Turbo shaft Engine
Turbo shaft Engine
A turboshaft engine might be defined as a gas turbine engine designed to produce only shaft power. Such engines find application in helicopters, where their light weight and small size compared with piston engines render them attractive. Turboprop engines are similar to turboprop engines, expect that the hot gases are expanded to a lower pressure in the turbine, thus providing little exhaust velocity.
Comparison of turbojet, Turboprop and Turbofan engines (Figure 8)
Turbojet
1. Low thrust at low forward speeds
2. Relatively high TSFC at low altitudes and low speeds. This disadvantage decreases as altitude and speed increase.
3. Long take-off road is required.
4. Small frontal area results in reduced ground clearance problem.
5. Lightest specific weight.
6. Ability to take advantage of high rams pressure ratios.
Suitable for high speed, high altitude long distance flights.
Turboprop
1. High propulsive efficiency at low speeds which falls off rapidly as speed increases this results in shorter take-off roads. The engine is able to develop very high thrust at low speeds because the propeller can accelerate large quantities of air at zero forward velocity of the airplane.
2. More complicated and heavier than the turbojet engine.
3. Lowest TSFC.
4. Large frontal area of propeller and engine combination necessitates longer landing gears for lowing airplanes.
5. Efficient reserve thrust possible.
Turboprop engines are superior for lifting heavy loads off short and medium runways
Turbofan
1. Increased thrust at forward speeds similar to a turboprop results in a relatively short take-off. But unlike the turboprop the turbofan does not decrease in thrust with increasing speed. It can be used approximately up to Match 1.
2. The weight lies some where between the turbojet and turboprop.
3. Ground clearances are less that for turboprop but more than that for turbojet.
4. TSFC and specific weight fall between turboprop and turbojet. Increased operating economy in 800 – 1200 km/h. range.
5. Considerable noise level reduction over equivalent turbojet engines.
6. Superior
7. Two thrust reverses are required if the fan air and primary engine exhaust through separate nozzles.
Fan engine is suitable for long range, relatively high speed flight.
Comparison is made by assuming that the engines are equivalent as to compressor ratio and internal temperatures and that the engines are installed in equal sized aircraft best suited to the particular type of engine being used.
Advantages and Disadvantages of the Gas Turbine as Aircraft power plant
Advantages
1. Freedom from vibration-permits lighter propeller sections and mounting structure.
2. Simplicity of control.
3. No radiators or other cooling surfaces.
4. Negligible cooling air required.
5. No spark plugs required expect for starting – once combustion is established, it is self supporting.
6. No carburetors.
7. Available supply of compressed air.
8. Decreased fire hazard – less volatile fuels are used.
9. Lower specific weight.
10. Lower oil consumption.
Disadvantages
1. High specific fuel consumption at low air speeds – applies chiefly to pure jet engines have performance comparable to reciprocating engines.
2. Inefficient operation at low power levels.
3. Slow acceleration from minimum to maximum power level – this condition applies chiefly to turbojet engines. Turboprop and turbofan engines are able to accelerate quite rapidly.
4. High starting power requirements.
5. High cost manufacture.
6. Susceptibility to damage by foreign material – such material is readily drawn into the air inlet.
Basic Differences – Gas Turbines Vs. Reciprocating Engines
Aerodynamic
Advantages – similar nacelles possible; negligible cooling power required; high speed jet a more efficient propulsive means than propellers at high flight speeds.
Disadvantages – A high speed jet is less efficient propulsive means than a propeller at lower flight speeds and burning take-off. The development of turboprop and turbofan engines has made it possible to combine the advantages of the turbine engine with the efficiency of the propeller for lower speeds and take-off.
Weight
Turbine engines are considerably lighter than reciprocating engines for the same power output
Turbine engines <0.13 N/N thrust.
Turboprop engines <2.36 N/kW
Piston engines <6.05
Fuel consumption
Neutral characteristics – Best SFC occurs near maximum output. (Best SFC of reciprocating engines occurs at about one-half maximum power). At a given flight speed, SFC of the turbojet engine tends to decrease with altitude. The SFC for turboprop engine is comparable to that of the reciprocating engines.
Disadvantages – Best fuel consumption is in general poorer for the turbojet engine.
Output
Neutral characteristics - Operation of the gas turbine engine at varying altitude is somewhere between that of an unsupercharged and a supercharged reciprocating engine. That is, the turbine engine is more adaptable to varying altitude than the unsupercharged engine and perhaps a little less adaptable than the supercharged reciprocating engine.
General
Advantages – Low power plant vibration, relatively constant speed over a wide range of output.
Disadvantage- High engine speed
References
- Compressible fluid flow - A. H. Shapiro
- Fundamentals of compressible flow with aircraft and rocket propulsion - S. M. Yahya
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