Large turbofan noise
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Thanks Gaspath for saving my already dodgy eyesight! Your information has, I suspect, also provided the answer to my original post. As I am not an engineer I may just have interpreted your numbers wrong, but assuming that N1 represents the speed of the inner, ie fan, shaft, then at 100% the Trent's fan is rotating a whole 50% faster than the fan on the GE90! If this is true then it is no surprise that it makes a very different noise. Since the difference in diameter is small (about 12%), the Trent's tip speed must be considerably greater than the GE's. I don't have the patience to do the calculation, but this alone, when converted to noise frequencies, would surely explain the difference? Over to the physicists/engineers......
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One of the most interesting points about hearing aircraft 'noise' with the human ear, is how different people hear the same noise. Those that have to handle noise reports from the citizenry (airports and councils) know that:
1) One person sitting in their garden will hardly notice the aircraft going overhead - whilst their neighbour will be going mental and on the phone to the local group. One man's buzz saw is another's mental note that a RR, rather than a GE product is at work.
2) When you plot reports of intrusive a/c noise onto a map, a village/town/city will be speckled with reports, rather than banded. One might expect bands to build up as you get towards the airport or centre of flight path but the plots are almost always speckled.
It goes without saying that some of us in here [peeks out of anorak to look around for fellow beings] are less likely to complain about a/c noise than others.
1) One person sitting in their garden will hardly notice the aircraft going overhead - whilst their neighbour will be going mental and on the phone to the local group. One man's buzz saw is another's mental note that a RR, rather than a GE product is at work.
2) When you plot reports of intrusive a/c noise onto a map, a village/town/city will be speckled with reports, rather than banded. One might expect bands to build up as you get towards the airport or centre of flight path but the plots are almost always speckled.
It goes without saying that some of us in here [peeks out of anorak to look around for fellow beings] are less likely to complain about a/c noise than others.
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Using gas path's numbers, and assuming approximate STP at sea level, the GE fan tips are traveling at 1213 feet/second, or about 1.055 Mach, at a primary acoustic frequency of 829 Hz at 100% n1. The RR is 1588 ft/sec, or 1.38 Mach, at 1430 Hz.
I estimate Mach at 1150 ft/sec, and define the primary acoustic frequency as the number of times per second a fan blade tip passes the 12 o'clock (or any other) position.
Local air conditions will change the Mach number, but not the other data. If the engines are synchronized, the primary frequency should be a primary factor in perceived "tone."
I estimate Mach at 1150 ft/sec, and define the primary acoustic frequency as the number of times per second a fan blade tip passes the 12 o'clock (or any other) position.
Local air conditions will change the Mach number, but not the other data. If the engines are synchronized, the primary frequency should be a primary factor in perceived "tone."
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Thanks Intruder - some interesting numbers there! The GE90 fan blades are barely supersonic - simple maths says only about the last 3 inches spanwise have supersonic flow. With the Trent on the other hand the last 15 inches or so of blade span are supersonic at 100% power.
Now, presumably the blades act like any other aerofoil travelling supersonic, ie: the leading edge and mid-chord shockwaves make a sonic boom. So the Trent's 15 inches of each blade are each creating a sonic boom as they rotate. The effect must be similar to a football fan's rotating rattle with its sounding board contacting a toothed wheel. The "buzzsaw" noise is therefore generated by the booms occurring at the "past 12 o'clock" frequency of the rotating blades.
On this basis, it is small wonder that the Trent sounds so different!
Now, presumably the blades act like any other aerofoil travelling supersonic, ie: the leading edge and mid-chord shockwaves make a sonic boom. So the Trent's 15 inches of each blade are each creating a sonic boom as they rotate. The effect must be similar to a football fan's rotating rattle with its sounding board contacting a toothed wheel. The "buzzsaw" noise is therefore generated by the booms occurring at the "past 12 o'clock" frequency of the rotating blades.
On this basis, it is small wonder that the Trent sounds so different!
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Unfortunately its not that simple for noise due to shock-wave propagation. The velocity of a shock wave is non-linear i.e. it varies with amplitude. Tiny blade to blade differences get exaggerated to large variations by the time that the shocks have propagated to the end of the duct, whereupon linear sound propagation takes over.
At the end of the duct, instead of a uniform set of shocks passing at the blade passage frequency, you get a very unevenly spaced set of shocks (in both spacing and amplitude), sometimes one shockwave will overtake another and the pressure patterns merge. The whole pressure pattern thus only recurs after a complete rotation of the fan. That is why the 'buzz-saw' noise has a fundamental frequency at the shaft rpm rather than at the blade passing frequency for the subsonically generated component.
When calculating the Mach no. you must also vector sum the axial velocity component in the intake (~0.5Mach) acting at 90 degrees to the tangential component (tip speed).
At the end of the duct, instead of a uniform set of shocks passing at the blade passage frequency, you get a very unevenly spaced set of shocks (in both spacing and amplitude), sometimes one shockwave will overtake another and the pressure patterns merge. The whole pressure pattern thus only recurs after a complete rotation of the fan. That is why the 'buzz-saw' noise has a fundamental frequency at the shaft rpm rather than at the blade passing frequency for the subsonically generated component.
When calculating the Mach no. you must also vector sum the axial velocity component in the intake (~0.5Mach) acting at 90 degrees to the tangential component (tip speed).
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Current thinking on the cause of "Bad" Buzzsaw is variations in surface near the fan, thus the hardwalling in front of newer inlets. Something like a rivet head protruding, probes, etc. cause an uneven airflow to the fan thus "stressing" it more in different areas. 
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I can remember a very good quote that can be used to amaze the general public on the topic of large jet engines........
"Consider an engine that is designed for one part to be subjected to supersonic pressure, another part to be bathed in hot gas, for 20 hours a day, for 25 years. And with proper maintenance, to not fail during that time."
Puts a Ford Focus into perspective.
"Consider an engine that is designed for one part to be subjected to supersonic pressure, another part to be bathed in hot gas, for 20 hours a day, for 25 years. And with proper maintenance, to not fail during that time."
Puts a Ford Focus into perspective.
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Been some time since I was involved with high by-pass engines.
Bearing that in mind, I recall that a major cause of cold stream noise was the interaction between the air leaving the fan and impinging upon the stators behind it, in the duct. The then characteristic "buzz-saw" noise was a major consequence of this.
The Americans are more into variable stators (OK, mainly in the core engine compressors). Do these apply in the fan duct, giving better matching off design point ? Just a wild guess.
I repeat, it's been a while since I've been involved, but there may be something useful in my ramble.
Be interesting to know the answer.
Bearing that in mind, I recall that a major cause of cold stream noise was the interaction between the air leaving the fan and impinging upon the stators behind it, in the duct. The then characteristic "buzz-saw" noise was a major consequence of this.
The Americans are more into variable stators (OK, mainly in the core engine compressors). Do these apply in the fan duct, giving better matching off design point ? Just a wild guess.
I repeat, it's been a while since I've been involved, but there may be something useful in my ramble.
Be interesting to know the answer.
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Hi Anthony,
You are right in thinking that stator-rotor interaction is a major component of fan noise, but it doesn't contribute to 'buzz-saw' noise, only to blade passing frequency harmonics.
It is caused by the fan wake interacting with the vanes, this wake decreases with distance, and the potential field (bow wave) of the stator can modulate the angle of attack of the fan and thus cause a BPF tone. Both of these mechanisms reduce significantly with bigger spacing, and by having more than twice the number of stator vanes to fan blades, the interaction will be rapidly attenuated in the duct (cut-off condition).
This mechanism is more important at subsonic fan speeds as in the approach certification condition.
I don't think variable as opposed to fixed guide vanes have much influence on the noise though.
You are right in thinking that stator-rotor interaction is a major component of fan noise, but it doesn't contribute to 'buzz-saw' noise, only to blade passing frequency harmonics.
It is caused by the fan wake interacting with the vanes, this wake decreases with distance, and the potential field (bow wave) of the stator can modulate the angle of attack of the fan and thus cause a BPF tone. Both of these mechanisms reduce significantly with bigger spacing, and by having more than twice the number of stator vanes to fan blades, the interaction will be rapidly attenuated in the duct (cut-off condition).
This mechanism is more important at subsonic fan speeds as in the approach certification condition.
I don't think variable as opposed to fixed guide vanes have much influence on the noise though.
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A very interesting read.
What was the original purpose that wide chord fan blades were introduced. Do they have any other effects other than what Mark 1 has described. I'm guessing that they would increase the pressure (increased bypass ratio) Any information much appreciated.
What was the original purpose that wide chord fan blades were introduced. Do they have any other effects other than what Mark 1 has described. I'm guessing that they would increase the pressure (increased bypass ratio) Any information much appreciated.
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Flight Safety:
Your information is completely incorrect. The CFM56 uses the F101 core. The GE90 core was based on a NASA-funded E3 development program(E3, pronouced as E cube, stands for Energy Efficient Engine).
Throughout this thread, there is some good, semi-good, and bad information. I have worked on engine acoustic predictions on a peripheral role. However, it isn't really my field of expertise. So, I won't add more half-truths to this thread. Just to reiterate some of the points that have already been made. The science of engine noise is very complicated. Fan size, tip speed, bypass ratio, rotor count, stator count, blade-row spacing, acoustic liner, jet speed, jet mixing, cowl length, chevron, pylon, tail plug geometry, etc., are just some of the things than can affect the noise characteristics of an engine. Unlike in aerodynamic design, aeroacoustic design still heavily rely on semi-empirical-based tools instead of numerical simulations.
One more thing:
Anthony:
The stators behind the fan rotor, or exit/outlet guide vanes, are there to straighten the flow, as the name suggested. So, they are not variable. P&W in the 90s had developed an ADP (Advanced Ducted Prop) that used variable pitch vanes behind the ducted prop fan. These vanes also served the purpose of a thrust reverser (thus, reduce engine weight, or rather compensate for the much larger and heavier prop fans). I believe this was the only commercial-type engine that used variable vane in the fan stream.
Your information is completely incorrect. The CFM56 uses the F101 core. The GE90 core was based on a NASA-funded E3 development program(E3, pronouced as E cube, stands for Energy Efficient Engine).
Throughout this thread, there is some good, semi-good, and bad information. I have worked on engine acoustic predictions on a peripheral role. However, it isn't really my field of expertise. So, I won't add more half-truths to this thread. Just to reiterate some of the points that have already been made. The science of engine noise is very complicated. Fan size, tip speed, bypass ratio, rotor count, stator count, blade-row spacing, acoustic liner, jet speed, jet mixing, cowl length, chevron, pylon, tail plug geometry, etc., are just some of the things than can affect the noise characteristics of an engine. Unlike in aerodynamic design, aeroacoustic design still heavily rely on semi-empirical-based tools instead of numerical simulations.
One more thing:
Anthony:
The stators behind the fan rotor, or exit/outlet guide vanes, are there to straighten the flow, as the name suggested. So, they are not variable. P&W in the 90s had developed an ADP (Advanced Ducted Prop) that used variable pitch vanes behind the ducted prop fan. These vanes also served the purpose of a thrust reverser (thus, reduce engine weight, or rather compensate for the much larger and heavier prop fans). I believe this was the only commercial-type engine that used variable vane in the fan stream.
Last edited by casual observer; 20th Jun 2004 at 16:56.
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CO is correct about the source of the GE90 core, so my guess about the whistle noise was incorrect. My apologies to Seloco and all Ppruners. I knew when I was creating that last post that I was in a hurry and didn't have the time to do the research, and I strive to be accurate when posting on this website. 
I thought I'd be bitten when I wrote that, and I was. Thanks CO for setting me straight.
PS - I decided to delete the entire post.
I thought I'd be bitten when I wrote that, and I was. Thanks CO for setting me straight.
PS - I decided to delete the entire post.
Last edited by Flight Safety; 21st Jun 2004 at 13:28.
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I read somewhere that recent versions of the Trent and other engines - including both 7E7 offerings are to have contra (or is that counter?) rotating spools. Is this driven by efficiency or noise considerations or both i.e. what exactly will this technology contribute?
747FOCAL
Er, methinks your are getting mixed up with the compressor stators. The coldstream outlet guide vanes (N1 fan stators) are fixed. They are there to 'straighten' the airflow. This means that the airflow is more axial than radial, increasing efficiency of the engine.
The compressor stators of most big fan engines (yes even the 3 spool Trent) are variable.
Back to the topic.
My understanding was that the buzz saw noise was caused by the fan blade's being supersonic. A320/CFM56 being a classic case.
The GE90 stators are variable.
The compressor stators of most big fan engines (yes even the 3 spool Trent) are variable.
Back to the topic.
My understanding was that the buzz saw noise was caused by the fan blade's being supersonic. A320/CFM56 being a classic case.
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those GE90s sound rather gimmicky more than anything else
with their delicate little EEEEEEEEEEEEEEEEEEEE. God I miss those DC9s. I love the fact that when youre sitting in the front of the 9s at takeoff its soo quiet. It is quite easy to forget that your plane is actually making quite a raucus down there 
wish i cpold say JT8D forever!
with their delicate little EEEEEEEEEEEEEEEEEEEE. God I miss those DC9s. I love the fact that when youre sitting in the front of the 9s at takeoff its soo quiet. It is quite easy to forget that your plane is actually making quite a raucus down there wish i cpold say JT8D forever!
