Engines are complex apparatuses equipped with numerous components and assemblies that allow them to work optimally. One feature in particular is called the air entrance, and it is designed to conduct incoming air to the compressor with a minimum energy loss, ensuring that air flow is free of turbulence. To achieve such ends, proper inlet design contributes significantly. It improves aircraft performance by increasing the ratio of compressor discharge pressure to duct inlet pressure.
Also called the compressor pressure ratio, this ratio is the outlet pressure divided by the inlet pressure. The amount of air passing through the engine is based on three factors. First, the compressor speed, which is measured in RPMs, next, the forward speed, and lastly, the density of the ambient air.
The turbine inlet duct type is typically determined by the type of gas turbine engine. For example, a high-bypass turbofan engine inlet is different from a turboprop and turboshaft inlet. Generally, large gas turbine-powered aircraft are furnished with turbofan engines and the inlet is bolted to the front of the engine. Meanwhile, the engines are mounted on the wings, or nacelles, on the aft fuselage, and a select few will have them on the vertical fin.
On most modern turbofan engines, the fan is the first part that receives incoming air, thus icing protection must be incorporated. Deicing systems prevent chunks of ice from accumulating on the leading edge of the inlet and damaging the fan. At the same time, warm air is bled from the engine’s compressor and ducted via the inlet to prevent ice from forming. The inlet guide vanes can straighten airflow and contain some sound-reducing materials that absorb fan noise.
Turboprops and turboshafts can utilize an inlet screen to filter out ice and other debris from making their way into the engine, or a deflector vane and a heated inlet lip can also be used. Military aircraft travel at speeds above Mach 1, so the airflow should be maintained below Mach 1 since supersonic airflow in the engine can destroy it. By using convergent and divergent shaped ducts, the airflow can be controlled. Additionally, supersonic inlets can be used to slow the incoming engine air below Mach 1 before it enters the engine. In the next section, we will cover the most common inlet mounting styles and inlet types.
Many large commercial and military aircraft utilize wing-mounted engines. For the DC-10 and L-1011, a combination of wing-mounted and vertical stabilizer mounted engines are used. In these instances, the air inlet duct is positioned in front of the compressor and is mounted to the engine. Integral mounting of the inlet with an engine reduces air inlet length, which in turn increases operational efficiency. Some commercial aircraft and a majority of small business jets may also be fitted with aft fuselage mounted engines wherein the duct is short and is mounted directly to the engine.
Aircraft with this mounting style feature air inlet ducts in the wing’s leading edge. Typically, wing-mounted inlet ducts are located near the wing root area.
Engines mounted inside a fuselage will generally use air inlet ducts located near the front of the fuselage. While using this type of air inlet duct permits the aircraft manufacturer to construct a more aerodynamically efficient aircraft, the increased length of the inlet duct introduces some inefficiency. For single- and twin-engine aircraft, the air inlet ducts are mounted on the sides of the fuselage. This configuration allows the duct length to be shortened without adding excessive drag to the aircraft. Unfortunately, this arrangement causes some flight maneuvers to become imbalanced.
Consisting of a fixed geometric duct with a diameter that progressively increases from front to back, this shape works similar to a venturi in that as the intake air spreads out, the velocity of the air decreases and the pressure increases. This additional pressure contributes to engine efficiency once the aircraft has reached cruising speed. At this speed, the compressor reaches its optimal aerodynamic efficiency and produces the most compression for best fuel economy.
On supersonic aircraft, air inlet ducts have a fixed or variable geometric design with a diameter that progressively decreases and increases from front to back. This design is utilized to slow the incoming airflow to a subsonic speed before it reaches the compressor. Furthermore, many supersonic inlet ducts take advantage of a movable plug that changes the duct geometry as per flight conditions, allowing it to accommodate a wide range of speeds.
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