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Sunday, November 4, 2012

Pull String Control System

A Pull String control system works very well.  Make sure you install the pull string BEFORE you cover the plane.  MUCH EASIER!!!

They are easier to adjust and don’t strip your servo as easily as the traditional push piano wire system does. In this write up I will explain how to install a String Pull system in your plane.  This is more commonly used on the elevator and rudder.  You just need some aluminum tubing and string.  The string can be purchased from any fabric store.  Be sure to get strong polyester string.  Pull on it hard to ensure it will not break easily. 

In the picture below you will see how the string looks after its installed, keep in mind you will need to build custom control horns as in my other write up.

In the next picture below you will notice that the string route through the servo horn and back to the other side of the control horn. 
Don’t get intimidated by this as once you do it the next time is a breeze and you will learn to love the system.  You can use small washers in place of these fancy custom plates that are mounted under the servo screw to hold the string in place.  To adjust all you need to do is loosen the screw and adjust.  Simple!! 

Now, here is the tricky part.  You are probably saying to yourself there are not enough strings to loop around each elevator horn, right?  Well let me explain.  The elevator servo will have two string ends coming from it.  You will need to tie a “Y” off of each string end.  One “Y” will route through to the top of the elevator and the other “Y” will route to the bottom.  Make sense? 

Here is another picture

Notice how the string has to hook to both sides of the horn.  This is because it is a Pull system not a Push system.  This is one side of the elevator.  You will need to do this to both sides.

To secure the string all you need to do is wrap around horn and then use a small drop of CA to secure the string.  Then tie a knot.  The CA allows you to pull the string tight and hold it in place while you tie the knot.  If you don’t CA before tying the knot you will be fighting to keep the string tight. Allow enough loop to be able to remove the string off horn if you ever need to.  I usually tie my knots about ½ to 1 inch from horn.

This will take patience and practice.  Use aluminum tubing to route the string through the fuse.  Be sure to debur the tubing so rough spots don’t cut the string. 


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