“What's that?” Asked my three year old daughter, pointing skywards as we walked the dog. The hills around us resonated with a rumbling drumbeat and four contrails trailed the beast overhead as it climbed towards the Atlantic.
It was a beautiful day, probably the first day of the year that really felt like spring. No leaves on the trees yet, but the air was warm and the sun bright. Two smaller planes followed the big one overhead along the same path, and a minute later their own tremulous signature bathed the Tipperary pastures. “It's a big plane.” I told my daughter. “Do you like it?”
She did.
Not everyone does though, and while those big birds were pretty high up as they climbed out of Cork or Dublin, -probably Dublin-, it would have been a lot less fun if the airport was next door.
I have, admittedly, limited sympathy for people near airports who complain about noise. This is not a new technology, and the airports predate most of the local residents; all of them in the case of Heathrow, Europe's busiest. You can hardly buy a house next to an international airport and be unaware that planes will fly overhead.
All the same, you don't want them waking the kids either, and as urban centres cluster around their airports, people are brought into close proximity. There's only so much that triple glazing will do, especially when the flights go in the evening or early morning.
So what are our options?
Those who have read this blog before will be unsurprised to find that there are technological solutions to this problem: Lots of them!
Let's take a look at what the engineers have been cooking up…
1: Nudge, don't shove
The aircraft of the 60s were different creatures from today, and not just because you could smoke on them. Marked out by classics like the Boeing 707, they don’t at first glance look too different to what we’re flying today, over half a century later, but small details matter, and the harder you look the more of them you notice.
One obvious feature of the early jet airliners is that their engines, though plentiful, are all a bit… small. And smoky. And, curiously, very noisy. A large part of this is because of a design philosophy that aimed at reducing fuel consumption also had the effect, over the years, of suppressing the acoustic signature of the classic jets.
In short, the fans at the front of the engine are much, much larger now.
The earliest jet engines were pure turbojets, in that every ounce of air that came in the front found its way through the compressors and the combustor to be accelerated and flung out the back. This tight, hot gas plume exerts powerful shear layers in the air behind the aircraft, where the high velocity jet frantically tries to shed its energy into the ambient air through the creation of unstable vortex structures. This powerful expanding and collapsing turbulence gives birth to a particularly noisome acoustic signature.
In simpler times I remember watching the big Russian fighters bend the air at Finningley Airshow in South Yorkshire, at the time the biggest one-day military airshow in the UK. It was completely captivating for a plane-obsessed young nerd like me, and the stars of the show were without a doubt the big Sukhoi ‘Flankers’, rearing up on their tails like a cobra in front of the crowd, while the sound of their engines split the air. I remember not quite hearing that sound, but feeling it vibrating deep in my lungs. I even watched the passenger window of my dad’s car visibly shake under the acoustic onslaught.
And that lung-quivering impact was entirely due to this effect.
Modern airliners, by contrast, have huge driven fans at the front supplying most of their power, primarily for fuel efficiency: It's easier to accelerate a huge volume of air through the fan gently than a small volume through the core harshly.
In short, it's easier to nudge than to shove.
And the benefit of this is that the throaty roar of the pure turbojet is quelled and tamed by the cool air of the fan and the gentler air mixing it inspires. So much so that in modern aircraft, unlike the savages of the 60s, the hot exhaust is now muffled and makes up a minority of the acoustic signature.
And so successful are the expanding fan stages that they are growing bigger and bigger, which brings its own problems…
2: Only Fans
Fans are good. More fan is better.
A modern jet engine, for all its power, teases the air rather than brutalising it, and over the years as the fans have grown so have the bypass ratios of new engines; the proportion of airflow that passes through the cool fan and not the incandescent core.
What this means is that the latest widebody airliner turbofan engines have a bypass ratio of 10:1, meaning that ten times more air passes through fan than core. Such engines are, in effect, ducted fan propulsion systems. This is great for noise reduction as well as efficiency, except that the fan is not exactly silent.
We've all heard it: An abrasive tonal buzz, like a giant alien wasp, that accompanies the rumbling of a big jet engine. That's from the fan stage, whose acoustic signature is less broad-band than jet exhaust and more tonal.
Unfortunately human beings don't like aggravating tonal noise.
There are a few solutions to this, which focus on either eliminating the root causes of that noise, or else damping it so it doesn't get into the cabin. We'll cover each approach in turn.
Zero-splice intake nacelles are one way of beating the buzz, as it's produced from discontinuities in flow caused by fan blades passing features at a fixed frequency. The intake nacelles generate this through a ‘splice’ where multiple assembled segments come together imperfectly. Improved manufacturing control can create ‘zero splice’ architecture which helps control fan noise. But there's more…
Aerodynamic optimization of blades for acoustics is possible, but only to a point as their primary goal is aerodynamic efficiency. What is a big plus for both acoustics and aero, however, is gearing the fan, a technique first commercialised by Pratt & Whitney with the PW1000G. What does this mean?
The fan stage is powered by the engine's low pressure turbine stage, where fast-moving, hot expanding gas yields its tremendous energy to rotating turbine blades, powering the fan through a central power shaft. The problem is that the ideal rate of rotation, from a purely aerodynamic efficiency perspective, is different for the turbine and the fan: The turbine would like to rotate faster and the fan slower.
A slower fan on big engines is both more aerodynamically efficient and quieter too. Win-win!
And our option list isn't even done, because you can damp the sound as well. One of the most common ways of doing this is with a Helmholtz liner in the engine nacelle: Enabled by the tonal nature of fan noise, the regular geometry of the resonator creates an antiphase excitation that helps to cancel out certain frequencies.
Some physics 101 here: When a reflected wave matches the waveform of the excitation signal it is in resonance, which amplifies it and can cause serious issues: The Tacoma Narrows bridge was a classic example of structural resonance causing mechanical failure. A flute is an example of acoustic resonance creating pleasant sounds.
What is not always appreciated is that this works in reverse, too: If the reflected waveform is half a wavelength out then is is in antiphase, and cancels out the original excitation. A helmholtz resonator can damp fan noise in this way in a narrow frequency range, and for other frequencies you can just add more layers of resonators.
However, inevitably in aeronautics there are headwinds…
The same efficiency-minded influence which wants to expand the fans and let us fly on the tiniest sips of aviation fuel can also act against our own interests.
As fans grow wider the weight of their enclosing nacelles bloats as well, and the only way to make that work for weight-sensitive aircraft is to shorten the nacelle. Crucially, while this is aerodynamically perfectly OK, it's an acoustic nuisance, as an extra layer of damping is removed.
And this is added to the removal of other sources of damping in the system, such as the weight saving replacement of kevlar blankets with carbon fibre once an engine has moved to lightweight carbon fiber blades. All very impressive and efficient, I'm sure, but passenger experience matters too.
But what about the hot heart of the engine? How do you silence a furnace in a hurricane?
Let's find out…
3: Cool it! Nozzle mixing.
It was a bright, cold day in Derby on my first year in Rolls-Royce and I was looking skywards in anticipation, like many in the crowd around me that day. We were stood in the fields near the massive Moor Lane offices and the fitness centre, close to the rugby grounds that I would grow to know well over the years.
But not yet, and not on this particular day, because we were there to see a flyover: One of the first Boeing 787 ‘Dreamliners’ to incorporate our Trent 1000 engines. It would pass over us in thanks, and whilst I was only just in the door and didn't feel at all worthy, I fizzed with excitement just like everyone else.
And here it came!
Straight overhead, low, white and beautiful. Its wings elegantly curved and engine chevrons glittering in the sun, and all I could think was “Wow. It's much quieter than I thought!”
And it was. The huge plane whispered its way overhead, and shortly afterwards, nodding to each other at a job well done, the engineers of Rolls-Royce went back to work.
What made it so quiet, and what made the jet planes of old so loud?
Our enemy here is velocity shear. The high speed hot flow leaving the jet engine core interacting with the slower-paced flow around it spawns turbulent structures in the shear layer which generate high frequency jet noise. Lower frequency jet noise is then generated when the mixing layer permeates throughout the core flow, creating large scale turbulent structures.
If we want to ease this, then we must improve flow mixing.
The chevrons are the back of the nacelles on that Boeing 787, and aircraft like it, are a clue. By exploiting pressure differences between the inner & outer face, exhaust chevrons can create stable vortex structures that enhance mixing and reduce the acoustic signature of the aircraft. This is an option, but the extra weight and drag is a cost you wouldn't want to bear if you didn't have to, so such measures are usually extended to only the most severe noise reduction challenges.
For example, the Airbus A321 used core exhaust chevrons for noise management at takeoff & landings, whereas the later A320neo, an otherwise similar aircraft, didn't as its use of quiet PW1000G geared turbofans meant that it didn't have to.
It's all compromise.
But what if it didn't have to be?
Crossflow microjets create flow mixing vortices without all that troublesome extra drag, and they can be turned off when no longer needed. An entertaining concept, but an early-stage one for the time being. Lots more development is needed before these go anywhere near a commercial aircraft.
So we have options on both the fan and the core, albeit not costless ones.
What about the rest of the plane?
4: Loud landings and ruffled feathers.
Years later and another airshow.
I stood at the Farnborough International Airshow in my Rolls-Royce garb, having volunteered to man the RR stand before the public open days started. I was slightly pissed off that my working hours had managed to overlap with the Eurofighter Typhoon flight, but had at least managed to enjoy the Red Devil parachute team and the always-spectacular F18 display.
Anyway, I could clock off just in time for the Big One.
It wasn't a Raptor or a stealth bomber, of course: Farnborough is commercial, generally ends with billions in purchase agreements and features an informal competition between Boeing and Airbus to see who can sell more airframes. Raptors and Spirit bombers aren't for sale to anyone.
But the A380 was.
The Airbus A380 ‘superjumbo’ is one of the very few aircraft that can make a jumbo jet look small. I've seen these things cut a cloud in two; they are genuinely colossal, so I wasn't expecting much out of the display. -And yet, after the roar of the takeoff run, all flaps extended, the monster left the ground, reared like a cobra and went up at an angle more familiar to fighter jets. It then executed what looked almost like a hammerhead manoeuvre, or stall turn, though I know that couldn't really be the case. I'm sure that giant aircraft, rearing commandingly before the crowd, was in control the entire time.
But its size rendered those movements glacial, impossibly slow looking. It looked like it was cheating physics.
And, once again, to ears trained in childhood to the planes of the time, it sounded strangely quiet for a monster of such size, doing something that preposterous.
Like a bird flaring its wings and extending its feathers as it alights on a tree, a big aircraft during takeoff or landing will throw clean cruise lines to the wind, extend flaps, spoilers and gears and will, briefly, cease to be an efficiency machine and start being a freeclimber, clawing at the air with every fingertip. Inevitably, this is noisy, and as the engines get quieter, the airframe noise looms larger near the world's airports.
There are two primary sources of airframe noise; landing gear and high lift devices (slats, flaps etc). On smaller aircraft it's the high lift devices that dominate airframe noise on approach and takeoff. On large aircraft it's the landing gear that make the bigger impact.
That the landing gear should be noisy should hold no surprises. I mean, look at them! but there are some things that can be done: Simply by covering cavities and adding rims on wheels the might A380 and A350 managed significant noise reductions. Likewise, moving pipework, obstructions and cabling into the slower flowing ‘quiet zone’ behind the strut makes a difference. And the European led SILENCER program for aircraft acoustics has even trialled smooth fairings encompassing the critical landing gear equipment, which remove multiple decibels all by themselves to the observer on the ground.
High lift devices are harder to silence, alas: Their function of reducing takeoff or landing speed means that any acoustic improvement that impedes their primary function becomes counterproductive, as faster moving aircraft are inevitably noisier. The A380 and A350 made some minor progress reducing slat and wing gaps, but otherwise it's slim pickings until morphing wings, porous edges and inflatable cuffs become a reality: Something that material limits will sadly slow down. Most candidate porous materials, for example, are just too brittle.
That said, morphing wings could yet be a thing. Airbus UpNext's revolutionary eXtra Performance Wing demonstrator will fly soon, which will trial precisely these technologies.
Where there's a will, and a wing, there's a way…
5: Noisy Conclusions.
Air travel has become a thread in the grand tapestry of our civilization, and I'd venture a vital one. Pull too long on it and the entire artwork would unravel into a depressing mess on a dirty floor.
It's not just about winter sun in the Canaries. It's about bringing families together, expanding opportunities and offering love across borders. My very own family owes its existence to this and would not exist otherwise. Romance forged by the white heat of the turbine blades and garish yellow & blue Ryanair seating: Is there any better way?
When next you are overcome by climate anxiety and travel guilt, remember what air travel does to pull us together, all over the world.
And remember, when next the wolds and fens echo to the rumble of contrails high above, that a thousand unseen engineers are working to make this moment a little bit more pleasant for you.
Figures taken from the exceptionally interesting paper An Overview Of Aircraft Noise Reduction Technologies, L. Leylekian.