Closed Loop (Pull-Pull) Questions
 

Okay, let me say up front that I realize the best way to do this is buy a Nelson bellcrank and drill it appropriately. But I have the stuff pictured below handy, and I would like to "do-it-myself", so to speak.

This is for a rudder connection on a Goldberg Ultimate. What you see is the kit-supplied tiller bar that attaches to the rudder, a DuBro heavy-duty servo arm, and some small bearings, washers, and a spacer from a helicopter mixing base assembly.

I understand how to do this on a helicopter; but I have found that the solid pushrods used in that application behave differently than flexible cable in a fixed-wing installation, in terms of binding and slack.

My question is this: given that none of the holes in both parts (servo arm and tiller bar) allow for a perfect parallelogram, should I hook it up so that the distance from center on the servo arm is GREATER or LESS than that on the tiller bar?

Steve

RE: Closed Loop (Pull-Pull) Questions
 

Steve:
Less than the tiller arm. When you give rudder, one side will go slack but the rudder has a load on it, so it wont hurt anything. When you go back to neutral, they will tighten up again. If you want, you could drill the servo arm.

RE: Closed Loop (Pull-Pull) Questions
 

If the attachment points for the pull-pull lines on the rudder horn are at exactly 90 deg to the hinge line, you don't need to have a perfect parallelogram to maintain proper line tension. Draw it out on a piece of paper and you'll see what I mean. No matter what angle the rudder is at, the lines will stay the same length to the servo arm. If the attachment point on the servo arm is a greater length than the rudder arm it just means that the amount of movement at the rudder is multiplied by the ratio of the two lengths, same as any control horn/servo arm setup. The lines will stay tight on both sides regardless of the ratio (as long as everything is on center).

RE: Closed Loop (Pull-Pull) Questions
 

Just like Curt said. Have done lots of pull-pulls on rudder. No bellcrank necessary but a good coreless servo is a good idea.

Clair

RE: Closed Loop (Pull-Pull) Questions
 

Thanks for the info.

Clair, the bellcrank is necessary because the servo must be off-set from center. This model uses a split elevator pushrod, and the elevator servo must be centered to avoid differential throw in the elevator halves.

I'm building a platform for the bellcrank which will sit under the elevator pushrod. The rudder servo will sit off to the side, driving the bellcrank.

Had to do the same thing on my H9 CAP...

Steve

RE: Closed Loop (Pull-Pull) Questions
 

I agree Steve that the servo needs to be centered, otherwise the bellcrank system, which I have had to do once or twice. Another answer on some planes that I have done a lot is to "step" the two servos.....elevator servo with the pushrod centered in the fuselage, and, looking down into the cavity (low wing), the rudder servo is in front and "higher" (i.e. cables are below the elevator pushrod if the plane is right side up) with the cables running over the elevator servo and pushrod. Hope this makes sense......written as though you are looking down into the radio cavity of a low-wing airplane with the fuselage upside down.

Clair

RE: Closed Loop (Pull-Pull) Questions
 

I understand your explanation; but since the bellcrank must be supported by a platform, and running the elevator pushrod "under" said platform would restrict accessibility, I arrange my set-up so that the elevator servo sits higher, with the elevator pushrod "over" the rudder control system (lower, when the plane is right side up).

Steve

RE: Closed Loop (Pull-Pull) Questions
 

Maybe I wasn't as clear as I thought.....my explanation is for a set-up where a bellcrank is not needed, and by "stepping" the rudder servo up the elevator pushrod is below the cables, and between them, so is fairly accessible. (Again, talking about the fuse upside down). It is possible this way to get the pushrod and the cables all centered on the fuselage center line, and keep the geometry pure.

Clair

RE: Closed Loop (Pull-Pull) Questions
 

Oh, I see. Both servos are basically on the centerline of the model, at different heights? But that would require placing one in front of the other, with the higher one behind (actually, toward the nose) the lower one, yes?

Sure, I can see where that would work fine. But unless you were building a model from scratch, or a kit, that would require some surgery to the servo tray. Probably about the same effort needed to fabricate the bellcrank assembly...;)

Actually, I do have my elevator servo "stepped"; it is raised off of the servo tray by spruce blocks.

Steve

RE: Closed Loop (Pull-Pull) Questions
 

You've got it Steve! Actually, not too much trouble to fabricate a custom servo tray of light play and basswood to make that system work. Probably about like the time required to mount a bellcrank!

Clair