The principal functions of a gear landing consist in allowing the evolutions on the ground until takeoff (towing, taxi…), the damping of the impact of landing, and, thanks to a brake associated, stopping of the plane at an acceptable distance.
The landing gear can sometimes be equipped with Ski S or floats if the apparatus must land on sea or land on the Neige.
During the phases of flight, if the landing gear does not fold up, the train is fixed, if not, it retractable and returned to decrease resistance to the air. It is placed then in the landing gear box which can be in the Fuselage or in the wing S.
HistoryThe history of the landing gear goes up with 1876. Alphonse Pénaud and Paul Gauchot, two French inventors, patents the plan of a Monoplan Amphibie two-seater revolutionist for his time. One of the innovations of this Aéroplane is that it has a retractable landing gear with compressed-air shock absorbers. But it is not before 1917 that the first apparatuses equipped with partially retractable landing gears will make their appearance and they will become common only at the end of the Twenties. At that time, the performances of the planes had improved so much that the aerodynamic advantage of the train retractable amply justified the complexity and the overweight of the system.
Various types of landing gears
There exist mainly two types of landing gear:
- trains “traditional”, (also called taildraggers ), which is composed of 2 principal trains in front of the auxiliary train and center of gravity to the back,
- the “three-wheeled” trains which are composed of 2 principal trains slightly to the back of the one caster and center of gravity to the front one.
The weight increasingly more important of the aircraft, the landing gears take into account wheels more and more. The Airbus A340 has a third train (called central train) between the two principal ones, the Boeing 747 has five landing gears: one with front, two pennies wings and two pennies the fuselage a little behind, like the Airbus A380, which has in all 22 wheels. The planes having 3 landing gears use the nose gear wheel to move when they are on the tarmac. The 747 and A380, them, also use both train interiors which can against-direct when the nose gear wheel directs, in the same way as for the cars with 4 driving wheels, the wheels are directed in the direction opposed to that of the nose gear wheels to facilitate the turns.
A dysfunction of the landing gear can lead so that one calls a Wooden horse and to have for consequence the destruction of the plane.
Certain planes use the wheels only for the Décollage and throw then them, to gain in weight, in place, since there does not need more mechanism of retraction and in simplicity. For these planes, the Atterrissage is done on skis, for example. Among the historical examples, let us quote the Messerschmitt Me 163 and the Messerschmitt Me 321.
Another example of unusual landing gear is the train “monotrace” which one finds on certain military aircrafts like SO.4050 Vautour or the Hawker Siddeley Harrier. On these planes, the landing gear is composed of two wheels principal rows under the fuselage (the train is called Tandem ), and of a small wheel close to the tip of each wing.
A train with multiple tandems was used on certain military jets in the years 1950, like the Myasishchev M-4, the Yakovlev Yak-25, the Yak-28 and the Boeing B-47 Stratojet, because it allows a great capacity of carrying between the principal wheels. Another variation of this Tandem is used on the Boeing B-52 Stratofortress which has 4 bogie trucks principal under the fuselage, and a small wheel supporting each wing. The train of the B-52 is single also thanks to the fact that each of the four bogie trucks is directional. That largely facilitates the landing in the event of wind of through (by using the technique known as of the crab landing).
Principle of an existing landing gear
principal structural elements (see diagram):
The box (barrel) constitutes the “body” of the undercarriage. It contains the shock absorber and ensures the transmission of the principal efforts towards the structure plane.
The main strut (drag strut ESA) makes it possible to transmit the axial loads coming from the center coils with the structure of the plane.
The compass (torque link ESA), on the principal trains, makes it possible to prevent the rotation of the sliding tube compared to the box. On the nose gear, it makes it possible to transmit the torque between the system of direction (steering) and the sliding tube.
The shock absorber (shock to absorb) makes it possible to absorb the energy of the impact of landing and supports the ground maneuvers while ensuring a maximum of comfort for the crew and the passengers. The shock absorbers are generally of oil and air type. There exist shock absorbers simple room or double room.
the system of extension/retraction
The sequence of extension/retraction of the trains is conditioned by the position of the stick control “sunken/left” the trains and by the answer the detectors of positions of the train and the trap doors. It orders successively the opening of the trap doors, the unlocking of the train, the extension of the train, the locking of the train in low position, then the closing of the principal trap doors (and conversely at the time of the retraction). The back trap doors are ordered directly by the undercarriage.
In normal mode, the exit of the undercarriage is ensured by the action of the jack of operation (actuating cylinder) which is used at the same time as shock absorber of end of the road in order to avoid a locking low too violent, while the jack of unlocking press the secondary brace (lock link) on its thrusts to prop up it. In help mode (known as “Free Fall”), the exit of the train is done by gravity, helped by the aerodynamic efforts. 2 draw springs (lock springs) ensure and maintain the locking of the undercarriage in low position, by blocking propped up the 2 arms of the secondary strut and by consequence that of the main strut (drag strut).
At the time of the retraction of the train, hydraulic flows supplying the actuators are reversed. The retraction is started by the action of the jack of unlocking which breaks the jamming of the secondary strut and consequently that of the main strut to which it is connected. The system thus unbolted went up in the box of train using the jack of operation. cases of fixing (uplock assembly) make it possible to lock the trains and the trap doors in high position.
the system of direction (Nose Wheel Steering)
The action of the pilot on the ordering of direction is transmitted to the jack of direction (steering actuator) via a calculator (BSCU) and of a hydraulic system. The jack is equipped with a toothed rack in catch with the pinion of the revolving tube. The rotation of the turning tube is transmitted to the axle via the compass. In other cases, and to reduce the obstruction, the system pinion/toothed rack is replaced by a concept using 2 jacks (known as “push/sweater”) operating directly the revolving tube.
Wheels and the brake
The rim ensures the support of the tire as well as the housing of the brake. The brake consists of multiple discs (generally out of carbon) and of clamps with pistons. The braking of a plane at the time of a phase of landing or of the interruption of a procedure of takeoff (Rejected Take Off), requires the dispersion of a very great quantity of energy. The brake constitutes in this phase a well of heat. The wheels are thus subjected to important constraints.
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