Mike3579

My Feedback: (56)

Setting up Pull Pull on my 30% Pilot RC extra 300 and I have some questions. I'm using the outer holes on the control horns which is about 4inches and I'm using an SWB 4inch offset servo arm. I have the cables crossed. Both cables are tight when ther servo is centered but when I go in either direction both cables get slack in them. Is it normal for both cables to go slack in either direction but to be tight in the center?

GVR-4579

Mike you want the cables to stay tensioned all the way both directions. Make sure the all the distances from the center lines of the pivots points are the same. Your 4" offset on the control arm also needs to take into account the thickness of the control surface to get the distance from the centerline correct. If that doesnt help make sure the pivot points are in the same relationship to the attachemnt points as well,

Hope that makes some sense and helps you out.

pe reivers

That answer prohibits crossed wires!
Search for Ackermann principle, and you will find an article of my good friend Brian Felice. Not exactly Ackermann, but close enough! (sorry Brian!)
It is not possible to get equally tight cables in all servo positions when the cables are crossed over.
aving said that, in many situations one can ask:"who cares?" because the aerodynamic load always tends to strain one cable only, so cable slack outside neutral is a non-issue in my humble opinion. I will don my armour just in case. Maybe someone will convince me of my errant ways.

PS, the link is: [link=http://members.cox.net/bdfelice/Pull-pull/pull-pull.htm]Ackerman[/link]

Taildragger726

How about using the rudder arm included with the kit. The offset will be correct.

JoeAirPort

My Feedback: (41)

It's fine. I have an EF Extra which that plane is a copy of. It's totally normal. You can go fly, don't worry about it.

PS: This is my second season flying mine with the same exact setup. I fly hard core 3D, my rudder is used as much or more than any other surface. Mine's the same way as yours. Just goes a little slack away from center. It's fine.

bgold

What will keep your cables tighter than any thing is to have the pivot point of the control horn connection right on the hinge line and then use a servo arm that will have exactly the same width as the rudder horns. I cross or don't depending on the angle I need to get on the cables so they don't rub the fuselage at entry point.

Mike3579

My Feedback: (56)

Thanks for your help guys! Yep I also crossed the cables to keep them from rubbing the fuse.

Taildragger726

Really???



Again, the included rudder horn/arm has the proper offset to keep your cables properly tensioned. Just glue and screw it to a servo wheel and you are set.

JoeAirPort

My Feedback: (41)

Yeah really. Come watch me fly sometime. You'll believe it's working correctly.

Taildragger726

Not doubting your abilities, just that asinine statement.

JoeAirPort

My Feedback: (41)

Well go ahead tell me why I'm wrong then. Insulting people is easy. Didn't you see that post from Chris Hinson when Pilot released their planes? The guy used to work with Chris.

Taildragger726

JoeAirPort

My Feedback: (41)

LOL

Mike3579

My Feedback: (56)

Ok so I ditched the SWB arm because I wasn't happy with the slack I was getting. I ended up putting the the rudder horn/arm that came with the plane and now it is much better! Both cables stay tight in both directions. Is that ok? How tight is to tight?

Taildragger you mentioned glueing the horn to the servo arm. With the 4 screws/nuts is glueing really needed? What should I use to glue them epoxy??

Thanks for all the help guys!

Taildragger726

Scuff with sandpaper first, epoxy is fine. I used flexable CA from Mercury. Hitec wheels, this was on a 35%. This was before adding screws.

Phantom Phixer

My Feedback: (3)

Mike:

Just had the same issue, and spent a couple of nights looking at the geometry, as did I do something wrong.

If both center distances on the rudder arm and the servo arm are equal, its has to stay tight provided you are also on the center of the pivot point.

I also found this comprehensive explanation of the pull / pull and sort of learnt something.

My solution was to make sure center distances were the same as my rudder horn is metric, and the control arm american standard.

http://members.cox.net/bdfelice/

krproton

A while ago I experimented with pull/pull rudder cable geometry simulating different scenarios (parallel cables, crossed cables, servo arm offset, etc.) in AutoCAD. I started with drawings from the Great Planes 27% Extra 330S ARF that has 27” cables. I used 50-degree servo throw (each way) to simulate maxed ATV’s for 3D flight (even though this may be way more rudder throw than needed). Below are my findings;

Example 1 illustrates perfect geometry; the cables are parallel, the mounting holes on the rudder horns align with the pivot point (leading edge) of the rudder and the distance between the cables at both ends is equal. The servo arm has no offset. As the drawing illustrates, even at full rudder deflection the distances between the rudder horns and the servo arm do not change, so cable tension remains the same and neither goes slack. This is the only scenario that provides perfect geometry.

I’ve heard it said that as long as the rudder horns are in line with the leading edge, and as long as the servo arm is the same length as the rudder horns, you can cross the cables and the tension will stay the same all the way to full deflection. But this isn’t true. In Example 2 the “idle” cable acquires approximately 3mm slack at full deflection. Most pilots might find some cable slack acceptable. But given the same scenario on an even larger plane, the amount of slack could be greater.

On some models (that have traditional bolt-on control horns instead of threaded torque rods or custom-made imbedded horns), the horns must be set back from the leading edge. On models like this, I’ve heard people say that as long as any offset in the servo arm is the same as the offset in the rudder horns, then cable tension will stay the same all the way through the throw and you won't get any slack. But this isn’t true either. In Example 3, the horns are 5mm back from the leading edge and there is 5mm offset in the servo arm. But the idle cable acquires approximately 3mm slack. Again, 3mm of slack may be okay for most pilots, but it isn’t perfect and would be even worse on a larger plane (about 10mm slack on a 35% model).

The idea of offset on the rudder servo arm caused me to think about Example 2 again (where the cable connection points on the rudder align with the pivot point, but the cables are crossed—probably the way most 35% and 40% kits/ARFs are set up). So I made another drawing (not shown), only this time with 5mm offset in the servo arm. Now, at full rudder deflection, any existing cable slack was immeasurable. This means that if your rudder cables cross, some offset will be needed in the servo arm even if the rudder horns are not set back from the leading edge.

Look at Example 4A. The offset on the “rudder end” is still 5mm, but now the offset on the servo arm is 10mm. (This is the exact setup as on the GP 27% Extra.) This time the idle cable has no measurable slack (.1mm), so the cable tension remains virtually the same even when the rudder is fully deflected.

This might lead you to believe that the formula for servo arm offset is double the offset on the rudder horn. But it doesn’t work. I drew another illustration in Example 4B with the servo located closer to the rudder. But with the same offset as example 4A, the idle cable now has 1.5mm slack, so you can’t simply multiply the cable mounting point offset on the rudder by some number—I guess there are just too many variables.

There may be a formula that describes the relationship between horn offset, servo arm offset, cable length and the width of the cable connection points on both ends, but I’m not even going to try to figure it out!

So here are my conclusions;
1. In order to get the pull/pull cable setup you want, you have to determine how much cable slack you can live with. I don’t know if there is a consensus on this—some seem to think a small amount of slack is just fine, but some don’t.
2. If your cables are crossed and/or if the cable connection points on the rudder are not in line with the pivot point, you will probably need an offset rudder servo arm—especially on larger models.
3. There isn’t a formula (that I know) for determining servo arm offset that can be applied to all situations. Practically, any offset needed would have to be determined through experimentation or the use of computer-aided design tools such as AutoCAD. (Or hopefully, your kit/ARF manufacturer has already done this for you!)
4. The only PERFECT scenario is when there is a parallelogram (where the cables are not crossed, the cable connection points on both ends are the same distance apart and in line with their pivot points). Otherwise, a rudder servo arm with some offset may be a good idea.

JoeAirPort

My Feedback: (41)

Nice work. I've never seen anybody do this. That was very interesting. Bottom line is that the ARF designer needs to do it right in a CAD package and tell us what to use to get it right. Of course there are imperfections in our installations such as gluing in phenolic horns slightly crooked, rudder horn rod holes drilled crooked. Or from the ARF mfg, the slots were cut imperfect. ARF fuses can be assembled and glued crooked, rudder hinges drilled crooked. I don't mind a little slack away from center. I use my rudder a lot in 3D and precision flying.

Thanks for posting this. It was VERY interesting and useful.

krproton

Hey, thanks!