Digital Servos vs analog servos on Ailerons

[Don Szczur]

I'm using digitals on the elevator and rudder.  My switch was four years ago
when I went from JR 4131 to JR 8411 servos.  I did have to increase the expo
for the same feel.  Pitch and Yaw are not as critical as roll (length of
fuselage has a lot of leverage and requires more to move it around than the
short chord of the wing.  Definitely recommend the extra torque from the
digitals- 8411 or 8611 for cleaner snaps and knife edge (rolling) loops and
circles.

Don
----- Original Message ----- 
From: <adriancwong at earthlink.net>
To: <discussion at nsrca.org>
Sent: Thursday, July 01, 2004 11:57 PM
Subject: Re: Digital Servos vs analog servos on Ailerons


> Don,
>
> Good report.
>
> What about digitals on rudder and elevator? I am currently using all
digitals, except on the throttle.
>
> Thx,
>
> Adrian
>
>
>
> -----Original Message-----
> From: Don Szczur <dszczur at maranatha.net>
> Sent: Jul 1, 2004 7:51 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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