beachbrada

My Feedback: (17)

How do I get the slack out of the wire opposite of the one being pulled?

pimmnz

Shorten it.
Evan, WB #12.

rmh

It is a geometry thing - and best bet - review the geometry with a sharp high school kid.
apparantly your geometry is off -

Rodney

Unless it is excessive, you do not want to get rid of it. You have positive Ackerman (a desirable trait for pull-pull). As mentioned above, it is a matter of geometrythe relationship of the hinge points relative to where the pull-pull lines are attached. Google "Ackerman" and delve through the list for where it applies to RC for more info. You may even find it if you do a search for Ackerman in these forums. Without some positive Ackerman, you will strain your servos and could even cause problems in excessive current drain at high surface deflections, even worse, negative Ackerman and you will have some Ackerman effect unless you managed to get every hinge point and attach point absolutely perfect (an almost impossible task).

rmh

The geometry thing is easy to setup IF you first take round servo wheels - one at the rudder pivot (hinge line) and one on the servo.
IF -all holes used in both wheels are not in same relative positions, the distance from servo to rudder connecting points will change

On a 40% model , as much as 1/8" offset on one hole will cause a noticeably uneven setup.
You can rig up a test setup to proove this, on carboard using wheels and some pin pivots and two lengths of piano wire to simulate the cables.. .

cyclops2

Why not use a continous loop of 20 # test Dacron fishing line that has been snugged up when everything is set to 0 degrees ? Then lock the Dacron in place with a extra loop in any hole & a TINY dot of CA on the extra loop threads.

Works for me.

Tall Paul

On nylon fish line pull cables, this "Spanish Windlass" process can tighten up the slack side.

rmh

I saw no mention as to the size of the model-
typically for aerobatic setup , slack of any sort is not desireable.
A very good cable source , is nylon coated leader -used for fishing . It is available in a wide selection of diameters. for very small models Spiderwire will suffice.

cyclops2

I have no idea how the Ackerman effect ( more travel in 1 direction than the other ) can take place if you do a Zero Setup with the servo horn at a 90 degree angle to the fuselage centerline & tail control horns also at a 90 degree angle to the fuselage centerline.

Equals always give equals.

If you do not set the transmitter computer to 0 offsets or the trim levers to 0 then you can build in a offset. True. some servo arms will not give a 90 degree angle to the centerline. Cure is to make a VERY THIN wood wedge to get it perfect. Is that being too fussy ? Maybe. But none of my planes fly with any trims. Computer or trim lever. I can hit DEFAULT all day & no plane changes trim.


Edit. I do have 1 plane, the Martin China Clipper that DOES have a built in Ackerman Effect do to a VERY HIGH elevator. Still did a Zero Setup for equal throws.

Mr Cox

The servo arm should of course be at 90° when centered so that the motion is symmetric, Ackerman serves a different purpose here. The rudder horns are intentionally not rotating around the exact center of the rudder hinges (instead it is further aft), so that the line that isn't pulling will give a slack. This way, it can be setup to have zero slack around the center position (where it is important) and a little slack when it doesn't matter, i.e. at large deflections.

cyclops2

Thank you explaining how to get the Ackerman Effect.

I have never in 55 years ever put a control horn..... NOT ON THE CENTERLINE of a hinge joint.

That is where I am in error.

I am NOT going to change now to join some others.

Put it on the center line of the movable surfaces hinges and life is simple.........No excuses.

Rich

rmh

Relax - the slight loosening of the non pulling side is desireable
We have setup countless (it seems) pull pull for competitive aerobatic stuff. Still doing it.
55 years?
I started in 1949 . Did a control line stunter with a Hassad Blue streak for my class project in shop.
also how come no one asked about relative arm lengths?
Hmmmmmmmmmmm?

cyclops2

All my relatives have the same arm lengths.

cyclops2

I have always pre-loaded the lines with 1# of tension to stop any chance of flutter due to plastic stretching from winter to summer temperture changes.

Rich

Rodney

The two most common mistakes that cause people to complain about their pull-pull systems is 1: No positive Akerman and 2: to much tension in the lines at neutral position. For those of you who do not know about Akerman, there is a wealth of information on it on many of the RC forums (use the search feature) or just Google "Ackerman" and sort through those sites related to RC. It's not complicated and can be the difference between a smooth well working pull-pull system and one which eats up battery life and wears out servos.

JimCasey

My Feedback: (1)

Did beachbrada ever come back? Or did he just push the doorbell button and run away?

Anyway, for a truly accurate linkage, everything should be stacked along the same centerline and more importantly, the linkage must be symmetrical.

If the servo shown in the illustration were an elevator servo mounted off center from the elevator centerline, then the control horns would need to be offset so that the cables are perpendicular to the ACTUAL lever arm at zero deflection. AND a line drawn thru the pivot points of the control horn must pass thru the hinge point.

That's the illustration with the green control horns. That would be extremely impractical and it's a major justification for computer radios.

Rodney

Again, I see some comments in some of the above posts that are not really correct. As one who has been setting up pull-pull on both elevator and rudder on dozens of large (1/4 scale or larger) with success, I will just mention some of the false statements.
You must always have equal lengths on the servo and on the horns on the surface: definitely not true, what you have to make sure of is that the degrees of rotation of the driving arms does not travel so far that the moveable surface horns are rotated past 180 degrees. i.e. as long as the surface being moved does not move past the point where the non pulling line starts to tighten. It is easy to demonstrate this with a compass and a sheet of paper, just draw the respective arcs and measure the distance between the fixed points on the arc for different degrees of throw.
You must always have the attachpoints at right angles to the hinge points of the surface being moved: again false, what you want is to have the attachpoint offset from the hinge point (that is how you get the desired positive Ackerman effect). Realize that if you get the attach point on the wrong side of the hinge point you will get negative Ackerman which can be very bad as both lines try to tighten as you move the surface away from neutral.
You must have straight line shots between the servo and moveable surface: again wrong, if you use the proper guides you can change directions a great deal to enable routing the pull-pull lines around structure or to make exit points in a better location.
It just takes a little common sense and a few sketches to set up a reliable pull-pull system that allows you to make the best use of available space for servos, unusual routing of lines etc. If in doubt, connect a dummy setup of what you want on a mockup which is easy to make with some scrap wood, some cardboard and a little time.

fokker fan

Yep, you're quite right. Have a look at the control runs on early fullsize aircraft (IE WW1) - aileron cables routed from the stick out through the wings, and tail control cables on pusher aircraft (IE DH2) routed out around the prop to the tail.