| Apparently when I said in my last post “ensuring that there was equal tension in each cable by the simple measure of ensuring that each elevator was at the same relative angle” it didn’t mean much; I knew what I was talking about but of course I’d done it! Rather than just explaining the tensioning bit I thought I might as well do a step-by-step account of the elevator set up. If you’re building an SE5a, or similar and want to incorporate the pulleys etc., or you’re a bit “nerdy” like me and like reading technical stuff then the next few paragraphs may be worth reading. Then again you might just be suffering from insomnia and what boring to sleep!
The cables each consist of a spring with “tails” attached to both ends. A short “tail” that is attached to the control horn on the same side as the spring, which I’ll call the sprung cable. A long “tail” that passes through the fuselage and attaches to the other control horn; I’ll call this the non-sprung cable although of course the spring does have exactly the same effect on it.
My original idea was to pass the non-sprung cable through the centre of the spring of the other cable but in practice this wasn’t a good idea. When some tension was applied to the spring and the coils opened slightly the non-sprung cable could get between the coils and this caused a lot of friction; running the cable on the outside of the spring, between it and the tube in the tail plane, caused no problems.
I threaded the cables through the 2 halves of the tail plane bringing the sprung cables out for the top control horns and the non-sprung cables out for the bottom control horns. The choice was purely arbitrary but if done the other way round the following steps would have to be reversed.
The model was turned upside down and the non-sprung cable pulled until the spring hit the fuselage side. With full up elevator (remember the model is upside down) I bent the cable back on itself ¼" past where it met the control horn thus ensuring that in practice the spring would never hit the fuselage side. The cable was threaded through the control horn and fixed by binding with thin copper wire and a drop of cyano. This was then repeated for the other side.
The model was then turned the right way up and the elevators temporarily joined by a length of spruce and clamps. One of the sprung cables was threaded through its control horn and pulled until there was sufficient tension to keep the cable tight and provide enough friction on the pulleys to make them rotate when the elevator was moved from full up to full down. This turned out to also be about ¼" but I tensioned it just that bit more to be on the safe side and then terminated the cable as before.
When I removed the spruce “joiner” the spring contracted and I had one elevator deflected up and the other deflected down.
Now we get the "equal tension" bit! I threaded the other sprung cable through its control horn and tensioned it until both elevators were in line. When operating the elevators together there’s no pulling against the springs as the tension of one counteracts the tension from the other. |