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Pull pull question

Old 06-28-2010, 09:45 AM
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pabrokenprop
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Default Pull pull question

Ok… I'm building a 30% tt toc yak and I seem to have trouble with my pull pull rudder.
At "zero" I have tight wires. When I deflect, the wires go slack. What am I missing here?
I have moved the linkage in to 3" on the rudder to try to make it a 1:1 ratio with my 3"
servo arm. I have also tried crossing and uncrossing the wires and still have slack. WTH am
I doin wrong?!?!

Please help me out here.
I'm ready to fly this plane BUT I'm
not willing to fly it without being sure
it's ready.
Old 06-28-2010, 09:54 AM
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MajorTomski
 
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Default RE: Pull pull question

The distance between the pivot point on the servo and the attach point of the cable on the servo arm, i.e. 1.4 inches on your 3" arm has to match the distance from the hinge line of the ruder to the attach point of the cable on the rudder control horn.

A TOC flyer I know doesn't mind if only the pull line remains taugh under load.

HTH YMMV
Old 06-28-2010, 09:54 AM
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Fleet
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Default RE: Pull pull question

Geometry of one of the cranks is deferent than the other. Rudder bellcrank must be aligned to the rudder hinge line. Any miss alignment will cause a slacking of one and tightening of the other. If your cabling at the servo is mounted directly to the servo, you've got perfect alignment unless your bellcrank there is bent or bending under load.

Hope this helps.

Ray W.
Old 06-28-2010, 11:03 AM
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pe reivers
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Default RE: Pull pull question

I like the situation that the cables go slack when the control surface is not centered.
Old 06-28-2010, 11:11 AM
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denoferth
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Default RE: Pull pull question

To keep slack out of the control wires the rudder bell crank must be aligned to the rudder hinge line AND the holes in the servo bell crank and its pivot-point must be in line. What is often overlooked is many servo arms have holes that are not in line with the pivot point. Lay a straight edge across the bell crank for a quick check. If the inner or outer holes don’t line up with the pivot point then you have found the problem.

Read more: http://www.rcuniverse.com/forum/m_98...#ixzz0sAEquRaD
Old 06-28-2010, 11:18 AM
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Fleet
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Default RE: Pull pull question

A cable that slacks would assure that the flight control surface would not necessarily travel in the opposite direction immediatly if commanded by the servo movement. In other words, the slacking must be taken up before the flight control surface begins its movement. In our modeling world it may not be such a big deal as it is in the full size world.

During flight manuvers with loose or slacked cables or vibration will allow the cable to whip about and, in by it's own movement, cause uncommanded and eratic flight control surface movement.

Ray W.
Old 06-28-2010, 12:30 PM
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JoeAirPort
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Default RE: Pull pull question


ORIGINAL: pe reivers

I like the situation that the cables go slack when the control surface is not centered.
Yep, me too. Way better situation than the opposite.
Old 06-28-2010, 01:16 PM
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jooNorway
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Default RE: Pull pull question


ORIGINAL: pe reivers

I like the situation that the cables go slack when the control surface is not centered.
In such cases I have had trouble holding highspeed knife-edges... []
Old 06-28-2010, 01:40 PM
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MTK
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Default RE: Pull pull question


ORIGINAL: pabrokenprop

Ok… I'm building a 30% tt toc yak and I seem to have trouble with my pull pull rudder.
At ''zero'' I have tight wires. When I deflect, the wires go slack. What am I missing here?
I have moved the linkage in to 3'' on the rudder to try to make it a 1:1 ratio with my 3''
servo arm. I have also tried crossing and uncrossing the wires and still have slack. WTH am
I doin wrong?!?!

Please help me out here.
I'm ready to fly this plane BUT I'm
not willing to fly it without being sure
it's ready.
Suggest you look Ackerman Principle up on WWW. It's a common geometry problem originally observed on turning cars I believe...the outside wheel skidded while turning.

Servo standard movement is, say, +/- 60 degrees; rudder movement is what? +/- 40 degrees? The pulling side will be tight while the "pushing" side will go slack, always

It isn't an issue in practice. You won't see a problem in flying
Old 06-28-2010, 03:39 PM
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slopoke
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Default RE: Pull pull question

Search for ackerman pull pull here on rcu. There is a good discussion and pics too.

slopoke

Old 06-28-2010, 04:04 PM
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pe reivers
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Default RE: Pull pull question

Brian, a good friend of mine from America's smallest state, mistakenly called it Ackerman. He did however put an excellent explanation on his web pages how to have equal cable tension in all servo positions. He builds the ugliest planes I ever saw, but without sacrifice to function and building integrity. See http://members.cox.net/bdfelice/Wedge/Wedge.htm
Still I deviate from the perfect to have some slack (not much) when not centered. That allows me to adjust cable tension in center position, without having to worry about extra servo wear and rudder horn failures. I cannot in the world imagine how this would influence knife edge, unless very sloppy building causes extreme tension differences resulting in gobs of slack at full deflection.
Old 06-29-2010, 02:27 AM
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Default RE: Pull pull question


ORIGINAL: pe reivers

. I cannot in the world imagine how this would influence knife edge, unless very sloppy building causes extreme tension differences resulting in gobs of slack at full deflection.
I experience a kind of wobbling.
Old 06-29-2010, 06:03 AM
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pe reivers
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Default RE: Pull pull question

Interesting.
How slack was your non-pulling cable? just free of tension, or could it be moved 5mm before it came up to tension?
Old 06-29-2010, 07:13 AM
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scale only 4 me
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Default RE: Pull pull question


ORIGINAL: pe reivers

I like the situation that the cables go slack when the control surface is not centered.
Me too, I've never noticed a little slack being a problem in flight or any issues with holding a knife

It's a pull/pull system after all,, not pull/push
Old 06-30-2010, 12:11 AM
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bgold
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Default RE: Pull pull question

You have to do 2 things to get rid of the slack.
1. Get the pivot point of the control horn on the hinge line exactly, not just close.
2. Make sure the servo arm is the same size as the distance between the two rudder control horn pivot points.
Old 06-30-2010, 07:38 AM
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Default RE: Pull pull question


ORIGINAL: scale only 4 me


ORIGINAL: pe reivers

I like the situation that the cables go slack when the control surface is not centered.
Me too, I've never noticed a little slack being a problem in flight or any issues with holding a knife

It's a pull/pull system after all,, not pull/push
The problem with a a pull-pull with slacks is when the rudder is to be returned from a deflected position. There will be a delay for the rudder to move back because of the slack. The delay probably is more when the rudder is further off the center. To fly precisely, you do not need the delay.

Old 06-30-2010, 07:45 AM
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Default RE: Pull pull question


ORIGINAL: bgold

You have to do 2 things to get rid of the slack.
1. Get the pivot point of the control horn on the hinge line exactly, not just close.
2. Make sure the servo arm is the same size as the distance between the two rudder control horn pivot points.
Some gas ARFs provide a servo arm extension (a V-shaped piece of thin metal plate) that allows the servo ends of the pull-pull cables to be connected at an angle less than the usual 90 degree between the cable and the servo arm center line. This will achieve the effect of 1, without really doing it.

Old 06-30-2010, 12:45 PM
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Default RE: Pull pull question


ORIGINAL: pe reivers

Interesting.
How slack was your non-pulling cable? just free of tension, or could it be moved 5mm before it came up to tension?
It could be moved, maybe as much as 5mm. The problem did disappear when I fixed the geometry.
When the cable is just free of tension it works superb, as you say.
Old 06-30-2010, 01:47 PM
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Default RE: Pull pull question


ORIGINAL: nonstoprc


ORIGINAL: scale only 4 me


ORIGINAL: pe reivers

I like the situation that the cables go slack when the control surface is not centered.
Me too, I've never noticed a little slack being a problem in flight or any issues with holding a knife

It's a pull/pull system after all,, not pull/push
The problem with a a pull-pull with slacks is when the rudder is to be returned from a deflected position. There will be a delay for the rudder to move back because of the slack. The delay probably is more when the rudder is further off the center. To fly precisely, you do not need the delay.


What delay?
In flight,,, as soon as you release the rudder the wind/air pressure moves the rudder
Old 06-30-2010, 07:09 PM
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Default RE: Pull pull question

If the other line is tight, the rudder servo can actually help with the return of the rudder. What I meant by delay is the speed delta of servo assisted pull vs. the air pressure only pull.
Old 07-01-2010, 05:06 AM
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pe reivers
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Default RE: Pull pull question

If you have a very slight slack, the servo will start pulling the control surface back immediately. Question is, is the servo as quick as aerodynamic forces will do the same?
Like in bowshooting. If you just relax the fingers and let the string pull them open the release is much better and quicker than when trying to stretch the fingers for the release.
Old 07-01-2010, 07:43 AM
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Default RE: Pull pull question

True. The aerodynamic force could be great when the plane moves at a fast speed. But it could be much less when the plane is near stall (i.g. stall turn). The help from the servo is much needed in the later case.
Old 07-01-2010, 08:06 PM
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Default RE: Pull pull question

typically... with cables that cross.... it isn't possible to have a (mathamatically) perfect setup.

The best you can get... is to have the two rudder horns set 180 derees apart ligned up on the hinge of the rudder and the tillar at the servo to have about 7 degrees of aft offset.....

if the cables don't cross... than the front has to just be identical to the back

it is OK for slack to develop... if it is tight from around plus 10 or 15 degrees to minus 10 or 15.... that is no prob... airloads will usually keep the pulling cable tight.....
Old 07-01-2010, 08:11 PM
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Default RE: Pull pull question



There is no delay if the plane is flying forwards... the airloads return the surface to neutral faster then the servo
could...... in other words... when going from deflected to neutral... the airloads push the servo....

slack isn't a problem unless it exists around center.....



ORIGINAL: nonstoprc


The problem with a a pull-pull with slacks is when the rudder is to be returned from a deflected position. There will be a delay for the rudder to move back because of the slack. The delay probably is more when the rudder is further off the center. To fly precisely, you do not need the delay.

Old 07-01-2010, 08:16 PM
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Default RE: Pull pull question

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