[Eye Can Fly]

I'm aware that there are several ways to achieve the task. There appears to be trends with which method is best, and possibly the application should be considered.

With my limited exposure to what other modellers are using, I figured I'd start a thread to discuss the issue. For the most part the manufacturer has determined where the servo(s) would be, but we all aren't going to use what they deemed to be best.

A classic scenario is where the assembler puts a larger engine in their plane and needs some tail weight to get the CG to where it has to be. I have a plane that is was done to. While I didn't start the assembly on that one, I was the guy that had to finish it!

As I understand there are three main ways to control a rudder:

1. Control rod
2. Pull Pull
3. Push Pull

I have used the first two extensively, and would entertain the third option, but have never seen it in person. Guess I could have scoured the net, but figured it would be a good topic to discuss here.

In an ARF to a certain size the control rod is quick and easy, especially if the plane was built with support for the rod. Some ARFs don't give you much room to get into it's guts to change this at assembly time.

The Pull/Pull has its own set of issues. Travel, and consistent tension as well as cable wear, just to name a few.....

And finally the Push/Pull which I know absolutely nothing about.....

Could someone enlighten me of the positives and negatives of this system? Please!

[Trumpetman]

I've used the push/pull approach in my DA200 powered Dalton. For this application it is perfect: it gets the weight of two servos way back in the tail (needed to balance the quad), the linkages are short and direct (zero slop) and the force is balanced on both sides of the rudder. The downside is you have to spend some quality time ensuring that the servos are working together at centre and all the way through the control throws. It's pretty easy to do with today's servos and radios but it does take some time.

[Eye Can Fly]

Have you got any pics of that Gord?

BTW How many people cross their cables in the Pull/Pull system?

My current assembly needs no additional tail weight (I think)!!!!

[Trumpetman]

https://www.rccanada.ca/rccforum/showthread.php?t=399354

If you check out post #15 you can see a push/pull setup on my friends CARF Extra 330. Again, this was a plane with a big quad up front and the servo weight in the tail was needed anyways.

[Eye Can Fly]

Thanks Gord. Could a Push/Pull be done with one servo?

[Trumpetman]

I suppose it could if you had a double servo arm that exactly matched the width of the rudder control horns. With the servo mounted exactly on the centreline and using identical control rods, it should all work. If the power of only one servo is needed though, I would simply stick with a single control rod operating a single control horn.

[Billpilotca]

Originally Posted by Eye Can Fly

Thanks Gord. Could a Push/Pull be done with one servo?

What size plane are we talking here ?

Fully agree with what Gord has already said. For 35-40% planes push/pull is the way to go and gives best rudder authority and holding power. But does take some fiddling with pushrod lengths, sub-trim and end points to make sure the servos don't fight each other.

For a 50 cc I would just use one good servo in the back on one side and not worry about synching two servos. It in effect becomes push pull because it is pushing in one direction and pulling in the other but not needing two servos on opposite sides.

But it depends on CG and what you need to do to make the plane fly optimally within your comfort level.

[Mike Sebastien]

Simplest easiest way is to put the Rudder Servo in the back. I am going to go against the grain here and say that you only need one servo for the rudder depending on what your flying style is. I have one really good servo on my 40% Carden Extra 260 and I have no blow back issues. Knife Edge loops and snaps are effortless. When I built the aircraft I did make a mount for second servo on the Fuselage for the rudder just in case.

New word "blowback" that is when the airflow overpowers the servo and forces the rudder back to neutral. Often seen in 3D and knife Edge flight

[Billpilotca]

Just saw your other thread re the AW 50cc Ultimate QB. If that is the plane you are talking about then if the CG would allow, I would throw that 7955 in the rear on a direct pushrod connected to the rudder on one side and really I don't think you would have any problems. But depends on the CG. Personally I would choose the rear servo direct pushrod connection and get rid of the pull/pull.

I flew an AW 35% Extra 260 for 5-6 years with pull pull since that is what it said in the manual when I put it together (way back when etc). However it also have the rudder servo slots already available under the covering. After futzing with batteries for several years and moving them backwards to try and make the plane fly more neutral (CG) I finally yanked out the pull pull and moved the servos to the rear. Plane flies better than ever and for sure there is no rudder blow back now, lots of holding power.

But again it all depends on the CG and how the plane flies.

[Eye Can Fly]

My largest planes (35% and 37%) are set up with a single large(ish) servo in the tail close to the work load. Two 30% planes used a pull/pull with the servo near the CG. The 30% planes had the cables crossed to the opposite side horns, which helped keep the tension issue to a minimum.

Bill this is that Ultimate Bipe which of course you have to do all the wiring and rigging through the opening the lower wing usually occupies. Keeping things like the elevator wiring clear of the cables has to be carefully considered. Fuel tank (hanging) always is fun when I'm trying to route my vent line too! Dyslexia isn't your friend when workin' on something upside down. Ha! I usually do stuff a$$ backwards even when its the right way up!!!

I have the cables and servo installed where AW thought is should go. Guess I will get the ignition, switch, UBEC batteries and fuel tank mounted up and see what the CG looks like. I prefer to move things around, rather than add useless weight to balance the plane. The bipe doesn't look like it is as easy to move stuff around as a work through the top mid-wing plane.... will see.