Pull Pull setups
 

Hi All,

I know this is gonna sound kinda dumb, but I would like some help understanding this Pull Pull setup technique. I have heard of everyone talking about Pull Pulls and I don't understand what the difference is between the Pull Pull Setup compared to regular pushrod setup. Can you guys explain the advantages/disadvantages of the both.

Thanks very much in advance, :)

Pull Pull setups
 

A pushrod setup has one control rod that connects between the servo arm and the control horn. The control rod both pushes and pulls the control horn.

The pull-pull setup is a set of either two control rods or two wires. On the flying surface is mounted two control horns, one on each side of the flying surface. Likewise both sides of the servo arm are used. One side of the servo arm is linked to one of the flying surface control horns and the other side of the servo arm is connected to the other flying surface control horn. This forms a 'rectangle'.

One advantage to the pull-pull is a possible weight savings in the tail which can be helpful in sailplanes. Another is that the pull-pull can eliminate any need to 'push' the control links ... rather it can 'pull' in either direction. If a rod is used for each of the pull-pull connections then one rod pulls and the other pushes.

One disadvantage is that if there is too much tension on the cables or too little tension then the control surface control could have problems such as 'flutter' if the cables are too loose or 'servo shaft binding' problems if the cables are too tight.

If rods are used between the servo and the flying surface control horns the loose or tight issue is not a large concern compared to using flexible cables.