Alan H. Epstein : Des avions plus silencieux

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Depuis l’apparition des avions à réaction, les moteurs d’avions sont devenus de plus en plus silencieux et le seront davantage grâce aux évolutions technologiques, indique Alan H. Epstein, qui a participé au développement de systèmes de propulsion des avions plus silencieux et économes en carburant.

Dr Alan H. Epstein est le vice-président pour la technologie et l'environnement chez Pratt & Whitney, un fabricant américain de moteurs d'avion. Cet extrait est issu d’un entretien avec Timothy Spence d’EURACTIV en amont du Salon international de l’aéronautique et de l’espace à Paris du 17 au 23 juin.

L’entreprise américaine fournit des moteurs à Airbus et d’autres fabricants d’avions. Ses nouveaux moteurs à turbofan à réducteur, les PurePower PW1000G, seront utilisés pour l’Airbus 320neo, le Bombardier canadien CS100 moyen-courrier, les avions de passagers CS300, l’Irkut MS-21 russe et le Mitsubishi Regional Jet. Le moteur PurePower PW1000G a terminé son premier essai en vol le 15 mai.

New engines about to come on the market are reportedly 75% quieter than those in the air today. How noisy is that on take-off?

In decibels, the number is about 85 – that’s like standing on a busy street corner in Manhattan or London or Paris, whereas the early jet planes were 120 and above – they were rock bands. Rock bands haven’t gotten any quieter, but airplanes have.

Anyone who lives near an airport or on a flight path might take a different view. Aren’t we looking at years before this technology catches up with what’s in the air today?

No, the technology is a real product. The Neo flew last week, the CSeries engine is certified by Transport Canada and the [CSeries] airplane is going to fly this month. It will take time as the new airplanes go into the fleet. You can only make so many airplanes per year, and it will take time for the old airplanes to come out.

Once Bombardier has several test airplanes flying and they start flying around for marketing purposes, they’ll fly it into noise-sensitive communities and there will start to be groundswells from people saying, ‘Why should we put up with these noisy airplanes when we could be flying on these things.’

Notwithstanding, people are still going to be complaining about noise.

You’ve gone from a rock band to urban traffic in terms of noise. Do you see a point where one of your engines is as quiet as an electric motor in a car?

It isn’t the motor that makes the noise anymore. It’s the pushing of the airplane. So in other words, I could conceive of an electric-powered airline. Now to do that, I would need the world’s longest extension cord, but as a thought experiment I could electrically power an airplane. All that means is I have an electric motor turning a shaft and I still need a propeller or fan that pushes the airplane.

What makes the engine so quiet?

An airplane needs a certain amount of thrust and you can make the thrust by moving a lot of air slowly, or little bit of air very fast. The noise gets made by the speed of the air being moved. So from a noise point of view, you want to move as much air as you possible can as slowly as you possibly can. But the inside of the engines – all those compressors and turbines – want to turn really fast.

In the past we’ve had a compromise between the parts that turned slowly and the parts that want to turn fast because they were all connected just by a shaft. Putting a gear – like the gear on your car – it lets the motor turn fast and the wheels turn at whatever speed they want to go at.

If you take the A320, the current engines flying on that [plan] are 63-inch-[160 cm] – diameter fans; the new Neo with the GTF [geared turbofan] has an 81-inch-[206 cm]-diameter fan, so there’s an enormous amount of area increase. We have some [illustrations] – there aren’t photographs yet because the airplane isn’t ready yet – and it looks a little strange because it looks like this little plane with these enormous engines on it.

Is there a correlation between noise reduction and fuel efficiency?

Turning the fan really slowly dramatically reduces the fuel consumption. So on the Airbus Neo, you are talking about a 16% reduction in fuel consumption that goes along with the noise reduction. My suspicion is that most of the airlines, if not all the airlines, are buying these as fast as they can to get the fuel consumption because fuel is now 50% of their direct operating costs. The noise is a bonus.

So the motivation is fuel efficiency?

That’s my interpretation of our customers’ actions. If you are running a business that has historically lost money for the last century and the price of fuel has ballooned so that it’s more than 50% of your operating costs, you’re desperate to reduce fuel burn and these new engines are an enormous step in that direction.

Is it possible to retrofit a plane with these newer, quieter, more efficient engines?

Yes, it’s physically possible. Historically it’s never been economically viable.

There is more and more effort in Europe to shift away from the traditional ‘stair-step’ approach to landing an airplane to a more direct approach. Would that have any impact on noise reduction?

There is a big effect. You’re talking about continuous descent arrival, or CDA, and the idea is that it keeps the noise off the ground because the airplane is higher for longer.

There is the noise you make but the noise you hear is a function of how far away you are. For a [Boeing] 747-400, for example, the noise contour on the ground is reduced by between 35 and 40% using a CDA approach [compared] to a standard approach.

You asked me a question that I did not answer, and that was is it possible to build a virtually quiet engine.

There was a study done by MIT and Cambridge University for a virtually silent airplane, and virtually silent means not that it didn’t make any noise, but if you were standing on the street and the airplane flew over, you wouldn’t hear it because the noise was below the ambient noise in an urban environment. You couldn’t do that in the country where the only noise is crickets at night, but you can in the city.

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