I am going to modify a Kyosho Cap 232 Arf with a pull-pull system on the rudder. My question is that, after thinking the process through(I have learned my lesson about being hasty), I thought about this...

In a pull-pull system, when the surface is in the neutral(center) position, unless the pull-pull is absolutely perfect, the surface would have slight slack in each wire, and therefore the surface would be prone to flapping slightly back and forth(flutter).

Is this a problem? I havn't even set up the pull-pull, I was just thinking it through, and thought about this potential problem.

Thanks,
Philip

HI philip. The cables should be snug so there is no slack at neutral. Also you should use a double ball bearing servo as to handle the tension on the servo shaft. Tim.

Hello!
I have used pull-pull systems from small fast airplanes like F3D pylonracers (both elevators and rudder) to large aerobatic airplanes (Rudder) like the SIG CAP 231 EX and I can asure you that this the way to go if you want a system that is both light and vibration free (for the servos).
The pull-pull system is very simple to use!
I attach the ends of the fishing lines( Yes I use the thinnest steel fishing line available) to ordinary treaded rods on which I screw plastic or metal clevesis which I mount on the servos. I drill a hole in the treaded rods and pull the wire through and then securing it with crimped on brass tube. At the back (rudder) I pull the wire through the controllhorn hole and crimp on a brass tube (these tubes are bought at fishing stores)

I do not tighten the wires much..you will see for yourself that this is not necesary! And no tightener is needed!!
Properly done there's no slack in the system...only be careful how you mount the controlhorns at the rudder. Aways mount them at 90 degress to the hingeline. that way no slack is built in the system
Offcourse you will have to mount the clevesis on the servo likewise! 90 degrees to the axle on the servo!

Regards!
Jan K
Sweden

You might also consider using a bellcrank/tillerbar to hook the cables to, and a short link from the servo to the bellcrank. That way you are not placing all the strain on the servo itself, and the cables can be run tighter. This setup also allows you to run 2 servos if necessary.

Thanks for all the info guys.

jaka, do you have any specific brands, and tension strength of the fishing wire you use?

DuBro makes a nice pull-pull set in both 2-56 and 4-40 sizes with nylon coated cable and hardware:

http://www.shopatron.com/product.pht...duct_id=DUB517
http://www.shopatron.com/product.pht...duct_id=DUB518

> Properly done there's no slack in the system...

Hi Jan,

Don't you get some slack in the non-pulling cable when turning the rudder or elevator? I don't see that this is a problem though because the tention on the pulling side keeps the rudder firmly in place.

Ernie

I'M BUILDING THE 31%EDGE 540 (AEROWORKS) i WAS THINKING OF USING ONE SERVO. THE HITEC HS 5735 MG AT 6 VOLTS, IT PUTS OUT 264 OZ OF TORQUE AT A SPEED OF .13 DOES THIS SOUND LIKE A GOOD SINGLE SERVO CHOICE?

joHN

If the control horns on the rudder are the same width as the servo control horn and the rudder horn holes are at the hinge line, you will have a parallelogram and will not get slack. Even if you do however, it's VERY uncommon for a rudder to flutter. I had a control rod fail in my Sporster and didn't know untill I tried to knife edge, then when I landed couldn't steer. Has anybody actually seen RUDDER flutter?

> Has anybody actually seen RUDDER flutter?

Can't say as I have *seen* it, but I don't know why a rudder would be any less prone to flutter than any other hinged control surface. There is a tiny bit of slop in the rudder linkage on my Super Stick, and it keeps loosening all of the servo screws on the rudder servo faster than I can keep them tight. (it must be some flutter?) Whether pull pull or not I think it is a good idea to use the holes furtherest away from the hinge lines and keep slop to a minimum when possible.

Ernie

You may be correct, but most flutter happens on control surfaces where there is a pressure difference between the two sides, The only time that happens with a rudder is in knife edge. I'm only asking because I've never heard of a rudder flutter, on either real or model planes.

but most flutter happens on control surfaces where there is a pressure difference between the two sides, The only time that happens with a rudder is in knife edge. I'm only asking because I've never heard of a rudder flutter, on either real or model planes.

High speeds will cause flutter also as will even small amounts of slop in the setup. Flutter can be heard during high speed passes sometimes and should be cause for getting on the ground right away!

Ernie

There is a pressure difference every time you slightly touch the rudder control. That is how all control surfaces do their work. High pressure on deflection side, low pressure on opposite. The surface is then "sucked" into the lower pressure pocket of air. This is not a knife edge phenomenon

I set up a pull/pull in my cap with eqal distance apart and on hinge line just like your suppose to. And when it turned one side got loose. I had to seperate the connections at the rudder further apart than the servo for them to stay tight. the parrelelogram theory didn't work. Why?

I have no answer for you. I've been doing it that way for a long time and never had the problem

The control horns might be ever so slightly canted. The holes in the horns need to be in a perfectly straight line across and exactly on the c/l of hinge. Doesn't take much to throw this off ( slightly tapered surface) . Slight shimming might be all that is required.

Ray

I think you just hit on the problem. Some guys use rocket city hardware on the Rudder (or simular), as such the drill and mount the hardware (control horn) too far behind the hinge line. I think Ideally, you want the control horn pivot point to be even with the hinge line.

Am I right?

JOhn

Much like some full size aircraft have the attachment points a certain distance behind the hinge line.

A bit off subject but, could anyone explain how(or give any links on RCU) to install a "Y" pushrod connection on the elevator to operate each half?

It looks like we are divided on whether some slop on the non pulling cable is a GOOD thing or a BAD thing. Obviously some slop there is better than going too tight as you turn. So far I am going with a little slop in the non pulling side because no matter what I do it just comes out that way....:-)

Ernie

If you're using Nyrod... Install two tubes, and near the servo, have them both aligned so that the pushrods will meet the servo arm at the appropriate location on the servo arm. Put one tube above the other at the mounting point (bulkhead) near the servo and secure the tubes. If you put them side by side, you WILL get more travel on one than the other as the servo reaches it's endpoints.

There are several methods for tying the pushrods together. You can wrap the ends with strong string and epoxy, then install a threaded connector in one pushrod and connect to the servo. You can also put threaded connectors in both pushrods, place a small 2 hole landing gear strap (with a third hole drilled in the middle) over both threads, secure it with 2-56 locknuts, then cut the excess thread off and install a third threaded connector between the middle hole and the servo arm.

There must be enough room between the servo arm and the hard point for the pushrod tube to allow for variations in pushrod spread as the servo moves and this is dependent on your connection, whether string/epoxy (pushrod tubes close together) or a landing gear strap (pushrod tubes farther apart). Just make sure that wherever your threaded servo connector ends up that it is even with the servo arm both horizontally and vertically, and take this into consideration before securing the pushrod tubes to the bulkhead.

Remember... Over/Under... NOT side by side.

Sorry, I have no pics.