Liquid-gas shock absorbers(Fig. 81) are telescopically connected cylindrical parts that form the working chamber. Typically, the upper part of the shock absorber 1 is fixedly attached to the aircraft structure, and the axle for the wheels is attached to the second, movable part 2. To prevent (for some struts to limit) rotation of the moving parts of the shock absorber around the vertical axis, a two-link chassis (spline-joint) is used. The working chamber of the rack is divided into two cavities by a diaphragm 4 with a calibrated hole.
The internal cavity of the rack is filled with a strictly dosed amount of liquid and gas under pressure.
Liquids poured into the rack must have a well-defined viscosity with as much constancy as possible even with significant fluctuations in ambient temperature in order to reduce the effect of changes in viscosity on the operation of the shock absorber. The initial gas pressure in shock-absorbing struts usually ranges from 15 to 50 kg/cm2, and for some aircraft it reaches several hundred atmospheres.
The tightness of the telescopic connection is achieved by installing sealing cuffs made of leather, rubber, or elastic plastic. During flight, the shock absorber strut is decompressed under the influence of gas pressure. When an aircraft lands and moves along the airfield, the strut has more or less compression, depending on the flight weight of the aircraft, landing conditions, runway surface and other factors. In this case, the liquid is placed in the lower part, and the gas is placed in the upper part, but when the shock absorber operates, the gas and liquid are vigorously mixed, forming a mixture.
When the wheels hit the ground, under the influence of the ground reaction force, the rod with the piston moves inside the stationary cylinder. The internal volume of the rack decreases and the liquid is pushed out at high speed through the hole in the diaphragm, and then passes through the holes in the pipe 6 of the plunger. Impact energy is spent on increasing gas pressure, overcoming hydraulic resistance when liquid passes through a calibrated hole and friction of sealing collars or rings in the rack. In this case, part of the energy is converted into heat. By selecting the area of the passage holes and changing them during operation, depending on the degree of participation of the liquid in absorbing impact energy, it is possible to obtain a shock absorber in which the main amount of energy is absorbed during forward stroke or only during reverse stroke, or equally during forward and reverse stroke.
For shock absorbers with main forward braking, the reverse motion of the shock absorber parts occurs vigorously, which causes the aircraft to bounce. In shock absorbers with main braking on the reverse stroke, the forward stroke uses mainly gas and partly liquid, which enters the cylinder cavity through a hole in the diaphragm. From the cylinder cavity located above the diaphragm, liquid through the hole in the piston head 5 enters the annular cavity between the rod and the cylinder, formed when the rod moves. In this case, the spool ring 3 is pressed down and allows the liquid to freely fill the annular cavity. On the reverse stroke, the flow area of the hole from the annular space decreases due to the upward movement of the spool ring, and the liquid converts most of the work accumulated by the gas during the forward stroke into heat. Such shock absorbers are called shock absorbers with primary braking on the reverse stroke. In modern aviation, shock absorbers with reverse braking are the most widely used.
Liquid shock absorbers Due to their small size and weight, they are being used more and more often. The elastic medium in such shock absorbers is liquid, which at high pressures can noticeably change its volume. The use of such shock absorbers became possible only after a reliably operating seal had been created that could withstand pressures of the order of 3,000-4,000 kg/cm 2 for a long time. The energy is absorbed due to the hydraulic resistance of the fluid flowing through small holes from cavity to cavity, as well as the frictional forces of the shock absorber parts as they slide mutually.
Rubber shock absorbers. In shock absorbers, rubber is used in the form of a cord consisting of individual rubber threads enclosed in a double braid of cotton threads, or in the form of plates of various thicknesses and shapes. The cord shock absorber works in tension, and the plates work in compression. The main disadvantages of rubber shock absorbers are low hysteresis, loss of elasticity at low temperatures, destruction under the influence of gasoline and oil, large dimensions and short service life. Currently, such shock absorbers are rarely used and only on light aircraft.
Oil-spring and oil-rubber shock absorbers. The creation of such shock absorbers was caused by the desire to eliminate the disadvantages inherent in rubber and steel shock absorbers - low hysteresis, large required stroke. Shock absorbers of this type existed before the creation of reliable seals, after which they were replaced by gas-liquid shock absorbers, which use compressed nitrogen or air instead of rubber or springs.
Literature used: "Fundamentals of Aviation" authors: G.A. Nikitin, E.A. Bakanov
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The two main supports of such a landing gear are located behind the center of mass of the aircraft, and the third support is installed in the forward part of the fuselage. To ensure controllability of the aircraft on the ground, this support is either freely oriented or equipped with a forced rotation system for the front wheels.
The scheme is characterized by the following parameters:
b - chassis base;
B - chassis track;
H - chassis height;
e - removal of the main supports;
g is the angle of extension of the main supports;
jo - tipping angle;
jst - parking angle.
These parameters are related to the landing angle α pos, the installation angle α mouth and the take-off angle α flap of the wing.
The take-off run of an aircraft with this landing gear configuration is performed in a three-point position when:
α break = jst + α set.
At the end of the takeoff run, by deflecting the elevator, the pilot tears off the front support, and then the main supports are lifted off the ground. The aircraft lands on the main supports with the wing attack angle
α pos = jo + α set. + jst
followed by transferring to the front support. The transfer condition is ensured by the offset angle g = jo + (1 - 2)о.
This condition gives the value of the relative offset e/b = 0.1 - 0.15, which shows the proportion of the load from the total force of gravity falling on the front support when parked.
The absence of sideways tipping is ensured by an angle e equal to (40 - 45)o, which corresponds to a relative track B/b = 0.7 - 1.2.
The front landing gear design offers the following important advantages:
Easier piloting technique for takeoff, landing and run;
Stability of movement during the takeoff and run, which is ensured by the application of friction forces of the wheels of the main supports behind the center of mass of the aircraft;
Improved visibility from the cockpit when driving on the ground;
Easy to maneuver when using the front wheel steering system;
More intense braking during the run and the possibility of high-speed landing, which is ensured by eliminating the danger of the aircraft capping;
The floor position of the passenger and cargo cabins, as well as the axes of the engines, is close to horizontal, which eliminates the blowing of the hot gases of the turbojet engine onto the GDP.
The disadvantages of the scheme include the greater weight of the chassis due to the longer front support and the possibility of self-oscillations of the front support of the “shimmy” type. To dampen these vibrations, the front support is equipped with hydraulic dampers - vibration dampers of the front wheels.
Bicycle chassis diagram.
The landing gear consists of a front landing gear, similar to the three-point front landing gear, and a rear landing gear, mounted on the fuselage behind the aircraft's center of mass. This design avoids installing the main landing gear on the wing. In this case, they are installed on the wing
only auxiliary supports, which, in the absence of aircraft roll, may not touch the ground
Main parameters of the scheme:
b - chassis base;
H - chassis height;
B" - track of underwing struts;
g - extension angle of the main support;
b - front support angle.
There are two types of bicycle chassis:
1) chassis with rear support angle g = (25 - 30)o and e/b = 0.1 - 0.15.
The parameters of such a chassis, except for the track, are selected similarly to the parameters of a tricycle chassis with a nose gear. The takeoff and landing of such an aircraft are no different from similar modes of an aircraft with the landing gear discussed above. 
2) chassis with g = (40 - 60)o and e/b = 0.4 - 0.5.
The impossibility of tearing off the front support during takeoff requires takeoff from both supports simultaneously, and the necessary increase in the angle of attack of the wing at the end of the takeoff is ensured either by lengthening the front support or by shortening (squatting) the rear support. The complexity of the design of such supports and the difficulty of piloting the aircraft during takeoff and landing limit the use of this landing gear design. It is usually only used on military aircraft.
Multi-leg chassis.
On heavy aircraft with a very high take-off weight, in order to reduce and more evenly distribute the load on the runway, it is necessary to increase the number of landing gear supports. A front support design may use three, four or more main supports. The number of front supports of more than two makes it very difficult to maneuver the aircraft on the ground, so even on very large aircraft more than two front supports are not installed. To improve maneuverability with a large number of supports, in addition to the steerable front supports, sometimes the main supports are also made controllable - all or only some of them (front, rear). The parameters of a multi-support chassis are selected in the same way as the parameters of a three-support chassis. In this case, the point of application of the resulting ground reaction forces on the wheels of the main supports when the aircraft is parked is taken as the rollover point.
When landing, an aircraft with a multi-leg landing gear first touches the ground with the rear wheels of the main landing gear, then rolls over onto the remaining main and front wheels. The rear shock absorbers, which are the first to touch the ground, are made softer than the others.
Chassis loads.
During takeoff and landing of an aircraft, as it moves around the airfield, and while parked, static and dynamic loads act on the landing gear wheels. Their magnitude and direction are determined by the landing gear layout, the conditions and nature of the landing, the type of runway, the characteristics of the shock-absorbing system, etc. These loads can be represented in the form of three component forces applied to the wheels, directed along the main coordinate axes of the aircraft:
Px - front impact force;
Py - vertical force;
Pz - side impact force.
The magnitude of these loads is determined by strength standards or aviation regulations (AR), which specify the main design cases of landing gear loading, overload and safety factor for each case, the magnitude of the load, its direction and distribution between the supports and wheels. Based on the loads found in this way, design diagrams are constructed and all the necessary strength calculations are carried out.
Structurally - chassis power circuits.
The chassis support consists of the main power element - the strut, a device for absorbing and dissipating the energy of shock loads - the shock absorber and support devices - the wheels.
The structural and power diagrams of the landing gear supports can be classified according to the following criteria:
The method of attaching the rack to the aircraft;
The method of placing the shock absorber on the support;
The method of attaching the wheels to the rack.
Methods of attaching the strut to the aircraft.
Based on this feature, a distinction is made between cantilever and strut-mounted rack mounting schemes.
With a cantilever design, the rack is rigidly fixed (pinched) in the upper fastening unit and, in terms of power, represents a cantilever beam working in bending. Rigid sealing is ensured by locking the rack in the released position with a mechanical lock of one design or another. The pinching of the non-retractable rack is ensured by the design of its fastening unit.
The main disadvantage of this scheme is that in the root part the rack absorbs large bending loads, which greatly increase its mass.
In the braced design, the strut (1) is equipped with additional struts (2) in one or two planes, which significantly reduce bending moments in the root part of the strut and, as a rule, provide an overall gain in chassis weight.
The struts can be folded to ensure cleaning. Chassis lifts are sometimes used as a brace. In both cases, reliable fixation of the rack in the released position must be ensured. The braced design, in addition to gaining weight in the structure, also provides a more rigid fastening of the strut to the aircraft, which has a beneficial effect on eliminating certain types of self-oscillations of the struts that occur when the aircraft moves on the ground. The strutted landing gear design is the most widely used on modern aircraft.
Shock absorber placement diagrams.
Depending on the location of the shock absorber relative to the power element of the support - the strut, there are telescopic (a), lever (b and c) and semi-lever (d) strut designs.
The telescopic (a) rack combines a power element - a tubular rack and a shock absorber. The strut pipe acts as a shock absorber cylinder, into which a rod with a piston fits, forming a telescopic pair with the cylinder. The wheels are suspended at the lower end of the rod. To prevent rotation of the rod in the cylinder, both of these elements are connected by a two-link joint (spline joint), which ensures only translational movement of the rod in the cylinder under the action of an axial compressive load. The disadvantages of this scheme include the lack of damping of lateral loads and front impact loads, as well as high friction in the axle boxes and shock absorber seal under the action of these loads. Partial absorption of the front impact with this scheme can be ensured by giving the strut a certain angle of inclination in the longitudinal plane parallel to the plane of symmetry of the aircraft. Greater participation of the shock absorber in the perception of front impact force can be achieved by using a swinging telescopic strut design. In this scheme, the rack is hingedly suspended in the upper fastening unit and is fixed in the released position by a rigid strut attached in front to the middle hinge of the two-link link. When there is a front impact on the wheels, the force in the strut causes the shock absorber to compress, which reduces the load and provides a softer transfer of the front impact energy to the landing gear and aircraft structure. When the shock absorber is compressed, the strut rotates (swings) relative to the upper hinge, which explains the name of this scheme.
The lever design of the strut is characterized by the fact that the wheels in this case are fixed to a lever, which is hinged to the strut or fuselage.
The shock absorber rod is connected to the lever by a spatial hinge, which completely eliminates the transmission of bending moments to the shock absorber and provides ideal conditions for the operation of the shock absorber seal and axle boxes. The following types of lever racks are used:
Lever strut with an internal shock absorber, which is located inside the strut (b);
Lever strut with a remote shock absorber attached to the outside of the strut (a);
Lever diagram without rack (d).
In addition to improving the operating conditions of the shock absorber, the lever circuit provides shock absorption for the front impact, during which the lever rotates and the shock absorber is compressed.
The semi-lever design (c) is a combination of telescopic and lever struts. In this scheme, the lever with wheels is hinged not to the strut, but to the shock absorber rod, and an additional link is installed between the lever and the strut at the front using two hinges - an earring, which ensures compression of the shock absorber when the wheels are loaded. The shock absorber comes into operation both with a vertical load and with a front impact on the wheels, but the force of the front impact itself is transmitted to the rod and causes it to bend.
Wheel mounting diagram.
Attaching the wheels to the shock absorber rod or to the lever can be done using a fork, half-fork, axle shaft or two axle shafts.
Placing more than four wheels on one axle makes it very difficult to maneuver the aircraft and place the wheels in the retracted position. Therefore, for four or more wheels on one support, multi-wheel carts are usually used, designed to accommodate four, six or eight wheels on two or three axles. The wheel axles are mounted on a power element - the trolley frame. The fastening of the axles to the frame can be fixed or movable (in plain bearings), depending on the method of transmitting braking torques from the wheels to the rack. 
To equalize the loads between the axles, the trolley is hinged to the stand, which requires the installation of an additional stabilizing shock absorber that sets the position of the trolley relative to the stand and dampens the vibrations of the trolley relative to the hinge.
The use of multi-wheeled chassis bogies requires a special method of transmitting the braking torques of the wheels to the rack. If the braking moments of the wheels are transferred to the axles of the trolley, then the trolley frame, under the influence of these moments, will rotate relative to the hinge of the trolley, increasing the load on the front wheels and unloading the rear ones.
This leads to uneven wear of the wheels and reduces braking efficiency while driving. To eliminate the influence of braking torques on the redistribution of load between the wheel axles, these moments are usually not transmitted to the bogie frame. In this case, the brake housing is movably mounted on the axle (or the axle together with the brake housing is hinged in the frame) and is kept from rotating during braking by a special rod attached to the strut (shock absorber rod) above or below the bogie suspension hinge. The location of such a brake rod should obey a simple rule - the axis of the rod should be directed to the point of intersection of the line passing through the bogie hinge axis and the wheel axis with the ground line when the wheel tires are compressed. If the bogie hinge and the wheel axles are located on the same horizontal plane, then the brake rod is located horizontally.
Features of fastening the front wheels.
The design features of the front landing gear are associated with the need to ensure controllability of the aircraft when moving on the ground. For this purpose, a free orientation mode is required for the front wheels. Stability of movement in this mode is ensured by the creation of a stability arm (t), at which the point of contact with the ground by the wheels is behind the turning axis of the wheels.
After the aircraft takes off from the ground, the freely oriented wheels must be automatically installed in a neutral position in the plane of symmetry of the aircraft. For this purpose, the design of the front support provides a special mechanism for setting the wheels to the neutral position. One of them is shown in the figure. This shock absorber has a pair of profiled cams, one of which is connected to the rod (upper), and the other to the cylinder. After the shock absorber is lifted off the ground by the charging pressure, the rod moves out and the upper cam, sliding along the lower stationary cam, sets the rod and wheels to a neutral position.
When an aircraft moves along the ground at high speed, the deformation of the wheels and struts under load causes sharp turns of the wheels in both directions.
Such self-oscillations of the front struts are called “shimmies”. To eliminate shimmy, the front wheels are equipped with special hydraulic dampers. When the wheels turn, the movement is transmitted to the piston or blades of this damper, which drive fluid from one cavity to another through small calibrated holes.
With rapid oscillatory movements of the wheels, the fluid resistance increases sharply, which eliminates the development of self-oscillations. When maneuvering an aircraft, the turning speed of the wheels is low and the damper does not have a significant effect on the taxiing qualities of the aircraft.
On heavy aircraft and on aircraft with a bicycle landing gear, the front supports are equipped with a system for forced rotation of the wheels at the pilot's commands. When this system is turned off, the wheels go into free orientation mode.
Main supporting elements of the chassis.
Aircraft wheels are the most common support elements for landing gear in modern land aircraft. The wheels on the main supports must be equipped with brakes. Tail legs, bicycle chassis auxiliary legs, and most front legs use non-braking wheels.
Aviation wheels.
The wheels are used to move the aircraft on the ground. The wheel consists of a tire, a housing and a brake.
Pneumatics.
A pneumatic consists of a tire and a tube mounted on the wheel body. Chamber 4 with valve 5 is placed inside the tire. 
A charging pressure p0 is created in the chamber through the valve. Recently, tubeless pneumatics, in which the volume between the tire and the wheel body is sealed, have become increasingly widespread. In such pneumatics, the inside of the tire is covered
sealing layer of rubber 7. Multi-layer pneumatic frame 3 is made of high-strength cord consisting of synthetic or steel threads. Stiffening rings 6, made of steel wire, are embedded in the sides of the frame. The outside of the frame is covered with a protective layer of rubber 2. A protector 1 made of high-quality rubber is applied to the rim of the tire. The outside tread has grooves of a specific pattern to increase traction with the airfield surface. Non-brake wheels can be manufactured with a smooth surface. On pneumatic tires used in winter, metal spikes can be installed to increase traction with the ground. Grooves on the surface of the tire ensure that water is squeezed out from under it when driving on a wet airfield, thereby eliminating aquaplaning (floating) of the wheels at high speed.
Pneumatics are characterized by:
Overall dimensions;
Outer diameter D;
The greatest width is B;
Cross-sectional shape:
Balloon,
Arched,
Round,
Charging pressure:
High pressure - more than 1.5 MPa,
Medium pressure - 1 - 1.5 MPa,
Low pressure - 0.5 - 1 MPa,
Ultra-low pressure - less than 0.5 MPa.
With an increase in charging pressure p0, the dimensions and weight of the tire decrease, the permissible load on the wheel increases, but its maneuverability deteriorates - the required strength of the soil or airfield runway surface increases.
Wheel housing.
The wheel housing (6) is made by casting from aluminum or titanium alloy. Recently, wheels have appeared with bodies of two stamped halves connected by bolts. Angular contact bearings are pressed into the housing hub on both sides.
The bearings are protected from dirt by a special seal 1. An adjustable spacer sleeve 2 is inserted between the bearings, calibrated to a certain tightening of the bearings. Pneumatics are mounted on the body and fixed on it with two flanges 3 and 4, one of which (4) is removable and consists of two halves, which are connected by special locks 5.
Brakes (7) are installed inside the wheel housing. Depending on the type of brake, steel finned brake jackets or
splines (8) for brake discs are installed
Wheel brakes.
Brakes serve to reduce the length of the flight after landing, ensure maneuvering of the aircraft during taxiing, its immobility during parking and when testing engines. The brakes must ensure the creation of a maximum braking torque on the wheel, determined by the maximum value of the coefficient of friction of the wheel on the runway surface, as well as the absorption and dissipation of the kinetic energy of the aircraft during the run.
Three types of brakes are in practical use: shoe, chamber and disc.
A brake shoe consists of two or more rigid brake pads coated with a special friction material (retinax), which has a high coefficient of friction and can withstand heating up to 10,000 C.
The pads are hinged on the brake body, which is fixedly mounted on the wheel axis. Outside, above the pads, there is a steel drum with fins (jacket), bolted to the wheel body and rotating with it. The brake pads are pressed against the drum by special hydraulic cylinders at the pilot's signals and brake the wheel. When the brakes are released, the springs return the pads to their original position.
The energy consumption of the shoe brake is low, so its use is justified only on light aircraft with low landing speeds.
The chamber brake consists of a brake housing 2 fixedly mounted on the wheel axis, on which a large number of brake pads 4, covered with friction material, are installed around the circumference.
Due to the radial grooves, the pads can move relative to the body only in the radial direction, and with special leaf springs 6 they are constantly pressed towards the wheel axis. On the brake body, under the pads, there is a flat annular rubber chamber 3, into which compressed air or pressurized hydraulic mixture is supplied from the aircraft braking system. The chamber, expanding and overcoming the action of the springs, presses the brake pads against a steel drum mounted on the wheel body and brakes it. This type of brake ensures uniform pressure of all brake pads against the drum and does not require adjustment of the gaps between the pads and the drum, but due to the presence of a rubber chamber that is afraid of overheating, its energy intensity is also low.
The disc brake operates on the principle of a friction clutch. It consists of a set of alternating movable and fixed discs mounted on the brake body.
The movable disks 1 are connected by splines to the wheel housing 2 and rotate with it. Fixed discs 3 are connected along the inner surface with keys to the brake housing 4, bolted to the wheel axle. From the end, the disk pack is compressed by an annular piston 5, creating a braking torque between the disks. When the brake pressure is released, the piston returns to its original position using special springs.
Disc brakes are compact, have high energy consumption, and do not require a precise concentric arrangement of the wheel and brake body, so they are widely used on modern aircraft.
Automatic braking is used to prevent the wheel from completely jamming and skidding when braking.
For this purpose, an inertial sensor is installed on the wheel, the housing of which is fixedly fixed to the brake body. A roller with small gear 1 rotates in the sensor housing. This gear meshes with large gear 2, mounted on the wheel body. When the wheel rotates, the sensor shaft rotates at a speed of several thousand revolutions per minute.
A flywheel is installed on the roller, which is connected to the roller by spring-loaded friction linings. The frictional forces in these linings spin the flywheel, and it rotates together with the roller. When skidding occurs, the wheel and sensor shaft begin to lose angular rotation speed. The flywheel, due to inertial forces and, overcoming frictional forces in the linings, rotates relative to the roller and, due to inclined bevels, moves along the axis. This movement is used to turn on the microswitch and send a signal to the solenoid valve, which relieves pressure in the braking system. This prevents wheel skidding and ensures highly efficient wheel braking while cruising.
Chassis shock absorbers.
During landing, an aircraft with a landing weight mpos approaches the ground with a certain vertical speed Vy. Kinetic energy of aircraft vertical movement
A = (mpos Vy2)/2 must be absorbed during the collision with the ground by those parts of the aircraft that are deformed under the influence of shock loads. Due to these deformations, the center of mass of the aircraft drops down to the ground, or we can assume that the wheels move upward relative to the center of mass of the aircraft under the action of the vertical reaction of the ground P. At the end of the impact, the vertical speed of the aircraft drops to zero, the ground reaction forces increase to a maximum value Pmax, and the work of these forces on the full movement of the wheels relative to the center of mass of the aircraft Hmax will be equal to the total kinetic energy of the impact A. The value of Pmax determines the overload and design loads for all elements of the aircraft during landing. For their
reduction, it is always desirable to reduce the value of Pmax, and this is only possible by increasing the displacement of Hmax during the collision of the aircraft with the ground. For this purpose, special elements are included in the landing gear design - shock absorbers, the main purpose of which is to increase the deformation of the aircraft supports and increase Hmax. In addition to shock absorbers, the movement of the aircraft's center of mass upon impact is significantly affected by the deformation of the wheel tires. Elastic deformations of the structure - wing, fuselage, etc. have little effect on the displacement of Hmax and are usually neglected.
Thus, the main property that a shock absorber must have is its elasticity - the ability to deform under load.
During the impact, the tire tires of the wheels and shock absorbers, deforming, absorb (accumulate) all the impact energy A. At the end of the impact, when the speed Vy is completely extinguished, the force Pmax, acting on the aircraft, begins to move it upward and return the energy accumulated in the tires and shock absorbers back to the aircraft . The energy accumulated by the pneumatics is almost completely returned to the aircraft during the return stroke. If the shock absorbers returned all the accumulated energy to the plane on the return stroke, then the plane would again take off from the ground and make such jumps for quite a long time. To prevent this from happening, the design of the shock absorber necessarily provides for the possibility of reducing the forces, and, consequently, the energy returned to the aircraft during the return stroke.
As a result, the shock absorber dissipates part of the impact energy, usually turning it into heat, completely eliminating repeated jumps of the aircraft during landing.
It follows that the second most important property of a shock absorber is its ability to dissipate impact energy, turning it into heat.
The elastic properties of a shock absorber are ensured by the inclusion in its design of special elastic bodies or elements - rubber, steel springs, springs, gas, liquid. In terms of specific (per unit mass) energy intensity, the most advantageous of them are gas and liquid, which are used in liquid-gas and liquid shock absorbers, which are widely used on modern aircraft. The liquid in these shock absorbers ensures energy dissipation due to its flow with high resistance from one cavity to another, which is accompanied by heating of the liquid and the conversion of mechanical energy into thermal energy.
Liquid-gas shock absorber.
The main elements of a liquid-gas shock absorber are cylinder 1, a rod 2 that moves progressively in it, a plunger 3, a profiled needle 4, a braking valve 6, a seal package 7, which ensures sealing of the internal volume of the shock absorber. The rod is supported on the cylinder by bronze axle boxes. The upper axle box 5 is connected to the rod and moves with it, and the lower one is fixedly fixed in the lower part of the cylinder. The shock absorber is filled with liquid through special valves to a certain level and charged with compressed nitrogen to the initial pressure po.
Under the action of compressive loads, the rod enters the cylinder, the volume of the gas chamber decreases, and the pressure in it and the load on the rod increase. Liquid from the lower cavity of the rod flows into the upper cavity of the cylinder through the annular gap between the needle and the plunger, experiencing great resistance. Next, the liquid passes through the holes in the axle box 5 into the annular cavity between the rod and the cylinder. At the same time, the ring valve 6 moves down and opens a free passage for liquid. The force P applied to the rod during the forward stroke is spent on compressing the gas Pg, overcoming the forces of resistance to the flow of liquid Rz, the friction forces in the axle boxes and seals Pm and the inertia forces Rin of the elements moving with the rod.
Rp.x. = Rg + Rzh + Rt + Rin.
The work of inertial forces is small and can be neglected.
The figure shows the nature of the change in the listed forces depending on the movement of the rod d when compressing the shock absorber.
Gas pressure and force Pr are determined by a polytrope with index k = 1.1 - 1.2. Pr is the force created by the pressure of the initial charging of the shock absorber. The force of resistance to fluid flow is directly proportional to the square of the ratio of the rod speed to the area of the passage holes for the fluid.
The shaded areas in this figure show the amount of energy absorbed by each of the listed forces.
The total work absorbed by the shock absorber is equal to the sum A = Ar + As + At.
It can be expressed through the maximum force Pmax and rod movement dmax
The work of friction and fluid forces turns into heat and is dissipated, and the work expended on gas compression is accumulated and returned to the aircraft on the return stroke. During the reverse stroke of the rod, which occurs at a lower speed, the liquid flows in the opposite direction. The ring valve rises upward with liquid and sharply reduces the area of the passage holes in the axlebox 5, which ensures energy dissipation during the return stroke. The change in force Pr during the reverse stroke occurs along the same polytrope as during the forward stroke. The forces of friction and resistance of the liquid are subtracted from the forces created by the gas P = Rg - Rzh - Rm.
The work of the forces of friction and fluid resistance also turns into heat on the return stroke and is dissipated.
In the shock absorber operation diagram, the area between the forward and reverse stroke curves shows the total work dissipated by the shock absorber DA = A1 - A2 (hysteresis loop). In modern shock absorbers, the total dissipated work is 50 - 60% of the energy A1 absorbed during forward travel.
Total absorbed impact energy during landing Adef. when the center of mass of the aircraft is lowered by an amount Ne due to deformations of the shock absorber, wheel tires and structure, it will determine the maximum load on the wheels SPke.
During a rough landing with increased vertical speeds, the fluid resistance increases sharply, which leads to an increase in the design loads on the shock absorber - the appearance of peak overloads (f). To eliminate this drawback, two-chamber liquid-gas shock absorbers were developed.
Double-chamber liquid-gas shock absorber.
The shock absorber parameters are determined based on the estimated vertical speed Vy and the corresponding impact energy during landing. However, most landings performed by experienced pilots occur at speeds Vy significantly lower than the calculated ones. In this case, it is desirable to have a softer shock absorber, which will provide less load during landing. For this purpose, it is desirable to reduce the initial charging pressure of the shock absorber. Typically it corresponds to a force equal to 0.5 - 0.6 of the parking load. A further decrease in po reduces the energy reserve of the shock absorber during the take-off run, when the load on the wheels is maximum and soft. 
The shock absorber will be heavily compressed. A compromise solution can be obtained by using a two-chamber shock absorber.
In such a shock absorber, two gas chambers are created, charged with different initial pressures - a low pressure chamber (H) and a high pressure chamber (B). At the initial moment of compression of the shock absorber, the low-pressure chamber comes into operation, and when the pressure in it becomes equal to the charging pressure of the second chamber, both chambers begin to work together. Due to the increase in the total volume of compressed gas, the compression polytrope becomes flatter. In a two-chamber shock absorber, the charging pressure in the first chamber (H) can be reduced to 0.1 - 0.15 of the parking load and a very soft shock absorber can be obtained when landing. If the parking load during the take-off run is chosen to be close to the load at the fracture point of the polytrope, then due to its small slope behind the fracture point, it is possible to obtain a sufficient supply of shock absorber energy capacity during the take-off and run to absorb shock loads when hitting bumps, especially at high speed at the end of the take-off run.
Diagrams of the operation of a two-chamber shock absorber are shown in the figures, which retain the same symbols as in the previous section. On these diagrams Rst.vzl - indicates the parking load on the shock absorber at the take-off weight of the aircraft.
Shock absorbers with unloader valve.
Due to the use of liquid resistance in the forward stroke, the liquid-gas shock absorber has a fairly high (up to 0.8 - 0.85) coefficient of completeness of the operation diagram, which ensures its high energy intensity with a small stroke of the rod. This energy intensity is needed only when the aircraft lands at the moment of its first impact with the ground. All other modes of aircraft movement on the ground - run, take-off, taxiing maneuvers - do not require high energy intensity of the shock absorber. In these modes, the shock absorber absorbs the energy of shock loads when the wheels hit bumps on the airfield. The energy of these impacts is small, but they are accompanied by sharp, high-speed movements of the shock absorber rod, which, with a high coefficient of completeness of the work diagram and at high speeds of the aircraft, leads to large peak loads transmitted to the landing gear and the aircraft. To reduce these loads, it is desirable to have a soft shock absorber, even with a lower energy intensity and a lower coefficient of completeness of the operation diagram. This can be achieved by reducing or even completely eliminating fluid resistance when the shock absorber operates at the above aircraft motion modes. This transformation of a hard liquid-gas shock absorber into a soft pure gas shock absorber is ensured by the inclusion in its design of a special unloading valve, which, when the aircraft first hits the ground, reduces the area of the passage holes for liquid, and when the aircraft moves along the ground when the shock absorber is parked, the valve opens additional channels for the flow of liquid , which turns the shock absorber into a gas one. Reducing shock peak loads when the aircraft is moving, especially during the takeoff and run, has a beneficial effect on the service life of the landing gear and other components of the aircraft.
Scheme of retracting and releasing the landing gear using the example of the landing gear of the An-26 aircraft.
The landing gear support is retracted and released by power cylinders. When retracting the main landing gear, fluid from the hydraulic system flows in parallel into the upper cavity of the power cylinder and the hydraulic cylinder of the expansion lock. In this case, the reverse arrow of the deflection is selected; the thrust does not subsequently interfere with folding the strut and retracting the shock strut. The power cylinder retracts the shock strut by rotating it until it locks into the retracted position.
When retracting the shock strut, using a mechanism kinematically connected to it, the front wings of the support compartment are opened and then closed. The doors open fully at a shock strut rotation angle of 35°, and begin to close 6° before the strut is fully retracted. In the closed position, the doors are locked with a mechanical lock, which is controlled from the shock strut retracted position lock.
When the main landing gear is released, fluid from the hydraulic system first enters the hydraulic cylinder of the shock strut retracted position lock, opening it and the associated door lock. Only after opening these locks does the liquid enter the lower cavity of the power cylinder, which, due to the damping device, ensures shock-free completion of the shock strut release. At the end of the release, the thrust links, under the action of their springs, are installed on a mechanical stop, forming a reverse deflection arrow, thereby fixing the support in the released position.
Opening and closing of the front compartment doors when releasing the shock strut occurs in the same way as when cleaning, but in the closed position the doors are not locked with a lock.
When retracting the front landing gear, fluid from the hydraulic system simultaneously enters the hydraulic cylinder of the lock in the extended position and the hydraulic cylinder for retracting and releasing the front landing gear. The lock opens, the shock strut begins to retract, and at the same time the centering device and the control mechanism for the front and middle flaps are activated, which open to an angle of 85° and allow the front shock strut to pass into the chassis compartment. At the end of cleaning, the lock of the retracted position is closed and at the same time all doors of the front support compartment are closed.
When releasing the front landing gear, the mechanisms operate in the reverse order. During release, the lock in the released position closes, and the front and middle doors close at the same time.
The Yak-18T aircraft has main landing gear of a single-column truss-beam design with side and rear struts and direct attachment of the wheel to the shock absorber rod. The main landing gear (Fig. 45, 46) are installed in the center section and consist of the following elements.
Post 1 is the main power element of the main leg, transmitting loads from the wheel to the aircraft. It experiences loads from forces and moments along all three axes. Like the front landing leg design, the main leg strut is integral with the shock absorber.
The folding strut 2 (side) absorbs the forces acting on the stand from the lateral force applied to the wheel and increases the rigidity of the stand structure in the lateral direction. Consists of upper and lower links. The rigid strut (see Fig. 45) 4 (rear) absorbs the forces acting on the strut in the plane of the wheel and increases the rigidity of the strut structure in the longitudinal direction.
The lift cylinder 6 and the retracted position lock 8 perform the same functions as similar structural elements of the front chassis leg.
Axle 5 and king pin 7 serve to attach and fix the shock absorber strut of the main landing gear leg in brackets located respectively on the rear spar and center section diaphragm; made of forging material 30ХГСА.
Shield 9 serves to partially close the niche when the main leg is retracted. Wheel 10 - support for the main landing gear leg, brake. To indicate the position of the main leg, a mechanical indicator 3 is mounted on it.
The main legs of the chassis in the retracted position are held by mechanical locks, in the extended position - by ball locks of the lift cylinders and side folding struts.
The shock-absorbing strut of the main landing gear leg (Fig. 47) consists of a steel cup (made of material 30KhGSA), a steel rod with an axle shaft for fastening the wheel, a splined joint that secures the rod from rotating around a vertical axis, and shock-absorbing parts. In the upper part, the glass 4 has eyes for the axle 14 and the kingpin 2, with the help of which the main strut is attached to the center section, as well as a bracket 1 for attaching the eye bolt of the lifting cylinder rod to the stand.
In the middle part of the glass, which is a thick-walled steel pipe, there is an upper charging fitting 3, attachment points for the shield rods and mounting eyes for the rigid and folding struts. In its lower part, the glass has an eye for attaching the upper link of the splined hinge and the suspension unit of the rack to the lock in the retracted position.
The suspension unit is an eye with a bolt 12 inserted into its holes with an internal spacer and an outer 11 steel bushing and two washers 10. The washers and paws of the eyes have a corrugated surface for adjusting the position of the bolt with the bushing. A nut is screwed onto the bolt and secured with a cotter pin.
Inside the glass in its lower part, using a nut 26 locked with a screw, a fixed axle box 23 with seals is installed, and using a locking ring 28, a seal 27 with an oil seal 25 is installed in the nut.
The shock-absorbing strut rod is hollow and made of 30KhGSA material. A unit with an axle shaft is welded to the lower end of the rod for attaching a wheel with a lower charging fitting and an eye for fastening the lower link of the spline joint. In the upper part, using a nut 20, locked with a cotter pin 21, a package of shock-absorbing parts is fixed, moving together with the rod and consisting of a movable axle box 16, a split ring 17, a valve 18, made in the form of a steel ring with three holes Æ 1.4 mm for leakage fluid, bushings 22 and 15. The movable axle box 16 and bushing 22 are made of BRAZHMTS material.
Using a nut 20, a piston 24 is installed on the rod, which has the ability to move inside the rod (stroke 120 ± 3 mm) and divides the cavity of the shock-absorbing strut into two chambers D and D, isolated from each other.
Through the lower fitting, chamber D is charged with nitrogen to a pressure of 65 ± 1 kgf/cm2, through the socket of the upper fitting, chamber D is filled with AMG-10 oil, and through the fitting it is charged with nitrogen to 24 ± 1 kgf/cm2. The design of the fittings is similar to the fittings of the front shock absorber strut. The tightness of the main shock absorber strut is ensured by the use of seals consisting of fluoroplastic washers and rubber rings located in the annular grooves on the inner and outer surfaces of the fixed axle box and the outer surface of the piston. The operation of the main landing gear strut is similar to that of the front strut.
The compression diagram of the main shock strut is shown in Fig. 48.
The work of depreciation during forward stroke is presented in the diagram as a curve abc. As in the diagram (see Fig. 39) of the compression of the front strut, the curve abc clearly breaks down into two sections: ab - shows the work of shock absorption during a normal landing (the work of the upper chamber D of the shock absorber strut); bc - the work of the lower chamber G. The latter comes into operation when the energy of a rough landing is absorbed or the aircraft overcomes a high obstacle when moving along the airfield. The share of work spent on overcoming the hydraulic resistance of the fluid in the total volume of work absorbed by the shock absorber during forward stroke is slightly higher than when compressing the front strut, which can be seen in section bc of the diagram characterizing the operation of the lower chamber of the shock absorber. Depreciation during the reverse stroke is carried out mainly by braking the fluid in the valve 18, which is pressed against the axlebox 16, and the liquid is forced out of the cavity between the cup 4 and the bushing 15 only through the holes in the valve and the axlebox.
The force curve ned when the rod moves downward, shown in the compression diagram of the main strut, consists of two sections that characterize the operation of the upper and lower chambers of the shock absorber.
Folding and rigid struts. Folding strut 2 (see Fig. 45) serves to fix the main landing gear leg in the extended position, transfers forces from the shock-absorbing strut to the center section assembly and, together with the lift cylinder, enters the mechanism for retracting and releasing the main landing gear leg.
The strut consists of upper and lower links stamped from 30KhGSA material, connected to each other by a bolt and nut.
The lower link of the strut is connected to the shock-absorbing strut, the upper link is connected to a bracket on the wall of the chassis niche. A ball insert is installed under the connecting bolt in the lower link of the strut. The nuts of the connecting bolts of the upper and lower links are locked with cotter pins.
The upper link of the strut is pivotally connected to a bracket on the wall of the landing gear niche and to a lifting cylinder. The connection to the lift cylinder is carried out using a special eye bolt, rotating in bronze bushings pressed into the boss of the upper link of the strut. Using a bolt and nut locked with a cotter pin, the strut eye bolt is connected to the eye bolt screwed into the lift cylinder rod.
An AM800K limit switch is installed in the bracket of the upper link of the strut, and an adjustable pressure screw is screwed into the bracket of the lower link. When retracting the landing gear, the strut folds, the pressure screw releases the limit switch rod from pressing, and the green signal lamp of the extended position of the main landing gear leg on the landing gear alarm panel in the cockpit goes out.
In the extended position of the main leg, the links of the folding strut are installed in the spacer and fixed in this position by a lift cylinder, the rod of which is locked with a ball lock, which prevents the strut from folding from external lateral forces acting on the chassis leg. The lower strut link pressure screw presses the limit switch rod and the green main leg extended position indicator light on the chassis indicator light illuminates. The reverse arrow of the strut deflection down from the straight line is 5 ± 0.2 mm.
Rigid strut 4 (see Fig. 45), connecting the axle to the stand, is a thick-walled steel tube with a diameter of 25X2, into which a fork and an ear are welded. Using a fork, the strut is attached to the axle, and using an ear - to the rack. The strut is fastened with bolted connections. The bolt nuts are secured with cotter pins.
The main landing gear retraction and extension cylinder is similar in design to the front strut lift cylinder. The eye of the lift cylinder is attached to an eye bolt installed on the upper link of the strut, and the rod is screwed into it with an eye bolt to a bracket (see Fig. 45) installed on the bolts securing the king pin to the shock absorber strut cup. The difference in the operation of the main leg lift cylinder from the front leg lift cylinder when the landing gear is extended is that the main leg is locked in the extended position and the ball lock is closed when the rod is retracted into the cylinder body.
Main landing gear shield. Shield 9 (see Fig. 45) serves to partially close the landing gear niche when the main leg is retracted. It consists of a casing and a rigidity stamped from D16 material welded to it. The rod is attached to the lower casing of the center section using a ramrod loop, and to the shock-absorbing strut using two length-adjustable steel rods. The rods connect the brackets on the shield with the units welded to the shock-absorbing strut cup. The nuts of the bolts connecting the rods to the brackets on the shield and the bolts connecting the rods to the shock-absorber strut cup are secured with cotter pins.
The lock for the retracted position of the main landing gear 8 (see Fig. 45) is secured with four bolts with anchor nuts to the wall of the niche of the main landing gear leg. In terms of the design of the elements and the principle of operation, the lock is similar to the lock for the retracted position of the front landing gear leg. When the lock on the landing gear signal board in the cockpit is open, the red signal lamp for the retracted position of the main landing gear legs goes out.
Wheel. A K141/T141 brake wheel is installed on each shock-absorbing strut of the main legs of the chassis.
The brake wheel (Fig. 49) consists of a wheel and a chamber brake. When installed on an aircraft, the brake wheel is assembled together with pneumatics measuring 500x150 mm. The wheel consists of a drum 3, which carries special structural units, and is a casting made of magnesium alloy ML4 or ML5. In the inner cavity of the drum there is a brake jacket 10, in which a chamber brake is located.
The flange 2 is made removable to facilitate the installation of the pneumatic 1 on the wheel. In the assembled wheel, the flange is held in the axial direction by two locking half-rings 9, and from rotation by bushings installed in the grooves of the flange and drum.
The wheel rotates on tapered radial-thrust roller bearings 5. Their outer rings are pressed into the drum hub socket. Internal races with rollers are mounted on the axle shaft 14 of the shock-absorbing strut rod and tightened with nut 6. On the outer sides, the bearings are protected from clogging and leakage of lubricant by a cap and a felt seal ring. The wheel is covered with a shield 7 to prevent dirt from getting into the internal cavities.
The chamber brake, located in the brake jacket 10, consists of a brake body 12, twelve pads 15, a brake chamber 17, a fitting 18 with a flange, return springs 16, a fairing 11, as well as fastening parts. Housing 12 is cast from magnesium alloy ML4 or ML5. With six bolts 13, the housing (and with it the entire brake) is attached to the flange of the shock absorber rod axle shaft. Pads 15 are reinforced - friction plastic is pressed together with a metal frame. The outer surface of the pads forms a friction pair with the surface of the jacket 10. The pads are able to move only in the radial direction under the pressure of compressed air supplied to the brake chamber 17 through the fitting and elbow 19.
Return springs 16 of the type of tape springs pass through the end grooves in the pads and move the pads away from the jacket after releasing pressure from the brake chamber.
The fairing 11 has four holes, closed with special covers and used to monitor the wear of the pads during operation.
When you press the brake levers installed on the control wheels, air enters the brake line and is distributed by the PU-8 (U138) differential into the brake chamber of the left or right wheel, depending on the position of the pedals. The pressure of compressed air supplied to the brake chamber creates a spacer force that moves the pads in the radial direction. The pads, moving, overcome the force of the return springs 16 and are pressed against the brake jacket 10, having previously selected the gap between the pads and the jacket. When they come into contact, frictional forces arise, creating a braking torque. When pressure is released from the brake chamber, the return springs press the pads away from the jacket to their original position. A gap is established between the brake pads and the wheel jacket to ensure free rotation of the wheel on the axle shaft.
The mechanical indicator of the position of the main landing gear (see, Fig. 45) consists of three main elements: an earring, a fork and the indicator itself 3. The earring stamped from AK-6 material is mounted on the bolt securing the rigid strut 4 to the axis 5 of the suspension strut hinge. Using a bolt with a nut secured with a cotter pin, the earring is connected to a steel fork, which is screwed directly into the indicator.
When the landing gear is extended, the indicator extends beyond the center section contours at a distance of 70 mm in front of the rear spar. The hole in the center section skin for the indicator exit is edged with a fluoroplastic piston. When retracting the chassis, axis 5 rotates in the main leg mounting bracket, and along with it the shackle changes its position. In this case, the pointer is retracted into the center section, and the pilot receives information that the struts are in the retracted position.
Landing gear on an airplane is not only connected through wheels (or
skis) aircraft with the surface of the earth, but also perform
a very important task - to absorb shocks and vibrations during landing,
takeoff and taxiing on the ground. Therefore, the landing gear represents
is a rather complex structure, with moving parts and
elastic elements. The last ones are hydraulic or
pneumohydraulic shock absorbers and have a very noticeable detail
– stock. According to the requirements for tightness, the rod is polished and shiny,
like... a mirror. Just look at the excavator, there is a lot
hydraulic cylinders with shiny rods, no matter how dirty and “dead”
neither was the car itself.
If on the prototype the shock absorber rod was not covered with corrugated
cover (as, for example, on the MiG-3), it is very noticeable and, if
neatly imitated, this greatly adds realism to the model
and entertainment.
When it comes to painting, there are many good ones.
metallic paints, for example, the “metal” series from Testors,
“silver” paint from the Super Zvezda series. And if it's your fault
manufacturer's part simulating a rod is not “quite round”
cross-sectional shape? Then you will have to do some modifications. Or a rework
if treatment with “little blood” does not produce results.
We will need drills (or rather, a set of drills of different diameters),
a not very sharp needle and a very sharp knife, preferably a vise and
metal tube of suitable diameter, for example, a needle
medical syringe. The company produces sets of excellent pipes
Model Point, there are diameters for all occasions in modeling life.
Separate the stand from the sprue.
Remove with a knife
trace of the junction of the mold halves and possible flash. 
First either
we cut or completely remove the hinge, the so-called. two-link 
If it is given
a separate part, just don’t glue it on yet. Cutting off the rod
not to the very “root”, i.e. not to where it starts
rack body, and leave ~0.5 mm of the former rod with each
sides. 
Carefully,
so as not to deform, clamp the stand in a vice and mark with a needle
the center of the future hole for the rod. Speaking in locksmith's terms,
we cap it. 
Now
the most interesting, but also the most crucial stage begins -
drilling We start with a drill with a diameter half the size needed,
that is, we make a centering hole. 
Need to drill
taking your time, constantly monitoring the process so that the drill does not “go away”
to the side, did not warp. After passing about 2-3 mm, you can
stop and start “drilling” with a drill of the required diameter,
those. equal to the diameter of the rod. In this case, the one who does not
cut off, a piece of the former stock. 
Drilling holes in both parts of the body
stand, take the tube and cut a piece a little longer
the length of the former rod by 3-5 mm, depending on the drilled
holes in the rack housing. The set of parts is ready! 
It remains
Having pre-painted the parts, assemble everything into a single structure.
The new rod is perfectly round in cross-section,
absolutely does not need painting and is pleasing to the eye, honest,
real metallic shine. 
The aircraft landing gear is designed for parking and movement on the ground. It is usually equipped with shock absorbers, which absorb shock energy when the aircraft lands and when moving on the ground, and brakes, which provide braking for the aircraft during its run and taxiing. In addition to wheeled landing gear, aircraft can be equipped with skis, floats (seaplanes), and tracks (all-terrain aircraft).
Comparative evaluation of various chassis schemes
For a stable position of the aircraft on the ground, a minimum of three supports are required. Depending on the location of the main and auxiliary supports relative to the center of gravity of the aircraft, the following basic schemes are distinguished: with a tail support, with a front support and a bicycle type. Airplanes equipped with landing gear with a tail support have the main supports in front of the center of gravity of the aircraft, located symmetrically relative to its longitudinal axis, and the tail support behind the center of gravity (Fig. 72, a).
For an aircraft equipped with nose gear, the main legs (legs) are located behind the aircraft's center of gravity, symmetrically relative to its longitudinal axis; the front support is located in the plane of symmetry of the aircraft, ahead of the center of gravity (Fig. 72, b).
For aircraft with a bicycle-type landing gear, the center of gravity is usually located at approximately equal distances from the wheels or wheeled bogies, which are located in the longitudinal plane of the aircraft, one behind the other (Fig. 72, c). The side supports located at the ends of the wing do not absorb shock loads during landing and takeoff.
Bicycle-type landing gear is used on high-speed aircraft, since it has become impossible to retract the landing gear into thin wings (the landing gear is retracted into the fuselage, and small side supports into the wing).
The most widely used on modern aircraft is a tricycle landing gear with a nose gear, which is explained by a number of advantages that an aircraft equipped with such a landing gear receives.
The advantages of this chassis scheme include:
the ability to land at a higher speed (while landing is easier and safer). This is explained by the fact that the nose strut protects the aircraft from capping (falling over the nose), which also allows the wheels to be braked more vigorously. Moreover, the “goat” of the aircraft is also prevented, since the center of gravity is located in front of the main wheels and when the main wheels hit, the angle of attack and c at the wing decrease;
the horizontal position of the fuselage axis provides good visibility for the crew, creates convenience for passengers, facilitates loading the aircraft with heavy cargo, allows jet engines to be placed horizontally, while the gas jet does not destroy the airfield surface; provides the aircraft with good stability during the run and take-off.
At the same time, the design of the landing gear with a front wheel has disadvantages: the difficulty of moving on soft and viscous ground, since the front wheel “buries”, there is a great danger when landing with a damaged front leg, the large weight of the structure, the difficulty of providing a significant volume in the front part of the fuselage for cleaning nose wheel.
Main parts and power circuits of the chassis
The main parts of the chassis leg are: wheels (usually brake wheels on the main supports), skis or tracks, shock absorber, side, rear or front struts, locks that lock the leg in
released or retracted position, a lift that allows the leg to be retracted and released.
The chassis is of a non-retractable type, which is rarely used nowadays, and does not have a lift or locks.
According to the structural and power schemes, the chassis can be divided into truss, beam and truss-beam.
The structure of the truss chassis (Fig. 75) is formed by a spatial truss to which the wheel axle is attached. The truss rods, which include the shock absorber strut, absorb compression and tension forces. Despite its light weight and structural simplicity, truss landing gear is currently rarely used and only on low-speed aircraft, since cleaning is difficult
whose chassis is extremely difficult.
The beam chassis (Fig. 76) is a cantilever beam with the upper end embedded in the wing or fuselage structure. A wheel or ski is attached to the lower end of the beam. The landing gear under the influence of the ground reaction force works in compression and bending. The maximum bending moment will be in the attachment point, so the attachment point for the strut to the aircraft must be strong enough.
Truss-beam chassis (Fig. 77) consists of one (single-post) or two (two-post) cantilever beams, supported by struts. The installation of struts relieves the strut from bending moments, the side strut - from the moment created by the lateral force, and the front or rear - from the moment of force directed along the axis of the aircraft.
In modern aviation, truss-beam landing gear is most widespread.
For aircraft with a large flight weight, a serious problem becomes the problem of reducing the specific load on the ground, since the maneuverability of the aircraft on the ground is inversely proportional to the specific pressure on the supporting surface of the landing gear. As the number of chassis wheels increases, the supporting surface increases. Therefore, chassis with paired wheels mounted on a trolley are widely used. The most widespread are multi-wheeled carts with the number of wheels from four to eight or more. There are airplanes that, to increase the maneuverability of the landing gear, have several wheels located along the fuselage in one or two rows.
In recent years, chassis with wishbone suspension have been widely used. In such a chassis, the wheel axle is not located directly on the shock absorber strut, but at the end of a fork lever (see Fig. 76), which is hingedly attached to the rigid strut.
The fork lever is also pivotally connected to the moving part of the shock absorber (its rod) using a connecting rod. Thanks to the articulated joint, the shock absorber absorbs only axial loads and bending of the shock absorber rod is thus eliminated. The lever suspension allows you to absorb not only vertical, but also horizontal forces. Due to the lever suspension, you can significantly reduce the required shock absorber travel and reduce the height of the landing gear.
The landing gear of an aircraft can be retractable in flight or non-retractable. Obviously, the design of a retractable landing gear is much more complex than a non-retractable one; the former also has more weight due to the mechanisms for lifting and releasing both the landing gear itself and the compartment doors and hatches designed for the retracted landing gear, locks and signaling the retracted and extended positions. At the same time, the aerodynamic drag of an aircraft flying with the landing gear retracted is reduced by 20-35% compared to an aircraft whose landing gear is not retracted. It is believed that for aircraft with a specific wing load exceeding 100 kg/m2, it is advantageous to use retractable landing gear.
The landing gear can be retracted into the wing, engine nacelles and fuselage. Sometimes special nacelles located on the wing are used to retract the main landing gear legs.
On airplanes with two to four engines mounted on the wing, the main landing gear legs are most often retracted into the engine nacelle compartments forward or backward and less often sideways (into the wing). With a “clean” wing, that is, when the engines are installed on the fuselage and the main legs are attached to the wing, it is advisable to move the legs to the side along the span, in this case the struts are retracted into the wing, and the wheels are retracted into the fuselage niches. The tail and front legs of the landing gear, fixed to the fuselage, are retracted into its compartments. It is advisable to move the front legs in the direction opposite to the direction of cleaning the main legs; for example, if the main legs are retracted forward, then the front leg should be retracted rearward, which ensures the least change in the aircraft's alignment when the landing gear is retracted and extended. Tail supports are usually removed with insignificant movement along the longitudinal axis and do not have a noticeable effect on changing the alignment of the aircraft. The landing gear retraction and release mechanisms are driven by hydraulic, gas, electric and mechanical drives; each leg of the landing gear has an independent power mechanism.
Chassis struts and trusses
Frontal and lateral loads acting on the landing gear leg, as well as twisting moments that arise when the aircraft turns on the ground, are absorbed by the attachment points of the strut to the aircraft and struts or trusses.
Struts are made from high-quality steel pipes or stamped profiles and, less commonly, from light alloys. At the ends of the struts, attachment lugs are welded to the aircraft components and to the landing gear components. Some struts are made to “break” to ensure retraction and release of the landing gear leg. In such struts, to prevent them from spontaneously folding when the chassis is extended, a lock is installed in the hinge. To eliminate the dynamic influence of frontal loads on the wheels, longitudinal vibration dampers are sometimes included in the design of the rear struts. The damper is a cylinder with a double-acting piston, held in a certain position by a spring or, more often, compressed gas. During a frontal wheel impact, the spring or gas in the damper is compressed and allows the wheels to tilt back. The liquid present in the damper flows from one cavity of the cylinder to another through a small calibrated hole and absorbs the impact energy.
Trusses are welded or bolted together from steel pipes and, less commonly, from profiles. Attached to the trusses are attachment points to the fuselage or wing, shock-absorbing struts, and in some cases, attachment points for lifts that ensure retraction and extension of the landing gear.
Literature used: "Fundamentals of Aviation" authors: G.A. Nikitin, E.A. Bakanov
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