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Old Jan 14, 2013, 02:55 PM
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Pasco Tri Cities, Washington, United States
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Pull-pull setup

I'm currently setting up the rudder control system in my Aura 110 pattern airplane. I've seen some folks use a wheel on the rudder servo for a pull-pull system. Using a wheel means the radius of the pull at the servo end is constant. However at the rudder end of the cables, the radius of the pull changes (reduces) as rudder deflection increases---unless a wheel of the same diameter isn't used at the rudder end--which I have never seen. Can anyone tell me why people would use a wheel on the servo instead of an arm the same width as the one at the rudder? I'm a mechanical engineer, so think I understand kinematics rather well. I'd like to install a system that will keep the cables taught throughout the movement of the servo and the rudder to keep the chance of flutter at a minimum.
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Old Jan 14, 2013, 03:39 PM
"...certainty is absurd."
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Quote:
Originally Posted by LRFitch View Post
I'm currently setting up the rudder control system in my Aura 110 pattern airplane. I've seen some folks use a wheel on the rudder servo for a pull-pull system. Using a wheel means the radius of the pull at the servo end is constant. However at the rudder end of the cables, the radius of the pull changes (reduces) as rudder deflection increases---unless a wheel of the same diameter isn't used at the rudder end--which I have never seen. Can anyone tell me why people would use a wheel on the servo instead of an arm the same width as the one at the rudder? I'm a mechanical engineer, so think I understand kinematics rather well. I'd like to install a system that will keep the cables taught throughout the movement of the servo and the rudder to keep the chance of flutter at a minimum.
The cables will stay taut (not taught) no matter whether a wheel is used, or whether the width is the same at the rudder, as long as the rudder horn holes are in-line with the hinge point and the servo holes are also in-line with the centre and equidistant on both sides. The radius stays constant no matter if an arm or wheel is used.

Having a longer servo arm just means you get more rudder deflection - the travel on each side remains the same, so the cables stay taut. The only time there is an issue is when the rudder horn holes, or the servo holes, are not inline with the pivot/hinge points, or the holes are not equidistant on each side of the pivot point.

Kevin
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Last edited by kcaldwel; Jan 14, 2013 at 04:47 PM.
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Old Jan 14, 2013, 06:15 PM
KE your cub.
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What he said. I have however noticed that some times you'll end up with you pull pull system having one slack line when hard over. I have to say that initially this bothered me, but these days I don't worry too much about it. It's highly unlikely that you'll experience flutter when hard over as the amount of airflow blow back, and the line in tension pretty much locks the control surface in place.

There may be a case for saying that you will get flutter with a huge counterbalance but most F3A aircraft don't have much of one, if any at all.
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Old Jan 18, 2013, 08:10 PM
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Quote:
Originally Posted by LRFitch View Post
unless a wheel of the same diameter isn't used at the rudder end--which I have never seen.
here ya go
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Old Jan 19, 2013, 05:22 AM
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Quote:
Originally Posted by kcaldwel View Post
The cables will stay taut (not taught) no matter whether a wheel is used, or whether the width is the same at the rudder, as long as the rudder horn holes are in-line with the hinge point and the servo holes are also in-line with the centre and equidistant on both sides. The radius stays constant no matter if an arm or wheel is used.

Having a longer servo arm just means you get more rudder deflection - the travel on each side remains the same, so the cables stay taut. The only time there is an issue is when the rudder horn holes, or the servo holes, are not inline with the pivot/hinge points, or the holes are not equidistant on each side of the pivot point.

Kevin
The centre distance between the servo arm cable attachment points, must be the same as the centre distance between the control horn cable attachment points, to keep both cables tight during movement.
As mentioned already, the pivot point must be on the hinge line as well.
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Old Jan 31, 2013, 10:09 AM
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Pasco Tri Cities, Washington, United States
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Thanks

Bill Clark,
That is what I'm talking about. What is the source for the "matched pair" of wheels? I've seen and made the servo wheel, but not seen the rudder "wheel".

The comment made about only being slack when "hard over" makes no sense to me. I'd guess the slack side is always slack when the rudder is moved--just not enough to notice---until fully deflected.
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Old Jan 31, 2013, 10:51 AM
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What kcaldwel said, the radius of the wheel does not have to be the same as the length of the horns on the control surface. As long as the holes on the horns line up with the hinge line and you do not pull the surface greater than plus or minus 90 degrees, no harm. I personally like to have a little positive Ackerman in my pull-pull systems as when the line not being pulled goes a little slack, no harm. In fact it will guarantee that you are not over taxing the servo which will happen if the opposite occurs; i.e. the non pulled line becoming more taunt which can easily happen if you do not make sure the holes on the horn are lined up properly with respect to the hinge line.
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Old Feb 01, 2013, 07:57 AM
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LR, I machined the rear pulley and the front is a hanger9. rear is larger giving 60/45deg servo/rudder ratio and of course no slack not that I think some slack matters.
on a conventional setup the rudder is loaded and the slack line is just along for the ride.
another thing to consider is if the servo arm is a different size than the rudder horn (like it should be unless you want +- 60deg of rudder throw) then having the horn over the hinge line theoretically will create some slackening because of the 2 different arcs
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