Misplaced Pages

#499500

57-549: The XA2J was intended to be a turboprop -powered derivative of the AJ Savage, with the design as initially proposed in December 1947 a simple modification of the Savage, with extensive use of components of the earlier aircraft. The design gradually evolved, however, to improve performance and increase compatibility with operations from aircraft carriers, as it was recognized that the AJ Savage

114-400: A Pratt & Whitney Canada PT6 , and an under-speed governor on a Honeywell TPE331 . The turboprop is also distinguished from other kinds of turbine engine in that the fuel control unit is connected to the governor to help dictate power. To make the engine more compact, reverse airflow can be used. On a reverse-flow turboprop engine, the compressor intake is at the aft of the engine, and

171-424: A propelling nozzle . Air enters the intake and is 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 stages, generating power at the point of exhaust. Some of the power generated by the turbine is used to drive the compressor and electric generator . The gases are then exhausted from

228-510: A bombing raid. In 1941, the engine was abandoned due to war, and the factory converted to conventional engine production. The first mention of turboprop engines in the general public press was in the February 1944 issue of the British aviation publication Flight , which included a detailed cutaway drawing of what a possible future turboprop engine could look like. The drawing was very close to what

285-499: A compressor and into a combustor where fuel is mixed with the compressed air and ignited. The combustion gases are expanded through a compressor-driving turbine, and then through a "free" power turbine before being exhausted to the atmosphere. The compressor and its turbine are connected by a common shaft which, together with the combustor, is known as a gas generator, which is modelled using the Brayton Cycle . The (free) power turbine

342-434: A ducted fan, followed by the unducted and much lighter F.5 . Development of these engines stopped abruptly owing to corporate takeovers, rather than technical reasons. Rolls-Royce continued with design studies for such engines into the 1980s, as did GE , but they have yet to appear as commercial engines. The advantage of the pusher propfan with a free power turbine is its simplicity. The prop blades are attached directly to

399-440: A gearbox failure, showed a free-turbine arrangement to be more at risk than a single-shaft turboprop. It could suffer a turbine overspeed to destruction after losing its connection to the propeller load. (In a single-shaft arrangement with a similar gearbox failure the turbine would still have most of its load from the compressor). Such a failure resulted in the 1954 accident of the second prototype Bristol Britannia , G-ALRX, which

456-442: A governor, and overspeed governor, and a fuel-topping governor. The governor works in much the same way a reciprocating engine propeller governor works, though a turboprop governor may incorporate beta control valve or beta lift rod for beta operation and is typically located in the 12 o'clock position. There are also other governors that are included in addition depending on the model, such as an overspeed and fuel topping governor on

513-430: A major market for turboshaft engines. When turboshaft engines became available in the 1950s, they were rapidly adopted for both new designs and as replacements for piston engines. They offered more power and far better power to weight ratios. Piston helicopters of this period had barely adequate performance; the switch to a turbine engine could both reduce several hundred pounds of engine weight, 600 lb (270 kg) for

570-410: A mode typically consisting of zero to negative thrust, is used for all ground operations aside from takeoff. The Beta mode is further broken down into 2 additional modes, Beta for taxi and Beta plus power. Beta for taxi as the name implies is used for taxi operations and consists of all pitch ranges from the lowest alpha range pitch, all the way down to zero pitch, producing very little to zero-thrust and

627-450: A number of engine-related mishaps, the XA2J project was abandoned and the second prototype was never flown. General characteristics Performance Armament Related development Turboprop A turboprop is a turbine engine that drives an aircraft propeller . A turboprop consists of an intake , reduction gearbox , compressor , combustor , turbine , and

SECTION 10

#1732848510500

684-581: A small amount of air by a large degree, a low disc loading (thrust per unit disc area) increases the aircraft's energy efficiency , and this reduces the fuel use. Propellers work well until the flight speed of the aircraft is high enough that the airflow past the blade tips reaches the speed of sound. Beyond that speed, the proportion of the power that drives the propeller that is converted to propeller thrust falls dramatically. For this reason turboprop engines are not commonly used on aircraft that fly faster than 0.6–0.7 Mach , with some exceptions such as

741-575: A test-bed not intended for production. It first flew on 20 September 1945. From their experience with the Trent, Rolls-Royce developed the Rolls-Royce Clyde , the first turboprop engine to receive a type certificate for military and civil use, and the Dart , which became one of the most reliable turboprop engines ever built. Dart production continued for more than fifty years. The Dart-powered Vickers Viscount

798-485: Is coupled to the turbine through a reduction gear that converts the high RPM /low torque output to low RPM/high torque. This can be of two primary designs, free-turbine and fixed. A free-turbine turboshaft found on the Pratt & Whitney Canada PT6 , where the gas generator is not connected to the propeller. This allows for propeller strike or similar damage to occur without damaging the gas generator and allowing for only

855-516: Is on a separate shaft. Turboshaft engines are sometimes characterized by the number of spools. This refers to the number of compressor-and-turbine assemblies in the gas generator stage and does not include the free power turbine assembly. As an example, the General Electric T64 is a single-spool design that uses a 14-stage axial compressor; the independent power shaft is coaxial with the gas generator shaft. One particular failure scenario,

912-407: Is that it can also be used to generate reverse thrust to reduce stopping distance on the runway. Additionally, in the event of an engine failure, the propeller can be feathered , thus minimizing the drag of the non-functioning propeller. 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

969-421: Is typically accessed by moving the power lever to a beta for taxi range. Beta plus power is a reverse range and produces negative thrust, often used for landing on short runways where the aircraft would need to rapidly slow down, as well as backing operations and is accessed by moving the power lever below the beta for taxi range. Due to the pilot not being able to see out of the rear of the aircraft for backing and

1026-635: The Napier Gazelle of the Westland Wessex , and also allow considerably more payload weight. For the Westland Whirlwind , this converted the inadequate piston-engined HAS.7 to the de Havilland Gnome turbine-powered HAR.9. As one of the first anti-submarine helicopters, the HAS.7 had been so weight restricted that it could carry either a search sonar or a torpedo, but not both. The free-turbine engine

1083-616: The P-3 Orion , and the C-130 Hercules military transport aircraft. The first turbine-powered, shaft-driven helicopter was the Kaman K-225 , a development of Charles Kaman 's K-125 synchropter , which used a Boeing T50 turboshaft engine to power it on 11 December 1951. December 1963 saw the first delivery of Pratt & Whitney Canada's PT6 turboprop engine for the then Beechcraft 87, soon to become Beechcraft King Air . 1964 saw

1140-830: The Piper Meridian , Socata TBM , Pilatus PC-12 , Piaggio P.180 Avanti , Beechcraft King Air and Super King Air . In April 2017, there were 14,311 business turboprops in the worldwide fleet. Between 2012 and 2016, the ATSB observed 417 events with turboprop aircraft, 83 per year, over 1.4 million flight hours: 2.2 per 10,000 hours. Three were "high risk" involving engine malfunction and unplanned landing in single‑engine Cessna 208 Caravans , four "medium risk" and 96% "low risk". Two occurrences resulted in minor injuries due to engine malfunction and terrain collision in agricultural aircraft and five accidents involved aerial work: four in agriculture and one in an air ambulance . Jane's All

1197-590: The Tupolev Tu-114 can reach 470 kn (870 km/h; 540 mph). Large military aircraft , like the Tupolev Tu-95 , and civil aircraft , such as the Lockheed L-188 Electra , were also turboprop powered. The Airbus A400M is powered by four Europrop TP400 engines, which are the second most powerful turboprop engines ever produced, after the 11 MW (15,000 hp) Kuznetsov NK-12 . In 2017,

SECTION 20

#1732848510500

1254-403: The Tupolev Tu-95 . However, propfan engines, which are very similar to turboprop engines, can cruise at flight speeds approaching 0.75 Mach. To maintain propeller efficiency across a wide range of airspeeds, turboprops use constant-speed (variable-pitch) propellers. The blades of a constant-speed propeller increase their pitch as aircraft speed increases. Another benefit of this type of propeller

1311-519: The Soviet Union had the technology to create the airframe for a jet-powered strategic bomber comparable to Boeing's B-52 Stratofortress , they instead produced the Tupolev Tu-95 Bear, powered with four Kuznetsov NK-12 turboprops, mated to eight contra-rotating propellers (two per nacelle) with supersonic tip speeds to achieve maximum cruise speeds in excess of 575 mph, faster than many of

1368-464: The World's Aircraft . 2005–2006. Free-turbine turboshaft A free-turbine turboshaft is a form of turboshaft or turboprop gas turbine engine where the power is extracted from the exhaust stream of a gas turbine by an independent turbine, downstream of the gas turbine. The power turbine is not mechanically connected to the turbines that drive the compressors, hence the term "free", referring to

1425-420: The additional expansion in the turbine system, the residual energy in the exhaust jet is low. Consequently, the exhaust jet produces about 10% of the total thrust. A higher proportion of the thrust comes from the propeller at low speeds and less at higher speeds. Turboprops have bypass ratios of 50–100, although the propulsion airflow is less clearly defined for propellers than for fans. The propeller

1482-414: The amount of debris reverse stirs up, manufacturers will often limit the speeds beta plus power may be used and restrict its use on unimproved runways. Feathering of these propellers is performed by the propeller control lever. The constant-speed propeller is distinguished from the reciprocating engine constant-speed propeller by the control system. The turboprop system consists of 3 propeller governors ,

1539-476: The engine nacelles to ease storage aboard ship. It had a crew of three: pilot, co-pilot/bombardier, and gunner who sat in a pressurised cabin in the nose of the aircraft. Up to 10,500 lb (4,800 kg) of bombs could be carried in a large enclosed bomb-bay in the center fuselage, while the planned defensive armament was a remotely controlled tail turret with two 20 mm cannon. Construction of two prototypes started 1 October 1948, but due to delays developing

1596-521: The engines, the first flight was not until 4 January 1952. The competing Douglas XA3D , the prototypes of which were ordered the year after construction had begun on the XA2J prototypes, first flew in October 1952. The A3D had far superior performance, which doomed the XA2J. The root cause for the failure of the XA2J was the protracted development and poor reliability of the Allison T40 engines. The T40 engine

1653-407: The exhaust is situated forward, reducing the distance between the turbine and the propeller. Unlike the small-diameter fans used in turbofan 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. Since it is more efficient at low speeds to accelerate a large amount of air by a small degree than

1710-493: The first jet aircraft and comparable to jet cruising speeds for most missions. The Bear would serve as their most successful long-range combat and surveillance aircraft and symbol of Soviet power projection through to the end of the 20th century. The USA used turboprop engines with contra-rotating propellers, such as the Allison T40 , on some experimental aircraft during the 1950s. The T40-powered Convair R3Y Tradewind flying-boat

1767-546: The first deliveries of the Garrett AiResearch TPE331 , (now owned by Honeywell Aerospace ) on the Mitsubishi MU-2 , making it the fastest turboprop aircraft for that year. In contrast to turbofans , 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. Modern turboprop airliners operate at nearly

North American XA2J Super Savage - Misplaced Pages Continue

1824-408: The free-turbine design has come to dominate and these designs are also mostly reversed overall, with their air inlet and compressor to the rear, feeding forwards to hot section and power turbine at the front. This places the turbine output close to the propeller gearbox, avoiding the need for a long driveshaft. Such engines are often recognisable externally, as they use external 'elbow' exhausts ahead of

1881-458: The future Rolls-Royce Trent would look like. The first British turboprop engine was the Rolls-Royce RB.50 Trent , a converted Derwent II fitted with reduction gear and a Rotol 7 ft 11 in (2.41 m) five-bladed propeller. Two Trents were fitted to Gloster Meteor EE227 — the sole "Trent-Meteor" — which thus became the world's first turboprop-powered aircraft to fly, albeit as

1938-444: The gas generator core and power turbine revolved in opposite directions, reducing the overall moment of inertia. For the helicopter engine replacement market, this ability allowed previous engines of either direction to be replaced simply. Some turboshaft engines' omni-angle freedom of their installation angle also allowed installation into existing helicopter designs, no matter how the previous engines had been arranged. In time though,

1995-459: The independence of the power output shaft (or spool). This is opposed to the power being extracted from the turbine/compressor shaft via a gearbox. The advantage of the free turbine is that the two turbines can operate at different speeds and that these speeds can vary relative to each other. This is particularly advantageous for varying loads, such as turboprop engines. A free-turbine turboshaft ingests air through an intake. The air passes through

2052-459: The main engine. A particularly common example of this is the PT6 engine, of which over 50,000 have been produced. An attractively simple configuration making use of the free turbine is the propfan engine, with a rear-mounted unducted fan in pusher configuration , rather than the more familiar tractor layout. The first such engine was the very early and promising Metropolitan-Vickers F.3 of 1942 with

2109-657: The most widespread turboprop airliners in service were the ATR 42 / 72 (950 aircraft), Bombardier Q400 (506), De Havilland Canada Dash 8 -100/200/300 (374), Beechcraft 1900 (328), de Havilland Canada DHC-6 Twin Otter (270), Saab 340 (225). Less widespread and older airliners include the BAe Jetstream 31 , Embraer EMB 120 Brasilia , Fairchild Swearingen Metroliner , Dornier 328 , Saab 2000 , Xian MA60 , MA600 and MA700 , Fokker 27 and 50 . Turboprop business aircraft include

2166-399: The move towards axial LP compressors and so smaller diameter engines encouraged a move to the now-standard layout of one or two engines set side-by-side, horizontally above the cabin. Turboprop aircraft are still powered by a range of free- and non-free turbine engines. Larger engines have mostly retained the non-free design, although many are two-shaft designs where the 'power' turbine drives

2223-608: The need for static vanes. The M1 Abrams main battle tank is powered by a Honeywell AGT1500 (formerly Textron Lycoming ) two-spool gas turbine engine. A commercial derivative has been designed as the TF15 for marine and railroad applications, and a flight-rated version, the PLT27, was also developed but lost a major contract to the GE T700 turboshaft. Turboshaft engines were used to power several gas turbine locomotives , most notably using

2280-439: The original Bristol Proteus and the modern TP400 have free turbines. The TP400 is a three-shaft design, with two compressor turbines and a separate power turbine. Where the turbine is at the rear of the engine, a turboprop engine requires a long drive shaft forwards to the propeller reduction gearbox . Such long shafts can be a difficult design problem and must carefully control any shaft vibration. For small turboprop engines,

2337-420: The outside of the rotating turbine disc. No gearboxes or drive shafts are required. The short length of the rotating components also reduces vibration. The static structure of the engine over this length is a large diameter tube within the turbine. In most designs, two contra-rotating rings of turbine and propeller are used. Intermeshed contra-rotating turbines can act as the guide vanes for each other, removing

North American XA2J Super Savage - Misplaced Pages Continue

2394-485: The power section (turbine and gearbox) to be removed and replaced in such an event, and also allows for less stress on the start during engine ground starts. Whereas a fixed shaft has the gearbox and gas generator connected, such as on the Honeywell TPE331 . The propeller itself is normally a constant-speed (variable pitch) propeller type similar to that used with larger aircraft reciprocating engines , except that

2451-460: The power turbine to accelerate and bring the rotor to its operating speed from stationary in just 15 seconds and a time from engine start to take-off of only 30 seconds. A further advantage of the free turbine design was the ease with which a counter-rotating engine could be designed and manufactured, simply by reversing the power turbine alone. This allowed handed engines to be made in pairs when needed. It also allowed contra-rotating engines, where

2508-551: The propeller and the low-pressure compressor while the high-pressure compressor has its own turbine. The first free-turbine gas turbine engine was the Bristol Theseus turboprop. This was the first Bristol gas turbine and its broad design had been produced by Frank Owner at Tockington Manor . It first ran in July 1945 and in December 1946 was the first turboprop to pass a 100 hour type test . Some large turboprop engines, such as

2565-522: The propeller to rotate freely, independent of compressor speed. Alan Arnold Griffith had published a paper on compressor design in 1926. Subsequent work at the Royal Aircraft Establishment investigated axial compressor-based designs that would drive a propeller. From 1929, Frank Whittle began work on centrifugal compressor-based designs that would use all the gas power produced by the engine for jet thrust. The world's first turboprop

2622-403: The propeller-control requirements are very different. Due to the turbine engine's slow response to power inputs, particularly at low speeds, the propeller has a greater range of selected travel in order to make rapid thrust changes, notably for taxi, reverse, and other ground operations. The propeller has 2 modes, Alpha and Beta. Alpha is the mode for all flight operations including takeoff. Beta,

2679-585: The same speed as small regional jet airliners but burn two-thirds of the fuel per passenger. Compared to piston engines, their greater power-to-weight ratio (which allows for shorter takeoffs) and reliability can offset their higher initial cost, maintenance and fuel consumption. As jet fuel can be easier to obtain than avgas in remote areas, turboprop-powered aircraft like the Cessna Caravan and Quest Kodiak are used as bush airplanes . Turboprop engines are generally used on small subsonic aircraft, but

2736-415: The turbine. In contrast to a turbojet or turbofan , the engine's exhaust gases do not provide enough power to create significant thrust, since almost all of the engine's power is used to drive the propeller. Exhaust thrust in a turboprop is sacrificed in favor of shaft power, which is obtained by extracting additional power (beyond that necessary to drive the compressor) from turbine expansion. Owing to

2793-493: Was an ambitious engine design with two power sections, (the T38 was developed from the T40 to assist in its development, by using a single power section with extension shaft and gearbox), driving two large contra-rotating propellers through a combining gearbox. Both the engines and the gearbox proved to be unreliable. The T40 engine was also used in the developmental of other aircraft . After

2850-468: Was deficient in performance and was a less-than-satisfactory carrier aircraft. The A2J was essentially an enlarged AJ Savage with the two reciprocating engines replaced with two Allison T40 turboprop engines and removal of the tail-mounted turbojet . Like the AJ, it was a high-winged monoplane with unswept wings. The wings were fitted with leading edge slats and large trailing edge flaps, and folded outside of

2907-557: Was designed by the Hungarian mechanical engineer György Jendrassik . Jendrassik published a turboprop idea in 1928, and on 12 March 1929 he patented his invention. In 1938, he built a small-scale (100 Hp; 74.6 kW) experimental gas turbine. The larger Jendrassik Cs-1 , with a predicted output of 1,000 bhp, was produced and tested at the Ganz Works in Budapest between 1937 and 1941. It

SECTION 50

#1732848510500

2964-469: Was fitted to prevent a similar reoccurrence. Writing in 1994, Gunston found it remarkable that protection was not common on free-turbine engines. However, certification regulations allow other methods for preventing excessive overspeed such as disc rubbing and blade interference. Most turboshaft and turboprop engines now use free turbines. This includes those for static power generation, as marine propulsion and particularly for helicopters. Helicopters are

3021-526: Was forced to land in the Severn Estuary . A failure in the Bristol Proteus propeller reduction gearbox led to an overspeed and release of the power turbine of Nº3 engine. It cut through the oil tank and started a fire that threatened the integrity of the wing spar . The pilot, Bill Pegg , made a forced landing on the estuary mud. The Proteus gears were redesigned and an emergency fuel shut-off device

3078-471: Was found to be particularly suitable. It does not need a clutch, as the gas generator may be started while the output shaft remains stationary. For the Wessex, this was used to give a particularly fast take-off from a cold start. By locking the main rotor (and the power turbine) with the rotor brake, the engine could be started and then, with the gas generator at a speed of 10,500 rpm, the brake released allowing

3135-460: Was of axial-flow design with 15 compressor and 7 turbine stages, annular combustion chamber. First run in 1940, combustion problems limited its output to 400 bhp. Two Jendrassik Cs-1s were the engines for the world's first turboprop aircraft – the Varga RMI-1 X/H . This was a Hungarian fighter-bomber of WWII which had one model completed, but before its first flight it was destroyed in

3192-635: Was operated by the U.S. Navy for a short time. The first American turboprop engine was the General Electric XT31 , first used in the experimental Consolidated Vultee XP-81 . The XP-81 first flew in December 1945, the first aircraft to use a combination of turboprop and turbojet power. The technology of Allison's earlier T38 design evolved into the Allison T56 , used to power the Lockheed Electra airliner, its military maritime patrol derivative

3249-593: Was the first turboprop aircraft of any kind to go into production and sold in large numbers. It was also the first four-engined turboprop. Its first flight was on 16 July 1948. The world's first single engined turboprop aircraft was the Armstrong Siddeley Mamba -powered Boulton Paul Balliol , which first flew on 24 March 1948. The Soviet Union built on German World War II turboprop preliminary design work by Junkers Motorenwerke, while BMW, Heinkel-Hirth and Daimler-Benz also worked on projected designs. While

#499500