I have been reading a bunch of threads on pull-pull systems and I am left confused.

1. Do the control arm and control horns have to be the same length? (I have read both equal and it does not matter)
2. Should you have some slack on the opposite side when control surface is being moved (I have read both; maybe this one is preferance?).

Now, everyone agrees if you do want slack, you need Ackerman. However,

3. Do you move control horn slightly fore or aft of hinge to get the correct Ackerman (I have read both options).

Lots of opinions (conflicting) so I am looking for the real answer. Thanks.

I don't know if this will help or not.
http://www.mindspring.com/~rellis2/rcpattrn/ppull.htm

Maybe

1 - The control horns do not have to be the same length, but you want them to be somewhat close.

2 - This is a bit of a misunderstanding - a typical example of taking a good idea to extremes. The point of using Ackerman is not so much to create slack as it is to prevent the wires getting too tight. Unless you have very large servo arms and control horns, there's really nothing wrong with putting the horns on the hinge line.

3 - They go AFT of the hinge line

Chemie,

Study these schematics and try to find the responses to your own questions.

This is why I said that while the arms don't need to be the same length, they should be close.

In Inewqban's diagram, he is measuring servo movement and not cable movement. When you measure cable movement, the differences are less.

So much less, that I blew it up 5 or 6 times to give you an idea.

In the illustration below, the servo arm is 10 inches long and the control horn is 20 inches long. With a 30° servo rotation, the cables will tighten by 1/32".

While 1/32" might be a cause for concern, when you scale it down to the norm, you're talking about less than .01 inch (Or less than two turns of a clevis) - and even THAT is if the control horn is TWICE the length of the servo arm.

Really not worth worrying about.