Closed loop...

[john stones 1 - Moderator]

I bought a second hand Pilot Extra, been setting it up and have come to a grinding halt after faffing for 2 days. The closed loop isn't right and I'm lost as to what to do ? Rudder centers with a right clatter when actuated, it's so bad I reckon It'll cause damage. Wires cross because they'd foul the formers otherwise, take the cables off the servo, move it by hand it's fine (tension on both cables) ? I believe I understand what's wrong, but don't know the solution (bigger horn on servo)? Help please.

[McG 6969]

HI John,

I surely would like to help but I'm afraid not to be able to.

One thing I've been reading on different occasions is to use a servo arm connected to a 'slave' straight bellcrank/horn to release any load from the looped wires to the servo.

No idea with the 'crossed' wires though.

Sorry, young man...

Cheers

Chris

[MaxG]

3 things occur to me with your set up.

1) Ideally the servo arm length should be the same as your rudder horn size. In your case 60mm.

2) If you are still using 35Mhz it pays to make sure the two cables are not touching as that can cause glitching. Not sure about 2.4 but to make sure anyway I always use nylon covered fishing trace. Because I have several varieties of trace as long as one is nylon covered all is ok.

3) Have you too much tension in your cables?

HTH

Maxg

[john stones 1 - Moderator]

It's geometry related I think, smaller servo arm cured nothing, not glitching either Max, solution, I've no idea.

[alex nicol]

Hi John,

As with above suggestion, do the control wires touch at the crossover point and are they insulated?

If they do touch I'd suggest a slight vertical offset to avoid this. Also if they have been touching is there any evidence of chaffing.

If you reduce the size of the servo arm can you achieve a straight cable run?

Good Luck

Alex

[john davidson 1]

As stated in the first reply the moment arms of both servo and rudder must be identical, introducing a spring for tension in one line reduces the pressure on both rudder arm and servo

[john stones 1 - Moderator]

There's a washer under one ball link, not chaffing, not possible to run straight, the formers stop that option, they have to cross.

[Don Fry]

The problem is, I think, your linkage is 10 mm behind the hinge. And as it swings, the distance alters. Hence the clatter, and tensions altering.

Try bolting on a pair of 10 mm extensions to the ruddder arms, to put the movement point in line with the hinge.

By bolting, I mean altering the shape, so it's in line, and bolted as in rigid. I think the original arm should have been a different shape, ending in line with the hinge.

Edited By Don Fry on 23/02/2020 19:34:56

[Former Member]

[This posting has been removed]

[Martin Harris - Moderator]

Can you modify the rudder horns so that the wire attachment points are in line with the hinge pivots? That should allow the tension to remain constant...

[john stones 1 - Moderator]

That's where I'm thinking Don, means surgery, little room between the horn and elevator, want to be sure before I cut.

[Denis Watkins]

Definitely the crossover John

As one side of the servo pulls, the rudder bar pulls harder at the opposite servo connection, opposing the movement

The crossover needs springing as suggested

Or another mechanism as a bellcrank

Edited By Denis Watkins on 23/02/2020 19:51:34

[Don Fry]

I don't think you need surgery. Bolt a pair of forward facing extensions to the existing arms. Prove this is the problem. And then a L shape extension can be made, 2 bolts onto the existing arm, bringing the junction between control wire and horn forward by 10 mm.

[Tim Flyer]

Yes the connection to the horns HAS to be on the hinge line or geometry is wrong, that’s why rudder control horns angle forward.  I got that wrong once on my LA7 and initially had awful rudder flutter until I fixed it and lined up the connection with hinge line. If the connection isn’t on the hinge line the line that is supposed to be slackening releases at a lower rate than the line that is pulling .. so the wires are “fighting each other “.

Edited By Tim Flyer on 23/02/2020 20:04:25

Edited By Tim Flyer on 23/02/2020 20:08:18

[john stones 1 - Moderator]

Models sat on a chair, so no pressure from the wheel, I disconnected it looking for the problem, the rudder horns will clash with the elevator if I alter them, however they t/planes removable so i can remove some elevator, just want to be sure of a cure before I cut anything. Rudder horns are f/glass insets those can be cut off and h/duty ones bolted on.

Edited By john stones 1 on 23/02/2020 20:08:40

[Tim Flyer]

what you’re set up is doing is making a “ virtual “ pivot point 10mm back from the actual one. That’s why one wire will try to over extend when the rudder moves

[john stones 1 - Moderator]

Posted by Tim Flyer on 23/02/2020 20:12:33:

what you’re set up is doing is making a “ virtual “ pivot point 10mm back from the actual one. That’s why one wire will try to over extend when the rudder moves

I agree, I want reassurance before I commit surgery, fitted many a closed loop, never encountered this before.

[Tim Flyer]

Hmm quite difficult space wise . One solution is using a a disk servo arm . Then you could set both pivot points a bit towards the nose of the aircraft that would mean the pulling wire will not overtake the releasing wire . You might need a large disk . 

Edited By Tim Flyer on 23/02/2020 20:26:02

[Tim Flyer]

That way there is no surgery and u can test it with fingers rather than powering up and stalling servos

[john stones 1 - Moderator]

I have a large curved horn that would move pivot points towards the nose, it has me flummoxed, would that negate the rudder end issue, is their non surgery hope.

[Tim Flyer]

Yes in theory if servo arm is same width as rudder horn and points are 10mm forward of servo arm centre that will offset your rudders 10mm rearward pivot point exactly

[Don Fry]

Can you make a L shaped block, mounted above the existing horn, to act as the hinge point. That control wire looks too close to the elevator, as in not in line with the exit from the fuselage.

then it all moves without clashing.

PS. Acquired projects come with issues.

[Tim Flyer]

In practice you will be using a wider arm than the tail so the released wire will be slacker . With a big arm you won’t need to move the servo pivot point the full 10mm forward... just a bit

[Don Fry]

Thinking sideways, why, if the servo end is moved towards the tail by 10 mm, i.e., offset in the servo wheel by 10 mm, would that not correct the geometry?

[john stones 1 - Moderator]

I have no idea Don, wish it was a servo in tail job.