In 2006, Boeing began using chevrons, or scalloped edges, on turbofans mainly for noise reduction. They added the chevrons to engines on the fan ducts on the 737 MAX, B787 Dreamliner, and 747-800. Airbus, Boeing’s main competitor, did not install chevrons and focused on other methods to make their engines quieter. Boeing has since abandoned the chevrons on new aircraft after testing and flight data revealed some problems in the design.
Noise Reduction: A Key Focus for the 737 MAX and B787
When Boeing was initially developing the 737 MAX and the 787, reducing engine noise was a top priority. The 737 MAX would use the CFM International LEAP engine, while the 787 offered two choices: the General Electric GEnX-1B and the Rolls-Royce Trent 1000. These engines were large and powerful—and very loud. To address this, Boeing started designing features specifically to quiet them down.
Modern jet engines create noise when cool air from the turbofan mixes with the hot air coming from engine core. The turbulent interaction between these two airflows generates much of the roar we associate with jet engines.
Boeing Partners with NASA on New Engine Designs
NASA teamed up with Boeing to tackle the noise issue and helped develop a promising solution: chevrons on engine exhausts. They found that the scalloped edges of this design create narrow air channels from the exhaust. These channels reduced the mixing of temperatures and helped make the engines noticeably quieter.
At first, despite hopes that the chevrons would work, NASA found it difficult to develop designs that actually worked.
“Early on, we didn’t have the advanced diagnostics, instrumentation, and insight to know what we had done to make it worse instead of better,” said James Bridges, the associate principal investigator responsible for coordinating aircraft noise research at NASA. “You have an idea, and then you cut out a piece of metal and try it. Sometimes, the kernel of the idea might have worked out, but the way you did it wound up causing more noise.”
Advanced Testing Proves Effectiveness of Chevrons for Noise Reduction and Efficiency
NASA eventually began using advanced testing methods in wind tunnels with lasers and high-speed photography to prove the effectiveness of their designs. Following successful testing, Boeing added chevrons to the 747-800, 737 MAX, and B787.
This effort by Boeing and NASA served some specific purposes. The primary objective was to reduce engine noise during takeoff and landing–not only to make flights quieter and more comfortable for passengers. It was also because many areas worldwide have issued stricter noise reduction policies. This has forced manufacturers to create new designs.
Boeing also anticipated even tighter restrictions in the future and wanted to stay ahead of the curve.
Noise reduction was not the only reason Boeing developed chevrons. Boeing also saw them as a means to improve fuel efficiency. By reducing the turbulence of the engines, the chevrons would improve airflow and reduce fuel consumption.
Another benefit of this technology is that it allows manufacturers to reduce weight. The chevrons allowed Boeing to remove some sound insulation from the aircraft, making them lighter and more fuel-efficient. For example, Boeing was able to remove about 600 pounds of sound insulation from the 787 Dreamliner while reducing the noise by 15 decibels.
Boeing Finds that Chevrons Reduce Thrust
While Boeing saw significant advantages of using chevrons for noise reduction and fuel efficiency, the corporation also discovered some problems. They found that the chevrons caused a slight reduction in thrust.
Petter Hörnfeldt, a 737 MAX pilot, explained this, stating, “Anytime a vortex is being created, it takes away energy from the object that’s creating the vortex. Since you’re adding these chevrons, and they are creating vortexes, they’re actually reducing the amount of thrust that the engine could take out … about 0.5% of the thrust.”
This may not be much, and with the added benefit of reduced engine weight, the impact of reduced thrust is even less. However, based on this and other flight data, Boeing abandoned its use of chevrons in aircraft models. It has not included chevrons on its newer 777X wide body.
Terry Beezhold, 777X chief project engineer and vice president at Boeing, explained, “We are replacing the chevrons with a new nozzle design technology. It provides equivalent levels of noise for the cabin and community but is lighter in weight and has lower drag.”
Airbus Develops Different Noise Reduction Designs
Boeing’s main rival, Airbus, has taken a different approach to engine noise reduction. While Boeing adopted chevrons–a technology it held patents for–Airbus has not used them on its aircraft. Instead, Airbus pursued its own noise-reduction strategies.
On the A350, for example, Airbus focused on developing enhanced insulation and high-bypass noise reduction. High-bypass designs allow more air to flow through the engine nozzle without entering the turbine core, slowing exhaust velocity and reducing noise.
Airbus reports that these concepts give its engines similar sound volumes and performance as Boeing’s designs.
