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Why is a helicopter

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So much louder approaching than descending?

Living near the hospital, they are more common than crows!

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Not loud at all compared to jets dog fighting and going straight up over you. Can be fun to watch as it looks like they will hit each other. But supper loud,I cannot understand someone just feet away yelling at me.ūüėĶ¬†Oh maybe I am just deaf,but I still hear them leaving.ūü§£

 

Yesterday got buzzed by a copter as we picked corn. I think it may have been the corporate billionaire that bought next door piece of land.

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17 minutes ago, MTO said:

So much louder approaching than descending?

Living near the hospital, they are more common than crows!

Do they take off faster than they land and it doesn’t seem as loud? When we set up LZ’s they seem to take forever to a set down but are gone in a flash. 

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5 hours ago, MTO said:

So much louder approaching than descending?

Living near the hospital, they are more common than crows!

Taking off they are generating off more thrust to ascend.  Descending the only thrust that is being generated is just enough to keep them from free falling.

In transit the thrust has to provide lift as well as directional movement simultaneously. ¬†I¬†would think that there¬†is not a lot of mechanical efficiency in the ‚Äútraction‚ÄĚ aspect of a rotor. ¬†So I am guessing that the additional ‚Äúslippage‚ÄĚ is being heard as additional noise.

My simple analogy is to a car rolling towards you versus a car power sliding towards you.  Which one is louder?

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My (fixed wing) flight instructor said helicopters can’t really fly.  They just beat the air into submission.

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1 hour ago, 1586 Jeff said:

My simple analogy is to a car rolling towards you versus a car power sliding towards you.  Which one is louder?

Ex girlfriend tried running you over much?

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Just now, Ihfan4life said:

Ex girlfriend tried running you over much?

Negative.

It was my wife that did that.  A Prius does not power slide and is silent until the engine starts.....

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Doppler effect.

 

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Not Doppler effect. Blade slap is the bulk of the noise on approach. The region of the most slap is low power descent around 65 knots, exactly what one is doing on approach. It's mostly caused by the blades flying to the tip vortice of the preceding blade. If you look at one of the newer machines you'll see the blade tips are no longer just squared off like older designs but have various profiles designed both to increase efficiency and reduce noise due to blade slap.

Doppler effect is more apparent when being overflown. It's, greatly simplified, the reason old large two blade designs like a Huey can be heard for miles coming then magically quiet after passing overhead.

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If you want to hear something weird sounding, check out the V-22 Osprey tilt rotor craft. They fly by here every so often. The first time I ever heard one approaching, I thought what the heck is that? They make a unique sound all their own. 

 

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1 hour ago, New Englander said:

Not Doppler effect. Blade slap is the bulk of the noise on approach. The region of the most slap is low power descent around 65 knots, exactly what one is doing on approach. It's mostly caused by the blades flying to the tip vortice of the preceding blade. If you look at one of the newer machines you'll see the blade tips are no longer just squared off like older designs but have various profiles designed both to increase efficiency and reduce noise due to blade slap.

Doppler effect is more apparent when being overflown. It's, greatly simplified, the reason old large two blade designs like a Huey can be heard for miles coming then magically quiet after passing overhead.

Thanks for the aerodynamic details of rotary-wing aircraft.  A brief research does show the sound profiles to be very complex and the majority of what we hear is as you have explained.

It appears that Doppler shift also contributes and plays a part in the noise received by our ears.  Specifically referenced in Sections 5.4 and 8 of the below.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880017523.pdf

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Descending was a poor word choice.

They are simply SO much louder coming than going. 

I guess I have a better understanding now that New Englander explained the blade air effects.

 

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l don't know much about rotor aerodynamics or doppler effect but l do know a Huey has a sound ll it's own.....coming or going.

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2 hours ago, Atilathehun99 said:

Thanks for the aerodynamic details of rotary-wing aircraft.  A brief research does show the sound profiles to be very complex and the majority of what we hear is as you have explained.

It appears that Doppler shift also contributes and plays a part in the noise received by our ears.  Specifically referenced in Sections 5.4 and 8 of the below.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19880017523.pdf

Ha! As I said: "greatly simplified"! The NASA paper you referenced looks like something Sheldon on Big Bang would be working on.

That paper is so old it's written on a typewriter but of course, still valid. In practical applications there are noise abatement techniques that can mitigate some of the problem but requires non-optimal descent profiles and flight inside of dead man's curve (height/velocity curve) that pilots are loath to enter unnecessarily. With a critical patient aboard time is of the essence. I  think rooftop helipads often don't lend themselves to those techniques either.

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5 hours ago, New Englander said:

That paper is so old it's written on a typewriter but of course, still valid.

Aye.

Newton prob used a quill and ink well to enlighten the world to his discoveries but as you've inferred no matter the technology of dissemination they are are true now (before and forever) as they were in 1687.

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.....sometimes I almost forget this is a tractor forum.

I enjoy it though. Lotta smart people here. 

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