The sawtooth edges around parts of a Boeing 787 engine nacelle look like a styling feature, but they were developed to solve one of jet propulsion’s most persistent environmental problems: noise created where fast engine exhaust mixes with slower surrounding air. These serrated edges are called chevrons. NASA and Boeing research showed that carefully shaped chevrons can smooth the mixing between high-velocity and lower-velocity air streams, changing the turbulence structures that generate noise.[1][2] The result is not a silent engine, and chevrons come with aerodynamic trade-offs, but they became one of the most visible noise-reduction technologies on the Dreamliner.
The short answer
Jet noise is generated partly by turbulent mixing between exhaust streams and the surrounding atmosphere. A sharp circular nozzle edge allows those streams to meet abruptly. Chevrons create controlled streamwise vortices that encourage the flows to mix more gradually. This can reduce the strength of some of the large turbulent structures responsible for low-frequency jet noise.
Where the chevrons are
On 787 engine installations, serrations are visible around the trailing edge of the nacelle. The exact geometry differs with engine installation and component because the acoustic and aerodynamic conditions around the bypass stream and core exhaust are not identical.
What a nacelle is
The nacelle is the aerodynamic housing surrounding a jet engine. It includes the inlet, fan cowl, structural and access components, and the aerodynamic surfaces guiding bypass airflow toward the rear. It also has to integrate thrust reversers, acoustic treatment, fire zones and maintenance access.
Why the nacelle matters to engine efficiency
A turbofan can have an extremely efficient core yet lose part of that advantage through poor nacelle aerodynamics. The inlet must deliver smooth air to the fan. The external cowl must produce low drag. The rear geometry must allow the fan stream and core exhaust to leave efficiently.
Where jet noise comes from
Aircraft engine noise comes from several mechanisms. The fan creates tonal and broadband noise. Compressor and turbine stages contribute their own frequencies. Combustion generates pressure fluctuations. Most relevant to chevrons, the high-speed exhaust produces turbulent mixing noise as it interacts with surrounding air.
Why older turbojets were especially loud
Early turbojets produced much of their thrust by accelerating a relatively small mass of air to very high velocity. High exhaust velocity creates strong shear with the surrounding atmosphere and intense jet noise. Modern high-bypass turbofans instead move a much larger mass of air by a smaller velocity increase, improving both propulsive efficiency and acoustic performance.
The 787 uses high-bypass engines
The Dreamliner can be powered by the General Electric GEnx-1B or Rolls-Royce Trent 1000 family depending on customer selection.[3] Both are modern high-bypass turbofans designed specifically for the aircraft’s long-range efficiency targets.
Bypass air and core exhaust
The large fan divides incoming airflow. Much of it passes around the engine core through the bypass duct and produces thrust without entering the combustor. A smaller portion enters the core, where it is compressed, mixed with fuel, burned and expanded through turbines.
At the rear of the engine, these streams eventually interact with one another and with ambient air. Their different temperatures and velocities create turbulent shear layers.
A clean circular edge is not acoustically perfect
Without chevrons, the boundary between streams develops turbulent instabilities as high-speed and low-speed flows meet. Large coherent eddies can form and break down downstream. Those fluctuating pressure fields radiate sound over a wide range of frequencies.
What a chevron does to the flow
Each tooth protrudes into or away from the shear layer depending on its geometry. This creates pairs of streamwise vortices that draw the two flow streams together earlier. Instead of waiting for natural instability to create large mixing structures farther downstream, the chevrons deliberately seed smaller-scale mixing close to the nozzle.
Why earlier mixing can reduce noise
Large turbulent structures are efficient producers of lower-frequency noise that can travel long distances. Breaking some of that mixing into smaller structures can reduce certain dominant noise components, even if turbulence itself is not eliminated.
Chevrons do not eliminate turbulence
The technology works by controlling turbulence rather than making exhaust perfectly smooth. In fact, the chevrons intentionally encourage mixing in a more controlled pattern. The acoustic benefit comes from changing the size, location and strength of turbulent structures.
NASA’s role
NASA says chevron research during the 1990s and early 2000s used computer simulation, ground testing and flight testing to determine which shapes reduced noise effectively.[1] The work involved NASA, Boeing, engine manufacturers and other industry partners.
NASA subsequently identified the 787 as one of the commercial aircraft using the resulting technology.[2]
Why computer modelling was important
Changing nozzle geometry affects an extremely complex turbulent flow field. Engineers need computational fluid dynamics to predict how a particular tooth length, angle and spacing will alter velocity gradients and pressure fluctuations before expensive full-scale hardware is built.
Wind-tunnel testing
Scale models allow engineers to measure flow and acoustic response under controlled conditions. Microphone arrays can identify where sound originates, while flow diagnostics show how the chevrons alter the exhaust plume.
Full-scale engine testing
Eventually the technology must be tested under representative engine pressure, temperature and thrust. A shape that works on a small model can behave differently once Reynolds number, exhaust temperature and full-scale structural limits are considered.
Flight testing matters too
Aircraft installation changes the acoustic picture. Wing reflections, pylon geometry, forward speed and atmosphere all affect what people hear on the ground. Certification noise measurements therefore use complete aircraft under defined takeoff and approach procedures.
Noise footprint
NASA has cited Boeing data indicating a substantially smaller 787 noise footprint compared with previous-generation aircraft, with chevrons forming part of the overall noise-reduction package.[2] It is important not to attribute the whole improvement to chevrons alone. Fan design, acoustic liners, engine cycle, aircraft aerodynamics and operating procedures all contribute.
What an acoustic liner does
Nacelle interiors contain sound-absorbing treatment formed from perforated facesheets and resonant cavities. These structures are tuned to absorb portions of fan and engine noise before the sound leaves the inlet or exhaust.
Chevrons and acoustic liners solve different problems
Acoustic liners absorb pressure waves. Chevrons change the flow mechanism generating some of those waves. Effective engine noise control uses both source reduction and acoustic treatment rather than relying on one technology.
Why chevrons have an efficiency trade-off
Mixing streams earlier can create additional drag or reduce propulsive efficiency under some operating conditions. A chevron large enough to maximise noise reduction could impose an unacceptable fuel-burn penalty. Engineers therefore optimise the shape for a compromise between acoustics and performance.
This is why the teeth are not enormous
If deeper serrations always produced better aircraft performance, designers could simply make them much larger. In reality the tooth length, penetration, spacing and curvature are tuned to produce enough flow control without excessive aerodynamic loss or structural complexity.
Why different engines can use different chevrons
The GEnx and Trent 1000 have different internal architectures, flow conditions and nacelle arrangements. Acoustic optimisation therefore depends on the specific engine-airframe installation. A chevron geometry cannot automatically be copied from one engine to another and expected to produce the same result.
Temperature changes the problem
Core exhaust is much hotter than bypass air or ambient atmosphere. Density and speed of sound differ between the streams, altering the way turbulence develops. High thrust during takeoff produces a different exhaust condition from low thrust on approach.
Why takeoff noise matters most to communities
Takeoff uses high engine thrust while the aircraft is relatively close to the ground. Community noise standards therefore place significant emphasis on departure measurements. Reducing engine-source noise during high-power operation can shrink the geographical area exposed above defined noise levels.
Approach noise is different
On approach, thrust is lower and aerodynamic noise from landing gear, flaps and airframe cavities becomes relatively more important. Engine chevrons can still contribute acoustically, but they do not solve noise generated by extended landing gear or high-lift devices.
Why the 787 is quieter inside too
Engine-source reduction can lower the sound entering the cabin, but cabin noise is also controlled by fuselage insulation, structural vibration management and interior acoustic treatment. NASA notes that chevron technology can reduce noise both outside the aircraft and within the passenger cabin.[2]
The pylon changes the exhaust flow
The engine pylon attaches the nacelle to the wing and interrupts the otherwise circular flow around the engine. This creates an asymmetric aerodynamic environment, so chevron design has to account for local interaction with pylon and wing rather than assuming the engine operates in free air.
Nacelle aerodynamic drag
At cruise the nacelle contributes skin-friction and pressure drag. Boeing’s overall 787 design therefore integrates inlet, cowl and rear nacelle geometry with the wing to minimise drag while meeting acoustic, structural and maintenance requirements.[3]
The nacelle also contains the thrust reverser
After landing, thrust reverser mechanisms redirect part of the bypass airflow to produce a rearward force on the aircraft. Chevrons have to coexist with the reverser structure and moving components without interfering with deployment or structural load paths.
The fan cowl must open for maintenance
Technicians need access to engine accessories and structure, so nacelle panels are designed to open. Latches, hinges and seals must restore the precise aerodynamic contour when the cowl is closed because high-speed airflow is sensitive to steps and gaps.
Why chevrons must survive vibration
The serrated trailing edge sits in an environment of vibration, pressure fluctuations, weather and temperature change. It must retain shape throughout repeated flights because acoustic performance and structural integrity both depend on correct geometry.
Composite materials
Modern nacelles use composite materials extensively because they provide high stiffness-to-weight ratio and allow complex aerodynamic shapes. Exact material construction varies between engine supplier and component, but composite nacelle panels are common across current large turbofans.
Erosion
Rain, dust and airborne particles can erode leading and trailing surfaces over time. Maintenance inspections therefore check nacelle condition and approved repairs preserve the contour rather than treating the chevron edge as an unimportant cosmetic feature.
Damage to a chevron
A cracked or damaged nacelle trailing edge requires maintenance evaluation under approved engine and airframe documentation. The concern can include structural integrity, loose material, vibration and aerodynamic performance.
Why noise certification is highly standardised
Aircraft cannot be compared reliably by standing near two random departures with a phone sound meter. Certification uses defined microphone positions, atmospheric corrections, aircraft weights and flight profiles so regulators can compare noise performance consistently.
ICAO noise standards
International civil-aviation noise certification is structured through ICAO Annex 16, with national authorities implementing the applicable requirements. New aircraft generations must comply with increasingly stringent limits, encouraging manufacturers to reduce source noise as well as fuel consumption.
Noise and fuel efficiency are often connected
High-bypass engines can improve both fuel efficiency and noise because they move more air at lower jet velocity. But some acoustic devices can trade slightly against aerodynamic efficiency, which is why aircraft design must optimise both rather than assume quieter automatically means more efficient.
Why later engines may use fewer visible chevrons
Advances in fan design, nozzle optimisation and variable geometry can reduce noise without relying on the same external serrations. The best solution evolves with engine architecture. Chevrons are one technology in a continuing progression rather than the final form of quiet propulsion.
Research continued beyond the 787
NASA’s work on quiet propulsion continues through computational acoustics, advanced nozzles, airframe integration and new propulsion concepts. The 787 chevron is therefore best viewed as a successful technology transition from research into service, not an isolated invention.[1]
Why the shape became a Dreamliner signature
Many noise-control technologies are hidden inside an engine. Chevrons are visible from the terminal, making them one of the easiest ways to recognise the 787’s propulsion installation. Their visibility has made a sophisticated fluid-dynamics concept unusually familiar to passengers.
The engineering lesson
The sawtooth edge does not work because it physically blocks sound. It works because sound begins as unsteady aerodynamic motion. By controlling how exhaust streams mix, engineers can change the turbulent structures that generate acoustic energy. This is a classic example of solving an environmental problem at its aerodynamic source.
Conclusion
The serrated chevrons on Boeing 787 nacelles are the visible outcome of years of NASA-industry noise research. They generate controlled vortices that encourage high-speed and low-speed exhaust streams to mix earlier and in smaller structures, reducing parts of the jet-noise spectrum. The geometry must be carefully limited because aggressive mixing can cost propulsive efficiency. Together with high-bypass engines, acoustic liners and the wider Dreamliner aerodynamic package, chevrons help make the 787 quieter without changing the basic physics of how a turbofan creates thrust.
Sources / Technical References
- [1] NASA, “NASA Helps Create a More Silent Night” — https://www.nasa.gov/aeronautics/nasa-helps-create-a-more-silent-night/
- [2] NASA, “NASA Contribution: Chevrons” — https://www.nasa.gov/image-article/nasa-contribution-chevrons/
- [3] Boeing, 787 Dreamliner By Design — https://www.boeing.com/commercial/787/by-design
- [4] Boeing, 787 Dreamliner official programme information — https://www.boeing.com/commercial/787
- [5] GE Aerospace, GEnx engine information — https://www.geaerospace.com/commercial/aircraft-engines/genx
- [6] Rolls-Royce, Trent 1000 engine information — https://www.rolls-royce.com/products-and-services/civil-aerospace/widebody/trent-1000.aspx
- [7] ICAO, Annex 16 — Environmental Protection — https://www.icao.int/environmental-protection/Pages/noise.aspx
Disclaimer: Cockpit King provides general aviation education and reference information. Nacelle construction, acoustic design, inspection limits and engine maintenance requirements vary by configuration and must always be verified against current approved Boeing, engine-manufacturer, operator and regulatory documentation. This article is not maintenance instruction.