I have setup a pull-pull rudder on a 37% Ultimate, and I have a question. Cables are tight on both sides when centered-- however when I start to deflect the rudder, the cable on the opposite side gets slack -- when fully deflected, the slack is very pronounced. My question: is this normal?

here are the details:

SWB self adjusting tray with offset bellcrank
Distance across the bellcrank is 2.5"
Control horn on rudder is 4" from end to end (using du bro HD 10-32)
The control horn pivots on the hinge line
Using cross over cables
Lenght of cables is approx. 30"

I want to know if I have done something wrong in my setup, or slack is normal in this configuration? This is the first time I have tried an offset bellcrank and cross voer cables.

thanks
Jim

Are you sure the bell crank is only 2.5 in? Should be the same as the rudder arm 4".
A little slack is normal on the opposite side of the cable.

Jim
Try a straight bellcrank the offset bellcrank should be used when you are not on the hinge line with control horns, you suggest that you are on the hinge line so you should use a straight bellcrank, it will take the slack out almost completely. Carroll-RCU is correct the bellcrank and the rudder control arm should be the same lenght.
Thanks

Hi Jim - Some slack should happen when the pull pull is set up properly. It sounds like you might have a little bit to much however.

Here is a link to some of the best pull-pull set up explanations I've found. The [link=http://members.cox.net/bdfelice/Ackerman/ackerman.htm]Ackerman[/link] effect.

Hope this helps a little, it sure did me when I needed it.

I have to disagree. Having the bellcrank and the rudder horn the same size will offer a 1:1 ratio. It is desireable to set your system up with a 1.5:1 ratio giving "mechanical advantage" to the setup. I have mine set up with a 3" servo arm and a 4.5" rudder horn. The geometry is perfect and there is little to no slack on the non pull side. I am sure that not everyone shares my views. On the other hand, slack on the non pull is nothing to be concerned about and to some people it is desired.


MRT750 is correct. If you are on the hinge line then a straight bellcrank would be needed to eliminate the slack but don't get too worked up over it.

See example B on this link.

http://www.swbmfg.com/rudex.html

"I have to disagree. Having the bellcrank and the rudder horn the same size will offer a 1:1 ratio. It is desireable to set your system up with a 1.5:1 ratio giving "mechanical advantage" to the setup. I have mine set up with a 3" servo arm and a 4.5" rudder horn."

the setup you describe gives you mechanical "disadvantage".... slop is multiplied and torque is reduced...

[:-]

FenceMAgnet wrote: I believe you are not entirely correct. By using a short servo arm and a long rudder horn you increase the torque available to deflect the rudder. The price you pay for this is reduced travel. Crash90 is correct.

In order to obtain the best possible resolution the servo should be operated with as much throw as possible. If the available angular deflection of the servo is larger than the desired angular deflection of the rudder one should shorten the servo arm (or even better, lengthen the rudder horn) rather than reducing the servo throw.

Whether or not "slop" is increased depend on what kind of slop you are talking about. In order to reduce effects of "slop" in the servo gear train one should aim at using the maximum servo throw. If necessary the rudder horn could be lengthened or the servo arm shortened. In order to minimize the effects of slop in links etcetera, all arms should be as long as possible.

My method to determine what arm lengths is:
a) Determine the desired angular deflection of the rudder, say +-45 deg..
b) Determine the maximum angular deflection of the servo, say +-60 deg
c) The ratio between (b) and (a), 60/45 = 1.33 is the necessary ratio between the lengths of rudder horn and servo arm.
d) Select the longest possible servo arm, say 2".
e) Calculate the required length of the rudder horn by multiplying servo arm length with the ratio in (c), e.g., 2"*1.33=2.67".

/Red B.

As a general comment I have made the following posting that will hopefully be of some help to people having problems with their pull-pull setups:
[link=http://www.rcuniverse.com/forum/fb.asp?m=3010132&key=]Pull-pull and Ackerman 101[/link]

/Red B.