Mike99

I have a SIG CAP 231EX w/G23 gas that I got from a friend. I am now going through all the linkage before my first flight with this plane. He had VERY mild rates set up on this plane, and had a lot of the geometry of the servos off. My question is this. On a pull pull rudder system. How do you keep the wires at the same tension throughout the full movement of the servo? I now have both ends (rudder and servo) at the same distance from the centerline, but at the recommended three inch throw of the tail one wire is still sloppy loose while both are tight at the centered position.
Thanks

majortom-RCU

My Feedback: (40)

As long as the active cable--the one doing the pulling--is taut, it's ok for the non-active cable to go slack. When the plane is in flight, the airstream (or even propblast) will try to blow the control surface to neutral, and all you need for control is sufficient tension on the pull cable. Having both cables remain taut throughout the control range is unnecessary, and more trouble than it's worth.

JimCasey

My Feedback: (1)

What majortom says is accurate, but there's more.
As you said, the geometry of the servos is off.
at the neutral position, the servo arm should be perpendicular to the centerline of the two pullpull cables. Generally this means straight across the airplane fuselage. Same with the control horn, but that MAY mean that you should find a way to tilt the control horn a bit to make it perpendicular to the actual force if the fairleads are offset below the stab, for example. The cables should run straight as possible, and exit the fuselage through fairleads (low-friction guides) Each cable should attach the same distance out on the servo arms (end hole preferred). Each cable should attach the same distance out on the control horn, but this distance does NOT have to be the same distance as it was at the servo.

Here's the trick part: If you draw an imaginary line through the holes you use in the control horn, it HAS to pass through the hinge line of the control surface. If the control horns are mounted too far from the hinge line (this is common), then the geometry is off and slack cable really does go droopy.

Here's what majortom left out: If your geometry is off and you get a droopy cable, you MAY get differential motion of the control surface. It may even be differential at low deflections with the end points coming out the same. Whether this is a problem depends on how accurately the airplane responds to what you tell it.

bgi

My Feedback: (7)

If it's not possible to line up the control horn holes on the hinge line, it's best to have them slightly BEHIND the hinge line. This enables the non-pulling side to go slack during deflection - and that's OK. Not perfect, but OK.

If the control horn holes are IN FRONT of the hinge line, the line tension will increase with deflection. This may break things. Not good!

bipeaddict

This is also know as Ackerman. Ever wonder why your car is able to make a turn out of your driveway? The first cars were not able to make the turn. For a full discussion and the impact on pull-pull setups see.

http://users.ids.net/~bdfelice/ackerman.html

RCPilot100

My Feedback: (4)

I've been using 200 oz servos on the tail - now Volz has a 300 oz one out - gotta order one - takes the work out of pull pull - just install one on the side and all is well. My buddy just put a Volz in his Edge - real nice way to do it.

Dan

Holeshot

Pardon me, bloke, but the URL:

http://users.ids.net/~bdfelice/ackerman.html

is a dead link.........carry on.


BTW


Google search

rogerswin

My Feedback: (2)

Hi Mike,

In your post when you say "wires", are you using ridged wires or a flexible cable? If using ridged wire you might want to replace it with a flexible cable system. Forget all the geometry about working out your pulls. Replace your servo arm with a cable pull-pull wheel similar to that as mfg. by "Hanger 9" and others. Run a continuous cable from one control horn (or linkage) around the servo wheel to the other control horn (or linkage). The slack is taken up at either end. Follow the directions with the servo wheel by using the clamp screw to keep the cable from slipping on the servo wheel. Simple and very positive in either direction.

Roger

ps
It should be noted, that it is still a requirement that the control horns be on the hinge line.

Prboz-RCU-delete

MMM interesting topic. Correct me if i'm wrong. If you have your servos connections the same distance from the center line of the servo, and the connections centered across the servo, and at the tail the same thing centered both ways you couldn't get a loose cable. if the servo pulls the cable forward 1/8 " then it is going to release the other side 1/8" so therefore you shouldn't be getting slack line, because no matter how far out you have your control horns (or in) it is still only going 1/8" both sides. You could only get a slack cable. you could only get a slack cable if the connects aren't across the pivot point of the servo or tail. by moving forward of the pivot point or back from the pivot point it is possible to get slack or even overtightened cables.

my 2c worth.

Prboz

bipeaddict

Well Gee, I guess it is a dead link. Sometimes that happens. Maybe bdfelice will find a new host. He has an eight page explanation. I will summarize.

Ackerman is rotary to linear differential. Prboz you are correct if everything is at 90 degrees the movement will be the same. So if the control horn is exactly over the hinge line and the control surface is square, not beveled, and the servo arm is at 90 degrees you are correct. If the horns are slightly behind the hinge line or your surface is beveled you will get positive ackerman. One of the cables will go slack. What you don't want is the horns in front of the hinge line (negative Ackerman). Both cables will go taunt and tear something up.

So the goal is not to have slack in the system. The goal is to have the system NOT tighten as the control surface moves away from neutral. However, having a little slack doesn't hurt anything.

Another way to see Ackerman in action is on a airplane with a single servo controlling the ailerons. Notice that each linkage is attached to a servo wheel forward of 90 about 30 degrees. That induces mechanical aileron differential. The surfaces move different distances.

WYDAH

Hi,

I have the bdfelice article bookmarked as:

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

Very good information.

Rgds,
-Fitz.

Holeshot

There ya go....good link. Most web pages dealing with Ackerman are in automotive terms and examples.
I just read the "jwatwood" explanation of Ackerman and was reminded of a bad driven sprocket that I installed on my 750 Honda a couple of decades ago. The center hole was off center, so when you rode you got the strangest oscillations as the chain went loose then tight.