Digital Servos vs analog servos on Ailerons

[Dean Pappas]

Hi Don,
I am pleased to see you confirming some of my long held statements about aileron servos. At each step of development, when a servo with better centering power became available, I have noticed better loop tracking, especially in square corners. If I used a 1-piece wing, I would consider an old fashioned linkage where lifting loads are not carried by serv torque, at all. This has all gotten more noticeable as ailerons have grown in size: mostly in chord. I also haven't really seen a change in how much expo I use. As strong as rudder servos now are, they still blow back a great deal. If you ever do tests with an aerodynamic counterbalance on the rudder, be ready to change all your rudder throws except for the spin! For some neat reading, try "the great rudder experiment" at http://www.mindspring.com/~rellis2/rcpattrn/rudder.htm 
This is interesting stuff.
Regards to All,
    Dean P.
 

-----Original Message-----
From: discussion-request at nsrca.org [mailto:discussion-request at nsrca.org]On Behalf Of Don Szczur
Sent: Thursday, July 01, 2004 11:51 PM
To: discussion at nsrca.org
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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