|
A turboprop engine is a turbine engine that drives an aircraft propeller.〔("Turboprop" ), ''Pilot's Handbook of Aeronautical Knowledge'', Federal Aviation Administration, 2009.〕 In contrast to a turbojet, the engine's exhaust gases do not contain enough energy to create significant thrust, since almost all of the engine's power is used to drive the propeller. The propeller is coupled to the turbine through a reduction gear that converts the high RPM, low torque output to low RPM, high torque. The propeller itself is normally a constant speed (variable pitch) type similar to that used with larger reciprocating aircraft engines. Turboprop engines are generally used on small subsonic aircraft, but some aircraft outfitted with turboprops have cruising speeds in excess of 500 kt (926 km/h, 575 mph). Large military and civil aircraft, such as the Lockheed L-188 Electra and the Tupolev Tu-95, have also used turboprop power. The Airbus A400M is powered by four Europrop TP400 engines, which are the third most powerful turboprop engines ever produced, after the Kuznetsov NK-12 and Progress D-27. In its simplest form a turboprop consists of an intake, compressor, combustor, turbine, and a propelling nozzle. Air is drawn into the intake and compressed by the compressor. Fuel is then added to the compressed air in the combustor, where the fuel-air mixture then combusts. The hot combustion gases expand through the turbine. Some of the power generated by the turbine is used to drive the compressor. The rest is transmitted through the reduction gearing to the propeller. Further expansion of the gases occurs in the propelling nozzle, where the gases exhaust to atmospheric pressure. The propelling nozzle provides a relatively small proportion of the thrust generated by a turboprop. Turboprops are most efficient at flight speeds below 725 km/h (450 mph; 390 knots) because the jet velocity of the propeller (and exhaust) is relatively low. Due to the high price of turboprop engines, they are mostly used where high-performance short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required. The most common application of turboprop engines in civilian aviation is in small commuter aircraft, where their greater power and reliability than reciprocating engines offsets their higher initial cost and fuel consumption. Turboprop airliners now operate at near the same speed as small turbofan-powered aircraft but burn two-thirds of the fuel per passenger.〔(More turboprops coming to the market - maybe )〕 However, compared to a turbojet (which can fly at high altitude for enhanced speed and fuel efficiency) a propeller aircraft has a much lower ceiling. Turboprop-powered aircraft have become popular for bush airplanes such as the Cessna Caravan and Quest Kodiak as jet fuel is easier to obtain in remote areas than is aviation-grade gasoline (avgas). ==Technological aspects== Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (up to that necessary to drive the compressor) from turbine expansion. Owing to the additional expansion in the turbine system, the residual energy in the exhaust jet is low.〔"(Turboprop Engine )" ''Glenn Research Center (NASA)''〕〔"(Turboprop Thrust )" ''Glenn Research Center (NASA)''〕〔(Variations of Jet Engines; Turboprop Engines )〕 Consequently, the exhaust jet produces (typically) less than 10% of the total thrust,〔"(The turbofan engine )", page 7. ''SRM University, Department of aerospace engineering''〕 and turboprops can have bypass ratios up to 50-100〔Ilan Kroo and Juan Alonso. "(Aircraft Design: Synthesis and Analysis, Propulsion Systems: Basic Concepts )" ''Stanford University School of Engineering, Department of Aeronautics and Astronautics'' (Main page )〕〔(Prof. Z. S. Spakovszky ). "(11.5 Trends in thermal and propulsive efficiency )" ''MIT turbines'', 2002. (Thermodynamics and Propulsion )〕〔 although the propulsion airflow is less clearly defined for propellers than for fans.〔"(Propeller thrust )" ''Glenn Research Center (NASA)''〕 Unlike the small diameter fans used in turbofan jet engines, the propeller has a large diameter that lets it accelerate a large volume of air. This permits a lower airstream velocity for a given amount of thrust. As it is more efficient at low speeds to accelerate a large amount of air by a small degree than a small amount of air by a large degree,〔Paul Bevilaqua : (The shaft driven Lift Fan propulsion system for the Joint Strike Fighter ) page 3. Presented 1 May 1997. DTIC.MIL Word document, 5.5 MB. Accessed: 25 February 2012.〕〔Bensen, Igor. "(How they fly - Bensen explains all )" ''Gyrocopters UK''. Accessed: 10 April 2014.〕 a low disc loading (thrust per disc area) increases the aircraft's energy efficiency, and this reduces the fuel use.〔Johnson, Wayne. (Helicopter theory ) pp3+32, ''Courier Dover Publications'', 1980. Accessed: 25 February 2012. ISBN 0-486-68230-7〕〔Wieslaw Zenon Stepniewski, C. N. Keys. (Rotary-wing aerodynamics ) p3, ''Courier Dover Publications'', 1979. Accessed: 25 February 2012. ISBN 0-486-64647-5〕 Since propellers are not efficient when their tips reach or exceed supersonic speeds,〔 reduction gearboxes are placed in the drive line between the power turbine and the propeller to allow the turbine to operate at its most efficient speed. The gearbox is part of the engine and contains the parts necessary to operate a constant speed propeller. This differs from the turboshaft engines used in helicopters, where the gearbox is remote from the engine.〔〔 Propellers lose efficiency as aircraft speed increases, so turboprops are normally not used on high-speed aircraft〔〔〔 above Mach 0.6-0.7.〔 However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching Mach 0.75. To increase propeller efficiency, a mechanism can be used to alter their pitch relative to the airspeed. A variable-pitch propeller, also called a controllable-pitch propeller, can also be used to generate negative thrust while decelerating on the runway. Additionally, in the event of an engine outage, the pitch can be adjusted to a vaning pitch (called feathering), thus minimizing the drag of the non-functioning propeller. While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compression. Centrifugal compressors have the advantage of being simple and lightweight, at the expense of a streamlined shape. While the power turbine may be integral with the gas generator section, many turboprops today feature a free power turbine on a separate coaxial shaft. This enables the propeller to rotate freely, independent of compressor speed. Residual thrust on a turboshaft is avoided by further expansion in the turbine system and/or truncating and turning the exhaust 180 degrees, to produce two opposing jets. Apart from the above, there is very little difference between a turboprop and a turboshaft.〔Nag, P.K. "(Basic And Applied Thermodynamics )" p550. Published by Tata McGraw-Hill Education. Quote: "If the cowl is removed from the fan the result is a turboprop engine. Turbofan and turboprop engines differ mainly in their bypass ratio 5 or 6 for turbofans and as high as 100 for turboprop."〕 Some commercial aircraft with turboprop engines include the Bombardier Dash 8, ATR 42, ATR 72, BAe Jetstream 31, Beechcraft 1900, Embraer EMB 120 Brasilia, Fairchild Swearingen Metroliner, Dornier 328, Saab 340 and 2000, Xian MA60, Xian MA600, and Xian MA700, Fokker 27, 50 and 60. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「turboprop」の詳細全文を読む スポンサード リンク
|