Notes to Self 3

[Ron Van Putte]

On Feb 10, 2005, at 10:17 AM, Jon Lowe wrote:

> 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 is correct.  If the distance the pushrod attachment on the servo 
arm is from the center of the servo is exactly equal to the distance 
the pushrod attachment to the control horn is from the control surface 
rotation point, there will be perfect linear motion of the servo arm 
relative to the control surface, assuming the servo arm and the control 
horn attachment points are perpendicular to the servo rotation point 
and the control surface hinge line, respectively.

Ron Van Putte

> -----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. 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&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 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 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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