Wednesday, May 16, 2012

Using model aircraft control linkage

My first exposure to model aircraft control linkage was in trying to simplify the switch machine connections for a double crossover in the hidden track on my old layout. This turned out to be one of those times you can benefit by wandering over to the “other side” of the hobby shop and having a look at the accessories for flying model airplanes. These linkage parts are intended to connect the control surfaces in the model airplane to the radio-controlled servos in the fuselage, and they are beautifully designed and built.
     The assemblies have nylon clevis connections for the servo, threaded steel wire for the long connection (4-40 threads if I recall correctly), and various bell cranks to transmit the desired motion. I purchased a handful of these parts and took them home to experiment. I quickly saw how nicely they can do the job, and went back to get enough more for my needs. I apologize that I no longer remember the brand name, but there are several brands out there today—try Googling the topic.
     Here is how the double crossover was linked when I was done. I had some old double-coil Walthers switch machines which were quite powerful (probably more than I needed), and wanted to see if they would work. Indeed they did. Here is a plan view of the completed linkage to one of the four switches; all four were similarly linked, to two switch machines, one on each side of the trackwork.

You can see here the flexibility of the parts, with the different adjustments to length and throw at the bell crank.
     The Walthers machine has a transverse brass throw bar, which moves back and forth horizontally in the photo above, and the bar has a hole in each end. It was easy to attach the nylon clevises, since they simply snap into these holes. I used a 6-32 screw up through the track board to pivot the bell crank, and set the height by arranging some nuts at the right height on this screw.
     Here is a closer look at the screw post, which could of course be arranged in a variety of ways. The nylon bell crank came with a brass bushing in it, to minimize wear, a nice design feature. The spring is a typical switch-machine connecting spring, of the kind used with twin-coil machines.

Finally, this may serve as a better look at the bell crank and its height adjustment screw (the nut beneath the bell crank can’t be seen here).  This linkage system was easy to install and easy to adjust, to optimize motion. In service, it proved utterly dependable and stable.

As I mentioned, this was hidden trackage, which is the reason for the gray-painted area immediately around the track, to suggest ballast, and the black color elsewhere.
     This is my only experience with this airplane control linkage arrangement, but I was most impressed and satisfied with the result. If you have a linkage problem anything like this, I would recommend you take a look at these convenient and well-made parts as candidates to solve the problem.
Tony Thompson


  1. I have long used the Gold-n-rod products which I originally found in the model aircraft section of a local hobby shop for turnout control where it would be awkward to use a hand throw. (The prototype Rigby Yard in Portland, Maine, had classification tracks that ran from both sides of a centre thoroughfare so there will usually cars standing in the way of a hand control on the opposite side of the centre track.) They are very flexible plastic rods inside an equally flexible plastic sheath, so the friction is negligible. They come in several lengths and diameters (smaller diameters are more flexible, of course, but larger diameters need less support. There are rods for 2mm, 4-40 and 2-56 connectors.

  2. Good suggestion. This product comes in solid rods of nylon or steel, and in steel cable form also. Check them out at:

    There are other brands also, and as far as I can tell from what I've seen, all are well made and not terribly expensive. Good equipment for the right job.
    Tony Thompson