Go Back  RCU Forums > RC Airplanes > Questions and Answers
Reload this Page >

pull pull geometry

Notices
Questions and Answers If you have general RC questions or answers discuss it here.

pull pull geometry

Old 12-13-2003, 01:02 AM
  #1  
kamakasi
Senior Member
Thread Starter
My Feedback: (3)
 
kamakasi's Avatar
 
Join Date: Sep 2003
Location: butternut, WI
Posts: 405
Likes: 0
Received 0 Likes on 0 Posts
Default pull pull geometry

I want to use a pull/pull set up on the rudder of my plane. After doing some research I have come up with some questions.
1: Does the cable spacing need to be the same on the control surface as on the servo horn?
2: How large of a servo horn is required? is a 2!QUOT! arm better than a 1!QUOT!
3: To add Ackerman or not. I have found conflicting views on this. Which is correct? To have the release side go a little slack or to maintain equal tension?
4: Will a nylon servo horn suffice or is a stronger one required?

This is a larger glider a Sailaire and the rudder is about 100 sq."

As always thanks for the input.
Old 12-13-2003, 02:09 AM
  #2  
rflasch
Senior Member
My Feedback: (8)
 
Join Date: Nov 2003
Location: Chico, CA
Posts: 132
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

OK Kamakasi ---I'll take a shot at it...I have pull/pull on a Midwest 300XS, DP Edge 540, and Goldberg Suhkoi

1. No the cables do not have to have the same spacing at servo and rudder, however--you do not want (and usually don't need) a lot of difference that would start to effect the mechanical ratio.

2. On non-ganged servo arms you do not want to EVER get out more than 7/8" on the arm. If you need more throw adjust the EPA or TTA up to 140-150%. It's also a good idea on the surface control horn to use the outermost hole.

3. I don't fly gliders but on our powered planes with the large control surfaces in vouge today, we use at least heavy duty arms and in most cases aluminum servo arms.

4. Ackerman can be debated here, I think it's great on my pickups front end!!! Again I am not familiar with the flight loads of gliders, but on my planes I never want slack on one side of the cables....slack=flutter. The great advantage of pull/pull to me is the preloaded cables always maintaining some tension so that my control inputs are precise and maintainable!

5. I am not a glider pilot nor do I pretense to be so ---so anything I just told you may or may not apply to gliders!!!
Old 12-13-2003, 07:05 AM
  #3  
CafeenMan
Senior Member
 
Join Date: Jan 2003
Location: Spring Hill, FL
Posts: 4,734
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

rflash - OK, I'm confused as to your answer to #1. If the holes are not spaced the same and centered identically around the pivot, then you don't have a parallelagram. If you don't have a parallelagram, then either one side will tighten or slacken depending on which way it is different.

So what I'm not understanding is how you can say that the holes do not need to be spaced the same, yet you don't want the tension to change. What am I not understanding?

I'm not giving you a hard time here. I just really don't understand how this works.
Old 12-13-2003, 08:07 AM
  #4  
JimTrainor
Senior Member
 
Join Date: Oct 2002
Location: Ontario, ON, CANADA
Posts: 1,309
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

I've only setup one pull rudder...

I concluded that the advantage of adding a tiny bit of positive Ackerman is simply to avoid accidentally introducing a bit of negative Ackerman. If you end up with some negative ackerman then the non-pulling cable gets increasingly tighter as the servo rotates - hence, the servo is working to stretch the cable (i.e. overcome the increasing tension on the non-pull cable) not simply working against the aerodynamic loads. It also puts unecessary load on the servo arm and control horn.

If your installation is perfectly symetric, then I don't see any need for positive Ackerman.
Old 12-13-2003, 08:31 AM
  #5  
Crash90
Senior Member
My Feedback: (17)
 
Join Date: Oct 2002
Location: Cressona, PA
Posts: 2,320
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

I agree with jim trainor. The only reason for setting up positive akerman is to avoid negative akerman. If you take your time and do it properly then akerman is not needed. I did however set up my Sukhoi with 1/16 positive akerman just to be safe.

A little slack on the non-pull side of a pull pull WILL NOT induce flutter.

Keeping the width of the cable at the servo the same as the rudder is important to maintain proper geometry. Furthermore, if the servo arm is wider than the rudder horns you will begin compromising mechanical advantage.
Old 12-13-2003, 09:45 AM
  #6  
ml3456
 
Join Date: Oct 2002
Location: Darien, IL
Posts: 626
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

How do you set up positive akleman? Is this where the rudder spacing is a little wider thann the servo arm, or vice versa?

Thanks, ML
Old 12-13-2003, 09:52 AM
  #7  
JimTrainor
Senior Member
 
Join Date: Oct 2002
Location: Ontario, ON, CANADA
Posts: 1,309
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

It is a simple matter of how you place the control horns relative to the hinge line. Assuming you have set up the servo arm, cables, and control horns to form a parallelogram, then you must place the horns slightly to the rear of the hinge line.

It is described here: http://members.cox.net/bdfelice/Ackerman/ackerman.htm
Old 12-13-2003, 04:25 PM
  #8  
Stick Jammer
Senior Member
 
Join Date: Nov 2002
Location: Crete, IL
Posts: 2,251
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

JimTrainor and CafeenMan are correct. If the servo arm length doesn't match the control horn length and the pivot point of the control horn is dead on the hinge line, the non pull cable will tighten as the servo travels. Not what you want. If you have a little positive Ackerman then this probably won't be an issue. IMO it's best to try and shoot for the perfect set up, keep the control horn and servo arm the same length and get the pivot point as close to the hinge line as possible without being in front of it. It's also important for the cables to exit the fuse where it's the same width as the servo arm/control horn and not rub on anything at full throw.
Old 12-14-2003, 08:58 AM
  #9  
Rodney
Senior Member
 
Join Date: Dec 2001
Location: FL
Posts: 7,769
Likes: 0
Received 1 Like on 1 Post
Default RE: pull pull geometry

I use pull-pull on all my big planes (6) on both elevator and rudder. I do not attach direct to the servo though. I put a large pulley (about 2 inch diameter) mounted very close to the servo. A short heavy (4-40) rod on ball joints connect to this pulley. Since a pulley always pays out as much line as it pulls in, you can now make the horns on the moveable surface about any length you want with no problems with line tension changing as long as you have the pivot points of the moveable surface horns on or slightly ahead of the hinge line. If it is slightly forward, the line not being pulled will have a little slack at full throw. This is okay and will NOT cause flutter. Make the cables just snug enough to not jump off the pulley, actually quite low tension. Doing it this way also allows you to place the pull-pull lines just about anywhere you want. You do not have to have it lined up with centerling of the surface and, if you use guides, can even turn fairly abrupt turns between the pulley and the surface being moved. It is also much less strain on the servo bearings.
Old 12-14-2003, 01:15 PM
  #10  
Stick Jammer
Senior Member
 
Join Date: Nov 2002
Location: Crete, IL
Posts: 2,251
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

Originally posted by Rodney:

It is also much less strain on the servo bearings.
At the risk of starting a huge debate, I'm curious as to why a lot of people feel there is any less strain on pull-pull servo bearings when using a bellcrank (or pulley) instead of a direct connection. The cables of a pull-pull don't need to be excessively tight at neutral, not enough to cause the bearings to wear out while just sitting there. Once the servo starts to travel and there is a flight load on the rudder (or whatever surface), that load is being equally transferred to the servo in either setup. The bearing of the bellcrank doesn't take any of the flight load. If the mechanical ratio of the setup is the same with or without a bellcrank of some sort, the flight load imparted on the servo ain't gonna change. If it did, then we should be able to use smaller, cheaper, less powerful servos for the same application. It's just not the case. I'd be curious to hear from anyone who has honestly wore out a servo bearing by using direct connection for a pull-pull.
Old 12-14-2003, 01:26 PM
  #11  
CafeenMan
Senior Member
 
Join Date: Jan 2003
Location: Spring Hill, FL
Posts: 4,734
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

Stick Jammer - I was of the same school of thought that having the cable hooked directly to the servo was a bad thing to do. A good buddy of mine does that and swears by it. His thoughts are the same as yours and he flies some very large aircraft. So, I'm doing away with bellcranks as well. I'm convinced that the servo doesn't unduly suffer with the direct connection.
Old 12-14-2003, 02:08 PM
  #12  
CurtD
My Feedback: (1)
 
CurtD's Avatar
 
Join Date: Dec 2001
Location: Dallas, GA,
Posts: 335
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

[quote]ORIGINAL: Stick Jammer

JimTrainor and CafeenMan are correct. If the servo arm length doesn't match the control horn length and the pivot point of the control horn is dead on the hinge line, the non pull cable will tighten as the servo travels.

I'm sorry but this is dead wrong.

If everything is centered properly then the control arm and the control horn do NOT have to be the same length. I've attached a small CAD drawing that illustrates this. the various lines A-A', B-B', C-C' will all be exactly the same length at any given servo arm position angle. The only difference between the servo arm and the rudder arm will be the amount of angular movement. Slack in the lines is almost certainly due to not having the line between the rudder horn attach points passing EXACTLY through the rudder hinge line. Even a small error here will cause some slackness in one line and tightness in the other.
Attached Thumbnails Click image for larger version

Name:	Ca82291.jpg
Views:	80
Size:	27.4 KB
ID:	82202  
Old 12-14-2003, 02:15 PM
  #13  
CafeenMan
Senior Member
 
Join Date: Jan 2003
Location: Spring Hill, FL
Posts: 4,734
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

OK, that looks good to me. Just to satisfy myself I'm going to do a mock up with some scrap plywood and mount it to a piece of pine and see what happens. Thanks for doing the drawing for us.
Old 12-14-2003, 06:24 PM
  #14  
Stick Jammer
Senior Member
 
Join Date: Nov 2002
Location: Crete, IL
Posts: 2,251
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

CurtD,
Sorry man, but I'm dead right. It's simple geometry. Opposite sides of a parallelogram can only remain equal if opposite angles remain equal. You have already observed that the angles will be different. This difference between opposite angles is not constant, it changes as the servo continues to rotate. This is the reason that the difference in cable lengths also continues to change with servo rotation. With the control horn pivot point dead on the hinge line, the non-pull cable will get increasingly tighter as the servo continues to rotate. How much tighter it gets is dependent on the length difference between the servo arm and the control horn. In most cases it's not a drastic change, but it's there none the less. Take that drawing a little further and rotate the servo arm 90 degrees from neutral so that it's now on the center line. I'll think you'll see the obvious difference in cable lengths at that point. The non-pull cable needs to be slightly more than 1/8" longer than the pull cable. I realize that nobody would ever travel the servo that far but it shows the problem a little more clearly.
Old 12-15-2003, 01:41 AM
  #15  
kamakasi
Senior Member
Thread Starter
My Feedback: (3)
 
kamakasi's Avatar
 
Join Date: Sep 2003
Location: butternut, WI
Posts: 405
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

This has been a good learning experience. I have figured out a way to keep the control horns the same width. I am going to try to set this up neutral but I will keep in mind that if I error to error toward the rear of the hinge line.
Old 12-15-2003, 03:05 PM
  #16  
CurtD
My Feedback: (1)
 
CurtD's Avatar
 
Join Date: Dec 2001
Location: Dallas, GA,
Posts: 335
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

ORIGINAL: Stick Jammer

CurtD,
Sorry man, but I'm dead right. It's simple geometry. ...
I don't disagree that at extreme angles it doesn't work properly but if you'll look more closely at the geometry of the drawing (it's more complex than you may realize) you will see that the lines will remain EXACTLY the same length through 90 degrees of total servo angular movement (45 each side of neutral). This exact setup (2" servo arm / 3" rudder horn) is in my Pitts and does not tighten or loosen even with the servo at its full travel position. It WILL work properly this way unless you want to go more than 90 deg total travel (I never do that). The other factor that determines total allowable angular movement is the ratio of the smaller to the larger horn.

I stand by my statement that to say it won't work unless the arms are equal length is wrong.
Old 12-15-2003, 07:03 PM
  #17  
Stick Jammer
Senior Member
 
Join Date: Nov 2002
Location: Crete, IL
Posts: 2,251
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

ORIGINAL: CurtD

I stand by my statement that to say it won't work unless the arms are equal length is wrong.
I don't believe that I ever stated that it wouldn't work unless the arms are equal in length. I did state that the non-pull cable will tighten if the servo arm is shorter than the control horn and the control horn pivot point is dead on the hinge line. A small amount of positive Akerman will usually take care of the problem. I stand by that statement.

ORIGINAL: CurtD

I don't disagree that at extreme angles it doesn't work properly but if you'll look more closely at the geometry of the drawing (it's more complex than you may realize) you will see that the lines will remain EXACTLY the same length through 90 degrees of total servo angular movement (45 each side of neutral).
I don't have a CAD program nor do I know how it works but I can tell you that even though it may appear that the cables remain the same length through 90 degrees of servo travel on your example, they don't. It's a very small change, but it is there. It's physically impossible by shear geometry for them to remain the exact same length. You even stated that over 90 degrees of servo travel will cause a problem. Well, guess what, that problem starts as soon as the servo begins to rotate and only gets worse the farther it travels. Don't mistake what I'm getting at here, a pull-pull system will work in less than a perfect setup. I'm only stating what the perfect setup should be. Keep in mind that most SA planes need 45 degrees of rudder deflection. This is easy when the servo arm is the same length as the control horn. 45 degrees of servo rotation = 45 degrees of rudder deflection. With a 2" servo arm and a 3" control horn, the servo would have to travel very close to 90 degrees in each direction to get 45 degrees at the rudder. Again, I don't know how CAD works, but I do remember having this debate a while back with a few others on RCU. One of the replies was from someone who works with CAD and his findings backed up what I had stated. He posted actual numbers to a few decimal places that showed how the non-pull cable would need to get longer as the system rotates.
Old 12-15-2003, 07:14 PM
  #18  
Stick Jammer
Senior Member
 
Join Date: Nov 2002
Location: Crete, IL
Posts: 2,251
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

Found the thread I was referring to if you want to take a look at the actual numbers. Post #27

http://www.rcuniverse.com/forum/m_12...tm.htm#1285514
Old 12-16-2003, 01:50 AM
  #19  
kamakasi
Senior Member
Thread Starter
My Feedback: (3)
 
kamakasi's Avatar
 
Join Date: Sep 2003
Location: butternut, WI
Posts: 405
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

o.k. this is getting over my head. but I have another question . I want to use a coreless servo for the rudder. But I can't seen to locate one that has metal gears. Using a torque calculator I find that at my flying speeds and rudder deflection at those speeds I only need 68 oz. of torque max. Are the metal gears really necessary? I like the smaller dead band and better centering of the coreless. Any thoughts?
Old 12-16-2003, 02:00 AM
  #20  
CafeenMan
Senior Member
 
Join Date: Jan 2003
Location: Spring Hill, FL
Posts: 4,734
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

I don't like metal gears. I bought a few thinking they would be really good, but they quickly developed a lot of slop.
Old 12-16-2003, 12:42 PM
  #21  
famousdave
Senior Member
My Feedback: (61)
 
Join Date: Sep 2003
Location: Bradenton , FL
Posts: 2,262
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

This was a great find. I was having some issues with my pull-pull resistance increasing near full travel - to the point where I was not able to make it full deflection (3" on my big monocoupe). I re-rigged the cables and introduced positive ackerman and voila!! Problem fixed. I now have a very smooth 3" rudder deflection with no binding. Wish I saw this thread weeks ago, I probably took a lot of life out of my servo!

This really works, you made a believer out of me!!

DP
Old 12-16-2003, 07:31 PM
  #22  
Stick Jammer
Senior Member
 
Join Date: Nov 2002
Location: Crete, IL
Posts: 2,251
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

kamakasi,
Hitec makes several coreless metal gear servos. 925MG and 945MG are coreless analog. 5925MG and 5945MG are coreless digital. The 5975HB is coreless with their new Karbonite gears. They claim the Karbonite gears are almost as strong as metal but don't wear out as metal gears do. You can check them out here:


http://www.hitecrcd.com/
Old 12-18-2003, 01:46 AM
  #23  
kamakasi
Senior Member
Thread Starter
My Feedback: (3)
 
kamakasi's Avatar
 
Join Date: Sep 2003
Location: butternut, WI
Posts: 405
Likes: 0
Received 0 Likes on 0 Posts
Default RE: pull pull geometry

Stick Jammer,
Thanks for the link. I like what I saw but don't know if I can justify the expense. Maybe Santa will be good to me.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.