Trax540

My Feedback: (1)

When setting up Pull pull elevator is it imperitive that the servo be on the same axis and also centered?
Or is there an alternative placement and routing of the wires.

Any help is appreciated

KitBuilder

I believe what's most important is the distance between the two wires being the same at the servo end and surface end so that they are parallel. Having the servo arm, wheel, etc.. in alignment with the horns is also good practice. The farther you get from opitmal alignment, etc the less mechanical advantage and more difficult to tune it will be. This is just my understanding. I have set up pull pull on rudder before but that's it.

KitBuilder

I believe what's most important is the distance between the two wires being the same at the servo end and surface end so that they are parallel. Having the servo arm, wheel, etc.. in alignment with the horns is also good practice. The farther you get from opitmal alignment, etc the less mechanical advantage and more difficult to tune it will be. This is just my understanding. I have set up pull pull on rudder before but that's it.

There is much info here:
http://www.mindspring.com/~rellis2/rcpattrn/ppull.htm

Mode One

I disagree with kit builder. If the servo arm is smaller, you simply wont get as much throw as you would if the arm was the same or bigger. There is mechanical advantage (more leverage) to a smaller servo arm, which is a good thing, if you end up with the amount of throw you need. Having the elevator servo upright is o.k.

Lnewqban

My response to your questions is yes.

Both must be in line and symmetry; rotated positions around that line are non-critical.

Check this out:

http://members.cox.net/bdfelice/Ackerman/ackerman.htm

Mode One

What a great post, full of information, Thank you for making this information available! I had never heard of the term "Ackerman" before and I'm sure most of the Ackerman's of the world have never heard their names being used like this either! The Author states in the beginning that the term being used here, is a misnomer! When discussing this information previously (servo geometry and control surface throw) I've always heard this to be described as differential.

Trax540 you do not want any "Ackerman" or differential in your system, as far as the geometry of pull-pulls are concerned.

Trax540

My Feedback: (1)

If i understand that link correctly, I can incorporate a SMALL amount of ackerman into my pull pull just to insure that the cables do not tighten while deflected, but are taunt at Neutral, Using that method for setup will not be an issue ???

Is that correct?

da Rock

A question often asked about pull/pull.....................

Since both the servo arms and the horns rotate, why would ackerman be needed?

As long as the distance from the center of rotation to the connect points for both the servo arms and horns are equal, ackerman isn't needed is the answer usually given.

Lnewqban

Trax540,

You can use it or not, just try it if a regular set up gives you problems.

Many problems with pull-pulls come from slight misalignments’ and different lever lengths’ for servo and horn.

Here is another good link with examples and pictures:

http://members.cox.net/bdfelice/Pull-pull/pull-pull.htm

Trax540

My Feedback: (1)

Well at the servo the attachment points are 1 1/2 " apart, so ive modified my Control horns and the clevis will attach the same 1 1/2"
apart.
Also my servo is centered in the fuse so if i align the horns onto the hingpoint center i should in theory have exact throw each direction. As long as i understand everything correctly

jaka

Hi!
Servo arm doesn't have to be same length as the rudder arm. But...it is vital that the rudder arm (wire attaching point) is not behind the pivoting point at the rudder. Otherwise the non pulling wire will tighten more than the pulling wire!

Jetdesign

My Feedback: (8)

From what I gather, Ackerman is optional, and it 'playing it safe'. It is ideal to have the cables stay taught throughout, staying at one constant tension. It is very difficult to do this, so to avoid having the cable get tighter (which is BAD), people employ the Ackerman principle to have one cable slacken just to be safe.

Trax540

My Feedback: (1)

By behind the pivot point i would assume you mean AFT of the hinge line?

Please clarify because i thought i read that Aft of the Hinge line is ok but forward of it would cause the tightening of the lines during deflection.

Lnewqban

If your levers are equal, there is nothing to worry about.
Right over the hinge line will be just find, sorry for the confusion.

Check the schematics:

jaka

Hi!

Front of the hinge line or right over the hinge line is Ok ...after is not!

Rodney

As one who has and still uses push pull on all my big models and most of the small ones, I can conscientiously give you the following advice that has always proven correct.
Number one and probably most important; use a bit of positive Ackerman.
Do not over tighten the lines, have them just barely taunt at neutral, thenon pulled line will slacken a bit as you pull off neutral.
You can have the arm length different between the servo and the surface, either way depending on whether you want more or less throw. Just make sure that, if the servo arms are the longest, you do not try to move the surfaces more than about plus or minus 90 degrees and even that will be a bit of a strain.
The servo and the surfaces do not have to be in the same plane or even on the same axis if you use good guides. I use nylon tubing to route the cables around and through obstructions with never a problem in either accuracy or ease of control. The secret is to have the guides well anchored where the lines enter them at the servo and also where they exit the fuselage at the tail as well as anywhere in between where you are changing direction. I use pull pull on both elevator and rudder as it is so much easier and more precise than push rods are, also lighter and more durable and completely free from vibration. If you are in doubt about Ackerman, Google the term, there is lots of info available.

Lnewqban

That is new and very interesting to me, Rodney.

Could you explain more about the material and diameters of the guides, cables, lubrication, adjusters, etc.

May be showing us some pictures?

Thanks

Trax540

My Feedback: (1)

Great information everyone exactly what i was hoping to come across.

Rodney i would like to machine some miniature pulleys to route the cables through the Fuse. Its great to hear i dont need to keep the servos on the same axis as i will now use dual servos for the Elevator.

Thanks very much everyone!!

jaka

Hi!
I agree fully with Rodney!
But I don't think Rodney speaks about pulleys...? At least I don't use pulleys in my planes! The only thing I do is to use thin (2mm) plastic Sullivan wire hoses as guides where the wires comes out of the fuselage.

Rodney

I wrote a rather detailed description of how to do pull-pull in the May 1993 issue of Model Airplane News. That item was also reprinted in their "Radio Control Airplane How To's Vol 2". The technique is still very appropriate and I think the Volume 2 How To is still available and well worth it as it has many good articles in it such as "21 ways to mount servos", "Biplane Secrets", "Flutter - - Causes and Cures" plus much more, well worth the price. If you build large models, you will get your monies worth even if you do not use the Pull-Pull information.

Lnewqban

You are welcome, Trax540; just keep us posted and show us pictures of the installation later at this same thread.
Could pulleys be made of hard wood with bronze bushings?
Remember that they will create a sideways force against the fuse frame.

Check this animation out:

http://en.wikipedia.org/wiki/Aircraf...ontrol_systems


Thank you very much, Rodney; it seems you have been cutting balsa for a while!

Trax540

My Feedback: (1)

Do i have to use Bellcranks before the control surface to keep tension off the servos??

Or can i run the cables directly from the servo, to the rudder, and Elevator, routed through the nylon tubes.

Ideally i would like to keep all my servos upright and just route the cables as you explained through tubing, for exits.
as i took into consideration the sideways force on the fuse as id rather not have that extra strain.

Lnewqban

Trax540,

I would keep it as simple as possible.

Pulleys will not create a lot of stress; we are talking about just the force created by a control surface deflecting in the air.
Just be aware to reinforce the point of installation accordingly.
The good thing is that the whole system can be tested for flaws before any flight.

I like the idea of the nylon tubes, but I couldn't find pictures of such a system, nor the magazine of reference.

Here you have some more info of what can be done:

http://web.archive.org/web/200702050...d/pullpull.htm

http://web.archive.org/web/200702052.../pullpull1.htm

http://www.mindspring.com/~rellis2/rcpattrn/ppull.htm

http://www.swbmfg.com/

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

http://www.hpcgears.com/calc.htm