The latest engine to roll out of Pratt & Whitney just might deliver what the
aerospace industry has sought in a commercial jet for more than a century _ some
peace and quiet.
"The tremendous racket that is at present associated with the aeroplane plays a considerable part in prejudicing the public against these machines," read a 1911 editorial in The Aero, an industry publication based in London, that advocated the "fitting of silencers" on engines.
Pratt's PurePower geared turbofan engine _ well-known for its efficiency with jet fuel _ is also a long stride toward quieter flight. With a noise profile that comes in 20 decibels below the most stringent airport standards, the engine is expected to be noticed by far fewer people on the ground.
And the result: Airlines have ordered more than 3,500 of the engines for new aircraft.
A twin-engine airplane flying 1,000 feet overhead comes in as loud as a noisy dishwasher to people on the ground, about 70 decibels. So living by an airport is like having a reprise of that dishwasher every few minutes throughout the day. Pratt's engine, at about 20 decibels lower, will be more like the hum of a refrigerator.
Since those early, loud days of jet flight, the aerospace world has cycled through its share of solutions. Mechanical ideas included putting mufflers on engines, but they proved to be heavy. Others focused on advanced materials that deaden sound. The latest advances have come from relying more on the quieter portions of the engine for thrust.
But with airplanes still considerably loud, airlines are forced to operate within noise budgets set by airports; pay noise fees; and jump through hoops to avoid being especially loud over populated areas. The major engine-makers, Pratt and General Electric, are both releasing engines that they say are significantly quieter than existing technologies.
"This is the first time in commercial aviation that you can't blame the engine guys anymore," Alan Epstein, Pratt's head of technology, said in an interview. He said it won't be long until "airplanes get to the noise levels of highways."
Analysts say the drop in engine noise _ Pratt, a division of United Technologies Corp., advertises a 75 percent reduction _ is a jump forward in an industry used to smaller advances. "This is a big change, not just an incremental improvement," said Richard Aboulafia, aerospace analyst with the Teal Group in Fairfax, Va.
The engine lines up against its noise-conscious competitor, GE Aviation's LEAP engine, which bets on the cumulative effect of a handful of material and design changes to reduce noise, Aboulafia said. The Pratt engine, on the other hand, is a more straightforward approach, accomplished by moving more air, more slowly.
"It doesn't depend on a host of new incremental improvements," he said. "That definitely has its appeal."
For airplane makers, like Airbus, the less noisy engines _ combined with more aerodynamic airframes _ will give their airline customers some wiggle room within the "noise budgets" at airports, which are set to decrease over time.
In the very least, a quieter engine "enables them to preserve their existing operations," said Raphael Sheffield, head of technical marketing at Airbus, which will have a version of Pratt's engine on its new A320neo single-aisle airliner.
And beyond that, the quieter engine will give airlines some room to possibly add more flights "within the existing noise budget."
With projections for airlines to see annual increases in passenger miles, airports and the entire industry are looking at how to build out capacity at airports, while remaining good neighbors to residents in surrounding areas. Noise and emissions are expected to be major issues in this expansion, and Pratt and other engine makers are competing to offer the least noisy engine.
"The biggest impediment to the expansion of air transportation in the developed world is noise," Epstein said.
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The Pratt engine is quieter than other engines, but it is also bigger.
An airplane engine gets its thrust from one of two related sections: the engine core, where the air compression and combustion occur, and the fan, which is driven by the engine core and pushes a large volume of air with large blades.
It's the engine's center core _ the jet portion where combustion occurs _ that makes it particularly loud.
Part of the air that enters an engine goes through the core, where it's mixed with jet fuel and ignited, but most of the air goes around the core. Over time, engine makers like Pratt have made smaller and smaller jet cores, while enlarging the size of the fan and its blades that give more thrust with less noise.
"You can have the same amount of thrust with a small amount of air going very fast, or a much larger amount of air slowly," Epstein said. "So more air means the engine needs to get bigger and bigger in diameter."
The more air moving through the fan, as opposed to through the engine core, the quieter _ and more fuel-efficient _ the engine will be. And that ratio of air that moves through the core compared to the air that moves through the outer fan is called the bypass ratio.
The company released the JT8D engine in 1963 for Boeing's 727. Its bypass ratio was 1, so equal amounts of air are sent through the engine's jet and fan. In 1970, Pratt's JT9D upped the ratio to 5 for the 747.
The latest PurePower engine out of Pratt has a bypass ratio of 12 _ for every 1 part of air that goes through the engine core, 12 others are sent through the much quieter fan.
"It's so quiet on approach that if it's coming for landing (and) you shut off the engines, people on the ground wouldn't know the difference," Epstein said.
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Noise limits are coming down each year, analysts said.
The limits, though set by each country, are based on recommendations from the International Civil Aviation Organization, a standard-setting agency of the United Nations.
To keep up with the falling noise ceiling, airports have two general tools: make less noise with a quieter engine or avoid flying over where people live.
The avoidance approach has airlines tweak their approaches and takeoffs to make less noise where it really matters.
For example, aircraft approaching Reagan National Airport outside of Washington, D.C., fly along the Potomac River rather than over densely populated areas. At LaGuardia Airport in New York planes taking off make a 180-degree turn to climb over the water rather than throttling the engines over the Bronx.
In some cases, pilots will stay higher in the air as long as possible before landing.
But with an overall quieter aircraft, it's possible that airlines could bring jets into smaller airports that are closer to sensitive, urban areas.
Porter Airlines, a regional airline based at Billy Bishop Toronto City Airport, ordered a dozen CSeries airliners from Bombardier _ with Pratt's geared turbofan engine _ to expand its service from a short-haul model to a coast-to-coast model with added destinations in Florida, California and British Columbia.
Currently, Porter only flies quieter propeller-driver turboprops to destinations about 2 hours away from Toronto, mainly because agreements between the city, the airport and the port authority have limited the airport to those aircraft.
The only problem is that the CSeries jets that Porter ordered are currently banned from flying at the airport that's just 1.5 miles from the center of Toronto and its 2.6 million residents.
"When we started, we didn't have the sense that we could operate jets from there because the technology didn't really exist," said Porter Airlines spokesman Brad Cicero. "But the CSeries program really changed that for us."
The airline's purchase is conditioned on local and national officials changing the airport's jet ban and approving a 367-yard extension to the runway, Cicero added that Bombardier has guaranteed that the dozen CS100s will be able to operate within the airports existing noise limits.
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