The strength of the helicopter design
For a combat mission and safety design of the helicopter shall be sufficiently strong and rigid. Under strength are due to the ability to perceive the structure without collapsing, given external loads occurring during operation. Under the ability to understand the rigidity of the structure to resist deformation under load.
In the operation of the helicopter is exposed to various PA nature and magnitude of the load: static (constant or slowly changing over time), dynamic (shock and vibration). Depending on the type of loading design or parts of it must have the appropriate type of strength.
The combination of different types of essential values of strength, ensuring the normal operation of the structure within the established limits and terms referred to operational strength.
During operation, the structural strength is not constant. Heavy loads are close to the limit, can cause permanent deformation in its elements. Small but repetitive stress cause the development of fatigue cracks that weaken the structure. There is a deterioration
rubbing parts, abrasion HB blades, vanes of gas turbine engines under the influence of dust and sand. Moreover, when maintenance of damage made in the form of dents, scratches, scratches, nicks and the like. D. All this leads to a gradual reduction of structural strength and force limit resource (plaque in hours) of the helicopter.
In operation, the design of permanent operating temperature changes, precipitation, dust, solar radiation, and so on. D. The impact of these factors causes corrosion of structural elements, cracking glass and other non-metallic parts, damage to protective coatings. As a result, necessary to limit the calendar time equipment operation (lifetime).
Thus, all of the above external factors reducing the strength and impairing the performance of the design, limit its durability. The durability of the aircraft called the property continue to operate based on service and repairs to a certain limiting condition in which violated the requirements of safety, reduced operational efficiency. The indicators serve as a resource of durability and service life.
One of the main tasks of technical operation of aviation technology is to maintain the necessary strength for the entire service life in a real operation.
General principles for the calculation of the strength of the helicopter
The Standards also provides strength: the negative effect of the overload = -0,5 when entering into the planning, energetic turns the helicopter hovering, the impact of the vertical and lateral gusts of air and other. Each of the estimated cases is crucial for the strength of any part of the unit or the helicopter.
Planters settlement cases are considering various options fit: for all the support, only for the main planting a side impact, and so on. D.
Land settlement cases considering the impact of the wind, towing the helicopter on unprepared ground, and others.
A particular difficulty of calculating the strength of the helicopter is that its basic load, such as force from the blades HB have variable magnitude and direction of the character, which causes vibrations of the blades themselves and the whole structure of the helicopter. This is called dynamic loading. With long-term effect of repetitive loads structural failure occurs at voltages much lower than with a constant static load. This is due to the phenomenon of material fatigue.
The strength standards are also given all the necessary data to calculate the stiffness of the structure, its dynamic strength and service life (life).
The concept of static strength calculation
If the load is constant or design changes slowly, deformation, and stress it will also be constant or vary gradually in proportion to the load, without oscillatory processes. This is called a static loading.
For helicopter static loads can be considered: traction and tail rotor; centrifugal forces of the blades; aerodynamic forces of the wing and tail.
Calculation of static strength comprises:
- - Determination in accordance with the standards of strength magnitude and distribution of the design loads;
- - Construction diagrams Q and transverse to the longitudinal force N, bending and torque for the consideration of the design of the helicopter;
- - Identification of the most loaded sections of the structure, in which the highest possible voltage;
- - Determination of the stresses in structural elements and compare them with destroyed.
Static structural strength is provided if the voltage in its elements do not exceed the destructive values.
However, the provision of static strength does not guarantee the safe operation of the helicopter, as under the influence of variable loads in its design having corresponding variable voltage. These voltage superimposed on the constant increase the total stress and can lead to fatigue failure of the structure.
AC loads helicopter
The main load of the helicopter are variable, they are constantly changing in magnitude and direction with a certain frequency.
The main sources of variable loads are main and tail rotors. The reason for the periodic change of forces acting on the rotor blade, is the continuous variation of the speed and direction of the incident flux on them in different azimuths in different sections during forward flight of the helicopter. When the blade is in its rotation moves toward the oncoming flow to the helicopter, the total rate of its flow is increased and in the reverse direction, on the contrary, decreases. Because aerodynamic forces are proportional to the square of the flow velocity, the lift and drag Street Chl blades are also constantly changing. This causes flapping of the blades in a vertical plane and in a plane of rotation.
When flapping centers of mass of the blades periodically approaching and moving away from the axis of the screw, which causes variable Coriolis forces acting in the plane of rotation. These forces also cause fluctuations in the rotation plane of the blades.
All of these variables forces are transmitted to the hub HB and further through the rotor shaft and gearbox to the fuselage of the helicopter, causing its oscillation in the vertical and horizontal planes. Amplitude of the alternating forces transmitted from the blades can be thousands of Newton, and heavy helicopters - tens of thousands. The frequencies of these forces are multiples of the product of the rotational speed of the screw in the number of blades.
Additional sources of variable strength may appear poor balance and nesokonusnost blades. Poor balance is unequal static moment of the blade, causing an unbalance of the centrifugal forces. Nesokonusnost manifested in different amplitudes of flapping of the blades due to differences in their external forms, torsional rigidity or inaccurate adjustment setting angles. For the same reasons there are variable forces the tail rotor.
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