Jan 31, 2013, 01:11 PM Registered User Germany Joined Sep 2010 80 Posts pull-pull crossed: how calculate bell crank? Hi, this is the first time I build an airplane with a pull-pull rudder and the wires crossed. As this is supposed to be the first time I build my own rudder horns using fiberglass I had a hard time trying to find out how to calculate the ratio of rudder horn/bell crank and bell crank offset. Tried so solve this problem geometrically using a drafting compass but I didn't find a solution. I also found out that the whole problem is covered by this Ackerman geometry stuff—but does anyone how to actually calculate this in order to get minimum wire slack? There must be a solution, or not? Given rudder horn width and distance to servo there must be a way to calculate this stuff, or am i wrong? Stefan
 Jan 31, 2013, 02:42 PM Illinois Joined Sep 2001 14,689 Posts You really DON'T WANT minimum wire slack! You want some differential so that the pull is pulling but the "push" isn't pulling back! If you make them equal, you'll almost be there. Make it so that the arms are slightly to one side of the pivot point, just 1/16" or so is usually enough. This will allow the one "pushing" to move slightly more than the one "pulling" - and if you get it wrong, rotate it 180 degrees and then you WILL have it. Andy
Jan 31, 2013, 02:47 PM
Registered User
Germany
Joined Sep 2010
80 Posts
I think I found a graphical solution. It seems that one of the crossed wires has projected to the line between hinge and servo center by 90 degrees. This is the point where the servo axis has to be placed.

But maybe one of you knows a mathematical way to do this..

Stefan

# Images

Last edited by sroge; Jan 31, 2013 at 02:54 PM.
 Jan 31, 2013, 03:45 PM Registered User Florida Joined Aug 2004 3,667 Posts Whether or not the lines are crossed has little to no effect on horn/hinge placement; it is the same either way. Just make sure you do NOT have any negative Ackerman, some positive is, IMHO, good as it guarantees that you will not have the non pulling line get tight and stress the servo. The radius of the point the lines attach to the servo and the radius of the point where the lines attach to the horns does not have to be the same. As long as you do not pull the moveable surface more than plus or minus 90 degrees (an extreme that should never be approached) there will be no problem in unequal radius between the two.
Feb 01, 2013, 02:08 AM
Registered User
Germany
Joined Sep 2010
80 Posts
Ok, thank you! It really seems that having slack (positive Ackermann) does not be something bad. But my first thought was: what happens during a tail slide?

But at least from math point of view this thing was quite interesting to me—unfortunately I could not find any information on this specific subject. So finally i figured out a mathematical solution for crossed wires that even works with different radius of horn and bell crank. Horns have to be aligned to the hinges.

So I got these parameters:

rb: half the span of the bell crank (this is actually not radius)
d: distance between hinge line and bell crank line
o: bell crank offset

So the equation

o= rb * sin ( 90 - atan (d / ( rh + rb ))

So for example with (mm)
rh=30
rb=20
d=800
you get an offset of 1.25 mm

Applied to another "experimental setup" from
rh=67
rb=38
d=254
I get an offset of
14.5 mm

This geometry is quite unrealistic, but it shows that it works.. It is also quite obvious that a longer distance between the hinges and the servo makes the slack effect more neglectable. But the closer both are to each other the more significant the offset will be as related to minimizing slack.

Of course the equation has to be transposed to have the servo pivot in a fixed place and determine offset and wire length. Now as it is the other way it is quite impractical.

Stefan

# Images

Last edited by sroge; Feb 01, 2013 at 02:27 AM.
Feb 01, 2013, 06:46 AM
An itch?. Scratch build.
South Wales U.K.
Joined Mar 2003
10,497 Posts
Or don't cross the wires to start with, (if possible of course).

By using a secondary bellcrank, any load on the servo has gone, plus the servo can drive it either side if restricted room is a problem.

# Images

 Feb 01, 2013, 08:30 AM AKA "Fingers" United States, IA, Rockwell Joined Jul 2011 2,158 Posts This might make your head hurt, but if you can sort through the "garbage" there is some good info. I'm kind of intrigued by using the pulley system. No slack at all..... if set up right Ken http://www.rcuniverse.com/forum/m_10...m.htm#10852484 Last edited by kenh3497; Feb 02, 2013 at 10:02 PM. Reason: corrected the link
 Feb 02, 2013, 03:43 PM Jim C Patrick Shenandoah County Joined Jan 2008 631 Posts Ken, your link got mashed up. I think you meant this one. I had a similar idea to use pulleys (actually a capstan) I posted here some time back. The RCU link says 'typical setup' on a sailboat, so I guess my idea wasn't so original. But the concept has a lot of advantages and is 100% linear, so I still plan on using it sometime. The straight servo-to-rudder pull-pull system is fine as long as Ackerman is kept in mind, that is: the pivot point on the rudder should be a tiny bit behind the string holes, and the distance between string holes must be the same at rudder and servo. This is especially true if you use crossed strings like the OP wanted to do.
Feb 03, 2013, 08:57 AM
Registered User
Florida
Joined Aug 2004
3,667 Posts
Quote:
 Originally Posted by jcpatrick Ken, your link got mashed up. I think you meant this one. I had a similar idea to use pulleys (actually a capstan) I posted here some time back. The RCU link says 'typical setup' on a sailboat, so I guess my idea wasn't so original. But the concept has a lot of advantages and is 100% linear, so I still plan on using it sometime. The straight servo-to-rudder pull-pull system is fine as long as Ackerman is kept in mind, that is: the pivot point on the rudder should be a tiny bit behind the string holes, and the distance between string holes must be the same at rudder and servo. This is especially true if you use crossed strings like the OP wanted to do.

The second comment "distance between string holes must be the same at rudder and servo" is not true. No harm if different as long as at maximum throw the moveable surface is not moved so far that binding occurs.
Feb 04, 2013, 08:10 PM
Jim C Patrick
Shenandoah County
Joined Jan 2008
631 Posts
Quote:
 Originally Posted by Rodney The second comment "distance between string holes must be the same at rudder and servo" is not true. No harm if different as long as at maximum throw the moveable surface is not moved so far that binding occurs.
Math says you aren't right. Be happy to see the proof otherwise.

# Images

 Feb 05, 2013, 04:46 PM Registered User Florida Joined Aug 2004 3,667 Posts Just think about it a bit. If all points are on center (no Ackerman) as much line will be played out as taken in at the power end (the servo). Now, if I pull the same distance on the horn that I play out on the other end of the horn, no problem. I do this all the time and, as long as you do not pull the horn through an angle greater than 90 degrees. Yes, the math support this. This is an easy way to adjust the amount of throw on the moveable surfaces. Now, if you have some positive Ackerman, the non pulled line will go a bit slack as the surface is moved away from neutral which is again, no problem.
Feb 05, 2013, 08:01 PM
Jim C Patrick
Shenandoah County
Joined Jan 2008
631 Posts
Quote:
 Originally Posted by Rodney Just think about it a bit. If all points are on center (no Ackerman) as much line will be played out as taken in at the power end (the servo).
Methinks you are "thinking" wishfully. No math (or drawing, model, or reality) supports the idea that a 1" crossbar will rotate correctly if pulled by strings from a 2" rotating crossbar. As the following shows, Ackerman —putting the driven axis slightly behind the string attachment points— makes no difference.

In the field different sized horns can work: With cheap and flexible plastic horns, rudder horns almost (but not quite) the same size as the servo horn, no hardpoint rudder hinges, not perfectly solid servo mounts (foamy anyone?), and stretchy monofilament strings. All adds up to a lot of slop and on a non-stressed plane it might work fine, but don't think about it for a hotliner or plane that has to hold up to high stress.

For an accurate and workable pull-pull, the distance between string holes must be the same at rudder and servo.