Thrust reversal
Thrust reversal is a method of braking or stopping a vehicle by deflecting the thrust against the direction of movement. One of the best-known uses of thrust reverser is its use in aircraft in order to brake them more quickly and more effectively after touching down on the runway. The thrust reverser is also used in boats and ships .
history
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Until the introduction of controllable pitch propellers in sea and aviation, there was no thrust reversal - at least in aviation. In shipping, there was the possibility - still common today with rigid shaft drives - to stop the main engine and start it again in the opposite direction in order to switch the thrust from forward travel to reverse travel.
The first step in a change came when the reversing gear invented by Citroën made it possible to reverse the direction of rotation of the drive screw without having to run the engine the other way around.
In the course of the Second World War , the controllable pitch propeller (although an invention already 25 years old) was introduced to all those involved in the war, first in aviation and then, after the problems with the much larger propulsion forces had been solved, also in seafaring.
The idea of the controllable pitch propeller was not initially to have a thrust reverser available, but rather, compared to an automobile , to be able to adapt the drive to the required speeds and external loads by changing gears. The pitch to change to a propeller means exactly what you wanted to be able to switch gears, so the same drive power other torques in push to implement. For aviation, it is extremely important that a controllable pitch propeller weighs significantly less than a gearbox .
This was accompanied by the invention of the " reverse gear " in aviation and shipping.
From there, in the US Navy at the end of the 1940s , it was only a small step in development until the reverse thrust flap was also found on the ever-larger passenger aircraft. At the end of the 1950s and the beginning of the 1960s, these had become so large and heavy that various options were tried out to relieve the wheel brakes.
The Boeing 707 was the first civil jet aircraft with a thrust reverser in 1957 , and in 1960 the French Caravelle was also equipped with a thrust reverser, which previously had a braking parachute.
The first water jet propulsion with reverse thrust flap came from the Australian manufacturer Hamilton in the 1950s. Today the thrust reverser has become an indispensable part of its use in sea and aviation.
Mode of action
aviation
In jet engines , flaps behind the thrust nozzle or in the bypass housing ( thrust reverser ) deflect the engine jet or the bypass jet against the direction of flight, thus causing the aircraft to decelerate. With appropriately equipped propeller engines, the propeller blades can be adjusted ( controllable pitch propeller ) in such a way that they generate a forward thrust. This is mainly done to shorten the landing roll distance and to relieve the wheel brakes of an aircraft.
To reduce aircraft noise , often only the thrust reverser flaps are activated while the engine power remains in the idle position. Often there are (depending on the type of aircraft different) limits for the lowest speed for the use of thrust reverser in order to prevent damage to the engine from small blown stones ( FOD ) or the suction of exhaust gases.
The thrust reverser is actuated either by the thrust levers themselves or with the help of additional levers that are attached to their front. Exceptions are e.g. B. the Blackburn Beverley and the C-17 , in which the thrust reverser can also be used for rolling backwards, as well as the Hawker Siddeley Trident and the Douglas DC-8 , in which it could or can be activated in flight. In order to prevent exactly this, the activation of the thrust reverser is impossible with other types of aircraft as long as there is no minimum weight on the wheels (Weight on Wheels, WOW), this for example also with the Airbus A320 combined with the requirement that the altimeter is less than 10 Feet (around 3 meters) and with the additional requirement that the wheels have started to turn, originally in the case of the A320 at a speed of no less than 72 knots . After the accident on Lufthansa flight 2904 , the link between the rotation of the wheel and the prevention of the activation of the thrust reverser was canceled, and the minimum weight on the wheels was reduced from twelve tons to two tons.
The thrust reverser is used on most medium and large commercial aircraft. Some military aircraft that are to be used from short runways also have a thrust reverser, for example some military transport aircraft, the Tornado and the Saab 37 Viggen .
The powerback process , which is used by reverse thrust, causes an aircraft to roll backwards under its own power, especially in the USA, but is rarely used today in the civil sector . B. to leave a "nose-in" parking position.
The automatic triggering of the thrust reversal of only one engine of a twin-engine Boeing 767-300ER of Lauda Air during the climb meant that the aircraft was unable to maneuver and crashed (see also Lauda Air flight 004 ) .
A similar accident occurred on October 31, 1996 after the thrust reverser activated automatically in the right engine of a Fokker 100 operated by the Brazilian airline Transportes Aereos Meridionais shortly after take-off; Here, too, the aircraft was unable to maneuver and crashed (see also TAM Linhas Aéreas flight 402 ) .
Another crash, to which it had been due to a one-sided deployed thrust reverser, occurred in 1978 with a Boeing 737-200 of Pacific Western Airlines (see also Pacific Western Airlines Flight 314 ) .
seafaring
In boats and ships that are propelled by controllable pitch propellers , the pitch of the propeller blades is adjusted so that the direction of thrust is reversed without changing the direction of rotation of the propeller.
The same applies to a Voith-Schneider drive in which only the blades are actuated differently to stop or reverse the boat / ship, but the direction of rotation of the drive does not change.
With a Schottel drive or a small outboard motor , the entire drive / motor is turned 180 degrees around the vertical axis in order to achieve the same effect, a thrust reversal, without changing the direction of rotation of the drive.
In the case of a water jet propulsion system, similar to the jet engine of an aircraft, a flap is moved into the propulsion jet in order to deflect the thrust in the opposite direction.
Paddle
The hand-held single, double paddle or oar of a rowing boat is called a counterstrike. This is done with or without a 180 ° turn of the blade, runs in the direction of travel and is used to brake or turn the boat.
Rating
The force with which a thrust reverser works is measured as a percentage of the propulsive force that is not deflected. Especially with the use of eyelids in jet engines and water jet drives this level of the thrust reverser is already in the construction of an important metric that determines yet in the landing of an aircraft, as will be short stopping distances (and be as long, therefore, the runway must) and Ships / boats, how fast they can still be traveling in reverse, i.e. how maneuverable a boat / ship is still moving with reverse thrust.
However, different drive manufacturers solve the problem in different ways, which means that the details of the shape of the thrust reverser flaps and the angles at which they (must) be guided into the propulsion jet, a certain secretiveness or patents certain constructions in detail protect.
Thrust kickback
When reversing thrust with a thrust reversing flap, the biggest problem is that the deflected propulsion jet can be blown in the direction of the inlet area of the engine. In the case of an aircraft, this can cause foreign objects to be blown up on the runway ( Foreign Object Damage ) and sucked in by the engine , causing serious damage.
It is essential to avoid this thrust kickback in aircraft. This is often achieved by the fact that the thrust reverser deactivates itself on aircraft below certain taxiing speeds. On the Airbus A380 , only the two engines are equipped with thrust reversers in the inner positions to prevent the outer engines, which are already located over the shoulders of the usually 45 m wide runway, from whirling up foreign objects from the adjacent, unpaved grass surfaces.
In the case of ships / boats, the sucking up of the deflected propulsion jet by the propulsion system can result in the total thrust dropping to zero. In the maritime sector, the zero thrust effect of the thrust kickback is used constructively, for example, when a possible forward or reverse thrust performance is fully available, to remain in place while idling and to start off suddenly with full thrust. Another application is sideways travel (or partial support) without a bow and / or stern thruster when there are several water jet propulsion systems .
For this reason, with water jet propulsion, an area is always provided in which a previously defined speed of the drive and a very specific position of the thrust flap lead to a thrust kickback and the associated cancellation of each propulsion.
literature
- Ernst Götsch: Aircraft technology . Motorbuchverlag, Stuttgart 2003, ISBN 3-613-02006-8 .
Web links
Individual evidence
- ↑ Boeing's Jet Stratoliner , Popular Science, Number 165, Jul. 1954, p. 121
- ↑ GlobalSecurity: C-17 Globemaster III. Retrieved June 29, 2010 .
- ↑ Bernd Vetter: Pioneers of the Jet Age, DC-8 , Gera Mond Verlag, Munich (2001) ISBN 3-932785-86-X page 86
- ↑ Roger Shaw (Hsg): Safety and Reliability of Software Based Systems: Twelfth Annual CSR Workshop (Bruges, September 12-15, 1995), Springer Science & Business Media, 2012, ISBN 9781447109211 , page 48