Rudder cable play

[Bruce Tuncks]

In my SK, the rudder is driven by a push-pull cable. I think the same system is used in all Jabirus. The cable itself has about 1.5mm play ( in and out of the outer sheath )  at the rudder horn, and nearly all of the rudder trailing-edge play comes from this.

 

At the other end, on the rudder pedals, there is no play that I can notice. The rudder pedal mechanism is much stiffer than the rudder itself, so this is probably what is hiding the play.

 

Now I can see how clearance between the inner and outer cables can cause play as the inner cable can move sideways. This play would be greater if the cable did a 90 degree turn. 

 

But in the rudder cable, it runs quite straight and I don't think this is the cause of the play.

 

So where does the play come from, and how do I get rid of it?

 

 

[Head in the clouds]

It's still because the inner is smaller than the outer so under compression the inner goes into a series of esses whereas under tension it straightens out.

 

You can't get rid of the play, and the more worn the liner becomes, the greater the play becomes, so it may be time to replace the cable.

 

A few years ago a new series of Morse/Teleflex cables came on the market which are more flexible, have less play due to a closer fit between inner and liner, and are also smoother due to less friction because the teflon liner bore is star shaped rather than circular, so there's less contact area.

 

They're not very expensive in the smaller sizes, about $100 for yours, so a new one shouldn't be too painful on the hip pocket. 

 

 

[facthunter]

 Be very careful how you secure the outer cable I would duplicate it if possible (the clamp at each end).  Note If pitch control is lost you're dead.  Rudder is not so critical. Play can cause flutter also , in some situations. It feels crook as well.  The best feel is rods.  Those  push pull cables are a practical cheap way of doing it. Nev

 

 

[Yenn]

I just wonder why they used those Bowden type cables in the first place. I think they weigh more than a cable each side from pedal to rudder and they introduce a lot more friction.

 

 

[Bruce Tuncks]

They ( we called them snakes in model planes ) were easy to install at building  time. Much easier than rods, but I agree they don't feel as good.

 

There is a bit of redundancy in the Jabiru setup with the elevator in that the trim works the same horn, only with springs as a buffer. I reckon you could land it roughly but ok on the trim alone. The flaps give some pitch control too, I hope never to have to try and fly with just flap and trim tho.

 

So don't worry Nev,  I would never  install push-pull cables without the proper end-clamping. 

 

I agree about the flutter, it is because of this consideration that I would like to have less free play at the rudder trailing edge.

 

 

[Jaba-who]

I have same sort of play in mine and I spoke to both Rod and Jamie at Jabiru about it way back when I built mine in 2008 or so. 

 

Both confirmed that it’s standard for those cables to have about that amount of play. Neither was concerned and even said that despite my concern about flutter they felt it wasn’t an issue. 

 

Mine has done about 600 hours on the same cables now and I don’t really think the play has gotten any worse. 

 

 

[facthunter]

They are all like that. If you had a grease nipple some where in the middle and injected light grease  it might help. Think they would have to be bad to get flutter but some planes rely on a "no Play" situation in the controls. I wouldn't like to have to land a plane with a spring in the trim only. Other more positive forms  are better. Tab or horiz stab moving.. Nev 

 

 

[Jaba-who]

Well, as so often happens - something is talked about on a forum. Never seen a problem before myself. Then next time I go flying the same problem jumps me!  

 

Only this time a bit more scary. In fact probably the scariest thing to happen to me for a long time. 

 

Took the wife and we were flying along happy as.  

 

Showed   her how the Dynon autopilot can execute a self controlled 180 degree turn.(to left)  As it rolled out I felt a lateral coarse shudder in the joystick. Only lasted a second and I can’t remember if it self terminated or it terminated when I turned the autopilot off and rocked the wings a bit. 

 

Anyway I rolled it a bit left and right and all seemed ok. 

 

Flew for a few minutes and thought I’d see if it did it again. So same auto pilot 180 turn. Smooth  all the way round and rolled out. Thought nothing was going to happen then suddenly felt a buzzing in joystick which rapidly increased to a shudder to a violent lateral movements and now involving the whole airframe. My glasses bounced off my nose. All this faster than I could get my thumb onto the AP disengage button. Once I hit it I pulled power off and continued the roll to the right. Got instantly worse so rolled it to the left and it stopped. 

 

At this point we were close to home so made a slow turning descent and landed without problem. 

 

Inspected wings, ailerons etc. no damage but left aileron had free movement from neutral to full up position. Locking joystick in position didn’t stop the free upward movement. Clamping a plate over ailerons outer ends  however locked  the joystick solidly. So not a broken inner cable. 

 

Then got missus to hand move left aileron up while I watched the inner end of cable on the bell crank. 

 

When the aileron   went up the inner end of the cable sheath came apart and the inner came out in a bend flapping free.

 

the cable consists (going from  outer plastic coated wound wire sheath toward the inner end) then a cast hub with grooves to accept the securing clamp then a stainless short  straight solid outer sheath. Out of that emerges the moving rod attached to the inner wire. 

 

In my case that last solid stainless straight sheath had disengaged from the bigger clamped hub. (Which was still clamped solid). When the turn in that direction had pushed it out of the hub the wire inner was no longer constrained and as was pushed back it simply buckled up in an arc floating in mid air. Nothing holding the aileron in place so it started to flutter and took off. 

 

I am sure I had aileron flutter given the feeling it was in the joystick and it was lateral coarse juddering. 

 

Hate to think what would have happened if it had gone on for even a second longer!! 

 

Now  got to talk to jabiru about what sort of inspections or potential damage the aileron has ( as well as replace the cable). 

 

 

[Head in the clouds]

Yup, they have a limited life. I've had quite a number of them fail in boats.

 

Usually they fail at the engine end and then it's easy to assume that they only fail due to rust and/or the constant movement of the outboard due to tilt/trim and steering function ... then I had a few failures at the remote control end which put the lie to the rust/movement aspect, and were clearly cases of wear and fatigue failure.

 

If I used them in a plane I would replace them every two years, they don't cost much. I fear we may see more failures as the fleet ages, I hope they have a happy outcome, like yours Jaba.

 

Sounds like you were very lucky. I've had a severe case of flutter to the point where my (aly) I beam sparred wings were flapping like a bird's, and the onset is frighteningly rapid, as you described - this happened to me before I knew what flutter was, so I did the wrong thing, I was on final approach and instead of slowing down I put the nose down to get on the ground asap, so the violent shaking continued all the way to the ground - gladly it was a strong airframe.

 

If your specs were shaken off I'd say you also had a case of aileron flutter induced wing flutter (not just aileron flutter), gladly the Jab wings are composite so they can cope with more flexing than one might imagine, like a fishing rod to a degree, but you'd probably do well to conduct an extensive tap test to check for delamination between the skins and the foam cores. If that has occurred it's not the end of the world, it can be fixed by injecting epoxy and using cinch straps and packers to apply clamping pressure while it sets, with a slight weight penalty.

 

Thanks for posting your experience, I sincerely hope it encourages others to replace their cables more frequently - by the way, what is the Jabiru factory's requirement for periodic cable replacement?

 

 

[Jaba-who]

Same as the CASA directive relating to stainless steel ended cables - 15 years. 

 

 

[Head in the clouds]

Same as the CASA directive relating to stainless steel ended cables - 15 years. 

That's mighty scary, because those cables fail due to wear and fatigue rather than time.

Consequently a plane that flies the average private ownership hours of 20-50 per year might have 300 to 750 hours of use in 15yrs, which, in a non-corrosive environment should be fine. But a plane in a training environment or being used on a station perhaps, operating 500 hours a year, might have 7500 hours of wear and fatigue on those cables before anyone was aware that they ought to have been replaced.

 

From personal experience, and anecdotally from many others' experiences, I would be replacing the cables at two years or 200hrs, whichever came first, if I wanted to be confident they wouldn't produce any unpleasant surprises.

 

There's another factor that massively affects the potential life of these cables. It's the angular deflection of the ends at full control movement. In some installations it will be minimal, in others it will be close to, or at, the maximum angular deflection available. If you have a look at the punishment the liner and inner have to absorb at full angular deflection under load (i.e. down aileron, up elevator or either rudder), it's significant. And in those airframes with a harsh geometric setup, the actual punishment of the cables is further determined by the style of the person operating the controls, some are smooth and gentle, others happily whack the controls about like it's a video game.

 

 

[jetjr]

the CASA directive does not apply to cables used in Jabiru. - From RAA Tech team

 

Its in regards to corrosion in ends of cable assembly. Jabiru use similar but not the same setup. 

 

These cables are expensive and a big job to replace. Have to be bought through Jabiru as maker wont discuss aircraft usage.

 

Jabiru claim very few problems with the system and the cables at supposed to be self damping preventing flutter.

 

The clamp security are a daily inspection item I thought. Doubt many do it well

 

Theres a few SB on these cables. Damage look familiar?

 

http://www.jabiru.net.au/images/JSB041-1_Jabiru__Elevator_Cable.pdf

 

 

[Nobody]

Well, as so often happens - something is talked about on a forum. Never seen a problem before myself. Then next time I go flying the same problem jumps me!  Only this time a bit more scary. In fact probably the scariest thing to happen to me for a long time. 

 

Now  got to talk to jabiru about what sort of inspections or potential damage the aileron has ( as well as replace the cable). 

Jabba-who, It sounds like you has a lucky escape, if the flutter had become divergent then the outcome could have been much worse.

 

Can you please report this through the appropriate channels (either RAAus or CASA, rego dependant)  so that others might be altered to the possibility of an issue.

 

 

[facthunter]

It's not a straight pull at the ends as it goes to essentially a  crank . even if it doesn't fully rotate. As said there is a lot of bending. from one direction to the opposite as you move the control. (Not just back and forward in the line of the cable.  This is likely to cause the failure experienced. If the clamp was further away and a short rod interposed to the actuating drank it would work better . ( less angle change) or some  alternatively,   an ability for the end  "outer" to pivot slightly. It's a bit of a compromise when you examine the set up more closely.

 

     You won't ever forget that one, Jaba. Control irregularity/failure and flutter are things you NOTICE.  I'm not very keen on putting the Autopilot through the normal control system. If you are flying it YOU felt things happening. IT doesn't. However the problem IS the cable end. You wouldn't want it to be the elevator.  Nev

 

 

[Jaba-who]

Same as the CASA directive relating to stainless steel ended cables - 15 years. 

I have to correct that statement of mine. 

I spoke to jabiru and got the lowdown on inspections, replacement of cables etc. 

 

it seems although The CASA directive applies to cables with specific stainless steel ends and is a replacement at 15 years there was more to the initial problem that generated that AD. Essentially  the problem was related to specific types of stainless steel though CASA initially suggested manufacturers should include all cables with stainless steel ends of any type. 

 

Somewhere along the way, the fact it was only certain cables with only certain types of stainless steel in the ends was realised and Jabiru have now removed their cables from the manufacturers self imposed inclusion in that AD. 

 

Jabiru now say that the cables have a 10,000 hour life and don’t normally require replacement before that. 

 

The likely culprit in my case was the clamp was slightly  on the wrong angle and at full rotation of the joystick (in the opposite direction ) caused the angulation of the culprit end outer sleeve of the cable to deform a little. That then allowed it to drop out of the swagged end and drop down exposing the loop of inner wire. 

 

 

[Jaba-who]

the CASA directive does not apply to cables used in Jabiru. - From RAA Tech teamIts in regards to corrosion in ends of cable assembly. Jabiru use similar but not the same setup. 

 

These cables are expensive and a big job to replace. Have to be bought through Jabiru as maker wont discuss aircraft usage.

 

Jabiru claim very few problems with the system and the cables at supposed to be self damping preventing flutter.

 

The clamp security are a daily inspection item I thought. Doubt many do it well

 

Theres a few SB on these cables. Damage look familiar?

 

http://www.jabiru.net.au/images/JSB041-1_Jabiru__Elevator_Cable.pdf

Yep. That’s exactly what happened with mine only difference being the cable was the aileron cable not the elevator. 

Although not not that expensive and not that difficult to replace. Replaced both aileron cables @ $160 ish each plus postage. 

 

Easy enough to replace. Undid both ends and stamped the new one to the old one and pushed/pulled the cables through till the new one was in position. 

 

And yes - can’t buy them off the manufacturer. I bought one for my autopilot a few years back and spoke to the manufacturers who were very happy and we were almost at the stage of giving him credit card details until he causally asked what it was going into and I said a jabiru aircraft. He instantly went cold and said he would not sell me or deal with me at all. I had to go back to jabiru and order it through them.  

 

 

[Bruce Tuncks]

Thanks Jaba-who.  Gosh its good to find out about these things the easy way.

 

For some reason I do not have that SB in my files, so there is some work to do to make sure I haven't missed anything else too.

 

 

[jetjr]

If you replace all 5 cables that's best part of $1K  plus fitting - fairly expensive if looking at every 2 years like suggested.

 

You also need a second inspection on this too.