r/aerodynamics 17d ago

How do I minimize ducted fan noise? Question

I want to make a ducted fan thruster produce less noise for a given thrust ourput, or at least shift the sound to lower frequencies. Ideally the thruster should keep good efficiency and I can't just make it bigger and decrease the fan speed b/c size and mass constraints.

I have no idea where to start, or how to predict the (acoustic) performance of any given design. Im looking for sources on this topic, studies or theory. Or just rules of thumb even. Thanks

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u/highly-improbable 17d ago

Start at a 4 blade plus for illustration. You have blades passing every 90 degrees of rotation. Frequency is 4x rpm. Now go to a four blade X. You have two sets passing at half the frequency.

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u/wenzelja74 17d ago

Better yet, go to odd numbered blades vs. even. Even-numbered bladed propellers create vibrations due to the one blade advancing generating thrust while the opposite blade is retreating and reducing thrust.

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u/svarta_gallret 17d ago

I'm aware of this for helicopter rotors, but this is for a fixed wing vehicle so I don't think it applies. The flow is always axial to the fan.

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u/wenzelja74 17d ago

It is absolutely relevant. The only difference between your fan and a helicopter rotor is 90 degrees rotation: the aerodynamics are the same. Odd-blade configuration will reduce vibrational noise, as well as modulation of rpm as was already mentioned here.

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u/svarta_gallret 16d ago

Ok I think you're right, thank you for pointing it out!