elmshoot

My Feedback: (6)

There ought to be a website that will allow you to imput the nubers and have it tell you how long to make the arms etc offset etc..
Excellent discussion!
I would prefer to cross my lines and you say that will affect the Ackerman?
Sparky

Bozarth

My Feedback: (15)

Figure 1 is with control horns on the hinge line...no tension or slack on the non pulling side.
Figure 2 is with control horns behind hinge line...slack on the non pulling side.

Larry882

To Speedracerntrixie,
I'm looking at the picture of your tiller arm and hardware. I see a red, looks like a 4-40 ball link, and a 4-40 threaded eyebolt, some looped cable and a metalic colored blob. What is that blob? It looks like lead. Did you make it?
Please answer this question.
Thanks,
Larry

speedracerntrixie

My Feedback: (29)

Sparky, crossing the cables will affect the Ackerman. How much depends on how long your fuse is and the width of the servo and control arms. Larry. what you see is indeed a red 4-40 ball link and a Sullivan 4-40 eye screw. in this case I used Kevlar cord that is nylon coated. The aluminum blob you see is a length of aluminum tube that the Kevlar cord gets looped into and then it gets crimped 3 times 90 degrees from the previous crimp. A couple drops of thin CA finishes it off.

Larry882

To Speedracerntrixie,
I have been using tennis racket Cat Gut for the cables. It comes in different colors and doesn't stretch. It is 1/16" diameter. For the crimps I use a 3/16" solid aluminum rod with 2 .067 holes drilled length wise on like .039 centers. I make them in a simple fixture in my milling machine vise and use my digital readout to locate the holes. I use 2 per connection or 8 of them per plane. I have never seen them stretch or come loose. It is a pain in the ass to install though.
Larry

sjhanc

My Feedback: (3)

I know you 3D experts won't like this but I first used it 15 years ago when I had the same problem as Roo Man with my first 3D plane when the cable runs in the ARF didn't line up. The pull pulls are Sullivan Carbon Fiber Reinforced control rods. They don't have to run straight as the outer housings keep them off the structure. They can be fine adjusted with very slight tension and they never go slack when the temperature changes like steel does, and they don't stretch like fishing line does. I used the 2-56 size here but you can use any size you want. The particular parts in the picture have been installed in three different airplanes over the years.

Bozarth

My Feedback: (15)

The mathematics/geometry would disagree...crossing the cables are still symmetrical and do not affect cable length differential, nor do width of servo and control arms. It is only due to hinge offset. Hinge offset is the only non-symmetrical part of the system.

Kurt

speedracerntrixie

My Feedback: (29)

Kurt, I understand what you are saying however when I would try to do a setup with different length arms or cross the cables I did not end up with satisfactory results. That's why for the past 10 years or so I have been doing everything that way I have illustrated. I suspect that it has something to do with having the cables at 90 degrees to the tiller arm and control horn. If you think about it having the cables crossed will result in the angles at which the cables meet either end to not be symmetrical between the two sides. This lack of symmetry has to result in one cable having less tension then the other unless Ackerman is applied correctly. Brings us back to where we started and that is by keeping everything on the center of rotation ( hinge lines ) and arm of equal length and non crossed cables makes setting up a pull pull much easier.

Bozarth

My Feedback: (15)

Hey Speedracerntrixie,

I hear you too. Can't argue with experience! I like the mathematics and geometry challenge. It simply comes down to 1) the x-component of movement of both servo arms being equal with each other. If they aren't, you will have slack/tension issues OR 2) the x-component of movement of both control horns being equal with each other. If they aren't, you will have slack/tension issues.

"Brings us back to where we started and that is by keeping everything on the center of rotation ( hinge lines ) and arm of equal length and non crossed cables makes setting up a pull pull much easier."
AGREED!

Cheers,
Kurt

Lone Star Charles

Kurt - What you posted (drawing) does make some intuitive sense; however, it is not correct. Speed is correct in saying that anything other than a parallelogram setup will require some 'tweaking' of offsets in order to avoid having one line either slacken or tighten. Installing lead-out guides that bend the cable as it passes through the side of the fuselage further complicates the issue. It's not usually a big deal and so most of us just live with it.

Bozarth

My Feedback: (15)

Hey Charles,

Where did I go wrong? A symmetrical trapezoid mathematically works. It simply changes the ratio of servo movement to control surface movement and is still symetrical...please let me know.

Kurt

Bozarth

My Feedback: (15)

For those interested:

Cable pull distance needed to deflect a control surface 30 degrees using a 1" long surface control horn installed 0.25" aft of the hinge line:
=offset + sin30 - offset(cos30)
=0.5335"

Cable slack needed to allow a control surface to deflect 30 degrees using a 1" long surface control horn installed 0.25" aft of the hinge line:
=sin 30 - offset + offset(cos30)
=0.4665"

This means you will have 0.067" of slack cable on the non-pulling side. It does not matter if the control horn is the same length as the servo horn, as long as both are symmetrical (both sides at the servo must match and must be 90 degrees at the servo as not to create differential).

We can scale this to any deflection angle, offset, or control arm length. I can post the diagram if anyone really cares - but High Plains might destroy my proof!

Kurt

Lone Star Charles

Kurt

It does seem as if a symmetrical trapezoid might work, but it doesn't. If you have some drafting software like AutoCAD, draw some examples and then measure the aligned distances between the arm attachment points - you will discover that the deflected distances are not equal to the neutral, non-deflected distances. Give me an example - servo arm length, control arm length, and distance between the two - I will draw it in AutoCAD and dimension it. I don't know if I can post it, but I can at least describe the result.

Charles

Bozarth

My Feedback: (15)

Hey Charles,

Sounds like a fun idea.

See what you get with the following info.
Servo arm length for each arm: 1"
Servo arm to rudder control arm: 24"
Control horn on the rudder hinge line (no forward or aft issue until later)
Rudder control horn length for each side from rudder center line: 2"

This should be a symmetrical trapezoid with a 2" side, two 24" sides, and a 4" side. Servo screw to rudder hinge line should be a little less than 24"
Deflect the servo 30 degrees and then 60 degrees and see what you get at the rudder. I'll do the old fashioned pencil and calculator method. More fun than working!

Kurt

Lone Star Charles

Hi Kurt,

I have completed the drawing using AutoCAD. As you can see, the arrangement that you describe will result in the lines getting a little to a lot tight. Although I am not very good at this, I will try to post.



Yes, it was kinda fun.

Have a Merry Christmas.
Charles

Bozarth

My Feedback: (15)

Charles,

Awesome! You've convinced me. I wish I knew AutoCAD! Great post.

Kurt

Roo Man

My Feedback: (1)

Lone Star,

Great drawings.

The $64,000 question [showing my age, LOL] is how do you make pull-pull work??????????????

Gary

Lone Star Charles

Roo Man

That question was answered pretty well in the first few posts by Speed and Leroy. If you make all of the servo arms and the rudder arms of equal length and if you can draw a straight line from each cable connect point that passes through the hinge then you have a parallelogram that will work perfectly. Anything else will require tweaking (offset of the connections) else you will have a system that either gets slack or becomes too tight. I'm too lazy to do the math, so I usually just draw it on AutoCAD until I get one that works pretty well and then I install that.

If you can put together a sketch of your fuselage assembly, along with dimensions of the distance from the servo post to the rudder hinge, and the arm dimensions, I'll try to make you a drawing like the one above.

Merry Christmas
Charles

Bozarth

My Feedback: (15)

I haven't yet got my brain around the trig as to why.

Kurt

Roo Man

My Feedback: (1)

Lone Star

Not a very good scan.

Tried to give you all the info you might need.

Forgot to note the rudder arm width. It's 3 1/8' same as servo arm.

Not possible to run down side, must cross cables.

Thanks for your help.

Will send PM with my phoned number incase you should want it.

Thanks,

Gary

elmshoot

My Feedback: (6)

What we need is a servo arm that we can infinitely adjust the Ackerman on.
Sparky

speedracerntrixie

My Feedback: (29)

if you use a screw type rudder horn it can be bent in the direction needed. You really only have to have the adjustment on one end. I once suggested an adjustable tiller arm to Scott at SWB. The said they had tried to develop one but could not come up with a design that would keep its adjustment.

Lone Star Charles

Roo Man

I started the drawing. Please double check it before I go further. Gotta go get grandchildren. Will have to put off the rest until Monday or Tuesday. Please review for correct so far.

Thanks
Charles

Roo Man

My Feedback: (1)

Charles,

Did you plan on showing the slots? If so you need to know that the fuse narrows to Just 1 1/4" at 24 3/4" from servo c/l. The slot is noted on the dwg it's the 1 7/16" dim. Starts 19" from servo ctr and ends at 20 7/16". Front of slot is at approx 2" width of the fuse the rear of the slot is approx. 2 1/4". Everything else is right on.

Sure do appreciate your efforts.

Gary

Lone Star Charles

Check this for accuracy. If correct, I will continue.