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

Servo geometry question

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

Servo geometry question

Old 02-23-2006, 06:20 PM
Senior Member
Thread Starter
My Feedback: (1)
Join Date: Oct 2002
Location: Somewhere
Posts: 655
Likes: 0
Received 0 Likes on 0 Posts
Default Servo geometry question

I just want to make sure I am on the right track with this setup. First of all, I have a profile plane that flys pretty well but it could be better. OK, say I want to set my elevator up for 45* throw, would I figure out what size control horn I need by setting my elevator at a 45* angle and checking to see if the control rod interferes with any part of the airplane on the way to the servo arm? Is that how you figure out what size control horn to use? Second I have read that you want a 1 to 1 ratio with the set up, so I would want an arm on the servo the same length as the distance from the hinge line on the elevator to the connection point on the horn that would give me my 45* deflection, right so far? If anything, maybe a shorter servo even, say 1/2 inch. Ok, now with my subtrims already at zero saying everything is matched up, I would make sure my ATV (Futaba system) is at 140% right? Now if ATV is for setting end points, do I just leave it alone? I think this is where I really get confused, if what I just did was for high rates, now what about low rates, if I only want say 10* up travel on the elevator, do I select my low rate switch a dial down my ATV to get it. What about staying above 80% on the dual rate thing? Or is it the dual rate that I am dialing down but ATV stays up? I want to keep my servo resolution, let me know if I am on the right track so far? I confusing myself now.
Old 02-23-2006, 07:28 PM
Senior Member
Join Date: Sep 2004
Location: Greensboro, NC
Posts: 3,312
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Servo geometry question

I prefer ATV in the 100% range for high rates, which should use 45 degrees of servo travel in each direction. I prefer for ATV to be above 70% for low rate.

When you dial down the low rate you are dialing down the ATV for low rate.

Old 02-23-2006, 08:28 PM
Senior Member
redfox435cat's Avatar
Join Date: Mar 2005
Location: Lompoc , CA
Posts: 1,469
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Servo geometry question

when you start geting into 45 degree throws I'm guessing for 3-d use your going to loose serve res. it's simply how it works. You can adjust your throws by adjust what hole the clevis is hooked to. I can't say I ever given any though to calcualting the horn length. just play with it. remember the further the clevis is from the servo the more the surface moves. the closer to the servo the lees the control suface moves. at the control surface the colser to the control surface the more the surface moves. the farther away from the the control surfae the less the surface will move. the one to ratio is a good rule of thumb for percise control. when trying to set up for 3-d your going to get persise control, you set your low rate for percise control. don't go over 110% on the AtV. the problem with ATVs is most if not all Tx don't evenly distribut the stick movement to servo movment when the ATV is moved. so keep it around 95-105 %, personally the only control I use ATV's for is throttle and flaps where you need a persise stop point. If you look really close at what the servo is doing it will almost look like you put expo into the program when the ATV is moved to an extream. on this kind of set up it is a good idea to set in some expo so you little movment near center and alot of movment toward the extream of the stick imput
with a 3-d set up your going to end up setting 30%-50 % for the low rates since your normal deflection is 10-20 degrees. 35 degree plus is for 3-d set ups. 80% low rate would be a normal hi-lo rate for a sport model

my 2 cents
Old 02-24-2006, 02:17 AM
Senior Member
My Feedback: (6)
JohnW's Avatar
Join Date: Mar 2002
Location: Lincoln, NE
Posts: 1,815
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Servo geometry question

You first part is basically correct. Most servo's have about 45 degrees of travel both directions. So if you want 45 degrees of throw, the hole on the servo arm needs to be at the same radius as the hole on the control surface with respect to hinge line. If you want less throw, then the servo hole radius needs to be less than the control radius.

You have the err direction correct. It is better to err on with the servo arm being shorter. If you go the other way (long servo arm, short control horn) you decrease servo authority and can induce flutter. But, don't get too short as that can induce flutter. I.E., You are better off using two long arms as opposed to two short arms.

As for binding, yes you need to be sure the linkage won't hit airframe, etc.

As others posted, I wouldn't go 140 on the ATV unless you have to. I believe it is OK to overdrive some, say to 120. If you overdrive too much, there isn't enough room for any mix headroom. Also, some servos stop moving if overdriven, i.e., they hit their end of movement say at 130, etc.

So, once you have your ATV set (say at 120), try to get your throw set mechanically without messing with ATV. This is almost obtainable with adjustable horns (like threaded horns), and messing with the servo horn orientation on the servo splines. If you use the fixed hole horns, just get it as close as you can. Then tweak with ATV. If you were close with the mechanical side, you shouldn't need to adjust your ATV by more than about 5 either direction.

Now your ATV is set. There is no need to ever mess with it again. For low rates, you just set the rate which is a percentage of the ATV you set. So if you want 10 degrees, and your ATV is set to 45, your D/R setting would be around 25%. But since servo's aren't linear in their movement, be sure to measure the movement. Up/Down DR may not be the same for 10 degrees. This is due to linkage issues and servo pot issues. So as you put it, you are dialing the D/R down and the ATV stays up.

This does reduce resolution, but as others posted, there is no way to avoid this on any radio. That is the penalty of having extreme throws.

Oh, and ditto on the expo. On Futaba, you'll want negative expo, JR is positive. If your radio supports different expo on high vs low rate, use it. Otherwise I'd set expo on elevator to something around -40 to -50 which should work OK in both high and low rates, but different expo in high vs low rate would be better.


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.