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Computer drawing of gas turbine schematic showing the equations
 for pressure ratio, temperature ratio, and spillage for an inlet.

Most modern passenger and military aircraft are powered by gas turbine engines, which are also called jet engines. There are several different types of jet engines, but all jet engines have some partsin common. All jet engines have an inlet to bring free stream air into the engine. The inlet sits upstream of the compressor and, while the inlet does no work on the flow, there are some important design features of the inlet.

Because the inlet does no thermodynamic work, the total temperature through the inlet is constant. Referring to our station numbering, free stream conditions are noted by a "0" subscript, the entrance to the inlet is station "1" and the exit of the inlet and entrance to the compressor is station "2". The inlet total temperature Tt ratio is Tt2 divided by Tt0 and is equal to 1.0

Tt2 / Tt0 = 1

The total pressure pt through the inlet changes, however, because of several flow effects. Aerodynamicists characterize the inlet's pressure performance by the inlet total pressure recovery, which measures the amount of the free stream flow conditions that are "recovered". The pressure recovery pt2 / pt0 depends on a wide variety offactors, including the shape of the inlet, the speed of the aircraft, the airflow demands of the engine, and aircraft maneuvers. On the slide we show some simple equations for the pressure recovery that are used as standards. Recovery losses associated with the boundary layers on the inlet surface or flow separations in the duct are included in the inlet efficiency factor ni:

ni = pt2 / pt1

For subsonic flight speeds, these losses are the only losses. For Mach number M less than 1, the Military Specifications (Mil. Spec.) value of recovery is the inlet efficiency:

Mil. Spec., M < 1 : pt2 / pt0 = ni * 1

At supersonic flight speeds, there are additional losses created by the shock waves necessary to reduce the flow speed to subsonic conditions for the compressor.

Mil. Spec., M > 1 : pt2 / pt0 = ni * ( 1 - .075 * [M - 1] ^1.35)

The Mil. Spec. loss is a good first estimate of inlet recovery. Actual inlet performance may be greater, but is usually less than Mil. Spec. The magnitude of the recovery loss depends on the specific design of the inlet and is normally determined by wind tunnel testing.

There is an additional propulsion performance penalty charged against the inlet called spillage drag. Spillage drag, as the name implies, occurs when an inlet "spills" air around the outside instead of conducting the air to the compressor face. The amount of air that goes through the inlet is set by the engine and changes with altitude and throttle setting. The inlet is usually sized to pass the maximum airflow that the engine can ever demand and, for all other conditions, the inlet spills the difference between the actual engine airflow and the maximum air demanded. As the air spills over the external cowl lip, the air accelerates and the pressure decreases. This produces a lip suction effect that partially cancels out the drag due to spilling. Inlet aerodynamicists account for this effect with a correction factor K that multiplies the theoretical spillage drag. Typical values of K range from .4 to .7. But for a given inlet the value is determined experimentally. The form of the theoretical spillage drag D spill is very similar to the thrust equation, with a mass flow m dot times velocity term V and a pressure p times area A term:

D spill = K * (mdot i * [V1 - V0] + A1 * [p1 - p0])

As the air is brought from free stream to the compressor face, the flow may be distorted by the inlet. At the compressor face, one portion of the flow may have a higher velocity or higher pressure than another portion. The flow may be swirling, or some section of the boundary layer may be thicker than another section because of the inlet shape. The rotor blades of the compressor move in circles around the central shaft. As the blades encounter distorted inlet flow, the flow conditions around the blade change very quickly. The changing flow conditions can cause flow separation in the compressor, a compressor stall, and can cause structural problems for the compressor blades. A good inlet must produce high pressure recovery, low spillage drag, and low distortion.

You can investigate the effects of inlet performance on total engine performnace by using the EngineSim computer program. Click on "Inlet" on the graphics panel and vary te amount of inlet recovery. Because an inlet is essentially a hollow tube, the weight considerations of the inlet are small compared to the compressor or turbine. For ramjet and scramjet inlets, the materials used in the inlet must withstand high temperatures.

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Editor: Nancy Hall
NASA Official: Nancy Hall
Last Updated: May 13 2021

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