 Motores a Jato |
| Texto contendo partes de documentos sobre motores a jato. Any of a class of internal-combustion engines that propel aircraft by means of the rearward discharge of a jet of fluid,usually hot exhaust gases generated by burning fuel with air drawn in from the atmosphere. The only practical means of propulsion so far devised is to take advantage of Newton's third law of motion. In its simplest form, this law states that action and reaction are equal and opposite. It is observed in nature that forces (e.g., "pushes" or "pulls") never occur singly but always in equal or opposite pairs. Thus the existence of a force on a body requires the coexistence of an equal and opposite force. If a pair of forces acts between two separate bodies (and provided that no other forces act), the bodies will be impelled to separate or come together depending on whether the force pair is repulsive (a push) or attractive (a pull). The combination of Newton's second and third laws shows that the resulting motion of the two bodies can be expressed by the statement that the change in momentum of the two bodies is equal and opposite and along the line of action of the force pair. Momentum is the product of mass and velocity, and, because (at aircraft velocities) the mass of any body remains constant, a force pair acting between two bodies produces a change in the velocity of each. It is clear that in order to propel a body it is necessary to find something to push against. All aircraft-propulsion devices push against the air itself. If the air is used as the propelling medium, it must experience a change in momentum--i.e., it has to be accelerated toward the rear of the aircraft and discharged rearward with enough velocity that the reaction produces an appreciable thrust in the opposite direction. From 1903 to the late 1930s the reciprocating internal-combustion engine with a propeller was the sole means used to propel aircraft. The first jet-powered airplane was finally introduced in 1939 in Germany. The jet engine, consisting of a gas-turbine system,significantly simplified the propulsion process and thereby opened the way to substantial increases in aircraft speed, size, and operating altitudes. Over the years, several types of jet engines have been developed. The principal varieties are (1) turbojets, (2) turbofans, (3) turboprops, (4)turboshafts, and (5) ramjets. Turbojets. The basic idea of the turbojet engine is simple. Air taken in from an opening in the front of the engine is compressed to 3to 12 times its original pressure in a centrifugal or axial compressor. Sufficient fuel is added to the air and burned in a combustion chamber to raise the temperature of the fluid mixture to about 600 to 700C (1,100 to 1,300 F). The resulting hot air is passed through a turbine, the sole function of which is to drive the compressor. If the turbine and compressor are efficient, the pressure at the turbine discharge will be nearly twice atmospheric, and this excess pressure is utilized in a propelling nozzle to produce a high-velocity stream of gas and hence thrust. Substantial increases in thrust can be obtained by employing an afterburner. This device capitalizes on the fact that less than one-third of the oxygen in the atmospheric air is consumed in the turbojet engine. It is a second combustion chamber positioned after the turbine and before the propelling nozzle. By means of this device the temperature of the gas ahead of the propelling nozzle can be increased considerably. The result of this increase in temperature is an increase ofabout 40 percent in thrust at take-off or a much larger percentage at high speeds. Turbofans. In a turbojet all the air entering the intake passes through the gas generator, which is composed of the compressor,combustion chamber, and turbine. In a turbofan engine, by contrast, only a portion of the incoming air does so. The remainder passes through a fan, or low-pressure compressor, and is ejected directly as a"cold" jet or mixed with the gas-generator exhaust to produce a "hot"jet. The objective of this sort of bypass system is to increase thrust without increasing fuel consumption. It achieves this by increasing the total air-mass flow and reducing the velocity within the same total energy supply. The fan is commonly placed at the front of the engine, and its size is determined by the bypass ratio. For large ratios only one or two fan stages are needed, but for smaller ratios a higher jet velocity and more compression are required. Turboprops. Like the turbojet, the turboprop engine consists of a compressor, combustion chamber, and turbine, but in this case nearly the full pressure drop is used to run the turbine, which thus develops more than enough power to drive the compressor. The excess power is utilized to drive a propeller through reduction gearing. Compared with a turbojet engine, the turboprop has better propulsive efficiency at flight speeds below about 800 km per hour (500 miles per hour). Modern turboprop engines are equipped with propellers that have a smaller diameter but a larger number of blades for efficient operation at much higher flight speeds than previously achieved.To accommodate the higher rotative and flight speeds, the blades are scimitar-shaped with swept-back leading edges at the blade tips. Engines featuring such propellers are called propfans. Turboshafts. This is another form of gas-turbine engine that operates much like a turboprop system; however, instead of driving a propeller, it powers a helicopter rotor. The turboshaft engine is so designed that the speed of the helicopter rotor is independent of the rotative speed of the gas generator. This permits the rotor speed to be kept constant even when the speed of the generator is varied to modulate the amount of power produced. Ramjets. The simplest of all aircraft propulsive devices is the ramjet. It is essentially a turbojet in which rotating machinery has been omitted. Its application is restricted by the fact that its compression ratio depends wholly on forward speed. The ramjet develops no static thrust and very little thrust in general below the speed of sound. As a consequence, a ramjet vehicle requires some form of assisted take-off, such as another aircraft. It has been used primarily in guided-missile systems. The Jet Age Beginning in the 1920s, steady advances in aircraft performance had been produced by improved structures and drag-reduction technologies and by more powerful, supercharged engines, but by the early 1930s it had become apparent to a handful of farsighted engineers that speeds would soon be possible which would exceed the capabilities of reciprocating engines and propellers. The reasons for this were not at first widely appreciated. At velocities approaching Mach 1, or the speed of sound (about 745 miles per hour at sea level and about 660 miles per hour at 36,000 feet), aerodynamic drag increases sharply. Moreover, in the transonic range (between about Mach 0.8 and Mach 1.2), air flowing over aerodynamic surfaces stops behaving like an incompressible fluid and forms shock waves; these in turn create sharp local discontinuities in air flow and pressure, creating problems not only of drag but of control as well. Because propeller blades, describing aspiraling path, move through the air at higher local velocities than the rest of the aircraft, they enter this turbulent transonic regime first. For this reason, there is an inflexible upper limit on the speeds that can be attained by propeller-driven aircraft. Such complex interactions in the transonic regime--and not the predictable shock-wave effects of supersonic flight, which ballisticians had understood since the late 19th century--presented special problems that were not solved until the 1950s. In the meantime, a few pioneers attacked the problem directly by conceiving a novel power plant, the turbojet. While still a cadet at the Royal Air Force College, Cranwell, in 1928, Frank Whittle advanced the idea of replacing the piston engine and propeller with a gas turbine, and in the following year he conceived the turbojet, which linked a compressor, combustion chamber, and turbine in the same duct. In ignorance of Whittle's work, three German engineers independently arrived at the same concept: Hans von Ohain in 1933; Herbert Wagner, chief structural engineer for Junkers, in 1934; and government aerodynamicist Helmut Schelp in 1937. Whittle had a running bench model by the spring of 1937, but backing from industrialist Ernst Heinkel gave von Ohain the lead. The He 178, the first jet-powered aircraft, flew on Aug. 27, 1939, nearly two years before its British equivalent, the Gloster E.28/39, on May 15, 1941. Through an involved chain of events in which Schelp's intervention was pivotal, Wagner's efforts led to the Junkers Jumo 004 engine. This became the most widely produced jet engine of World War II and the first operational axial-flow turbojet, one in which the air flows straight through the engine. By contrast, the Whittle and Heinkel jets used centrifugal flow, in which the air is thrown radially outward during compression. Centrifugal flow offers advantages of lightness, compactness, and efficiency--but at the cost of greater frontal area, which increases drag, and lower compression ratios, which limit maximum power. Many early jet fighters were powered by centrifugal-flow turbojets, but, as speeds increased, axial flow became dominant. Development of Jet Engines. Like many other inventions, jet engines were envisaged long before they became a reality. The earliest proposals were based on adaptations of piston engines and were usually heavy and complicated. The first to incorporate a turbine design was conceived as early as 1921, and the essentials of the modern turbojet were contained in a patent in 1930 by Frank Whittle in England. His design was first tested in 1937 and achieved its first flight in May 1941. In Germany, parallel but completely independent work followed issuance of a patent in 1935. It proceeded more rapidly, and the very first flight of a turbojet-powered aircraft, a Heinkel HE-178, came in August 1939. By the end of World War II these prototype aircraft had developed into a few operational turbojet squadrons in the German, British, and U.S. airforces. In the military area, jet fighter aircraft developed rapidly and were in use during the Korean War (1950-53), flying at speeds of 1,000 kilometres per hour. During the next decade they overcame the sound barrier and established normal operations up to more than twice the speed of sound (Mach 2). Bomber and transport jet aircraft were also able to reach and cruise at supersonic speeds. (see also Index: military aircraft) The first civil jet transport, the British de Havilland Comet, flew in 1949, and regular transatlantic jet services were started in 1958 with the Comet 4 and the American Boeing 707. By 1974 more than 90 percent of hours flown throughout the world were flown by jets; the first supersonic airliner, the British-French Concorde, flying at more than twice the speed of sound, entered regular service in January 1976. During the 1980s various major aircraft manufacturers undertook programs to develop fuel-saving propfan and unducted-fan propulsion systems. Some authorities believe that the next generation of commercial air transport may very well be powered by such advanced-technology propeller engines.
Source: Encyclopedia Britannica
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