Turbine Engines

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TURBINE ENGINES

INTRODUCTIONEfforts to design a working gas turbine engine had been under way for years prior to World War II. Engineers eventually succeeded in placing a few engines in combat aircraft briefly during the closing stages of the war. The war effort had brought about many advances in gas turbine technology which could now be used for commercial aircraft design. Turbine engines offered many advantages over reciprocating engines and airlines were interested. Increased reliability, longer mean times between overhaul, higher airspeeds, ease of operation at high altitudes, and a high power to engine weight ratio made turbine power very desirable. Aircraft such as Lockheed's Super Constellation represented the practical limits of piston power technology and required frequent engine maintenance; therefore, air carriers turned to gas turbine engines for solutions. During the decade of the 50's, a gradual transfer from piston power to gas turbine jets and turboprops started taking place. Old workhorses such as the Douglas DC-3 and DC-7 gave way to the Boeing 707 and Douglas DC-8.

DESIGN AND CONSTRUCTION

Newton's third law of motion states that for every action, there is an equal and opposite reaction. Jet propulsion applies this law by taking in a quantity of air and accelerating it through an orifice or nozzle. The acceleration of the air is the action and forward movement is the reaction. In nature, a squid propels itself through the water using a form of jet propulsion. A squid takes sea water into its body and uses its muscles to add energy to the water, then expels the water in the form of a jet. This action produces a reaction that propels the squid forward. [Figure 3-1]

Figure 3-2. Hero's aeolipile, conceived long before the acceptance of Newton's Laws of Motion, proved that power by reaction was possible. Figure 3-1. Many technological developments were made by observing nature in action. A squid propels itself through the water by jet reaction in much the same way a turbojet engine propels an aircraft.

HISTORY OF JET PROPULSIONThe history of mechanical jet propulsion began in 1900, when Dr. Sanford Moss submitted his masters thesis on gas turbines. Later, Dr. Moss became an engineer for the General Electric Company in England. While there, Dr. Moss had the opportunity to apply some of his concepts in the development of the turbo-supercharger. This unique supercharger consisted of a small turbine wheel that was driven by exhaust gases. The turbine was then used to drive a supercharger. Research done by Dr. Moss influenced Frank Whittle of England in the development of what became the first successful turbojet engine. Dr. Whittle was granted his first patent for the jet engine

As early as 250 B.C., a writer and mathematician named Hero devised a toy that used the reaction principle. The toy, called the aeolipile, consisted of a covered kettle of water that was heated to produce steam. The steam was then routed through two vertical tubes and into a spherical container. Attached to the spherical container were several discharge tubes arranged radially around the container. As steam filled the container, it would escape through the discharge tubes and cause the sphere to rotate. [Figure 3-2] A more modern example of Newton's reaction principle is observed when the end of an inflated balloon is released. As the air in the balloon rushes out the opening, the balloon flies wildly around a room. In spite of the everyday examples, scientists' efforts to apply Newton's reaction principle to mechanical designs met with little success until this century.

Turbine Engines

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Figure 3-5. First flown in 1942, the Bell XP-59 was the first American jet-powered aircraft.

Figure 3-3. Dr. Frank Whittle of England patented the first turbojet engine, the Whittle W1, in 1930. Its first flight occurred in a Gloster E28/39 aircraft in 1941.

efforts were being directed toward the development and production of high powered reciprocating engines. However, in 1941 the General Electric Company received a contract to research and develop a gas turbine engine. General Electric was chosen for this important project because of its extensive experience in building electrical generating turbines and turbo-superchargers. The result was the GE-lA engine, a centrifugal-compressor type engine that produced approximately 1,650 pounds of thrust. Two of these engines were used to power the Bell XP-59 "Airacomet" which flew for the first time in October 1942. The Airacomet proved the concept of jet powered flight, but was never used in combat due to its limited flight time of 30 minutes. [Figure 3-5]

in 1930 and eleven years later, his engine completed its first flight in a Gloster model E28/39 aircraft. The engine produced about one thousand pounds of thrust and propelled the aircraft at speeds over 400 miles per hour. [Figure 3-3] While Whittle was developing the gas turbine engine in England, Hans Von Ohain, a German engineer, designed and built a jet engine that produced 1,100 pounds of thrust. This engine was installed in the Heinkel He-178 aircraft and made a successful flight on August 27, 1939. As a result, it became recognized as the first practical flight by a jet propelled aircraft. [Figure 3-4] In the United States, research in the field of jet propulsion was lagging. Most of the country's

JET PROPULSION TODAYToday, the majority of commercial aircraft utilize some form of jet propulsion. In addition, there are currently several manufacturers that produce entire lines of jet powered aircraft that cruise in excess of 600 miles per hour and carry more than four hundred passengers or several tons of cargo. Another step in the progression of commercial and military aviation was the ability to produce an engine that would propel an aircraft faster than the speed of sound. Today, there are several military aircraft that travel at speeds in excess of Mach one. One such aircraft is the SR-71 Blackbird which flys in excess of Mach five. In commercial aviation however, there is currently only one aircraft that flies faster than Mach one. This aircraft, the Concorde, was built by the British and French and placed into service in the mid seventies. Currently, there are more than ten Concordes in service that are capable of flying at 2.2 times the speed of sound. In addition to military and commercial aviation, jet propulsion has become extremely popular for use on business jets. These two and three engine aircraft have become extremely popular in recent years due in part to the efficiency and reliability of jet engines.

Figure 3-4. German engineer Hans Von Ohain designed and built the turbojet engine that powered the Heinkel He-178 to the world's first jet-powered flight in 1939.

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Turbine Engines

TYPES OF JET PROPULSIONNewton's reaction principle has been applied to several propulsive devices used in aviation. All produce thrust in the same manner, they accelerate a mass of gases within the engine. The most common types of propulsive engines are the rocket, the ramjet, the pulsejet, and the gas turbine.ROCKET

A rocket is a nonairbreathing engine that carries its own fuel as well as the oxygen needed for the fuel to burn. There are two types of rockets in use: solid-propellant rockets and liquid-propellant rockets. Solid-propellant rockets use a solid fuel that is mixed with an oxidizer and formed into a specific shape that promotes an optimum burning rate. Once ignited, the fuel produces an extremely high velocity discharge of gas through a nozzle at the rear of the rocket body. The reaction to the rapid discharge is forward motion of the rocket body. Solid fuel rockets are used primarily to propel some military weapons and, at times, provide additional thrust for takeoff of heavily loaded aircraft. These booster rockets attach to an aircraft structure and provide the additional thrust needed for special-condition takeoffs. [Figure 3-6] The second type of rocket is the liquid-fuel rocket, which uses fuel and an oxidizing agent such as liquid oxygen. The two liquids are carried in tanks aboard the rocket. When the liquids are mixed, the reaction is so violent that a tremendous amount of heat is generated. The resulting high velocity gas jet behind the rocket provides enough thrust to propel an object. RAMJET A ramjet engine is an athodyd, or aero-thermodynamic-duct. Ramjets are air-breathing engines with

Figure 3-7. As a ramjet moves forward, air enters the intake and proceeds to a combustion chamber where fuel is added. Once ignited, the heat from the burning fuel accelerates the flow of air through a venturi to produce thrust.

no moving parts. However, since a ramjet has no rotating compressor to draw air into the engine, a ramjet must be moving forward at a high velocity before it can produce thrust. Once air enters the engine, fuel is injected and ignited to provide the heat needed to accelerate the air and produce thrust. Because ramjets must be moving forward to produce thrust, they are limited in their use. At present, ramjets are used in some military weapons delivery systems where the vehicle is accelerated to a high initial velocity so the ramjet can take over for sustained flight. [Figure 3-7] PULSEJET Pulsejet engines are similar to ramjets except that the air intake duct is equipped with a series of shutter valves that are spring loaded to the open posti-tion. Air drawn through the open valves enters a combustion chamber where it is heated by burning fuel. As the air within the combustion chamber expands, the air pressure increases to the point that the shutter valves are forced closed. Once closed, the expanding air within the chamber is forced rearward to produce thrust. A pulsejet is typically considered more useful than a ramjet because pulsejets will produce thr