One of the unique features of Boeing’s 787 Dreamliner is the sawtoothed edges on the engine’s outlets. You can see them in this photo, on the bypass duct and also on the central turbine duct. Apparently they provide noise reduction benefits by helping to mix gas streams of very different velocities just a little more gradually than conventional straight-edged ducts.
As big jets go, the only other aircraft I’ve seen with this feature is Boeing’s 787-800. However, I’ve been seeing it on some regional jets which, AFAIK, are not made by Boeing.
This Quora article claims a “less than 0.25%” penalty in engine performance with the chevron feature. But “less than 0.25%”, on a long-haul flight, works out to a few hundred pounds of fuel. If true, this is not insignificant.
Some questions:
-Why are manufacturer(s) adding this feature to these aircraft? The reflexive answer is of course “to make them quieter,” but if planes are already meeting noise regulations without these features, then how is the extra cost/decreased performance justified to the airlines that are buying these aircraft?
-is the chevron feature patented by Boeing (or RR), or is the knowledge all public domain, courtesy of NASA?
-If the chevron design really is the cat’s meow, AND if it’s not patented, should we expect to see such technology retrofitted to existing planes (just as winglets were)?
Exactly. I don’t remember exactly what they are called, but basically, they are acoustic baffles designed to reduce engine noise by smoothing the mix of gases and reducing turbulence from the engine IIRC. By reducing noise it will reduce weight which will save a lot more fuel than the loss of efficiency.
IIRC, this was part of a NASA program initially to reduce noise, but I might be misremembering. ETA: Yeah, from here:
I see several 787 and 747-8F departures nearly every afternoon & evening at work (I work on an international airport in the Middle East; our hangar is about 400m from the runway). They are definitely some of the quietest aircraft that I have ever experienced.
Kind of remind me of the various hush kits that were developed over the years to bring older engines into compliance with newer noise regulations. Controlling the mixing of engine exhaust with the surrounding air stream seems to be the key.
The lack of sawtooth doesn’t mean its a noisy engine. low bypass, with the large ones on the A380, 787 being high bypass - 90% bypass - 10 % through, makes for lower noise, for example.
Such noise reductions are going through to regional aircraft.
They are also working on aerodynamics of the whole aircraft, as the flaps and landing gear make a high pitch scream. Just like your roof racks on your car make a whistle, flat surfaces with straight edges held in the air flow makes for noise. Add more curves and variation… This also applies to the blades in the engines… curves and variations to avoid the vortex screaming at full tilt.
The noise regs have a long history of being tightened up continuously. There are already standards that take effect in the future that are tighter than the standards in effect today.
The manufacturers have to continuously improve just to stay abeam of the regs. For a machine that you hope has a 30 year economic life you need to build in a lot of future proofing.
Also, it’s not clear what you mean by “retrofit”. The vast majority of winglets on 737s were installed when that particular airplane was built. But they’re definitely an afterthought versus the original 737-100 product design. So is that a retrofit, or an improvement on a later variant model of the same basic product?
The reason this distinction is relevant is the 747-8 has chevrons, as does new the 737 Max series. Although no older 747s or 737s do. The current A350 has them. The soon to fly A330NEO will have chevrons although the current A330 doesn’t.
In the very beginning some outfits sold winglet kits that were retrofitted to existing in-service airplanes. That’s definitely a retrofit. Nacelles and pylons are amazingly complicated machines. IMO we’re not going to see bolt-on chevron kits. Also, there are specialized manufacturers of these things. They might hold the relevant patents rather than either the airframers or the engine manufacturers.
Manufacturers want to sell more airplanes, airports want to handle more traffic, and airlines want to fly more passengers (and cargo). Two ways to support an increased demand in aircraft use and sales are to either build more airports, or (more commonly) expand the size of current airports. However, the public is more likely to vote in favor of (or at least not resist as strongly) a larger, expanding airport if the airplanes are less noisy. Quieter airplanes allows for larger airports supporting more aircraft. That’s worth it to everyone involved. Including the residents who have to live nearby.
I have some familiarity with vehicle exhaust emissions regulations, which have likewise undergone a downward trend over the decades - but the EPA doesn’t require a car sold this year to meet regulations that go into effect next year; it only ever has to meet the regs that were in effect the year it was sold.
Does the FAA (EPA?) write future noise regs such that a plane sold this year will be required sometime in the future to meet new, tighter noise regulations?
The difference that high-bypass makes to noise is incredible, something for which everyone should be grateful who lives within a hundred miles or more of a major airport. The whole principle of operation is different from the old zero-bypass turbojets – as befits the name “turbofan”, the thing is basically a giant ducted fan powered by a turbine, getting most of its thrust from the fan. I remember watching old low- and zero-bypass commercial jets taking off years ago, and it sounded like the very fabric of the atmosphere was being torn apart. From inside the plane, once those turbojets revved up to full power, they emitted a mighty roar that sounded like you were inside a rocket launch bound for the moon! Today’s jets, in contrast, emit something more like a pitiful whine. Air travel may be going down the tubes in overall inconvenience, but real progress has been made in noise reduction and fuel economy.
One of the first appearances of blended winglets was on the Boeing Business Jet, where both the BBJ and BBJ2 were retrofitted with them before they became standard on the 737 airframes on which the BBJs are based. IIRC, not only were the BBJ winglets retrofits, but they were (again, IIRC) sent out by Boeing to be retrofitted on new BBJs by a third party – I think it was these guys, who today are offering to replace the now-standard blended winglets with super-winglets for even higher efficiency.
A couple weeks ago I was a few miles from Logan and I heard an airplane taking off. I looked up, and I could see it had the Delta name and logo painted on the bottom of the fuselage. I can’t decide if that’s brilliant, because it gets their branding into the marketplace essentially for free, or stupid, because people will only see it when the planes are overhead being loud and annoying and no one wants their brand associated with that.
These are not FAA but rather international regs. In general they say things like “Any airplane noisier than X dB may not fly past Jan 1 2020. Any airplane noisier than (X-10)dB may not fly past Jan 1 2025.”
So anyone building an airplane today with an expected lifespan past 2025 needs to comply with the 2025 standard today. Or have at least an outline of a plan for how to retrofit the airplane later to meet the future standard. Even if today they don’t know exactly what that retrofit will be.
As well, on several occasions in the past the regs got changed to add a new lower threshold tacked on the end. i.e. Pretend the above is the current state of regs then tomorrow the governments agree to add a third sentence. “Any airplane noisier than (X-20)dB cannot fly past Jan 1 2030”. Again if you’re building an airplane today that you expect to have a 30 year productive life you need to plan for the full 30 years of ever-tightening regs.
The critical difference with cars is the average lifespan of a car is much shorter than of airplanes. And the regulatory attitude does not extend to economically killing the older cars by banning them past a certain age. For airplanes, since the beneficiaries are consumers and the “victims” are big business, it’s (more) OK to sunset assets early.
Of course, consumers have votes and big business has lobbyists. So the battle for “how tight how fast” is not as one-sided as all that.
Another key difference is that EPA regs are pretty universally US only. Which is less “green” than Europe. The Europeans have been leading the charge on worldwide noise regs for decades now. With the US lagging behind and dragging its feet as much as possible. A similar battle is going on right now over limiting carbon emissions. Which is a whole 'nuther thread.
The reg-writers are sensitive to the lead times appropriate to the industry. But at the same time, they have a political charge to “force the pace”; to ensure the industry has its feet held to the fire to progress faster than mere corporate convenience would dictate.
Airlines are sorta in the middle. Improved noise tends to go with improved fuel economy. Which is good. The low interest rates of the last few years have caused a lot of youngish airplanes to be retired or scrapped for marketing reasons: the passengers love new shiny as long as the tickets are cheap enough. Many pundits are arguing now that the first-tier economic life of airliners is more like 10 years than 30 and everyone should plan on the machines being depreciated to near scrap in at most 15 years. Which, if true, will incentivize the reg-writers to be even more aggressive on forcing the pace.
My personal view is the current 15-year life mania is a product of the current mix of low fuel prices and low interest rates. And will instantly evaporate when either or both of those things do.
All quite true and nicely explained. But high-bypass started with the 747-100 in 1969. Speaking only about the non-Third World the last zero bypass airplane was gone by the early 1980s and the last low-bypass airplanes by the mid-late 90s. The last medium-bypass aircraft still in commercial service is the MD-80 and those are rapidly disappearing now. I remember when those were almost the quietest thing in the sky. Now they’re by far the noisiest.
In other words, high bypass is real old news.
Moving forward, the noise reduction of a 787 over a 747-100 is astonishing. Some of it is indeed even greater bypass. But a lot of it is other things. Each minor but they add up.
Much of current NASA & manufacturer research is different yet again. For the latest models in production most of the noise of an airliner on approach is not from the engines; it’s from the airframe. Takeoff noise has a greater engine contribution, but the bottom line is that the current lowest-hanging fruit is reductions in airframe noise.
The problem is that the easy cleanup has already been done. Next steps are things like totally different airplane shapes, different engine mounting locations, mostly shrouded landing gear, continuous wings & flaps with no gaps, slots, or bumps, etc. In other words, we’re getting into big changes with big side effects. Which equals technological and business risk.
There is one more big bump in potential engine tech just ahead: open rotors AKA propfans. Which is a step-change in engine tech and will require a step-change in airframe design to accommodate it. Which is doubly risky. It remains to be seen when or if this tech emerges from the lab into production.
Quite right. The first examples were pure retrofit, a bolt-on accessory created by a third party to be added to airplanes already in service.
As of today, those pioneers are a tiny fraction of the installed base of winglets. The vast, vast majority of wingletted aircraft came that way from the factory using fully factory-designed components. Maybe it was just me, but my reading of the OP’s phraseology made me think he thought almost the opposite. Hence my “clarification” that may have muddied the waters instead.
Quite true, but I can relate these anecdotes because I, too, am real old!
Which train of thought got me wondering when those noisy old jets like the DC-8 and the 707 were in mainstream service, which in turn led to the unexpected discovery that the DC-8 (and 707) weren’t pure turbojets as I had inferred from the engines’ appearance and the sheer noise they produced. Most, like the ones that Air Canada flew (1960-1983, for anyone wondering) used low-bypass fanjets like the Pratt & Whitney JT3D which had a bypass ratio of 1.42:1 – very low by today’s standards, but bypass nonetheless. The JT3D was apparently used on 707s as well. Bypass or not, I can tell you they were noisy as all hell! AIUI, modern fanjets commonly have bypass ratios of 5 or more; most models of the GE90 are typically 9 or close to it, and some other engines are even higher.
Yeah. It’s amazing how much history I remember these days because I was there when it happened. Unfortunately it’s getting harder and harder to remember it correctly.
The original KC-135, B-52s prior to the H model, early 707-100 series, 720A, and DC-8 10/20/30 series aircraft all had variants of the military J57 = civil JT3 pure turbojets. Which were, as you say, noisy as hell. Exceeded only by the mighty Convair 880 & 990 powered by the extra noisy civil CJ-805 = military J79 which added massive extra smokiness for maximum environmental vandalism. The early 1960s were a “special” time.
The B-52H, later 707-100 series, 707-300 series, 720B, and DC-8 Series 40 and up had variants of the military TF33 = civil JT4 turbofans. Many KC-135s were later re-engined with utterly obsolete former civil JT4D or later turbofans off run-out 707s. Which greatly reduced the noise & helped some on the smoke.
The 727, 737, and DC-9 started out with low bypass JT8 turbofans.
A decade or two later the rest of the surviving KC-135s were re-engined with a variant of the fairly high bypass CFM-56 engines used on the 737-300 and up.
The era of pure turbojet airliner production pretty well ended in 1962ish. Although they stayed in use into the late 1960s and maybe a few into the early 1970s.
I live and work close enough to a major airport that I could probably hit airplanes with rocks if I wanted to. You get very used to them after a while. Really, the only time I notice airport sound is when there’s something really loud taking off. Fighter jets, large helicopters, maybe a plane that misses the approach so the sound of it landing is different (since it didn’t actually land). I’m thinking that if you’re in a position to see airplanes, you’re probably used to it. Also, if I’m booking a flight, how loud the planes are wouldn’t even enter into my thought process, but ‘all else being equal, I do see a lot of Delta planes’ might.
In the 60’s, American advertised its new 727’s as “Whisperjets”. Times do change, and so does technology.
The chevrons provide a way to increase the shear area between the higher-velocity fan discharge and the ambient air, reducing the turbulence noise from that interface, without a real penalty in either weight or performance.
I would think low fuel costs would make older planes more attractive. Why retire a fifteen-year-old 737 when a new replacement is only a few percent more efficient? When fuel is cheap, a few percentage points won’t cost too much. When fuel is expensive, every gallon you save adds up.
As you say, they reduce the operating cost advantage of new low-burn planes over older higher-burn planes. Which, as you say, tends to support keeping the oldies in service.
The other thing they do is flatter the whole economy which leads to increasingly happy consumers, more ability and desire to buy leisure travel, more business travel, increased profitability for airlines, and all the rest of the good stuff.
Which leads to airlines both growing which means getting additional new jets, and airlines competing on “luxury” and “service”, such as it is within the industry. Which is to say shiny new jets with shiny new interiors, new and faster wifi, etc. Which jets, oh by the way, happen to be more fuel efficient. But fuel efficiency isn’t why they’re being bought. They’re being bought because it’s springtime in the industry to borrow Reagan’s memorable tagline.
During the good times IMO/IME the second effect outweighs the first. Hence my contention in my earlier post.
If fuel prices spike but the economy holds mostly steady you’ll definitely see what you predict: massive parking of the few remaining oldest high-burn jets. OTOH, if recession hits and fuel prices stay low you’ll also see the really old jets retired first in response. If industry disaster hits, e.g. another 9/11 or 2007 stock market crash, after the first wave of retirements you’ll also see a lot of newer, next-oldest jets get parked at least for the duration of the downturn.
Air travel may be going down the tubes in overall inconvenience, but real progress has been made in noise reduction and fuel economy.
