How to stop electrical whining noise?

On 2018-12-27, Jeff Liebermann <jeffl@cruzio.com> wrote:
> On Wed, 26 Dec 2018 21:43:53 +0200, Tauno Voipio
><tauno.voipio@notused.fi.invalid> wrote:
>
>>On 26.12.18 20:45, Jeff Liebermann wrote:
>>> On Wed, 26 Dec 2018 11:01:56 -0500, bitrex <user@example.net> wrote:
>>> 
>>>> Here's some interesting papers on sources of noise from small DC motors:
>>>> <https://www.mdpi.com/2072-666X/9/2/84/pdf>
>>>> <http://downloads.hindawi.com/journals/ijrm/2006/063214.pdf>
>>>> The first seems to indicate a significant source of noise at high freq
>>>> in small motors is excited resonance of the motor housing itself
>>> 
>>> Suggestion:  Remove the propellers from the quadcopter and run the
>>> motors without the props.  The residual noise should be 100% chopper
>>> noise.  I did that in the past on my Walkera XR-350 clone:
>>> <http://www.learnbydestroying.com/jeffl/pics/drivel/slides/Walkera%20QR%20X350%20Drone.html>
>>> and heard very little remaining noise from the motors.  However, my
>>> hearing is not the best, and I didn't bother making any measurements
>>> with an SPL meter.  If I find time, I might try it again later today.
>
>>If you remove the propellers, you should substitute a
>>comparable amount of torque load to keep the operating
>>conditions. The noise will be significantly lower when
>>there is no torque load.
>
> I could do that using a disk (washer) with a radius of about 30% of
> the propeller radius (center of mass) and having the same weight as
> the four propellers.  I think I have some spare props left over from
> my last attempt at indoor flying (before I added the shrouds).  It's
> not a good equivalent load torque because it's not pushing air, but
> methinks the inertial load of the disk (washer) will be sufficient to
> have the chopper produce some vibrations.

Props effectively have negative weight in flight. if you're looking for
bearing noise. I guess you can run it upside down with the weights 
summing to the craft's normal weight

-- 
  When I tried casting out nines I made a hash of it.