Uh huh. Uh huh. Okay. Um, can you repeat the part of the stuff where you said all about the...things? Uh... the things?
Its a H9 .60 CAP! It will not matter if there is a bit of slack or not.

Can you figure out this one as quickly?

"Please resist the urge to curse, flame, degrade, insult or embarrass someone in your post."

Look at post 29. Scale 4 me posted a picture of a "pulley". What is commonly refered to as a pully would be what you see there. Or if you open the hood of your car and look at the the belsts that drive the power steering, the belt goes around the "pulley". Now, according to Browing MFG, (supplier of power transmissin parts to industry) a "pulley" has a flat friction surface with no grooves and a slight crown to help keep the flat belt centered on the pulley. A "sheave" has a groove for the belt to rid in. The shape of the belt can be in a "V" shape, round or have mulitple V's.

A pulley and a flat belt


A Sheave

Not too technical for some of us who enjoy getting to the nitty-gritty technical details that accurately explain the issues - toys or not. Call it a sickness[8D]
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I started playin' with toys back in "55" or so. Don't figure I'll ever stop! It kind of looks like I'm just as sick as the rest of you. LOL

You have to love this hobby. Those school trained engineers don't have a thing over us back yard engineers!! We just get our schoolin' by Hard Knocks

Ken

The irony,,,,

Dooh, now I'm guilty too

I have a neat solution for the worry of the wires touching. Install one of the servo ends as normal but install the other upside down which will give you about 1/2" separation of the wires. It's not a big deal to do this. Try it, you might like it.

Yup.

You've obviously carefully setup some pull pull systems.

The pulley neatly assures that the point the cable is let out is colinear with the point cable is taken in (as well as the pivot center). A decent servo arm attached to a well centered servo achieves the same geometrical symmetry.
At the control surface, the same applies... the attachment at either arm or horn must be colinear with the hinge line.
Also, at the control surface, the spread of the attachement points must be equidistant from the hinge line. (Each of the attach points CAN be 3" from the hinge line, and the spread at the servo can be 20" or 1/2"... Just so long ast the attach points are equidistant from the hinge/pivot line).

The big clue that a pull-pull system is out of geometrical symmetry is that the cable tension is tight.

A well rigged pull-pull system will require just minimal tension.

Once again, you don't have to use a pulley at the servo to get "equal movement" of the cable ("pay out as much line as you retract"). A traditional servo ARM will do the same if symmetrically installed.

Kurt

I think that the point most people miss when the set up a pull pull system is that they do not allow for a little positive Ackerman. Having the non pulled line go a bit slack is an advantage, not a disadvantage. And yes, most set the line up much to tight, you want them just tight enough to not droop at neutral.

I've never had any problems setting up pull-pull using servo arms on my pattern planes.
As metioned previously, it's all about the geometry, get that set and you're sorted.
Control horn/conection point width must be identical to the servo hole width or visa verse.
Use a screw adjustable horn(s) at the controll surface end and adjust it so its Identical to the servo arm width.
Next check the offset. If the conection hole at the controll surface end isn't directly in line with the hinge line( where it shoud be if you built it in corectly) you'll have to offset the holes on the servo end accordingly. Once everthing matches, it's a done deal.

Regarding straight or crossed cables... it's precisly the same deal crossed or straight.
The only difference is the crossed method allows the cables to exit at a slightly more acute angle that can better suite different aircraft design.
Another point worthy of discussion is the use of the tiller bar. Cables connect from the surface to the tiller arm, then a standard push-pull conection to the servo. This method allows you to get a considerable tension on the cables without over stressing the servo assembly/gears etc. As with any pull-pull the geo' will have to be spot on if you don't want slop.

Excellent answer David[sm=thumbs_up.gif]

If the geometry is good and the control horn holes align with the hinge line, the control horn length does not have to be the same as the servo horn.

Kurt

Why is so important that the control horn holes align with the hinge's line?

It is better to have the line go slack on one side, rather than get tighter. Both of my pull-pull setups go slack with rudder input due to imperfections in symmetry.

I still don't see the advantage of the servo 'wheel' other than changing the rate at which the rudder moves.

Call me biased, but I think the best engineers are the ones who grew up with hammers in their hands and ended up graduating from an engineering school.

See drawing attached. Because the horns are not aligned with the hinge line, length A does not equal length B for any angle of deflection. In this scenario, the cable on the non-pulling side will have slack. A couple of trig calculations prove it.[X(]

Kurt

I agree with you completely.

Kurt

Right, slack is better than getting tighter. If the attach point on the horn is aft of the hinge point, that is positive Ackerman (where the non pulled line get a bit of slack as you move off neutral). If in front of the hinge point, the line not being pulled will get tighter as you move away from neutral. Depending on how much offset is present, this can cause the servo to be overloaded and will certainly result in high servo current being drawn

http://www.qmfc.org/school/ackerman.htm

A great article.

Kurt

[sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif]

nuff said

Gentlemen, without wanting to sound like a real *****... Slack is bad!
Can you imagine your elevator/aileron having a slick side? It'ld be unacceptable. Same applies to rudder.
Really, it isn't difficult. Line it up, keep it strainght and even. Job done.

Regarding servo/horn width. Strange things start happening when you use different servo arm-rudder horn width IMO.
Non proportional response being one. That's why I keep everything equal.
I must mention that you can't just use a standard servo arm... Grab some of those large aftermarket, beefed up jobbies.
Adjust the rudder horn to match the outside holes on the servo arm and spend your time positioning the horn so the conection points are over the hinge.

Excellent bit of info. As shown, positive Ackerman is good. That slack in the line not being pulled has absolutely no bad effects on response and does not induce flutter as some have predicted. As one who has been using pull pull on all sizes of models for the last 20 years and on many models I am convinced it is the most trouble free and reliable way to control elevator and rudder control. I've also tried it on ailerons but did have some problems when the wing deflection (spar bending or deflecting under G loads) caused excessive changes in line tension if trying to use a single servo (long lines) for aileron control.

But the article proposes to move the horns aft the hinge line. Why?

I was thinking this 3 days ago

Why Joe?

This is elementary trigonometry that is learned in high school.

It doesn't require engineering studies to understand that rotational to linear movement creates an asymmetry.
That is why pistons moved by a crankshaft have a max and a min translation speed, while the crankshaft keeps constant rpm's.

Using a pushrod is not problematic because the other side is free to move as it pleases.
As long as the pushrod is perpendicular to the lever from the point of rotation, the horn will deflect the same in both directions, without any differential.

Using two cables instead of one pushrod, forces us to compensate for that asymmetrical movement of one side respect to the other.
How do we do it? Introducing another asymmetry, by making the lever from the point of rotation non-perpendicular respect to the cables.

In other words, positioning the holes of the horn in line with the hinge works for pushrods, but not for pull-pull.
Crossed and parallel cables require different geometry, as the schematics below show.
The greatest the distance between servo arm and horns, the less evident it is.
The practical difference in slack and movements may be small and we may live with it, but it is there.

Why the top and bottom sections of a belt moving two pulleys, or a chain moving two sprockets never develop any slack, regardless of the direction of the rotation?
Because the belt or chain is always connected to the wheels at the points at which they are perpendicular to the lever from the point of rotation.