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Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 12:42 am
by Farsight
Hi

I was looking into unducted fans the other day and it got me thinking...

Where is the line drawn between increased fuel efficiency and noise? Fuel efficiency is obviously crucial, and in an age where were spending tens of millions in R&D to eak out a 5% advantage...is a subsequent 10% increase in noise deemed acceptable? And vice versa.

Looking at the new 777x taxi tests, the ge9x seems very quiet and I’d imagine its well within noise regulations...well within.

Im sure GE know better than me, but it seems to a casual observer that they have more ‘headroom’ to design into?

Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 12:54 am
by Farsight
And...if we were to come up with some new technology that boosted fuel effiency by 20% but took us back to pre hush kit 727 sound levels (acceptable in the 70’s apparantly), would we ease noise regulations to accomodate it?

My question is long winded but hopefully you can see where im going!

Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 2:09 am
by Starlionblue
At a guess, given how turbofans work, more fuel efficiency tends to give lower noise levels. Bigger fans are more efficient. They increase the bypass ratio so you get more low speed "quiet" air in proportion to high speed "noisy" air. In effect, bypass air acts as a noise blanket.

BTW most approach noise is aerodynamic, from high lift devices.

Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 2:31 pm
by kalvado
In principle, one may expect less noise = less energy waste = more fuel efficiency.
So I was totally surprised to learn that chevrons on 787 engines actually increase fuel burn.
I don't really understand how unducted turbofan works. Duct reduces tip vortices, so less blade drag, and less energy sent out as a sound, a win-win. Well, at a cost of a heavy duct.

Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 3:34 pm
by Farsight
Thats what I'm getting at. Chevrons on the 787 was a surprising example of prioritising noise suppression over fuel consumption.

That case is especially strange considering the engines were probably also within current noise regulations without the chevrons....

And although yes it is correct to say that the most efficient possible engine should be quieter by nature (noise is wasted energy), there is the potential for new technologies that save fuel and emit more noise than previous generations. Open rotor engines are an example, more fuel efficient, more noise.

Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 4:12 pm
by flipdewaf
Farsight wrote:
Thats what I'm getting at. Chevrons on the 787 was a surprising example of prioritising noise suppression over fuel consumption.

That case is especially strange considering the engines were probably also within current noise regulations without the chevrons....

And although yes it is correct to say that the most efficient possible engine should be quieter by nature (noise is wasted energy), there is the potential for new technologies that save fuel and emit more noise than previous generations. Open rotor engines are an example, more fuel efficient, more noise.

From what I remember the A380 engines were about 3” over what they should have been for maximum economic use due to wanting lower noise levels. In this case the trade off is normally heavier vs noisier.

Fred


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Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 4:40 pm
by Farsight
This is what I don't understand, if the engines are already meeting noise regulations, why bother making them quieter at the expense of fuel economy??

Re: Fuel efficiency vs Noise reduction

Posted: Sun Jun 30, 2019 6:46 pm
by Armadillo1
Noise regulation may change as planned or some unplanned, not for general but for some new measure technique. Better to secure a gap after regulations.

Also, a specially for a380 with bar and shovels , noise still a competing advantage

Re: Fuel efficiency vs Noise reduction

Posted: Mon Jul 01, 2019 7:34 am
by CowAnon
flipdewaf wrote:
Farsight wrote:
Thats what I'm getting at. Chevrons on the 787 was a surprising example of prioritising noise suppression over fuel consumption.

That case is especially strange considering the engines were probably also within current noise regulations without the chevrons....

And although yes it is correct to say that the most efficient possible engine should be quieter by nature (noise is wasted energy), there is the potential for new technologies that save fuel and emit more noise than previous generations. Open rotor engines are an example, more fuel efficient, more noise.

From what I remember the A380 engines were about 3” over what they should have been for maximum economic use due to wanting lower noise levels. In this case the trade off is normally heavier vs noisier.

Fred

For the open rotor, the changes were/are even more extreme.

  • When GE was designing the UDF for the MD-90 series in the 1987-1988, the best design was a 128-inch fan that had 85% propulsive efficiency, but it just missed compliance with the then-current Stage 3 regulations.
  • The engine would have complied using a 140-inch fan with 83% efficiency.
  • However, McDonnell-Douglas decided to go with a 140-inch fan with 80% efficiency that was 10.5 effective perceived-noise decibels (EPNdB, which is the sum of the decibel levels at takeoff, sideline, and approach) below what Stage 3 required. Since Stage 4 was enacted in 2006 and had a standard 10 EPNdB below Stage 3, this engine probably wouldn't have needed modification for the new requirement.
  • A GE study in 2013 predicted that a 168-inch fan could achieve 85% efficiency at a level 15-17 EPNdB under Stage 4 requirements. Stage 5 was enacted beginning in 2017 and is 7 EPNdB below Stage 4, so this fan would comply with Stage 5. If the next requirement drops the Stage 5 requirement by the same difference or less, the fan would still comply without modification.

So that's a 40-inch diameter increase (1 meter, and about 30% above the original diameter) to meet new or expected noise requirements spanning 30 years or more, for the same efficiency (85%) and thrust (25,000 pounds). Still, I don't think size or noise regulations were or would be the deciding factor in rejected an open rotor. A 128-inch fan is too big to be a practical fit on the wings in the typical low-wing configuration, and a 168-inch fan isn't oversized for an engine mounted on a high-wing configuration or on the rear fuselage. The larger hurdles are questions about reliability and maintenance costs of a new type of engine, and airline/passenger perceptions (paranoia) about blades coming off or engine failures.

Re: Fuel efficiency vs Noise reduction

Posted: Mon Jul 01, 2019 10:18 am
by Armadillo1
Open rotor can be placed under or on wing like common turboprop. Plus a little slats issue