The flight inlet duct, which serves as the entry point for atmospheric air into the aircraft, is generally considered part of the airframe rather than the engine itself. However, it is commonly designated as engine station number one. Understanding the purpose and operation of the inlet duct is essential when studying gas turbine engine design and performance, as it plays a critical role in delivering smooth, efficient airflow to the engine. Figure 1 illustrates several examples of aircraft inlet ducts positioned in different locations.
To maintain smooth airflow with minimal turbulence, the inlet duct should be kept in near-new condition. When repairs are required, flush-mounted patches must be expertly installed to avoid creating drag. Additionally, using an inlet cover helps maintain cleanliness while preventing corrosion and abrasion.
Air enters the aerodynamically contoured inlet at ambient pressure and begins to diffuse, arriving at the compressor at a slightly higher static pressure. Usually, most of the diffusion occurs in the front portion of the duct, while the airflow past the engine inlet fairing (also called the inlet center body) remains relatively constant in pressure as it reaches the compressor. This design ensures the engine receives smooth, low-turbulence airflow at a uniform pressure, which is critical for optimal compressor operation.
As the aircraft accelerates to cruising speed, the increased inlet pressure boosts mass airflow into the engine. At cruise, the compressor operates at its aerodynamic design point, providing maximum compression efficiency and best fuel economy. At this stage, the flight inlet, compressor, combustor, turbine, and tailpipe function together as a coordinated system. Any mismatch—caused by damage, contamination, or unusual ambient conditions—can negatively affect engine performance.
The turbofan inlet is similar to the turbojet inlet, but only a portion of the air entering the fan passes into the engine core; the remainder is accelerated through the fan discharge duct.
Figure 3 illustrates two common turbofan inlet designs. The full duct design, shown in Figure 3A, is used in low- and medium-bypass engines and features a longer duct that reduces surface drag on the fan discharge air, thereby enhancing thrust. In contrast, the short duct design, shown in Figure 3B, is typical of high-bypass turbofans. Older high-bypass engines could not use long ducts because of the excessive weight associated with wide-diameter ducting; however, advances in lightweight materials and modern design techniques now allow newer engines to incorporate longer ducts, benefiting from drag reduction without a significant weight penalty.
While stationary, an aircraft inlet usually does not achieve 100% pressure recovery, meaning the air pressure at the compressor inlet is slightly below ambient pressure. For example, if the ambient pressure is 14.7 psi (absolute), the compressor inlet pressure will be slightly below this value. As the aircraft accelerates on the runway toward takeoff, ram compression gradually increases the inlet pressure until it approaches ambient levels. In a typical subsonic inlet, this condition is reached at approximately Mach 0.1 to Mach 0.2.
Figure 4 illustrates this effect, showing gauge readings rising from negative to positive as the aircraft transitions from a static ground condition to flight. As speed increases further in flight, the inlet generates additional ram compression, allowing the engine to take advantage of the higher inlet pressure. This leads to an increase in compressor pressure ratio, producing greater thrust with improved fuel efficiency.
The ram compression (or ram pressure ratio) at any flight Mach number can be calculated using the following formula:
Example:
Consider a business jet flying at Mach 0.8 at an altitude of 31,000 feet. Using the formula above, the pressure ratio between the total (engine inlet) pressure and the ambient pressure can be calculated as:
This means that at Mach 0.8, the total pressure at the inlet is approximately 1.524 times the ambient pressure—a 52.4% increase due to ram compression alone. At much higher speeds, the effect becomes dramatic; for instance, the Concorde at a cruise speed of Mach 2.2 achieved a ram compression ratio of about 10.7:1.
This formula represents Pt2/Pam, which is the total pressure at the engine face divided by ambient pressure, giving the inlet compression ratio. It is applicable because Pt2 is essentially equal to Pt1, the total pressure at the lip of the flight inlet, expressed as Pt in the formula.
As air moves through the inlet (assuming 100% efficiency), the total pressure remains constant, while static and ram pressure components adjust: static pressure increases as air slows down in the diffuser, and ram pressure decreases correspondingly.
If the air density (p) in pounds per cubic foot and velocity (v) in feet per second are known, the total pressure in the inlet can also be calculated as:
This approach allows engineers to quantify the increase in inlet pressure due to aircraft forward speed, which directly contributes to engine thrust and efficiency.
Example:
Consider an aircraft flying at an altitude of 25,000 feet, cruising at 550 miles per hour (≈806 ft/sec). At this altitude, the static pressure in the flight inlet is 5.454 psi, and the air density, derived from a standard atmosphere chart, is 0.034267 lb/ft³.
We’ll calculate:
Solution 1: Total Pressure (Pt)
Substitute the given values:
So, the total pressure (Pt) in the inlet is 7.85 psi.
Solution 2: Inlet Pressure Ratio (Cr)
Hence, the inlet pressure ratio (Cr) is approximately 1.44 : 1.
To adjust the inlet’s shape, a movable restrictor is often used to form a variable convergent-divergent configuration. This design is necessary to reduce supersonic airflow to subsonic speeds. At subsonic flow rates, air behaves similarly to an incompressible liquid, but at supersonic speeds, it becomes highly compressible, creating shock waves.
Figure 5 illustrates a fixed-geometry C-D duct, where supersonic airflow is slowed by air compression and shock formation at the throat. After decelerating to Mach 1, the flow enters the subsonic diffuser section, where velocity decreases further and static pressure rises before entering the engine compressor. Some military aircraft capable of speeds up to Mach 2 use fixed inlets; however, fixed geometry is not always operationally practical due to stagnation pressure limitations at supersonic speeds.
An inlet shock wave is similar to those formed on aircraft wings or other airfoils. It occurs when sound wave energy accumulates because the waves are held stationary by the oncoming airflow. The beneficial effect of a shock wave is that it slows the air passing through the high-pressure region, aiding in supersonic-to-subsonic deceleration (Figure 6).
Figure 7 shows a supersonic diffuser inlet, which combines shock wave formation with a variable convergent-divergent shape to accommodate flight conditions from takeoff to cruise. Behind the final shock wave, airflow typically slows to approximately Mach 0.8, and further reduces to about Mach 0.5 through radial diffusion. At higher speeds, such as Mach 2 to Mach 2.5, multiple shock waves form within the inlet. Early shocks at the front are oblique, slowing the airflow while remaining supersonic. At the throat (the narrowest point where the duct transitions from convergent to divergent), a final normal shock reduces airflow to subsonic speeds, after which the diverging section behaves as a subsonic diffuser.
The movable wedge design in Figure 7 illustrates different wedge positions and their roles in convergence, divergence, and shock wave formation. It also includes a spill valve to regulate airflow at varying speeds. Many high-performance aircraft encounter variations in engine mass flow under different operating conditions and therefore require an onboard computer to monitor inlet conditions and control the positions of the inlet’s movable components.
During engine calibration on ground test stands, bellmouth inlets are often used, sometimes fitted with an anti-ingestion screen. Because duct losses are minimal, they are typically considered negligible. Engine performance data, such as rated thrust and trimming, are measured using a bellmouth compressor inlet (Figure 8).
Figure 9 demonstrates the effect of aerodynamic efficiency and duct loss. A rounded leading edge (Figure 9A) allows the airflow to fully utilize the inlet’s cross-section, while a sharp-edged orifice (Figure 9B) significantly reduces the effective diameter, increasing losses.
Some sand and ice separators include a movable vane that extends into the airstream. This causes the engine inlet air to make a sudden turn, while sand or ice particles, due to their higher momentum, continue in a straight path and are removed from the airflow. The movable vane is pilot-operated via a control handle in the cockpit (Figure 10C).
Earlier aircraft addressed this issue with a vortex dissipator, also known as a blow-away jet. In this system, a small jet of compressor discharge air is directed at the ground beneath the inlet from a nozzle located on the lower engine cowl. The system is usually activated by a landing gear switch, which opens a valve connecting the compressor bleed port to the dissipator nozzle whenever the engine is operating and weight is on the main landing gear (Figure 12).
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| Figure 1. Common engine inlet location |
Principles of Operation
The flight inlet duct in a turbine engine is designed to provide a uniform and steady supply of air to the compressor, ensuring stall-free operation and optimal performance. It must also produce minimal aerodynamic drag. Even minor disruptions in airflow can lead to a significant loss of efficiency and various engine performance issues.To maintain smooth airflow with minimal turbulence, the inlet duct should be kept in near-new condition. When repairs are required, flush-mounted patches must be expertly installed to avoid creating drag. Additionally, using an inlet cover helps maintain cleanliness while preventing corrosion and abrasion.
Subsonic Flight Inlet Ducts
Subsonic flight inlet ducts, typically found on business jets and commercial airliners, have a fixed geometry and a divergent shape. A diverging duct gradually increases in diameter from front to back, as illustrated in Figure 2. This type of duct is often called an inlet diffuser because it gradually increases the static pressure of the incoming air. |
| Figure 2 |
Air enters the aerodynamically contoured inlet at ambient pressure and begins to diffuse, arriving at the compressor at a slightly higher static pressure. Usually, most of the diffusion occurs in the front portion of the duct, while the airflow past the engine inlet fairing (also called the inlet center body) remains relatively constant in pressure as it reaches the compressor. This design ensures the engine receives smooth, low-turbulence airflow at a uniform pressure, which is critical for optimal compressor operation.
As the aircraft accelerates to cruising speed, the increased inlet pressure boosts mass airflow into the engine. At cruise, the compressor operates at its aerodynamic design point, providing maximum compression efficiency and best fuel economy. At this stage, the flight inlet, compressor, combustor, turbine, and tailpipe function together as a coordinated system. Any mismatch—caused by damage, contamination, or unusual ambient conditions—can negatively affect engine performance.
The turbofan inlet is similar to the turbojet inlet, but only a portion of the air entering the fan passes into the engine core; the remainder is accelerated through the fan discharge duct.
Figure 3 illustrates two common turbofan inlet designs. The full duct design, shown in Figure 3A, is used in low- and medium-bypass engines and features a longer duct that reduces surface drag on the fan discharge air, thereby enhancing thrust. In contrast, the short duct design, shown in Figure 3B, is typical of high-bypass turbofans. Older high-bypass engines could not use long ducts because of the excessive weight associated with wide-diameter ducting; however, advances in lightweight materials and modern design techniques now allow newer engines to incorporate longer ducts, benefiting from drag reduction without a significant weight penalty.
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| Figure 3 |
Ram Pressure Recovery
When a gas turbine engine operates while stationary on the ground, the inlet typically experiences a negative pressure due to the high-velocity airflow through the duct. As the aircraft begins to move forward, a phenomenon called ram pressure recovery occurs, in which the pressure inside the inlet rises toward ambient pressure. This increase, known as ram compression, results from the combined effects of air velocity and diffusion—as air spreads radially, its axial speed decreases, causing static pressure to rise.While stationary, an aircraft inlet usually does not achieve 100% pressure recovery, meaning the air pressure at the compressor inlet is slightly below ambient pressure. For example, if the ambient pressure is 14.7 psi (absolute), the compressor inlet pressure will be slightly below this value. As the aircraft accelerates on the runway toward takeoff, ram compression gradually increases the inlet pressure until it approaches ambient levels. In a typical subsonic inlet, this condition is reached at approximately Mach 0.1 to Mach 0.2.
Figure 4 illustrates this effect, showing gauge readings rising from negative to positive as the aircraft transitions from a static ground condition to flight. As speed increases further in flight, the inlet generates additional ram compression, allowing the engine to take advantage of the higher inlet pressure. This leads to an increase in compressor pressure ratio, producing greater thrust with improved fuel efficiency.
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| Figure 4. Ram pressure recovery |
The ram compression (or ram pressure ratio) at any flight Mach number can be calculated using the following formula:
Example:
Consider a business jet flying at Mach 0.8 at an altitude of 31,000 feet. Using the formula above, the pressure ratio between the total (engine inlet) pressure and the ambient pressure can be calculated as:
This means that at Mach 0.8, the total pressure at the inlet is approximately 1.524 times the ambient pressure—a 52.4% increase due to ram compression alone. At much higher speeds, the effect becomes dramatic; for instance, the Concorde at a cruise speed of Mach 2.2 achieved a ram compression ratio of about 10.7:1.
This formula represents Pt2/Pam, which is the total pressure at the engine face divided by ambient pressure, giving the inlet compression ratio. It is applicable because Pt2 is essentially equal to Pt1, the total pressure at the lip of the flight inlet, expressed as Pt in the formula.
As air moves through the inlet (assuming 100% efficiency), the total pressure remains constant, while static and ram pressure components adjust: static pressure increases as air slows down in the diffuser, and ram pressure decreases correspondingly.
If the air density (p) in pounds per cubic foot and velocity (v) in feet per second are known, the total pressure in the inlet can also be calculated as:
This approach allows engineers to quantify the increase in inlet pressure due to aircraft forward speed, which directly contributes to engine thrust and efficiency.
Example:
Consider an aircraft flying at an altitude of 25,000 feet, cruising at 550 miles per hour (≈806 ft/sec). At this altitude, the static pressure in the flight inlet is 5.454 psi, and the air density, derived from a standard atmosphere chart, is 0.034267 lb/ft³.
We’ll calculate:
- The total pressure (Pt) in the inlet, and
- The inlet pressure ratio (Cr).
Solution 1: Total Pressure (Pt)
Substitute the given values:
So, the total pressure (Pt) in the inlet is 7.85 psi.
Solution 2: Inlet Pressure Ratio (Cr)
Hence, the inlet pressure ratio (Cr) is approximately 1.44 : 1.
Supersonic Inlet Ducts
All supersonic aircraft require a convergent-divergent (C-D) inlet duct, which may be fixed or variable in geometry. For example, supersonic transports are designed with inlets that slow airflow to subsonic speeds before it reaches the engine, regardless of aircraft speed. Subsonic flow into the compressor is essential to prevent shock wave accumulation on the rotating airfoils, which can disrupt the compression process.To adjust the inlet’s shape, a movable restrictor is often used to form a variable convergent-divergent configuration. This design is necessary to reduce supersonic airflow to subsonic speeds. At subsonic flow rates, air behaves similarly to an incompressible liquid, but at supersonic speeds, it becomes highly compressible, creating shock waves.
Figure 5 illustrates a fixed-geometry C-D duct, where supersonic airflow is slowed by air compression and shock formation at the throat. After decelerating to Mach 1, the flow enters the subsonic diffuser section, where velocity decreases further and static pressure rises before entering the engine compressor. Some military aircraft capable of speeds up to Mach 2 use fixed inlets; however, fixed geometry is not always operationally practical due to stagnation pressure limitations at supersonic speeds.
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| Figure 5. Supersonic convergent-divergent inlet |
An inlet shock wave is similar to those formed on aircraft wings or other airfoils. It occurs when sound wave energy accumulates because the waves are held stationary by the oncoming airflow. The beneficial effect of a shock wave is that it slows the air passing through the high-pressure region, aiding in supersonic-to-subsonic deceleration (Figure 6).
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| Figure 6. Shock wave formation |
Figure 7 shows a supersonic diffuser inlet, which combines shock wave formation with a variable convergent-divergent shape to accommodate flight conditions from takeoff to cruise. Behind the final shock wave, airflow typically slows to approximately Mach 0.8, and further reduces to about Mach 0.5 through radial diffusion. At higher speeds, such as Mach 2 to Mach 2.5, multiple shock waves form within the inlet. Early shocks at the front are oblique, slowing the airflow while remaining supersonic. At the throat (the narrowest point where the duct transitions from convergent to divergent), a final normal shock reduces airflow to subsonic speeds, after which the diverging section behaves as a subsonic diffuser.
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| Figure 7. Supersonic airplane movable wedge inlet |
The movable wedge design in Figure 7 illustrates different wedge positions and their roles in convergence, divergence, and shock wave formation. It also includes a spill valve to regulate airflow at varying speeds. Many high-performance aircraft encounter variations in engine mass flow under different operating conditions and therefore require an onboard computer to monitor inlet conditions and control the positions of the inlet’s movable components.
Bellmouth Compressor Inlets
Bellmouth inlets, commonly used on helicopters, feature a converging shape that produces very thin boundary layers and consequently low pressure losses. While this type of inlet generates a relatively large drag factor, its high aerodynamic efficiency at low speeds outweighs the drag penalty.During engine calibration on ground test stands, bellmouth inlets are often used, sometimes fitted with an anti-ingestion screen. Because duct losses are minimal, they are typically considered negligible. Engine performance data, such as rated thrust and trimming, are measured using a bellmouth compressor inlet (Figure 8).
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| Figure 8. Bellmouth compressor inlet (with screen) |
Figure 9 demonstrates the effect of aerodynamic efficiency and duct loss. A rounded leading edge (Figure 9A) allows the airflow to fully utilize the inlet’s cross-section, while a sharp-edged orifice (Figure 9B) significantly reduces the effective diameter, increasing losses.
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| Figure 9 |
Low-velocity entry flow through round and sharp edge orifice
When installed, inlet screens can be positioned internally or externally at the inlet duct or directly at the engine compressor inlet (Figure 10A and 10B). Many of these separators are removable for maintenance or operational flexibility. In the sand separator shown in Figure 3-10B, suction at the inlet directs sand and small debris into a sediment trap using centrifugal forces.
Compressor Inlet Screens, Sand, and Ice Separators
Compressor inlet screens are primarily used on rotorcraft, turboprops, and ground-based turbine installations. Although all gas turbines are highly sensitive to debris such as nuts, bolts, and stones, screens are rarely used on high-subsonic flight engines. Past attempts showed that icing and screen fatigue failures caused significant maintenance problems, leading to their general abandonment in fast-moving aircraft.When installed, inlet screens can be positioned internally or externally at the inlet duct or directly at the engine compressor inlet (Figure 10A and 10B). Many of these separators are removable for maintenance or operational flexibility. In the sand separator shown in Figure 3-10B, suction at the inlet directs sand and small debris into a sediment trap using centrifugal forces.
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| Figure 10 |
Some sand and ice separators include a movable vane that extends into the airstream. This causes the engine inlet air to make a sudden turn, while sand or ice particles, due to their higher momentum, continue in a straight path and are removed from the airflow. The movable vane is pilot-operated via a control handle in the cockpit (Figure 10C).
Engine Inlets and Ground Effect
Some gas turbine engine inlets are prone to forming a vortex between the ground and the flight inlet. The suction created by this vortex can lift water, sand, small stones, nuts, and bolts, directing debris into the engine and potentially causing severe compressor erosion or damage (Figure 11). This phenomenon is particularly pronounced for wing pod-mounted engines with low ground clearance, as found on many modern high-bypass turbofan aircraft. To reduce ground effect, some inlets have been redesigned with a slightly flattened or out-of-round bottom. |
| Figure 11. Water vortex during test cell run up |
Earlier aircraft addressed this issue with a vortex dissipator, also known as a blow-away jet. In this system, a small jet of compressor discharge air is directed at the ground beneath the inlet from a nozzle located on the lower engine cowl. The system is usually activated by a landing gear switch, which opens a valve connecting the compressor bleed port to the dissipator nozzle whenever the engine is operating and weight is on the main landing gear (Figure 12).
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| Figure 12. Vortex dissipater |