My servos came with three different styles.. What makes the difference? I guess I should ask which work best where also?
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You just use what works. Normally you need a single arm for the elevator and throttle, a double arm for rudder and nosewheel together, and either a double arm or wheel if you have one servo for both ailerons. The wheel is handy for that because those offset holes let you have some aileron differential which smooths out your turns a little.

Thanks... I wasn't sure if there was some reason each servo came with the choice.

edit.. I do have to ask about the differential.. I'm trying to picture how that would work ? One aileron would move further up than the other down and then vice versa ?<br type="_moz" />

Yes you have it correct. The idea is for the upward moving ailerron to travel further than the downward moving aileron. The purpose is to minimise adverse yaw, inwhich the downward moving aileron causes more drag than the upward moving aileron. This in turn causes the airplane to yaw away from the direction of the turn. And it is this which gives the airplane that awkward staggering look during a turn.

What is involved in using that round arm for a single servo aileron setup is causing the differential by deliberately drilling pushrod holes not straight across at ninety degrees as in most cases but drilling holes for the two pushrods (in the case of a high wing airplane) forward of that line straight across so the pushrods are a little longer.

This geometry will induce aileron differential (more up than down) and this in turn will minimise that adverse yaw so your turns will be a lot prettier.

Of course this is also commonly done now with two aileron servo setups in the transmitter electronicaly and folks are forgetting these old mechanical setups which work just as well.

If you want to do this mechanically on a low wing with one aileron servo the opposite is done with pushrods shorter to not quite be lined up ninety degrees to the output bushing.

Sounds complicated but its really pretty simple, just kind of difficult to explain without a diagram. I think I still have an old photo somewhere showing this on a trainer somewhere and will see if I can find it.


John

Found it, this is a photo of a high wing trainer using a single servo that has mechanically induced differential. Note the pushrod holes in the round arm are forward of the output bushing of the servo.

Note I do not reccomend the use of EZ-adjustors on these pushrods as in the photo I much prefer a 'Z' bend on these rods and a clevis on the other end at the aileron torque rod horns.


John

Thanks for the details and the photo John.

You Bet[8D]

John

Do you make your own Z bends? Seems I've seen a YouTube on it, doesn't look all that difficult, but I don't have one to compare to either

Dubro bender is the way to go for Z bends


Terry

This is the orginal Dubro Z bend tool. It was a wonderful tool. A very easy two step tool that gave perfect z bends every time when used with Dubro 2-56 rod end soft wires. It only retailed for around eight bucks. Production was stopped perhaps two years ago and a new bench mounted substitute was introduced which does work well but its expensive and required screwing down to a bench.

When I asked why the original was discontinued at the Dubro booth last year in the industry show at Las Vegas, It was explained that the fellows will tend to use hardened music wire and that just will not work well and in some cases folks used metric wire which is slightly smaller and the bends just do not work unless the intended rod is used[]

Anyway that is the reason given but that little jewel is a treasure if you find one somewhere snag it.

John