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Setting up pull/pull system

Old 01-30-2003, 07:54 PM
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Default Setting up pull/pull system

I am setting up a homemade pull/pull system and have a question.

I originally used a threaded rod thru the elevator with nylon wingnuts on each end. The wingnuts arms were drilled to provide a connection point for the pull cable. The horizontal position of the drilled hole was just behind the hinge line, giving an 'Ackerman' effect (when a cable is pulled, the opposite cable goes slack.

I used a second nylon nut on the just below the wingnut to both lock the wingnut in place and to secure the pull cable (the cable goes thru the wingnut arm's hole and around the rod shaft between the wingnut and the lock nut).

I realized that the system could be simplified by eliminating the wingnut and just wrapping the pull cable around the threaded rod between two nylon nuts. The nuts would then be tightend together to secure the cable. This system would be simple, secure, and very adjustable.

My question is:

What is the effect of moving the connection point further back from the hingeline (Adding more Ackerman effect) on my pull/pull system?

With Wingnut (x - hinge, o - cable connection point)

``` +--o
``` |
----+---x------------------------------

Without Wingnut

``` o
``` |
----+---x-------------------------------


Thanks,
Old 01-31-2003, 01:23 AM
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Default Pull Pull System

If it were me, I would be adjusting the set up so that there is no "Ackerman" effect. I have set up cable control systems in models and full scale aircraft, and I can tell you that it's not good practice and you don't want the cables going slack at all! Your'e inviting control surface flutter with loose cables. You shouldn't be concerning yourself with mathematically working out how much slack there is going to be, but rather making sure that there isn't any at all.

I hope this helps!

Cheers, Nigel

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