Campgems
Posts: 1931
Joined: 2/18/2006
From: Arroyo Grande,
CA, USA
Status: offline
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quote:
ORIGINAL: shd3920
Thank you for your quick responses. A friend of mine was wondering if it was possible to do a pull-pull on the rudder and elevator of a Piper Cub kit he was planning on purchasing.
quote:
by Campgems:
"but it requires a bit of geometry to make it work well.The basic idea is to translate the movement of one end of the servo arm to an equal movement of the rudder or elevator horn. This requires a couple things. First, the center of the servo arm to the attachment point has to be the same as the hinge line to horn attachment point. 1/4 " of movement on the servo arm attachment point has to equal 1/4" of movement on the horn. If you don't have this setup correct, one side of the cable is going to go slack. This results in fluttering some times.
The second thing is not to build any Ackermond (spelling??) into the linkage. This requires that the center of rotation of the servo and it attachment points or both sides be on a single line, and the same goes for the control surface. Right horn attachment point through hinge line to the Left hor attachmen are all on one line. This ensures that the control surface exactly follows the servo movement and that both cables remain tight throughout the move. "
According to the above detailed explaination it would be way beyond mine or my friend's understanding or abilities.
It's really not that dificult. Take a ruller and draw a straight line. Put a tick near the center and one 1" on either side of that tick on the same line. The center tick is your servo and the two at 1" out is the pull, pull attachment to the servo. Draw a second line and do the same tick in the center and one, one inch away on either side. This is your control surface, Rudder or elevator. You draw a line from each of the outside ticks, these are your cables. You adjust the length of the cables so the two lines, servo and control surface are parallel. You hafe just setup your pull pull system. As long as the two endpoints of the lines are equal length and the lines are equal length, then you have a tight pull pull where the control surface will follow the servo exactly. The lines can cross each other, in this case the movement of the servo is reversed in the control surface, or the lines can be twisted 90 degrees apart, IE for the elevator.
Where you get into the goofy stuff is if the center point isn't on the same line as the two end points, either on the servo end or on the control surface. In this case, you get an uneven movement on oppsite ends of the control horns or servo arms, resulting in one line going slack. This results in uneven control surface movement and slop in the linkage. Imagine if your push rods in a standard setup allowed for a three or four degree movment of the control surface without movement of the servo. This is what a slack pull line gives you.
Don
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