Mechanical advantage to crossing the rudder cables?

[flatfuse]

Is there any mechanical advantage to crossing the rudder cables. It seems like there could be for large rudder movement but I'm not sure.

[Biff]

There's not a mechanical advantage so much as a practical advantage. If you didn't cross the wires, your exit holes for the cables would be just aft of the wings trailing edge (okay not quite that bad!) By crossing the wires your exit point goes further back. Looks nice and neat. Make any sense?

[Dennis C]

	Quote: Originally Posted by flatfuse
	Is there any mechanical advantage to crossing the rudder cables. It seems like there could be for large rudder movement but I'm not sure.

If you want the correct answer instead of some mumbo jumbo, send scott a e-mail @ http://swbmfg.com/

He is very helpfull.

[flatfuse]

Biff, I know that there are advantages to crossing the cables including the exit location and smaller exit holes. My question is, at rudder movement extremes the control horn is moving toward the fuse as much as forward. It seem that a crossed cable will give you some mechanical advantage, however the servo horn/Bell crank is now moving in the opposite direction as the rudder horn, so my question does it really give you some mechanical advantage?

[VIPER69]

Hey Flat,

The answer to your question is no. There is no mechanical advantage no matter which side you pick. The connection points for rudder and servo arms are exactly the same in both scenarios, so there is no difference in lever arm required for the advantage.

[Pro1Foam]

No, no, no and no. It's all in the geometry dude. Got to side with Biff on this one.

[Vtailman777]

The correct answer is yes there is ...draw a simple diagram of the rudder horn and servo arm...assuming a pull-pull....as the rudder moves from center to either side, the distance from the end of the rudder horn connection to the center line of the plane becomes less....therefore, the moment arm for the rudder is less and the force in the pull cable has to increase to maintain the same torque to the rudder...the same is happening at the servo....the moment arm is becoming less and less as the servo rotates....so both are losing arm length....however the torque rating on the servo remains....since the force(tension) in the cable becomes greater at some point the servo will stall if, in fact, the torque limit is reached.
The same thing happens with the cross cables,,,however, to a lesser extent...all you have to do is do a layout and you can see how this works....the main advantage of the cross cables is that you can run a large horn on the rudder without slicing the side of your plane fuselage front to back....a larger horn permits a lower force(tension) in the pull-pull cable for a given servo torque limit...in all cases the rudder as a system needs to be balanced for your application....on person that is content with normal straight lines...both vertical and horrizontal...may fine less rudder authority okay...whereas one who does snaps and blenders will want more rudder authority....my $0.02...V.

[VIPER69]

I still am not convinced.
The length of the lever arm never changes, though the angle of the force exhibited on the arm is somewhat different (though very slight) for each setup.
The most efficient force would be where the pull is normal (90 degees) to the line from hinge to pull connection. The straight setup (not crossed) is slightly better at holding the rudder in the straight position, but as the rudder is defected the forces are looking slightly better for the crossed pull-pull system. And when rudder begins to deflect is where you want the most force. It does not take much to hold the rudder straight, but as we fly 50mph and you want to deflect the rudder, you need the most force you can generate.

Without laying it out and doing some math (simple, but still requires the work) I would still argue that they are almost identical, and any advantage would be slightly given to the crossed setup when the rudder begins to deflect.

[warren cross]

Does it REally make a hoot of a differance??#1 does it work both ways??#so one cable goes a LITTLE slcak,does that matter(not much). #3 Is thare sufficient tourque to do a knife edge loop?? What more do you need? Case solved.

[paffy]

Here's a comparison of straight vs. crossed setup.

[VIPER69]

Good work Paffy. Did you do this for your Edge or what?
Looks like the difference is squat, with a very slight nod to the straight crowd.
I assume the difference actually goes even lower as the length between servo and hingeline increases and the angles become closer to each other.
Me likes it!

[paffy]

	Quote: Originally Posted by VIPER69
	Good work Paffy. Did you do this for your Edge or what? Looks like the difference is squat, with a very slight nod to the straight crowd. I assume the difference actually goes even lower as the length between servo and hingeline increases and the angles become closer to each other. Me likes it!

That's correct. The only difference is in the amount of slack. But with properly designed servo arm angles, it can be kept down below 1mm.