Go Back  RCU Forums > RC Airplanes > RC Jets
Reload this Page >

Lil' Help--Pull-Pull Steering Linkage

Notices
RC Jets Discuss RC jets in this forum plus rc turbines and ducted fan power systems

Lil' Help--Pull-Pull Steering Linkage

Old 05-22-2023, 05:53 PM
  #1  
YellowAircraft
Thread Starter
 
Join Date: Jun 2018
Posts: 156
Received 13 Likes on 8 Posts
Default Lil' Help--Pull-Pull Steering Linkage

Hi,

I'm trying to demonstrate to a certain engineer that a rigid pull-pull steering setup (3mm pushrods) is problematic when the geometry isn't exactly symmetrical. In this case, the servo output shaft is offset from the center of the strut pivot point AND the steering arm is forward of the strut pivot. Additionally, the span of the steering arm is shorter than the span of the servo arm. His belief is that the fact that everything is on ball joints mitigates all this. That's not correct.

Anyway, I wonder if anyone has any links to any videos showing a pull-pull setup where one cable is going slack upon deflection? Or maybe someone could post a clip of such a setup if you have one? I'm in Asia right now and don't have access to my own stuff, but I kinda need to short-circuit this issue for this fella before he goes into production with this built-in servo-burner-upper.
Old 05-22-2023, 06:52 PM
  #2  
mongo
My Feedback: (15)
 
mongo's Avatar
 
Join Date: Dec 2001
Location: Midland, TX
Posts: 3,521
Received 82 Likes on 72 Posts
Default

well,
sometimes experience is the best teacher, and bad experience teaches the best lessons...
Old 05-22-2023, 07:08 PM
  #3  
Shaun Evans
 
Shaun Evans's Avatar
 
Join Date: Jan 2002
Location: San Diego, CA
Posts: 7,137
Likes: 0
Received 1 Like on 1 Post
Default

Originally Posted by mongo
well,
sometimes experience is the best teacher, and bad experience teaches the best lessons...
Haha

Yes, indeed. But THIS time, I'd just like to circumvent all that. Every once in a while, we gotta be that parent that prevents the child from touching the hot stove. So... got anything for me (besides that nugget of Yoda-ish wisdom)?

Old 05-23-2023, 09:22 AM
  #4  
causeitflies
 
causeitflies's Avatar
 
Join Date: Jan 2003
Location: EASTERN OHIO
Posts: 2,437
Received 42 Likes on 32 Posts
Default

A little sketch of the setup would help. It doesn't have to be precise. It sounds like a double push-pull system instead of pull-pull. As you stated, symmetry is the key. What kind of engineer is he?

Last edited by causeitflies; 05-23-2023 at 09:25 AM.
Old 05-23-2023, 09:35 AM
  #5  
BarracudaHockey
My Feedback: (11)
 
BarracudaHockey's Avatar
 
Join Date: Jul 2003
Location: Jacksonville, FL
Posts: 27,050
Received 358 Likes on 288 Posts
Default

Correct me if I'm wrong (i'm not an engineer) but rigid means push/pull correct? Pull pull would be wires not pushrods each putting tension on when it pulls. I'd think the easiest way to eliminate binding would be to use 1 pushrod otherwise anything less than perfect geometry would cause binding.
Old 05-23-2023, 11:05 AM
  #6  
FlyJ
 
Join Date: May 2007
Location: San Francisco, CA
Posts: 112
Received 10 Likes on 8 Posts
Default

I put this in CAD for you and it shows the geometry problem well.

Setup:
4" servo arm
2" control horn
18" wires

When both arms are horizontal, hole-to-hole lengths are 18" (this is set by the wire length). When you turn the servo arm 20, one side remains 18" (set by the wire in tension), but the opposing side increases to 18.02". The reason for this is because of the pivot arm length. If the arms are the same length, then they will remain parallel through all angles. If the arms are different lengths, they will not remain parallel through their travel and this will result in slack on the non-tension side


The following users liked this post:
YellowAircraft (05-23-2023)
Old 05-23-2023, 11:50 AM
  #7  
mongo
My Feedback: (15)
 
mongo's Avatar
 
Join Date: Dec 2001
Location: Midland, TX
Posts: 3,521
Received 82 Likes on 72 Posts
Default

Originally Posted by Shaun Evans
Haha

Yes, indeed. But THIS time, I'd just like to circumvent all that. Every once in a while, we gotta be that parent that prevents the child from touching the hot stove. So... got anything for me (besides that nugget of Yoda-ish wisdom)?

then,
my next step would be to ask him why he does not use a rigid dual push/pull on his elevator, rudder and even ailerons. make the point that if a single pull push is ok for flight controls, it is more than adequate for nose steering.
The following users liked this post:
YellowAircraft (05-23-2023)
Old 05-23-2023, 12:07 PM
  #8  
mongo
My Feedback: (15)
 
mongo's Avatar
 
Join Date: Dec 2001
Location: Midland, TX
Posts: 3,521
Received 82 Likes on 72 Posts
Default

i guess, in order to be certain,
is this one of those set ups where the steering servo "rides" through the retraction motion with the gear leg?

in that situation, i can see a rigid dual push/pull being able to work with minimal binding at the ends of the throws. probably produce excess current draw and most likely premature servo failure. with short links and the arms kept closer to the same length, it could work, for a while.
Old 05-23-2023, 01:54 PM
  #9  
YellowAircraft
Thread Starter
 
Join Date: Jun 2018
Posts: 156
Received 13 Likes on 8 Posts
Default

Originally Posted by causeitflies
A little sketch of the setup would help. It doesn't have to be precise. It sounds like a double push-pull system instead of pull-pull. As you stated, symmetry is the key. What kind of engineer is he?
Hi,

Yeah, you're correct. Double push-pull. Aero-Engr. What's not clear from the photo is that the servo output shaft is also offset from center relative to the center of the strut pin.


Last edited by YellowAircraft; 05-23-2023 at 01:57 PM.
Old 05-23-2023, 02:29 PM
  #10  
YellowAircraft
Thread Starter
 
Join Date: Jun 2018
Posts: 156
Received 13 Likes on 8 Posts
Default

Originally Posted by BarracudaHockey
Correct me if I'm wrong (i'm not an engineer) but rigid means push/pull correct? Pull pull would be wires not pushrods each putting tension on when it pulls. I'd think the easiest way to eliminate binding would be to use 1 pushrod otherwise anything less than perfect geometry would cause binding.
THIS!

Sadly, the problem I'm trying to solve isn't how to eliminate the natural binding this asymmetry is causing. It's trying to convince the boss that the problem exists.
Old 05-23-2023, 05:46 PM
  #11  
mongo
My Feedback: (15)
 
mongo's Avatar
 
Join Date: Dec 2001
Location: Midland, TX
Posts: 3,521
Received 82 Likes on 72 Posts
Default

if he will believe in electronic measurements:

you can put an amp meter on the steering servo and show him the amp load increase as the system is moved about.
how it stays relatively constant around center and climbs as the deflection increases

Last edited by mongo; 05-23-2023 at 07:50 PM. Reason: i HATE auto correct
Old 05-23-2023, 06:01 PM
  #12  
speedracerntrixie
My Feedback: (29)
 
speedracerntrixie's Avatar
 
Join Date: Jun 2010
Location: Happy Valley, Oregon
Posts: 9,527
Received 177 Likes on 152 Posts
Default

Simple, let him set it up his way but disconnect one ball link. Demonstrate that at neutral everything lines up, then show him that it doesn’t line up at the end points. Hopefully then he will be more receptive to the solution.

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 - Manage Preferences - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.