I have read a lot of stuff about pull-pull geometry, ackerman, etc, etc and the need to get everything just right. There is one issue that still baffles me. Cable slack and whether or not it is a real issue.

When your rudder is all the way over, air pressure pushing against the rudder keeps it tight. If you have slack in the other cable it seems that air pressure keeps the rudder taught until it approaches center and the slack goes away. As the rudder swings the other way that cable gets tight and air pressure again maintains the tension so it seems that slack in the non pulling cable should not be an issue. What am I missing or where have I gone wrong. thanks.

in normal flying it is not a problem but when doing acrobatics if you tail slide for instance it would cram your rudder over and might break something.if you have two inch arms on the servo,you need two inch horns on the rudder and also be on the hindge line

I just made a drawing on AutoCAD and What I got is really interesting.

With 60 degrees on the servo each side and a 3" servo arm (1 1/2" each side) and a 3 1/2" control horn on the rudder 48" from the servo (hinge line to servo output spline). The results are 0.009" slack in the cables ----> negligible. This was done with cables "parallel", not crossed. I get about 45 degrees at the rudder.

Now I can use a single 5955 on the rudder of my big plane.

I think if you cross the cables, that .009" becomes .009" of tightening... you would need to offset the control horn behind the hinge-line by .009".

I am using a 3.5" servo arm and a 4" control horn with my JTEC extra. I will use 3/8" of offest in the servo arm and the control horn will be offset .19" behind the hinge line. Should keep things tight. It is 36" from the servo hub to the hinge line.

Could you plug that into autocad?

Assuming the 3/8" offset from the rudder is toward the rudder, not the spinner, you get 0.025" slack in your cables --> really not bad for crossed cables as previous tests showed something like 1/8" to 3/16" slack in crossed, straight servo and control arms in longer-than-36" setups (which diminishes the effects of wrong geometry).

Rules of thumb :
- If you cross cables, use the same lenght on the control and servo horn, all straight. The drawback is that you can't get mechanical advantage without going through was Sylliness did. If you cross cables with straight servo and control horns, you get the worst possible slack (not saying that the slack obtained in innacceptable).

- If cables are not crossed, more room for error and mechanical advantage is possible without fussing too much. but some slack in cables will happen by the maximum throw.

- The farther away you are from the servo the better. It has to do with the angles of the cables relative to each other I think.

I think I would trust what SWB says as the geometry of pull pull systems is not that complicated using a computer...

Do you build with that kind of precision [X(] ??

Oh and one last note, with 60 degrees each side on the servo, you should get about 51 degrees each side on the rudder. This thing will ROCK !!

Splais,

Can you recommend any on-line resources that discuss setting up a pull-pull for a 50cc size plane?

Thanks,

tom

This is a great guide.

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

Also Scott the owner of SWB is an awesome individual. He will spend time on the phone helping you. He flies at our indoor fun flies in the golf dome and he just as nice if you don't buy stuff from him. His products are top notch.

Joe,

Thank you very much. I found the SWB guide quite helpful.

v/r

tom

3D Joy... thanks for the validation of my math... I haven't set it up yet but now I will with confidence. The only reason I'm using a 3 1/2" Servo arm with 3/8" offset is because it's what I have sitting here from Nelson. I designed the rest of the system around the servo arm. I could move the control horn offset slightly toward the hinge line to eliminate all slack (since I haven't drilled the Dubro 866 control horns yet).

I won't buy SWB anymore after he copped an attitude with me on the phone several times. The first time (many months ago) he had an attitude until I volunteered my credit card information... then he was sweet as pie. The second time I had a 1.5 minute conversation gathering the dimensions of the 4.5" offset arm, in which he was short with me, AND he didn't know most of the dimensions. I called back in 10 minutes to check the distance from control horn to servo that the 4" arm is good for, and he about snapped. I don't care who he is or how bad his day is... he's in the business of customer service and is required to treat his customers with respect, especially when he has no idea who is on the other end of the line. Plus I have issues with his pricing policies. Adding $75 to the price of a 2 servo rudder tray to add a couple pulleys and a couple inches of cable (self ajusting setup) is crap. Personally, the man's a crook. On the other hand, I've had no issues with Nelson and he's been very cordial the couple times I've spoken with him. Plus he publishes his friggen specs, so it makes things easy.

Anyway, now that I've exercised my right to free speech I'll go on about my day.

3dd, you hit on something I didn't think of. You are correct, in a tail slide the air forces are acting on the "loose" cable. this could cause an over displacement of the rudder leading to damage.

Tomriddle, if you do a seach on RCU there dozens of posts on this issue.

Splais,

Thank you. I did and you are right. Quite helpful. Your post gave me serval key words that allowed me to successfully "google" some more info from the web.

Interesting that there does not seem to be a comprehensive on-line resource for something that has generated so many questions, opinions, and varieties of set ups.

Happy flying,

tom

There are a lot of guys here on RCU that just can't wait to get their slide rules out .

lol - I was quite impressed at the number of autocad engineers who were 'wrassling' with rotation angles etc.

Perhaps there is a "Red Schofield" equivalent out there with a tutorial on setting up, optimizing and maintaining pull-pull systems <grin>

tom

Ah... you're just jealous. The great thing is that the nugget is multi-talanted... it's great at reeling in the women too.

Next time, try posting something useful.[:@]

Sillyness,

Sorry if I did not post anything useful. Thank you for your input.

v/r

tom

I know this isn't useful either... but where did you get Tom Riddle? Is that your name or is that the evil dude from Harry Potter? I read those books when I was sick for a month in "another land".

I'd like to think that Mrs Rowling owes me a royalty check or something

Here are a couple principles that will keep you out of trouble when setting up a pull-pull system:

1. Parallel members should be equal in length.
2. If the holes on the servo arm are offset from the centerline of the servo, the pivot point of the cable at the control surface needs to be offset from the hinge line the same distance and direction.

Pauly,

Thank you. When does a "crossed pull-pull" configuration make sense? What are the trade-offs?

v/r

tom

I wouldn't be too concerned there. You usually don't do a tail slide with full over rudder. Anything less than full over and the cable slack will then tension up (gently mind you as the plane falls back rather slowly at first) and keep the rudder from doing any serious damage. If you can get the perfect geometry then that's great but I wouldn't loose a ton a sleep over it if you have a little bit of sag in the opposite line.

OnTheEdge said exactly what I was thinking as I read through this thread. Typically you're not going to do a tail slide with the rudder fully deflected and even if you do, the opposing air pressure on the rudder will be very gradual as there is zero air speed when the plane begins to slide. Highly doubtful this will even be close to doing any kind of damage.

Great minds thing alike

I like crossed cables... I think you can get a little more power out of them because of the direction that the cables are pulling on the control horn and on the servo arm... at full deflection on the servo you almost get an "over-center" effect which increases the linear force applied to the control horn, especially if your servo arm is shorter than the control horn (must be a properly designed system). Also, with parallel cables, at 45 degrees rudder deflection 70% of the force exerted by the servo is trying to pull the control horn out of the rudder and 70% of the force is used to deflect the rudder (vector math... 70% + 70% = 100%... gotta love the stuff). With crossed cables, you get a slightly higher percentage of the force trying to deflect the rudder because of the angle between the cable and control horn.

All that being said, I know most won't agree and most will say its negligible... they may be right. The REAL reason... a smaller exit hole and avoiding having to cut into any of the fuselage formers. For me, that alone makes it worth it. You just have to set it up with the right amount of offset to keep from getting excessive slack (you can get a LOT of slack if done wrong).

splais, this seems to be a carry-over form our conversation at dod. I go by just kevin b over there. Does anyone know what "ackerman" is?