Turboprop ( suitcase word from turbojet and propeller ) is a common name for propeller turbine air jet engine (abbreviated PTL ), often also simply referred to as propeller turbine . A turboprop is a heat engine with continuous internal combustion (thermal fluid machine ) and is mainly used as an aircraft engine. Colloquially, an aircraft powered by PTL is often referred to as a "turboprop" (just as an aircraft with turbine jet propulsion is often referred to as a "jet").
Application and use
This type of engine is characterized by a relatively low specific fuel consumption , which is why it is mainly used in transport and short-haul aircraft. Regional airliners with straight, non-swept wings and a cruising speed of 500 to 700 km / h are the main area of application for turboprop engines in civil aviation today. Aircraft with these engines are limited to flight speeds of up to 80 percent of the speed of sound ( Mach 0.8 ), which corresponds to about 870 km / h at an altitude of 8,000 m under normal conditions . In this speed range, turboprops work more economically than pure turbine engines.
Although modern high-performance turboprops can also reach altitudes above 10,000 m (30,000 ft), for certification reasons they are usually limited to a maximum of 8 km altitude (according to Section 21 LuftBO, all members of the flight crew on duty must have oxygen masks to hand and flights over 6,000 m (20,000 ft) require a pressurized cabin and passenger oxygen systems on aircraft for commercial passenger transport ). Due to the lower air pressure in the higher layers of air, the efficiency of turboprop engines also decreases, which is why more and more aircraft with turbofan jet engines are used there.
Another civil application is smaller business aircraft (example: Beechcraft King Air ) and agricultural aircraft . In military terms, turboprops are used in tactical transport aircraft such as the C-130 Hercules or in school and training aircraft such as the Pilatus PC-21 .
Structure and functionality
The turboprop engine consists of a gas turbine that drives a propeller, usually via a speed-reducing gearbox . The thrust of the engine is largely generated by the propeller - the working gas leaving the outlet diffuser carries only max. 10% of the total thrust, which means that the propulsion principle differs significantly from turbojet engines and is similar to the turbofan. To generate thrust, a very large amount of air is moved by the propeller (also in comparison to the amount of working gas flowing through the engine itself), but it is only slightly accelerated. In the case of pure turbojet jet engines , on the other hand, much smaller quantities of the propulsion medium are accelerated much more strongly.
The angle of attack of the propeller blades is changed depending on the flight speed, flight altitude and load , so that both the turbine and the propeller work as consistently as possible in the optimum speed range ( constant speed prop , variable pitch propeller ).
The gas turbine provides the energy to drive the propeller. It sucks in air that is compressed in an axial or radial, mostly multi-stage turbo compressor. Then it goes into the combustion chamber, where the fuel burns with it. The now hot, high-energy combustion gas flows through the turbine, which is mostly axially and multi-stage, expanding and cooling. The energy transferred to the turbine drives the turbo compressor via a shaft on the one hand and the propeller via a gear on the other . The exhaust gases are expelled backwards.
Compared to a conventional drive via piston engines , propeller turbines have the advantage of lower weight with the same output (approx. 0.23 to 0.27 kg / kW) and a smaller frontal area with less air resistance.
Kerosene is commonly used as a fuel for turboprop engines in aviation .
Some turboprops have a free-wheeling turbine. Here the turbine stages that drive the compressor and the auxiliary units and the actual power turbine are not coupled to one another on a common shaft. The gas generator, which consists of the compressor and its own turbine (stage), generates a gas flow that acts on a mechanically separated turbine stage that drives the propeller. With these engines, the speeds of the gas generator and the useful power level can therefore vary considerably depending on the operating status. In the smaller versions, the power turbines are one or two-stage; the compressors can be combined with radial and axial compressors. This type of construction is often used in helicopters because it allows the engine or gas generator to be started while the rotor is held in place by a rotor brake (e.g. CH-53G ). This makes the use of detachable mechanical couplings between turbine and load superfluous, since the rotor system does not have to be rotated by the starter due to the turbine stages, which are only connected by gas dynamics but not mechanically. This also applies to the starting process of free-wheeling propeller turbines, since the power turbine, propeller gear and propeller do not have a direct frictional connection with the starter motor, which can bring the gas generator shaft to starting speed more easily.
The concept is also used in fixed-wing aircraft, such as the Pilatus PC-7 , which is powered by a Pratt & Whitney Canada PT6 propeller turbine. In some designs, drives with a free-wheel turbine are traversed by the gas flow against the actual direction of flight (air entering the compressor from the rear), usually with distinctive exhaust pipes in the front part of the engine cowling. The advantage here is that the gas generator rotor (compressor and turbine) does not have to be designed with a hollow coaxial shaft, but only single short shafts are required to transfer the drive forces generated by the respective turbine stages to the rear of the compressor and to the front of the propeller gear to transfer, which significantly simplifies the construction of such engines.
Prop drive mechanisms
In modern engines, propellers with five or often more overlapping blades are increasingly used, which are usually sickle-shaped and wide, have a very thin profile and sometimes even work as a pair of opposite directions on two coaxial shafts of a turbine. As a result, the same air throughput can be achieved with a shorter blade length and / or a lower speed. The lower speed at the blade tips results in a lower noise level. In addition, this is the only way to keep the current speeds at the blade tips below the range of the speed of sound and reduce the enormous frictional forces involved. Such engines are often referred to as prop fan engines . The naming is based on the turbofan engines , which have a fan ( blower stage ) placed in front of the turbine core to guide a multiple of the air mass flowing through the turbine around it. While the bypass ratio in modern turbofan engines is around 9: 1, it is around 20: 1 in propeller engines and around 100: 1 in conventional turboprop engines. The share of turbine thrust in the total propulsion is thus higher with propfans than with conventional turboprops.
Intensive research efforts were made in the 1980s to optimize the number, shape and arrangement of the airfoils. Externally, prop drive mechanisms usually only differ from conventional turboprops in the design of the propellers. However, the graft drives, in which the blades are arranged behind the turbine, leave a rather unusual visual impression.
History and examples
- Designed by György Jendrassik in 1937 and successfully commissioned in 1940, the Jendrassik CS-1 was the world's first functioning turboprop engine. The engine was supposed to be used in a Hungarian reconnaissance aircraft, but due to stability problems it only achieved 400 HP instead of the originally planned 1000 HP. The project was discontinued by the Budapest company Ganz & Co when the Hungarian military decided in favor of licensed products from Messerschmitt AG .
- The first German turboprop engine ( BMW 109-028 ) was planned for February 1941
- The world's first flight with a turboprop aircraft was carried out on September 20, 1945: A Gloster Meteor was equipped with Rolls-Royce Trent (RB.50) engines . It was a modified Rolls-Royce Derwent that powered five-bladed propellers via a gearbox .
- The first mass-produced turboprop airliner was the Vickers Viscount (first flight in 1948).
- The largest aircraft with this type of engine is the Antonow An-22 , whose Kuznetsov NK-12 MA engines, developed by German Junkers experts under Ferdinand Brandner , with an output of 11,188 kW (15,211 hp) each, are the most powerful turboprop engines ever built.
- The fastest and largest turboprop passenger aircraft in service was the Tupolev Tu-114 . The aircraft was also equipped with Kuznetsov NK-12 engines. (Cruising speed 770 km / h)
- The fastest turboprop aircraft is the Soviet Tupolev Tu-95 bomber , which - also equipped with a variant of the Kuznetsov NK-12 engine - reaches a maximum of around 900 km / h.
- The smallest series turboprop machines are also flown by holders of a private pilot license (PPL) with appropriate “high-performance instruction” and are often Piper PA-46 Malibu Meridian or conversions of the Cessna 205/206/207 .
- The Airbus A400M military transport aircraft is powered by four TP400-D6 turboprops , each of which, with 8,200 kW, is one of the three largest of its kind and uses sickle-shaped propellers. In 2011, they were the first military engines to receive direct civilian approval from EASA.
- Willy JG Bräunling: Aircraft engines: basics, aero-thermodynamics, cycle processes, thermal turbo machines, components and emissions. (VDI book), Springer, 2004, ISBN 3-540-40589-5 , p. 23; limited preview in Google Book search
- Helmut Kreuzer: All propeller airliners since 1945. Air Gallery Edition, Erding 1999, ISBN 3-9805934-1-X , p. 164.
- FlugRevue July 2011, p. 16, EASA approval for TP400