Pull/Pull Cables (ERCITS)
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Pull / Pull Cables

By John Rimmer

In case you haven't heard, Spectra has got to be the best way to control your tail feathers as it is the lightest, strongest, most positive, and smoothest of all servo to elevator and rudder connections. Spectra is a non-stretch bullet- proof-vest fiber that is sold under the name "spider wire" where fishing gear is sold. It is slippery, small in diameter, and STRONG. A CA'd square knot secures it well. I have used small pieces of Teflon for turns of more than 90 degrees. The neatest thing about this is the decrease in weight. The cable or push rod system you replace may weigh up to two ounces. This will require as much as 4 ounces to be removed from the nose. That adds up to as much as 6 ounces total decrease in weight. With all this you might think it is difficult to install or requires maintenance - not true. I find it quicker to install than any other type of tail feather control and in more than a year have had not one problem. I find it much more positive than any type of cable or pushrod.

Installation could not be simpler. Finish the plane including covering or painting before installing the pull/pull cables. You can use two 1/2 A control horns on the rudder and elevator, but I use 1/32 plywood, or thin glass/epoxy circuit board. Cut a slot through the control surface at the appropriate place and use thick CA to hold the horn in place. Since full servo travel is usually 60 degrees most control surfaces should move about 30 degrees each way. I like to make the holes in the control surface horns about double the distance between the holes in the servo arm.

Tie the spider wire to the control horn with a square knot leaving some slack for movement and secure the knot with CA. You can also use thin CA to firm up the spider wire to make it easier to poke through the holes. It is a nice touch, but not necessary, to insert plastic tubing in the fuselage where the spider wire will enter going to the servo. Carefully route the spider wires through the fuselage to the servo, taking care not to twist them creating additional drag.

Using your Exacto knife remove the sharp edge on both sides of the servo arm or wheel made by the holes; push the spider wires up through these holes and wrap them once around the screw holding the arm to the servo. I like to use a little thin CA on the spider wire that will go around the screw to prevent any possibility of slippage and loss of tension. Later you may simply loosen the set screw on the servo arm and adjust the cables for better control surface neutralization.

Don't be worried about slack in the cables because the leverage of the system seems to almost eliminate control surface sloppiness you might think would result.

Good luck with your flights and have fun soaring!

Reprinted courtesy of the Deaf magazine.

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This page created and maintained by Al MacDonald. Updated January 21, 2002.

ERCITS1 , ERCITS2 OR ERCITS3

Suggestions for improvements or additions to this page are always welcome.