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Old Mar 18, 2015, 11:59 PM
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Discussion
TX Dual Rates Setting vs Servo Control Horn Position

What is the difference between setting the range of control surface travel via using the Dual Rates on the Transmitter versus moving the z-bend of the Control Rod into a different position on the Servo Control Horn?
In other words, if I wanted to reduce the travel on my Ailerons of a new plane, should I change to Low Rates on my Tx (say to 50%) or move to comparable inner position on my Servo Horn? Which would be the better way and why. Would there be any difference for a 3D plane?
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Old Mar 19, 2015, 01:10 AM
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1) Changes in transmitter have more "granularity", ie, you can make a change in increments of 1%. Changes made at the servo arm position are of fixed amount, and probably much larger than 1%
2) It's been multiple decades since my statics & dynamics class, but I recall there's a mechanical advantage you get by being further out on the arm. Bringing the pushrod in closer would reduce that. I suspect most servos are powerful enough that this is not a problem, in practice
3) It's possible you gain a little control resolution (may not be the correct term) as you get in closer. For a 1 degree change in the position of the shaft, the closer in the pushrod is, the less the rod will move. Therefore, a 1 degree change results in less control surface movement, meaning finer control. I suspect this difference is insignificant for most fliers
4) Probably the reason people make the changes in the transmitter: It's really easy! You don't have to : expose the servo; some how get the rod off of the arm (unscrew; remove servo arm; remove rod, place it in different hole, replace?). Let's say you do that; and then don't like that (need more, need less). So you have to do it all over again. On the transmitter, it's push a few buttons...

I don't do 3d; but I believe 3d is possible due to the large control surfaces, large surface movements, and gobs of power.
- Large control surfaces implies you want as much torque as possible; you may be going the wrong way by bringing the pushrod in closer.
- Large movements? Well, you lose that by moving the pushrod in
- Gobs of power; this would have no impact on that aspect
- Fine control resolution; I suspect 3d planes really need to have this, so moving in closer would get you that. But I suspect the better solution is a really good servo made for the job

Regards
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Last edited by Joe Perrone; Mar 19, 2015 at 01:15 AM.
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Old Mar 19, 2015, 03:57 AM
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The way I see it, a servo signal from the transmitter goes in increments, usually small enough that the servo movement appears to be smooth. When you reduce the rates on the transmitter, you're reducing the number of increments and, as a consequence, the total distance the servo arm travels. Reduced number of increments will mean in theory you've got less-precise control over the position of your control surface.

So if you want to permanently change the travel of a control surface, the best way is to adjust the links at the servo arm or at the control surface, and leave the transmitter rates at 100%. Dual rates should be kept for instances where you want to be able to change the control deflection from your transmitter.
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Old Mar 19, 2015, 07:51 AM
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Agreed. The point of dual rates is to allow you to have loads of movement sometimes and less at other times.

If you just want less movement overall then you do it by either changing the mechanical set up or by setting the overall travel amount in the transmitter. No need for two rates.

Steve
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Old Mar 19, 2015, 08:13 AM
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If you do it mech on the plane you can't change rates while it flight.

For 3d don't reduce endpoints on tx do it in the d/r.
http://www.rcgroups.com/forums/showp...count=22436%22 #7 endpoints on tx "endpoints should be very close to maxed out. Some radios will go to 120%, some to 140 or 150. Whatever, it just needs to be maxed out."

I had a link but lost it on geometry of servo vs control horn. The gist of it is you loose power from the servo if the geometry is wrong.
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Old Mar 19, 2015, 08:21 AM
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ahh found it

http://www.modelairplanenews.com/blo...set-up-servos/
Quote:
By having full travel to drive the control surface, you maximize the servo’s precision and power. This setup works best with the pushrod in the outer servo-arm and control-horn holes.
Quote:
Avoid moving the pushrod closer to the surface on the control horn; moving it would reduce the leverage applied by the servo
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