Closed loop rudder issues

[Kris S]

I've just set up my first closed loop rudder system and both lines are going slack but only in 1 direction.
Both horns are exactly the same length from centre to hole and lines are running parallel to each other.
I've read about people having one line go slack but not both.
I've spent 2 hours trying to suss this out with no luck
Please help.
Thanks.

[john stones 1 - Moderator]

Any chance of a picture Kris ?

John

[Ultymate]

The system is a "Pull Pull" system, it's only the line that's pulling that's doing any work. If both none pulling lines go slack in their moving arc it's of no great significance so long as the two lines are tensioned "slightly" at centre point. The slackness I think you have usually relates to the geometry of the rudder horns in relation to the hinge point of the rudder

[Biggles' Elder Brother - Moderator]

Another point to consider Kris - picking up on Ulty's point - the system only really works if there is some resistance to the rudder movement. In the air this is of course air pressure - but on the ground that doesn't exist. So to test it you need to provide so resistance - a little gentle pressure from your hand resting against the side of the rudder is enough. It shouldn't go slack then - does that still give you adequate throw without a big "dead zone" is then the question. If the lines are reasonably taut (not tight) at neutral and give positive rudder movement you should be OK.

BEB

[Kris S]

Both lines are tensioned at centre point and stay tensioned through the whole arc of right rudder but when it gets to 50% throw going left both lines go slack.
Maybe I need to try a different horn on the rudder as it's as far forward as it can go but holes aren't quite over hinge line.
I just think it's odd that it's only in 1 direction.

[Kris S]

BEB, thanks I will try that.

[Keith Berriman]

Hi Kris S do have the open area to be able to cross the lines ?? so that the right servo lines goes to the left rudder arm and visa versa I use this method and find I have a closer control on the rudder

[Biggles' Elder Brother - Moderator]

Yes, its its generally a good idea to cross the lines if you can - I've never been entirely sure why this works, but it does tend to produce a better system. Maybe it gives a more "natural" run to the cables or perhaps the fact that the pull is now "off axis" from the horn/servo-arm provides a more positive leverage? Anyway - whatever the reason it does work!

BEB

Edited By Biggles' Elder Brother - Moderator on 17/08/2014 17:20:02

[Ultymate]

Crossing the lines does work "sometimes" though not always, basically it just changes the geometry slightly, nothing to lose if you have the room give it a try

Edited By Ultymate on 17/08/2014 17:25:28

[cymaz]

Half way down on THIS page is a good diagram. Basically. both lines should be all the same lengths from the hinge line to the servo pivot points.

[Kris S]

Crossed the lines and it's improved.
Lines are at constant tension one direction and acceptable in the other. Guess I am probably being over fussy as I tend to be.
Thanks everyone for your help, very much appreciated.

[Chris Barlow]

As mentioned above try applying a little resistance to the rudder. I did find on my Puppeteer that the furry type mylar hinge was springing the rudder to the left so when moved in that direction the line never seemed tight. This was cured by carefully removing the now cyano'd fur from the hinge line but with hindsight wasn't really neccesary

I also agree with crossing the lines inside the fuselage if you can and if it suits the model since it keeps them closer to the center of the fuze and increases the angle to the horns giving a more linear pull although this can also be done using guides .

[J D 8]

Flair closed loop control compensator is a handy device,sort of spring loaded wheel that keeps tension on the lines.Seen some on flebay.

[PatMc]

The distance from servo to control horn is so much greater than the size of servo arm & control horn that it doesn't make any difference if the lines are crossed or not.
It doesn't matter if the lines go slack when off centre as Ultymate said it's the pull line that's doing the work (That's why it's called "pull-pull" and in fact it's the prefered system.
The diagrams below show the 5 possible set-ups giving 3 different geometries. A 2:1 gear down ratio is shown but it could be any ratio.

a) is acceptable but the lines are always under tension putting strain on the servo bearings & control surface hinges even when the model isn't in use.

b) & c) have the same geometry achieved from alternate ends. Both lines slacken when off normal but in flight air pressure keeps the pull line active. When the model is stored it's best to leave the servos a little off centre to relieve the constant strain on servo & hinges.

d) & e) again similar geometry achieved from alternate ends. This time both lines will tighten up when off normal & get tighter as servo movement increases. This means the servo & control surface will be pulled towards each other & cause jamming. Not a workable system.

Kris, the problem you're having is a bit puzzling. The only explanation I can think of for the lines staying tight in one direction whilst slackening in the opposite is some sort of assymetry in the geometry.
E.G. in diagrams b) & c) if the lines were tensioned with the rudder was central but the servo arm off neutral I think this would cause what you're experiencing. I would need to sit down with a sketch pad, compasses & ruler to confirm this but I'm fairly confident it's the case.

[Gary Manuel]

Good sketches and advice Pat

[Kris S]

Thank you Pat I appreciate your help.

[ROY DAVIES 1]

On full sized non powered controls this problem was cured with a 'Pacific tensioner' And if my memory serves me right Flair produced one years ago. Dudley Pattison of Flair was a full size home builder so maybe he was inspired to produce a smaller copy. The full sized consisted of a round pulley (bellcrank) at the cockpit end with both cables from the rudder horn doing one turn around the rim on the 'left' and the same the 'right' both cables met at the middle of the pulley where there was a slot with a tension spring attached to the pulley and on to a sliding block in the slot to which the cables were attached The cables were pre tensioned to a given value so as not to have a too slack or tight cable. The tensioner was connected to the pedal via a rod to the rim of the pulley. A similar system was used on old radios to keep the tuning needle cord tensioned.

I made some from 2" x 1/8" nylon discs 40 years ago worked great with no 'blow by' on the rudder. Simpels tchk*

[Dave Miller]

The opening posts problem may be due to the lines being twisted around the elevator push rod.

I must add that I think that the "e" sketch above is the correct layout and the one that I have used for many years. It becomes important to use this sort of linkage when using closed-loop for elevator control as well as the rudder.

The idea of crossing the lines is simply to get a better exit of the lines from the fuselage.

[Gary Manuel]

I agree with Pat, that "b" and "c" are the correct geometry. "d" and "e" results in the wires becoming too tight when moved, and will in fact cause damage. The important thing to note is that the centre dot in the rudder end on the sketches is the hinge line, not the horn connection point.

"b" and "c" will result in slackening away from the neutral point, but will always be pulling against the air flow.

"d" and "e" will result in tightening away from the neutral point, and could result in damage.

"a" will result in the lines being the same tightness throughout the movement - the problem with this setup is that the slightest of error in setting up, could inadvertently result in arrangement "d" or "e".

Dave,

The shape of the horns may indeed look like "e", but the actual connection point of the wire must be either on (a) or behind (b) the hinge line to prevent damage.

 

Edited By Gary Manuel on 18/08/2014 09:26:35

[Dave Hopkin]

I am probably being thick here (I usually am!) But in Pats Diags, they look identical, the only difference I can discern is (maybe) the distance from servo center to the pull line attachment... if that's correct, is there an optimum ration between rubber horn and (used) servo arm length?

Or am I missing something......

[Bob Cotsford]

In 'D' the servo connections are behind the servo shaft and in 'E' the rudder horns are positioned in front of the rudder's pivot point, in both cases the angle between the cable and the arm is greater than 90 degrees.

Edited By Bob Cotsford on 18/08/2014 10:21:36

[Dave Hopkin]

Posted by Bob Cotsford on 18/08/2014 10:20:27:

In 'D' the servo connections are behind the servo shaft and in 'E' the rudder horns are positioned in front of the rudder's pivot point, in both cases the angle between the cable and the arm is greater than 90 degrees.

Edited By Bob Cotsford on 18/08/2014 10:21:36

Ahhhhhhhhhhhhhhhh missed that ta

[Kris S]

My geometry is as diagram B above with the exception that the lines are parallel due to both rudder and servo horns being the same length. I will experiment with different set ups and see what happens and may try Roy's idea if I don't improve the system.
Thanks again Pat for the excellent diagrams.

[Kris S]

Thanks also to John for suggesting originally the device Roy suggested.

[Gary Manuel]

Kris,

If using arrangement b, I think the key point to getting the left and right tensions the same is to ensure the SERVO arm is at 90 degrees to the centre line when everything is neutral. Ideally, this would be by playing around with the arms on the servo splines, and then if necessary by adjusting the sub-trims. Once you have the servo arm centred like this, adjust the rudder on the clevises to get it centralised.

You should then find that the wires are tight(ish) in the centre and slicken off as you move either right or left. It you have everything set up symetrical, the slackening on each side should be equal.