fastplane

I am doing my first rudder & elevator pull-pull setup. This is on a 90 size plane. It has split elevator.
I am using Du Bro 4-40 plastic coated steel cable and couplers.
All of the cables are run and connected. The cables are crimped at the servo arm. Using one servo with arm horizontal.
I am having trouble setting the tension on the cables for the elevator halves. It seems that the goal is to get the cables past the 'slack' stage but not guitar tight.
What is the correct method / procedure to get all of the cables same tension and the elevator halves at neutral?
I seem to get one side set properly and then working on the other side the elevator half will be not be at neutral.
When this is completed -- if the elevators deflect either up or down the cable not being pulled is slack. Is this a problem?
Thanks.

jaka

Hi!
Why have a split elevator ??? Makes things harder to adjust! Why not glue the halves together with a aluminum arrow shaft (I use black Easton arrow shafts, lighter and harder than carbon)
I always have the adjustment at the servo arm, inside the fuselage. At the rear I just crimp the wire which minimises the ugly factor.
Here are some pictures of my modified U-Can Do .60 and some other airplanes

Regards!
Jan K
Sweden

majortom-RCU

My Feedback: (40)

I will concede that Jaka's approach has its merits. However, to answer the question as it has been put:

Are you using some sort of threaded connector in your linkage? If so, set up the length and crimp so that it's as close as you can get to equal and minimal tension on both sides. Minimal tension means no sagging, but not tight. Then make your final adjustments by turning a few revolutions on your threaded connectors, then lock them with a jam nut when they're taut and neutral. A dab of thread lock will keep them set.

When you do the rudder, the hinge axis is vertical. Gravity does not deflect it one way or the other. Set the tension on either side so the rudder is at neutral with servo at neutral. Now if you roll the model 90 degrees, gravity will pull the rudder slightly down. Disregard this effect, as it will disappear in flight.

When you do the elevators, roll the model so the elevator hinge axis is vertical and adjust tension same way you did for rudder. Now when the model is upright, the elevators will sag a bit. Again, this effect will disappear in flight (or at worst, you'll need to nudge the trim button a bit).

fastplane

Majortom, those are some good thoughts. Yes, there are threaded connectors at each control surface. I have adjusted the cables on one half of elevator for equal tension at neutral position. Setting the other elevator half to equal tension and neutral has been the problem. So far when one side is neutral the other side is off a small amount enough to disturb the flight. I am not sure I want any noticeable sag in the cables. That would induce slop in the control system. I am trying to eliminate slop. Perhaps I need to be more patient and careful to connect and measure as you suggest. Thanks.

Ernie Misner

The right amount of tension is when you grab the rudder (or ele) and attempt to wiggle it back and forth.... and there is no feeling of slop. If the arms are long enough, it takes very, very little tension to achieve this.

If the holes in the control horn are slightly behind the hinge line, this causes one line to go slack with movement. This doesn't seem to hurt anything though because the force of blowback in flight keeps things tight. Sometimes it is difficult to get the holes exactly on the hinge line, so this is preferable to having them in front where the lines would get overly tight with motion.

I put a drop of CA inside of the pinch tube, but did not go completely through them twice as suggested. That is kind of a pain, and perhaps necessary when fishing for 200 lb. halibut.

Ernie

feep

Fastplane, when setting up pull/pull systems I think the most important factor is keeping the geometry equal. For instance, if your servo is off center and you come directly off the servo to the elevator than the distances are not equal and you will not have equal tension. When off center installs are neccessary they use tillers, which are free moving arms which have the pull/pull lines coming off to the control surface and are actuated buy control arms from the servo.

On the elevator end,If your horns are not centered over the hingeline than you will have slack on the non tension line. This is not a problem as Ernie stated unless you have a full flying elevator. This problem has a name it's called "ackerman".

I have split elevators on a 1/4 scale Fokker Eindecker and I use a servo for each with a tiller for each. Works for me now but I had the ackerman problem and in the neutral position the plane would porpoise. In up or down I had no problems because of "blowback".

It seems to me that you don't need top be off by much to have a tension problem.

fastplane

Thanks Feep and all the others that have given me some help.
I think I am understanding what some of the problems are and how to avoid them now.
Things are looking much better now on the plane.

chris_in_montreal

My Feedback: (1)

This link really helped me understand the importance of correct geometry in pull-pull setups:

[link=http://members.cox.net/bdfelice/]pull-pull geometry[/link]

tommy s

My Feedback: (55)

The easiest way to get the cables at equal tension and everything centered is
to not cut the cables at the servo end. Make one continuous loop from the rudder,
or elevator, control horn through a crimp sleeve at the servo arm, then through
the servo connector, back through the crimp sleeve, then back to the control horn
at the rear. Adjust the tension and center the control surface before you crimp the
sleeve at the servo. Simple and quick.

tommy s