I've been reading up on the use of Ackerman (geometry) in pull-pull systems. To make a long and complex story short, it seem that what it needed is to have the connection point on the horns be just slightly behind the pivot point.

Well, it's too late for that on my rudder; the control horn is perfectly centered as I thought it was supposed to be. The question is, can I still work some Ackerman into the system by altering the setup up of a bell-crank at the other end -- the servo end of things?

I hadn't previously heard of the Ackerman geometry with pull-pull controls. After glancing over some references very briefly I still don't quite understand the definate advantage of an Ackerman setup over a linear system, i.e. where the connection points on the horn are on the same plane as the pivot point.
Is it because the pull-pull system is analogous to a cars steering geometry where there is a differential change or different distances that each wheel (or servo arm) travels?
Would there be any disastrous effects with a non-ackerman setup?

http://members.cox.net/bdfelice/Ackerman/ackerman.htm Here is some info. I found at "Alan's RC Links"

If it's too late for your rudder, I don't see any reason why you can't do it on your servo. Just remember that the Ackerman offset is always in the direction away from the cables. If you add Ackerman to the rudder (typical case), you would offset your control horns behind the hinge line. If you add Ackerman to the servo (your case), you would offset the connection point in front of the pivot point.

I made a pic. The top is a typical Ackerman setup. The bottom would be your Ackerman setup. The left side is the servo. The right side is the rudder.

I annotated it with measurements to show how Ackerman works. In both cases, there is an extra 0.06 units of length on the slack cable. And of course, the cable that is pulling has no slack (0.00 units).

Juice

Juice, thanks, man -- the pix were tremendously helpful!

BTW, as far as I can tell from studying the photos on the Fokker Team Eindecker CD, the original EIII didn't have any Ackerman built into the system but then the angles in the photos might not tell the whole story.

Juice:

Slight dispute, sir. The Ackerman effect will show up in any linkage of this sort without parallel arms, it's just that with angles leading into the opposite end will result in tightening the cables with deflection, and we really don't want that. But it's still "Ackerman."

Bill.

OK, I'm confused again. My understanding is that we definitely want a slight degree of Ackerman in a pull-lull system and that is accomplished (typically) by placing the connection point in the flying surface just slightly behind the hinge pivot point. Am I right so far? If this is not possible (as in the case of my rear flying surfaces where the control horn is build concentric with the tubing that forms the pivot point), can I, as Juice suggested, add Ackerman to the system by altering the geometry of the bellcrank (or servo arm) at the servo end? According to Juice's diagram this would be accomplished by moving the connection point FORWARD of the "hinge point" (in this case either the rotation point of the bellcrank or the center post on the servo itself).

Bill, could you clarify your objection a bit more?

'Bu:

You have the right idea, and the right method. All I was pointing out was that the Ackerman effect is not limited to that one application. As a matter of fact the basic and most common application of the Ackerman effect is in the car you drive every day. It is what makes the wheel on the inside of the turn attain a greater steering angle than the wheel on the outside - the inside wheel has to roll in a smaller circle than the outside wheel.

Bill.