Air intakes: meaning, requirements and types. Air intake: fresh air for engine operation

Supersonic aircraft must have the appropriate type of air intakes, because the front part of the compressor cannot cope with supersonic flow. At subsonic speeds the intake must have the pressure recovery properties of a subsonic intake, but at supersonic speeds it must reduce the air flow rate below the speed of sound and control the formation of shock waves.

Supersonic cross-sectional area diffuser from the front to the rear gradually decreases, which helps to reduce the flow speed below 1M. A further reduction in speed is achieved in a subsonic diffuser, the cross-sectional area of ​​which increases as it approaches the compressor inlet. To properly slow the flow of shock waves, it is very important to control their formation in the air intake. The use of air intakes of variable geometry allows for proper control of shock waves; they may also have bypass flaps to bleed air from the air intake without changing its speed.

Rice. 2.2. Variable throat air intake (based on original Rolls-Royce drawing)

Rice. 2.3. External/internal compression air intake (based on original Rolls-Royce drawing)

Movable air intakes

For movable air intakes, the inlet cross-sectional area (Concorde) is changed by means of a movable central cone (SR 71). This allows the seal shock(s) at the compressor inlet to be controlled.

Operational calculations

Takeoff. The engine air intake is designed to maintain a stable air flow at the compressor inlet; Any disturbance in the flow that causes turbulence can cause stall or compressor surge.

The air intake cannot cope with high angles of attack and maintain stable air flow. One of the most critical moments occurs during engine acceleration to takeoff thrust. Intake airflow can be affected by any crosswind, especially on tail-mounted engines with S-shaped intakes (TriStar, 727). To prevent possible flow stall and surge, there is a procedure in the operating manuals that must be followed. It usually consists of progressive movement of the aircraft before smoothly increasing the operating mode to takeoff speed, approximately 60 - 80 knots (takeoff without stopping).

Icing. Under certain conditions, icing of the air intake may occur. This typically occurs when the outside air temperature is below +10°, there is visible humidity, standing water on the runway, or runway visual range is less than 1,000 m. If these conditions are present, the pilot must turn on the engine anti-ice system.

Damage. Damage to the air intake or any roughness within its passage can cause turbulence in the incoming air flow and disrupt the flow in the compressor, causing stall or surge. Be alert for damage and uneven surface roughness of the trim panels when inspecting the air intake.

Suction of foreign objects. Suction of foreign objects while the aircraft is on or near the ground will inevitably cause damage to the compressor blades. Give sufficient attention to the area on the ground in front of the engine air intakes before starting them to ensure that there are no loose rocks or other debris. This does not apply to tail-mounted engines, whose air intakes are located above the fuselage; they suffer much less from the absorption of foreign objects.

Turbulence in flight. Severe turbulence in flight can not only cause coffee to spill, but also disrupt air flow in the engines. Using the turbulence speed specified in the owner's manual and the correct RPM/EPR will help reduce the likelihood of compressor failure. It may also be advisable or necessary to activate continuous ignition to reduce the likelihood of engine flameout.

Ground operations. Most compressor damage is caused by foreign objects being sucked in. Damage to the compressor blades leads to changes in the geometry of the system, which can lead to deterioration in performance, stalled flow in the compressor, and even engine surging. To prevent such damage from occurring, it is important to take preliminary measures to remove debris from the parking area. Next, during the pre-flight inspection, the pilot must ensure that there are no foreign objects in the engine air intakes. The responsibility does not end there; after the flight, it is necessary to install plugs on the inlet and exhaust ducts to prevent the accumulation of contaminants and autorotation.

During startup, taxiing, and reversing thrust, foreign objects may be drawn into the air intake and a minimum amount of thrust should be applied to prevent potential damage.

During gas turbine engine operation, serious damage and some fatalities occurred due to personnel being sucked into the air intakes. If it is necessary to carry out work in close proximity to a running engine, special care must be taken.

CHAPTER 3 – COMPRESSORS

Compressor

· List of compressor purposes.

· Description of centrifugal and axial types of compressors used for aircraft engines.

· Name of the main components of the compressor stage and a description of their functions.

· Description of changes in gas parameters (p, t, v) in the compressor stage.

· Definition of the term “pressure increase ratio” and indication of its value for the stage of centrifugal and axial compressors.

· Indication of the advantages of a two-stage centrifugal compressor.

· Listing the advantages and disadvantages of a centrifugal compressor compared to an axial compressor.

· The name of some engines having axial and centrifugal compressors.

· Explanation of the narrowing of the annular air channel in an axial compressor.

· Indication of the input and output speed of the axial compressor stage.

· Indication that axial compressors have pressure ratios up to 35 and outlet temperatures up to 600°C.

· Description of the reason for twisting of compressor blades using speed triangles.

· Indication of the purpose of the VNA.

· Indication of the reason for the compressor clicking when rotating on the ground, i.e. due to autorotation.

· Description of the design of two- (and three-) shaft compressors of modern engines, principles of their operation and advantages.

· Definition of the terms “compressor stall” and “surge”.

· Indication of the following conditions that cause flow stall and surge:

o a sharp increase in fuel consumption with increasing speed (RPM);

o low speed, i.e. small gas;

o strong side wind on the ground;

o icing of the engine air intake;

o contamination or damage to the compressor blades;

o damage to the engine air intake.

Description of the following stall and surge indicators:

o abnormal noise in the engine;

o vibration;

o RPM fluctuations;

o increased EGT;

o Sometimes burning gases escape from the air intake and exhaust device.

· Listing the pilot's actions in the event of a flow stall.

· Description of design methods to minimize the likelihood of flow stall and surge.

· Indicate measures for the pilot to prevent stall and surge.

· Description of the compressor diagram (surge range) with lines of RPM, stall limit, stable operation and acceleration.

021 03 03 03 Diffuser. Description of diffuser functions

Compressor types

Before fuel is added to the combustion chambers and the subsequent expansion of combustion products in turbines, the air must be compressed.

There are two main types of compressors used in engines today: one creates axial flow through the engine, and the other creates centrifugal flow.

In both cases, the compressors are driven by a turbine, which is connected to the compressor impellers via a shaft.

To operate the internal power engine, air is required, which is taken from the atmosphere using a special device - an air intake. Read the article about what an air intake is and why it is needed, what types it is and how it is designed, as well as about the correct selection and replacement of this part.

What is an air intake?

Air intake (air intake) is a part of the power supply system for vehicles with internal combustion engines; pipes of various shapes, cross-sections and designs for air intake and its directed supply to the air filter and then to the carburetor or throttle assembly.

The air intake has several functions:

• Selection of atmospheric (cold) air for supply to the engine;
• Selection of warm air to power the engine during a cold start and during warm-up (mainly in the cold season);
• Directed air supply to the filter regardless of its location (this allows for convenient placement of the filter and other parts of the power system);
• Some types of air intakes protect the engine power system from water and dirt entering it;
• In some cars and during tuning, it serves as a decorative element.

Air intakes are important parts of the engine power supply system, since the volume and stability of the air supply to the engine depend on their design, installation location and general technical condition. Therefore, if this part breaks, it must be repaired or replaced. To make the right choice of air intake for a car, you need to understand their types, design and features.

Types, design and applicability of air intakes

Structurally, all air intakes are the same - it is a pipe of a round, rectangular or more complex cross-section, which is installed on one side on the air filter housing, and the other goes to the most convenient place inside the body or outside the car. Under the influence of vacuum that occurs in the intake tract of the engine power supply system, air is sucked through the outer part of the intake, enters the filter and then into the system.

Air intakes can be divided into two groups according to their installation location on the vehicle:

• External;
• Internal.

External intakes are installed outside the car body - above the hood, above the roof, behind the rear surface of the cabin, etc. For installation, a place is selected where there is normal or increased air pressure while the vehicle is moving, avoiding areas of turbulence (vortices) with low pressure.

Internal intakes are located in the engine compartment in close proximity to the engine. To supply air to the engine compartment, there are holes in the hood, fenders or other body parts. These air intakes are divided into two types according to their purpose:

• For cold air intake;
• For intake of warm air.

Intakes of the first type are located at some distance from the engine, providing air supply to the filter at ambient temperature. Intakes of the second type are located at the hottest parts of the engine (usually mounted directly on the exhaust manifold), providing warm air to the filter. A system of two air intakes facilitates winter operation of the engine by speeding up its warm-up. As a rule, such a system contains a thermostat with a damper, by changing the position of which you can mix warm and cold air to achieve the optimal temperature of the fuel-air mixture entering the cylinders.

Diagram of the air tract of the passenger car engine power supply system

Diagram of the air path of a truck engine power supply system

External and cold air intakes are divided into two groups according to the method of air supply:

• Passive;
• Active.

Passive air intakes are simple devices in the form of plastic or metal pipes of various configurations that only provide air supply to the filter. Most air intakes on passenger cars and many trucks have this design. On the outside of these devices there can be various auxiliary devices - “fungi” to protect against dust and dirt, resonators to form an air flow of a certain structure, mesh, blinds, etc.

Active air intakes are more complex devices that not only deliver air to the filter, but also solve one or more auxiliary tasks. The most common types of active air intakes are:

• Monocyclones are intakes with swirlers (fixed blades located transverse to the axis of the air flow), which impart rotation to the air flow for additional dust removal (due to centrifugal forces) and better filling of the power system. An example of a monocyclone is the typical air intake of MTZ tractors in the form of a fungus; modern intakes of trucks intended for operation in dusty conditions are also equipped with several cyclones;
• Rotating intakes are devices on the outer side of which a rotating mesh drum with an impeller and a swirler is installed. The drum begins to rotate under the influence of the oncoming air flow, due to this, large debris is sifted out and a swirling air flow is formed in the power system. Rotation also ensures self-cleaning of the outer surface of the drum from stuck particles of contaminants, so these devices are used on cars and various equipment (tractors, combines) operated in dusty conditions.

Both of these air intakes, as well as all intakes with mesh at the inlet, are considered coarse air filters, which eliminate the penetration of large particles (stones, grass, etc.) into the power system and significantly extend the life of the air filter.

A separate group includes special-purpose air intakes - snorkels (snorkels). These devices are used on SUVs and other equipment that, during operation, have to overcome deep water obstacles and drive off-road (military equipment, rally cars). The snorkel is a sealed pipe placed at the level of the car roof - its location at the highest point of the car provides protection from water and dirt. Typically, snorkels are equipped with a rotating intake, which can be deployed along or against the direction of travel of the car; it has a mesh and can be equipped with auxiliary parts (for draining water, for swirling air, etc.).

Air intake on the hood

Finally, there is a large group of hood air intakes of passenger cars, which perform two functions - the formation of a directed air flow and decoration. These devices have a variety of designs and add new notes to the appearance of the car, and, at the same time, provide an intensive supply of air into the engine compartment or directly to the internal air intake. But today, purely decorative air intakes have become widespread, which help give the car a more aggressive, sporty look, but have virtually no effect on the operation of the air tract of its power system.

Questions regarding the selection and replacement of air intakes

During vehicle operation, the air intake is not subjected to heavy loads, but it can be damaged due to impact (to which external intakes of trucks, tractors and other equipment are especially susceptible) or vibrations, or lose its characteristics due to aging (plastic parts are especially susceptible to this). If there is a malfunction, the part must be replaced, otherwise the operating mode of the engine may be disrupted, the rate of filter clogging may increase, etc.

For replacement, you should choose only those air intakes that are suitable for a given car or tractor - this can be easily done by type and part number. Replacement is possible only in cases where the same parts are used on different equipment - such as, for example, the intakes of all KAMAZ vehicles, “fungi” for air intakes, monocyclones and rotating intakes of many tractors and trucks, etc.

Replacing the intake usually comes down to dismantling the old part and installing a new one; this requires unscrewing several screws, removing a couple of clamps and removing one or two seals. During installation, you should ensure that the seals are installed correctly and ensure maximum tight installation to avoid air leaks through the cracks. All work should be performed in accordance with the vehicle repair and maintenance instructions.

The choice of a decorative air intake comes down to selecting a part that is suitable for the installation location and appearance. Installation of the intake can be carried out in various ways, including without drilling the hood and other body parts - in each specific case, you should follow the attached instructions.

With the correct selection and replacement of the air intake, the engine will receive the required amount of air and operate normally in any conditions.

The air intake of an aircraft with a turbojet engine has frames, outer and inner surfaces and external supply flaps, which in the closed position are sections of its outer surface, characterized in that the said flaps are fixed relative to the air intake frames with the possibility of rotation and, together with these frames, form a fragment of the said outer surface In addition, there are internal supply flaps, also fixed relative to the air intake frames with the possibility of rotation and, together with these same frames, forming a fragment of the inner surface of the air intake. Part of the frame structure, together with the feed flaps in their open position, forms a channel with a profiled aerodynamic surface. There are stops to fix the opening angle of the valves. Part of the frame structure in the area where the doors are installed is made in the form of a hollow profile. The proposed solution provides access to the engine of the required air flow while ensuring the necessary strength of the air intake without increasing its weight.

The utility model relates to aviation technology, more precisely to the air intakes of aircraft with turbojet engines equipped with make-up flaps, and can be used on various aircraft.

Replenishment flaps to ensure the necessary air flow during takeoff and landing modes are available in the air intakes of many aircraft engines (Air fleet technology. 1991. No. 4, p. 52; Nechaev Yu.N. Theory of aircraft engines. VVIA named after N.E. Zhukovsky, 1990, pp.255-259).

Various designs of air intakes with make-up flaps, which are distant analogues of the proposed solution, are described in A.S. USSR No. 315650 (B 64 D 33/02), RF application No. 94022790 (B 64 D 33/02), RF patent No. 2088486 (B 64 D 33/02), a.s. No. 912040 (B 64 D 33/02), US patent No. 4203566 (US cl. 244/57), etc.

Placing make-up flaps in the quantity necessary to capture the air flow of the required size, in structures such as those described in RF application No. 94022790 or in RF patent No. 2088486, may require their placement almost up to the engine compressor, which will negatively affect the quality of the air flow in front of the compressor and reduce the service life engine.

The closest to the proposed solution is the air intake of the Su-27 aircraft (A. Fomin “Su-27. History of the Fighter”, M.: “RA Intervestnik”, 1999, p. 218). It is adjustable, has a nearly rectangular cross-section, and is equipped with a retractable mesh to prevent foreign objects and small birds from entering the engine air tract during takeoff and landing. The replenishment flaps are located on the lower side of the air intake and, in the closed position, form its outer surface in the area where the protective mesh is placed and are designed to open and close under the influence of differential pressure. The inner surface of the air intake channel above the flaps is formed by the said protective mesh in the position when it is lowered. The doors can open both when the net is extended and when the net is retracted.

The disadvantage of this design is that when placing a large number of make-up valves it is compact, i.e. in the form of blinds, the dimensions of the cutout for the flaps in the air intake skin increase, the integrity of the frames crossing the cutout for the flaps is disrupted (there are no frames at the cutout site), which negatively affects the overall strength and rigidity of the air intake. Due to the resistance of the protective mesh and poorly streamlined frames in the area where the wings are located, the effective thrust of the power plant and its gas-dynamic stability are reduced during takeoff and landing modes, which affects flight safety.

The objective of the utility model is to increase the reliability of the operation of the aircraft's power plant during takeoff and landing modes by optimizing the design of the air intake make-up flaps.

To solve this problem, an air intake is proposed that has frames, outer and inner surfaces and external supply flaps, which in the closed position are sections of its outer surface, characterized in that each of the mentioned external flaps is fixed relative to one of the air intake frames with the possibility of rotation, the mentioned external the supply flaps together with these frames form a fragment of the said external surface of the air intake; in addition, there are internal supply flaps, each of which is also fixed relative to one of the mentioned air intake frames with the possibility of rotation; the internal supply flaps in the closed position together with these same frames form a fragment of the internal air intake surface.

Part of the structure of each of the mentioned frames, together with the corresponding mentioned external and internal supply flaps in their open position, forms a channel with a profiled aerodynamic surface.

To fix the opening angle of the external and internal replenishment flaps, there are stops.

Part of the structure of the frames in the area where the replenishment flaps are installed is made in the form of a hollow profile.

The proposed solution makes it possible to provide access to the engine of the required air flow while ensuring the necessary strength of the air intake without increasing its weight.

The utility model is illustrated with figures.

Figure 1 shows a longitudinal section of the proposed air intake.

Figure 2 shows a cross-section of the air intake in the area where the replenishment flaps are located.

Figure 3 shows on an enlarged scale a fragment of the longitudinal section of the air intake in the area where the recharge flaps are installed.

The air intake of an aircraft engine contains external 1 and internal 2 surfaces, frames 3, as well as external supply flaps 4 and internal supply flaps 5, and one of the flaps 4 and one of the flaps 5 are fixed relative to each of the frames 3. The lines 4 are fixed with the possibility of rotation relative to axis 6, and the shutters 5 - with the possibility of rotation relative to axis 7.

The external replenishment flaps 4 in the closed state, together with the outer flanges 8 of the frames 3, form part of the outer surface 1 of the air intake, and the internal replenishment flaps 5 in the closed state, together with the internal flanges 9 of the frames 3, form part of the inner surface 2 of the air intake. In this case, the distance between the replenishment flaps 4 is determined by the width of the outer shelf 8, and the distance between the inner flaps 5 is determined by the width of the inner flange 9.

Flaps 4 and 5 open almost synchronously under the influence of a pressure difference, allowing a flow of additional air into the air intake.

Part 10 of the design of each of the frames 3 between the points of contact of the frame 3 with the flaps 4 and 5, together with these flaps in their open position, forms a channel with a profiled aerodynamic surface so that when the flaps 4 and 5 are open, air flows inward without disruption.

To fix the opening angle of the external replenishment flaps 4 and the internal replenishment flaps 5, there are stops 11 located on the frames 3.

The part of the structure of the frame 3, located in the installation area of ​​the doors 4 and 5, is made in the form of a hollow tubular profile.

The air intake works as follows.

When the aircraft power plant is started, a vacuum is created in the air intake channel and, under the influence of a pressure difference, the outer replenishment flaps 4 and the internal replenishment flaps 5 open. The opening angle of the flaps 4 and 5 is fixed by stops 11. Additional air enters through the holes formed by the flaps 4 and 5 on one side and profiled aerodynamic parts 10 frames 3 on the other side, inside the air intake.

After the aircraft takes off and its speed increases, the vacuum in the air intake channel decreases, and the flaps smoothly close under the influence of their own weight and pressure inside the air intake channel.

In the event of surge when the make-up flaps are open, the air wave moves forward from the engine compressor and acts on flaps 4 and 5, trying to deflect them to a larger angle. This is prevented by stops 11. The torque arising in this situation is resisted by the hollow profile of frames 3 (this profile has increased torsional strength).

The connection between the essential features and the technical result is as follows:

Placing the flaps in such a way that the replenishment flaps, together with the external flanges of the frames, form part of the outer surface of the air intake, and the internal flaps, together with the internal flanges of the frames, form the inner surface of the air intake, allows you not to violate the integrity of the frames and place the flaps compactly with a sufficiently large total replenishment area in the most convenient place , providing maximum effect;

Part of the frame structure with a profiled aerodynamic surface ensures continuous flow around the frame;

Stops for fixing the opening angle of the valves prevent the valves from breaking out and overturning when exposed to a back pressure wave during engine surge;

Making the structure of the frames in the area where the feed flaps are installed in the form of a hollow profile increases their rigidity, prevents them from twisting from the effects of torques created by the forces acting on the flaps, makes it possible to reduce the width of the internal and external flanges of the frames while ensuring the necessary strength, and thereby increase the throughput ability of recharge flaps placed between the frames.

Utility model formula

1. An aircraft air intake having frames, outer and inner surfaces and external replenishment flaps, which in the closed position are sections of its outer surface, characterized in that each of said outer flaps is fixed relative to one of the air intake frames with the possibility of rotation, said external replenishment flaps together with these frames they form a fragment of the mentioned outer surface of the air intake; in addition, there are internal supply flaps, each of which is also fixed relative to one of the mentioned air intake frames with the possibility of rotation; the internal supply flaps in the closed position, together with these same frames, form a fragment of the internal surface of the air intake .

2. The air intake according to claim 1, characterized in that part of the structure of each of the mentioned frames, together with the corresponding mentioned external and internal supply flaps in their open position, forms a channel with a profiled aerodynamic surface.

3. The air intake according to claim 1 or 2, characterized in that there are stops to fix the opening angle of the external and internal supply flaps.

4. The air intake according to claim 1 or 2, characterized in that part of the structure of the frames in the area where the supply flaps are installed is made in the form of a hollow profile.

5. The air intake according to claim 3, characterized in that part of the structure of the frames in the area where the supply flaps are installed is made in the form of a hollow profile.

IEDs of Tu-160 bomber engines.

Today we'll talk about air intakes. This topic is quite complex (like many things in aviation). I will try, as always, to simplify it more for general acquaintance... We'll see what comes of it :-)...

About what happened...

The beginning of a fine summer day in 1988 was no different from many of the same weekday days in the 164th Orap (Brzeg, Poland). It was a daytime flight shift. The weather scout has already returned, and the flights of all squadrons have begun according to the planned flight tables. The afterburner roar of taking off aircraft excited the surroundings, and even in the hangar parking lot of the TECH, its impressive power could be clearly felt.

I was then acting as head of the engine regulations group. Immediately after the general formation, the head of the TECh rushed to me and took me aside for a conversation. The news was, to put it mildly, unpleasant. One of the MiG-25s got into a difficult situation while accelerating at supersonic speed.

First, the pilot felt strange shocks, then the afterburner of the right engine went out and almost immediately after that it switched off automatically. The launch attempt was unsuccessful, the pilot stopped the mission and, continuing the flight on one engine, returned to the airfield. The landing was completed successfully, without any problems, however, there was a serious flight accident.

We, the engine engineers, together with the AO specialists, after transporting the aircraft to the TEC, began searching for the cause of what happened. During a preliminary inspection, it was discovered that the entire afterburner chamber, within the range of visibility of its elements, was wet from fuel. does not evaporate so quickly, especially the type (quite heavy) that was then used on the MiG-25 (T-6).

MiG-25RB aircraft.

However, this does not happen during a normal engine shutdown, because it is performed by stopping the supply of fuel to the combustion chamber (throttle throttle at STOP), and the remaining fuel from the fuel manifolds, after the cessation of combustion and atomization, is drained into the drain tank.

This means that turning off the afterburner and stopping the engine probably occurred suddenly due to the extinction of the flame in the FCS and OKS, and the fuel continued to flow and be sprayed by the injectors for some time until the throttle was set to “Stop”. And the reason for the extinction was apparently problems with air flow.

Literally immediately after the start of the checks, a failure of the right air intake control system was detected . As a result, during acceleration at an already sufficiently high supersonic speed, a air intake surge, which caused the extinguishing of both combustion chambers (OKS and FKS) and, as a consequence, the engine stopping.

A fairly lengthy description of the circumstances surrounding the flight accident was required because its cause directly relates to the topic of today’s article. In this case air intake– this is not just a pipe passing air. This is a serious, working element of the power plant of an aircraft with a turbojet engine (D, F), the creation of which must comply with a whole set of norms and rules. Without them, its correct operation and, ultimately, the efficient and safe operation of the entire propulsion system is impossible. Incorrect operation of the air intake (IA) of a turbojet engine can cause a serious and even, in special cases, severe flight accident.

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The name itself, however, does not give any hints in this regard. Word "air intake" means a special structural unit that, using high-speed pressure, “takes air” from the atmosphere and supplies it to specific components of the aircraft. By the way, not only aircraft, but also, for example, various, especially quite high-speed, cars.

The purpose of air intake may be different. Basically, they can be divided into two groups that differ significantly from each other.

First. Outside air on fast-moving vehicles (primarily aircraft) is convenient for cooling certain components, instruments, assemblies and their structural parts or technical special fluids (working fluids) used for the operation of systems that heat up during operation. For reasons of streamlining, such systems and assemblies are mostly located inside (and even deep inside) the structure of the aircraft.

They exist to supply air to them. special air intakes, combined, if necessary, with air ducts that form and direct an air stream to the desired location. In this case, cooling fins, special radiators, both air and liquid, or simply parts and housings of units can be subjected to cooling for the purpose of cooling.

There are enough such structural units on every aircraft. And, in general, they are not anything particularly complicated. Of course, all air channels must be correctly profiled, mainly in order to maintain a minimum of drag and supply a sufficient amount of air for blowing.

Air intakes for cooling equipment on the Su-24MR aircraft.

However, incorrect operation of such air intakes, as a rule, does not lead to immediate disruption of the operation of ventilated aircraft components and, even more so, to any serious or fatal consequences for the aircraft.

An example is the air intakes for cooling units of the Su-24M aircraft.

Second. But poorly performing air intakes belonging to the second group may well be the reason for this. This air intakes air-jet engines. The air that they pass through is supplied to the input of these engines and serves as a working fluid for them (further turning into gas).

The characteristics and efficiency of the engine (including thrust and specific fuel consumption), and therefore, ultimately, the entire aircraft, depend on the parameters and quantity of incoming air, the quality and condition of the air flow. After all, the engine, as you know, is its heart. The condition of this heart is largely determined by the correct operation of the most important unit of the power plant - the air intake, which is otherwise (and deservedly) called input device gas turbine engine (GTE).

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The importance of proper operation of the air intake directly depends on the flight speed. The higher the speed capabilities of the aircraft, the more complex the design of the turbojet engine and the higher the requirements for it.

When the engine is operating under starting conditions, air enters the air intake mainly due to the vacuum created by the gas turbine engine compressor at the inlet. In this case, the main task of the air intake is to direct the air flow with the least loss.

And with increasing speed, when flying at high subsonic and, especially, supersonic speeds, two more tasks are added to this task, and both of them are main. It is necessary to reduce the flow speed to subsonic, and at the same time effectively use velocity pressure to increase the static air pressure before entering the engine.

Exactly this usage consists of converting the kinetic energy of the oncoming flow (velocity pressure) during its braking into potential energy of air pressure. Quite simply, this can be said as follows.

Since the total pressure of the flow (according to Bernoulli’s law) is a constant value and equal to the sum of static and dynamic pressure (we can ignore the weight pressure in our case), then as the dynamic pressure drops, the static pressure increases. That is, the inhibited flow has a higher static pressure, which is the basis of the work air intake.

That is, the VZ essentially works like a compressor. And the higher the speed, the more impressive this work is. At speeds of 2.-2.5 M, the degree of pressure increase in the air intake can be 8-12 units. And at high supersonic (and hypersonic) speeds, the operation of the air intake is so efficient that the need for a compressor virtually disappears. There is even such a thing as “ compressor degeneration"at high supersonic speed. This is the same process when a turbojet engine gradually turns into a ramjet.

It should be noted that in real air intakes with such dynamic compression, not all the kinetic energy of the flow is used to increase pressure. There are inevitably losses (so-called total pressure losses), which depend on many factors and differ for different air intakes.

Types of modern input devices.

In relation to the speed (maximum) of the aircraft on which they are used, VZs can be subsonic, transonic and supersonic.

Subsonic…

Currently, these are most often the input devices of high-bypass ratio turbofan engines. They are typical for modern subsonic passenger or transport aircraft. Such engines are usually located in separate engine nacelles, and are air intakes They are quite simple in design, but not so simple in terms of the requirements placed on them and, accordingly, their implementation.

As a rule, they are calculated for cruising flight speeds of about 0.75...0.85M. They must have a relatively low mass, provided that the required air flow is ensured. A very important requirement for them is to ensure low energy losses of the air flow (internal losses), which they direct into the engine through their channel, as well as losses to overcome external resistance (external losses).

Scheme of flow and changes in flow parameters in a subsonic gas turbine engine.

This is ensured by correct profiling of the internal channel and external contours, which reduces drag and improves flow. In addition, the leading edges of the inlet device most often have a fairly thick profile, taking the shape in the longitudinal (meridional) section of the channel.

This allows for continuous flow around surfaces, which minimizes losses and, in addition, provides another useful effect. When flowing around a thick entrance edge, an aerodynamic force similar to lift arises.

And its horizontal projection is directed along the flight and is a kind of additive to thrust. This force is called “suction”, and it very significantly compensates for the external resistance of the air intake.

Flow around a subsonic air intake. Action of suction force.

The conversion of dynamic pressure into static pressure in this type of air intake occurs as follows. The design of the channel is calculated so that in its inlet section the flow velocity is less than the flight speed. As a result, the flow before entering the air intake has the shape of a diffuser (“diverges” to the sides), which inevitably entails braking and an increase in pressure (the aforementioned Bernoulli’s law).

That is, compression from the high-velocity pressure mainly occurs even before entering the air intake (the so-called external compression). Then it continues on the first section of the channel, which is also profiled in the form of a diffuser. And in front of it, the channel most often still has a small confusive section (that is, a tapering section). This is done in order to equalize the flow and velocity field.

Subsonic air intake with make-up flaps and a beveled entrance plane.

Entry plane air intake often sloping. This is done to ensure efficient operation of the air intake (and engine) at high angles of attack, when the inlet is obscured by the lower part of the engine nacelle housing.

In design input device of this type, for some engines, so-called . When the engine operates at elevated speeds under starting conditions (that is, there is no speed pressure or is quite low), it is not always possible to provide the required air flow.

Preliminary external compression in such modes is practically absent, and the inlet section of the air intake simply cannot pass all the required air, since its dimensions do not allow it.

Airplane Yak-38. Takeoff mode - make-up doors are open.

Flaps for additional air supply in starting conditions (taxiing). Airplane Tu-154B-1 engine NK-8-2U).

Therefore, additional windows can be made on the air intake shell, which open at the desired mode (usually due to vacuum in the air intake channel) and close after gaining speed. An example is the Tu-154B-1 aircraft. The video clearly shows the opening of the feed flaps on the left engine.

Transonic.

Such input devices radical In general, there are few structural differences from subsonic ones. However, their flow conditions are already more stringent, because they are used in aircraft power plants with maximum flight speeds of up to 1.6...1.7M. Up to these speeds, the use of an air intake with a constant flow path geometry does not yet lead to a large increase in losses as a result of dynamic compression.

Such air intakes have sharper edges compared to subsonic air intakes to reduce wave drag, which, as is known, manifests itself in the transonic and supersonic flow regions. To reduce losses due to stall when flowing around sharp edges and to ensure air flow at low speeds and under starting conditions, additional make-up windows can also be used on these air intakes.

Subsonic and transonic air intake. Position of the direct shock wave.

In front of such an air intake during supersonic flight, a direct shock wave(I wrote about the formation of shock waves). For sharp edges it is attached. When passing through it, the pressure in the flow increases (external compression). A further increase in pressure occurs in a diffuser-type channel.

To reduce the flow velocity before the shock wave input device advantageous to be located in the so-called slow flow zone, which is formed when the flow flows around the structural elements located in front of the air intake (adjacent air intakes - more about them below).

Transonic air intake of the Su-24M. The plane of the PS drainage device and the perforation of the PS suction device are visible.

These are, for example, side (Su-24M, F-5)) or ventral entrance devices (F-16). Structurally, they are usually moved away from the fuselage to form a kind of slot channel 50–100 mm wide. It is needed so that the boundary layer growing on the front of the fuselage surface does not fall into the air intake channel and disturb the uniformity of the flow, increasing losses. It seems to “merge” further into the stream.

Su-24M bomber during taxiing. The make-up valves are open.

Ventral transonic air intake of an F-16 aircraft.

A device for draining the boundary layer on the air intake of an F-4 "Fantom" aircraft.

Supersonic.

The main difficulties begin for input devices when using higher maximum flight speeds - 2.0...3.0M or more. At such speeds transonic air intake cannot be used due to the large increase in the intensity of the direct attached shock and, accordingly, the increase in total pressure losses, which negatively affects the engine parameters (in particular, thrust).

High compression efficiency here is achieved using supersonic input devices (SVU). They are more complex in design and are used to increase pressure. shock system.

To control the process of flow deceleration (and therefore increase the pressure in it), the so-called so-called braking surface , having a specific profile. This surface, when interacting with a supersonic flow (high-speed pressure), creates conditions for the formation of shock waves.

As a rule, there are several of them, that is, a system of shocks is created, including two, three (or even four) oblique and one direct shock (the so-called head wave), which is trailing. When passing through oblique shocks, the decrease in speed and loss of total pressure is less than when passing through straight shocks, the change in parameters is less sharp, and the final static pressure is higher due to lower losses.

In general, the more oblique shocks, the less pressure loss in the flow. However, their number is determined by the design of the air intake, designed for certain maximum speeds.

Passing through such a system, the flow reduces its speed to approximately 1.5...1.7 M, that is, to the level of transonic air intakes. After this, it can pass through a direct shock with relatively small losses, which is what happens, and the flow becomes subsonic, acquiring a certain amount of pressure, and then passes through a narrowing channel into its smallest section, called the “throat”.

The braking surface can have different shapes, but most often it is in the form of a wedge or cone (depending on the shape of the air intake). A wedge (cone) usually has several surfaces (or steps) articulated with each other. Oblique shock waves are formed at the junction points (corners).

Their inclination depends on the Mach number of the flight and the inclination angles of the individual stages. These angles are selected to create flow conditions that are closest to optimal in the design mode.

Depending on the location of the braking surface relative to the air intake body (its shell), as well as its configuration, the shock waves can be positioned differently relative to the entry plane air intake.

Types of VCA: a) external compression: b) mixed compression: c) internal compression.

This, in turn, determines the type of braking process and, accordingly, the type of the supersonic input device itself. First type– VCA with external compression. All of his oblique shocks are located in front of the plane of the entrance to the air intake (that is, outside), and the throat is located in close proximity to it.

Second type – VCA with mixed compression. Here, part of the oblique shocks is located outside, up to the entrance plane, and part inside, that is, behind it. The throat is moved further away from the entrance edges, and the channel from the entrance to the throat is narrowed.

Third type– VCA internal compression. In it, all shock waves are located inside the air channel behind the inlet plane.

In practice, VCAs with external compression are mainly used. The use of two other types, theoretically more effective for compressing flow at high supersonic speeds, in practice encounters various technical difficulties.

There is also a division of air intakes into types according to design characteristics:

According to the shape of the inlet section.

These are the so-called flat and spatial (usually axisymmetric).

Flat intakes (sometimes they are box-shaped or scoop-shaped) have an inlet section in the form of a rectangle, sometimes with roundings at the corner points. The channel itself from the rectangular entrance gradually changes its cross-section to round before entering the engine.

Controllable air intake of an early series Su-24 aircraft. The hinge for turning the vertical panel is visible. Perforations for boundary layer suction are also visible.

The braking surface of a flat air intake is made in the form of a wedge with a special profile. If the air intake is controllable (more on this below), then the flat one has good opportunities for this, namely the possibility of a sufficiently large change in its geometry, allowing you to create a system of shock waves of varying intensity.

U axisymmetric air intake to create such a system, a cone is used, also profiled in a special way (stepped). The inlet cross-section of such an air intake is circular. The cone is the central body in the first section of the internal channel; then the channel also has a circular cross-section.

Frontal axisymmetric air intake with a conical adjustable braking surface, on the MiG-21-93 aircraft

There are also so-called sector air intakes, the inlet section of which is a part (sector) of a circle. And their braking surface is also a part (sector) of the cone. They are usually located on the sides of the fuselage according to the lateral principle (more on this below) and compete with them in terms of reducing total pressure losses. An example of such structures is air intakes Mirage series aircraft, bomber F-111, Tu-128 interceptor, experimental MiG-23PD.

Mirage 2000-5 aircraft with traditional sector IEDs.

For modern aircraft (fifth generation), spatial air intakes with different shapes of the inlet section are designed (for example, T-50; F-22 - parallelogram) with the so-called spatial compression. Here, not only braking surfaces, but also specially profiled shell edges participate in the creation of a whole complex of shock waves.

Tu-128 aircraft with sector IEDs (museum).

By location on the fuselage.

These are frontal and adjacent. Frontal air intakes are installed either in the forward part of the fuselage or in separate engine nacelles. Thus, they operate in an undisturbed air flow. They are most often axisymmetric in shape.

MiG-15 fighter with a typical frontal subsonic air intake.

Adjacent airborne objects are located (adjacent) near any part of the aircraft surface. As a result, the air flow entering them is already slowed down due to its flow around the aircraft elements located in front. This means that the size of the required pressure ratio is reduced, which makes it possible to simplify the design of the air intake.

However, in this case one has to deal with the growing boundary layer, which tends to get into the air intake from the same elements located in front (most often from the fuselage). Usually the boundary layer is simply “drained” through a channel formed when the air intake is located at a certain distance from the aircraft structure (50...100 mm - already mentioned above).

A device for draining the boundary layer of the Eurofighter Typhoon fighter.

Nevertheless, a certain degree of unevenness of the flow at the entrance to the channel is still formed. And it cannot always be productively corrected due to the rather short length (according to the aircraft layout) of the air duct.

Adjacent air intakes There are lateral, ventral and underwing. The braking surface almost always takes the form of a stepped wedge (horizontal or vertical). The exception is the above-mentioned sector air intakes, in which the braking surface is the cone sector (Mirage aircraft).

MiG-31 fighter during taxiing. Adjacent air intakes. The open flaps of the shell are visible.

Some features of VCA with external compression.

The VCA is designed for certain flight Mach numbers, usually close to the maximum. Based on this, design parameters are selected for the design mode. These are the areas of the inlet, throat and outlet, the angles of the braking surface panels (cone surfaces), the locations of the kinks of these panels, the angles of the shell (in particular, the “undercut angle”).

Undercut angle in the front air intake. 1,2 - braking surface, 3 - edge of the shell, 4 - air intake body.

For the design mode, there are two schemes of oblique shock waves. In the first, oblique shock waves are focused on the leading edge of the shell. The direct shock (head wave) is located in the channel behind the throat. The flow is organized in such a way that it enters the channel at supersonic speed and can become subsonic only by passing through this shock.

The disadvantage of this scheme of input devices is the interaction of such a direct shock with the boundary layer near the channel walls. This leads to layer separation and pressure pulsations, as a result of which the outlet flow may not be sufficiently uniform and stationary. However, this type of air intake has less external resistance compared to the second type.

In the second scheme, the direct shock (head wave) is advanced in front of the entrance to the air intake, being partly in the internal flow (in front of the channel), partly in the external one, and has different intensities along its length. Before entering the internal channel, it represents an almost straight shock, which only slightly bifurcates near the braking surface, becoming λ-shaped. In the external flow, it bends to the side against the flight, turning into an oblique one.

VCA with defocusing oblique shocks (second scheme). The slit for draining the PS, the perforation for its suction, as well as the principle of forming the spreading resistance are shown.

To prevent the head wave from destroying the system of oblique shocks in the immediate vicinity of the entrance to air intake, these shocks are slightly shifted and slightly defocused in relation to the input edge of the shell (due to the choice of the angles of location of the panels (β) of the braking surface), that is, simply put, they do not all (three) converge at one point of this edge, but continue further into external flow.

In calculations, however, such a scheme can be replaced with a sufficient degree of accuracy by a simplified one, when it is assumed that the system of oblique shocks is focused on the leading edge and is closed by a direct shock, also located directly on the edge of the shell.

VCA with shocks focused on the shell (first scheme). β - angles of location of adjustable panels.

This shift and defocusing has become the reason for the second type of input devices being most often used in practice. The fact is that this arrangement of shocks significantly reduces the possibility of their destruction by the head wave, which can move during operation to the inlet and outlet along the channel when the air intake operates in various off-design modes.

That is, the stability of the air intake, and therefore the engine as a whole, increases. However, resistance input device there is more of the second type. This is due to the emergence of the so-called spreading resistance, which does not exist for the first type.

A little about spreading resistance.

IN air intake of the first type, the flow immediately enters at supersonic speed (as mentioned above). And in the second type, where the head wave is located almost at the entrance to the air intake, the flow enters the channel already subsonic. Due to the location of oblique shocks, the flow at the inlet, passing along the stagnation surface, is formed in such a way that its outer layers spread out to the sides without falling into the air intake channel.

That is, the actual entrance area becomes smaller than the constructive one (in the figure above F H< Fвх ) поэтому и действительный расход воздуха через air intake is also getting smaller. That is, part of the air, slowed down, which has already passed through oblique shocks, and therefore energy (of the engine) was spent on increasing the pressure, does not enter the engine itself and does not participate in the creation of thrust.

There is even such a parameter to characterize the operation of the air intake as air flow coefficient, equal to the ratio of actual flow to the maximum possible. If this coefficient is less than unity, then there is a spreading of the flow at the inlet, which causes spreading resistance.

In general, at the same time, at the same time, for the air intake, in addition to the spreading resistance, other types of external aerodynamic resistance are also considered, the reduction of which must be strived for. This is important because the so-called external resistance of the inlet device is a force directed against the flight, which means it reduces the effective thrust of the entire power plant, which, in fact, includes the air intake.

In addition to the aforementioned spreading resistance, the external resistance of the air intake also includes shell resistance and various bypass valves (if any) are the so-called excess pressure forces, as well as friction forces in the flow.

Additional losses during flow passage in the channel are associated with the viscosity of the gas, as well as with the configuration of the channel itself. The harmful influence is expressed in an increase in the thickness of the boundary layer and an increase in the probability of flow separation due to the rather complex shape of the braking surface.

The shape of the canal and the area of ​​the throat are adjusted to suit the purpose. reduce harmful effects. The flow makes a fairly sharp turn when entering the internal channel. To avoid flow separation, the channel itself is first made confuser (narrowing) and after turning, diffuser (expansion).

The flow reaches its highest speed (subsonic) in the throat. From the point of view of suppressing separation, the most advantageous speed in the throat becomes . If the flow speed in the throat is equal to the speed of sound, then the throat is called optimal.

The harmful effects of viscosity (boundary layer) are overcome using various technical devices. These include: the use of perforations in areas of the braking surface for suction of the boundary layer or special cracks near the throat to drain it. These techniques make it possible to reduce the size of the emerging separation zones, thereby streamlining the flow at the exit from the air intake.

To activate the boundary layer, special turbulators installed behind the throat are also used. They create small vortices that help mix the boundary layer with the main flow and thereby speed up the process of equalizing the flow velocity field in the channel.

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Returning to the above two types of VCA with external compression, we can say that despite the greater external resistance and lower actual throughput (flow coefficient less than unity) in the design mode, air intakes with defocused oblique shocks are generally more preferable to use than the VZ of the first scheme.

This is due to the fact that defocusing allows you to significantly increase stock of stable work air intake, which is quite important for safe operation in various operating modes, even with a slight decrease in efficiency.

During flight, the speed, altitude, temperature and density of the air and, of course, the operating mode of the engine itself, to which the air intake supplies air, change. Sometimes this air is needed a lot, sometimes not enough, and this (at a constant flight speed) will certainly affect the change in operating mode input device.

With a constant flight Mach number (for example, equal to the design one) and a change in the engine operating mode, three types of air intake operating modes can be distinguished.

The first mode is supercritical . In this case, there is a supersonic flow zone behind the throat. When switching to higher modes, the engine increases speed and needs a lot of air. It is clear that it intensively takes air from the air intake. In this case, the back pressure, which always exists in stationary mode at the end of the air intake channel (repressed air with already increased pressure, ready to enter), decreases.

Scheme of flow movement and changes in parameters in the VCA. Supercritical mode. The feed and bypass valves are shown.

As a result, the head wave shifts slightly towards the entrance (along the flow), and the flow itself in the channel accelerates and, when passing the throat, becomes supersonic with further acceleration in the expanding channel. A process occurs that is basically similar to the process in .

However, since the back pressure at the end of the channel (in front of the gas turbine engine compressor), although reduced, remains, then at some distance behind the throat a shock wave (S) is formed, during the passage of which the flow becomes subsonic. This jump can have a different position and intensity depending on the operating mode of the engine, and therefore its need for air.

Second mode. When the engine is throttled and, therefore, the required amount of air is reduced, the back pressure at the end of the inlet device channel increases and shifts the shock S towards the throat (against the flow). If the throat is optimal (mentioned above), then moving into it the jump disappears. This mode of operation of the air intake is called critical.

The third mode is subcritical . This mode is possible with further throttling of the engine. Now the flow along almost the entire length of the air intake channel becomes subsonic. This means that the action of backpressure from the end of the channel extends over its entire length. The consequence may be a shift of the head wave against the flow closer to oblique shocks (sometimes they say the wave is knocked forward - “knocked out wave").

At the same time, due to a general decrease in flow speed, friction losses fall, which in itself. Certainly. Fine. But there is also “bad”, the harmful impact of which can be significant. The knocked-out bow wave can move so much against the flow that it begins to destroy the system of oblique shocks. The result may be an increase in losses, a decrease in efficiency and, most importantly, a decrease in the stability of the operation of the air intake, which can result in such an unpleasant phenomenon as air intake surge.

Unstable operating modes of a supersonic input device.

1. Surge.

The term “surge” was already encountered earlier when we got acquainted with gas turbine compressors. This word itself comes from the French pompage - “pump” or “pumping”. Therefore, it is applicable not only to aircraft compressors and pumps. It means the phenomenon of instability, non-stationary flow (gas or liquid), accompanied by low-frequency fluctuations in parameters, in particular pressure and flow (air for us).

The definition of surge mainly applies to blade machines. Such a machine, in particular, is the TRD axial compressor. Air intake, of course, does not belong to this type of mechanism, but is essentially a compressor and is fundamentally susceptible to such a phenomenon as surging.

Mechanism of occurrence.

Conditions for the occurrence of air intake surge can only appear at sufficient supersonic levels (M > 1.4...1.5). In this case, the operating mode should be subcritical, when the air intake channel is filled with excess air, which the engine is not able to let through, usually due to sudden throttling (reduction in speed).

Due to this overflow, the back pressure from the outlet of the air intake to the inlet increases. Because of this, the head wave is squeezed out (knocked out) against the flow and begins to destroy oblique shocks, first their part closest to the entrance to the air intake.

As a result, layers with lower total pressure appear in the air flow. These are those layers that did not pass through the shocks (due to their destruction, usually these are the outer layers) and those that touch the braking surface (due to losses in the near-wall boundary layer - usually these are the inner layers). The resulting so-called weakened zones (in Figure I, II, III).

Picture of the occurrence of IED surge. - b). Destruction of a system of oblique shocks knocked out by a wave - a).

And so, through these zones, with further throttling of the engine, the increased back pressure breaks out of the air intake channel. That is, compressed air is released into the atmosphere, or, more precisely, it is intensively released. At the same time, it pushes the head wave even further, which completely destroys the system of oblique shocks.

This position is maintained until the pressure in the air intake duct becomes lower than the inlet pressure (due to the release of compressed air through weakened zones). Then the air begins to move in the opposite direction - into the channel. The movement is so fast that the IED goes into supercritical mode. At the same time, a jump S appears in the space behind the throat.

Then, as the air intake channel is filled with air, back pressure appears and grows, which shifts this shock to the throat and the system transitions to a subcritical mode. This again creates the initial conditions for repeating the surge cycle and everything starts all over again. That is, there are fluctuations in air flow and pressure in the supersonic air intake.

These oscillations are low-frequency, usually from 5 to 15 Hz. Moreover, they have a fairly large amplitude and are very sensitive for the aircraft and crew. They appear in the form of shocks due to fluctuations in engine thrust (change in flow rate), as well as popping and shaking of the structure, especially in the air intake area.

The amplitude of such oscillations depends on the M number and can reach 50% of the pressure before surge at M > 2. That is, their intensity is quite high and the consequences for the power plant can be serious.

Firstly, the engine compressor may begin to surge, which can lead to its (engine) failure. Secondly, due to a sharp periodic decrease in air flow (that is, a sharp decrease in the amount of oxygen - especially at high altitudes), both afterburner and main burnout can occur, that is, the engine switches off automatically.

This is exactly what happened in the case of the MiG-25R aircraft mentioned at the beginning of the article, when at high supersonic speed due to a failure of the air intake control system, the controlled wedge suddenly straightened out completely, opening the entrance to the air intake to a large amount of air.

In addition, if pressure fluctuations are sufficiently intense, then the lining of the air intake channel can become deformed or even collapse with all the ensuing consequences. And the longer the channel, the higher the inertia of the flow and the stronger the surge phenomena.

Prevention (elimination) of surge.

Due to such serious possible consequences of surge, it is unacceptable in operation. If it does occur, then the main and main way to stop it is as quickly as possible. speed reduction. As mentioned above, the speed conditions for surge occurrence are M > 1.4...1.5.

If the flight takes place at a lower speed, then the oblique shock waves are less intense and are located at a greater angle to the braking surface (that is, less inclined), and therefore are located further (relatively of course) from the entrance plane and the air intake shell. In this case, the head wave, when exposed to backpressure, can move against the flow without the risk of destroying the shock system. That is, surging does not occur even with a large degree of engine throttling.

There are also constructive and technical ways to prevent this phenomenon. The simplest one – the use of so-called bypass flap. The principle here is clear: surging is prevented (or eliminated) by bypassing “extra” air from the air intake channel behind the throat. This reduces the back pressure that knocks out the head wave. Or, to put it simply, overflow of the air intake is eliminated.

Second constructive method is associated with a change in the throughput of the input device or, more precisely, the throughput of the shock wave system at the inlet to the air intake. But more on this below, but for now about one more unstable mode of operation of the air intake.

2. Itching of the entrance device.

The name is funny, but it's spot on. Itching is in some ways the opposite of surging, although it has virtually no effect on air flow. It represents pressure fluctuations with a fairly high frequency (100...250 Hz) and low amplitude (5...15% of the initial pressure). It occurs only in deep supercritical operating modes of the air intake, when the engine requires a lot of air and the air intake does not meet these needs.

As already mentioned, in this case, a supersonic flow with a shock wave S appears behind the throat. The interaction of this shock with the boundary layer of the flow becomes the reason for its non-stationarity. The further along the channel the shock is located, the thicker the boundary layer and the higher the intensity of the shock. Separation zones appear and increase, increasing the unevenness of the flow.

Diagram of the occurrence of air intake itching.

In these zones, periodic pressure fluctuations occur with a fairly high frequency. These pulsations are joined by high-frequency oscillations of the shock itself. They, in turn, affect the cladding and structural elements. It is these structural vibrations that “itch”, and quite unpleasantly.

Itching air intake Compared to surge, it is not so dangerous, however, due to the unsteadiness of the flow generated by it, it negatively affects the operation of the compressor in terms of reducing the stability of its operation. In addition, high-frequency vibrations can disrupt the operation of instruments and units located in the airborne area, and physiologically have an unpleasant effect on the pilot, whose workplace is most often located close to their source.

Itching is eliminated by throttling the engine, that is, reducing its need for air and eliminating the acceleration of the flow behind the throat. And it is prevented by using drainage and suction of the boundary layer, as well as its turbulization. Devices for this were mentioned above.

Another effective method is similar to the second method of dealing with surge. This is a change in the air intake capacity. That is, the use of the so-called adjustable input device.

Adjustable supersonic air intakes.

All previous descriptions of air intakes and their features implied that they have a stationary, unchangeable geometry. That is, initially, during design, the input device is calculated for a specific operating mode, which is called the design mode (the shock waves are focused on the shell). During operation, its geometric dimensions and shape do not change.

However, in actual operation, the air intake does not always operate at its design level, especially for maneuverable aircraft. Atmospheric parameters and flight parameters, air intake and engine operating modes are constantly changing, and their combination most often does not fit into the concept of “calculated”.

This means that for the power plant as a whole, sufficiently high performance cannot always be achieved. Therefore, the goal of the designers (in our case, the designers of the air intake of the turbojet engine) is to achieve the maximum possible coordination of the operating modes of the air intake and the engine in order to obtain the best possible efficiency characteristics of the entire power plant and at the same time ensure stable and safe operation of the VCA in all combinations of modes possible in operation engine operation, parameters and flight conditions.

It is worth noting that the words “if possible” are used here for the reason that the requirements for maintaining high efficiency indicators (low total pressure loss, high pressure ratio, low resistance and sufficient flow) at the same time as a large margin of stability are contradictory.

For example, from the point of view of maintaining high efficiency and the absence of flow pulsations due to the interaction of the boundary layer with the shock S, the subcritical operating mode of the air intake is more advantageous. However, the stability is low, disturbances can propagate against the flow (subsonic in the channel), and the operating parameters approach the surge limits.

On the contrary, in the supercritical regime the bow wave is far from the system of oblique shocks, and the stability of the air shock is high. But on the other hand, the efficiency decreases, in particular due to the effect of the S jump on the boundary layer. With deep overcriticism, this jump is so close to the exit from the OT that the likelihood of itching increases significantly.

Therefore, in practice, one has to choose something in between and often allow for some reduction in efficiency for reasons of ensuring stable operation modes of the air intake. This is facilitated, in particular, by the shape of the flow part (like a Laval nozzle), which, in principle, is more conducive to operation in a supercritical mode.

For traditional air intakes with a constant geometry, the possibilities for achieving the above-mentioned coordination of operating modes are not too high, especially if the aircraft are designed for operation at high supersonic speeds (M>2). This means that the speed range of the aircraft on which they are installed will not be very wide.

Therefore, almost all modern supersonic input devices equipped with a geometry changing system in order to ensure coordinated work with the engine throughout the entire speed range.

Physical meaning of IED regulation is to ensure compliance of the air intake capacity with the engine capacity in all modes of its operation and all operational Mach numbers of the flight. The capacity of the air intake is determined by the capacity of the jump system and throat.

Regulation occurs due to the movement of the so-called wedge, consisting of several panels - for flat (box-shaped) air intakes, or due to the axial movement of a special stepped cone (central body) - for axisymmetric air intakes. In this case, the position of the shock waves and the area of ​​the throat change, and therefore the throughput and stability margin.

Picture of flat air intake regulation. The rotating edge of the shell is shown.

Picture of the regulation of the frontal axisymmetric air intake. The feed and bypass valves are shown.

In a simplified form, extending the wedge with increasing speed looks like blocking the air intake channel (or its throat) so as not to allow excess air to pass there.

In fact, with this extension and the corresponding change in the position of the shock waves (inclination angles), the cross-sectional area of ​​the air stream captured by the air intake decreases, because the air, passing through the shock waves and moving parallel to the braking surface, spreads to the sides. Because of this, part of the jet (outer layers) simply does not enter the channel. As a result, the volume of air entering the air intake decreases (mentioned above).

For an axisymmetric VCA, the control process is similar. Only when the cone is extended, the oblique shock waves do not change their inclination and relative position. However, in exactly the same way there is a decrease in the cross-sectional area of ​​the air stream captured by the air intake, and a decrease in the throat area due to the so-called “ undercut angle» shells, because the throat itself moves towards the entrance when the cone is extended.

Physical picture of the control of the VCA (axisymmetric with a cone is shown). There is a decrease in the actual air intake capacity.

Control elements can also be additional flaps on the front edge of the shell ( rotary shell) And bypass flaps, which for different types of air intakes help solve the problem of maintaining the required flow rate and stability margin.

For example, for axisymmetric (head-on) IEDs, in which the extension of the cone, according to design conditions, ends before the aircraft reaches the maximum flight Mach numbers, the opening of the bypass valves located behind the throat prevents excessive removal from the entrance of the head wave, thereby reducing drag and increasing the stability margin input device.

On other aircraft, the bypass flaps play the role of an anti-surge device and operate only under certain conditions: deep throttling of the engine, turning off the afterburner, etc.

During takeoff and low-speed subsonic flight, it is important to open the throat as much as possible to ensure increased air flow, as well as to reduce the possibility of flow stalling from the sharp edges of the shell. Therefore, the wedge panels (or steerable cone) are set to the fully retracted position.

In addition, for starting conditions in VCA with similar purposes, the ones already mentioned above (for subsonic and transonic VZ) can be applied. additional air supply flaps, installed behind the throat of the air intake.

These flaps open inward under the influence of vacuum created in the air intake channel when the engine is running at takeoff or in flight at low speeds. When the required speed is reached and the vacuum decreases, the flaps close. It is also possible to automatically open and close such doors from hydraulic (Su-24M) or electrical systems.

Su-24M aircraft on landing course. Transonic air intakes. The open right recharge flap is visible.

The use of such flaps ensures a reduction in thrust losses during takeoff (there is enough air) and makes it possible to increase the stability of the compressor by reducing the intensity of stall phenomena at sharp inlet edges (for SVU and transonic air intakes).

For flat air intakes The existing possibilities for air flow control are significantly wider, so they often do not require the use of bypass flaps (as well as make-up flaps).

MiG-31BM. The rotating edge of the shell is clearly visible.

In addition, such air intakes have the ability to deflect the leading edge of the shell (change the “undercut angle”), which allows you to change the geometric area of ​​the entrance. Inward deflection reduces it and allows the head wave to be kept near the leading edge of the shell at moderate supersonic speeds, which increases the stability of the IED.

IED of the prototype E-155M aircraft. The removed wedge and traces of its movement are visible (on the outer wall). As well as perforation and a rotating edge of the shell (bottom edge).

And the outward deflection ensures a smooth flow entry into the channel and reduces losses associated with its separation. This is important, as already mentioned, in take-off conditions (low speed and high angles of attack), when large losses are possible due to flow disruption from the sharp leading edges of the IED shell. In particular, the MiG-25 and MiG-31 aircraft have such an air intake.

IED of a MiG-25 aircraft with an open shell flap.

IED of the MiG-25 aircraft. The perforation, the rotating edge of the shell (below) and the trace from the movement of the wedge (retracted up) are visible.

In air intake control systems, in principle, separate control of surge capacity and throat area can be used, when each panel is controlled separately according to its own program. This is the so called multi-parameter control.

However, in this case the system turns out to be too complex. Therefore, in practice it is used single-parameter control, when all panels are connected kinematically and controlled by the movement of only one main hinge. That is, some average control mode is selected - single-parameter.

The control of the air intake mechanization elements is automatic, but manual control is also available, used only in emergency cases. A special control program takes into account external flight factors (Mach number, air temperature) and engine rotor speed. Usually the program is formed according to the already specified engine consumption parameters.

Influence of angles of attack and slip.

Supersonic input devices quite sensitive to change angles of attack and slip. The final response of different types of air intakes may differ, but in general such a change is harmful. An increase or decrease in flow angles changes the position and intensity of shock waves, which affects the throughput, the amount of losses and the stability margin air intake.

For example, for frontal axisymmetric input devices at large positive or negative angles of attack, the symmetry of the flow around the braking surface changes significantly. On the windward side, the intensity of the shocks increases, which means the pressure in the flow behind the shocks increases. On the leeward (shaded) side the process is opposite, here the degree of pressure increase decreases.

Flow around a frontal air intake at high angles of attack.

As a result, a transverse flow of flow occurs in the channel and on the braking surface from areas with lower pressure to areas with higher pressure, which causes the boundary layer to flow down, thicken and separate. The consequence is unsteadiness of the flow, a decrease in stability and actual air flow.

For flat air intakes, the degree of influence of changes in angles of attack is largely determined by the location of the air intake relative to the aircraft structural elements.

To improve performance air intakes at positive angles of attack (both frontal and flat), their geometric axis is often located at some negative angle to the horizontal plane of the aircraft. This angle is called " wedging angle" It is usually -2 ˚…-3 ˚. This measure makes it possible to reduce the magnitude of the incoming flow angles when flying at high angles of attack.

A similar angle of inclination is often formed on low-speed airways. For example, on subsonic air intakes (passenger aircraft), the entrance plane can be inclined with the upper sector forward (mentioned above).

Similar measures for turning the geometric axis can be used for more comfortable flow when flying with a glancing angle.

In some air intakes, special partitions are installed at the initial section of the internal channel to level the flow and streamline the velocity field.

Input devicesDSI .

For modern fighter aircraft, their practical speed is usually limited to a Mach number of 2 (or even less). This also applies to the recently introduced fifth generation aircraft. In this regard, the ideas of using uncontrolled air intakes for them are being considered and are already being put into practical application (F-22, F-35).

The point is also that air intake control systems complicate the design, thereby reducing reliability, and add weight. In addition, the complex spatial airspaces of new aircraft often make it difficult to effectively control surfaces of complex configurations.

However, the rather high requirements for such air intakes, based on the high specified characteristics of newly developed equipment, especially 5th generation fighters, force us to look for ways to improve them and improve the parameters that they always had on aircraft created in previous years.

Options such as low radar signature And supersonic cruising flight(albeit not too large) are normal requirements for a 5th generation aircraft. This means that all design features that increase radar visibility should be leveled out if possible. The total pressure loss in the air intake must also be reduced.

An important step on this path was the relatively new input device, so-called air intake DSI. In particular, it uses two ideas to improve the air intake by reducing pressure losses.

First– this is an increase in the number of compression shocks. The more there are, the smaller the losses. Theoretically, increasing the number of shock waves to infinity reduces the total pressure loss to zero.

Second. Shock waves generated by a cone have a smaller angle of inclination than shock waves generated by a wedge (the angles at the apex of the cone and wedge are equal). Therefore, from the point of view of total pressure loss during braking in the air intake, a frontal axisymmetric air intake is considered more advantageous. However, it cannot always be arranged in a design.

Experimental MiG-23PD with sector air intakes.

A compromise in this sense was the so-called sector air intakes(mentioned above - aircraft such as Mirage, F-111, MiG-23PD, Tu-128), in which the central body is in air intake a part (sector) of the cone protrudes. The efficiency of such air intakes can be higher than that of conventional flat side ones.

F-111C with sector air intake.

In the DSI air intake, a new element is the so-called ramp, which is a braking (compression) surface at the entrance to the air intake and has a shape similar to that of part of the cone surface. That is, the flow here is also conical (optimal for the air intake).

Conical braking surface of the DSI air intake.

In addition, the special swept (or oblique) edges of the shell of such an air intake also create multiple compression waves (in other words, a fan of compression waves (or shock waves in supersonic conditions)).

As a result, in addition to the so-called spatial compression, these waves, in interaction with the conical flow on the ramp, under certain conditions, have unfolding action in the transverse direction on the streamline on it, that is, on the boundary layer running from the fuselage elements located in front of the air intake. It drains outside the air intake, which reduces total pressure losses and increases operating stability.

Pattern of boundary layer streamlines for a DSI air intake.

With sufficient supersonicity, that is, in the design mode, depending on the shape of the air intake edge, under the influence of compression waves from it, a larger volume of the boundary layer can be drained outside the air intake. For an oblique edge at M1.25 - up to 90%, for a swept edge in the shape of a “fang” - at M1.4 - up to 85%.

The actions to drain the boundary layer are reflected in the very abbreviation of the name of such an air intake - DSI (diverterless supersonic inlet). Literally translated, this abbreviation means something like “air intake without diverter.” The word “diverter” here, of course, is artificial and means the traditional channel for draining the boundary layer, which is available on aircraft with adjacent air intakes(mentioned above).

This channel is quite wide and significantly increases radar signature airplane. Thus, DSI air intakes provide an advantage in this regard, since they do not have a special channel for draining the PS, which, by the way, has a positive effect on reducing aerodynamic drag. In addition, the ramp protrusion significantly blocks the air intake clearance, reducing the direct visibility of the blades of the first stage of the engine compressor, which is also quite important from the point of view of reducing radar signature.

Experimental XF-35. The ramp and the edge of the DSI fang-type air intake are clearly visible.

F-35 fighter with DSI air intakes. The conical braking surface - the ramp - is clearly visible.

An example of this type of air intake can be the air intake of F-35, XF-35 aircraft. The XF-35 has a fang-type air intake lip.

In fairness...

Still, it is worth noting that the calculation and design of new spatial uncontrollable air intakes and air ducts are a complex and expensive matter. Such, for example, as the F-22, which also has S-shaped air channels from the air intake to the engines.

Fighter -22 with spatial unregulated air intakes.

In the off-design mode, the operation of such air intakes, despite all their advanced technology, will necessarily be accompanied by losses, which means less efficiency of the power plant. But there are many such modes.

Controllable air intakes these losses, one might say, do not exist. In this case, the operation of the air intake-engine system is optimized for all modes, is quite predictable, controllable and has high efficiency parameters.

Therefore, choosing the type of air intake is a kind of compromise that forces you to take into account many, often conflicting, factors. For example, the T-50 fighter has adjustable spatial compression air intakes. The F-22 has spatial unregulated air intakes.

Airplane T-50. Controlled VCA with spatial compression.

At the same time, the Russian fighter is a worthy competitor to the American (even superior in many respects) despite the lower stand thrust of the engines, and even at a significantly lower cost. It is likely that the efficiency of the F-22 power plant in off-design modes (especially during fast maneuvering) is not as high as stated in open sources.

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We'll probably end here. I hope that the main provisions of this, in fact quite difficult to understand and extensive topic, have ceased to be incomprehensible. Thank you for reading to the end. Until new meetings and articles.

At the end I will add pictures that “didn’t fit” into the main text.

Frontal axisymmetric air intake of the Su-17 aircraft.

Mechanics of adjustment of axisymmetric and flat air intakes.

Feed flaps on the NK-8-2U engine (Tu-154B-2 aircraft). Opened during takeoff.

MiG-21-93 fighter. Frontal axisymmetric air intake with adjustable cone.

Replenishment flaps on a Harier fighter.

Sector IED of the F-111 aircraft.

F-22 air intakes.

F-5 aircraft with transonic air intakes.

Use: on aircraft of various types and purposes operated from ground airfields. The essence of the invention: in the front part of the air intake channel there is an additional upper entrance, equipped with a protective device in the form of a solid flap hinged in the upper part of the channel, interacting with the upper additional and main entrances, and the make-up flaps are located in the upper part of the air intake channel behind the additional upper entrance. 2 ill.

The invention relates to aviation technology and can be used on aircraft of various types and purposes operated from ground airfields. During the operation of aircraft with gas turbine engines in ground conditions in engine operating modes on the ground and during takeoff and landing modes, various foreign objects that find themselves on the runway (grains of sand, gravel, concrete fragments, random metal parts, etc.). The entry of such objects into air intake ducts can cause significant damage to aircraft engines. Considering the difficulty of ensuring the absence of foreign objects on the runway, partly resulting from the destruction of the runway itself during its operation, for airfields intensively operated in various weather conditions, and the dangerous consequences for the aircraft and its crew, there is a need to develop various devices to protect aircraft air intakes from foreign objects entering them. Known protective devices for the air intakes of gas turbine engines of aircraft against the ingress of foreign objects prevent the tossing (or reducing the height of the throwing) of foreign objects from the surface of the runway and their further suction into the air intake channel during engine operation (jet protection systems), and carry out the separation of solid particles that have entered the air intakes with their removal from the air flow entering the engine (separator protection systems) or mechanically do not allow foreign particles exceeding certain geometric dimensions to pass into the air intake channels, mesh protection systems (Airkraft Flight Conference Zhukovksy, Russia, August 21 September 5, 1993, TsAGI, with .148-156). The disadvantages of jet protection systems that blow air jets onto the surface of the airfield and prevent the formation of a vortex that throws foreign objects to the entrance of the air intake are the dependence of the degree of protection of the air intake on the size and weight of foreign particles, on the presence and strength of the side wind above the surface of the airfield, as well as the practical impossibility protection using such systems from foreign objects thrown by the chassis wheels. The disadvantages of separator systems for protecting air intakes, based on the use of the inertial properties of foreign particles trapped in the air intake channel and moving with the air flow, are the need for special profiling of the air intake channel with the formation of special additional channels for removing part of the air with separated particles from the main channel, as well as the dependence the degree of separation from the specific gravity of foreign particles entering the air intake channel and changes in air flow through the air intake channel, which, in turn, depend on the engine operating mode and often cause the difficult to implement need to regulate the separation process. The disadvantages of mesh protection systems are the possibility of providing protection using such systems only from foreign particles exceeding the size of the cells of the meshes used, the danger of icing of the protective meshes under certain weather conditions and significant pressure losses entering the air intakes caused by the hydraulic resistance of the meshes and increasing with decreasing the sizes of their cells. To improve the characteristics of air intakes during takeoff and landing modes, make-up flaps are used, located on the side (Air fleet technology. 1991, N4, p. 52) or bottom (Nechaev Yu.N. Theory of aircraft engines. VVIA named after N. E. Zhukovsky, 1990, p.255-259) side of the air intakes. The closest to the proposed one is an air intake with a mesh protection system (US patent N 2976952, class B 64 D 33/02 (F 02 C 7/04), 1961), containing the main entrance, make-up flaps, panels forming the air intake channel, and rotary protective device installed in the channel. The disadvantages of this technical solution are the implementation of protection against foreign particles that can enter the air intake only from the side of the air intake entrance and only those exceeding the size of the cells of the meshes used, the danger of icing of the protective nets under certain weather conditions and significant pressure losses of the air entering the air intakes caused by hydraulic mesh resistance and increasing with decreasing cell sizes. However, this technical solution does not provide protection against foreign particles entering the air intake channel through the openings of the make-up flaps. The purpose of the invention is to increase the efficiency of eliminating the entry of foreign objects into the air intake channel when working on site and during takeoff and landing modes. The goal is achieved by the fact that the air intake channel is made with an additional upper entrance in the front part of the channel, the protective device is made in the form of a solid flap, hinged in the upper part of the channel with the ability to interact with the upper additional and main inputs of the air intake, the make-up flaps are located in the upper part of the air intake channel after the additional upper entrance. Making an air intake channel with an additional entrance in the front part of the channel and making a protective device in the form of a solid flap hinged in the upper part of the channel with the ability to interact with the upper additional and main inputs of the air intake and placing the make-up flaps in the upper part of the air intake channel in neither the patent nor the technical literature were not found, and therefore it is concluded that the invention meets the criteria of “novelty” and “significant differences”. In fig. 1 shows a diagram of an aircraft air intake; Fig. 2 is a graph of the dependence of the values ​​of the total pressure recovery coefficient in the section of the air intake channel corresponding to the plane of entry into the engine compressor, in the modes of coordinated operation of the air intake with the engine and comparison of the obtained values ​​with the level of their standard values ​​in takeoff and landing flight modes corresponding to the range of Mach numbers flight M 0.0.25. The air intake 1 of the aircraft (Fig. 1) contains the main entrance 2, make-up flaps 3, panels 4 forming the air intake channel, ending with the plane 5 of the entrance to the engine compressor, a rotary protective device 6 installed in the channel and an upper additional entrance 7. When working on site and during takeoff and landing flight modes, the rotary protective device 6 rotates and closes the main entrance 2, opening the additional upper entrance 7; the replenishment doors 3, located behind the additional upper entrance, open. When leaving the range of takeoff and landing flight conditions, the rotary protective device 6 rotates and closes the additional upper entrance 7, opening the main entrance 2, the make-up doors 3 are closed. In Fig. 2, curve 8 is the dependence obtained in experimental studies, line 9 is the standard dependence of the level of values ​​( Nechaev Yu.N. Theory of aircraft engines. VVIA named after N. E. Zhukovsky, 1990, p. 287). The use of the proposed technical solution ensures that when working on site and during takeoff and landing flight conditions, foreign objects do not enter the air intake channel, since for this technical solution in the operating modes under consideration, air is taken into the air intake channel from the upper hemisphere of the surrounding space, and not from the lower, as in technical solutions of analogues and prototypes. This ensures that the total pressure recovery coefficient is at or above its standard values.

Claim