Adjusting control horns, linkages and servo arms

[Simon Hall 2]

Just wondering what is the best way of doing this. I have felt that I had to adjust the threads on the rods of my Radian to get the rudder and elevators neutral. They were slightly off centre/level. Is this normal? Does trimming solve this anyway? I also moved a hole in on the rudder horn as it seemed a straighter run for the cable. Not flown yet though! Am I just being stupid and should leave as it was?

[Dave Hopkin]

With the servo arm centered aim to get the control surfaces in a neutral position by adjusting the length of the linkage (typically you will have at least one clevis on a thread that will give length adjustment)

Moving the linkage on the horns will alter the throw distance - Outer Servo, Inner horn gives the max throw

[john stones 1 - Moderator]

No you're not being stupid Simon, if it's not right correct it and as Dave says adjust the clevice horns etc, then trim on test flight with Tx.

John

[TigerOC]

Hi Simon,

Tip from another relative novice rc flyer. Use the flat side of a ruler to line up the active surface and the stabilizer. Adjust the clevis until it lines up with the hole on the control horn. Attach the clevis to the control horn and check that the surface is level with the ruler again. You may need have to adjust by half turns to get it right.

Check the throw of each surface that you need. You can then use the appropriate hole on the control horn to match the throw you need. Outside hole gives least movement and moving in gives the most on the active surface. You can then fine trim the throw on your tx as needed.

You can also assign switches on your tx for throws as well. As a rookie you want to turn down the throws a bit so reduce these on the normal position of the switch and then you can also have normal throws in the down position of the switch which you can use periodically in flight as your confidence improves. Also use slightly higher expo than recommended until you get more confident. This will smooth your flight considerably.

Rob

[Ross Clarkson]

Posted by Dave Hopkin on 14/09/2014 20:02:14:

With the servo arm centered aim to get the control surfaces in a neutral position by adjusting the length of the linkage (typically you will have at least one clevis on a thread that will give length adjustment)

Moving the linkage on the horns will alter the throw distance - Outer Servo, Inner horn gives the max throw

Some great advice from all, brilliant stuff. Just goes to show you learn something new everyday. It seems so simple now that it is written down but 'Outer Servo, Inner horn gives the max throw' is a great tip.

Thanks.

[Engine Doctor]

Hi Simon . Setting the control surfaces centered /neutral mechanically with the servos centered is always good practice .It will give you more adjustment with the radio trims if needed and equal throw in either direction To hold the surfaces at neutral I use a plastic building clamp and some scrap balsa sheet either side of elevator,aileron or rudder. This holds all the flappy bits firmly while making the adjustments . Once set and the model has been test flown and trimmed I then put a drop of cyano on adjuster threads to lock them up as some of the supplied hardware has such poor fitting threads they can some times slip .

[Dave Hopkin]

You should aim to get all control surfaces set up so that with the servos powered up and in neutral (centre stick) the control surfaces should be flat - as suggested use a straight edge to achieve this for Ailerons feel the junction between the fixed part of the trailing edge and the end of the moving surface

The Transmitter Trim should be used for in flight trimming to fine tune the trim rather than "zeroing" the throws

[Gregg Veasey]

Posted by Dave Hopkin on 14/09/2014 20:02:14:

Moving the linkage on the horns will alter the throw distance - Outer Servo, Inner horn gives the max throw

It will give you the most throw but geometry wise its incorrect. You should aim to have the shortest servo arm and the longest surface horn possible.

[Biggles' Elder Brother - Moderator]

Posted by Gregg Veasey on 18/09/2014 11:07:28:

Posted by Dave Hopkin on 14/09/2014 20:02:14:

Moving the linkage on the horns will alter the throw distance - Outer Servo, Inner horn gives the max throw

It will give you the most throw but geometry wise its incorrect. You should aim to have the shortest servo arm and the longest surface horn possible.

I wouldn't completely agree with this TBH. I can't see in what way its "incorrect". It might sometimes be less than optimum, but "incorrect"? I don't see it.

I think most of the advice above is spot on. Good practice says get the control surfaces centred when the servos are centred by mechanical means - ie adjusting the linkage length. I would go further and say it is also good practice to select the servo/control horn combination that gives you a control throw which is just a tad larger than the max throw you require - then fine tune with small adjustments on the Tx end points.

In my view poor practice is centring the control surface using trim or sub-trim - its not what they are for! And setting throws way-way to large and then ending up with end point settings that only allow 50% servo throw. You paid good money for that Tx/servo resolution its daft to throw it away!

BEB

[Gregg Veasey]

Just trying to highlight something that MANY MANY don't understand and get wrong. I totally agree with you about using sub trim and end points. I've seen all to often heavy duty servo's fail because of poor geometry and flutter too because the inner hole was used on the control surface.

[Steve Hargreaves - Moderator]

Also worth remembering that moving the linkage to the furthest hole on the servo arm will reduce the available torque or "pushing power" the servo can provide to the control surface.

As BEB sez...get it right mechanically first with the ATV at 100% then fine tune with the Tx....

[Gregg Veasey]

Yep agreed, thats what I originally said

"You should aim to have the shortest servo arm and the longest surface horn possible."

[IanR]

The aileron servo arms on my Parkzone F4U Corsair (original one without the flap and retract option - just taken it out of its box) were not centred when my DX7 transmitters trims were zeroed. So I've used the TX's sub-trim function to zero the servo arms and have moved the clevis into the surface control arm hole which most closely corresponds to the recommended maximum throw.

On high rates the instructions call for 15mm up and 10mm down. Low rates 12mm and 8mm. I presume this is to give some differential.

I have 2 questions which I would be very grateful to receive advice on -

1) Is this correct use of sub-trim? I have used 22 "beeps" of sub-trim on one aileron and 76 "beeps" of sub-trim on the other aileron. Will this reduce the amount of ordinary trim I can use when flying?

2) Secondly, should I use the TX's maximum throw settings, up and then down, to achieve differential or should I set up and down throws equally and then use the TX's Differential Function to dial in a percentage of differential?

Many thanks for any help. Ian

[Martin Whybrow]

Oh dear, people confusing torque and force again

The torque delivered by a servo is the same regardless of how long the servo arm is; what does change is the force transmitted by the linkage. The main reason for using the longest servo arm possible is to maximise the distance that the linkage moves as this alleviates any slop between the clevises and the servo arm and horn.

So, aim for the longest servo arm you can consistent with using the full 100% travel of the servo to deliver the correct control surface deflection; adjust the throw by moving the clevis at the horn and only then use the TXs travel limit adjustments to fine tune the deflection; if you're using the threaded post style horns, you can achieve a very fine level of adjustment without resorting to adjusting the TX travel limits.

[Gregg Veasey]

The longer the servo arm the harder it has to work to move the control surface. (pick up something heavy with your out stretched arm, then try again with your arm bent i.e shorter)

At the surface end, the shorter the horn the harder it is to move the surface. (push a gate open from its hinge point then again from the opposite end)

[Martin Whybrow]

Sorry Greg, you are mistaken; you are forgetting that the long servo arm goes with a long control arm; the torque required by the servo does not vary if the translation of degrees of rotation at the servo to degrees of deflection at the control surface is maintained.

[Biggles' Elder Brother - Moderator]

Posted by IanR on 18/09/2014 12:20:56:

The aileron servo arms on my Parkzone F4U Corsair (original one without the flap and retract option - just taken it out of its box) were not centred when my DX7 transmitters trims were zeroed. So I've used the TX's sub-trim function to zero the servo arms and have moved the clevis into the surface control arm hole which most closely corresponds to the recommended maximum throw.

On high rates the instructions call for 15mm up and 10mm down. Low rates 12mm and 8mm. I presume this is to give some differential.

I have 2 questions which I would be very grateful to receive advice on -

1) Is this correct use of sub-trim? I have used 22 "beeps" of sub-trim on one aileron and 76 "beeps" of sub-trim on the other aileron. Will this reduce the amount of ordinary trim I can use when flying?

2) Secondly, should I use the TX's maximum throw settings, up and then down, to achieve differential or should I set up and down throws equally and then use the TX's Differential Function to dial in a percentage of differential?

Many thanks for any help. Ian

well let's not get too prescriptive about all this - but strictly speaking, no that is not the proper use for sub-trims - although I have no doubt that it is what a lot of folks do!

What I aim for before a maiden flight is this: all control surfaces at neutral with the sticks in the middle, the trims at zero and the sub-trims at zero. Then fly it. I might need to add some trim in flight to get it to fly straight and level "hands off". Now the question is what do you do now? You have three choices:

1/ Leave it alone - afterall it flys OK. But the problem with that is I'm permanently taking up some of my trim range. If due to say a slightly different battery, or battery position or maybe I want to fly faster at cruise throttle because its a bit of a windy day, and I have less scope for adjustment because I'm using some of it to just get the plane nominally right.

2. Transfer those "permanent" trim settings to the sub-trims thus re-zeroing the main trims. This is better as I have my full trim range back. But the slight problem now is my servos are all slightly off centre at "neutral" and so I have more servo movement available in one direction than the other and so I'm not making the full use of the sensitivity of my servos. Do I do it? Yes of course I do - sometimes, when I'm feeling lazy!

3. Transfer the control surface offsets required for hands-off S&L to the mechanical system by adjusting the control linkage lengths thus putting the trims back to zero. I may not be able to quite do that perfectly, the adjustment in my control linkage length may not be fine enough. So make any very fine final adjustment via the sub-trim. So I will end up with a model that flies S&L hands off, with the main trims at zero and just a very small sub-trim offset. You might wonder "how do I accurately transfer the bit I can't get via control linkage adjustment to the sub-trim?" Well with a Taranis Tx this easy now. You fly the model, put in the very small trim adjustment you need to fine tune then when you land simply transfer that to the sub-trim.

BEB

[Biggles' Elder Brother - Moderator]

Ian - regarding the differential. Well again this can be done mechanically - but let's not go there as we'll end up joining Martyn and Gregs discussion!

The real answer is - the two alternatives you cite amount to the same thing! The TX's differential function just resets the end points which you are doing manually the other way. It depends which way you find easiest, but it ends up in the same place!

BEB

[Gregg Veasey]

Posted by Martin Whybrow on 18/09/2014 12:53:46:

Sorry Greg, you are mistaken; you are forgetting that the long servo arm goes with a long control arm; the torque required by the servo does not vary if the translation of degrees of rotation at the servo to degrees of deflection at the control surface is maintained.

I am highlighting a general rule. I am showing that it is bad practice to use a long servo arm with a short control horn.

[Martin Whybrow]

Posted by Gregg Veasey on 18/09/2014 13:28:42:

I am highlighting a general rule. I am showing that it is bad practice to use a long servo arm with a short control horn.

But that's not what you said

[IanR]

Thanks, BEB. That will do me. The reason for using sub-trim to zero the servo arms is that they were "off" out of the box. The only way I could see to adjust manually would be to remove them from the wing and make a right old mess of the tape and paint that surrounds them. I'd never get it to look right again. I note that using sub-trim doesn't alter the amount of trim available in flight. Good.

Thanks, again. Ian

[Gregg Veasey]

Ah, sorry, I thought it was.

My 'gate' and 'something heavy' examples were independent examples for the servo arm and control horn to try and explain in simple terms why that general rule should be followed.

Oh well, I tried to help 

 

Edited By Gregg Veasey on 18/09/2014 13:50:12

[Peter Beeney]

Interesting ideas here; although I would wholly agree with Greg. I always try and use the innermost hole on the servo arm and the outermost on the control surface as I consider this gives the best degree of control. You need the servo to be in charge of the control surface, not the other way round. Of course, it’s not always practicable to get it exactly as good as this, a compromise is invariably needed to get the necessary amount of control throw, but it’s possible to get the nearest best fit.

PB

[Simon Hall 2]

The reason for my original post was because I took out the servo arms and rods from 2 servos in a hurry to make it easier to remove the one faulty servo. I was not thinking about the positions or the arms and the rod notches. Please bear with this stupid mistake as I am a total newbie to anything RC.

Anyway, I have looked online for pics of the original servo arm positions and have matched this up as best I can. I fitted the exact same replacement servo and glued it in the same position. I have adjusted the threads near the control horns so that the control surfaces are as flat as I could get them when I power everything up. They are also taught with no play.

Below are pics of how they are when everything is powered up for flight. Is this good enough? The rudder and elevators seem to move freely and are equal in up/down, left/right movement.

**LINK**

[Biggles' Elder Brother - Moderator]

Yes that looks OK -not perfect but OK! I assume that front servo arm doesn't foul the back servo body at full throw?

Looking at the elevator it looks to be not quite straight? Pointing very very slightly down? It might just be the angle the photo has been taken from and be OK in reality? Just have another good look at it to be sure.

BEB