The nacelle is an aerodynamic enclosure that surrounds and protects the aircraft's engine, yet it remains a housing separate from the fuselage. Streamlined to minimize drag, this smooth structure is mounted on slender pylons and encloses the engine and its components. Because nacelle shape affects the power produced, designers treat its geometry as a decisive parameter.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
What is an aircraft engine nacelle?
An aircraft engine nacelle refers to an aerodynamic enclosure or housing that surrounds and protects a part of an aircraft, typically an engine, propeller, or turbofan.
The smooth structure that surrounds the jet engine is known as the nacelle. It is a streamlined enclosure and a housing that is separate from the fuselage. A nacelle functions as a container for aircraft parts.
On most single-engine aircraft the engine and nacelle are located at the forward end of the fuselage, while on multiengine aircraft engine nacelles are attached to the fuselage at the empennage or built into the wings. Engines are mounted in individual nacelles, or, as in the B-52 Stratofortress, two engines are mounted in a single nacelle. Engine pylons, which attach an aircraft engine to the wing or fuselage, must support engines that can weigh over 6-8 tons, withstand immense forces during flight including engine thrust, turbulence, and extreme weather conditions, and still keep vibration and movement within safe limits. Nacelles help maintain aerodynamic balance.
The nacelle shields the engine from foreign object ingestion, debris, lightning, and bird strike, while the outer shell protects the engine from dust, rain, and harsh environmental conditions, assists with heat control to guarantee safety and optimal performance, and minimizes noise during operation. A cowl or cowling is any part of the engine nacelle that opens or can be removed for inspection. The removable cover of the engine allows access to the engine and components inside. The primary design issue is streamlining, and the nacelle provides an aerodynamic shell for minimum drag. A firewall is incorporated to isolate the engine compartment from the rest of the aircraft, and noise-reducing features comply with environmental regulations.
What is a nacelle interface unit?
A nacelle interface unit ensures the transfer of power and fluids between the engine and the aircraft via tubing and connections for electrical, hydraulic, pneumatic, and fuel flow functions that are performed using the Engine Build Units (EBU) hardware. It contributes to the aircraft's safe braking with the deployment of the nacelle's thrust reverser.
A nacelle interface unit is the Remote Interface Unit (RIU) located inside the engine nacelle. The RIU digitizes voltage, current, frequency and temperature signals coming from nacelle sensors and communicates the digitized data to aircraft systems over the MIL-STD-1553 bus, giving the nacelle a flexible, network-linked sensor interface.
What are the main components of an aircraft nacelle?
The main components of an aircraft nacelle are listed below.
- Pylon
- Inlet
- Fan Cowls
- Thrust Reversers
- Exhaust Cone
- Exhaust Nozzle
- Ventilation System
- Anti-Ice System
- Firewall
A nacelle is built around a framework of structural members - longerons, stringers, rings, formers and bulkheads - that transfer flight and thrust loads to the engine mount. A firewall isolates the engine compartment from the rest of the aircraft, while the exterior skin and cowling provide aerodynamic smoothness and protect internal parts. The inlet is an aerodynamic and acoustic unit and its lip is often an aluminum inlet lip for reduced aerodynamic drag. It contains sound reducing materials that absorb fan noise. Large two-piece fan-cowl doors facilitate access to the engine for inspection and maintenance. The thrust reverser redirects engine exhaust to help decelerate the aircraft during landing, providing an important safety feature on wet runways and reducing wear and tear on airplane brakes. The exhaust nozzle forms the final part of the engine exhaust system. Upper and lower ventilation air intakes provide airflow to cool engine components, while the 360-degree single-piece extended inner barrel integrates advanced acoustic protection for reduced engine noise.
What materials are used to construct an aircraft nacelle?
The materials used to construct an aircraft nacelle in light aircraft include chrome/molybdenum 4130 or 4140 steel tubing, while in larger aircraft nacelles are forged from chrome/nickel/molybdenum assemblies. By the 1970s designers had turned to composites for the weight savings necessary for the large nacelles required by the new generation of high-power turbofans. Today's nacelles are made primarily from a variety of composite materials rather than metals. Composite materials made from carbon and glass fibers, aramid papers and high-temperature resins have changed modern nacelle construction, providing substantial weight and cost savings over conventional metal materials.
Most of the structures for a test nacelle are hand-laid on open molds with carbon fiber prepreg and honeycomb core, then cured under vacuum in an oven. Manufacturers choose to use Kevlar honeycomb core structures because they are far stronger and lighter than earlier designs with an aluminum core. Primary bellmouth, cowl, boattail and substitute thrust reversers are built from a combination of Toray 2510 carbon fiber 12K plain weave prepreg plies and Flexcore honeycomb core.
The fire bulkhead is usually a stainless steel or titanium bulkhead to prevent fire from spreading throughout the airframe. Nanoparticulate silica attached to the carbon fibers turns into glass at fire temperatures, increasing burn-through time. The market for aircraft nacelles is classified into composites, stainless steel, aluminium alloys, titanium alloys and nickel-chromium but materials vary depending on temperature requirements needed.
What is the effect of nacelle shape on drag?
Nacelle shape systematically affects every major drag. Wave drag is a strong function of the ratio of maximum-to-capture cross-sectional area, of boattail area and of the axial position of maximum cross-sectional area. Nacelles designed such that the maximum cross-sectional area occurs at or near the nozzle exit yielded the lowest wave drag. Nacelle shape parameters were systematically varied and consequences of these variations on friction drag were determined. Maintaining laminar flow over as much of the nacelle as possible results in lower skin-friction drag, while favorable interference effects produced by carefully designed nacelle shapes nearly offset the friction drag of the nacelle itself.
Induced drag rises when a nacelle is added to a wing because the installation causes a loss of lift. Cooling drag is dependent on the external nacelle shape, yet adjusting the shape and exiting cooling flow over the afterbody reduces the nacelle drag. If the nacelle terminates at or before the trailing edge, separation is likely with a resulting increase in drag. Rounded front corners or suitable changes to the cowling contours reduce the nacelle drag and result in a corresponding increase in aerodynamic cleanness. An increase in nacelle diameter will cause greater nacelle drag and interference with the wing, while the presence of a screening surface increases frontal drag of a nacelle model by approximately 14% and flow detachment increases frontal drag by approximately 2.5 times. Because nacelles contribute over 30% of the total airplane drag, optimal nacelle shape defines the geometry with the lowest drag and contributes to the best aircraft performance.
What is a nacelle chevron?
A nacelle chevron is a sawtooth edge found on jet engine nacelles and/or nozzles, which Boeing introduced as a design change. Chevrons reduce engine noise outside and inside the cabin, resulting in a 69% smaller noise footprint on the 787 and a 29% smaller footprint on the 747-8.A nacelle chevron is a V-type serrated edge formed on the trailing edge of the engine nacelle and on the exhaust nozzle. These saw-tooth patterns were developed jointly by Boeing, General Electric and NASA, and they first appeared on the CF34 8C5 engine of a Bombardier CRJ900 in 2003.
Chevrons act as a fan-air / exhaust-gas mixer: they smooth the mixing of hot core air with the cooler bypass air that has passed through the fan. By controlling this shear layer they weaken the shock waves in the exhaust plume, cutting jet-blast noise by roughly 2-3 dB and shrinking the 787's noise footprint by 69 % and the 747-8's by 29 %.
The gain comes at negligible cost. Flight tests show thrust drops only 0.25 %, while mixing increases propulsive efficiency and lowers fuel use. Because they reduce noise production chevrons are now a visible signature on the nacelles of Boeing 787, 737 MAX and 747-8 aircraft.
