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thermal engines for propulsion of aircraft (airplanes, helicopters, dirigibles, and the like). Airplane engines have to meet very stringent requirements: maximum power (or maximum thrust) combined with minimum mass per unit of power (or thrust) and minimum size (particularly frontal cross section, in order to minimize drag); minimum consumption of fuel and lubricant per unit of power; reliability; long service life; and ease of operation combined with low manufacturing cost. The process of development of aircraft engines has gone through several stages. The first aircraft engine was a steam engine installed on the aircraft built by A. F. Mozhaiskii (1885). Subsequent airplane engines were designed, in all countries, on the basis of the piston-powered internal combustion engine. The basic factors governing the development of aircraft engines has been the need to increase speed and increase the load-carrying capacity of the airplane, requirements which have rapidly become increasingly stringent. The gasoline engine was chosen as the basic aircraft engine because it weighed the least. Improvements in gasoline engines included making all engine parts lighter through the use of high-strength materials, boosting engine performance (a supercharger to boost the engine has been designed for that purpose), and increasing the efficiency of the propeller. To attain maximum propeller efficiency as engine shaft speed continually increased, a reducing gearbox was coupled to the engine shaft, slowing down the frequency of rotation of the propeller. By the 1940’s, piston aircraft engines were attaining the limits of their capabilities: the sound barrier stood in the way of any further increase in speed, and a formidable increase in the output of airplane engines was required to overcome that barrier. Such a breakthrough became possible as a result of conversion to the gas turbine and jet engine.
Different classes of airplanes require different kinds of aircraft engines, both in power output and in the type of thrust. Existing aircraft engines are divided into propeller engines, which generate thrust by rotating a propeller; jet engines, in which thrust is generated through the expulsion of gases from a jet nozzle at high speeds; and compound engines—turboprop engines—where the basic thrust is generated by a propeller and some considerable augmented thrust (8–12 percent) is generated through expulsion of combustion products.
Some improved piston aircraft engines, brought to a high degree of perfection, have pushed airplane speeds up to 750 km/h. Still higher speeds could not be attained because of the high specific mass (mass per unit of power) and the need for a propeller, whose efficiency decreases with increasing flight speed. Piston aircraft engines are still installed on airplanes of moderate flight speed, corresponding to Mach 0.2–0.5, that is, 200–500 km/h, and also on helicopters, while turboprop airplane engines are installed on aircraft with flight speeds of Mach 0.5–0.8, that is, 500–800 km/h, and on helicopters. The first turbojet engines, which appeared at the end of the Great Patriotic War, made it possible to boost speeds up to 960 km/h.
The specific mass of piston aircraft engines is 540–680 g per kilowatt (kW) (400–500 g per horsepower [hp]); the specific mass of turboprop aircraft engines is 140–400 g/kW (100–300 g/hp). If the mass is divided not by the unit of power, but by the unit of thrust generated by a propeller, then the specific mass of a piston-engined aircraft will vary as the flight speed varies, in response to the variation of the propeller efficiency, whereas the specific mass of a turbojet aircraft engine will remain practically constant over the entire speed range up to 750 km/h (see Table 1). This is what makes the turbojet aircraft engine the most favored one at. high flight speeds.
Extremely lightweight turboprop engines for vertical takeoff and landing aircraft (VTOL) made their appearance in 1965–67. Their specific mass is in the range 6–7/g per newton. Double-flow (bypass) turbojet (turbofan) engines have been designed on the basis of turbojet and turboprop aircraft engines. Their special feature is the generation of two jet streams: one internal, or central, jet stream consisting of high-temperature combustion products sent to the jet-propulsion nozzle of the gas turbine and a second stream surrounding the first stream concentrically and consisting of air driven by the second-flow compressor.
|Table 1. Approximate values of specific masses of aircraft engines—masses per units of power (grams per newton), as related to the uses of the engine|
|Engine operating conditions||Piston||Turboprop||Turbojet|
|Takeoff conditions ...........||33||20||17|
|360 km/h .................||57||35||17|
|750 km/h .................||180||110||17|
Double-flow turbojet aircraft engines are used on subsonic aircraft; because of the low rate of fuel consumption, they are competitive with conventional turbojet and turboprop airplane engines.
The thrust developed by a turbojet aircraft engine at supersonic flight speeds is increasing. The specific mass of turbojet aircraft engines has been substantially lowered over the 1939–67 period.
The layouts of turbojet aircraft engines are different for subsonic and supersonic aircraft. At supersonic flight speeds, the temperature of air and gas is very high in turbojet aircraft engines. To ensure use of the ram pressure of the air to maximum advantage with minimum power loss, the air intake must be made with variable dimensions and shape. Afterburners are employed to boost the thrust of aircraft engines. The jet propulsion nozzle is also made with variable dimensions and shape.
Because the aircraft engine is an automatic system, the pilot is freed from the job of handling and controlling the engine in flight. The fuel pressure, the temperature of gases upstream of the turbine, and other parameters are maintained at a specified level automatically, no matter what the flight altitude.
The further development of aircraft engines envisages continuing the basic trends on which the principal efforts of airplane engine designers in various countries have been concentrated: higher speeds and higher flight altitudes and also continual increase in the load-handling capacity of the airplane. This latter imposes the requirements that aircraft engines develop greater thrust with a smaller rate of fuel consumption and lower specific mass and that they have a long engine operating life (that is, a lengthening of the operating time of the engine between repairs, usually expressed in hours). All this calls for raising the temperature of the gas upstream of the turbine, which promotes the use of cooled nozzle (guide) vanes and rotor blades. On the other hand, efforts are made to lower the power drain in all aircraft engine components, which requires stepping up the efficiency of the compressor, turbine, afterburner, and so forth. The temperature of the gases can be raised by using refractory materials (niobium, molybdenum) in the turbine blading and in other parts coming in contact with high-temperature gases. Reductions in specific mass can be achieved through the use of low-density materials (titanium alloys, beryllium alloys). On large planes for passengers or transport, it is feasible to install either afterburning turbofan jet engines, thereby achieving a higher range of flight speeds, or turbofan aircraft engines with a high bypass ratio (the ratio of the air flow of the first and second streams), in the range of six or eight to one, in order to obtain high thrust values at low cost.
REFERENCESInozemtsev, N. V. Aviatsionnye gazoturbinnye dvigateli: Teoriia i rabochii protsess. Moscow, 1955.
Teoriia reaktivnykh dvigatelei. Moscow, 1958.
Konstruktsiia aviatsionnykh gazoturbinnykh dvigatelei. Moscow, 1961.
Skubachevskii, G. S. Aviatsionnye gazoturbinnye dvigateli: Konstruktsiia i raschet detalei, 2nd ed. Moscow, 1965.
Aviatsiia i kosmonavtika, 1963, no. 3, pp. 6–13; 1966, no. 2, pp. 60–64; 1967, no. 7, pp. 57–61.
S. K. TUMANSKII and G. S. SKUBACHEVSKII