Thread: Discussion Multiplex Fun Cub
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Old May 29, 2011, 07:08 AM
SKYPILEIT is offline
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did a reversed Immelman..once.
Novi, Michigan
Joined Nov 2002
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Originally Posted by chiloschista View Post
Hi all,

I know motor vibration was discussed a lot here, but I would like to share another experience.
After tenth of successful flights I started to experience motor vibrations.
I balance propellers, but even with unbalanced propellers I don't believe vibrations could become so strong and bad.
I cross swapped three different motors, three different ESCs, and three propellers. Always the same problem.

So I did a little of reading here and found someone had similar problems.

My guess is that after sometime the original motor mount deforms (temperature, air, humidity, etc. Do you know plastic loose weight becoming dry ?) and will no more hold the motor flat and stiff.

Guess what ? I fixed the motor with two more screw (so four now) and all the vibrations went away.
So what's that now ?

Even a very small amount of unbalanced propeller / motorcase will cause a resonance, or better said a wave, from front side (propeller) and backside (motorcase), the motormount being the nodal point (are you familiar with sound physics ?). It is almost impossible to stop a single wave vibration / sound in a nodal point. Fortunately sounds have harmonics which will not all pass through the same nodal point. I guess a motor - motormount - propeller system does not have noticeable harmonics.
So if the motor is not very stiff it will vibrate like a jackhammer when resonance is induced.
I had exactly the same problem in an Acromaster, but there I fixed it fixing the motor mount with four screw.

Btw on the FunCub too I discarded the two fixing screw and fixed now the motor mount to the plastic in the fuselage with four Parker like screw and washers, instead the four adjustement screw, to have the right amount of side / down thust.

Here is a link about nodal points

Thanks to everyone who wrote about this (and all the rest too ) and hope this will help others.

Best regards,

Good info to share..Thanks Ric..!
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