Pull Pull Geometry problem
 

I am having problems with my pull pull set up. I have measured and Re-measured several times to make sure everything is where it should be.

When I move the rudder one way the wires stay nearly the same tension. When I move it the other way, they loose their tension. Any suggestions as to what might be causing it to be completely fine one way and not the other.

Thanks

RE: Pull Pull Geometry problem
 

The distance between the wires at the servo horn should be the same measurement as the distance between them at the control horn.

Find the distance between the wires at the servo horn. Make it the same on the control horn.

Did you cut the holes in the fuse to the same distance? The holes in the fuse where the wires exit need to be spaced at the same width as the horns.

RE: Pull Pull Geometry problem
 

This is on an QQ 102" arf. The exit holes were pre cut. I have measured and re measured the distances from the servo arm connections and the control horn connections and they are the same.

On the cutouts, I can see where at the absolute extreme throw the wire would hit the side but I am loosing tension, only in one direction at just before half of the full throw. The other direction, the tension remains nearly the same. I can't figure out why one way would be fine and the other not.

RE: Pull Pull Geometry problem
 

It sounds like the rudder bevels are not completly centered or one rudder horn is just a tad longer than the other.


Shawn

RE: Pull Pull Geometry problem
 

Or your clevis pin is not on the hinge line. If the cables are not hitting the slots then you have some imperfection in the air frame, rudder, control horn etc etc. Maybe your servo arm has offset in it?

RE: Pull Pull Geometry problem
 

My servo arm does have offset in it as suggested in the Manual by QQ. That is what confused me too as the control horns look to be on or almost on the hinge line. Are you thinking that a straight arm?

RE: Pull Pull Geometry problem
 

If it's a straight cable setup the servo arm should NOT have offset, it should be straight. Also the clevis pin should be right on the hinge line. Is yours a crossed or straight cable system?

RE: Pull Pull Geometry problem
 

Well, the QQ Yak calls for a crossed cable system. This is why the servo arm needs to be offset. if your cables are not crossed in the fuse, that is a problem. However, your problem seems more like either the control horn or servo arm is crooked, i.e, one side angled slightly forward and the other slightly aft.

RE: Pull Pull Geometry problem
 

They are crossed.

In looking at the rudder bolt, it may be slightly crooked. It looks as if the left horn is a bit forward of the right, very slight but I think noticeable. I am guessing though over a span of 4 ft, slight becomes major. I will work on straightening that and see how it works.

RE: Pull Pull Geometry problem
 

a little slack in one or both lines under tension is not going to have any impact in flight. the side that is tight is the one that is loaded, and as soon as it experiences any pressure the other side will start to get some slack anyway. my advice...leave it be.

RE: Pull Pull Geometry problem
 

I've noticed this too. And, I agree--leave it alone.

You'll NEVER be able to tension the cables enough to keep them tight in all situations. When you give right rudder, that side will get tight and the other side may get a little loose. So what? The right side is pulling the rudder over. Can't go anywhere, even if the left side is slack.

Just try and get equal tension on the wires when the rudder is centered and go fly it.

RE: Pull Pull Geometry problem
 

I agree. I usually play with the geometry a little until it's as good as I can get it and then I go fly. These planes are not perfect, we are not perfect, so you are going to always get some degree of slack. the real thing to avoid is when it gets tighter as you deflect the rudder/surface.

But the general rules need to be followed as well as possible. For example if it's straght cables, no offset in the arm or control horn (clevis pin on hinge line). If it's a straight cable and there is offset in the control horn, there needs to be offset in the servo arm. If they are crossed cables there needs to be offset in the servo arm.

Take a look at SWB pull pull examples. It really summarizes it all.

http://www.swbmfg.com/rudex.html

RE: Pull Pull Geometry problem
 

Thanks guys I appreciate it.

After working more on it, I now have it where there is equal tension at neutral, and I do get some slack at full 3D deflection but with the slightest pressure on the rudder the pulling cable is nice and tight.

It did throw me for a loop though when in one direction everything stayed tight and in the other it was loose.

Thanks.

RE: Pull Pull Geometry problem
 

That's an EXCELLENT link. I don't ever use pull-pull. Well, not NEVER, but I avoid it for the most part.

I usually put my rudder servo in the tail, because I use larger engines, and I'll use a single push/pull setup. Or use 2 rudder servos in the tail and rig them both push/pull.

I didn't know why people used offset horns and how the geometry all worked with straight or offset arms.

Very informative.

RE: Pull Pull Geometry problem
 

Well, you could make the servo arm parallel to the rudder control arm's current position ( you know / / ) to get the geometry more consistant if you wanted, but I wouldn't worry about it too much, unless it's real bad and you like to do a whole lot o tailslides, which is the only place you might have a problem.

RE: Pull Pull Geometry problem
 

Scott at SWB is the pull pull master IMO. I actually think pull pull is a pain in the butt. There is always some former rubbing on the cables, or the cables are rubbing on the slots, the ARF rudder is hinged crooked from the plane mfg, the cable slots have to be really long for straight cables, the cables hit the elevator servos, many more things too long to list. Then there's me drilling the rudder horn hole crooked. All sorts of things can go wrong.

Anyway, on my big ARF's pull pull rudder is necessary for balance but I don't really enjoy setting them up because it just never seems to turn out perfect or even close. Hopefully I will nail the perfect pull pull rudder setup one of these days. :)

RE: Pull Pull Geometry problem
 

Now I need help guys:
I am assembling the Kangke Laser 2000 ARF right now. The control horns are nice aluminum machined parts--but they aren't perfect. When you drill them through the control surface and then screw on the plastic piece with the hole in it--my pivot point ends up being about 1/8" off the hinge line.

Look at the link that JoeAirPort posted. My rudder control horn looks like the third picture. It's offset from the actual hine line. Do I absolutely need an offset servo horn?

If I do need an offset servo horn, whats the offset in a 3" SWB horn? And, whats the distance between the holes? My control horn holes are 3" apart--so does that mean the servo horn holes have to be exactly 3" apart too?

Thanks

RE: Pull Pull Geometry problem
 

You could get away with no offset on the servo arm but there will be slack at max deflections. My buddy did a really cheap fix to the same setup you have (well close they are Nelson horns). He just cut up some Hitech blue anodized alum servo arms (I have like 50 of them) and bolted them to the ends of his non-offset servo arm. He made it so the offset ends are bolted onto the end of the straight arm at right angles. So he basically fabricted his own offset into the servo arm. He all but eliminated the slop and I was pretty impressed with how it looked. I think he had to use about 4 servo arms that he cut the hubs off (or maybe left them on can't remember, you get the idea). He needed two per side for the the offset and two more as cross braces to keep the offset pieces from turning. I will attach a stick sketch I made up in Microsoft Paint. If you like the idea credit goes to Todd (Twistawrecka) on RCU. His were 3/8 but you would need to measure your distance from the control horn flag hole to the hing line and that's the offset you need.

Oh and by the way that's a good question how long the servo arm has to be with the offset, maybe Scott at SWB could answer that. I think it should be 3 inches though, same as control horn.

RE: Pull Pull Geometry problem
 

This idea is from Gene at Troy Built Models

I can take some plate aluminum--1/8" thick--and cut it to bolt right on top of a HD DuBro plastic servo horn. Bolt the aluminum plate on top of the plastic horn, and just drill the offset holes out where I need them.

I need 1/8" offset.

In the pic I've attached--the black outliine is a DB HD plastic horn with 4 holes in it. The red line represents the aluminum plate that I would bolt on top. Use a couple 2/56 bolts to secure the plate on top of the plastic horn and them drill my offest holes out on the end.

Kind of a home-made Nelson horn.

Crude Paint drawing attached: [:-]

RE: Pull Pull Geometry problem
 

That looks great. Go with it.

PS: Is it 3 inches long end to end?

RE: Pull Pull Geometry problem
 

The length of the Dubro plastic arm doesn't matter. I think it's about 2.5"

I'll make my aluminum plate about 3.25" long and drill my holes exactly 3" on center to match the rudder horns.

I'll just run a couple 2/56 bolts through the aluminum plate and down through the plastic DuBro arm. Then put nylon lock nuts on the bottom of the assembly.

Thanks to Gene from Troy Built Models. He's the one who gave me this idea. [sm=thumbup.gif]

RE: Pull Pull Geometry problem
 

It is no problem to get both wires tight at all positions i got my wires crossed and with offsets and both wires are tight in all throws,

look at this link to get it right the ackerman principle.. http://members.cox.net/bdfelice/Ackerman/ackerman.htm


and another thing flutter will be no issue then, otherwise it will remember that..

RE: Pull Pull Geometry problem
 

Here's what we're doing on our new 37% Yak Pull Pull Rudder, for what it's worth. ;) :)We've studied the Ackerman principle, and studied the SWB diagrams. We're using a slightly modified "Example B" from SWB. We have an offset SWB Servo Arm which is about 4 1/4 inches across from cable to cable. It is offset 5/16 of an inch to the rear. On the rudder we have a Control Horn which measures 4 3/4 inches across from cable to cable. Cables will be crossed. The modifier will be: The control horn is offset to the rear about 1/8," per Mr. Ackerman, to help keep both cables under tension all the time. We made the control horn slightly longer than the servo horn so that we can observe the amount of rudder throw we will get, and then be able to "dial it in" by rotating the connectors on the ends inwards for more throw. :D

RE: Pull Pull Geometry problem
 

I think you'll find the offset of the control arm rearward is not required. Ackerman has been introduced by the servo arm in SWB example "B" (5/16") adding another 1/8" will bring the total calculated rear offset to 7/16". The SWB example is calculated for a specific amount of rearward offset based on arm dimensions and the rudder control arm being centered over the hinge line..

Your proposed arm lengths 4.25-4.75 @

60* servo angle will net 50* rudder throw.
50* servo angle will net 43* rudder throw.

Your unequal arm lengths may create a problem with slack too. A calculated fixed value will minimize slack, any other value will provide for same.

RE: Pull Pull Geometry problem
 

Shoe,

I ran some numbers your proposed setup will provide .15 slack in the slack cable and provide .24 tension in the pull cable at extremes.

Best case setup would be to eliminate the rearward offset at the rudder control arm (center the cable pivots over the hinge-line), and use equal length arms 4.25-4.25, there is about a .05 differential in cable lengths at extreme throw with this setup…

Rudder deflection will be 1:1, 60* servo angle will net 60* rudder throw.