Connecting two 10-32 control rods (a challenge...)

[bdflyer]

So, I have a push-pull cable that controls the pitch anti-servo tab on the plane. I have removed the manual (and poorly placed) handle and opted for a Ray Allen T3-12A servo, commonly used in this type of application. The 12A uses a 10-32 threaded rod from the included (plastic) clevis to push/pull on the rod. That rod needs to connect to a corresponding 10-32 threaded rod at the end of the said push-pull cable.

The alignment of the servo and the push pull cable is slightly off center and the server has to be higher then the cable by a few degrees (nothing all that significant). This means that the server needs to be mounted a bit away from the cable as to reduce the angle at which the control rod has to travel between the servo and the cable.

Normally, one would leverage a MW-3 rod end to account for the vertical and slight horizontal alignment (perhaps one degree) needed for one side of the connection. It is the other side that is an issue: I would normally expect a clevis to connect to the MW-3 to complete the coupling. However, the width of the MW-3 (and most other ends I have reviewed) is .321", which is far (far) wider then any 10-32 clevis I have looked at (and I made quite a few calls). I also tested an inline ball joint from Midwest Controls that worked precisely as needed (Link), but the quality of the part is more akin to use in a small building damper and less so for a control surface of an aircraft. I was able to get it to fail by hand, though only with pressure only to the side.

Here is a link to a photo that shows the assembly (in test-fit mode, so pls excuse the lack of pins for instance): (Link)

I am open to suggestions here - it would be nice to have a ball joint to be frank, as it is very stable and very effective. However, I will take any workable option at this point. Too many hours testing and researching on this one... One hesitation with a rod end and clevis is that I do expect that there will be some "wobble" pushing toward the cable from the servo as the assembly will want to push downward or upward before continuing its horizontal line of motion, especially as the anti servo tab is loaded at speed, and the cable has more resistance as a result.

I was also trying to avoid installing the servo in the tail because of clearance and concerns (perhaps unfounded) that the tab would be a bigger problem if the servo shaft were to break there then if the shaft were to break at the end of a 20' cable (with its own resistance to dampen the effect).

Thoughts?

[Dana]

If the misalignment is only one degree you might be able to use just the clevis on the servo end and a rigid coupling nut; there will normally be some slop in the clevis. Or if you really want or need to use a ball end, you could simply make your own clevis to fit it.

[cub builder]

FWIW, I've done this with both of my planes. In both cases, I ended up with the servo mounted in the elevator.

On one of the planes, I originally had a push/pull bowden cable to the trim tab. The problem is that when the tab pushes up (nose down trim), the cable is loaded in compression and eventually started flexing back and forth, then in a matter of maybe a minute, completely failed. When the only cable to the tab broken, the tab went into a violent flutter, making the aircraft difficult to fly and causing some minor damage to the aircraft before I could get it onto the ground. This is a fairly common problem with trying to control a trim tab with a single bowden type cable. I have seen this happen on several aircraft types. In another case (again trimmed nose down), the bowden cable only flexed under load, but that was still sufficient to cause flutter even though the cable did not break. In that case, the pilot pulled nose up and trimmed nose up, which stopped the flutter once the cable was loaded in tension. Give it some serious thought if you are going to attempt to control your tab with a single cable. I know it's a lot of work, but mounting the servo in the tail is a much safer option.

-Cub Builder