What is the Purpose of Air Inlets?

The air inlet of an aircraft, also known as an intake, is an opening or structure on the body of a plane or jet that allows for air to enter into systems. As jet engines and supersonic speed aircraft were realized and improved upon, such air entrances became a required design to allow for the correct amount of air to flow into the engine. In this blog, we will discuss some of the various inlets that may be present on an aircraft, as well as how they benefit flight operations.

In general, the main types of inlets are nose inlets, wing inlets, annular inlets, scoop inlets, flush inlets, and bellmouth inlets. Nose inlets are those that are placed on the nose of the aircraft fuselage, nacelle, or powerplant pod. Wing inlets are placed alongside the leading edge of a wing, and they are most often found at the wing root on single-engine configurations. Annular inlets are those that encircle the fuselage of the aircraft entirely or partly, or are at the power plant pod or nacelle. Scoop inlet types are installed near the surface of the fuselage or nacelle and flush inlets may be recessed within the fuselage, nacelle, or power plant. Lastly, bellmouth inlets are funnels that may be mounted to the front of the aircraft engine.

No matter the type of air inlet, most follow two designs which are single and divided entrances. When an aircraft features an axial-flow engine, the single entrance design is most beneficial, allowing the engine to achieve maximum ram air pressure. To achieve such pressure, inlets are placed in an area of the wing or an installation where the entrance is short, straight, and unobstructed. Divided entrances may be used to diffuse air, allowing low velocity air to enter the plenum chamber to efficiently operate a double-entry air compressor. Aircraft that cannot achieve straight ducts due to their design also benefit from such air inlet designs.

To prevent ice buildup within the air inlets, the duct inlet will feature anti-icing systems. Within turbine engine aircraft, air inlets may prevent ice formation through the use of hot bleed air and/or hot engine oil. Guide vanes are also beneficial for anti-icing systems, directing flow towards the air compressor at an optimal angle. As many aircraft vanes are hollow, hot air and oil may flow to prevent ice buildup.

If an aircraft is operating at subsonic speeds, the best aircraft inlet is a pod installation, such as is seen on many modern jetliners. This is because such inlets allow for sufficient ram air pressure while minimizing the effect of air on the aerodynamics of the jet. When the aircraft is smaller in size, multiple inlets may be placed around the fuselage to ensure that airflow is not decreased. At higher supersonic speeds, inlets with variable geometry are most beneficial to ensure that the flow is supplied at subsonic speeds.