Turbine engine inlet systems direct the correct quantity of clean, uniform airflow to the engine compressor under a wide range of operating conditions. Aircraft maintenance technicians should understand the different inlet designs, airflow characteristics, and protective features to ensure safe operation and optimum engine performance.
The engine inlet of a turbine engine is designed to provide a relatively distortion-free flow of air, in the required quantity, to the inlet of the compressor. [Figure 1]
 |
| Figure 1. An example of a turbine engine inlet |
Many engines use inlet guide vanes (IGV) to help straighten the airflow and direct it into the first stages of the compressor. A uniform and steady airflow is necessary to avoid compressor stall (airflow tends to stop or reverse direction of flow) and excessive internal engine temperatures in the turbine section.
Normally, the air-inlet duct is considered an airframe part and not a part of the engine. However, the duct is very important to the engine’s overall performance and the engine’s ability to produce an optimum amount of thrust.
Aerodynamic Performance and Ram Recovery
A gas turbine engine consumes considerably more airflow than a reciprocating engine. The air entrance passage is correspondingly larger. Furthermore, it is more critical in determining engine and aircraft performance, especially at high airspeeds. Inefficiencies of the inlet duct result in successively magnified losses through other components of the engine.
The inlet varies according to the type of turbine engine. Small turboprop and turboshaft engines have a lower airflow than large turbofan engines which require a completely different type of inlet. Many turboprop, auxiliary power units, and turboshaft engines use screens that cover the inlet to prevent foreign object damage (FOD).
As aircraft speed increases, thrust tends to decrease. However, as the aircraft reaches a certain speed, ram recovery compensates for the losses caused by the increases in speed. The inlet must be able to recover as much of the total pressure of the free airstream as possible.
As air molecules are trapped and begin to be compressed in the inlet, much of the pressure loss is recovered. This added pressure at the inlet of the engine increases the pressure and airflow to the engine. This is known as “ram recovery” or “total pressure recovery.” The inlet duct must uniformly deliver air to the compressor inlet with as little turbulence and pressure variation as possible. The engine inlet duct must also hold the drag effect on the aircraft to a minimum.
Configuration and Velocity Management
Air pressure drop in the engine inlet is caused by the friction of the air along both sides of the duct and by the bends in the duct system. Smooth flow depends upon keeping the amount of turbulence to a minimum as the air enters the duct. On engines with low flow rates, turning the airflow allows the engine nacelle to be smaller and have less drag.
On turbofan engines, the duct must have a sufficiently straight section to ensure smooth, even airflow because of the high airflows. The choice of configuration of the entrance to the duct is dictated by the location of the engine within the aircraft and the airspeed, altitude, and attitude at which the aircraft is designed to operate. To accomplish this, inlet ducts are designed to function as diffusers with a divergent shape, decreasing the velocity and increasing the static pressure of the air passing through them.
Divided-Entrance Duct
The requirements of high-speed, single- or twin-engine military aircraft, in which the pilot sits low in the fuselage and close to the nose, render it difficult to employ the older type single-entrance duct, which is not used on modern aircraft. Some form of a divided duct, which takes air from either side of the fuselage, has become widely used. This divided duct can be either a wing-root inlet or a scoop at each side of the fuselage. [Figure 2]
 |
| Figure 2. An example of a divided-entrance duct |
Either type of duct presents more problems to the aircraft designer than a single-entrance duct because of the difficulty of obtaining sufficient air scoop area without imposing prohibitive amounts of drag. Internally, the problem is the same as that encountered with the single-entrance duct: to construct a duct of reasonable length with as few bends as possible.
Scoops at the sides of the fuselage are often used. These side scoops are placed as far forward as possible to permit a gradual bend toward the compressor inlet, making the airflow characteristics approach those of a single-entrance duct. A series of turning vanes is sometimes placed in the side-scoop inlet to assist in straightening the incoming airflow and to prevent turbulence.
Variable-Geometry Duct
The main function of an inlet duct is to furnish the proper amount of air to the engine inlet. In a typical military aircraft using a turbojet or low bypass turbofan engine, the maximum airflow requirements are such that the Mach number of the airflow directly ahead of the face of the engine is less than Mach 1. Airflow through the engine must be less than Mach 1 at all times.
Therefore, under all flight conditions, the velocity of the airflow as it enters the air-inlet duct must be reduced through the duct before the airflow is ready to enter the compressor. To accomplish this, inlet ducts are designed to function as diffusers, decreasing the velocity and increasing the static pressure of the air passing through them. [Figure 3]
 |
| Figure 3. An inlet duct acts as a diffuser to decrease the airflow velocity and to increase the static pressure of air |
As with military supersonic aircraft, a diffuser progressively decreases in area in the downstream direction. Therefore, a supersonic inlet duct follows this general configuration until the velocity of the incoming air is reduced to Mach 1. The aft section of the duct then increases in area, since this part must act as a subsonic diffuser. [Figure 4]
 |
| Figure 4. The aft section of an inlet duct acting as a subsonic diffuser |
In practice, inlet ducts for supersonic aircraft follow this general design only as much as practical, depending upon the design features of the aircraft. For very high speed aircraft, the internal configuration of the duct is changed by a mechanical device as the speed of the aircraft increases or decreases. A duct of this type is usually known as a variable-geometry inlet duct.
Military aircraft use the three methods described above to diffuse the inlet air and slow the inlet airflow at supersonic flight speeds. One is to vary the area, or geometry, of the inlet duct either by using a movable restriction, such as a ramp or wedge, inside the duct. Another method uses a variable airflow bypass arrangement, which extracts part of the inlet airflow from the duct ahead of the engine. In some cases, a combination of both systems is used.
The Shock Wave Diffusion Method
The third method is the use of a shock wave in the airstream. A shock wave is a thin region of discontinuity in a flow of air or gas, during which the speed, pressure, density, and temperature of the air or gas undergo a sudden change. Stronger shock waves produce larger changes in the properties of the air or gas.
A shock wave is intentionally set up in the supersonic flow of the air entering the duct, by means of some restriction or small obstruction which automatically protrudes into the duct at high flight Mach numbers. The shock wave results in diffusion of the airflow, which, in turn, decreases the velocity of the airflow.
In at least one aircraft installation, both the shock method and the variable-geometry method of causing diffusion are used in combination. The same device that changes the area of the duct also sets up a shock wave that further reduces the speed of the incoming air within the duct. The amount of change in duct area and the magnitude of the shock are varied automatically with the airspeed of the aircraft.
Compressor Inlet Screens
To prevent the engine from readily ingesting any items that can be drawn in the intake, a compressor inlet screen is sometimes placed across the engine air inlet at some location along the inlet duct. Engines that incorporate inlet screens, such as turboprops [Figure 5] and APUs [Figure 6] are not as vulnerable to FOD.
 |
| Figure 5. An example of a turboprop engine that incorporates inlet screens |
 |
| Figure 6. An example of an inlet screen on an APU |
The advantages and disadvantages of a screen vary. If the engine is readily subjected to internal damage, as would be the case for an engine having an axial compressor fitted with aluminum compressor blades, an inlet screen is almost a necessity. Screens, however, add appreciably to inlet duct pressure loss and are very susceptible to icing.
Failure due to fatigue is also a problem. A failed screen can sometimes cause more damage than no screen at all. In some instances, inlet screens are made retractable and may be withdrawn from the airstream after takeoff or whenever icing conditions prevail. Such screens are subject to mechanical failure and add both weight and bulk to the installation.
In large turbofan engines having steel or titanium compressor (fan) blades, which do not damage easily, the disadvantages of compressor screens outweigh the advantages, so they are not generally used.
Bellmouth Compressor Inlets
A bellmouth inlet is usually installed on an engine undergoing testing in a test cell. For this reason, the bellmouth inlet duct is most often used on helicopter airframes. [Figure 7] It is generally equipped with probes that, with the use of instruments, can measure intake temperature and pressure (total and static). [Figure 8]
 |
| Figure 7. A bellmouth inlet used during system tests |
 |
| Figure 8. Probes within a bellmouth inlet used to measure intake temperature and pressure |
During testing, outside static air must be allowed to flow into the engine with as little resistance as possible. The bellmouth is attached to the movable part of the test stand and moves with the engine. The thrust stand is made up of two components, one nonmoving and one moving. This allows the moving component to push against a load cell and measure thrust during the testing of the engine.
The bellmouth is designed with the single objective of obtaining very high aerodynamic efficiency. Essentially, the inlet is a bell-shaped funnel having carefully rounded shoulders which offer practically no air resistance. [Figure 7]
Duct loss is so slight that it is considered zero. The engine can, therefore, be operated without the complications resulting from losses common to an installed aircraft inlet duct. Engine performance data, such as rated thrust and thrust specific fuel consumption, are obtained while using a bellmouth inlet.
Usually, the inlets are fitted with protective screening. In this case, the efficiency lost as the air passes through the screen must be taken into account when very accurate engine data are necessary.
Turboprop and Turboshaft Compressor Inlets
The air inlet on a turboprop is more of a problem than some other gas turbine engines because the propeller drive shaft, the hub, and the spinner must be considered in addition to other inlet design factors. The ducted arrangement is generally considered the best inlet design of the turboprop engine as far as airflow and aerodynamic characteristics are concerned. [Figure 9]
 |
| Figure 9. An example of a ducted arrangement on a turboprop engine |
The inlets for many types of turboprops are anti-iced by using electrical elements in the lip opening of the intake. Ducting from either the engine or the nacelle directs airflow to the intake of the engine.
Deflector doors are sometimes used to deflect ice or dirt away from the intake. [Figure 10] The air then passes through a screen and into the engine on some models.
 |
| Figure 10. Deflector doors used to deflect ice or dirt away from the intake |
A conical spinner, which does not allow ice to build up on the surface, is sometimes used with turboprop and turbofan engines. In either event, the arrangement of the spinner and the inlet duct plays an important function in the operation and performance of the engine.
Turbofan Engine Inlet Sections
High-bypass turbofan engines are usually constructed with the fan at the forward end of the compressor. A typical turbofan intake section is shown in Figure 11.
 |
| Figure 11. A typical turbofan intake section |
Sometimes, the inlet cowl is bolted to the front of the engine and provides the airflow path into the engine. In dual compressor (dual spool) engines, the fan is integral with the relatively slow-turning, low-pressure compressor, which allows the fan blades to rotate at low tip speed for best fan efficiency. The fan permits the use of a conventional air inlet duct, resulting in low inlet duct loss.
The fan reduces engine damage from ingested foreign material because much of any material that may be ingested is thrown radially outward and passes through the fan discharge rather than through the core of the engine. Warm bleed air is drawn from the engine and circulated on the inside of the inlet lip for anti-icing. The fan hub or spinner is either heated by warm air or is conical as mentioned earlier.
Inside the inlet, near the fan blade tips, is an abradable rub strip that allows the fan blades to rub for short times due to flightpath changes. [Figure 12] Also, inside the inlet are sound-reducing materials to lower the noise generated by the fan.
 |
| Figure 12. Rubber stripping inside a turbofan engine inlet allows for friction for short periods of time during changes in the flightpath |
Fan Specifications and Exhaust Routing
The fan on high-bypass engines consists of one stage of rotating blades and stationary vanes that can range in diameter from less than 84 inches to more than 112 inches. [Figure 13]
 |
| Figure 13. The air that passes through the inner part of the fan blades becomes the primary airstream |
The fan blades are either hollow titanium or composite materials. The air accelerated by the outer part of the fan blades forms a secondary airstream, which is ducted overboard without passing through the main engine. This secondary air (fan flow) produces 80 percent of the thrust in high-bypass engines.
The air that passes through the inner part of the fan blades becomes the primary airstream (core flow) through the engine itself. [Figure 13]
The air from the fan exhaust, which is ducted overboard, may be discharged in either of two ways:
- To the outside air through short ducts (dual exhaust nozzles) directly behind the fan. [Figure 14]
- Ducted fan, which uses closed ducts all the way to the rear of the engine, where it is exhausted to the outside air through a mixed exhaust nozzle. This type of engine is called a ducted fan and the core airflow and fan airflow mix in a common exhaust nozzle.
 |
| Figure 14. Air from the fan exhaust can be discharged overboard through short ducts directly behind the fan |