*"Ceramic matrix composites take flight in LEAP jet engine.
A quarter-century ago, the Department of Energy began a program to support U.S. development of ceramic matrix composites. In 2016 a new aircraft engine became the first widely deployed CMC-containing product.
The CMC needs less cooling air than nickel-based super-alloys and is part of a suite of technologies that contribute to 15 percent fuel savings for LEAP over its predecessor, the CFM 56 engine."*
Obviously these components are more fuel-efficient. I’m wondering if they sound the same as immediately previous engines? I understand that jet engines are significantly quieter these days – when I was growing up near SFO, teachers would have to stop lectures until planes going by overhead had passed.
But is the sound of these CMC engines distinctive in some way?
Each engine type has it’s own sound. Even the same engine installed on a different type of aircraft has a distinct sound - the engine cowling is specific to the aircraft type, and changes the sound characteristics.
The CMC parts are generally in the hot section of the engine - the combustion section and the high pressure (HP) and low pressure (wait for it… LP) turbine sections. They’re not likely to be shaped very differently than their metallic counterparts in the actual power-producing airflow area of the part, so their impact on the engine’s sound will be minimal at best, and probably not something that will be audible to the human ear.
most of the sound from a gas turbine engine is the whine from the compressor inlet, and with a jet there’s also the exhaust blast. Modern passenger jets are so much quieter because they keep cranking up the bypass ratio; rather than expelling just a small-quantity high-velocity stream of exhaust like older jets (which get really loud as the exhaust reaches sonic speeds) they rely on the fan to move a huge quantity of air but more slowly. plus the bypass air shrouds and blends into the exhaust stream to mask some of its sound.
Agree with JHBoom. Beyond the individual aircraft+engine differences already mentioned, it’s real unlikely there’s any qualitative difference in the nature of the sound due to the CMC parts. The CMC parts are buried way deep in the engine. The engine may well have other distinctive notes from other things, notably fan blade count and RPM.
Here’s the engine manufacturer’s page on their product: LEAP Engines – CFM International Jet Engines CFM International. They’re real proud of this thing and rightly so. They make no mention of it being inherently quieter. So I bet it probably isn’t.
Here’s Boeing’s page on the 737MAX, one of the aircraft using these engines. http://www.boeing.com/commercial/737max/by-design/#/noise-footprint. They say the 737MAX will be quieter although the difference is not huge. Some of that difference may be engine itself; more of it will be the nacelle and installation, plus other airplane changes unrelated to the engines.
Here’s Airbus’ page for the A320 with the new engines (A320 NEO) : http://www.aircraft.airbus.com/aircraftfamilies/passengeraircraft/a320family/a320neo/. They mention a 50% reduction in engine noise. Although as I suggested above, the CMC stuff is probably a negligible contribution to that. Interestingly, they make no mention of noise reduction on the A321 page. On the 319 page they say “some reduction”. It’s also worth pointing out that the A320NEO has two choices of engines: the LEAP with the CMC innards and the PW1000G (see below). It’s not clear how those engine options affect the sound level.
You didn’t ask, but here’s info on a different engine innovation: http://www.purepowerengine.com/quieter.htm. The front fan is driven by a ~3:1 reduction gear system. Pratt & Whitney has been working on this for well over a decade and it’s finally coming to fruition. IMO this has a lot more potential to produce engines that are not only quieter, but sound qualitatively different as well.
Within a couple to few months there will be enough of these new engines flying around the US that people will get a chance to listen for any distinctive notes and for overall noise/quietness.
The most consistently wonderful thing about this board is that there are always a half-dozen people who are far more informed than I about anything.
The reason I ask is because lately, in the past year or so, I have noticed a very distinctive change in the sound of jet engines as they fly overhead. (I live nearer to Oakland Airport now.)
I just want to make fairly sure that I have explored the more mundane explanations first, because Occam’s Razor.
Thereafter if warranted, I will expound my theory, probably over in MPSIMS.
Edit: I want to make clear that I am asking about jet engines already extant for 12 to 24 months.
dB are a complicated unintuitive unit of measure, and not one I’m competent to explain well. 20dB may not be as amazing as all that.
In general engines aren’t silent yet, but a lot of the remaining noise comes from the turbulence caused by the airplane’s passage, not the engines. Said another way, the low-hanging fruit of engine noise has been mostly picked with these latest designs coming online now and any further *significant *reductions in total noise must come from a combo of engine and, increasingly, airframe changes.
NASA and their European counterparts is engaged in a bunch of early stage research on fundamentally quieter airplane designs. As are the aircraft manufacturers. I expect we’ll see the earliest fruits of that effort deployed 15 or 20 years from now.
It is always amazing to me that when I started in the biz in the late 80s the fairly new MD-80s were pleasantly quiet vs. the ear-splitting roar of the older designs. Now they’re instantly recognizable by their ear-splitting roar compared to later designs. They make exactly as much noise as they ever did. Our standards and expectations have changed that much. First to worst in 30ish years. And that’s before this latest generation gets deployed in any quantity.
just like with gasoline and diesel piston engines, it’s more efficient to have a big engine turn slower to make the power you need. in the case of turbofans, big engines turning slower with the fan doing most of the work is what you want, so long as you stay subsonic. look at how tiny the engine nacelles are compared to the fuselage on the 707, and then look how enormous they are on the 787.
Heck, does the engine core produce any appreciable thrust anymore on a modern high-bypass turbofan?
all else being equal,* a reduction of 20 dB means it’s 10% as loud as before. However, not all noise sources are created equal, which is why we have things like A- and C-weighting, and loudness units like sones. the frequency content of the sound matters. If you use an unweighted measurement, a modern jet engine is still pretty loud compared to an old one, but since you’ve eliminated the screech from the compressor and the crackle from the exhaust jet, you’ve reduced the loudness in the midrange (where our hearing is most sensitive and most prone to damage) and reduced the apparent loudness considerably.
when it comes to human hearing, a dB is not a dB. 100 dB of a 1 kHz tone will be deafening and provoke insanity. 100 dB of broad-spectrum noise will not be.