Many individuals have likely observed the unique sawtooth patterns on the rear of engines found on planes like the Boeing 787 Dreamliner or Boeing 737 MAX, yet few understand their purpose. Referred to as chevrons, these serrated trailing edges were created to address one of the major environmental issues in commercial aviation: noise pollution from aircraft.
Rather than serving as a design element, they are the result of extensive collaboration among Boeing, GE Aerospace, Honeywell, and NASA, aimed at reducing jet engine noise during takeoff and landing. Their introduction came at a time when aircraft manufacturers faced mounting pressure to comply with increasingly stringent international airport noise regulations while still maintaining the fuel efficiency that airlines require.
Boeing’s solution was not merely to redesign the engine itself but to reconsider how the exhaust exited the nacelle. This led to one of the most recognizable characteristics of several contemporary Boeing aircraft, although it also involved compromises that would influence future engine development.
What Are Chevrons & How Do They Mitigate Engine Noise?
A modern turbofan engine emits two distinct streams of air at its exhaust. The hot, high-speed core exhaust flows out through the center of the engine, while a cooler bypass airflow envelops it. When these two streams converge behind the engine, the significant difference in speed results in turbulent mixing, which is a primary contributor to jet engine noise.
This is especially crucial during takeoff and initial ascent, when engines operate at high thrust levels, and aircraft are nearest to surrounding communities, making noise reduction a vital concern for manufacturers and regulators. Chevrons assist in minimizing this noise by creating small vortices that promote a more gradual blending of the hot and cold airflows.
This process diminishes the intensity of turbulent mixing and reduces the acoustic energy generated by the exhaust. Prior to the technology being implemented in airline service, engineers from Boeing, GE Aerospace, and NASA tested various chevron shapes, depths, and spacing configurations to find the optimal balance between acoustic performance and aerodynamic efficiency.
Since each engine produces unique airflow characteristics, the final chevron design was customized for each engine type instead of applying a uniform design across all aircraft. The outcome was a noise-reduction solution that could be incorporated into existing turbofan designs, enabling aircraft to comply with increasingly strict noise regulations without necessitating a complete overhaul of the engine architecture.
The Trade-Off of a Quieter Engine
While chevrons do reduce noise, they accomplish this by intentionally modifying the manner in which exhaust gases exit the engine. The serrated edges generate controlled vortices that facilitate a more gradual mixing of the hot core exhaust and cooler bypass airflow, thereby decreasing the turbulence that produces noise. However, these vortices also result in additional aerodynamic losses compared to a smooth nozzle design, leading to a slight reduction in thrust and an increase in fuel consumption.
Although the effect is minimal, it represents a permanent efficiency trade-off since the chevrons remain in the exhaust stream throughout all phases of flight, including cruise, when their noise-reduction advantages are less critical. The exact performance impact has never been publicly disclosed by manufacturers.
Modern engines are already engineered to function at high efficiency levels, meaning even minor aerodynamic drawbacks can affect fuel consumption, payload capacity, and operating expenses. For airlines, even fractions of a percentage point are significant, as aircraft spend thousands of hours in the air annually.
The influence of chevrons is particularly pertinent for high-frequency operators flying aircraft like the 737, where numerous short sectors mean that the small performance penalty is repeated across many flights. Ryanair, one of the largest operators of the 737, sc
Source: Original article
