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Monday, September 8, 2014

Pull Spring Servo linkage for DLG type glider rudder

My Gambler discus launched glider had a rattling rudder pushrod in the carbon boom. Also, it didn't move that smoothly. So, I decided to convert it to a pull-spring rudder. The first thing was to cut the old pushrod at the pod end, and tack some strong thread on it with CA glue. Pulling the pushrod out from the tail end allowed me to thread string through the boom with no fuss at all. I'm using BCY archery serving, which seems really strong, light and abrasion resistant. This is the thread emerging from the end and you can see where I tacked it to the old pushrod. 
The spring is simply really thin wire, sorry not sure the diameter, bent into a U shape with one leg off at an angle. (The torque is determined by the length of the spring and the thickness of the wire. I just guessed). So when you place the U on a table, one of the legs is flat to the table and the other one sticks up. I then carefully pushed each leg into the rudder near the horn and boom exit. Now, three things are important here. First, it's crucial to have the rudder horn on the other side of the plane from the throwing peg. That was already the case for me, so all good. Second, you need to think carefully about which orientation the spring goes, so that the servo and string are pulling in the right direction. Third, you should really have the bevelled edges of the rudder and fin exposed in order to push the spring legs in. However, the bevel side was on the other side, and I didn't want to remove the strong 3M Blenderm hinge tape. So, I went for it as it was. Here I've pushed one leg in, and am pushing the other in, feeding a drop of CA in as I push:
This is it pushed home. So to recap, one leg into the fin, the other into the rudder. 
Then tie off the string to the horn, a little drop of CA to secure the knot and I put a small piece of cloth tape over the spring - probably not really necessary:
Same thing at the other end. I kept a bit of wire leading to the servo, in order to have some adjustment for finding throw centre. 
It works really well. Much smoother and a tad daresay lighter than the pushrod arrangement. And happily, the rattle of the rods in the boom has gone! No issues (e.g. buzzing) due to the servo arm being under tension. That must mean I got the spring tension about right. So far, it's been very reliable. 

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