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Old 02-09-2017, 03:51 AM   #1
dereckbc
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Default Servo Linkage Ratio Fact Check

OK if one sets up the Servo Linkage Ratio to 1:1; it should not make any difference if you used either the inner or outer most position on the Servo Arm and Control Horn linkage?

True or False.

If False please explain. I do not think it makes any difference and is a matter of cosmetics or you might need clearance and would choose the outer holes. As long as the distance from the between the Control Pivot to Pivot Point are equal distance on both Servo and Control Horn. If the servo deflects 10 degrees, so does the Rudder Elevator, or Ailerons.
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Old 02-09-2017, 07:09 AM   #2
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Geometrically you are correct.
The assumption in that example is that there is no slop in the linkages.

If you have a little slop, maybe 0.010" and it is close to the center of the pivot, it will allow the surface to move a greater distance without direct control, than if you were at the outer end of the servo arm or control arm. This can start a surface into flutter, if there are other conditions which would contribute to it.

Since it is hard to get no slop and still free movement, it is usually better to be at the outer holes.

On 3D aircraft, sometimes we want the surface, lets say the rudder, to move 60 degrees each way, yet some servo only move 30 degrees each way.

Obviously in that case we must set a 1-2 ratio. The pushrod at the servo would connect to a point twice as far from the center as the connection at the control horn would be from the hinge line.

I hope that makes sense.

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Old 02-09-2017, 12:00 PM   #3
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As long as the arm and horn pivot length are the same and parallel the ratio of movement is 1:1 but the forces in the link rod (and the hinges) are inversely proportional to the arm lengths and as Wildflyer states so is any free play.
So to reduce the forces involved, which can themselves generate 'free' movement, and to minimise the effect of any free play already in the linkage the longer the arms the better.
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Old 02-09-2017, 03:57 PM   #4
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Originally Posted by Wildflyer View Post
I hope that makes sense.
Yes Sir it does, thank you.

I am using digital servos on a new 3D Plane. The servos come factory set to +/- 60 degrees (120 total) nand can go a full 180. The plane, an Extreme Flight Edge Demonstrator control surfaces will travel +/- 45 degrees.

My initial thought was 1:1 ratio and limit servo travel to +/- 45 degrees. Second thought for maximum mechanical leverage was set the servo range to +/- 60 and use 1.5:1 ratio. I gain Leverage but the trade off is Speed.

Anyway what had me questioning my logic is why I kept reading to use the holes farthest away from the Servo and Control Horns you can get. That did not make sense to me because doing so on the Servo Arm would mean you loose leverage or torque.

So now from your explanation I understand why the authors said that. It also explains why 3D planes need fast and strong servos because some models I have seen require some insane throws of +/- 160 degrees and that would require a ratio of less than 1:1 something like .75:1

THX
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Old 02-10-2017, 12:04 AM   #5
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I think you are really overthinking the entire linkage setup thing, just set the airplane up as instructed in the manual then go fly the snot out of it

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Old 02-10-2017, 12:16 AM   #6
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Originally Posted by Xpress.. View Post
I think you are really overthinking the entire linkage setup thing, just set the airplane up as instructed in the manual then go fly the snot out of it
Yes Sir, that is how I learn. That is what engineers do. Plane calls for 1:1 ratio. and 45 degree deflection.
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