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Servo fitting and horns

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Dove from above17/10/2019 21:42:54
20 forum posts
3 photos

I have never had to work out where to fit servos in a model before.

I need to fit 2 servos close together for rud and ele. The fuse is fairly narrow so tight to work in.

I want to try and get the placement right first go and I can see the only reason I would get this wrong would be getting the servo horn landing at a good distance to allow some adjustment of holes inward and outwards.

So what I am trying to say badly is, if I get the linkage rod sitting in the correct position should I be looking at getting the middle of horn for connection BUT saying that what length of horn is correct??? that's hard to say I know.

for instance I just pick up aileron and rudder horns from my LHS ,i got these because they looked "about" the right size and the shop only had them or 1/4 scale looking one! I just thought i would start in the centre hole and adjust if needed. Woud i do the same with the servo horn. Use what came with the servo and aim for centre hole?

Cor, sorry for the rambling,hard to describe that.

Doc Marten17/10/2019 22:32:04
378 forum posts
4 photos

I start with inner hole on the servo horn and outer hole on the control surface horn for leverage purposes.

Edited By Doc Marten on 17/10/2019 22:33:23

Nigel R18/10/2019 00:43:02
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3099 forum posts
479 photos

What doc said, for elevator.

For rudder I would start at the outside servo hole and outer hole on the horn.

Mark Stevens 118/10/2019 03:07:55
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126 forum posts
48 photos

A very simple but effective guide for you - Link

Mark

Martin McIntosh18/10/2019 19:28:15
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2947 forum posts
1083 photos

You must always start with the outer horn in the servo and control surface. The further out you go the less the play in the servo gears will be amplified and any slop in the holes (there should be none anyway). If you are new to this you can use a Z bend and a kwik link but make sure that each end is free but with no slop. I do not generally use kwik links any more and favour ball links which with a little fettling will give a firm but slop free linkage. Some soldering skills may be required here to get a proper 2mm thread on each end of the rod.

Dove from above18/10/2019 20:19:05
20 forum posts
3 photos

Thanks guys, I know its hard to comment on something that you can not see and each set up will be different BUT with these 19g servo`s I have the horns vary 1" to 2 1/4 ". When/why would you pick the smaller over the larger dia horns,because I could use any of the 3 supplied different sizes.

Martin McIntosh18/10/2019 22:02:33
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2947 forum posts
1083 photos

About 3/4" to 1" out from the centre screw is a good starting point for many models. Surface throw can usually be adjusted later on the Tx. but it is always best to set it up mechanically as close as possible to that recommended for the design. Use a ruler to measure the linear travel of the servo on a particular hole and compare this with similar on the control surface. Difficult to describe without a picture of your servos/model.

Andrew Ray18/10/2019 23:58:35
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715 forum posts
19 photos

Inner hole on servo output for better resolution and less slop/play. You don’t want to reduce the servo movement at the transmitter unless absolutely necessary as less servo movement results in poorer accuracy and ultimately jerky movement of the control surface. Just look at the way moulded gliders are set up. The inner hole on the output arm is too far out and to use maximum servo movement very often a hole is drilled 5 or 6 mm from the centre of the arm and the clevis is ground away to clear the output shaft flange or a purpose designed clevis/connector is used.

In fairness this is taking it to the extreme but as has been said in an earlier post, inner hole on the servo output and outer on the horn and adjust until you get the movement you need but only reduce the servo travel at the Tx as a last resort.

Edited By Andrew Ray on 19/10/2019 00:03:31

Martin McIntosh19/10/2019 09:04:16
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2947 forum posts
1083 photos

I have to disagree Andrew. Some gliders use that method to keep the dangley bits as close in as possible so as not to interfere with the airstream. 0.1mm of slop in the gears or linkage is 2% of total at 5mm radius. At 20mm this becomes only 0.5%. You just use longer control surface horns to compensate.

100mm rudder servo pull-pull horns are commonplace on 3D models, with correspondingly long rudder horns.

Andrew Ray19/10/2019 09:27:47
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715 forum posts
19 photos

You only have to try it to see the difference Martin. If you have, let’s say one degree of free movement due to slop then the amount of slop that is transmitted to the surface increases the further away from the centre. Your percentages don’t tell the story. It is more important with smaller servos as the size of the gears and corresponding number of teeth on each gear will effect the amount of free play in the gear set.

No doubt the 3D machine you are talking about doesn’t use standard servos?

Dove from above19/10/2019 10:38:59
20 forum posts
3 photos

Once again thanks very much for all the help, I am new at building so the set up is new also.

The build is nothing difficult ,it is a "mini Frantic" its from the "avicraft" stable,this model is a single wing Panic basically. large control surfaces.

Martin McIntosh19/10/2019 14:23:30
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2947 forum posts
1083 photos

Sorry Andrew, but you still have this wrong. Let us say that the servo is 100% accurate and the play is only in the linkage which is what the OP is not familiar with. My argument is now obviously valid.

I am just completing one of those awful DWH Storches. A guy doing a video on the build tried it as supplied and soon found out that a very short moment arm on the surface horns gave about 30% play in the movement. I tried it and he was correct, resulting in much longer horns being required all round. I used to fly F3A in the 70`s and in those days we did not have the advantages of the equipment now available and linkages were of paramount importance to get accuracy, so as a former British champion and twice team member in that in that discipline I believe that I know what I am talking about.

No hard feelings I hope since everyone on this forum no doubt have their own views.

Frank Skilbeck19/10/2019 16:54:12
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4488 forum posts
101 photos

What you actually want to achieve is the required control surface movement using as much as the servo rotation as possible and move the control connection in/out to achieve this. What you don't want to do is find that you are reducing the end points/rates on the transmitter by a large amount to make the control surface move the required distance.

Martin McIntosh19/10/2019 17:53:58
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2947 forum posts
1083 photos

Agreed.

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