Pull-pull rudder servo installations seem to vary widely -- from the disc/braced installations to keep rudder geometry constant at all angles (and avoid slack cables), through to simply looping pull-pull wiires through a standard, tough horn.

First question is who supplies the braces and disc based horns if you go that way? I haven't been able to find these at any of the local Oz suppliers, or the better known international ones.

Any other hints from the experts on robust and durable rudder installations and the benefits of complex versus simple installations (I recognise there will be many opinions -- but I haven't found a thread that contains them!)

Hi David,

I have attached some pictures of my setup which I think works well for me. My cables are always taunt no matter were my rudder is including extreme deflections. Firstly the rudder horn must allow the pull pull cables to terminate over the hinge line. I also use the MK rudder horn attached to a standard horn on the servo which offsets the cable terminations slightly. The distance between cables on the servo end as well as the rudder horn end must be identical. I use the MK ball bearing fittings at least on one end to reduce friction and allow surface to centre exactly. Ensure that cables are parallel and straight. Any kink will result in excess cable guide wear which in turn will result in loose cables that require periodic re-tensioning. This sounds like a bit of work but the results are worth it.

Regards,

Peter

Hi Peter!
Have you found it necessary to offset the cable terminations on the servo in order to maintain tension at large deflections?
I'm asking out of curiosity since I do not do that, unless there is a need to compensate for rudder control horns that terminate behind the hinge line (which should be avoided if at all possible)
In principle the set up you are using will increase the wire tension slightly as the ruddder is deflected. I prefer setups that leave the tension of the wires constant, independent of the deflection or systems that have the highest wire tension at the centered position.

/Red B.

Looks to me like his control horn is right on the hinge lines . Actually most people have a
tendency to run their wires too tight anyway. They should not pull hard on the servo as
this will cause the output shaft bearings to wear prematurely. If you use quality pull-pull
cable that doesn't stretch it doesn't have to be as tight as a guitar string. Too tight makes
digital servos buzz all the time also.

tommy

Note the servo arm end...it has the takeoff points slightly aft of the centerline of the output shaft, thus introducing the Ackerman needed....(great thread recently on this topic on the NSRCA mailing list)

VERY nice install, Peter!!

Peter,

Your setup looks very nice but I agree with Red that having the servo horns offset behind the wheel and the rudder control horn exactly over the hinge line will cause increased tension in the cable opposite the side that is pulling. In other words, if you input right rudder as the surface deflects to the right the left cable will increase in tension over the right. This is very undesirable and if the offset is too much it *will* cause a flutter situation at high speeds when the cables begin fighting each other as one side deflects and the other side gets tighter thus pulling to the opposite side. I have a friend that snapped his stab twice when flutter occurred due to the control horns being in front of the hinge line (same effect as having the servo control arm behind the servo center line).

Your offset looks very minor so you may not experience any flutter, but I thought I'd warn you.

As to the distance between the control horns and the servo arm being the same, I've heard this too but believe it's a misnomer. I've experimented with this and I believe that different distances between the two will simply result in the end with the least distance separating the right and left having to move further, but I don't think there is any other adverse effect. Where this was useful for me was in the case of the elevator. It's better if our servos operate at near 100% throw between full up and full down so we have the max precision. My last elevator pull-pull setup only had the servo moving about 30%-50%. To increase this I used a smaller space between the cables at the servo and a larger one at the elevator. This meant that the servo had to move further to accomplish less throw at the elevator. As far as I can tell the the geometry works out because the distance one side moves is equal to the opposite side even if the other *end* is moving a different amount.

Here’s a great article on pull-pull geometry. I learned a lot from this web page: [link=http://members.cox.net/bdfelice/Ackerman/ackerman.htm]members.cox.net/bdfelice/Ackerman/ackerman.htm [/link]

KeithB

Thanks guys,

Now I am beginning to get more of the picture -- and understand what Ackerman means!!

I had always aimed for a perfect parallelagram -- so if the rudder horns came forward to the hinge line, the servo horn connections should be in line with the servo output shaft (as in Peter's install); and if the rudder horn was back from the hinge line (as with an IM style rudder horn -- lets say 20mm), you put the same offset between the servo output shaft and the connections on the horn (as I did in the model below -- sorry it isn't the greatest photo).

I would worry about the distance between the wires at the servo end being different to that at the rudder horn end -- that breaks the parallelagram. If the servo end were the smaller distance would't the 'non pulling' wire always be in extreme tension and stop movement?

The argument was that it is better to keep both wires taught -- as having one slack could allow flutter. Seems that may not quite be right and it is better to always allow a little slack.

A couple of other questions: a) what about the braces we sometimes see used from the servo output shaft back to the fornt of the servo -- I assume they take some of the load off the servo bearings; and b) the merits of ball or other hinged joints vs wires straight through the servo arm (which is what I noticed on JAS's Impact? -- light and failure proof perhaps?)

David, FWIW - I have done many airplanes with pull-pull elevators and rudders, and keep putting the cable through the servo arm. Saves weight, eliminates a failure point, and lets you balance the tension on installation more easily. At least in my experience.

Of course, I'm also a guy who has ignored Ackerman for a few decades and didn't have a tail flutter off yet....although I did stay at a Holiday Inn Express the other night

David, you're safe if the control horn is behind the hinge. This will cause slack on the side opposite to the pulling cable, but this is not a problem. Some people may not like the slack but it doesn't cause any catastrophic effects because the air flow will be pushing the surface back thus creating the tension needed by the pulling cable. So even though the opposite cable is not pulling back the air flow does the job.

Personally I think you can setup a pull-pull just about any way you want to and not have any major problems as long as you don't created the dreaded anti-Ackerman by having the cables shorter than the distance between the hinge centerline and the servo centerline.

KeithB

Bob, are you saying that you've commonly had the cables shorter than the distance between the hinge line and the servo rotation point, or longer. Longer is OK, shorter causes the flutter.

KeithB

yes

Hi all,

Just wondering, what about the Pull-Pull wheel.
Is it better ? [sm=confused.gif]
Just curios, I’ve give it a try in my fist 2M Bird.
Could some one explain the deference between these 2 types off set-up?


Regards

Danny Koolman

A wheel on the servo will give you linear pull. It will give you the same pull for each degree of rotation of the wheel. An arm will give you non-linear pull. You will get less pull in the direction you want to go and more pull (so to speak) toward the center of the plane the further the arm turns. Kind of like reverse expo. Less sensitive at extreme throws. I have used both a wheel and arm and can't really tell the difference. Probably because I don't need extreme throws for what I do.

It would seem that an arm at each would be better than a wheel at the servo and arm/horn at the surface..right? The 2 arm setup would create the most similar geometry as the attach point of the wire is traveling through a similar arc at each end. With the wheel, that point always stays at the same tangent point, creating the true linear pull while the other end travels through an arc causing less linear pull distance????....help....I think i is cornfused..

From what I have seen, those who use wheels/discs use them at both ends -- on the servo and also a half disc at the rudder end. But I am not sure who supplies them? In theory it would be a better solution because it removes the built in expo of the arm/horn solution, and as the wires always stay a constant distance apart would mean smaller or point exits in the fuz could be used (the arms in theory need a longer exit slot in the fuz to allow for the distance between the wires narrowing under deflection of the rudder).

David

Thanks David. I have modeled that in 3D but have never seen it in real practice.

I use a Hanger 9 wheel on my rudder and can't really see any tension change when
the rudder is deflected. Works great and very easy to set up tension and to center.

tommy s