pushrods? What else should I be checking? it's weird because at times, they push fairly freely but then get "sticky" again.

Thanks to anyone who can give me some advice.

Joe
Old 01-21-2011, 02:25 AM
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MinnFlyer
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Default RE: stiff pushrods - help!

I would recommend bringing it to your local field and let your clubmembers have a look. It's impossible for us you say if it's right or wrong by your description alone, and if there IS a problem, you'd need someone to give it a "hands on" look to find out what it is
Old 01-21-2011, 03:56 AM
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FlyingPilgrim
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Default RE: stiff pushrods - help!

Joe, If they are too tight to use, you might try substituting metal 2-56 pushrods inside the plastic guide tubes you already have installed. Sullivan Golden Rods may work also.
Old 01-21-2011, 05:36 AM
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Default RE: stiff pushrods - help!

You might try sanding the inner rod, it will make it so less surface area touches the outer tube making it easier to move. You want a scuffed surface not a smooth one.

You can also do like the Germans do with Bowlen (SP?) cables. You use the outer tube like you would normally, the you put the inter-rod in, but don't hook it to anything. You use a piece of wire inserted into the inner-rod from the servo to the surface, connect them any way you would normally. What this does is it gives you a thin steel rod that is supported the complete run the inner-rod keeps it straight and acts as a sleeve bearing. You get the added benefit of the steel rod being very temperature stable so your trims don't change with the temperature, not much of a problem with a trainer, but in sailplanes trim is everything.
Old 01-21-2011, 05:57 AM
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Default RE: stiff pushrods - help!

Hi!
Do you use Sullivan "Golden rods"??
If not...Use them!
Metal rods inside a plastic tube is no good!
Oil will sooner or later get inside the plastic tube and create even more friction.

Best system is to use steel fishing wire...but that is not for newbies!
Old 01-21-2011, 06:47 AM
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joescottsdale
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Default RE: stiff pushrods - help!

Thanks all for the help so far.

As a follow up, the elevator pushrod, if twisted while pulling and pushing back and forth, becomes very easy to slide. So I have a feeling that the pushrod is stiffer the way it is installed than it is if turned a little bit. This doesn't seem like a good situation. It also leads me to believe that maybe there is just too much friction inside the tube.

I haven't heard of Sullivan "golden rods" but will look them up.

I might try sanding to reduce the rod's diameter. That seems like it would help.
Old 01-21-2011, 06:54 AM
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outdoorhunting
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Default RE: stiff pushrods - help!

Probably the "sleeves" are moving around a little. Even though they are almost straight, they tend to move. When they move, you'll get a little binding. Make sure you glue the sleeve to your bulkhead braceing keeping it as straight as possible. Good luck
Old 01-21-2011, 07:36 AM
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dbacque
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Default RE: stiff pushrods - help!

Sounds like the pushrod runs are not straight enough. More or less straight is not good enough as every bend creates friction. If rotating the inner pushrod frees things up it then you've probably found the spot where the natural curve of the inner rod is more closely matching the curve of the outer rod and reducing the friction.

The real answer is to make your pushrod runs straight, arrow straight. This does two things. It reduces friction and it eliminates slop in the connection.

As a last resort, if you absolutely can not straighten the pushrod run, you can switch to a metal pushrod. But it must be a good fit in the outer sleeve or you'll have even more slop in the surface. So you need to cut short sections (1/4" or so) of the inner pushrod and slide them onto the wire pushrod every few inches and fix them in place with a drop of CA. This will keep the wire from wiggling around in the outer tube. If the pushrod still feels stiff, try to bend the wire to match the bend in the outer rod to reduce the friction even more.

Dave
Old 01-21-2011, 08:54 AM
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ARUP
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Default RE: stiff pushrods - help!

I don't use those tube in tubes any more. When I did use them I would insert a music wire into the outer portion then glue the 'outer' tube to the fuse. When glue cured I would remove the music wire and then put the 'inner' tube and its connectors in place. This gives a very straight run to the control surface.
Old 01-21-2011, 12:56 PM
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Default RE: stiff pushrods - help!

Hi!
Why not!
Sullivan "Golden rods " (Red or Blue ones) have been useed by modelers world vide for decades!
Old 01-21-2011, 01:26 PM
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Default RE: stiff pushrods - help!

I have found that binding the steel rod ends to a piece of stiff 1/4" square balsa strip and adjusting the length of the whole thing just long enough to reach from the servo to the moving surface horn results in a friction free pushrod every time. I'm told that the idea is really old fashioned and not much used these days, as the modern stuff is so much better, but since the wooden pushrod expands the same rate as the fuselage, so no trim changes with temp changes, absolutely no friction other than the clevis pin friction in the horn and servo arm, and really light weight, I find that as a solution it is somewhat better than anything else...Sometimes 'New and Improved', isn't. And I don't have to get down the fuselage to attach it to wherever to keep it straight...
Then again, I might just be a bit of an old Luddite...
Evan, WB #12.
Old 01-21-2011, 02:47 PM
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Default RE: stiff pushrods - help!


ORIGINAL: pimmnz

a bit of an old Luddite...
Sounds like some of your forefathers got free transportation to the South Pacific.
Old 01-21-2011, 02:49 PM
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Edwin
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Default RE: stiff pushrods - help!

I respect your opinion Jaka, but I disagree with you on this one. I prefer steel rod in tube over nyrods on a control surface anytime. I use nyrods from servo to throttle. I have found on several of my planes as they got old (5 years or more) the nyrods got sticky. Just my opinion, not trying to change anybodys mind.
Edwin
Old 01-22-2011, 06:37 AM
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ARUP
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Default RE: stiff pushrods - help!


ORIGINAL: jaka

Hi!
Why not!
Sullivan ''Golden rods '' (Red or Blue ones) have been useed by modelers world vide for decades!
Their length changes with temp, they tend to bind, difficult to eliminate flexing so have to use more support structure which is heavier and cost more $$$. I like plain old home made pushrods.
Old 01-22-2011, 06:45 AM
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Default RE: stiff pushrods - help!


ORIGINAL: ARUP


ORIGINAL: jaka

Hi!
Why not!
Sullivan ''Golden rods '' (Red or Blue ones) have been useed by modelers world vide for decades!
Their length changes with temp, they tend to bind, difficult to eliminate flexing so have to use more support structure which is heavier and cost more $$$. I like plain old home made pushrods.

i wonder if this is an isolated issue with some modelers...........i've not ever experienced any changes like this.
Old 01-23-2011, 03:21 AM
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Default RE: stiff pushrods - help!

I think a lot has to do with how precise the plane is. I have used Sullivan and DuBro flexible pushrods for over 30 years in hot and cold weather and never noticed trim differences.

And if I did, Uh... isn't that what the trim buttons on your transmitter are for?
Old 01-23-2011, 04:57 AM
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Default RE: stiff pushrods - help!


ORIGINAL: MinnFlyer
And if I did, Uh... isn't that what the trim buttons on your transmitter are for?
I don't think you would see a change in a LT-40, I only mentioned it for his future reference for high-end sailplanes or acrobatics. Personally I used plan old balsa pushrods in my LT-40.
Old 01-23-2011, 06:26 AM
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Rodney
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Default RE: stiff pushrods - help!

I agree with Minniflyer, I've been using Golden Rods for years with never a trim problem due to temperature.
Old 01-23-2011, 10:08 AM
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DGrant
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Default RE: stiff pushrods - help!

As for the original issue, it sounds like a simple binding that's going on, of which theres a few ways to deal with it.. One being simply hunt down the area of the pushrod that's binding, usually it at one end or the other of the outer tube exits, especially if you have relatively straight shot through the fuse. I've used a little graphite or silicone(wd40) in this area, and even put a very thin coat on the wire itself, that lends nicely to a very smooth assembly. As its only a plastic tube, with a solid inner wire, there won't be alot there to figure out...and...

I used Sullivan NyRods for quite a while, I found them extremely flexy, and needing more support to stay true then other set-ups. On the operation of them.. the "pull" side(when they're pulling) is reasonable, and I don't think there's any stretch going on.. but getting them to "push" a given surface is another story, and they tend to buckle and give more then a solid wire would.. and I think with these anomalys' it really doesn't make this hardware the best for any given set-up where you need rigidity and strength.

When the NyRod type of system is combined with a solid wire though it seems much more durable, stiff, and the control surface is very sturdy, with little chance of blow-back, which is the whole idea behind whatever system you use.... none of them are any good if you can simply and easily move those surfaces against the servo rotation.......

I use the solid wire inside, and cut what I'll call "bushings" and tack them to innerwire, the bushings are about 1/2" and I space them about 8"+ or so... this seems to keep everything solid, with less drag/binding, while still taking advantage of the easier installation of this type of rod system.

I've seen it both ways, with and without a bushed center rod, and I've used the plastic center rod, but overall I prefer a metal rod in a plastic guide.. it seems more precise for aerobatic applications.. that's for sure.
Old 01-23-2011, 11:51 AM
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Default RE: stiff pushrods - help!

I have read this thread from the beginning.

I did not see anything about the hinges that could be the real cause of the problem.

Some principles I follow ___

1 - Use hinges that have negligeable friction. (pinned hinges)
2 - Make sure the axis ofrotationis in a straight line.
3 - Make sure the control surface fall to their limit of travel freely just by their own weight.
4 - Install control linkages that do not bend under compression (pushing).
5 - Assure these linkages (whatever type you use) can take the full force of the servos while in compression without bending.
6 - A bit of imagination to get the straigtest routing posssible.
7 - After linkage installation and connected to the control surfaces, moving the control surface at the servo end of the linkages should feel nearly no friction at all (you knew that and that is the reason you posted). NOTE: I am writing for all readers and in particular beginners.
8 - Same if moving the trailing edge of a control surface with fingers; the opposition to motion should be mainly the weight of the control surface rotating around the hinges.
9 - The friction in the linkages should be nearly negligeable.
10 - The linkages should have no twist in their function.

The above principles will also assure to have the best possible resolution of the servo rotation.

Do I have to mention that no slop should be tolerated ?

Have fun with setting this all up and ending up with good contol surfaces operation.

You have received lots of good advice on the linkage part of the control installation.

Ragards from Zor.

Old 01-23-2011, 05:48 PM
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outdoorhunting
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Default RE: stiff pushrods - help!

Bingo !!! You hit the nail on the head ZOR. If his list is followed, no more binding !!! Great points !!! Ya get a 5 for that !!
Old 01-25-2011, 07:47 AM
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Default RE: stiff pushrods - help!


ORIGINAL: Rodney

I agree with Minniflyer, I've been using Golden Rods for years with never a trim problem due to temperature.
I prolly fly a little different than most folks. I set my airplanes up so that I can kill the engine at altitude to try and thermal. Don't laugh- I've thermaled a Stick 40 for 45 minutes before and I've slope soared trainers (to show the trainee what can be done) in front of tree rows! I like to get 'em up there then just watch and , to me, looks very realistic. It's very relaxing and control inputs need to be minimal- thus the detection of changes in plastic tube pushrod lengths and flight trim. If you are a flip-flop flyer or just like to yank and bank then trim prolly doesn't matter. Don't mean to hijac thread- apologies!
Old 01-25-2011, 10:16 AM
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Zor
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Default RE: stiff pushrods - help!


= ARUP

I prolly fly a little different than most folks. I set my airplanes up so that I can kill the engine at altitude to try and thermal. Don't laugh-
>
>
>
I gess you intended to write "I probably fly a little ____"

I certainly am not laughing at you.
Is there a better way of setting (balancing) a model airplane ?

I do not endorse shutting the engine completely.
With electric it is easy to restart but with glow or gas it is not practical to have a heavy starting system on bboard to start the engine while airborn.

I believe it is preferable to set thngs up so that at best gliding speed the prop rotation has zero angle of attack so that it does not provide thrust nor drag.

To each his own outlook.. I am not telling anyone what to do, only what I do.

Regards de Zor

Old 01-26-2011, 02:50 AM
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Default RE: stiff pushrods - help!

I installed nyrods in my 1/3 scale Spacewalker, just yesterday. Last time I looked, I wasn't a flip/flop or yank and bank flyer.
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Old 01-27-2011, 07:21 AM
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ARUP
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Default RE: stiff pushrods - help!

Hi Tom- yep, more than one way to skin a cat! Sure is a lot of work supporting those nyrods when a pushrod will do but everybody has their preferred method. Over to you...

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