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motor vibrating uncontrollably at high throttle

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Old 07-20-2017, 07:24 PM
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Corsair013
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Default motor vibrating uncontrollably at high throttle

Why is my rc airplanes motor vibrating uncontrollably at high throttle? I am running a 1500kv Turnigy SK3 spinning a 8x6 prop.
Motor runs offa 30amp esc, and i use a 2200 3cell 25c lipo.

it runs fine up to just above half throttle, then it vibrates uncontrollably and once even broke the mount!

Prop is balanced, nothing is bent.

What is going on!!!!??

BTW, I am using the motor in this plane: Click image for larger version

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Thanks
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Old 07-21-2017, 06:02 AM
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Post an image of the motor mount with and without the motor attached. Dan.
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Old 07-21-2017, 08:00 AM
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You say the prop is balanced and nothing is bent but it really sounds as if something is bent or out of balance
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Old 07-21-2017, 12:12 PM
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Quote:
Originally Posted by All Day Dan View Post
Post an image of the motor mount with and without the motor attached. Dan.
Here you go, pictures of it with and without the motor:
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Old 07-21-2017, 12:14 PM
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Quote:
Originally Posted by BarracudaHockey View Post
You say the prop is balanced and nothing is bent but it really sounds as if something is bent or out of balance
Brand new prop, checked it was balanced. Also tried the motor with different props and same result. Runs fine without any though.

No evidence of shaft bending, all straight as far as I can see.
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Old 07-21-2017, 01:19 PM
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Try it with just the hub and spinner. Sometimes over tightening mashes things out of place.

Dennis
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Old 07-21-2017, 02:16 PM
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Thank you, will try now.
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Old 07-21-2017, 02:31 PM
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Quote:
Originally Posted by Propworn View Post
Try it with just the hub and spinner. Sometimes over tightening mashes things out of place.

Dennis
Just tried it now, works just fine all the way up to full throttle.

Would a too small prop do this? (vibrate on a too large motor?)
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Old 07-21-2017, 04:07 PM
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Quote:
Originally Posted by Corsair013 View Post
Just tried it now, works just fine all the way up to full throttle.

Would a too small prop do this? (vibrate on a too large motor?)
Don't think so ducted fans are even smaller and rev higher without problems. A prop can be balanced statically but be out of balance dynamically. First try a different prop preferably different brand. We had problems with hobby king props vibrating right off the motors on our indoor.

Try taking the motor out of the airframe and mount to a solid block of wood and run it up with the prop and spinner. If there is no vibration then I would suspect what they call soft foot which is a soft, flexible or weak motor mount structure on the airframe. Try re gluing and some re-enforcement in places you suspect may be weak.

Dennis
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Old 07-21-2017, 04:55 PM
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That's one long cantilever you have there. All those lightening holes, lite plywood and no gussets add up to a weak structure. Add the weight of the motor and torque from the prop it's no surprise you experienced your problem. As a last resort you may want to consider beefing up the structure with some aircraft plywood, hard wood triangular stock and plenty of epoxy. Dan.
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Old 07-21-2017, 05:28 PM
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Quote:
Originally Posted by Propworn View Post
Don't think so ducted fans are even smaller and rev higher without problems. A prop can be balanced statically but be out of balance dynamically. First try a different prop preferably different brand. We had problems with hobby king props vibrating right off the motors on our indoor.

Try taking the motor out of the airframe and mount to a solid block of wood and run it up with the prop and spinner. If there is no vibration then I would suspect what they call soft foot which is a soft, flexible or weak motor mount structure on the airframe. Try re gluing and some re-enforcement in places you suspect may be weak.

Dennis
Ok, will do. The mount seemed to be a bit weak looking.
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Old 07-21-2017, 05:30 PM
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Quote:
Originally Posted by All Day Dan View Post
That's one long cantilever you have there. All those lightening holes, lite plywood and no gussets add up to a weak structure. Add the weight of the motor and torque from the prop it's no surprise you experienced your problem. As a last resort you may want to consider beefing up the structure with some aircraft plywood, hard wood triangular stock and plenty of epoxy. Dan.
Yup, structural reinforcement seems to be the answer.

Thanks!
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Old 07-23-2017, 10:17 AM
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Quote:
Originally Posted by Corsair013 View Post
... Would a too small prop do this? (vibrate on a too large motor?)
No, not possible. It is impossible for motor to run faster than Kv voltage.

Construction is strong enough, it can handle the vibrating, but it is not stiff enough.
Motor mound is already strengthened at bottom, I would strenghten the motor mount on top as well, close the box as it were, with horizontal piece of ply.
Glue outside the gap is useless, adds only weight. Remove excess glue before it sets.

Vriendelijke groeten Ron

Last edited by ron_van_sommeren; 07-23-2017 at 10:26 AM.
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Old 07-23-2017, 07:09 PM
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ok, thanks
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Old 07-24-2017, 09:33 AM
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Your prop may be balanced but is it running centered & true on the motor shaft?
With the prop running slowly and looking directly from the side do both prop tips pass in exactly the same place?
If they do not then the motor will 'feel' an out of balance force that gets worse with speed and any 'give' in the motor mount will likely make it even worse..
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Old 07-24-2017, 11:32 AM
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ok, will try that.
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