Red B.

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.

RTK

My Feedback: (1)

Thanks Red, does this all also apply to crossed cables?

DKjens

My Feedback: (50)

In most of my set ups, despite being ever so careful with the geometry, my non-pulling wire has always seemed to go a little slack, when rudder is deflected. I have always been of the opinnion, that this slight slacking is of negligable importance, since the only time the rudder will ever see a force from that side, would be in a tail slide. During normal flight, hovering and 3D, the force exerted on the deflected side of the rudder, will surely keep the pulling cable tight, even when rudder is returned from extreme deflection, to neutral.
DKjens

Red B.

Removed due to dupliate posting.
/Red B.

Red B.

DKjens, I agree with you, a little slack when the servo is deflected is of no concequence.
What you do not want to end up with is a geometry where wire tension increases with deflection.

RTK, I will look into that question.

/Red B.

FBaity

My Feedback: (2)

I have a question for this thread. I have a Goldberg Sukhoi and the rudder cables are crossed and overdriven ( servo horn is larger than the rudder horn ). This aircraft will fishtail when flown in fairly strong cross wind consitions regardless of cable tension.. I have a lot of rudder travel in high rate ( will almost touch elevator ) but have found I need it for blender to flat spin. Will reducing the servo authority, smaller arm, eliminate the fish tailing condition? Are the crossed pull,pull cables the problem? Would appreciate some ideas, thanks.

Frank Baity
AMA 38026

FenceMagnet

My Feedback: (2)

dual rates and/or exponential will help you greatly !

[:-]

FBaity

My Feedback: (2)

I am using dual rates and expo, also have digital JR 8411 0n rudder.

DKjens

My Feedback: (50)

FBaity,
What you are describing there is a common problem with the Composite-ARF 3.0m Extra330. It doesn't matter how tight your pull-pull cables are or what and how many servos you use on the rudder, at high speed, it will fishtail as well. On this plane it is acredited to the very thin trailing edge on the rudder, and the fix is to widen the trailing edge. People do this in different ways, I bought a pack of Dave Brown control rods. These are something like 3/8" fiberglass rods. I cut one to length, cut a slot all the way down through it, slide it on the trailing edge and secure it with glue. This fixed my problem, you may try something similar.
DKjens