Noise reduction methods

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SamP

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I've been reading about articles about how for delivery drones and EVTOLs to see widespread use, they need to address how noisy they are. Trying to think of ways to make them quieter.
Reducing propeller speed is probably the biggest. I would imagine that would mean more blades for a given engine / motor. This is probably first order. Would one big motor be quieter than many motors?
How about airframes? A more efficient airframe means that you would need less power to push it through the air. Would a conventional design be less quiet than a flying wing?

I've read that morphing wings reduce noice because there are no gaps from the ailerons and flaps, which create a lot of noise.

Other ways to make airplanes in general more quiet?
 

jedi

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Shut off the engine!

Next best thing is to reduce the power required. After that move more air more slowly.

Did you ever notice that birds in flight are much more quiet than the typical airplane. They are so quiet in flight that they have to crow, squawk or honk in order to be noticed.
 

Brünner

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According to my very limited experience, on a typical C172 the biggest noise generators are the propeller and the engine.
Can't do much about the prop, and the engine can be silenced with an extra muffler attached at the exhaust. One of my friends has a Gomolzig on its plane and he's happy with it although the engine might be producing a bit less power.
 

John.Roo

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Shut off the engine!

Next best thing is to reduce the power required. After that move more air more slowly.

Did you ever notice that birds in flight are much more quiet than the typical airplane. They are so quiet in flight that they have to crow, squawk or honk in order to be noticed.
Absolutelly right 👍
No rotating prop = no noise ;)
According to my experience is source of noise propeller (I suppose we speak about propeller drive airplanes).
You can notice sound of engine only up to some RPM. Than noise produced by prop.
Ideal way how to reduce noise = decrease RPM of prop ;)
 

Geraldc

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Did you ever notice that birds in flight are much more quiet than the typical airplane. They are so quiet in flight that they have to crow, squawk or honk in order to be noticed.
Maybe deliver pizzas with birds.
 

henryk

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Maybe deliver pizzas with birds.
-or man powered orni...


"BOLS" service !

PS=not so "offtop"=glider "powered" by dusts energy (not electric !)

 
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Victor Bravo

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High aspect ratio glider-style airframes with better aerodynamic shape will reduce noise, power requirement operating cost, fuel, etc. Propellers or rotors that also are high aspect, slower, cleaner will achieve the same results as wings. Using the Bowers/Prandtl twist distribution on rotors and propellers will likely further reduce noise and power requirements. A properly designed large single rotor helicopter drone will make less noise than the equivalent quad or multi-copter drone.
 

BJC

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High aspect ratio glider-style airframes with better aerodynamic shape will reduce noise, power requirement operating cost, fuel, etc. Propellers or rotors that also are high aspect, slower, cleaner will achieve the same results as wings. Using the Bowers/Prandtl twist distribution on rotors and propellers will likely further reduce noise and power requirements. A properly designed large single rotor helicopter drone will make less noise than the equivalent quad or multi-copter drone.
One of my college professors was doing research aimed at making military helicopter rotors quieter.


Always wondered what he discovered. Any former military helicopter pilots here know?


BJC
 

SamP

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Thanks all for the replies, especially the funny ones. :)

High aspect ratio glider-style airframes with better aerodynamic shape will reduce noise, power requirement operating cost, fuel, etc. Propellers or rotors that also are high aspect, slower, cleaner will achieve the same results as wings. Using the Bowers/Prandtl twist distribution on rotors and propellers will likely further reduce noise and power requirements. A properly designed large single rotor helicopter drone will make less noise than the equivalent quad or multi-copter drone.
Thanks for your reply. I saw a lecture ppt that showed how the B-47 and the Avro Vulcan wound up with similar L/D. One with higher Aspect Ratio, the other with less wetted area. http://www.dept.aoe.vt.edu/~mason/Mason_f/SD1L3.pdf

If same L/D, why would a high AR design be quieter?

Can you also explain why a quadrotor would be noisier? Just because it has more blades beating the air? I could imagine lift distributed across more blades would enable lower endtip velocity, and reducing noise there.

Thanks again for your thoughts!
 
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