Nacelles are aerodynamic housings distinct from the fuselage that surround exterior components on an aircraft. Nacelles most commonly protect instruments and equipment located along an aircraft's wingspan, such as engines, fuel tanks, or weapons. In aviation history, nacelles have also been used to house cockpits and other equipment in unconventional plane designs. Nacelles serve a variety of purposes including drag reduction and directing airflow for the purposes of engine cooling and use in the combustion reactions inside engines.
Nacelles are also used on horizontal axis wind turbines (HAWT). These implementations protect sensitive turbine components, such as crankshafts and gearboxes, from the environment.
Aircraft nacelles commonly include the following components:
Engine cowlings such as inlet and fan cowls are designed to protect aircraft engines and reduce parasitic drag, including form drag and skin friction drag. Form drag can be reduced by making the cross-sectional size of the nacelle presented to the air flow as small as possible. Skin friction drag is reduced by promoting laminar flow with a smooth evenly transitioned cone shape to prevent flow separation.
Thrust reversers are mechanisms that serve to redirect some of the air flowing out of the engine to the sides and partially forward, resulting in a net flow that creates significant drag, slowing the aircraft’s forward motion, and aiding in braking procedures.
The exhaust system including the exhaust cone and exhaust nozzle typically are covered with a nacelle.
Wind turbine nacelles must accommodate a variety of internal components, such as the gearbox, low- and high-speed shafts, generator, controller, brake, and yaw drive.
In designing nacelles, engineers seek to minimize the overall size while providing sufficient space for the necessary equipment in addition to ventilation for cooling purposes. A particular design challenge arises in developing the narrow pylon connecting an engine nacelle to the main aircraft. A large quantity of connecting lines need to fit in this space, including instrumentation leads, power cables, pneumatic lines, fuel lines, and engine controls.
Acoustic performance is also considered in nacelle design. Acoustic liners on the internal surfaces of an engine nacelle can reduce the noise generated by the engine. Consisting of a sheet perforated with small holes over a honeycomb structure, these liners exploit the Helmholtz resonance principle to dissipate acoustic energy.
Design parameters of particular importance when choosing materials for engine nacelles include weight and temperature resistance. Composites sandwich structures, such as epoxy systems and bis-maleimide (BMI) resins, are utilized extensively to minimize weight. Titanium and superalloys like Inconel can endure the high temperatures close to engine components.