I am putting together my first pattern plane that has pull-pull setups (rudder and elevators). There seems to be lots of discussion about getting the pivot over the hinge, wires too loose/tight, etc but I have not seen many people using a pull pull wheel like this one: http://www.hangar-9.com/Products/Def...ProdID=HAN3515 It would seem that this would be the ideal way to set up pull-pull - wires don't move laterally as the arm moves, you avoid the mechanical exponential, you only have one wire to adjust, etc. But I don't see many people using this setup. What are the pros and cons of using a pull-pull wheel?

Thanks,

Alan

If you work through the geometry, you'll see that in order to exactly duplicate the movement at the control surface it will need a round control horn. Otherwise the take-up side of the wheel pulls more cable than the other side needs reeled out.

Another solution is to modify the servo wheel to have a 'D' shape.

Some argue that a slack line on the feed side is no big deal, but I disagree since occasionally aerobatic planes can back up (tail-slide, stall turn).

The linkage system can be symmetric even with classic horns, you have to pay attention to the clevis point and control horn placement to get this correct.

Contrary to the common belief, it is also false that the servo arm and control horn should be of equal length in order to avoid slack cables: in my plane I computed the arm length in order to have a given amount of throw, say 40-45 degrees: with the servo travelling at +/-60 deg, this way we can have a greater mechanical advantage.

Very good linkages tutorial, a must read for pattern airplanes, is available at centralhobbies site: thanks to Troy Newman!

You are correct that it is a common misconception that arm length has to be the same on both ends. It does not.

But I still maintain that if the holes of a 'classic' rudder horn are lined up with the hinge you will get slack with a round servo output disc like the one abuckner asks about. That wheel has a slot around the edge that holds the cable like a reel.

You can build a model of the round-wheel-to-arm setup to see the result or better yet use a drawing program that will show dimensions such as Corel or Visio.

Thanks for the answers. I read through the CentralHobbies.com articles and they really helped clear things up - "must reads" indeed. It also sounds like the pull-pull wheel/pulley isn't the best answer. Thanks!

I've been using the wheel for years in my pattern planes, I like it better is smother in the way the cables moves, I still use arms on my big gassers, there are no big wheels for that and is unpractical, about tension just enough so you cannot move the rudder ( I only use pull-pull on the rudder) in any direction, of course unless you force the servo, I mean no flexing.

Regards

apereira,

I agree with you, I've used the wheel on just about every pattern ship I've flown in recent
years and I like it very much. It sure makes it easy to set up and center and tension is no
problem. The side that is not pulling loses a very tiny amount of tension, not enough to worry
about. I had the medium size Hanger 9 wheel in my Excelleron 90 and currently use the same
size in my OTOP with .015 control line wire.

tommy s