Ola Fremming's homepage


MK dual coupler and Pull-pull

Personally I prefer to use one normal size servo (up front) for the elevator instead of the option with two smaller ones back in the tail.  There are several reasons for this, here are some:

  • The bigger servo's are more reliable and robust
  • The total weight are equal or lower (long wires are surprisingly heavy)
  • It's generally better to have as much weight as close to the CG as possible, not at the far extremes of the airframe.

With the servo up front there are several methods to connect the servo to the elevator, I prefer to use the dual-couplers from MK. As they come the concept involves a long pushrod from the servo to the coupler, and while this makes for a precise and strong connection I have some bad experience with severe elevator trim change as a result of changing temperatures.  To avoid this I have been using pull-pull wires between servo and coupler.  Below are pictures and descriptions of how I do it.

Here are all the parts that goes into the tail, since the MK coupler comes with a one-sided arm only I make my own double arm from fiberglass board. I use a CNC router, but manual labor with basic tools should to the job also.

 

The double arm are screwed and glued to the center part of the MK coupler

 

The connections between the wires and links are easy to make; just bend the metal rod around a steel pin, cut it, solder it together and cut threads.

 

The threads are being cut.

 

The four connections are ready to be installed.  I keep two of them adjustable, while the two that ends up in the tail of the airplane has the link soldered in place.  Take a not of the tubes to be crimped onto the wire, they are from copper not brass.  Brass is not really suited for this as brass is brittle and tend to crack when crimped, copper on the other hand is softer and actually hardens from the mechanical work of being crimped in place.  When they are crimped in place I secure it with CA-glue just to be double sure that this will not come loose.

 

Everything is in place, here seen ate the servo.  To reduce wear I have the connection between wire and link inside a shrink-tube.  This may not be so important anymore as the engine vibrations that used to destroy everything is not present in a electric powered model.