Closed loop rudder issues

[Biggles' Elder Brother - Moderator]

Pat, I totally agree with your comment - theoretically it makes no difference whether the cables are crossed or not. Indeed in my response on that I said I can see no clear explanation or justification for crossing the cables.

But,...where I would slightly disagree is your comment to the effect that "it makes no difference". It makes no difference theoretically, but experience tells me that it does make a difference practically.

As stated I am unsure why this should be so. But if pressed to speculate on a reason I would suggest it is down to the fact that in the crossed over configuration the cables can exit the fuselage at a larger angle - whereas in the non-crossed-over case the angle of exit is extremely shallow. It is difficult in practice to achieve such shallow angles of exit without introducing a "kink" where the wire exits. This kink is obviously undesirable.

So, yes from a purely control geometry point of view - it makes no difference and theoretically there should be no benefit - but my experience means I will continue to cross the wires and if asked I would advise others to do the same!

BEB

[kc]

I wonder if the reason crossing the wires keeps them tensioned is that, if they actually touch, the one pulling tensions the slack one by friction. (pulls it out of line a bit, taking up the slack)

I have not tried crossing closed loop wires yet, but I always cross 'snakes' so the run is as straight as possible

[Gary Manuel]

Really sorry to be so anal about this, but there is actually a geometric advantage as well as a practical advantage for crossing the wires. The effect is more pronounced on shorter runs, but is always present.

Consider the sketch above: I have simplified matters by making the rudder and servo arms the same length.

The green lines represent uncrossed wires and the red lines represent crossed wires.

The green lines are tangential (at 90 degrees to the radius) to the circles. For a given rotation of the servo (from the neutral position) in either direction, the green wire will move a certain distance. This distance will be exactly the same as the distance the other green wire moves in the other direction. This will therefore result in no tightening or slackening of the wires. This arrangement is represented by "a" in PatMC's sketches.

Now consider the red crossed wires. If the servo rotates clockwise, it will result in the rudder moving anti-clock. The pulled wire is moving away from the tangent at the servo and towards it at the rudder end. The same is true for the opposite direction of rotation from neutral. This results in different distances of movement of the two red wires and results in slackening away from the neutral position.

Now rather than getting bogged nown in complex trigonometry, compare this with PatMCs sketches "b" to "e".

At the servo end, we are closest to "c" and at the rudder end, we are closest to "b". By crossing the wires, we have moved each end from acceptable arrangement "a", towards the Best Geometry "b" and "c" without actually changing any of the connection points. This actually allows straight servo arms and hinge-line rudder horns (geometry "a" ) to be used without the risk of tightening.

It also increases the angle the wires exit the fuselage as BEB has said.

Edited By Gary Manuel on 18/08/2014 13:55:35

[john stones 1 - Moderator]

Geometry aside, I tend to cross the wires myself, simply to get a better exit. As long as the rudder centres correctly I don't worry about the rest.

John

[ROY DAVIES 1]

When I was fiddling with full size piston aero engines many moons ago we were taught about 'ineffective crank angles' This was the point when the piston was at top dead centre there was no movement of the piston to left or right of about 5deg' same on model ones too. I believe that this is what we are seeing with the slack rudder cable with the rudder horn hard over. I have countered this in the past by placing the cable attach' holes slightly behind the rudder post to compensate for the lack of the positioning of the rudder and horn on the pivot centre of the rudder as is the practice in general on full size aircraft. I have also made other tensioners (compensators) to fit in both lines using ballpoint pen compression springs or similar and piano wire these work quite well in taking up the slack. They can be made with limited compression to limit 'over stretch' and are set up not to overload the servo. The servos of yesteryear were a bit less powerful than todays. I only had 3 or 4 in those

! days got a few now though.

[ROY DAVIES 1]

I will add a sketch or images of a sample I could make up of the above tensioner if you like. They are simple to make.

[Kris S]

Yes please Roy that'd be great, thank you.

[PatMc]

Posted by Biggles' Elder Brother - Moderator on 18/08/2014 12:25:32:

Pat, I totally agree with your comment - theoretically it makes no difference whether the cables are crossed or not. Indeed in my response on that I said I can see no clear explanation or justification for crossing the cables.

But,...where I would slightly disagree is your comment to the effect that "it makes no difference". It makes no difference theoretically, but experience tells me that it does make a difference practically.

BEB

BEB,
I was commenting from practical experience, not theory. In any case in most models the wires do not exit the fuselage in a direct line between servo & horn so there has to be a “kink”. In fact as soon as the servo & rudder are off neutral the wires can't maintain a straight line unless they exit through a slot or large hole.
In every model that I’ve used pull-pull connections each wire has exited through a short piece of thin plastic tube glued to the fuselage. On models with both rudder & elevator pull-pull the servos, or intermediate fulcrums, are both mounted in the same orientation, so one pair of wires couldn’t be crossed anyway.

[PatMc]

Posted by Kris S on 18/08/2014 10:27:12:
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,
If you follow Garry's advice re lining up the servo arm that should solve your problem. The fact that the wires go slack either side of neutral doesn't matter, using a tensioner is unecessary. I think they are only used in some full size aircraft in case there's a risk of a wire jumping a pulley & causing it to jam.

Re the length of servo & rudder horns. If they are equal you will have far too much rudder throw (Unless it's a 3D model) IMO anything over 30 - 40 degrees each way is excessive, the servo arm length should be about half the rudder horn.

[Kris S]

Thanks Pat, its a mini panic, not sure if you'd call that a 3D model? but I'm sure I've read somewhere to have lots of rudder throw.
I'm redoing it so will post results soon.

[Mike Smith]

Hi Kris,

I remember Rob from Avicraft once telling me that the Panic can have anything up to 90 deg throw on the rudder!!

I don't know if it helps, but this is how my Mini Panic is set up (the wires do not cross)

[Kris S]

Thanks Mike I'm glad you confirmed what I'd read. That's about how much throw I've got with my current horns so I'm happy with that.