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  1. #1
    michael wood's Avatar
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    post style control horn set up

    what will give me more throw on this control horn closer to the control surface or farther away I have the pushrod set on the outer hole on the servo arm and the control horn set at 1/2 inch above the aileron

  2. #2

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    Move the pushrod closer to the control surface to get more throw.
    It\'s like a Koala Bear crapped a rainbow in my brain!

  3. #3
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    It's good practice to get as much as you can from servo throw before gaining throw by mechanical means. Since a 1:1 setup can give you 35 degrees of throw each way with a proper mechanical setup, it's very rare that someone needs to go closer at the control surface than they have it at the servo.

    And yes, there are disadvantages to moving the horn closer at the surface. First, it reduces your effective servo torque, which means you may not be able to actually get the throw in flight that you are setting up for. Second, it magnifies any slop in your system. Slop in your gears, all of your linkages, and any flex will add up. And third, less pushrod movement translates to less overall precision in your control system. If your servo doesn't center perfectly or if you are using analogs, you'll find that the plane starts to feel disconnected from your inputs. That's very frustrating when you want to fly with any degree of precision.
    No kid, I said break ground and fly into the wind!
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  4. #4

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    Hi!
    I agree fully with Jester!
    The general rule when setting up an airplane is to have as short servo arm as possible and as long control arm (elevator) as possible.
    That's the rule if you want a sloop free set-up.
    Jan Karlsson - Supplier MVVS Products

  5. #5
    Pylonracr's Avatar
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    Wrong, Use as long a servo arm as possible, and reduce your throw with a longer control surface horn length. This will minimize any slop or dead band in your linkage. Read what Jester wrote again. As much as it pains me to say, he is right (Yes, we go back a ways).

    Scott
    Revver Brother #260

  6. #6
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    What can I say? Even a blind hog finds an acorn once in a while.

    I think Jaka is saying basically the same thing though. I generally start with the connection at the outermost hole on the control surface that is practical to use, then move it in at the servo arm as far as needed to get the throw close to where I need it to be.
    No kid, I said break ground and fly into the wind!
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  7. #7
    Pylonracr's Avatar
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    Jester, either I read it wrong or I am just thinking backwards. My method is as long of servo arm as possible and move the linkage away from the control surface to reduce throw. That always seems to minimize slop in the linkage since the pushrod moves further for the same control surface throw. Or am I wrong? I am barely halfway through my first cup of coffee, so it could go either way.

    Scott
    Revver Brother #260

  8. #8

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    Hi!
    Sorry ! But you are wrong!
    The reason for having the servo arm as short as possible is that the gear sloop in the gear box inside the servo will be minimized that way. Test by mounting a long servo arm (around 30mm long) on the servo and notice how much play you get at the tip of the arm. Then take a short servo arm (around 10mm long) and do the same test.
    On my pylon racers where the elevator throw is around 3mm up and 3mm down measured at the trailing edge of the elevator I have the linkage mounted on the servo arm approx 7-10mm from the servo center. The result is an elevator without any flex at all.

    The opposite is when it comes to the control surface, here we want to have as long control arm as possible to reduce the play in the linkage and hinges (the closer you put the linkage to the hinge line the more play you build in)

    Remember that the balance of he plane (C of G) is involved too in setting up an airplane and the rule of thumb here is to balance the plane as far back as is possible and at the same time have the servo arm as short as possible and the control arm as long as possible. Of coarse there is a limit to how far back you can have the C of G but this you have to find by trail and error! A little nose heavy first then move servos and batteries rearward until you find the "sweet spot".

    This is the rule of thumb when it comes to setting up any airplane!
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    Last edited by jaka; 03-18-2017 at 12:33 PM.
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  9. #9
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    You are definitely right Jaka about minimizing the effects of gear slop, which I honestly had not thought about until you gave your explanation above. What I was addressing, and I think pylonracr was too, is connector slop cause by clevises and Z bends. The most slop free setup possible would be snug fitting ball links on both ends of the pushrod with the horn in the closest servo arm hole and then moved out as far as possible on the control surface. That's assuming extra stiff linkages too. But for those who want to go with a Z bend and plastic clevises, I think connector slop is a bigger concern than gear slop and so going for the most pushrod movement would be better.
    No kid, I said break ground and fly into the wind!
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  10. #10
    Pylonracr's Avatar
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    You are kind of right Jester. When I read that post by Jaka I almost sprayed my wine all over my keyboard. I honestly thought it was a joke. Jester, you are correct in that the longest servo arm and long control horn will give you the most slop free setup in regards to any play in the linkage. A 1 to 1 ratio will always give you the same control surface throw, but with more pushrod travel any linkage play will be minimized. Jaka is completely wrong in his statement that a shorter servo arm will help in any way. Any geartrain slop will present itself in servo horn movement. A 5 degree movement in geartrain slop will be a 5 degree movement in the servo horn, which will be a 5 degree movement in control surface movement, Period. The short servo horn will only hurt you in that any pushrod play which will be amplified with reduced pushrod movement for any given control surface throw. Always use the longest servo horn possible, and adjust the control surface throw mechanically at the control horn. The only way to get rid of control surface movement due to geartrain slop in the servo is to replace the servo with a quality servo with no play in it.

    Scott
    Revver Brother #260

  11. #11
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    If you want to get technical about it, it's all about the ratio of the servo arm to the control surface. I think we've agreed that you want the attachment to be as far out as possible on the control surface. Where I see Jaka's point is that that 5 degrees of slop in the gears (it's never that much of course, but I'm using the term you used) creates a bigger linear movement the farther out on the servo arm you go. But it's the ratio of length that mitigates that. Farther out on the servo arm means more slop from a linear movement perspective, but then farther out on the control surface means that the linear slop makes less of a difference.
    No kid, I said break ground and fly into the wind!
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  12. #12
    Pylonracr's Avatar
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    Jester, you have the right idea. I used your example of a 1 to 1 ratio, so 35 degrees at the servo = 35 degrees at the control surface. Longer arms that maintain that ratio still give the same surface throw, but do it with more pushrod travel. Any play in the pushrod will be minimized. Any slop in the servo will be transmitted to the control surface at the same ratio of 1 degree servo slop = 1 degree control surface movement. This can not change. Of course you can alter the ratio to achieve the overall travel you desire, but servo geartrain slop will be transmitted to the control surface at that ratio. A shorter servo horn will NOT minimize this, but will only maximize any play in the pushrod.

    Scott
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  13. #13

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    Hi!
    As the rule says! The servo arm should be as short as possible and the control arm as long as possible! That's the fact!
    That way you get the least play. This is despite what kind of linkage you use, ball link, Z-bend or what ever.
    This could easily be seen . Put the linkage rod at the center of the servo arm (in theory, as this is kind difficult to do in practice ) and you get 0 degrees of play in the elevator surface (theoreticly)...the further out you go on the servo arm the more play you get in the elevator surface.( with the same control arm lenght) if you on the other hand shorten the control horn so much as nothing of it protudes above the hinge line you will get lots of play.

    As pictures below show.
    Short servo arms...and long control arms (on ailerons and elevator) that way you minimize play!
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  14. #14

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    Two other important factors for short servo arm/long control arm are that they maximize the effective torque that you get from the servo and maximize the granularity/precision you get from the control stick.

  15. #15

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    Pylonracr I disagree with your methodology. For maximum servo torque and resolution the closer to the center of the servo horn the better. Using the 5 degree arc of slop in the servo gear train example the linear movement of a connection point located closer to servo center will be less than the linear movement of a connection point farther out.
    I once built a training aid using two protractors and bell crank mounted pointers attached to identical servos mixed to the elevator channel on my computer radio. One servo was set up with the shortest servo arm and the longest bell crank connection to give one inch of throw. The other was set up with the longest servo arm and the bell crank connection that also gave one inch of throw. All connections were made using ball links for slop free connections. Care to guess which set up gave the least amount of slop at the end of the pointer? Care to guess which one gave the most precise and smooth response to trim input?

  16. #16
    Pylonracr's Avatar
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    Quote Originally Posted by pkoury View Post
    Pylonracr I disagree with your methodology. For maximum servo torque and resolution the closer to the center of the servo horn the better. Using the 5 degree arc of slop in the servo gear train example the linear movement of a connection point located closer to servo center will be less than the linear movement of a connection point farther out.
    I once built a training aid using two protractors and bell crank mounted pointers attached to identical servos mixed to the elevator channel on my computer radio. One servo was set up with the shortest servo arm and the longest bell crank connection to give one inch of throw. The other was set up with the longest servo arm and the bell crank connection that also gave one inch of throw. All connections were made using ball links for slop free connections. Care to guess which set up gave the least amount of slop at the end of the pointer? Care to guess which one gave the most precise and smooth response to trim input?
    Pkoury
    I find your test results quite interesting, and I am quite curious to see more. When I became a competent enough pilot to require accurate servo linkage I researched the best method and found what has been considered the best arrangement for the past 25 years or so. Use the longest servo horn possible, with the corresponding length control horn to achieve the needed control surface travel. This simply provides the most linear pushrod travel for any given control surface travel, thereby minimizing the effect of any linkage play. Applied servo torque, of course, remains the same as you still use the same servo travel to obtain the same control surface travel. This method has been used by nearly every TOC, IMAC, and pattern competitor for over 20 years.

    Now you have evidence that proves there is a better way. Will you please share it with us? I am very interested in high resolution photos of your test. What test instrumentation was used? What were the differences between the Control Subject and the Test Subject? How were the tests documented? I admit I never built a bench model to test this. I had one of my grad students in theoretical physics model the test in 3D Force/Vector plotting software and document the results. We tested using servo arm lengths of 1/4", 1/2", 1" 1 1/2", 2", and 3" while introducing linkage play of .030". The results were staggering to say the least. With the shortest servo arm the free travel at the control surface was over 2 degrees, and with the longest arm the movement was less than 0.01 degrees. Now, if there is a better way, there are hundreds of competitive pilots, myself included, that would like to know how to achieve better control surface resolution.

    Please share your testing and documentation with us so we can all improve our aircraft.

    Scott
    Revver Brother #260

  17. #17

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    I never said your method was wrong I said that I disagree based on my experience. That rig is long gone. The whole thing was put together as a club meeting presentation because so many new (and some old members) were having set up troubles thanks to the new computer radios. The common theme was over relying on the computer radios with sub-trim, end point adjustment, and expo to overcome improper mechanical setups on sport fliers with baseline sport servos on sport aircraft.
    The system I use has served me well so I stick with it.

    Your 20 years reference brought a smile to my face when I think about back when;
    You needed to order reverse rotation servos (or reverse them yourself) because TX servo reversing did not exist.
    You sawed the wing of a new pattern model in half to change the dihedral to tune out unwanted coupling because there was no TX mixing.
    You removed the servos to clean the pots before a pattern meet to ensure the best response from control inputs.

  18. #18
    Pylonracr's Avatar
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    The biggest mistake I see people making is using a long servo horn and short control horn, giving them way too much travel. Then they use the end point on the radio or dual rates to lower the throw, using about half of the servo travel. This cuts the servo torque way down while lowering resolution. Exactly the opposite of a properly set up linkage.
    Revver Brother #260

  19. #19

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    Hi!
    ?????But that is what I have been saying all the time! Long servo arm and short control arm gives too much play!
    That's why you should use as short servo arm and long control arm as possible ...to obtain stiffnes in the flying surface.

    Click image for larger version. 

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    Last edited by jaka; 03-23-2017 at 08:32 AM.
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  20. #20
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    At the end of the day it's is the ratio that matteres the most. Scott ( pylonracer ) is correct that a longer servo arm and an even longer control horn will yield the least amount of play in the linkage however it will show gear wear more. I assume that Scott like myself use good enough servos where gear play is usually not an issue. I just clicked a couple pictures of my latest pattern airplane. While being an older design it has had some updates and flies rock steady. Note that I use relitivly long servo arms but also really long control horns, to the point that if you look closely at the elevator servo, it is mounted on a wedge to keep the pushrod parallel with the servo. On all surfaces the ATV is over 100% and resolution is very good.
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    Of course it's true, I read it on the Internet.

  21. #21
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    I’d like to try and resolve this conversation if I can.
    If I understand it right, Jaka’s point is that you’ll feel less wiggle in the pushrod when you connect it to the inner hole of the servo. He’s right about that.
    Pylonracr’s point is that more pushrod movement for any given surface throw means minimizing the effect of linkage slop. He’s also right.
    Where I see a disagreement, and correct me if I’m wrong on this guys, is in the idea that one must pick one benefit over another. I say you don’t. There is an absolutely best way to do it, and I’m going to try to explain why.
    In an RC system, we start with angular motion at the servo, it gets converted to linear movement at the pushrod, then converted back to angular movement at the control surface. So let’s talk about gear slop. Gear slop would be measured in degrees, so let’s say a fairly loose servo had a degree of slop in its gears plus the amount of deadband it has designed in. You could do some basic trigonometry and figure out how much linear movement that degree would become at the pushrod, but we really don’t need to be that precise. It’s easy enough to see that the farther out you go on the servo arm the more linear movement that degree of slop is going to create. If I understand it right, that’s Jaka’s point. But that’s not the end of it.

    Since we convert that linear movement back to angular movement at the surface, the only thing that really matters is the angular ratio between the servo and the control surface. I’ll admit it took me a day or two to think this though myself, but I’m sure about it. If you were in a servo arm hole 15mm from the center of the arm (axis of rotation to be technical about it) and connected the other end of the pushrod 30mm away from the center of the hinge, you’d have a ratio. So that degree of servo gear slop would translate to degree of slop at the control surface. It would change absolutely nothing about the effect of gear slop to switch it to 10mm and 20mm, or 40mm and 80mm. The lengths don’t matter. It’s all about the ratio.
    But then there is another source of slop-linkages. Linkages create linear slop that gets added onto servo gear slop. An understanding of ratios is helpful here too. If you use cheap linkages and get 1mm of total slop, and your linear pushrod movement is 20mm, then 1/20th of the actual control surface’s movement will be seen as slop at the control surface. But if you use those same linkages but with longer servo arms and control horns to create the same ratio (minimizing gear slop too), then maybe you could wind up with 40mm of pushrod movement. That would mean only 1/40th of the control surface’s movement would be slop, resulting in an overall tighter setup.
    Bottom line: Use the longest control horns on your surfaces that are practical, use all of your available servo movement to minimize the effect of gear slop, then adjust the pushrod at the servo for the throw you need. That will give you the tightest possible setup and will also minimize the loads on your pushrod leading to less flex and possibly even minimizing the chances of having linkage fail.
    No kid, I said break ground and fly into the wind!
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  22. #22
    Pylonracr's Avatar
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    Well Jester, Crap: I have to agree with you Twice on one thread??!! You are correct and stated what I have been saying since the start of this thread. Shawn said the same thing in different words. The ratio will determine final control surface throw - Use the entire servo travel. The longest servo arm will utilize the longest pushrod travel and thereby minimize the effect of linkage play. Simple.


    Scott
    Last edited by Pylonracr; 03-24-2017 at 06:56 AM.
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  23. #23

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    I think we can agree that for any desired amount of control surface travel (say 1"), there are any number of servo arm/control arm length combinations that will work.

    It is easy to achieve zero linkage slop by using quality products like bolt-on swivel links and titanium pro-links. If you use z bends, soft metal rods that flex, and/or loose-fitting clevises, then you deserve what you get. However, if your linkage is sloppy anyway, then the shorter servo arm/longer control arm combination will result in less control surface movement.

    Gear slop is not a factor. Regardless of how long your servo arm is, gear slop will result in the same amount of control surface movement for the desired amount of control surface travel you've chosen. Longer servo arm/shorter control arm means the servo arm connection point moves a lot but the control surface moves less. Short servo arm/long control arm means the servo arm connection doesn't move much but the control surface moves more. The net result is that the control surface moves the same amount in either case. So, there is no benefit to going short servo arm or long servo arm as it regards servo slop.

    That leaves effective torque. A servo arm that is shorter than the control arm is more effective in moving the control surface and resisting blow back than the converse. Simple mechanical advantage.

    I would also argue that the shorter servo arm is also more precise in that for any amount of stick movement, the servo connection point moves less, thus you can more precisely position the servo connection point with your stick. Granted, that doesn't change the amount that the control surface would move as compared to a equivalent longer servo arm/shorter control arm, but I'd rather put as much precision/granularity at the front end as possible.
    Last edited by RBACONS; 03-24-2017 at 02:29 PM.


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