[NSRCA-discussion] Ackerman question

[Troy A. Newman]

Lance,

The surface control horn and the servo do NOT need to be the same distance 
from the pivot point. Rather they only need to be the same "offset" from the 
pivot point. A one to one ration is definitely not needed to maintain the 
cable tension on the trailing cable. I do it all the time. My maximum travel 
on the rudder is usually about 25-30degs the servo will easily do 50degs 
each side. I use rather large control horns on the surface side and then use 
the smallest servo wheel I can get use to the maximum throw I need. As I 
said usually only 25degs. My D/R values are sitting around 70-80% on the 
lowest rate I have. If I find that I'm flying on less than about 80% with no 
need to go to 100% at anytime I change out the servo wheel to a smaller one 
or go in a hole or two to get maximum resolution on the rudder that I can. 
This makes rudder inputs smoother and more consistent. It also allows me to 
run a right tight linkage that has no spongy or springy feel to it. This 
helps in rudder corrections, finesse rudder inputs like rolling loops and 
circles, and it also helps in snaps. A tight rudder linkage that is getting 
good leverage on the rudder will exit snaps cleaner and not give a wiggle 
when they are done. A spongy rudder linkage will give tail waggle on stall 
turns, in point roll transitions, rollers, and snap exits.

Also using more of the servo will lessen any effects of servo gear and pot 
wear, and will maximize trim resolution so that very fine movements are 
possible. This stuff shows up in all surfaces and will translate in the 
amount of mix being needed. Not the percentage of mix but how minute a 
movement can be controlled in a mix situation. Basically this all allows you 
to fine tune your model more closely.

see this article I wrote a while ago regarding the proper setup of a rudder 
pull-pull system It explains it pretty well. Not a bunch of theory and 
Ackerman stuff. Just the meat and potatoes of the deal. By the way it was 
published in the KFactor and then posted on Central Hobbies website. 
Specifically look at Figure #1 and #2

http://www.centralhobbies.com/instructional/linkage2/link21.html

Troy Newman

----- Original Message ----- 
From: "Lance Van Nostrand" <patterndude at tx.rr.com>
To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
Sent: Monday, January 15, 2007 9:19 PM
Subject: [NSRCA-discussion] Ackerman question


> OK, this ackerman thing got me thinking again about this and a question
> comes to light:
>
> Many times we say that the servo arm should match the control arm to
> minimize cable slack.  However, if we use a smaller servo arm than the
> control arm I am now thinking that this does not exacerbate the cable
> tension problem. In fact, it does nothing except reduce the surface
> deflection.
>
> If the goal is to use 100% of the servo travel (for precision) than an
> equally spaced servo/control surface would only let the servo rotate +-
> about 15 degrees for the average elevator.  This is less than 50% of the
> servo's capability.  I always used smaller arms at the servo, or use a 
> wheel
> if I end up with too much slack.  I thought I was making the best of a bad
> situation, but now I am thinking that there is no bad situation.  If the
> clevis is on the hinge line (or the servo connection and control horn are
> equally offset in the same direction) then you should get a constant 
> tension
> regardless of the mismatch in servo/control arms.  Is this right?
>
> --Lance
>
> ----- Original Message ----- 
> From: "Richard Lindberg" <rclindberg at comcast.net>
> To: "NSRCA Mailing List" <nsrca-discussion at lists.nsrca.org>
> Sent: Monday, January 15, 2007 11:32 AM
> Subject: Re: [NSRCA-discussion] Pull Pull Cable Tension Question
>
>
>> Here's the URL for Ackerman setups:
>>
>> http://members.cox.net/bdfelice/Ackerman/ackerman.htm
>>
>> FWIW, Bob Pastorello, et al,  discussed this on the list in November
>> 2004...!
>>
>> Richard
>>
> /listinfo/nsrca-discussion
>
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