64 MM EDF Motors

[Jack]

Was wondering if any of the members have tried in runner motors in this size. I'd like to find an in runner/12 blade fan combo comparable to the FW 3500kv/12blade unit. Please don't get me wrong, the FW units are great. I use them in all my 64's. Just like to get away from the hi pitched whine, organic to the out runners. If anybody has any idea's , please let me know. Thanks much......Jack

[Konrad]

Hi pitch whine of the out runners?

Most of the whine you hear is the on off wake turbulence from the rotor hitting the stators. Higher blade counts minimize the on off rotor load as more rotor blades are in line with the stators at any one time. Also most high blade count fans place the rotor farther away from the stator dampening the acoustical print from the rotor wake hitting the stators. This is done at the cost of flow efficiencies.

I suspect you are concerned with mechanical resonance in the rotating group. This is best controlled by balancing. With in-runners this is a bit easier as the rotating mass isn't as far away from the axis as an out runner. But a balanced out-runner and a balance in-runner system (impeller and motor rotor) should have close to the same whine assuming they are running at the same RPM.

I've found that the FW 64mm 12 blade rotor is a bit too flexible for more power than what FW gets with the 3500KV motor.

[Jack]

Thanks Konrad for the info. So let me see if I get this right. When you speak of balancing, you are referring to the balance between the fan and the spinning part of the outrunner, not the actual balance of the fan itself. All of my fans have been balance to the tee. I also find that at lower rpms the whine is not too bad. I'm a simple guy trying to get this right and really thanks for the information Konrad.

[fhhuber]

The whine is going to be virtually the same at the same RPM regardless of the motor.
Its the "siren" effect of the airflow being "chopped" by the fan and stator vanes.

You can change it by adding more stator vanes...

[Konrad]

Balancing of the component (the fan/impeller) can be done two ways. Single plane (static) like on a hobby prop balancer. Dual plane balancing (dynamic balancing) is done by spinning the impeller (fan) and removing measuring the weight (force) from both the front side and back side. Then in our hobby what is called dynamic balancing is really what is known as resonance suppression of an over hanging system, or trim balancing. This involves the motor rotor as well as the impeller (fan) spun at or near operating speeds.
Maybe some of the Google search images for "dual plane balancing" will help in understanding the difference between single and dual plane balancing

We are lucky that today the accelerometers in our smart phones can be used to do these dynamic/trim balance procedures.

Single plane is best done on narrow systems like the prop and relatively low rpm systems. Our EDF units are best balanced with the principles of dual plane (dynamic) balancing.

All the best,
Konrad

[Jack]

Thanks yall, so is what I am looking to do even possible without a big prob ? The tech info is great . Jack

[Konrad]

Thanks yall, so is what I am looking to do even possible without a big prob ? The tech info is great . Jack

What is that? Running in-runners in 64mm fan units? Not much of a problem.

Getting rid of the whine using in-runners or any other motor and NOT dynamically balancing the rotating mass? That will be an issue, as it is not likely that all the factors will line up properly to result in little or no whine.

[fhhuber]

Spend no money...

Experiment with a cheap ARF EDF from a crashed plane...

Test run and note the sound.
Pull the fan out.
Grind 1/16 inch off each vane increasing space between the vanes and the fan.
Reassemble.
Run it and note the change in tone.

Change to another (same dia) EDF's fan blade (different blade count preferably lower load)
Run it again.
Note that the sound changed again.


The whine is mostly the fan and stators interacting... not the motor.

[Konrad]

I think his whine issue is the resonant issue (rotor whip), not the wake turbulence issue we see between the 5 bladed impellers and the high blade count impellers 9 to 12.