I have read, and heard many stories on how to set up a pull-pull system. One of the most common and false statements that I hear is that if you want to cross the cables, you will have to use an offset servo arm. This is false, and I will try to explain why below. I will also try to explain the correct way to set up a pull-pull system.
All we are trying to achieve in a pull-pull system in exact geometry between the control surface control horn and the servo arm. This exact geometry is measured in an arc. The arc is measured from the pivot points, to the clevis connection points.
Now...if the rudder control horn, and the servo arm are swinging the same sized arc, it does not matter wether the cables are running parallel, or crossed. The reason is, the cables do not change the geometry of the rudder control horn, or the servo arm. If you look at SWB's example on their website http://swbmfg.com/rudex.html, you will notice that example B is an example of an incorrect set-up, and it states this in their description for Example B. I keep on seeing people refer to Example B as being a correct set up, and necessary if the cables are going to be crossed. This is incorrect. Example B will keep the cables tension as much the same as possible throughout the whole spectrum of travel. This is not an ideal setup, and should only be used if you don't have any other choices.
The only time an offset servo arm is needed, is when the distance from the pivot point (hinge centerline) to the clevis connections center is not the same as the servo arms pivot point to clevis connections center. i.e. the distance from center as measured from the center of the pivot point out to the center of the clevis connection point, or the center of the pivot point back, or forward of the clevis connection points are not equal to that of the control horns pivot point. The reasons that these dimensions are important, is to maintain the same arc travel throughout the entire travel of the servo arm, and the rudder control horn.
I cross the cables in every aircraft that I install a rudder pull-pull set up in. The reason for crossing the cables is to create the smallest cable exit hole possible. Parallel cables will require a longer exit slot than crossed cables. Also, when crossing cables, you want to make sure they don't rub together. This can wear them out, and cause possible interferance.
I have heard guys say that it is o.k. to have the non-tensioned cable go slack, because the air-pressure during flight will push on the control surface. This is true until you reverse the direction of the control surfaces travel. When reversing the control surfaces travel, you will have a delay in travel until the slack in the cable is taken up. Once the cable is tightened, the control surface will start to travel. This can cause premature servo gear wear, or breakage. It can also make the feel of the airfraft to be less than desireable.
When a pull-pull cable system is set up properly the cable tension will remain the same throughout the entire travel of the control surface.
This was not meant in anyway to bash anybody, or say I am right, and you are wrong. I am just trying to help people better understand the correct way to set up a pull-pull system, since it is such a popular system to being used in so many aircraft today.
Bron: Pull-Pull cables, to cross, or not to cross - FlyingGiants
All we are trying to achieve in a pull-pull system in exact geometry between the control surface control horn and the servo arm. This exact geometry is measured in an arc. The arc is measured from the pivot points, to the clevis connection points.
Now...if the rudder control horn, and the servo arm are swinging the same sized arc, it does not matter wether the cables are running parallel, or crossed. The reason is, the cables do not change the geometry of the rudder control horn, or the servo arm. If you look at SWB's example on their website http://swbmfg.com/rudex.html, you will notice that example B is an example of an incorrect set-up, and it states this in their description for Example B. I keep on seeing people refer to Example B as being a correct set up, and necessary if the cables are going to be crossed. This is incorrect. Example B will keep the cables tension as much the same as possible throughout the whole spectrum of travel. This is not an ideal setup, and should only be used if you don't have any other choices.
The only time an offset servo arm is needed, is when the distance from the pivot point (hinge centerline) to the clevis connections center is not the same as the servo arms pivot point to clevis connections center. i.e. the distance from center as measured from the center of the pivot point out to the center of the clevis connection point, or the center of the pivot point back, or forward of the clevis connection points are not equal to that of the control horns pivot point. The reasons that these dimensions are important, is to maintain the same arc travel throughout the entire travel of the servo arm, and the rudder control horn.
I cross the cables in every aircraft that I install a rudder pull-pull set up in. The reason for crossing the cables is to create the smallest cable exit hole possible. Parallel cables will require a longer exit slot than crossed cables. Also, when crossing cables, you want to make sure they don't rub together. This can wear them out, and cause possible interferance.
I have heard guys say that it is o.k. to have the non-tensioned cable go slack, because the air-pressure during flight will push on the control surface. This is true until you reverse the direction of the control surfaces travel. When reversing the control surfaces travel, you will have a delay in travel until the slack in the cable is taken up. Once the cable is tightened, the control surface will start to travel. This can cause premature servo gear wear, or breakage. It can also make the feel of the airfraft to be less than desireable.
When a pull-pull cable system is set up properly the cable tension will remain the same throughout the entire travel of the control surface.
This was not meant in anyway to bash anybody, or say I am right, and you are wrong. I am just trying to help people better understand the correct way to set up a pull-pull system, since it is such a popular system to being used in so many aircraft today.
Bron: Pull-Pull cables, to cross, or not to cross - FlyingGiants
