setting up pull-pull on a rudder
 

I have a 60 sized seagull edge 540 i'm going to setup as a pull-pull rudder. can i just use standard control horns bolted back to back on each side of the rudder and a standard servo arm? I haven't ever attempted to set one of these up.

Derek

RE: setting up pull-pull on a rudder
 

I just did the same thing and it worked great,, there is a little slack at full throw but nothing to worry about

RE: setting up pull-pull on a rudder
 

No problem, I buy the longer servo arms though so I have more adjustment option.

RE: setting up pull-pull on a rudder
 

Two things to to know

1 The horns need to be installed so that the holes line up with the hinge line on the rudder.

2 The distance between the holes that you use on the HORNS needs to be the same as the distance on the SERVO ARM holes. Most people use an extra long servo arm to help with this.

Lots of stuff on the web on this subject but the above points are the key to getting it working right.

RE: setting up pull-pull on a rudder
 

In a thread right above yours is a thread with this link. It explains the pull-pull geometry. http://www.qmfc.org/school/ackerman.htm

RE: setting up pull-pull on a rudder
 

Like mentioned above, check out the Ackerman bit, it is very important to NOT have any negative Ackerman in your setup, some positive (where the line goes slightly slack on the non-pulling side as you go off neutral). And, the length of the horns and the length of the servo arms DO NOT have to be the same length. As long as you do not get any binding at the extreme ends of the throw, you can safely have quite a bit of difference. I use pull-pull on both rudder and elevators on most of my planes and always have different lengths due to whatever is required to get the desired amount of throw. Check out Alan's web site for some good info on using pull-pull. http://homepages.ihug.co.nz/~atong/

RE: setting up pull-pull on a rudder
 

The slack is from the mounting point of the wires. If the holes used in the rudder horns are 2" apart for example, then use the holes 2" apart on the horn. Make sure you figure the thickness of the rudder into your measurement so you have the total distance between the holes. That being said a lot of guys have slack in their cables at full deflection. Most of the time it's not a big deal.

David

RE: setting up pull-pull on a rudder
 

The thread link posted by airbusdrvr has some excellent info. I will use the article's advise and mount the rudder horns slightly behind the hinge line to ensure that the "back side" horn / wire will have slack when tension is applied to the "front side" horn. If turning right, the right rudder horn is the "front side" horn and the left is the "back side".

RE: setting up pull-pull on a rudder
 

Slack should be avoided if at all possible even at maximum deflection because of the possibility of control surface flutter.

I agree that with small surfaces and slow speeds then flutter is unlikely but if you explore the envelope fully esp. in 3D style models then flutter might just bite one day.

My OMP YAK with a 120 AX would do climbing spins on full throttle both upright and inverted but I had to fix the slack in the elevator linkage to stop the whole tail fluttering. I was able to see it happen and get out of the flutter situation instantly so there was no damage.

RE: setting up pull-pull on a rudder
 

Slack will not cause any flutter. I speak from many years of experience on using pull-pull on many planes of all sizes. You will not get any flutter when the none pull line goes slightly slack during movement away from neutral. In fact, if you do not have a bit of slack as you go off center (positive Ackerman) you are far more apt to be in trouble with excessive servo wear and high current draw. I always make sure that I have a bit of slack as the control is moved away from it's neutral position.

RE: setting up pull-pull on a rudder
 

All I can say is that I experienced flutter and could repeat the situation. When I set up the elevator linkage correctly to avoid the slack in the non pull side I no longer got any flutter. I went on to fly that model for another 18 months without seeing a repeat of the flutter.

RE: setting up pull-pull on a rudder
 

What rodney is saying makes sense.On a silly note think about the poor people flying big scale yaks and the three rudder servos all looped up with stainless wire humming and we're still on the ground after switching on.Thers six strings there and if you pluck 'em hard enough sounds like the theme from deliverance..da da da dum dang dum ding dam dang who?)cheers

RE: setting up pull-pull on a rudder
 

Hi!
Rodney is right!
Slack in one wire does not induce flutter! What causes flutter is a too flimsy stab ...or elevator surface itself.

RE: setting up pull-pull on a rudder
 

I suspect that everybody here is sort of right about flutter. If you have a sufficiently sturdy control surface, you shouldn't get flutter, even though you have some slack in the one wire. If your control surface has some give in it, having no slack, if you can get there, might help keep that surface from flexing, and so reduce the chance of flutter. This is more a gut reaction than an analysis. Seems possible, though.