The name "Chassis" says that the landing gear designed for taking off and landing of helicopters. In addition, the chassis of helicopters used for parking and movement of the helicopter on the ground (on the water) during taxiing or towing.
The chassis must provide a helicopter in the expected operating conditions:
- Control of the helicopter during the takeoff run, run, taxi, towing and turns;
- Amortization of dynamic loads encountered during landing, taxiing, takeoff run;
- Tow, excluding the possibility of the landing gear on the Earth.
The characteristics of the wheels, brakes and landing gear tires must comply with takeoff and landing characteristics of the helicopter while providing:
- Long-term parking Locked wheels and steering the helicopter with a maximum permissible mass;
- Ups (landing) with a maximum permissible mass and speed.
By using a helicopter equipped with retractable landing gear emergency release system.
The damping characteristics of the spring strut must ensure the safety of the oscillations of the "earth" response.
When planting (ups) helicopter with a horizontal velocity should be excluded the possibility of dangerous vibrations to the strength of the "shimmy" front wheel at all speeds run (run-up).
The strength of the chassis unusual circuits (skid, float, etc.) Is tested in accordance with specific guidelines.
The efficiency of the wheel chassis should be determined for the two main types of landing:
- With a hover (when available power HB);
- From planning at the one engine inoperative unless its failure does not lead to the need to shutdown all engines.
In addition to these types of planting to ensure the safety of passengers and crew should be considered as cases of emergency landing of the helicopter (including the failure of all engines).
Cruising speed reduction Fy (landing hover) for supplying power to NV for the front and rear chassis and the same should be determined by the formula.
LOADING CHASSIS at landing
To exclude the possibility of touching the surface of the landing site during normal operation of RVs, or other structural elements installed on the helicopter tail safety support. Its static strength is tested for load selected on the basis of experience of design and operation, taking into account load cases during landing.
There are several schemes chassis wheels, skid, float, boat, etc .. landing gear can be carried in a combination of the two schemes - the so-called composite chassis. For example, by helicopter amphibious landing gear are boat and wheeled chassis; ski rack can be performed in combination with the wheel; on the ship's helicopter variant ballonets set to perform emergency landings on water.
Landing gear and their components
The presence of the helicopter emergency landing ballonets not exclude the possibility for the helicopter to land and transport on its wheels on the airfield and deck.
As a rule, used in helicopters wheeled chassis. In the parking lot helicopter usually has three points of support. If the wheels are located behind the main leg center of mass, its third pillar (or two legs) is installed under the nose of the helicopter and the system is called the chassis with the front, or bow, wheel.
Chassis nosewheel helicopter single-rotor design is at the end of the tail boom support with cushioning. The transport helicopter with a cargo hatch it is expedient to remove, to ensure easy access to the equipment loaded loading ramp.
0 tipping angle is determined from the safety considerations landing helicopter ARF NV. Parking £ angle (horizontal between the construction and the surface of the fuselage of the helicopter landing area) is required to facilitate the roulette helicopter (£ = 2-3 °) or loading through the rear cargo door (then £ negative angle).
If the bottom surface of the forward fuselage of the helicopter for structural or operational reasons, you can not install the nose landing gear, then the scheme is applied to the rear rack. In this case, before boarding a helicopter in autorotation must be correspondingly reduced pitch angle of the fuselage.
Landing gear, whose main chassis legs are arranged in front of the center of mass of the helicopter and the third pillar - in its tail part is called tailwheel landing gear.
In the civil helicopter is used, usually the chassis with nose wheel. This scheme has significant advantages compared with tailwheel landing gear, among them - a safe and easier landing of helicopters in poor visibility conditions and good directional stability during the run-up and run.
To ensure the stability of the movement and turns the helicopter on the airfield on the front (or tail, depending on the circuit chassis) front mounted casters. As a result, the orientation of the free wheels on the nose landing gear may be cross-self-oscillation - "shimmy." This form of oscillations is caused by the interaction of forces from the landing site with inertia and elastic design forces the nose landing gear.
Wheel main undercarriage made no reference points, with the brakes. Brake less powerful than that of aircraft, since helicopter landing mainly inhibits HB and for turns at roulette uses directional control. The brakes on the wheels main undercarriage serve to reduce the length of the path of the helicopter after the landing, the helicopter fixation on sloping ground and on the deck of the ship.
STRUCTURAL-power circuits CHASSIS
Structurally-power scheme is selected based on the chassis of the requirements:
- Maintenance (determining the type of landing gear axles: wheel, boat, floats, Polozkov, etc.);
- Layout (depending, in particular, from the unit that receives the load of the chassis: the fuselage, wing).
Subject to the requirements of aerodynamics, operation and providing an emergency landing while flying at low altitude, decide to landing gear in flight.
The helicopter used by the pyramid, console (girder) piramidalno- parallelogram linkage schemes and chassis.
Pyramid scheme consists of three bars. Two lower rigid rod pivotally attached to the force elements underside of the fuselage. They perceive the frontal and lateral loads from the wheel. The kinetic energy of the helicopter landing perceived cushion built into the core of the third. The length of the rod is determined by the design considerations, taking into consideration the requirement to ensure minimum weight belt the fuselage, which is exposed to this load. As a rule, a mount on the power of the frames, load bearing on HB. This constructive solution determines the position of the damper rod with respect to the axis of rotation of the two lower rods. Typically the shock absorber rod plane not perpendicular to the axis of rotation of the lower rods. Therefore, in order to crimp the shock absorber rod when loaded with only a longitudinal force at both ends 4, 7 established cardan nodes.
When the shock absorber crimping wheel axle travels in an arc of a circle. To the end crimping wheel had a maximum
area of contact with the ground, it is in no condition must be swaged camber f. This angle must not exceed a value at which possible disruption of the tread in compression damper by changing the lateral force gauge chassis.
Console (girder) scheme of the main chassis used in the case where the layout of the helicopter airframe eliminates the pyramidal structure of the chassis. Landing gear are cantilever beams, pylons attached to the fuselage.
Beam rack attached to the airframe of the helicopter so as to be perceived with all kinds of load rack on the frame of the helicopter (vertical, longitudinal and lateral forces and torque). The main feature of the chassis beam (in addition to its compactness) is a condition of the loading rod damper, is perceived not only in the axial load compression damper, but also longitudinal and transverse forces. At certain ratios of removal of the wheel axis relative 4 the mount chassis elements of the airframe, the value of the console and the distance between the chassis b the Bucs with fully released damper rod may jam due to a large friction in the journal boxes and cuffs.
To eliminate the turn of the wheel relative to the rack-mounted slot hinge 2. With no load on the wheel angle between the links shlits- hinge should not be more than 150 °. In this case, excluded jamming shlits- hinge when compression damper.
Beam chassis are also used in the nose and tail supports. In this case, the kinematics of the landing gear itself should provide orientation of the wheel when maneuvering the helicopter on the landing pad. The landing gear assembly is provided for locking the wheel in the direction of the plane when the flight is fully released the damper rod.
If design methods can not exclude the possibility of jamming or rack layout considerations need to reduce the height of the chassis legs, use the lever diagram.
The arm can have a different scheme of kinematic embodiments. The lever rod unloaded from the rack transverse loads, which improves the quality of cushioning on the chassis relative to the beam pattern.
When selecting the kinematic scheme for the helicopter deck must take into account the specific conditions of loading gear in contact with the landing area. The deck of the ship is moved in the horizontal and vertical planes and rotated relative to the axes XYZ. The stability of the helicopter on the deck, and the elimination of "earthly" resonance kinematics are directly related to the main and nose landing gear.
Pyramid chassis have a "ship" lack - for large vertical movements of the chassis of the helicopter there are significant lateral movements of the wheels Az, leading to a change in the track compression damper. In order to prevent slipping helicopter flight deck of the ship while pitching its surface covered with a special anti-slip mastic, and the surface of the landing area is stretched network. Those measures impede the free movement of wheel bearings pyramid schemes sideways, which could lead to the shutdown of the work gear shock absorber, ie to increase the loads on structural elements of the chassis, to reduce the overall system damping "chassis - HB", which is fraught with consequences provocations "earthly" resonance on the deck.
Console (girder) kinematics characteristic of great moments in the piston rod, which leads to considerable largest responses in the journal boxes absorbers. This deteriorates the dynamic characteristics chassis amortization both during landing and during absorption of energy in the "terrestrial" resonance.
Unsatisfactory in terms of the ship's conditions, operating properties has kinematics linkage chassis. In this scheme, the transverse vibrations of the helicopter caused by the lateral rolling of the ship and exchange rate, lead to yaw the helicopter (due to the asymmetry of compression damper main landing gear). This, combined with the very landmarks nose landing gear wheels of the front part of the helicopter moves in the direction of the roll and causes a tendency to slide into the deck.
Piramidalno- parallelogram kinematics best meets the specific conditions of operation of the helicopter on the ship. The use of such kinematic scheme of the main chassis provides a large wheel travel in a vertical plane with almost constant track. Another advantage of the scheme is that the landing gear shock absorber only accept axial loads that can make it sensitive to the characteristics of small loads.
When casters nose landing gear comes swinging bow of the helicopter, which in turn contributes to a shift in the direction of its roll.
To expand the range limit pitching angle of the ship, which are likely to rise and .posadka helicopter on the moving deck, must be entered into the design of the front landing gear unit, fixing them in the longitudinal axis of the helicopter in the parking load.
In terms of weight efficiency, the use of the material instead of BT-22 ZOHGSNA in the construction of power most profitable parts of the chassis. Alloy BT-22 satisfactorily welded, which allows to obtain the details of the chassis complex spatial form large size, their welding of the parts. For parts made of VT-22, there is the problem of protecting structures from corrosion (especially places are constantly exposed to water, mud, sand).
Certain limitations of application of titanium alloys in the construction of the chassis is not good enough they work in pairs of friction - in places it is necessary to apply special anti-friction coating.
SETTING THE ELEMENTS landing gear
As the kinetic energy absorber used special units, dampers and air wheels. For all types of shock absorbers necessary to have a damping and elastic elements.
Appointment of the elastic element of depreciation - the accumulation of kinetic energy is often perceived to restore the original operating state of the shock absorber.
Elastic cushioning members may be air (nitrogen), a spring or rubber. Aircraft landing gear wheels are also elastic elements depreciation. In addition to absorbing the kinetic energy they provide the necessary maneuverability of the helicopter on the ground and fixing it in the parking lot.
Purpose of the damping element - to absorb the kinetic energy and convert it to heat. In addition, the damping characteristics of the spring strut must ensure the safety of the self-oscillation of the "earth" response.
Damping is done by the friction forces in the throttling device, the journal boxes (bearings) and the material under strain.
Depending on the used elastic damping elements and shock absorbers are: gas-liquid, spring, spring-liquid, rubber plate, rubber Cord, spring-friction.
Rubber dampers are used in both in the tension and compression during operation. In the first case a rubber cord used in the second - the rubber plate.
The rubber cord is composed of yarns of rectangular cross section, stretched to 150-200% initial length and fixed in such a state of the textile braid. The coefficient of hysteresis cord does not exceed the 0,18.
Plate dampers (buffers) are recruited from the rubber plates. The coefficient of hysteresis 0,25-0,5 when compressed on 25-50% free height. The destruction (cracking) of rubber observed in 50-60% reduction. Buffer absorbs a lot of work, if the plates during compression are free to expand laterally. To this end, the plates lay smooth thin metal washers.
Rubber shock absorbers the following disadvantages:
- Low hysteresis;
- The loss of the natural qualities of the depreciation of the atmospheric conditions (aging);
- The negative impact of low temperatures;
- The destructive action of oils, etc.
The coefficient of completeness diagram of a rubber shock absorber T | = 0,4. Operating temperature range up to + 60 -45 ° C.
Liquid-gas shock absorbers and springs are the main types of shock absorbers used on helicopters.
Depending on operating conditions, the load acting on the spring strut, given the characteristics of stiffness value of the design and possible stroke kinematic scheme chassis selected KSS strut.
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