Pull-Pull and Ackerman 101
I have noticed that there are a lot of questions about the effects of different geometries when using a pull-pull arrangment to deflect rudder or elevators. Exact calculations are a little bit involved but here are som basic principles that hopefully will be of some help:
As an example I will use a pull-pull rudder with wires used in a non-crossover fashion.
The simplest arrangement that will maintain constant wire tension, independent of the angular deflection, is a set-up where the servo arm and rudder horns are of equal lengths. The servo arm should be straight, with no offset and the rudder horn should aligned with the hinge line.
Now, what will happen if we make changes to this geometry? Here are a few scenarios and its effect on wire tension:
a) Rudder horn is moved behind the hinge line:
This will cause wire tension to decrease when the rudder is deflected (the non-pulling wire will go slack).
b) Rudder horn is moved in front of the hinge line:
The opposite will happen. Wire tension will increase when the rudder is deflected from neutral (BAD!)
c) The wire attachment point on the servo arm is offset away from the rudder:
Wire tension will decrease when the rudder is deflected from neutral.
d) The wire attachment point on the servo arm is offset towards the rudder:
Wire tension will increase when the rudder is deflected from neutral.
e) The servo arm is made shorter than the rudder horn:
Wire tension will increase when the rudder is deflected from neutral.
f) The servo arm is made longer than the rudder horn:
Wire tension will increase when the rudder is deflected from neutral.
N.B. Sometimes it is desireable to use a servo arm that is shorter than the rudder horn in order to increase the mechanical advantage, i.e., trading rudder deflection angle for an increase in the torque that deflects the rudder. In that case the effect described in (e) and (f) can be almost completely compensated for by moving the rudder horn slightly behind the hinge line as described in (a).
g) A Pull-Pull cable servo wheel is used instead of a servo arm:
This is an interesting one! Wire tension will increase when the rudder is deflected. The effect however is VERY small!
/Red B.