C5LOAD From United States of America, joined Sep 2008, 917 posts, RR: 0 Posted (3 years 3 months 1 week 6 days 1 hour ago) and read 5354 times:
I was looking at pictures of the 787 and the new 747-800 and was curious as to the cookie-cutter design in the back of the engine? Is it something to do with its performance or is for looks? I tried posting the pics, but it won't work for some reason so here are the links.
BMI727 From United States of America, joined Feb 2009, 14328 posts, RR: 26 Reply 1, posted (3 years 3 months 1 week 6 days ago) and read 5329 times:
Quoting C5LOAD (Thread starter): Is it something to do with its performance or is for looks?
The chevrons are there to reduce noise. The shapes at the back of the pylon help mix the ambient air and the air from the engine more smoothly and effectively. The interactions between these two streams of air is responsible for the roaring sound you hear from an engine and the chevrons will help reduce that.
This is the same reason why higher bypass engines are quieter as well. The bypass air shrouds the exhaust.
Why do Aerospace Engineering students have to turn things in on time?
skyone From Mexico, joined Feb 2001, 409 posts, RR: 0 Reply 2, posted (3 years 3 months 1 week 6 days ago) and read 5316 times:
Don´t know, but it shure looks nice!!! I really like it. It is one of those things that makes things look more modern than its similars of the past, like in the case of the 747-800 vs. 747-400.
BrouAviation From Netherlands, joined Jun 2009, 981 posts, RR: 1 Reply 4, posted (3 years 3 months 1 week 5 days 17 hours ago) and read 4835 times:
I also believe it adds a little bit of fuelburn, as induced drag increases, and vortices will form easier.
Quoting BMI727 (Reply 1): The interactions between these two streams of air is responsible for the roaring sound you hear from an engine and the chevrons will help reduce that.
You are correct, but:
Don't want to nitpick, but doesn't this cause the screaming sound? The roar is only heard in front of the engine, and is actually caused by the engine itself?
Of course it depends on what you define as 'screaming' and 'roaring'.
Never ask somebody if he's a pilot. If he is, he will let you know soon enough!
MadameConcorde From San Marino, joined Feb 2007, 10243 posts, RR: 40 Reply 5, posted (3 years 3 months 1 week 5 days 17 hours ago) and read 4806 times:
This is one of the first things I noticed on the new aircrafts, the jigsaw design on the engines and I had not the slightest idea why they had done this. My first idea was so they looked different, though appartently there is a truly practical reason to this "cookie-cutter" design. Now I know more, thanks to this thread! Â Â ^^^^^^^^^^
There was a better way to fly it was called Concorde
747classic From Netherlands, joined Aug 2009, 1782 posts, RR: 11 Reply 7, posted (3 years 3 months 1 week 5 days 16 hours ago) and read 4696 times:
Quoting BrouAviation (Reply 4): I also believe it adds a little bit of fuelburn, as induced drag increases, and vortices will form easier.
At the moment approx 0.25 % specific fuel consumption (SFC) loss, caused by chevrons, is demonstrated.
In the future this loss can be eliminated by temperature depended chevrons. Using shape memory alloys, the chevrons will be able to bend in and out of the flow exactly as needed. Activated by temperature, the tabs can be told when to change shape based on the atmospheric conditions at takeoff and cruise altitude. Already they've been tested on a GE-90 engine on a Boeing 777, the first commercial application of a morphing structure.
BMI727 From United States of America, joined Feb 2009, 14328 posts, RR: 26 Reply 9, posted (3 years 3 months 1 week 5 days 11 hours ago) and read 4179 times:
Quoting BrouAviation (Reply 4): The roar is only heard in front of the engine, and is actually caused by the engine itself?
I think you have it backwards.
The two components (which I will scientifically refer to as the scream and the roar) have two causes. The scream, which is heard more in front of the engine, is due to the interation between the air and the fan (or compressor in a turbojet) blades. This is the sound that can turn into a loud buzzing sound with the blade tips go beyond the speed of sound.
The roar is caused by the turbulent interaction between the ambient atmosphere and the jet exhaust and is heard mostly from behind the engine. This sound is alleviated by having a high bypass ratio now by adding chevrons to the rear of the nacelle, which helps the streams of air mix more smoothly and quietly.
Why do Aerospace Engineering students have to turn things in on time?
Birdwatching From United States of America, joined Sep 2003, 3573 posts, RR: 52 Reply 11, posted (3 years 3 months 1 week 5 days 10 hours ago) and read 4005 times:
Why are you calling this a "cookie cutter" design?
It has no similarity to a cookie cutter.
Soren  Â
All the things you probably hate about travelling are warm reminders that I'm home
C5LOAD From United States of America, joined Sep 2008, 917 posts, RR: 0 Reply 12, posted (3 years 3 months 1 week 5 days 10 hours ago) and read 3968 times:
Quoting Birdwatching (Reply 11): Why are you calling this a "cookie cutter" design?
What would you call it? Sawblade?
"But this airplane has 4 engines, it's an entirely different kind of flying! Altogether"
Birdwatching From United States of America, joined Sep 2003, 3573 posts, RR: 52 Reply 13, posted (3 years 3 months 1 week 5 days 10 hours ago) and read 3939 times:
Quoting C5LOAD (Reply 12): What would you call it? Sawblade?
Much better. I tried to figure out how you would cut cookies with this. Not possible. Your "engine cookies" would still stick to the rest of the dough around them. In fact you could cut cookies much better with a conventional engine design.
That being said... mmmmm, cookies.
Soren
[Edited 2010-02-10 10:01:52]
All the things you probably hate about travelling are warm reminders that I'm home
keta From Germany, joined Mar 2005, 440 posts, RR: 0 Reply 16, posted (3 years 3 months 1 week 5 days 7 hours ago) and read 3724 times:
Quoting BrouAviation (Reply 4): Don't want to nitpick, but doesn't this cause the screaming sound? The roar is only heard in front of the engine, and is actually caused by the engine itself?
Quoting BMI727 (Reply 9): The roar is caused by the turbulent interaction between the ambient atmosphere and the jet exhaust and is heard mostly from behind the engine. This sound is alleviated by having a high bypass ratio now by adding chevrons to the rear of the nacelle, which helps the streams of air mix more smoothly and quietly.
  The exhaust causes a low frequency (which is what I understand as roaring) noise due to the turbulence of the mixing flow. Higher bypass reduces exhaust speed and chevrons improve the mixing conditions, both of which help reduce noise.
Quoting ArabAirX (Reply 8): Does anyone know if such a design will be used on the Trent XWB engines for the A350?
Airbus' renderings of the A350 don't show any kind of serrations on the engines, but who knows...
On the other hand, I recall Airbus stating that they achieved some noise reduction in the A380 by an improved engine inlet, which did not have a negative effect on fuel consumption. If this works, they'll include this design in the A350. I don't have more info, though...
Quoting 747classic (Reply 7): At the moment approx 0.25 % specific fuel consumption (SFC) loss, caused by chevrons, is demonstrated.
In the future this loss can be eliminated by temperature depended chevrons. Using shape memory alloys, the chevrons will be able to bend in and out of the flow exactly as needed. Activated by temperature, the tabs can be told when to change shape based on the atmospheric conditions at takeoff and cruise altitude. Already they've been tested on a GE-90 engine on a Boeing 777, the first commercial application of a morphing structure.
Stitch From United States of America, joined Jul 2005, 26700 posts, RR: 83 Reply 17, posted (3 years 3 months 1 week 5 days 7 hours ago) and read 3710 times:
Well they seem to work, as the 747-8 was quieter than the 777-300ER that departed before it. It also sounded quieter than I recall the 787-8 sounding.
Starlionblue From Hong Kong, joined Feb 2004, 15870 posts, RR: 66 Reply 19, posted (3 years 3 months 1 week 5 days 4 hours ago) and read 3619 times:
Quoting 747classic (Reply 7): At the moment approx 0.25 % specific fuel consumption (SFC) loss, caused by chevrons, is demonstrated.
In the future this loss can be eliminated by temperature depended chevrons. Using shape memory alloys, the chevrons will be able to bend in and out of the flow exactly as needed.
Wow. That sounds pretty sci fi.
I, for one, welcome our new memory alloy overlords.
Quoting Birdwatching (Reply 11): Why are you calling this a "cookie cutter" design?
It has no similarity to a cookie cutter.
This did indeed confuse me, especially as "cookie cutter" is an expression referring to something unoriginal and/or uninspired.
[Edited 2010-02-10 15:44:42]
"There are no stupid questions, but there are a lot of inquisitive idiots." - from Citadel by John Ringo
BMI727 From United States of America, joined Feb 2009, 14328 posts, RR: 26 Reply 20, posted (3 years 3 months 1 week 5 days 1 hour ago) and read 3544 times:
Here are some clips that might shed more light on the subject.
When you watch the PIA 777 take off (about 1:10), you will notice that you hear more of the higher pitched sound in front of the plane and the roar of the exhaust becomes more prominent after it passes by.
jetlife2 From United States of America, joined Jul 2006, 214 posts, RR: 25 Reply 21, posted (3 years 3 months 1 week 5 days ago) and read 3524 times:
This article describes the acoustic testing we did putting various chevrons (including the actuated shape memory material) on a GE90-115B/777-300ER, as well as other engine and airframe modifications
keta From Germany, joined Mar 2005, 440 posts, RR: 0 Reply 22, posted (3 years 3 months 1 week 4 days 18 hours ago) and read 3426 times:
Quoting jetlife2 (Reply 21): This article describes the acoustic testing we did putting various chevrons (including the actuated shape memory material) on a GE90-115B/777-300ER, as well as other engine and airframe modifications
MD11Engineer From Germany, joined Oct 2003, 13336 posts, RR: 64 Reply 24, posted (3 years 3 months 1 week 4 days 11 hours ago) and read 3350 times:
I´ve seen a few AN-124s at our airport using similar reverser cowls, but I didn´t take a picture.
Jan
25 Kit777: You seem to be forgetting the most beautiful airplane of them all, the A380 . Back on topic though, I have also seen these chevrons on A320 series ai
26 pilotpip: The 170/190 and CRJ-900 CF-34s have them on the exhaust as well. None on the cowl like the 340 above.
27 Kit777: Yeah, I was thinking of the exhaust section. I'm pretty sure some AF A320s have those too. Thanks Kit
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 The exhaust causes a low frequency (which is what I understand as roaring) noise due to the turbulence of the mixing flow. Higher bypass reduces exhaust speed and chevrons improve the mixing conditions, both of which help reduce noise.
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