Digital Servos vs analog servos on Ailerons

[Andre Bouchard]

I have used 9411's on ailerons and 3421's on elevators (metal gears) for quite some time.  I find these servos to be excellent.  The 9411's on the ailerons do get lose fairly quickly, and metal gear replacements are pricey.  I investigated converting both servos (9411 and 3421) to SA type, but I had second thoughts when I looked at the size of the plastic gears, particularly on the 3421.  I suggest caution here.

Regards,

Andre'

----- Original Message ----- 
  From: Keith Black 
  To: discussion at nsrca.org 
  Sent: Friday, July 02, 2004 8:19 PM
  Subject: Re: Digital Servos vs analog servos on Ailerons


  Don,

  Interesting report. 

  If you get a chance I suggest you try out the JR 9411SA's (nylon gears), they are incredibly tight with absolutely no slop. I flew the 9411 for about 200 flights and they developed quite a bit of slop. I then switched to 9411SA and was amazed how tight they were, much tighter than the 9411 brand new. It made a noticeable difference in how steady the plane flies. Also did the same on my elevator servos in the stab, changed the 3421's to the 3421SA's. The 3421's had also developed a lot of slop after 200 flights. For the rudder I run the 8411SA. Unless I run into problems with the nylon gears stripping I'm really sold on the SA's. Of course, flying FAI you put a lot more stress on your servos. 

  On my second plane I run pull-pull on the elevator and use the JR 8417 which seems to be the defacto digital elevator servo in the JR line. It's a great servo but even brand new it's not nearly as tight and slop free as the SA's. I'm considering changing the elevator to the 8411SA, though it's not as fast as the 8417.

  Keith Black

    ----- Original Message ----- 
    From: Don Szczur 
    To: discussion at nsrca.org 
    Sent: Thursday, July 01, 2004 10:51 PM
    Subject: Digital Servos vs analog servos on Ailerons


    At the urging of Dave, I have finally switched to digital servos on the ailerons.  After flying analog servos for 25 years, it was explained to me that digital servos on the ailerons help looping and radius of maneuvers.  Here is my assessment.

    The baseline servos were JR 7005.  The digital servos changed to are JR 9411 servos.

    Loops and lines.  The first thing I noticed was that, just as Dave mentioned, loops and loop segments are more locked-in.  The 9411 servos made the wings more stable, requiring less effort on the maneuvers involving loop segments (most all of the maneuvers).  Analysis- why this occurs- the theory is that during loops, wing panels generate quite a bit more lift.  since the digital servos lock, and hold center much more agressively than analog servos (holding torque) this provides very little give in the ailerons during the loop.  Regardless, its a noted difference and clear advantage of these servos.

    Rolls and rolling segments.  This part of the analysis was quite a bit more complex.  Clearly the digital servos have more speed and resolution than the analog servos.  The holding torque and contol surface power requires that control throw be reduced to attain the same roll rate.  However, the analog servos would "blow back" at high speed, but stay deflected at lower speeds.  Incredibly, the advantage to this is a more constant roll rate- independent of speed.   The ailerons  (roll rate) would feel more the same at low speed (like the top of an immelman, loop or vertical line).

    Snap rolls.  This is what I call the "rubber band" effect.  The analog servos would deflect quite a bit during snap rolls, requiring one to "unload" the snap (start coming out of the ailerons before the snap is complete).  Otherwise, the snap would be over-rotated.  Actually, this was not too hard to get used to, since its a very consistent effect.  With digital servos, however, there is much less deflection and thus a more "honest" snap roll.  It does snap differently, but the exit is much cleaner (or easier to have a clean exit).

    Exponential.  Here is another item of interest.  My initial perception was that I would have to add additional expo to get the same feel for the digital servos.  Since control thow was reduced (moved in one notch on the servo arm) there was still more roll rate at full stick deflection (since the ailerons are no longer blowing back like the analog servos), but not as much roll around neutral.  I actually had to cut my expo in half.  

    So, the overall analysis is that the digital servos provide greater stability of flight, more consistent snap exits and loop segments that are more locked in.  Analog provide more control roll authority at slower speed and more forgiveness with shaky hands (the rubber band effect does dampen shaky thumbs).  Conclusion- overall benefits outweigh analog servos- favor the use of digitals.  Will stay with the 9411's.

    Don 
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