Balanced 3 blade EDF still vibrate

[Babylon5]

Hi, has anyone encounter this phenomenon. After I have balance a 64mm 3 bladed EDF impeller and attached the impeller to the motor.
The impeller runs very smoothly , no vibration all the way to 32,000rpm outside the shroud. After I re-install the blade and motor into the EDF shroud and check to ensure wall to blade clearance of about 1mm all round and no loose parts .
When I ran the test again at around 3/4 power there is a noticeable vibration. Sound like some sort of resonance.
I am puzzle

Can anyone explain this phenomenon? Is there any remedy?

Thank you all in advance.
Cheers

[Warbird]

maybe the resonance created by the stators? You changed the blade count, correct?

[Babylon5]

Hi warbird , thank you for your feedback but I am still puzzle.
I have cut down to 3 blade with SFM impeller (super flying model) with 4 stator no problem(no vibration). When I replaced the impeller with the GWS64 cut down to 3 blade to try out then I see this problem.

What I am experiment is to compare which blade is more efficient since the GWS64mm impeller fit nicely into the 68 mm EDF from super flying model.

Another difference is I use a slender motor B20-25 series (20mm diameter) instead of the 24-25mm diameter motors so there are gaps and hollow at the motor mount between the motor and the motor holder.

Cheers

[Warbird]

Maybe your motor is not properly secured?

what does this mean: Another difference is I use a slender motor B20-25 series (20mm diameter) instead of the 24-25mm diameter motors so there are gaps and hollow at the motor mount between the motor and the motor holder.

The stators are build to take the twist out of the airflow generated by the rotor. different rotors need different stators (this also applies for the number of rotor blades). A proper designed fan has his stators matched to the number and pitch of the rotor blades to give best efficiency at flying speed.

A possible cause for the vibration:
If you fix a different rotor into the fan the airflow from rotor to stator might be turbulent resulting in pressure differences which in return can stall the rotor blades or create resonance - bouncing the blades over the pressure areas (similar to flipping a stone over the water).

[Babylon5]

Thanks Eric, Oh I see.
OK I will try something else tonight.


Cheers

[Babylon5]

Hi I replace the cut down GWS 3 blade to the original 6 rotary bladed of SFM EDF . Cut off the 3 blade and balance on a prop balancer .

Ran the test from low to high trottle. Bingo! no noticeable vibration, so looks like I have to stick to the original rotary blade. Ok to cut down from 6 blade to 3 blade.(This is nothing new as lots of EDF hobbyist are doing it with the SFM EDF on the internet for faster flight at lower power).

I will be running more test with this setup on other motors .More on this later.