pull-pull system setup
 

Just finishing up a Hog Bipe, and was planning on using a pull-pull rudder system. I've searched the forums here and at other sites, but haven't found much info. Does anyone know of a "how-to" sorta thing for pull pull systems. a couple of questions I have... Run the cables straight to the servo bellcrank, or use an arrangement similar to the giant scale setups? Do you cross the cables? any help, or pointing me in the right direction would be great!
Thanks!
mcmike

RE: pull-pull system setup
 

Mike, I've got several planes with pull/pull set-ups. On the WWI fairly low speed stuff I run the cables right off the servo, but on a couple of higher powered acrobatic planes I use a belcrank to carry the loads. Ball bearing servo are best if you are going to hook up directly.

The route of the cables isn't critical, most of mine cross, but the distance between the cable ends at the horns and at the servo or belcrank must be identical or you will get changes in cable tension. Also you don't have to have the cables bow string tight, just make sure it is the same for both cables.
Walt

RE: pull-pull system setup
 

Mike,
Look here:
[link]http://www.swbmfg.com/rudex.html[/link]

and here:
[link]http://members.cox.net/bdfelice/Ackerman/ackerman.htm[/link]

Ray [sm=thumbup.gif]

RE: pull-pull system setup
 

thanks guys. I had figured that the distances from the cable ends as Walt described was probably the case. I've heard that it is ok to have "pushing" cable go a bit slack when you deflect the rudder, but wonder about flutter in such a case. Granted, the sig pig isn't a full-on aerobatic or extremely fast plane, but I do like to throw them around the sky a bit :D.
What's the advantages of a pull-pull system over a good, stiff pushrod?

mcmike

RE: pull-pull system setup
 

On push-rod setups the servo mount has to deal with bidirectional loading; pull-pull implementations involve unidirectional loads.

Pragmatically, pull-pull systems are easier to implement because getting the cable length right is nothing more than pulling out the slack and locking the cable, whereas push-rod systems require fitting the link precisely. Providing for adjustment is the same in both systems, so that part does not factor with respect to implementation.

Push-rod systems are heavier, so if a few extra grams are of concern pull-pull would be the preferred method.

RE: pull-pull system setup
 

Unlike rigid pushrods which are set up with the (one) control horn's holes perpendicular to the hinge line for symmetrical up/down motion, you can set up pull-pull with both control horns set back a little. Cables that are tight at neutral will have the non-working cable slacken slightly when the control deflects (either direction), because of the slightly different horizontal movement of the holes. So you can eliminate flutter at center, and still have slight slackening when deflected (or at least not increased tension). You can see the affect by drawing it out carefully on paper.