Hi there
Can anyone tell me is it necessary to put a larger servo horn ie a extension on to a rudder servo with a closed loop (pull pull system) to make sure the clevises on the horn are the same distance apart as the clevises that are attached to the rudder

I know if you don’t you get slack in one of the cables when the rudder operates
But the one that is pulling is controlling the surface so is it necessary to use a extension ?

Thanks
Geoff

You don't need a larger horn unless you want more throw on the the rudder. Make sure your cables are attached in the same position on each side of the servo and at the rudder or you will get uneven throw. The same rules for travel apply to cables and push rods.

The tension on each of the cables is the same when centered. No extensions are necessary because the distance between the servo horn and the rudder horns remains constant throughout the travel, the only thing that changes with input is the location of the of the horns.

Thanks for that bearmech

Thats what i have always thought
I have always used standard servo horns on a closed loop
but somebody at my flying club uses extensions and i queried why

I guess thats why some use springs to take up the slack...

Springs will put less stress on the servo, but your rudder movement will not be as crisp. Glad I could help!

http://webpages.charter.net/bobad/pullpull.htm

http://www.geocities.com/roger_forgues/PullPull.html

http://members.cox.net/bdfelice/Pull-pull/pull-pull.htm

Check out these links!
Hope they are helpful....
Cheers
Hisham

Thanks again chaps
neat setups on the links....

Do a search on the Ackerman principle also so you can have another thory on horn placement