aileron servo instl vs vibration

[Dean Pappas]

Hi Jim and All,
Don't you just love this stuff?
So we can adjust the mass-balance by looking at a current meter,
placed inline with the aileron servo, and setting the idle to the resonance.
Adjust the weight/length until the current is at a minimum, right?
Dean P.

-----Original Message-----
From: JOddino [mailto:JOddino at socal.rr.com]
Sent: Saturday, February 21, 2004 1:48 PM
To: discussion at nsrca.org
Subject: Re: aileron servo instl vs vibration


Years ago most servos had the pot wiper driven directly by the servo output
shaft.  Pot wear was so bad the famous Giesendanner (Sp?) pot wipers were an
after market product all the serious pilots used.  We started using EK push
pull linear output servos that used a gear to drive the pot shaft.  It was
geared so the travel on the pot was about 180 degrees for a normal output
movement of plus and minus 9/16 inch as I recall.  Pot wear was no longer a
problem, which tells me Jerry's theory is correct.  The control surface back
driving the servo at the frequencies of vibration and aero buffeting is the
culprit.  If the turbines and the electrics don't show the pot wear then the
aero buffeting must not be the major contributor.  I've always wondered why
someone didn't make a servo that you couldn't back drive.  I guess the
efficiency would drop but we don't seem too concerned with that anyway.  The
servo also wouldn't blow back if it lost power but how often to we depend on
that.
Jim


----- Original Message ----- 
From: "Jerry Budd" <jbudd at QNET.COM>
To: <discussion at nsrca.org>
Sent: Saturday, February 21, 2004 9:11 AM
Subject: Re: aileron servo instl vs vibration


> Hi Xavier,
>
> A while back Dave von Linsowe tried side mounting the aileron servos
> to see if the servo pots would be more tolerant of the motor
> vibration.
>
> The theory was that the rotary acceleration forces induced by the
> motor on the airframe translate into increased vertical acceleration
> at the aileron servos causing the pot wiper preload to vary
> cyclically, resulting in increased rate of wear.  It was hoped that
> the pot wear could be reduced or eliminated by orienting the plane of
> rotation of the pot wiper in the vertical axis.
>
> It didn't help.  Tony Frackowiak also tried it on his Gator G-202 and
> it didn't work there either.
>
> And that makes sense, as the pot wiper preload is not likely to be
> affected by the inertial forces acting on the very low mass of the
> pot wiper.
>
> The current thought is that the inertial forces acting on the
> ailerons are back fed into the aileron servo gear train causing the
> aileron servos to have to work almost constantly to resist the
> uncommanded movement.  The servo moves slightly (but nearly
> continuously) around the commanded position to resist causing
> excessive wear on the pot and gears (similar to servo buzz).  Since
> the servo spends most of its time around neutral, that's where most
> of the wear occurs.
>
> Interestingly, the ePartner Tony Frackowiak is flying shows no
> aileron servo pot wear through ~40 flights (go figure!).
>
> Jerry
>
>
> >Is there a prefered method to install the aileron servos and protect them
> >from the high level of vibration someone was mentioning?
> >Could the servo be mounted on his side with the arm parralele to the ribs
> >wihout risking more wear of the gear ?
> >I suppose the vibration on the wing servo is mainly up-down
> >
> >Thanks
> >
> >Xavier
>
> -- 
> ___________
> Jerry Budd
> mailto:jbudd at qnet.com
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