What Is a Propeller Turbine?

A gas turbine engine commonly used in aircraft. The driving principle of a turboprop engine is roughly the same as that of a traditional propeller aircraft using a piston engine as a source of power, and the force generated when the propeller rotates is used as the propelling force of the aircraft. The main difference between it and the piston propeller is that in addition to the power source that drives the central axis of the propeller, the propeller of the turboprop engine usually runs at a constant rate, while the propeller of the piston power will vary according to the engine speed And there is a change in the speed. Represented by the Russian HK-93 function road propeller fan engine. ^[1]

Chinese name: Turboprop
Foreign name: Turboprop Engines
nickname: Turboprop

Short name: Turboprop
Rotating speed: About 1000 rpm
Output: Propeller shaft power

Construction of a turboprop engine

The construction of this engine is basically the same as that of a turbojet engine, except that it has one or two additional components. As shown in the figure to the right, it also has an air inlet, a compressor, a combustion chamber, a turbine, and an exhaust nozzle. These components also work similarly to turbojet engines. When it starts, the turbine starts to work, which drives the front compressor to rotate and sucks a large amount of air from the intake duct. The compressed air enters the combustion chamber from the compressor, where it is ignited and burned with the injected fuel, generating gas and emitting a large amount of heat. The hot gas then enters the turbine and blows it at high speed. The gas passing through the turbine finally passes through the tail nozzle and is ejected from the nozzle.

However, the turboprop engine must not only drive the compressor and various accessories, but also the propeller in front. Because the speed of the propeller is much lower than that of the turbine, which is generally about 1,000 revolutions per minute, a set of reduction gear must be installed on the engine. Depending on it, the speed of the propeller can be greatly reduced. So this engine is actually based on the turbojet engine, with two additional components-a pair of propellers and a set of reduction gears. ^[2]

Turboprop Engine Power Source

The turboprop engine is mainly based on the pulling force (thrust) generated by the propeller, and the thrust generated by the jet is very small, accounting for only about one-ninth of the propeller. ^[2]

Installation position of turboprop

For pull-in aircraft, the engine is mostly installed on the nose or the leading edge of the wing. This can reduce the load on the wing. Because the gravity of the engine is opposite to the direction of the lifting force, the external Bending moment. The other is the propulsion type, that is, the engine is installed at the trailing edge of the wing or the rear fuselage. This arrangement makes the wings and fuselage out of the slipstream of the propeller, and the resistance will be reduced, but the main landing gear is higher, the weight is increased, and the cooling conditions of the engine working on the ground are poor, so it is currently used less . ^[2]

Turboprop engine turboprop

One of the characteristics of the fixation of a turboprop engine is that the propeller will cause vibration when it is working, so a damping device should be used in the connection joint between the fixing device and the engine. This type of engine is usually fixed by an engine frame, and the engine frame is connected to the force frame of the fuselage or nacelle, such as a reinforced bulkhead. ^[2]

Advantages and disadvantages of turboprop engines

Turboprop advantages

Although the combustion chamber of a turboprop engine is similar to a turbojet engine,

Turboprop However, in order to recover more power from the exhaust gas to drive the propeller, the number of blade stages of the turbine end of the turbo engine is relatively high. In contrast, as the main propulsion of a turbojet engine comes from the reaction force generated by the direct discharge of hot gas into the atmosphere, the smaller the number of blade stages on the turbine side is, the better it is to maintain sufficient recovery power for driving. The fan blades on the compression side are sufficient. ^[1]

Turboprop disadvantages

In fact, turboprop engines are also more efficient than turbofan engines, but aircraft with turboprop engines are usually slower than turbofan-powered aircraft. The reason is that the bypass ratio of a turboprop engine is usually higher than that of a turbofan engine, but it also causes the blade tip part to have a high speed, which may cause shock waves. In addition, because the turbine rotates fast, it is necessary to have a variable speed gear between the turbine and the propeller to reduce the speed of the propeller so that the blade end does not exceed the speed of sound. So airplanes using propeller engines will weigh multiple gears. ^[1]

Applications of turboprop engines

Turboprop The turboprop engine not only works well at sea level and aircraft take-off conditions, but also at a height of about 10,000 meters, its working performance is still very good. Due to the limitation of reduced propeller efficiency, it cannot be used for high-speed flight. At low and medium speeds, turboprop engines are appropriate. At present, such engines are often installed in medium-speed long-range transport aircraft, passenger aircraft, maritime patrol aircraft, and anti-submarine aircraft, and even some long-range bombers also use turboprop engines. ^[2]

Analysis of differences in turboprop engines

Differences between turboprop engines and turbofan engines

Although the working principle is similar, the turboprop engine and turbofan engine are very different in generating power. The main power output of the turboprop engine is the propeller shaft power, and the gas thrust from the tail nozzle is extremely small. It only accounts for about 5% of the total thrust. In order to drive high-powered propellers, the number of turbine stages is also greater than that of turbofan engines, generally 2 to 6 stages.

Due to the large bypass ratio, the efficiency of turboprop engines is higher than that of turbofan engines at low speeds, but due to the influence of propeller efficiency, its applicable speed cannot be too high, generally less than 900km / h. It is widely used in low- and medium-speed aircraft or patrol, anti-submarine or fire suppression aircraft with strict requirements on low-speed performance. ^[1]

+ Difference between turboprop engine and piston engine + propeller

Turboprop engines have many advantages. First of all, its power is large, and the power-to-weight ratio (power / weight) is also large. The maximum power can exceed 10,000 horsepower and the power-to-weight ratio is 4 or more. The piston engine has a maximum power of 3,000 to 4,000 horsepower and the power-to-weight ratio is about 2. Secondly, due to the reduction of moving parts, especially pistons without reciprocating motion, the turboprop engine has good running stability, low noise, long working life, and low maintenance costs. In addition, because the core uses a gas generator, the applicable altitude and speed range of a turboprop engine are much higher than those of a piston engine. In terms of fuel consumption, the two are similar, but the kerosene used by turboprop engines is cheaper than the gasoline of piston engines. ^[1]