Notes to Self 3

[J.Oddino]

MessageSeems to me if the servo arm and the control horn are the same length you get a degree of control surface movement for every degree of servo movement.  However this is not the usual linkage configuration.
Jim
  ----- Original Message ----- 
  From: Jon Lowe 
  To: discussion at nsrca.org 
  Sent: Thursday, February 10, 2005 8:17 AM
  Subject: Re: Notes to Self 3


  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. 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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