Amortizartory helicopter chassis
The strength of hitting the ground is perceived Pneumatics quantitatively converted and transmitted through the shock to the structure of the helicopter, where is balanced by the weight and inertia.
A work attributable to the shock is determined by the normalized difference about work and absorbed pneumatics for a given overload:
The proportion of the impact energy absorbed by pneumatics, should not exceed 25-40% of the kinetic energy of the helicopter landing. On heavy helicopters, designed for small overload when landing (about 2), you can prevent the absorption of pneumatics half normalized work.
The direction of travel is determined by the wheel axle kinematics compression landing gear.
Shock absorbers should be placed so that the angle (3 in all possible directions normal strike (ua landing three points on the main wheels, etc.) as small as possible (for beam racks to 25 °, for lever (3 = 30-40 °).
During the compression strut lever ratio should not increase by more than 35-50%. When fully crimped damper lever rack should be in a horizontal position. It is recommended to use the starting angle of the lever to the horizontal and = 35-45 °.
It is necessary to avoid the separation of the wheels from the ground during the back shock-absorber rod. For this parking shrinkage shock absorbersYogi main chassis to be as much as possible (up to 2 / 3 calculated speed). Work hysteresis for forward and reverse strokes should be about 80% of the absorbed energy absorber.
Pneumatic friction on the ground, the force of inertia wheel spin when landing with mileage, elastic deformation under load rack cause the appearance on the wheel of alternating horizontal force. When calculating the depreciation is usually considered a vertical impact of the helicopter on the ground without the horizontal forces; a lot of moving parts strut ignore.
With pneumatic compression curve and summarizing the movement of the wheel axle, you can get a general diagram of pneumatic compression and shock absorber.
In practice movement of the main axis and the front wheels in a vertical plane depending on the mass of the helicopter and its operating conditions it is in the following range:
- The main pillar - 200-600 mm;
- A-pillars - 200-400 mm;
- Tail support - 200-400 mm.
This stroke damper depends on the gear ratio t. For a given compression height of the chassis and shock absorbers and wheels to the maximum values must be provided gaps between the structural elements and the surface of the helicopter landing pad.
The geometrical dimensions of the cross section of the cylinder rod and the shock absorber should be determined on the basis of their calculation for strength to solicit the greatest value of the diameter, the size of which is limited only by the terms of local buckling. In this case, the wall thickness of the cylinder and the rod sections will be minimal and, accordingly, their mass - the smallest. The outer diameter of the rod is chosen by minimizing the friction forces at the bottom of the journal box damper.
Adjusting the parameters and characteristics CHASSIS
Stability reserve screws on an elastic foundation can be obtained by increasing the damping of oscillations of the blade or the fuselage, ie increase the damping capacity of the chassis. However, these possibilities are practically limited. Dampers blade chassis and perform several other functions not related to the "earthly" resonance. The damper blades are running in forward flight, the helicopter and load the butt of the blade variable bending moment, depending on the degree of damping. The strength of the butt of the blade and the hub and the mass is determined by the presence of the damper.
For normal single-rotor helicopters circuit track chassis should be selected so that the natural frequency of the transverse oscillations of the helicopter on the run with broken columns (work only pneumatics) was approximately 20% higher operating speed of the screw:
Wheels with high-pressure pneumatics little crimped (have a greater stiffness), and therefore it is advisable to apply to the helicopter deck.
If the size of the holes, through which the slurry during the suspension, choose from the condition of a lack of "earthly" resonance, as a rule, the work of the shock absorber during landing is unsatisfactory (there excessive force when hitting the ground). When selecting them from landing conditions get too: small damping for transverse vibrations of the helicopter, it is insufficient to eliminate the "earthly" Resonance (damping in pneumatics is virtually absent).
Stiffness characteristics of the shock absorbers are determined by the initial pressure and volume r0 Fq airbags. The damping characteristics are determined by the law chosen in the shock absorbers changes of sections traversed by the liquid.
When designing the spring strut main landing gear (and with four pillars - and the A-pillars), special attention is paid to the substantial increase in damping generated amortization racks when a helicopter vibrations of the "earth" response. For this purpose, used suspension struts with a large stroke and a maximum reduction of efforts to ensure the inclusion of the work of the shock absorber damping and the emergence of virtually the moment the wheels touch the ground. The increase in damping contributes to a reduction of dry friction in the strut through the use of axle boxes instead of bronze.
To cushion the chassis tonnage helicopter initial force (pre-tightening) p in 0 5-6 times less than the land the helicopter landing gear shock absorber.
An effective means of incorporating a shock absorber in the work is the introduction of rezinoftoroplastovyh packing seals with low friction, reducing the diameter of the damper shaft, use the links in the kinematic nodes instead of the chassis bearing bronze bushings.
Deviation from the self-oscillation "shimmy"
As a result of the interaction of elastic and inertial forces on the nose landing gear may have transverse oscillations.
Deviation from the self-oscillation of the "shimmy" by a corresponding removal would lead to a substantial increase in weight and dimensions of the nose landing gear. Therefore, this problem is solved comprehensively. Helicopter maneuvering at the airport by samoorientirovaniya wheels pillar. On the chassis beam scheme this requirement by reaching the wheels back on the amount of I. At the front lever value of I is selected by konstruktivno- kinematic reasons. For the purpose of damping oscillations
about the axis of the front axle wheel amortstoyki established two parallel pneumatic arch type. If these measures do not lead to the elimination of self-oscillation, then on the front rack mounted damper "shimmy."
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