Charlie757
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Buzzsaw sound on takeoff

Thu Jun 20, 2019 6:54 pm

Recently took two flights on an A321 and B77W sitting in front of the engine and noticed that even when power is reduced after takeoff the buzzsaw sound never seems to go away it just fades out slightly?


I always thought the buzzsaw noise went away after decel but does the noise just get drowned out/carried away by wind noise outside the aircraft?

May sound like a stupid question but I’m just curious as I don’t sit forward of the wing much.
 
11C
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Re: Buzzsaw sound on takeoff

Sat Jun 22, 2019 2:13 pm

I’ve read that that is the sound of the fan blade tips exceeding the speed of sound. The bigger the fan gets, the more speed at the tip (high bypass engines have progressively gotten bigger). Definitely noticeable at takeoff power. That problem goes away with geared turbo fan engines because the fan speed is lower.
 
N766UA
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Re: Buzzsaw sound on takeoff

Sat Jun 22, 2019 2:48 pm

Even CRJ’s have a noticable buzz.
 
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767333ER
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Re: Buzzsaw sound on takeoff

Sat Jun 22, 2019 3:50 pm

As the engine spools up the buzzing starts as a sine wave like low hum but as the engine speed increases sonic booms in the ends of the fans get more powerful it transforms into a buzz. Even on decent and approach you’ll hear a hum, this is the same thing. Regular fans and geared fan should alike do it, but when you get higher and faster as you may imagine the conditions are different and you can only hear the sine wave like sound rather than the buzzing when you are cruising or in later stages of climb.
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slcguy
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 1:45 pm

On some engines there is growling sound before the fans have even spooled up as well as at full power when it is growl and buzz. Anybody that ever flew on an L-1011 knows that sound. The buzz only happened at takeoff but the growl happened anytime during flight when power was increased. Those early big fan RB-211s were great to listen too. You could be a half mile away on the airport and hear that growl and know an L-1011 was starting up.
 
11C
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 1:59 pm

767333ER wrote:
As the engine spools up the buzzing starts as a sine wave like low hum but as the engine speed increases sonic booms in the ends of the fans get more powerful it transforms into a buzz. Even on decent and approach you’ll hear a hum, this is the same thing. Regular fans and geared fan should alike do it, but when you get higher and faster as you may imagine the conditions are different and you can only hear the sine wave like sound rather than the buzzing when you are cruising or in later stages of climb.


I read some info on the Pratt website that seems to indicate that this problem is eliminated in GTF engines. The whole reason for the gear box is to optimize the two fan speeds, allowing the front fan to turn much slower, optimizing fuel consumption, eliminating some compressor sections, and also reducing the noise signature. I’ve ridden in a Hawaiian 321 NEO, but haven’t sat forward of the engines yet. From the back of the cabin the engines are very quiet.
 
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Veigar
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 2:17 pm

You flew an A321 and a 777. Try the buzz sounds from JT8D or RB211 on the S80 and 757 respectively.

Power is reduced after the airplane is in cruising, I believe.
 
SpaceshipDC10
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 2:43 pm

slcguy wrote:
Anybody that ever flew on an L-1011 knows that sound. The buzz only happened at takeoff but the growl happened anytime during flight when power was increased. Those early big fan RB-211s were great to listen too. You could be a half mile away on the airport and hear that growl and know an L-1011 was starting up.


Yep.

https://youtu.be/EawJatx3caQ?t=637


The CF6-50 is quite something too on the matter of buzzsaw:

https://youtu.be/N8JXK283zhM?t=932

https://youtu.be/tVTz06lP77k

https://youtu.be/mYTH57Mcgdc

It growls too at engine start: https://youtu.be/cQH48XCQJDs I always loved that sound
 
11C
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 2:59 pm

The best information I can find shows that the V2500 fan turns at 5650 rpm, while the PW1100G turns at 3200 rpm (at takeoff power, as best as I can tell). Obviously the 81” fan size on the PW1100G increases the distance each blade tip travels in one revolution versus the V2500. There is a website that provides the formula for those interested in calculating the distance traveled of the respective blade tips. Apparently, the PW1100G has various other enhancements to reduce the acoustic signature.
 
Waterbomber2
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 4:50 pm

Veigar wrote:
You flew an A321 and a 777. Try the buzz sounds from JT8D or RB211 on the S80 and 757 respectively.

Power is reduced after the airplane is in cruising, I believe.


My god... Some uneducated replies here.
For turbofans the measurable quantity is thrust, not power.
Thrust is not reduced per se, the design of engines and the lower air density at altitude result in lower thrust.

The GTF engines also have the buzz saw sound.
The fan turns slower but the fan is bigger.
You just need to find one taking off at a high thrust setting.

The Max N1 on the PW1100G is 3350RPM, the fan diameter 2.06 meters, resulting in 21669m/min tip speed.
ISA speed of sound is 343m/s or 20580m/min, if colder lower than that.
Plus the air around a fan blade is accelerated by its shape, so shockwaves are generated before that.

Generally, it's assumed that the supersonic airflow at the fan blade tip causes the noise.
SNECMA took a patent on some circular protruding shrouds to be placed on the intake ahead of the fan to keep the normal shockwaves from exiting from the front of the engine. It was counter productive though as it blocked off the airflow to the fan tips.

I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.

IMO the noise is owed to the bearings and general mechanical vibrations owed to imbalances of the rotating parts of the engine, combined with air vibrations caused by vortices slapping fan blades.
In the below video you can realise that the sound exacerbates when the aircraft rotates and starts climbing.
In this scenario, due to the relative angle of the airflow, the rotating parts are even more imbalanced, causing even more vibration and noise. There is no evidence to suggest that the pilots increased the engine RPM during the climb.

https://youtu.be/7qXewp-SYuw

If you rev up any car engine towards their RPM limits, you will hear very similar sounds, no supersonic blades involved there.
 
AirFiero
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 5:12 pm

slcguy wrote:
On some engines there is growling sound before the fans have even spooled up as well as at full power when it is growl and buzz. Anybody that ever flew on an L-1011 knows that sound. The buzz only happened at takeoff but the growl happened anytime during flight when power was increased. Those early big fan RB-211s were great to listen too. You could be a half mile away on the airport and hear that growl and know an L-1011 was starting up.


I always thought the best buzzsaw/growl was from the L-1011. The DC-10 and early 747s were good, too.

https://youtu.be/378zRym9jrc

https://youtu.be/TG4_7dvjpQM
 
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DocLightning
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 9:58 pm

Waterbomber2 wrote:
I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.


Close. Once the engine is turning at even idle, the centrifugal force of the blades will cause their roots to settle into position and not rattle.

On a thread in Tech/Ops from a few years back, we learned that the cause of the buzz is not the blade-passing frequency (which would be too high to hear) but rather the fundamental harmonic is N1. Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

For 3350RPM, that works out to 55.8 RPS or 55.8Hz (55.833333333...Hz if sig figs don't mean anything to you). The RB211-524 series of L-1011 fame had a maximum N1 of 3900 RPM for a beautiful 65Hz buzzsaw (assuming 100% N1, on one video I was able to match it to 60Hz because they rarely used 100%N1). Of course, as manufacturing practices have gotten better and blades are now made of composite, those irregularities are being smoothed out and that's why the buzzsaw is a lot quieter in the newer engines than in the older ones where the blades were titanium with a manufacturing process that was more prone to minor variations.
-Doc Lightning-

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acjbbj
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 10:50 pm

DocLightning wrote:
Waterbomber2 wrote:
I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.


Close. Once the engine is turning at even idle, the centrifugal force of the blades will cause their roots to settle into position and not rattle.

On a thread in Tech/Ops from a few years back, we learned that the cause of the buzz is not the blade-passing frequency (which would be too high to hear) but rather the fundamental harmonic is N1. Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

For 3350RPM, that works out to 55.8 RPS or 55.8Hz (55.833333333...Hz if sig figs don't mean anything to you). The RB211-524 series of L-1011 fame had a maximum N1 of 3900 RPM for a beautiful 65Hz buzzsaw (assuming 100% N1, on one video I was able to match it to 60Hz because they rarely used 100%N1). Of course, as manufacturing practices have gotten better and blades are now made of composite, those irregularities are being smoothed out and that's why the buzzsaw is a lot quieter in the newer engines than in the older ones where the blades were titanium with a manufacturing process that was more prone to minor variations.

C2 is 65.406 Hz.
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ikramerica
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Re: Buzzsaw sound on takeoff

Sun Jun 23, 2019 11:07 pm

DocLightning wrote:
Waterbomber2 wrote:
I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.


Close. Once the engine is turning at even idle, the centrifugal force of the blades will cause their roots to settle into position and not rattle.

On a thread in Tech/Ops from a few years back, we learned that the cause of the buzz is not the blade-passing frequency (which would be too high to hear) but rather the fundamental harmonic is N1. Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

For 3350RPM, that works out to 55.8 RPS or 55.8Hz (55.833333333...Hz if sig figs don't mean anything to you). The RB211-524 series of L-1011 fame had a maximum N1 of 3900 RPM for a beautiful 65Hz buzzsaw (assuming 100% N1, on one video I was able to match it to 60Hz because they rarely used 100%N1). Of course, as manufacturing practices have gotten better and blades are now made of composite, those irregularities are being smoothed out and that's why the buzzsaw is a lot quieter in the newer engines than in the older ones where the blades were titanium with a manufacturing process that was more prone to minor variations.

This adds to what I was going to mention about the long period and sometimes variable period womp, womp, womp some twins experience. Its also a harmonic when the two engines, not spinning at the same exact speed, create a not so pleasant combination.
Of all the things to worry about... the Wookie has no pants.
 
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DocLightning
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 12:23 am

ikramerica wrote:
This adds to what I was going to mention about the long period and sometimes variable period womp, womp, womp some twins experience. Its also a harmonic when the two engines, not spinning at the same exact speed, create a not so pleasant combination.


You mean a beats frequency?

https://en.wikipedia.org/wiki/Beat_(acoustics)
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AWACSooner
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 1:01 am

RB211 is, IMO, the best buzz going in the industry.

And then there was the old C-5’s!
 
FlyingLaw1
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 1:32 am

 
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767333ER
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 6:09 am

11C wrote:
I read some info on the Pratt website that seems to indicate that this problem is eliminated in GTF engines. The whole reason for the gear box is to optimize the two fan speeds, allowing the front fan to turn much slower, optimizing fuel consumption, eliminating some compressor sections, and also reducing the noise signature. I’ve ridden in a Hawaiian 321 NEO, but haven’t sat forward of the engines yet. From the back of the cabin the engines are very quiet.

On high enough thrust it still exists. The RPM is no slower than it has to be to create the 35000lb of thrust it can make with its fan gear or no gear. The gear really just speeds the rest of the engine up to the optimal speed much like 3 spool engines.
Waterbomber2 wrote:
Veigar wrote:
You flew an A321 and a 777. Try the buzz sounds from JT8D or RB211 on the S80 and 757 respectively.

Power is reduced after the airplane is in cruising, I believe.


My god... Some uneducated replies here.
For turbofans the measurable quantity is thrust, not power.
Thrust is not reduced per se, the design of engines and the lower air density at altitude result in lower thrust.

The GTF engines also have the buzz saw sound.
The fan turns slower but the fan is bigger.
You just need to find one taking off at a high thrust setting.

The Max N1 on the PW1100G is 3350RPM, the fan diameter 2.06 meters, resulting in 21669m/min tip speed.
ISA speed of sound is 343m/s or 20580m/min, if colder lower than that.
Plus the air around a fan blade is accelerated by its shape, so shockwaves are generated before that.

Generally, it's assumed that the supersonic airflow at the fan blade tip causes the noise.
SNECMA took a patent on some circular protruding shrouds to be placed on the intake ahead of the fan to keep the normal shockwaves from exiting from the front of the engine. It was counter productive though as it blocked off the airflow to the fan tips.

I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.

IMO the noise is owed to the bearings and general mechanical vibrations owed to imbalances of the rotating parts of the engine, combined with air vibrations caused by vortices slapping fan blades.
In the below video you can realise that the sound exacerbates when the aircraft rotates and starts climbing.
In this scenario, due to the relative angle of the airflow, the rotating parts are even more imbalanced, causing even more vibration and noise. There is no evidence to suggest that the pilots increased the engine RPM during the climb.

https://youtu.be/7qXewp-SYuw

If you rev up any car engine towards their RPM limits, you will hear very similar sounds, no supersonic blades involved there.

You are pretty much right about the PW1000G. Calling people uneducated for using the term power however may seem right if all you’re doing is looking at manuals, but from a physics standpoint power is somewhat correct as power is what the engine makes to turn the fan to create thrust. If you increase the thrust of an engine the power output of the engine increases to do so as the engine has to “do work” to get the fan spinning faster.

DocLightning wrote:
Close. Once the engine is turning at even idle, the centrifugal force of the blades will cause their roots to settle into position and not rattle.

On a thread in Tech/Ops from a few years back, we learned that the cause of the buzz is not the blade-passing frequency (which would be too high to hear) but rather the fundamental harmonic is N1. Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

For 3350RPM, that works out to 55.8 RPS or 55.8Hz (55.833333333...Hz if sig figs don't mean anything to you). The RB211-524 series of L-1011 fame had a maximum N1 of 3900 RPM for a beautiful 65Hz buzzsaw (assuming 100% N1, on one video I was able to match it to 60Hz because they rarely used 100%N1). Of course, as manufacturing practices have gotten better and blades are now made of composite, those irregularities are being smoothed out and that's why the buzzsaw is a lot quieter in the newer engines than in the older ones where the blades were titanium with a manufacturing process that was more prone to minor variations.

I like your use of math here. I do want to point out one thing though, if you are referring to the sound such as a propellor makes when you say blade-passing frequency then those are very audible on a turbofan. I will use a CFM56-5B for example, it maxes out at 5000 rpm N1 which means the fan buzzes at 83.3(...)Hz which sounds right to me. Now a propellor makes that sound that we all know and some of us love and some of us hate. Take the Dash 8-300 for example when the props run at 1200 rpm for takeoff, 1200/60 is 20Hz*4(blades) which then is 80Hz which is exactly the right frequency of noise the Dash 8 makes that feels like it’s vibrating the entire space time continuum. Apply this principal to what is basically an enclose propellor with many more blades in this case 36 on the CFM and the math goes as such: 5000rpm/60=83.3Hz -> 83.3*36(blades)=3000Hz. Now the buzz’s fundamental frequency is 83.3Hz, but of course we call it a buzz because it has many more other frequencies blended it that make it more like a sawtooth sort of wave. Otherwise you’ll hear this high droning whistle at, you guessed it, 3000Hz. This is whistle the sound you hear when the engine is spooling up and it’s the same howling sound you hear on wider chord fans like the famous howl/whine of a GE90 but of course it has fewer blades and is much bigger so lower frequency. If blade-passing is referring to something else than this is something different.

Take this video for example: https://www.youtube.com/watch?v=tY9Ygvwn29E&t=26s
As the engine spools up here you hear that fan blade whistle climbing towards nearly 3000Hz and then the buzz kicks in with the principal frequency hitting nearly that 83Hz.

And for the record, here’s proof the PW1100G can buzz: https://www.youtube.com/watch?v=D-YwHDnLz_o&t=4m26s

Remarkably quiet, but it’s there.
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FatCat
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 9:07 am

I was on a V2500 equipped LH A321 some times ago, row 6, the metallic noise at take off was very very loud. Some of the passengers even got worried about it...
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flysenior
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 9:29 am

Since last year I have experienced 4 or 5 times with Air Asia flights A320 that the buzzsaw sound suddenly quietens to no sound at all about 3 minutes after take off and in full climb. The aircraft then usually banks to the right and after about 1 minute (which feels like an eternity to me) the engines go to full power again.
The last time, I asked cabin crew why this happened. They had no idea and went to ask the Captain. The response was that they had to slow down on speed for traffic and that everything was safe.
I always thought that an aircraft was only cleared when its flight path was clear too. So I am still wondering.. why in full climb and only 3 minutes after take-off, speed should be reduced for traffic.
Anyone has any confirmation or other explanation to this.
It happens flying out of Kuala Lumpur, which is busy but I do not expect it to be so busy that climbs have to be delayed?
It once also happened on a flight with Cathay Pacific out of Hong Kong.
Would appreciate any comments. It will calm me down, next time it happens. .
 
Valdarez
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 9:45 am

flysenior wrote:
Since last year I have experienced 4 or 5 times with Air Asia flights A320 that the buzzsaw sound suddenly quietens to no sound at all about 3 minutes after take off and in full climb. The aircraft then usually banks to the right and after about 1 minute (which feels like an eternity to me) the engines go to full power again.
The last time, I asked cabin crew why this happened. They had no idea and went to ask the Captain. The response was that they had to slow down on speed for traffic and that everything was safe.
I always thought that an aircraft was only cleared when its flight path was clear too. So I am still wondering.. why in full climb and only 3 minutes after take-off, speed should be reduced for traffic.
Anyone has any confirmation or other explanation to this.
It happens flying out of Kuala Lumpur, which is busy but I do not expect it to be so busy that climbs have to be delayed?
It once also happened on a flight with Cathay Pacific out of Hong Kong.
Would appreciate any comments. It will calm me down, next time it happens. .

Takeoff thrust is typicaly limited to 5 minutes of usage ,at acceleration altitude the fmc will reduce the thrust to CLB thrust or derated CLB 1/2 hence the quitening of the engine buzzsaw,as you climb higher it will gradually increase its N1 limit as air density decreases (you need more"pull" the less dense the air is)
Do correct me me if im wrong guys :)
 
masi1157
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 10:22 am

DocLightning wrote:
Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

It is even worse: It is not just an irregular sine wave but rather a buzz saw shaped time signal which contains a strong peak at the fundamental frequency N1 and a large number of higher harmonics. This is caused by the fact that the shocks from each blade are not only slightly different, but also so strong that their propagation speed is no longer constant but depends on their amplitude. Larger shocks travel faster and "eat up" smaller shocks in front of them.


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Stickpusher
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 5:03 pm

Waterbomber2 wrote:
Thrust is not reduced per se, the design of engines and the lower air density at altitude result in lower thrust.


I'm not discussing fan engines here, but it may be of general interest anyway.

The old Comets used by Dan-Air on their longer routes (e.g. UK down to Tenerife) used to cruise with two Avon engines doing all the work and the outer two at flight idle. I never discovered whether it was fuel economy or engine life or what the reason was, and there's always the chance that it depended on upper air conditions. As a kid I used to ride in the jump seats every few weekends and this snippet came my way more than once. Since I was across from the FE all the time I got to hear all kinds of arcane stuff like that. Happy days.

The Comet was rather overpowered, a bit like the 757, so with 4 engines I guess they could run on two without the asymmetry once at cruise!

Of course with turbojets you don't get the buzzsaw at all, and their spool-up time was more rapid, with Concorde perhaps being the most impressive of the lot.
 
AirFiero
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 5:38 pm

767333ER wrote:
11C wrote:
I read some info on the Pratt website that seems to indicate that this problem is eliminated in GTF engines. The whole reason for the gear box is to optimize the two fan speeds, allowing the front fan to turn much slower, optimizing fuel consumption, eliminating some compressor sections, and also reducing the noise signature. I’ve ridden in a Hawaiian 321 NEO, but haven’t sat forward of the engines yet. From the back of the cabin the engines are very quiet.

On high enough thrust it still exists. The RPM is no slower than it has to be to create the 35000lb of thrust it can make with its fan gear or no gear. The gear really just speeds the rest of the engine up to the optimal speed much like 3 spool engines.
Waterbomber2 wrote:
Veigar wrote:
You flew an A321 and a 777. Try the buzz sounds from JT8D or RB211 on the S80 and 757 respectively.

Power is reduced after the airplane is in cruising, I believe.


My god... Some uneducated replies here.
For turbofans the measurable quantity is thrust, not power.
Thrust is not reduced per se, the design of engines and the lower air density at altitude result in lower thrust.

The GTF engines also have the buzz saw sound.
The fan turns slower but the fan is bigger.
You just need to find one taking off at a high thrust setting.

The Max N1 on the PW1100G is 3350RPM, the fan diameter 2.06 meters, resulting in 21669m/min tip speed.
ISA speed of sound is 343m/s or 20580m/min, if colder lower than that.
Plus the air around a fan blade is accelerated by its shape, so shockwaves are generated before that.

Generally, it's assumed that the supersonic airflow at the fan blade tip causes the noise.
SNECMA took a patent on some circular protruding shrouds to be placed on the intake ahead of the fan to keep the normal shockwaves from exiting from the front of the engine. It was counter productive though as it blocked off the airflow to the fan tips.

I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.

IMO the noise is owed to the bearings and general mechanical vibrations owed to imbalances of the rotating parts of the engine, combined with air vibrations caused by vortices slapping fan blades.
In the below video you can realise that the sound exacerbates when the aircraft rotates and starts climbing.
In this scenario, due to the relative angle of the airflow, the rotating parts are even more imbalanced, causing even more vibration and noise. There is no evidence to suggest that the pilots increased the engine RPM during the climb.

https://youtu.be/7qXewp-SYuw

If you rev up any car engine towards their RPM limits, you will hear very similar sounds, no supersonic blades involved there.

You are pretty much right about the PW1000G. Calling people uneducated for using the term power however may seem right if all you’re doing is looking at manuals, but from a physics standpoint power is somewhat correct as power is what the engine makes to turn the fan to create thrust. If you increase the thrust of an engine the power output of the engine increases to do so as the engine has to “do work” to get the fan spinning faster.

DocLightning wrote:
Close. Once the engine is turning at even idle, the centrifugal force of the blades will cause their roots to settle into position and not rattle.

On a thread in Tech/Ops from a few years back, we learned that the cause of the buzz is not the blade-passing frequency (which would be too high to hear) but rather the fundamental harmonic is N1. Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

For 3350RPM, that works out to 55.8 RPS or 55.8Hz (55.833333333...Hz if sig figs don't mean anything to you). The RB211-524 series of L-1011 fame had a maximum N1 of 3900 RPM for a beautiful 65Hz buzzsaw (assuming 100% N1, on one video I was able to match it to 60Hz because they rarely used 100%N1). Of course, as manufacturing practices have gotten better and blades are now made of composite, those irregularities are being smoothed out and that's why the buzzsaw is a lot quieter in the newer engines than in the older ones where the blades were titanium with a manufacturing process that was more prone to minor variations.

I like your use of math here. I do want to point out one thing though, if you are referring to the sound such as a propellor makes when you say blade-passing frequency then those are very audible on a turbofan. I will use a CFM56-5B for example, it maxes out at 5000 rpm N1 which means the fan buzzes at 83.3(...)Hz which sounds right to me. Now a propellor makes that sound that we all know and some of us love and some of us hate. Take the Dash 8-300 for example when the props run at 1200 rpm for takeoff, 1200/60 is 20Hz*4(blades) which then is 80Hz which is exactly the right frequency of noise the Dash 8 makes that feels like it’s vibrating the entire space time continuum. Apply this principal to what is basically an enclose propellor with many more blades in this case 36 on the CFM and the math goes as such: 5000rpm/60=83.3Hz -> 83.3*36(blades)=3000Hz. Now the buzz’s fundamental frequency is 83.3Hz, but of course we call it a buzz because it has many more other frequencies blended it that make it more like a sawtooth sort of wave. Otherwise you’ll hear this high droning whistle at, you guessed it, 3000Hz. This is whistle the sound you hear when the engine is spooling up and it’s the same howling sound you hear on wider chord fans like the famous howl/whine of a GE90 but of course it has fewer blades and is much bigger so lower frequency. If blade-passing is referring to something else than this is something different.

Take this video for example: https://www.youtube.com/watch?v=tY9Ygvwn29E&t=26s
As the engine spools up here you hear that fan blade whistle climbing towards nearly 3000Hz and then the buzz kicks in with the principal frequency hitting nearly that 83Hz.

And for the record, here’s proof the PW1100G can buzz: https://www.youtube.com/watch?v=D-YwHDnLz_o&t=4m26s

Remarkably quiet, but it’s there.


Great explanation, thanks!

Referencing the first video, I’ve noticed there seems to be a volume component to the buzzsaw sound. Notice the sound gets louder as the airplane accelerates, with the same thrust setting? The speed of the air entering the fan blade section at the front must be a factor as well?
 
Waterbomber2
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 6:54 pm

flysenior wrote:
Since last year I have experienced 4 or 5 times with Air Asia flights A320 that the buzzsaw sound suddenly quietens to no sound at all about 3 minutes after take off and in full climb. The aircraft then usually banks to the right and after about 1 minute (which feels like an eternity to me) the engines go to full power again.
The last time, I asked cabin crew why this happened. They had no idea and went to ask the Captain. The response was that they had to slow down on speed for traffic and that everything was safe.
I always thought that an aircraft was only cleared when its flight path was clear too. So I am still wondering.. why in full climb and only 3 minutes after take-off, speed should be reduced for traffic.
Anyone has any confirmation or other explanation to this.
It happens flying out of Kuala Lumpur, which is busy but I do not expect it to be so busy that climbs have to be delayed?
It once also happened on a flight with Cathay Pacific out of Hong Kong.
Would appreciate any comments. It will calm me down, next time it happens. .


It's probably a noise abatement procedure.
Full thrust to gain sufficient altitude over a noise sensitive area, reduce thrust over the sensitive area, then go full thrust again.
 
ikramerica
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 7:12 pm

[twoid][/twoid]
DocLightning wrote:
ikramerica wrote:
This adds to what I was going to mention about the long period and sometimes variable period womp, womp, womp some twins experience. Its also a harmonic when the two engines, not spinning at the same exact speed, create a not so pleasant combination.


You mean a beats frequency?

https://en.wikipedia.org/wiki/Beat_(acoustics)

What is your link to?
Of all the things to worry about... the Wookie has no pants.
 
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 7:17 pm

I prefer to call it the "stuck moo" sound, and found it to be most noticeable on old-school A300s, followed by the L1011 and B757.
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 7:35 pm

767333ER wrote:
Take the Dash 8-300 for example when the props run at 1200 rpm for takeoff, 1200/60 is 20Hz*4(blades) which then is 80Hz which is exactly the right frequency of noise the Dash 8 makes that feels like it’s vibrating the entire space time continuum.


:mrgreen: :rotfl: I've had always searched for the right words to describe the take off sound of a Dash 8, you just put it perfectly !
 
marcogr12
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 7:45 pm

Unfortunately the long-haulers of today are just too quiet for my taste..Not even a hint of screaming during spool-up, just the familiar "howling" sound and the buzzsaw that has expanded from the 777 to the A350 and now it "plagues" even short-haul a/c like the A320neos and the A220 and E2-190 which have become soooo quiet it's gets almost boring to fly with them, if you're an avgeek..Oh bless the airlines that still have oldies like the 752/3 and 763,747s and we can still enjoy some real avgeek takeoff..

p.s. Is it my idea or did the DC-10 have similar engine sound like the A300-B2?
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Waterbomber2
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 7:48 pm

masi1157 wrote:
DocLightning wrote:
Every blade is shaped very slightly differently, no matter how hard the manufacturer tries to make them all identical. This causes subtle changes in shock strength and shape generated by each blade resulting in an irregular sine wave with a fundamental harmonic of the rotational speed of N1.

It is even worse: It is not just an irregular sine wave but rather a buzz saw shaped time signal which contains a strong peak at the fundamental frequency N1 and a large number of higher harmonics. This is caused by the fact that the shocks from each blade are not only slightly different, but also so strong that their propagation speed is no longer constant but depends on their amplitude. Larger shocks travel faster and "eat up" smaller shocks in front of them.


Gruß, masi1157


What evidence is there that shocks eat up other shocks? I can't imagine how that can happen at all.
A shockwave propagates in a normal or oblique direction and its impossible for a shockwave to catch up to a other one after they are emitted at different moments in time.

The noise is as said, a combination of
1. vibrations owed to the engine's inherent instability during high speed rotation, not limited to the fan but the entire unit, causing a resonating sound,
2. units like bearings roating at a high speed within metal enclosings definitely contribute to vibrations as well. I've done engine runs after maintenance and you can't even touch anything from how those things are shaking.
3. Vortices caused by fan blades (essentially they are like rotating wings) hitting the following blades and causing small slaps to the blades as air's compressiblity does its work. You can isolate this phenomenon more clearly on helicopters as someone pointed out earlier. It's called blade slap

The difference in shape of the fan blades due to production inacurracies make a difference in 1. but saying that they are becoming better at making blades is an overstatement. Blade shapes are becoming more complex, with the recent ones being twisted. Balancing is becoming more and more challenging, not the other way around.

I think that one of the main reason engines are bcoming quieter is because the low pressure spool is rotating slower and slower. This reduces a lot of the vibrations. Another one would be improved bearing and gear designs.
 
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727tiger
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 7:53 pm

FlyingLaw1 wrote:



I lived on Vinings Mountain in Atlanta in 1985-86, and the sound of those C-5s turning and climbing out of Dobbins AFB was spectacular.

Of course, that was nothing like the old F-4s that flew out of Dobbins at that time. They would truly rattle windows.
 
masi1157
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 9:13 pm

Waterbomber2 wrote:
What evidence is there that shocks eat up other shocks? I can't imagine how that can happen at all.

You can observe something similar on the beach. Larger waves can travel faster than smaller ones and "eat up" the smaller ones in front of them.

Gruß, masi1157
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Chemist
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Re: Buzzsaw sound on takeoff

Mon Jun 24, 2019 10:20 pm

Today's cruise ships are designed with hull shapes that create secondary waves that self cancel the primary wake. The result is a relatively tiny wake that would otherwise be huge and a large disruption to other ships.
 
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 1:00 am

[quote="marcogr12"

p.s. Is it my idea or did the DC-10 have similar engine sound like the A300-B2?[/quote]
You are correct the CF6-50 series found on the A300-B4, DC-10-30 and 747 Classics all sound very similar on taxi, start up and takeoff! The version on the A300 did make some different unique sounds (bell type sound) shortly after a decent amount of power was applied!
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DocLightning
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 2:56 am

Chemist wrote:
Today's cruise ships are designed with hull shapes that create secondary waves that self cancel the primary wake. The result is a relatively tiny wake that would otherwise be huge and a large disruption to other ships.


The reason for the bulbous bow is that it makes a wave in front of the bow that interferes destructively with the bow wave. However, waves in and of themselves do not cause drag. Rather, in a conventional bow, the water piles up in front of the bow. That means that the bow is basically climbing into a mountain of water that it has to push aside.
Image

A bulbous bow raises a wave in front of the bow one half of a wavelength ahead of where the bow wave would be so that the bow is sailing into a trough. This cancels out the bow wave and reduces the height of the "mountain" into which the bow must sail.

Image

That alone can reduce fuel consumption by ~15%. But this kind of destructive interference would not be easy to apply to engine fan blades.
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flysenior
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 5:31 am

It's probably a noise abatement procedure.
Full thrust to gain sufficient altitude over a noise sensitive area, reduce thrust over the sensitive area, then go full thrust again.[/quote]

I fly regularly the same routes with Air Asia and it only happened 4 or 5 times over 1.5 years out of maybe 40 flights.
If a noise abatement procedure, I would think it would happen every flight?
In the beginning I was really worried, certainly the first time it occurred.
Now, I would like to understand why. It would still sort of reduce my anxiety.
 
masi1157
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 7:08 am

DocLightning wrote:
However, waves in and of themselves do not cause drag..

Waves generated by the ship carry energy away that is provided by the movement of the ship. So they do cause drag.

However that has not much in common with the generating mechanism of buzz saw noise.


Gruß, masi1157
508 different segments on 100 airlines to 211 airports in 55 countries
 
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DocLightning
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 7:45 pm

masi1157 wrote:
DocLightning wrote:
However, waves in and of themselves do not cause drag..

Waves generated by the ship carry energy away that is provided by the movement of the ship. So they do cause drag.


You can get into some strange cause/effect loops in fluid dynamics, but a wave traveling away from the ship cannot act directly on the ship. It is the *production* of the wave that causes drag by applying a force counter to the direction of motion on the hull.
-Doc Lightning-

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masi1157
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 7:50 pm

DocLightning wrote:
It is the *production* of the wave that causes drag by applying a force counter to the direction of motion on the hull.

That is quite exactly what I tried to say.


Gruß, masi1157
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Revo1059
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Re: Buzzsaw sound on takeoff

Tue Jun 25, 2019 8:44 pm

slcguy wrote:
On some engines there is growling sound before the fans have even spooled up as well as at full power when it is growl and buzz. Anybody that ever flew on an L-1011 knows that sound. The buzz only happened at takeoff but the growl happened anytime during flight when power was increased. Those early big fan RB-211s were great to listen too. You could be a half mile away on the airport and hear that growl and know an L-1011 was starting up.



Oh yeah, I worked with ATA and Rich International L-1011s at ORD in the 90s/00s. I knew as soon as they started what it was. Also fun in the colder months watching the plume of smoke belch out on startup.
 
Gasman
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 9:40 am

SpaceshipDC10 wrote:
slcguy wrote:
Anybody that ever flew on an L-1011 knows that sound. The buzz only happened at takeoff but the growl happened anytime during flight when power was increased. Those early big fan RB-211s were great to listen too. You could be a half mile away on the airport and hear that growl and know an L-1011 was starting up.


Yep.

https://youtu.be/EawJatx3caQ?t=637


The CF6-50 is quite something too on the matter of buzzsaw:

https://youtu.be/N8JXK283zhM?t=932

https://youtu.be/tVTz06lP77k

https://youtu.be/mYTH57Mcgdc

It growls too at engine start: https://youtu.be/cQH48XCQJDs I always loved that sound


The CF6-50 buzzsaw is so awesomely badass that it's impossible to believe it happened by accident. The engineers must've piped it in.
 
Amiga500
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 10:44 am

Waterbomber2 wrote:
Generally, it's assumed that the supersonic airflow at the fan blade tip causes the noise.

I think though that the origin of the rattling are not the shockwaves. Dozens of blades rotating 50 times per second would not be perceptible to the human ear/brain as a high frequency rattling/buzz saw but as a constant sound.
Plus the noise is most perceptible at a normal angle rather than from the front.

IMO the noise is owed to the bearings and general mechanical vibrations owed to imbalances of the rotating parts of the engine, combined with air vibrations caused by vortices slapping fan blades.
In the below video you can realise that the sound exacerbates when the aircraft rotates and starts climbing.
In this scenario, due to the relative angle of the airflow, the rotating parts are even more imbalanced, causing even more vibration and noise.


On rotation and at higher angles of attack (i.e. climb), flow onto the fan face is distorted - which results in deformations of the blades - with each respective blade distorting in different directions depending on where they are in their rotation (i.e. if blade 1 is at top then blade 11 distorts very differently in a 22 blade fan disk).

This moves the shape of each blade away from uniform and causes the shockwaves to be different for each blade at each position in the engine - it is then this "distribution" of different tonal noises that increases the buzzsaw noise you perceive - particularly as it is moving away from what the acoustic liner is designed to best attenuate.


I've never seen any research yet that indicated bearings are a substantial source of noise on turbofans. Perhaps it has just not been considered but...
 
mchei
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 10:50 am

The C-Series alias A220 also makes a sound when power is applied coming from idle. I find it pretty annoying on the ground when the aircraft needs to stop and increases power to be able to taxi. Especially because the aircraft is the opposite of being noisy this draws my attention every time.
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masi1157
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 11:05 am

Amiga500 wrote:
I've never seen any research yet that indicated bearings are a substantial source of noise on turbofans. Perhaps it has just not been considered but...

If they were, they would have been considered. And if they really would generate any considerable noise I would not worry about the noise but rather about their reliability.


Gruß, masi1157
508 different segments on 100 airlines to 211 airports in 55 countries
 
brainstorm21
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 11:10 am

That problem goes away with geared turbo fan
 
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NearMiss
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 1:41 pm

Speaking of great Buzzsaw sounds

https://www.youtube.com/watch?v=On3I6HqJNtI

I've always loved the tiny buzzsaw sound from CFMs

https://youtu.be/zvSr970oMKE
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crownvic
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 1:47 pm

I'm amazed this thread has survived on the Civil Aviation forum and not the Technical/Operations forum..Mods are always quick to move threads...
 
AirFiero
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 2:09 pm

masi1157 wrote:
Amiga500 wrote:
I've never seen any research yet that indicated bearings are a substantial source of noise on turbofans. Perhaps it has just not been considered but...

If they were, they would have been considered. And if they really would generate any considerable noise I would not worry about the noise but rather about their reliability.


Gruß, masi1157


Yeah, this didn’t make any sense. There is no way in hell that engine would be viable if it’s bearings were creating such an amount of noise.
 
masi1157
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 2:20 pm

brainstorm21 wrote:
That problem goes away with geared turbo fan

No, it doesn't. But it does get a little bit less pronounced.

Gruß, masi1157
508 different segments on 100 airlines to 211 airports in 55 countries
 
JesseCasserly
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Re: Buzzsaw sound on takeoff

Wed Jun 26, 2019 3:14 pm

This is something I've been trying to research for years. This article contained the most logical and detailed explanation, even if it's a little confusing to understand :D

https://www.southampton.ac.uk/engineeri ... stics.page

Now here's my question that I've been trying to figure out and there's definitely ZERO information on it, but maybe you guys can chip in: What about the sound beyond the buzzsaw range?
Let me explain. In general, there's about 4 stages to a high bypass turbofan sound. There's the idle sound (can be a whistle like on the CF6 or a howl like the PW4000), then there's the low spool sound (like that loud whistle the GE90-115b makes in the first few seconds after the throttles are advanced), then there's the whine sound (need I explain this one?! Think RB211!!), then finally is the buzzsaw sound.
Lets take for example the IAE V2500 and the GE90-115b for example. They're vastly different engines, but in general they both display these characteristic sounds in their own unique way and at very different frequencies. During most takeoffs, you hear the buzzsaw (well, more of a "grindsaw" on the GE90 because of the lower frequency), but during certain takeoffs, its totally whacked up! I first noticed a totally different takeoff sound 3 years ago when sitting in row 3 of an AA A321 that was departing from LAX to Lihue, Hawaii. This is a 6 hour flight so the A321 was, I'm guessing, near MTOW. Which also meant it was a TO/GA power takeoff, not very common for normal flights around Europe etc. What I noticed was the engine sound went through all of the ranges like I mentioned above, but shot past buzzsaw into just, roaring noise I guess. There was no buzzsaw, and it only returned after the reduction to climb power. Lucky for us all, I took a video :lol:

https://www.youtube.com/watch?v=n7nviNII520

Notice how the buzzsaw only returns after the reduction to climb power at 1:17 in the video. Here's another video I took last year coming back from another trip of the same thing:

https://youtu.be/nWMdrGZwVxA?t=94

I also mentioned the GE90, it does the same thing at very high power settings (not common with 95% of takeoffs, only go arounds really, like in this video):

https://youtu.be/C5dIrEYmqPI?t=156

Notice that the grindsaw sound only returns after the powerback a few seconds later.
I'm just trying to understand what the heck is going on in the world of physics during all of this :bouncy:
So we have those shockwave that create the buzzsaw sound, yes. But apparently we can spin an engine so fast that they can't keep up somehow. is that whats happening???
I would LOVE to hear everyone else's opinion!!

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