Notes to Self 3

[Jon Lowe]

What he is saying is this:  As the servo arm rotates from 90 degrees, the
effective length of the servo arm gets shorter, meaning that for each
incremental degree of rotation of the servo arm from 90 degrees, you get
less incremental movement of the pushrod.  Therefore the control is
inherently more sensitive around neutral than it is at the ends of the servo
throw, assuming linearity in the rest of the system.  Expo can be used to
counter this natural tendency.
 
HOWEVER, the effective length of the surface control horn also gets shorter
as the surface moves from neutral, increasing movement with each incremental
movement of the pushrod.    So it is a race between decreasing effectiveness
of the servo output arm vs. increasing effectiveness of the control horn.
Then you get into non-linearity of the aerodynamic surface effectiveness
itself as it moves from neutral, and you can get confused real quick.  You
are dealing with several areas of non-linearity, and trying to compromise
between them all.
 
Jon Lowe 



-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org] On
Behalf Of Tim Taylor
Sent: Thursday, February 10, 2005 8:28 AM
To: discussion at nsrca.org
Subject: Re: [SPAM] Re: Notes to Self 3



Troy let me get this straight in my mind, You said
 
"The reason is the servo wheel is round and its Y axis distance changes for
each degree of travel as the servo rotates around the back side."
 
Now It took me a few readings and some pondering to understand what you
said. It doesn't matter what type arm you use,correct so far? What I take
this to mean is as the servo arm moves the point at which the servo is
connected to the driving rod and the center arm screw is at it's farthest
point at 90 degrees to the servo case (assuming the servo case is 90 degrees
to the hinge line) as it moves forward or backwards the distance between the
pivot points decreases with each degree of rotation. Correct?

----- Original Message ----- 
From: Troy A.  <mailto:troy_newman at msn.com> Newman 
To: discussion at nsrca.org 
Sent: Friday, February 11, 2005 11:05 PM
Subject: [SPAM] Re: Notes to Self 3

8103, 9303 and 10X expo are the same Positive is softer and Negative is
harder feel. I'm writing an article for the KF right now that talks about
Expo and how to use it. When to use it. And the effects of it. Sometimes a
reverse expo (faster response) at center can be of benefit. We have played
with this in snaps and found that it has some promise...Lots of regular expo
on the ailerons and rudder and reverse expo on the elevator...The result is
the model will pitch harder on entry before the rotations.
 
Another thing depending on the radio it takes 20-30% expo just to get the
output to be linear. The reason is the servo wheel is round and its Y axis
distance changes for each degree of travel as the servo rotates around the
back side.
 
This is March KF stuff so I won't elaborate here but I have some data to
back up my words. 
 
Troy
 
 

----- Original Message ----- 
From: T <mailto:tncbrown at charter.net> &C Brown 
To: discussion at nsrca.org 
Sent: Wednesday, February 09, 2005 8:42 PM
Subject: Re: Notes to Self 3

That is odd Tim.  I wonder why JR decided after all these years to change
from a pos. setting to a neg. for a "softer feel".  I'm a firm believer in
"If it ain't broke, don't fix it"  
I made the same mistake a few years back when I switched from a Futaba 7UAP
to a JR 8103 and inadvertently put neg. expo in the 8103.  Talk about pucker
factor for a few minutes. 
Maybe Fu-tuh-boo-boo is rubbing off on JR!  (BG)
 
T-Bone
 

----- Original Message ----- 
From: Tim  <mailto:twtaylor at ftc-i.net> Taylor 
To: discussion at nsrca.org 
Sent: Wednesday, February 09, 2005 12:18 PM
Subject: Re: Notes to Self 3

That's the whole point in the story Ron, I ASSUMED I was right and didn't
NEED to RTFB and could have screwed up big time. I've been flying JR radios
for years but always a PCM10 of some sort, this 9303 is a new radio for me.
What I did was get in a hurry, I wanted to fly and trim this airplane and
didn't do what I should have done first. I hope my not so comical errors
will help someone else from making the same mistake I did.

----- Original Message ----- 
From: Ron Van  <mailto:vanputte at cox.net> Putte 
To: discussion at nsrca.org 
Sent: Wednesday, February 09, 2005 12:01 PM
Subject: Re: Notes to Self 3


On Feb 9, 2005, at 8:28 AM, Tim Taylor wrote:



As our Continuing story unfolds........
 
Timmy T is looking out of the shop door and sees that the weather is just
perfect for some trim flights
 
So we pack up the trailer and go for some trim flights. Recalling the last
flights  all control throws too sensitive so I dial in less rates on low and
put high rates to what low was before. Ok time to dial in some Expo on all
surfaces, pop up the screen on the 9303 and dial in 30%. Ok time to fly.
Airplane seems pretty stable now but the controls feel a bit "Twitchy" hmmm
no problem continue the flight land and have a look. Ok maybe I didn't get
enough expo dialed in so add some more. Next flight about the same but the
controls seem a bit more "twicthy" hmmm something's not right here. Ok times
up for flying for today pack up and head home. On the way home I get to
thinking about the last flight and how the airplane changed the "feel" after
I added the expo. Ok I get to wondering where the heck did I put the book
for this new radio, Ok lets review the book on page A20 is the expo
directions, it states "NOTE: Use only positive percentages unless there is a
distinct need to do otherwise". Hmmmmmm that don't sound right! Turn the
transmitter on and look at the expo I've got dialed into the radio,
*&%^$*&#)(#&%^%)@(_*&_(^#  Everything is done to the NEGATIVE !!!!!! No damn
wonder! So "Note to self" today's lesson is READ THE FRIGGIN BOOK !!!!!!!!!
 
Ok while I'm at it lets program a little down elevator  trim at low idle, I
wonder what page that is on in the book.................. "Fade to Black as
Timmy T reaches for the book."



My guess is that he's using a JR radio, because negative exponential is what
you want on a Futaba transmitter to soften the control response. That's why
well-meaning people at the field need to find out what brand of transmitter
the guy being counseled has before telling him how to change the expo. 

I had a similar situation when I was helping a novice trim out his airplane.
After I flew a trim flight for him, I told him he needed 50 - 60%
exponential on elevator and aileron. I asked him if he knew how to do that
and he said that he did. The airplane was a handful on the subsequent
flight, because he put in the wrong expo!

Ron Van Putte


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