What's the difference between turbojet and turbofan? What are their respective definitions?

turbojet

history

Turbojet engine is a kind of turbine engine. It is characterized by completely relying on airflow to generate thrust. Usually used as power for high-speed aircraft. Fuel consumption is higher than that of turbofan engine. There are two types of turbojet engines: centrifugal and axial. Centrifugal type was patented by British Sir Frank Whittle in 1930, but it was not until 194 1 that an airplane equipped with this engine went to heaven for the first time. It did not participate in World War II, but was born in Germany, and participated in 1945 as the first practical jet fighter Me-262. Compared with centrifugal turbojet engine, axial flow has the advantages of small cross section and high compression ratio. Today's turbojet engines are all axial flow.

Reply: turbojet engine

structure

admitting port

The main structure of axial-flow turbojet engine is shown in the figure. Air enters the inlet first, because the flight state of the aircraft is changing, and the inlet needs to ensure that the air can finally enter the next structure smoothly: the compressor. The main function of the air inlet is to adjust the air to the state that the engine can run normally before entering the compressor. When flying at supersonic speed, both the nose and the inlet will produce shock waves, and the air pressure will increase after passing through the shock waves, so the inlet can play a certain role in precompression, but the improper position of the shock waves will cause uneven local pressure and even damage the compressor. Therefore, there is a shock wave adjusting cone at the inlet of supersonic aircraft, and the position of shock wave is adjusted according to airspeed.

Aircraft with air intake on both sides or belly will be affected by the boundary layer (or boundary layer) of the fuselage because the inlet is close to the fuselage, and a boundary layer adjustment device will be installed. The so-called boundary layer refers to a layer of air flowing close to the surface of the fuselage. Its velocity is much lower than the surrounding air, but its static pressure is higher than the surrounding air, forming a pressure gradient. Because of its low energy, it is not suitable for entering the engine and needs to be eliminated. When the aircraft has a certain angle of attack (AOA), due to the change of pressure gradient, boundary layer separation will occur at the part where the pressure gradient increases (such as leeward side), that is, the boundary layer that was originally close to the fuselage will suddenly separate at a certain point and form turbulence. Turbulence is relative to laminar flow, which is simply an irregular moving fluid. Strictly speaking, all flows are turbulent. At present, the mechanism and simulation of turbulence are not clear. But that doesn't mean turbulence is not good. In many parts of the engine, such as the combustion process, we should make full use of turbulence.

compressor

The compressor consists of stator blades and rotor blades. A pair of stator blades and rotor blades is called the first stage. The stator is fixed on the engine frame, and the rotor is connected with the turbine through the rotor shaft. The active turbojet engine is generally an 8- 12 compressor. The more stages, the greater the pressure. When the fighter suddenly performs a high-g maneuver, the air pressure flowing into the front stage of the compressor will drop sharply, and the pressure in the rear stage will be very high. At this time, the high-pressure air in the rear stage will expand reversely, and the engine is extremely unstable, which is called "surge" in engineering. This is the most fatal accident of the engine, which is likely to cause shutdown or even structural damage. There are several ways to prevent "surge" Experience shows that surge mostly occurs between the 5th and 6th stages of the compressor, and a bleed ring is set in the second interval to release the pressure in time when the pressure is abnormal, so as to avoid surge. Or the rotor shaft is made into two concentric hollow cylinders, which are respectively connected with the front-stage low-pressure compressor and the turbine, and the rear-stage high-pressure compressor and the other turbine. The two rotor groups are independent of each other, so the speed can be automatically adjusted when the pressure is abnormal, and surge can also be avoided.

Combustion chamber and turbine

Air enters the combustion chamber after being compressed by the compressor, and is mixed with kerosene for combustion, and expands to do work; And then flows through the turbine to drive the turbine to rotate at high speed. Because the turbine and the compressor rotor are connected on the same shaft, the rotation speed of the compressor and the turbine is the same. Finally, the high-temperature and high-speed gas is ejected through the nozzle, and the power is provided through the reaction. At first, the combustion chamber was several small cylindrical combustion chambers, which were juxtaposed in a ring around the rotor shaft. Each cylinder is not sealed, but a hole is opened in a proper place, so that the whole combustion chamber is connected. Later, it developed into a compact annular combustion chamber, but the overall fluid environment was not as good as that of a cylindrical combustion chamber, and a combined combustion chamber combining the advantages of the two appeared.

Steam turbines always work under extreme conditions, and there are extremely strict requirements for their materials and manufacturing technology. At present, powder metallurgy hollow blades are mostly cast in one piece, that is, all blades and disks are cast at one time. Compared with the early days, each blade and disc were cast separately and then connected by tenon, which saved a lot of connection quality. The manufacturing materials are mostly high temperature resistant alloy materials, and the hollow blades can be cooled by cold air. The new engine developed for the fourth generation fighter will be equipped with ceramic powder metallurgy blades with more outstanding high temperature performance. These measures are aimed at improving one of the most important parameters of turbojet engine: the pre-turbine temperature. High pre-swirl temperature means high efficiency and high power.

Nozzle and afterburner

The shape and structure of the nozzle (or nozzle) determine the airflow state that is finally eliminated. Early low-speed engines used simple convergent nozzles to improve speed. According to Newton's third law, the greater the ejection velocity of gas, the greater the reaction force the aircraft will get. However, the growth rate of this method is limited, because the final airflow speed will reach the speed of sound, and then there will be a shock wave to stop the increase of gas speed. Supersonic jets can be obtained by using convergent-divergent nozzles (also known as Laval nozzles). The maneuverability of the aircraft mainly comes from the aerodynamic force provided by the wing surface. When the maneuverability is high, the thrust of the jet can be directly used. There are two schemes in history, that is, installing gas control surface at the nozzle or directly using deflectable nozzle (also known as thrust vector nozzle or vector thrust nozzle), and the latter has entered the practical application stage. The superb maneuverability of the famous Russian Su -30 and Su -37 fighters benefited from the AL-3 1 thrust vector engine of Rurika Design Bureau. The representative of the rudder is the American X-3 1 technical verification machine.

After the high-temperature gas passes through the turbine, it still contains some oxygen that is not consumed in time. If kerosene is continuously injected into this gas, it can still burn and generate additional thrust. Therefore, some high-performance fighter engines add afterburner (or afterburner) behind the turbine to achieve the purpose of greatly improving the engine thrust in a short time. Generally speaking, afterburner can increase the maximum thrust by 50% in a short time, but the fuel consumption is amazing. Generally, afterburner is only used for take-off or fierce air combat, and cannot be used for long-term supersonic cruise.

Reply: turbojet engine

service condition

Turbojet engines are suitable for a wide range of navigation, from low-altitude subsonic to high-altitude supersonic aircraft. MiG -25 is a legendary fighter in the former Soviet Union. It used the turbojet engine of Liurika Design Bureau as the power, and once set a fighter speed record of Mach 3.3 and a ceiling record of 37,250 meters. This record is unlikely to be broken for some time.

Compared with turbofan engine, turbojet engine has poor fuel economy, but its high-speed performance is better than turbofan engine, especially at high altitude and high speed.

Basic parameter

Thrust-to-weight ratio: Thrust-to-weight ratio represents the ratio of engine thrust to the weight of the engine itself. The greater the thrust, the better the performance.

Compressor stage: indicates how many stages there are compressor blades. Usually, the larger the stage, the greater the compression ratio.

Turbine stage: How many stages are there in the turbine blades of a turbine?

Compression ratio: the ratio of the pressure of intake air compressed by compressor to the pressure before compression. Usually, the greater the pressure, the better the performance.

Maximum net thrust at sea level: at sea level and conditions, when the speed difference (airspeed) between the engine and the outside air is zero, the thrust generated by the engine running at full speed. Units used include kN (kilonewtons), kg (kilograms) and lb (pounds).

Fuel consumption per hour per unit thrust: also known as specific thrust, the ratio of fuel consumption to thrust, metric unit kg/N-h, the smaller, the more fuel-efficient.

Pre-turbine temperature: the temperature before the high-temperature and high-pressure airflow after combustion enters the turbine. Generally, the higher the temperature, the better the performance.

Gas outlet temperature: the temperature at which the exhaust gas leaves the turbine and is discharged.

Mean time between failures: the total average time between two failures of each engine. The longer the time, the less likely it is to fail, and usually the lower the maintenance cost.

Turbofan

A gas turbine engine in which gas discharged from a nozzle and air discharged from a fan generate a reverse thrust. The turbofan engine consists of a fan, a compressor, a combustion chamber, a high-pressure turbine driving the compressor, a low-pressure turbine driving the fan and an exhaust system. Compressor, combustion chamber and high-pressure turbine are collectively called core engine. Part of the gas discharged from the core engine can be transferred to the low-pressure turbine-driven fan with energy, and the rest is used for accelerating the gas discharged from the nozzle. The rotor of the fan is actually a compressor with 1 stage or several stages of long blades. After the air flows through the fan, part of it flows into the core machine, which is called internal airflow, and is discharged from the nozzle at high speed, generating thrust, while the other part flows around the periphery of the core machine, which is called external airflow, which also generates thrust. This kind of internal and external ducted turbofan engine is also called internal and external ducted engine. The ratio of air flow through the outer culvert and the inner culvert is called culvert ratio or flow ratio. The bypass ratio has a great influence on the performance of turbofan engine. The bypass ratio is large and the fuel consumption is low, but the windward area of the engine is large. When the bypass is small, the windward area is small, but the fuel consumption rate is high. A turbofan engine with two air streams discharged into the atmosphere is called a split turbofan engine. When the two air streams inside and outside the duct are mixed with each other in the mixer behind the internal turbine and then discharged into the atmosphere through the same nozzle, it is called a hybrid turbofan engine. Turbofan engine can also be equipped with afterburner, which becomes afterburner turbofan engine. The afterburner on the split-row turbofan engine can be installed behind the internal turbine or in the external duct, or behind the mixer on the mixed-row turbofan engine.

When the core engine is the same, the working fluid flow of turbofan engine is between turbojet engine and turboprop engine. Compared with turbofan engine, turbojet engine has higher working fluid flow, lower injection speed, higher propulsion efficiency, lower fuel consumption and higher thrust. The first generation turbofan engine developed in 1950s has lower bypass ratio, compressor pressure ratio and gas temperature, and its fuel consumption is only about 25% lower than that of turbojet engine, which is about 0.06 ~ 0.07 kg/n-h (0.6 ~ 0.7 kg/kg-hr). At the end of 1960s and the beginning of 1970s, the second generation turbofan engine with high aspect ratio (5 ~ 8), high pressure ratio (25 ~ 30) and high gas temperature (1600 ~ 1750k) was developed, and the fuel consumption was reduced to 0.03 ~ 0.04kg/n-h (0). Compared with turbofan engine, Gao Han Road has lower noise and less exhaust pollution, and is mostly used as the power plant of large passenger aircraft. At the altitude of 1 1 km, the cruising speed of this passenger plane can reach 950 km/h. However, this kind of turbofan engine with high bypass ratio is not suitable for supersonic aircraft because of its low exhaust injection speed and large windward area.

Some fighters use turbofan engines with small bypass ratio and afterburner, but don't use afterburner when flying at subsonic speed. Fuel consumption and exhaust temperature are lower than those of turbojet engines, so the infrared radiation intensity is weak and it is not easy to be hit by infrared guided missiles. When flying at the speed of sound with more than twice the additional force, the thrust generated can exceed that of the afterburner turbojet engine, and the thrust-to-weight ratio has reached about 8 under the standard atmospheric conditions on the ground. Some fighters use turbofan engines with small bypass ratio and afterburner, but don't use afterburner when flying at subsonic speed. Fuel consumption and exhaust temperature are lower than those of turbojet engines, so the infrared radiation intensity is weak and it is not easy to be hit by infrared guided missiles. When flying at the speed of sound with more than twice the additional force, the thrust generated can exceed that of the afterburner turbojet engine, and the thrust-to-weight ratio has reached about 8 under the standard atmospheric conditions on the ground.

When the aircraft speed is lower than about 450 mph (724 km/h), the efficiency of pure jet engine is lower than that of propeller engine, because its propulsion efficiency depends largely on its flight speed; Therefore, pure turbojet engine is most suitable for higher flight speed. However, due to the airflow disturbance caused by the high tip speed of the propeller, the efficiency of the propeller drops rapidly above 350 mph (563 km/h). These characteristics make some medium-speed aircraft use a combination of propeller and gas turbine engine-turboprop engine, rather than a simple turbojet device.

Definition and concept of turboshaft engine;

Aviation turboshaft engine is a kind of gas turbine engine with air as working medium. It is mainly a gas turbine engine driven by output power, which drives most of the effective power (above 95%) of the power turbine through the output shaft. Turboprop engine is a gas turbine engine that uses a gas turbine to drive a propeller. The principle of aerodynamic thermal cycle of turboshaft/turboprop engine is basically the same as that of large turbojet/turbofan engine. Although the technical achievements and experience gained from the development of large gas turbine engines can be used for reference, the turboshaft/turboprop engine belongs to small gas turbine engines, so it has its own unique aerodynamic and structural characteristics:

(1) The "size effect" caused by small flow rate and small passage has adverse effects on compressor, turbine performance and cooling.

(2) High rotational speed-High rotational speed brings a series of new problems to critical vibration, high-speed bearing, shafting, support and fatigue strength of bladed disk.

(3) The flow is complicated-the short blade profile of the small turbine blade increases the flow turning point, and the three-dimensional characteristics and viscosity are prominent;

(4) The cooling effect is poor-the small turbine blades are short and thin, and the relative external surface area is large, but the internal cooling holes are difficult to arrange, the cold air flow is short, and the cooling effect decreases with the size reduction;

(5) Air inlet protection device (particle separator) is required.

The advantages of turboshaft engine are:

High power to weight ratio (500-600kW engine, almost 2 times higher than piston engine); Simple engine maintenance (especially at low temperature, no need to warm up and start); Small vibration (no reciprocating parts, high balance accuracy of engine rotor); The smaller maximum cross section improves the aerodynamic performance of the helicopter. Therefore, since the 1950s, the turboshaft engine has gradually replaced the piston engine and become the main power plant of the helicopter. Of course, it also has some shortcomings: the high speed of power turbine and the large reduction ratio of transmission rotor make the reducer large and complicated; The fuel consumption rate is generally slightly higher than that of piston type; Surrounding media (dust, humidity and temperature in the air) have great influence on its work; There are also small-sized turboshaft engines that are difficult to produce. After more than 40 years of continuous research, development and upgrading, modern turboshaft engines have the following characteristics:

(1) advanced performance: take-off fuel consumption is 0.267-0.358kg/(kW/h); (kw/h); (kW/hour); The power-to-weight ratio is 4-8kw/Dan;

(2) Good economy: the fuel consumption can reach 0.299-0.367kg/(kW/h) during cruising, with low maintenance cost and long service life (3000-5000 h per unit life);

(3) High reliability: low engine replacement rate in advance, long mean time between failures and low performance attenuation rate;

(4) Technical development potential: good power coverage and modification possibility;

(5) Strong environmental applicability: The power of the armed helicopter has strong sand prevention ability (generally with particle separator), infrared suppression ability, combat damage resistance and crash prevention ability.

Since1953; Since the Datt engine of Romanian company was put into use, turboprop engine became an important power device for civil and military transport aircraft at that time. The largest is the former Soviet Union's HK 12MB, and the takeoff power is 1 1000kW. Compared with piston engine, turboprop engine has high reliability and light weight, but its fuel economy is far lower than that of early pure jet engine. Due to the appearance of turbofan engine in 1960s, turboprop engine gradually withdrew from the field of large transport aircraft, but it is still widely used in the field of small and medium-sized aircraft.

Overview of foreign countries:

From T53, the first mass-produced engine developed by Lycoming Company in 1953 to today, three generations of turboshaft engines have been put into use, and the fourth generation is under development. The first generation refers to the turboshaft engine put into use in the 1950s, the second generation refers to the 1960s, the third generation refers to the turboshaft engine put into use in the late 1970s and early 1980s, and the fourth generation refers to the turboshaft engine put into use in the late 1990s or early 20th century.

After more than 40 years of development, the technical level of foreign turboshaft engines has been greatly improved;

(1) The fuel consumption rate decreases. The fuel consumption of the fourth-generation turboshaft engines, such as American T800 and Western Europe MTR390, is about 8% lower than that of the third-generation turboshaft engines with the same power level, reaching 0.273kg/(kW/h).

(2) The unit power is increased. Because the power levels of the third and fourth generation turboshaft engines are not the same, it is the best scheme to use the unit power as the performance index of turboshaft engines. For more than 40 years, the unit power has been growing steadily. For example, the unit power of T58 engine is166 kW/(kg/s) for products produced in the 1950s in the United States. The unit power of the second generation T64 turboshaft engine is197 kW/(kg/s); The unit power of the third generation T700 engine is 267 kW/(kg/s); The unit power of the fourth generation T800 engine reaches 300 kW/(kg/s), which is 8 1% higher than that of the first generation, 52.3% higher than that of the second generation and 2.4% higher than that of the third generation.

(3) The life cycle cost is reduced. Life cycle cost is an economic index to comprehensively measure a new engine. Compared with the predecessors, the new third generation has greatly reduced its life cycle cost, for example, T700 has reduced its life cycle cost by 32% compared with T58. The cost reduction mainly comes from the structural design of the unit and the reduction of fuel consumption.

(4) The general power reserve of the fourth generation turboshaft engine is 10-20%. Under the condition that the overall size of the engine is unchanged, the power can be improved by increasing the flow rate and turbine inlet temperature, or increasing the size appropriately, that is, adding a zero-stage compressor in front of the compressor.

(5) Adopt an integrated particle separator to improve the sand prevention ability of military forces.

(6) The compressors are all two-stage centrifugal, with good rotor stability, few parts, easy maintenance, corrosion resistance and strong foreign body damage resistance.

(7) A backflow annular combustion chamber and a pneumatic atomizing nozzle are adopted.

(8) Air-cooled turbine stator and rotor blades were used for the first time in engines with power less than 1000kW, which increased the turbine inlet temperature to1420 k. ..

After entering the 2 1 century, turboshaft engines will develop in two directions: one is to continue to improve the cycle parameters and component efficiency of turboshaft engines, to develop engines with better performance, and the other is to develop high-speed rotor propulsion technology. At the beginning of the next century, the pressure ratio of turboshaft engine will reach 16-26, and the temperature before turbine will reach 1500- 1920K. This kind of engine may still adopt a three-stage axial flow plus 1 stage centrifugal compressor with a total pressure ratio of 18. The flame tube of the combustion chamber has a multi-layer cooling structure. The turbine can adopt radial air intake with complex cooling channels. At present, the high-speed tilting rotorcraft T406 developed by Allison Company in the United States has a top speed of 600 kilometers per hour ... The next top speed to be achieved is above 800km/h, mainly including tilting rotor, folding rotor and rotor type.

So far, two generations of turboprop engines have been successfully developed and used abroad in terms of civil regional power. The third generation is under development. The first generation refers to turboprop engines put into production before 1970s, mainly including Datt, PT6A, TPE33 1. The power range is 500- 1500kW, the fuel consumption rate is 0.35-0.40kg/(kW/h), and the renovation life is 8000- 14000h, which is mainly used for 12-60-seat regional aircraft. The second generation was put into production at the end of 1970s, mainly including PW/KOOC-0/00, CT7 and TPE33/KOOC-0/-/KOOC-0/4//KOOC-0/5, pressure ratio/KOOC-0/-/KOOC-0/7, and turbine. The third generation was put into use in 1990s, mainly including AE2 100 and TPF35 1-20. AE2 100 is a turboprop engine with power of 4474kW developed by Allison Company on the basis of T406, which is used to compete for the next generation of high-speed regional aircraft. The main feature of this engine is that it has enough development potential, such as the power can be increased to 5880kW with the improvement of high-pressure turbine; The power under the static standard state of sea level will not be reduced because of hot weather and high altitude; High climbing power can shorten the climbing time of aircraft. TPF35 1-20 is a propulsive turboprop engine developed by Garrett Company of the United States for 20-39 regional aircraft, with a power of 1566kW. Compared with the company's early engines, due to the increase in size and compressor improvement, the fuel consumption decreased by 25% and the power-to-weight ratio increased by 53%. TPF35 1-20, as a stand-alone design, adopts many mature technologies, such as the compressor technology of F 109 turbine engine (a new compressor is being developed, and the power is increased by 25%, reaching 1870kW), TPE 331-0.

At present, in order to reduce the development cost and maintenance cost, many foreign small turbine engine manufacturers are trying to develop the "universal core machine" technology of turboshaft, turboprop and turbofan engines by using mature research and application experience, that is, based on a mature turboshaft engine, the corresponding turboprop and turbofan engines are developed. For example, AE2 100 turboprop engine of Allison Company in the United States was developed on the basis of T406 turboshaft engine "universal core engine" produced by Allison Company, which greatly reduced the development risk and cost. This has become the overall development trend of developing small gas turbine engines abroad. In addition, the development and production of turboshaft/turboprop engines abroad have separate plans, which are completed by specialized manufacturers or specialized small gas turbine engines, and there are test equipment and production equipment independent of large gas turbine engines.

Key technology of turboshaft/turboprop engine

(1) combined compressor

The turboshaft/turboprop engine requires the compressor to have a high total pressure increase ratio in order to obtain high thermal efficiency and unit power. With the increase of pressure ratio, the structure of compressor has changed from pure axial flow to multi-stage axial flow and one-stage centrifugal combined compressor. This is mainly because for the small turboshaft/turboprop engine with high pressure increase ratio, the increase of axial compressor stage makes the "size effect" of the last compressor stage more obvious, the airflow loss increases and the aerodynamic performance decreases significantly; Moreover, the rotor span of multistage axial compressor is large, which will also bring difficulties in rotor dynamics. Because the rotor structure of centrifugal compressor has better rigidity and stronger resistance to foreign objects, the size effect has little influence on centrifugal compressor, which is beneficial to replace the axial compressor behind it. In the case of extremely small size, a full centrifugal compressor system must be adopted.

(2) Combustion chamber

With the development of turboshaft engines to the third and fourth generations, most combustion chambers adopt recirculation annular combustion chambers. With the continuous improvement of turboshaft engine performance, the inlet temperature of combustion chamber and the temperature rise through combustion chamber are required to increase accordingly. As the temperature of hot gas approaches the temperature limit point of turbine materials, it is particularly important to maintain uniform combustion. Therefore, it is necessary to adopt a new fuel nozzle with large adjustment ratio coefficient to obtain uniform circumferential and radial temperature distribution coefficients. Higher combustion temperature and higher high-pressure thermal radiation will make the combustion chamber flame tube bear greater thermal load. At the same time, because more airflow is used for combustion, less airflow is used for cooling, and the increase of inlet airflow temperature reduces the heat absorption capacity of cooling airflow, making the traditional flame tube cooling technology no longer effective. It is urgent to improve the cooling of the flame tube and study more heat-resistant materials. In recent years, the research on new nozzle and improving the cooling of flame tube has been regarded as the research focus of improving the combustion chamber performance of small gas turbine engine abroad. In addition, this paper also introduces the development direction of the new combustion chamber, that is, replacing the traditional combustion chamber with the first wave rotor.

(3) Turbine

There are two main ways to improve the turbine inlet temperature of turboshaft engine: one is to seek high temperature resistant materials; The second is to adopt turbine cooling technology. In terms of adopting new materials, single crystal materials have been widely used at present, and the next step is to study anti-oxidation and anti-corrosion metal and ceramic coatings. In terms of cooling technology, the fourth generation turboshaft engines T800-LHT-800 and MTR390, which represent the highest level of turboshaft engines, adopt two-stage air-cooled single crystal blades and single-stage transonic air-cooled blades for their gas generator turbines respectively. It can be seen that the air-cooled turbine blades used in high-power turboshaft engines such as T700 and RTM322 have been applied to the turbine design of medium-power turboshaft engines, and the turbine inlet temperature has been increased to above1480 k ... However, because the turboshaft engines generate relatively small power, the required air flow is small, and the axial speed of the intake air flow is not much different from that of large engines, so the cross-sectional area of the flow passage is correspondingly small, resulting in shorter lengths of the stator and rotor blades. This makes it difficult for turbines to use air-cooled blades. At present, the pre-research of radial air-cooled steam turbine is being carried out abroad. Compared with axial flow turbine, radial flow turbine has smaller cooling gas flow and leakage and higher efficiency, and its size is suitable for small gas turbine engines.

(4) high-speed rotor dynamics

For the turboshaft engine with concentric rotor shafting and forward power output shaft, the power turboshaft must pass through the inner cavity of the gas generator rotor and extend to the front of the engine, so the span between the power turboshaft supports is longer and the shaft diameter is smaller. Early turboshaft engines (such as T53 engine) have low pressure ratio and low speed, and their power turboshaft still works in subcritical state. However, the working speed of the rotor shafting of modern small and medium-sized turboshaft engines with high speed and pressure ratio is likely to approach or exceed the critical speed, and some even exceed the third-order critical speed. When the engine speed is very high, the vibration amplitude of the rotor is required to be very small, which makes the rotor dynamics problem very difficult. Supercritical rotor support system is often used to make the rotor support system work smoothly above the critical speed of each rigid vibration mode, while the rotor shaft bends and deforms greatly below the critical speed. Reasonable selection of rotor support scheme, strict control of rotor axial size, correct use of elastic support and damper, and reasonable selection of materials will directly affect the dynamic characteristics of rotor support system.

(5) particle separator

Because helicopters are often used in bad landing conditions, it is easy for the rotor to suck up a lot of dust and gravel when flying and hovering at ultra-low altitude. After these impurities are inhaled into the engine, they will corrode the compressor, leading to performance degradation or compressor surge margin reduction, or even early repair. In addition, it will damage the blades, damage the engine and cause flight accidents. Therefore, in order to ensure the safe and reliable operation of turboshaft engine, intake air purification device must be adopted. Intake air purification devices can be divided into two types: blocking filter and inertial particle separator. Due to the low separation efficiency and large energy loss of equipment, the blocking filter has been replaced by an inertial particle separator which is more suitable for dust removal of turboshaft engine intake. At present, the inertial particle separator has been developed from the early integral part of the engine (such as T700 engine on the Black Hawk helicopter) to the outside of the helicopter, such as AH-64 Apache external air particle separator (EAPS). Experiments show that the sand removal efficiency of EAPS exceeds 90% when the energy loss is less than 3%, which better reflects the current design requirements for particle separators: to improve the separation technology level as much as possible on the basis of meeting the specific minimum aircraft performance. The fourth generation turboshaft engine T800 adopts an integral but separable inlet particle separator, with the highest separation efficiency in the industry. Through the C-class fine sand test on the test bench, it is proved that the separation efficiency is as high as 97%.

(6) infrared suppressor

With the rapid development of optoelectronics in the 20th century, the developed infrared imaging technology can identify the target at a long distance, that is, destroy the aircraft by tracking the infrared signals emitted by the aircraft, which makes the infrared suppression technology become important. Engine is the largest infrared radiation source of helicopter and the main tracking target of infrared missile. Therefore, it is necessary to install an infrared suppressor on the engine to reduce the temperature of the hot parts of the engine and the exhaust heat flow. For example, the infrared radiation is blocked or shielded by using a heat-insulating baffle in the nozzle, and the infrared wavelength is changed by using a special-shaped nozzle, so that the infrared detector is detuned; The radiation wavelength is changed by jet filtration; A two-dimensional nozzle with a non-circular cross section is used to filter 90% of infrared radiation. At present, the infrared suppressor mainly uses the injection principle to inject the surrounding cold air into the high-temperature tail flame to dilute the concentration of carbon dioxide, so as to greatly reduce the infrared radiation of the exhaust tail flame. American AH-64 helicopter gunship is equipped with infrared radiator and three rectangular ejector suppression devices. After installing this suppression device, compared with cooling the engine heat source with a cooling fan, the weight of the aircraft is reduced by 182kg, and the vertical climbing speed is increased by 76m/min. The infrared signal is only 6% of that without suppression device, while the infrared signal of exhaust heat flow is 10%. Application and influence:

Turboshaft/turboprop engines include light attack/anti-tank helicopters, special armed helicopters, tactical transport aircraft, anti-submarine attack aircraft, border patrol aircraft, light attack aircraft and primary trainer aircraft.