-
(1) rear three-point landing gear
* Rear three-point: This landing gear has a tail strut and two main landing gears. And the center of gravity of the plane is behind the main landing gear. The rear three-point landing gear is mostly used for low-speed aircraft.
-
-
The rear three-point landing gear is simple in structure and suitable for low-speed aircraft, so it was widely used before the mid-1940s. At present, this landing gear is mainly used for light and ultra-light low-speed piston engine aircraft.
The rear three-point landing gear has the following advantages: First, it is convenient to install the tail wheel on the aircraft. Compared with the front wheel, the tail wheel has simple structure, smaller size and mass. Second, during normal landing, three wheels land at the same time, which means that the attitude of the plane when it falls (the fourth stage of the landing process) is the same as that when the ground taxiing stops. That is to say, the angle of attack of ground taxiing is large, so it can use large aircraft resistance to slow down, thus reducing the landing and taxiing distance. Therefore, most of the early aircraft were equipped with rear three-point landing gear.
-
-
However, with the development of aircraft and the continuous improvement of flight speed, the rear three-point landing gear has exposed more and more shortcomings:
(1) When driving at high speed, it is easy to stand upside down (commonly known as holding the top) when encountering front impact or strong braking. Therefore, in order to prevent handstand, the rear three-point landing gear is not allowed to brake strongly, thus increasing the taxiing distance after landing.
-
-
(2) If the actual landing speed is greater than the specified value, the phenomenon of "jumping" will easily occur. Because in this case, the actual angle of attack when the plane is grounded will be less than the specified value, so that the tail wing will lift and only the main wheel will be grounded. At the moment of landing, the impact force acting on the main wheel will produce the moment of raising the head and increase the angle of attack. At this time, the actual speed of the aircraft is greater than the specified value, which will cause the lift to be greater than the gravity of the aircraft and make the aircraft rise again. After that, the speed of the plane dropped rapidly and the plane fell again. This phenomenon that the plane keeps taking off and landing is called "jumping". If the actual speed of the plane when landing is much higher than the specified value, the jump height may be high, and the plane may be damaged if it falls from this height.
(3) Instability during takeoff and landing. For example, during taxiing, some disturbances (crosswind or uneven road surface, which makes the resistance of wheels on both sides unequal) make the aircraft rotate at a certain angle relative to its axis, and then the friction force formed on the strut will produce a moment relative to the center of mass of the aircraft, which will make the aircraft turn at a larger angle.
(4) When stopping, taking off and taxiing, the front fuselage is lifted, and the downward vision is not good.
Based on the above shortcomings, the dominant position of the rear three-point landing gear is gradually replaced by the front three-point landing gear. At present, only a few small low-speed aircraft are still using the rear three-point landing gear.
(2) Front three-point landing gear
* Front three-point landing gear: This landing gear has a front strut and two main landing gears. And the center of gravity of the plane is in front of the main landing gear. At present, the front three-point landing gear is widely used in high-speed aircraft.
-
-
The front three-point landing gear is the landing gear arrangement mode adopted by most aircraft at present. Compared with the rear three-point landing gear, the front three-point landing gear is more suitable for the take-off and landing of high-speed aircraft.
The main advantages of the front three-point landing gear are:
* easy to land, safe and reliable. If the actual landing speed is greater than the specified value, when the main wheel hits the ground, the impact force acting on the main wheel sharply reduces the angle of attack, so it is impossible to produce a "jump" phenomenon like the rear front three-point landing gear.
* Good directional stability and safer crosswind landing. Control and turning are more flexible when sliding on the ground.
* There is no danger of handstand, so forced movement is allowed. Therefore, the running distance after landing can be reduced.
* Because the aircraft fuselage is in a horizontal or nearly horizontal state when stopping, taking off and landing, the downward horizon is better, and the gas discharged from the engine on the jet plane will not be directly sprayed on the runway, so the impact on the runway is less.
-
-
However, the front three-point landing gear still has many shortcomings:
* The front landing gear is difficult to arrange, especially for single-engine aircraft, and the remaining space in the front of the fuselage is very small.
* The front landing gear has a large load, a large volume, a complex structure and a large mass.
* When landing and taxiing, it is at a small angle of attack, and air resistance cannot be fully utilized for braking. The ability to surpass obstacles (ditches, mounds, etc.) when sliding on uneven runways. ) is also poor.
* The front wheel will produce shimmy, which requires equipment and measures to prevent shimmy, which increases the complexity and weight of the front wheel.
-
-
Nevertheless, due to the high landing speed of modern aircraft, and ensuring the safety when landing is the primary determinant of landing gear form, the front three-point landing gear has obvious advantages compared with the rear three-point landing gear in this respect, so it is widely used.
(3) Bicycle landing gear
* Bicycle type: This landing gear has a main landing gear before and after the center of gravity of the aircraft, and an under-wing strut, namely the auxiliary wheel, under the left and right wings of the aircraft.
-
-
Whether it is the front three-point landing gear or the rear three-point landing gear, the main wheel is arranged under the wing, so the main wheel is included in the wing when flying. However, the wings or other structural equipment of some aircraft are so thin that it is difficult to put the main landing gear into the wings. This kind of aircraft (especially the upper wing bomber) often uses bicycle landing gear, such as the American "Stratospheric Fortress" B-52. Because the two main wheels of the bicycle landing gear are on the axis of the fuselage, they are directly stored in the fuselage when flying, and only a smaller auxiliary wheel is installed under the left and right wings.
-
-
Although the bicycle landing gear solves the problem of retracting the main landing gear, it also brings many shortcomings:
* The front landing gear bears a large load, which increases the size and mass.
* It is not easy to get off the ground when taking off and running, which increases the distance between taking off and running. In order to make the aircraft reach the takeoff angle of attack, it is necessary to rely on special measures, such as extending the length of the front landing gear strut or shortening the length of the rear landing gear strut during takeoff and taxiing.
* can't use the method of main wheel braking, but must use steering control mechanism to realize ground turning, etc.
-
-
Due to the above unfavorable factors, bicycle landing gear is generally not used unless it is impossible. At present, only a few aircraft use this landing gear layout, such as the American Harrier AV-8 VTOL aircraft.
* multi-column:
The layout of this landing gear is similar to that of the front three-point landing gear. The center of gravity of the aircraft is in front of the main landing gear, but there are several main landing gear struts, which are generally used in large aircraft. For example, American Boeing 747, Soviet C-5A (military transport plane) and Il 86 (take-off mass 206 tons). Obviously, the use of multiple struts and wheels can reduce the pressure of landing gear on the runway and increase the safety of takeoff and landing.
-
-
Among these four arrangements, the first three are the most basic landing gear forms, and the multi-strut type can be regarded as the improved form of the first three points. At present, the most widely used landing gear layout on modern aircraft is the first three-point layout.
③ Structural classification of landing gear
* Frame landing gear
The main feature of frame landing gear is to connect the wheel with the wing or fuselage through the load-bearing frame. The bars in the load-bearing frame are hinged with the shock-absorbing struts. They only bear axial forces (along their respective axes) and do not bear bending moments. Therefore, the landing gear with this structure is simple in structure and light in weight, and was widely used in light low-speed aircraft in the past. However, due to the difficulty in retracting, modern high-speed aircraft are basically not used.
-
-
* Pillar landing gear
The main feature of strut landing gear is that shock absorber and bearing strut are integrated, and the wheel is directly fixed on the piston rod of shock absorber. The connection form between the upper end of the shock absorber strut and the wing depends on the shrinkage requirements. For retractable landing gear, strut can also be used as retractable actuator. Torque can be transmitted through the torsion arm or through the spline connection between the piston rod and the inner wall of the cylinder of the shock absorber strut. This type of landing gear is simple and compact in structure, easy to put away and light in weight, and it is one of the widely used forms in modern aircraft.
The disadvantage of strut landing gear is that the piston rod bears not only axial force, but also bending moment, which is easy to wear and jam, making the sealing performance of shock absorber worse and unable to adopt large initial pressure.
-
-
* rocker landing gear
The main feature of rocker landing gear is that the wheel is connected with the piston rod of shock absorber through a rotatable rocker arm. Shock absorbers can also be used as load-bearing columns. This type of piston only bears axial force, not bending moment, so it has good sealing performance, can increase the initial pressure of shock absorber to reduce the size of shock absorber, overcomes the shortcomings of column type, and is widely used in modern aircraft. The disadvantage of rocker landing gear is that its structure is complex and the joint is stressed heavily, so the wear is also great during use.