The blades of a helicopter rotor
Operating conditions helicopter rotor blade largely different from the operating conditions of an aircraft wing. The main feature is that the load acting on it are variable in time. Therefore, when choosing the blade elements of the material as the main put forward the following demands:
- Fatigue resistance: crack resistance (resistance to fatigue crack propagation) and low sensitivity to stress concentrators;
- Invariability mechanical properties of material elements and their compounds from the predetermined operation time, temperature and atmospheric environment;
- Technological requirements: the possibility of production to ensure the set-sectional shapes of structural elements; increase resource design elements hardening methods; control over the quality of the connection and the specified geometry
sizes in the manufacture of construction elements in the blade assembly; maintainability blade structure during its operation.
Besides the above, it is necessary to take into account the cost of material and manufacturing process blade process and cost of its operation.
In view of the above requirements are choosing the material, and which has the highest strength to weight ratio - and a specific modulus of elasticity E - p.
When forming the spar of the blade hybrid composites strive for maximum compatibility with the matrix material, for example, the magnitude of the dynamic elongation, degree of adhesion, and the coefficient of linear volume expansion, water capacity, aging time, the sensitivity to shock.
The sensitivity to impact is determined by the value of toughness. For fiber composites toughness is characterized by an attitude. One way to improve the toughness of the composites is the introduction into their structure more stable and less rigid fibers, such as glass or organic - in carbon fiber.
In the development of helicopter main power element of the blade - spar - carried out of wood, alloy steels, aluminum alloys, stainless steel, titanium alloys. It is now widely practiced manufacturing the spar from composite materials.
frame units - lining, ribs, tail stringer previously manufactured from plywood, fabric, aluminum alloys, in modern blades are made as of the CM.
Wood has been used in practice Ukhtomsky Helicopter Plant. Y.I. Kamov in the period of its formation. Decisive in the selection of the material the following considerations were: wood is insensitive to stress concentrators, crack resistant; it does not require complex technological equipment in the production of the spar and the blade frame; the cost of manufacturing the blade is not large.
The central part of the spar was made of wood delta (glued thin sheets of wood), the nose of the profile consists of a set of glued pine slats. The tail section was a frame made of plywood paneling, glued to the foam. the blade surface is covered with a cloth and water-resistant varnish.
During the operation revealed significant deficiencies wooden blades:
- Despite the water-resistant surface coating blade design elements saturated with moisture, leading to a change in the cross section center of gravity (shifted back) and reducing the flutter vane critical speed;
- Impregnation with antiseptics is not eliminated in operation putrid wood failure, despite the fact that its mechanical properties deteriorate.
In the practice of the Moscow Helicopter Plant. ML Mile HB blades used mixed construction - spar was made from a steel pipe and frame elements used wood and canvas.
The requirements of strength, stiffness and aerodynamic considering technological possibilities have led to the need to change the spar section forms a cylindrical radially into elliptical. Iron and steel industry did not have the equipment for the formation of the spar from a single blank. Therefore, designers have had to introduce telescopic joints connected by rivets steel, with strengthening technology (dornirovanie holes), smooth transitions rigidity at the joint, a longitudinal grinding inner and outer surfaces of each portion of the spar.
Given the nature of the aerodynamic loads on the chord of the profile, the profile of the front part of the blade was made of plywood and the back - from the web to the inboard part of the blade and plywood sheathing in the middle and end parts.
Aerodynamic load and the centrifugal force acting on the frame, through the ribs were transmitted to the spar. The transfer of forces and moments on the spar carried by flanges riveted to the wall of the spar and ribs.
During the operation revealed a number of shortcomings adopted a constructive force scheme blades. The presence of joints and rivets greatly complicated the process of achieving the necessary resource blades. Use in the rear casing without torque (leaf) leads to the fact that due to external aerodynamic forces and centrifugal force WHOspirit, inside the frame, significantly distorted the blade profile, which degrades its aerodynamic characteristics.
Introduction of the drain holes on the bottom surface of the blade at the end led to local losses of air to flow inside the frame under the influence of centrifugal forces. Eliminating this disadvantage by eliminating the web and jump na plywood sheathing on the entire surface of the blade significantly increase the weight of the blades and shift the center of mass of the blade back. As a result of joint activity of designers, engineers and metallurgists to address these shortcomings was created spar given variable section without joints, and the tail of the blade began to carry out dyuralyuminevoy plating, supported by mobile unit does not change shape under the influence of aerodynamic loads.
For tubular spar usually used tube of high-alloy steel or type ZOHGSA 40HNMA, hardened and tempered for strength (a ^ = 1100-1300 MPa). After the hot and cold rolling, forming and hardening the outer and inner pipe surface polished. The outer and inner surfaces of the spar is created hardening of vibro-impact way to boost endurance limit and w = 280-300 MPa mi »at constant part load atm = 200-250 MPa.
In the blade construction based on steel pipe is usually protected by the frame spar and can not be mechanically damaged in operation.
The use of extruded profile of duralumin material is allowed to form the spar profile with the most appropriate section (2.3.1). The use of closed profile, obtained by pressing (extrusion), limited range of use of existing duralumin alloy. In the process of pressing the material is separated into two parts, thus forming a profile tool (die), these two parts must be joined and welded pressure. To the material structure in the field of welding would not be impaired, it is necessary to use a material with high corrosion resistance, fatigue strength duralumin spar may be reduced due to the defects that arise in the process of pressing profile and machining .lonzherona. It is therefore necessary not only external but also the internal surface of the spar reinforcing vibro-impact way. endurance limit may be increased to a = 55-60 MPa at about t = 60 MPa. To exclude the possibility of minimum corrosion damage to the molded side member during manufacture and in operation is necessary to apply electroplating (for example, anodizing) operations after the intermediate processing.
pressing process does not allow to change the shape of the cross section for a given law, so the desired height profile along the length of the blade can be achieved only through the outer surface of the milling. As a result, the designer is able to design structurally-power blades scheme only rectangular in plan (the restriction r | = 1).
Spar contact surface with the air flow led to the need to protect the surface of the erosion damage.
forming blade spar attempt was made of a thin laminated sheet of stainless steel, connected by means of a monolith gluing. It was supposed the creation of the design, which has high resistance to fatigue crack propagation. Organic drawback of this design was the inability to provide quality bonding and elimination of identified defects adhesive surfaces.
The blades of a closed-form spar allow the use of technical means of continuous monitoring of the spar fatigue failure of the material. damage-metal spars alarm system consists of an air-pressure alarm and the plugs at the ends of the spar (2.3.2). The internal cavity of the spar is filled with pressurized air, above the pressure start alarm status.
In the event of a crack in the spar air pressure therein decreases. Information about the spar cavity depressurization supplied from a pressure annunciator extension bellows cap red specified in the butt portion of each blade.
Air pressure display in the longitudinal members in the cockpit is not output, because crack growth process before the destruction of the spar is several times longer than the maximum possible length of the helicopter flight. Control of the blade is carried out between the flight inspection on the Status of alarm.
The air pressure in the spar is created based on the ambient temperature and pressure, taking into account the beginning of the alarm status.
The blades of the helicopter Mi-26 steel tubular spars on the outer surface are lined with glass tape, whereby in the event of a crack in the spar exclude the possibility of detection of damage to the spar with a pneumatic alarm system. To ensure reliable operation of the system failure alarm spar over the entire length of its outer surface fluoroplastic double stacked cords (2.3.3) and after winding ribbons of glass produced in the polymerization mold. PTFE cords are stretched, forming air ducts diameter
2 mm opened from the outer surface of the pipe member. The appearance of fatigue cracks in the area of air channels leads to a pressure drop in the cavity of the spar and alarm actuation. The channels run double for technological reasons - there is always a chance of PTFE cord breakage when it is pulled out of the cavity length 14 m.
The anisotropy of composite materials has opened wide possibilities of their application in the blades HB. The application allows the CM directed to form stiffness characteristics of the blades (bending and torsion) by appropriate orientation of the reinforcing fibers of the composite, given the complex nature of its load.
Helicopter is the most advanced aviation equipment industry, it began to feel free to use the CM in such an important and difficult loads the unit as HB blade.
The effectiveness of CM in the power element of the blade is defined by a number of advantages of these materials compared to metals. In particular, the aerodynamic and aeroelastic parameters of composite blades can be chosen without regard to the limitations caused by technological processes produce rolled, extruded (pressed) or machined metal components.
Composite structures can be given a sophisticated aerodynamic shape and adjustable anisotropic material allows you to create the required rigidity within the specified aerodynamic and aeroelastic parameters. The result is greater aerodynamic efficiency screws determined ratio lift to the aerodynamic resistance.
Using CM having a high specific strength, the blade is made smaller mass than metal. Reduced weight blades, in turn, influences pas centrifugal force, the inertia of the rotor, the frequency and other characteristics.
Adjustable widely KM anisotropy produces the necessary structural blades and damping parameters.
The natural frequency of oscillation of the blade can be changed not only the redistribution of weight, but also a choice of reinforcing fibers having a low or a high modulus of elasticity, including hybridization (mixing), the degree of reinforcement and orientation of the reinforcing fibers with respect to the blade axis. The torsional stiffness of the blades can be substantially increased through the addition of layers oriented ± 45 ° relative to the blade span with little change in the frequency of the longitudinal oscillations.
One of the possible optimality criteria panels KM, providing at least its weight, is the condition of coincidence with the trajectory path of reinforcing the maximum principal stress. Typically, the CM is a set of unidirectional or fabric layers with different thicknesses and angles of orientation of the fibers. The properties of this material are determined by the properties of the individual layers and structure.
Effective implementation of the advantages of composites in the construction of the blades requires solving complex problems associated with the selection of a mutually agreed initial components (fiber and matrix), the definition of a rational structure of the material and the type of external loads and other impacts, taking into account the specific properties of the material and technological limitations in the design of the blade elements.
The mechanical behavior of the CM determined high strength fiber reinforcement, stiffness matrix and bond strength at the "matrix - fiber".
The largest application was received by the CM on fiberglass epoxy matrix. This is primarily due to the low cost of fiberglass. Further development of KM blade design involves the use of hybrid compositions
- Combinations of carbon with organovoloknom and other similar options.
CFRP, possessing high strength, insensitive to shock loads. The introduction of less rigid material and the protection of the spar surface from any damage provides features extensive use of such compositions.
Spar with a closed box-section £) -shaped can be made by winding unidirectional tape on the mandrel. This method of manufacturing the blade spar is widely used in large-scale series production, where it is advisable to automate the manufacturing process. In practice OKB NI Kamov chosen technology for manufacturing the spar parts by calculations of different fabrics or unidirectional tapes of the material on the mandrels.
Sheets spar material collected in bags and subjected to preliminary crimping of the autoclave at a low temperature. Sheets with the stick, packages become necessary to further build the shape and stiffness and virtually no polymerization of the binder takes place. After crimping the packages are open path profile.
Then the bags are collected together with the centering loads, heating element and butt plates in one unit, which is located inside the process chamber rubber press. package unit with a press camera placed in a special mold, the inner contour of which corresponds to the outer contour of the bow of the blade.
The press chamber is supplied with compressed nitrogen, and the mold is heated. This spar acquires the desired shape, the binder cures and all the elements of the spar is firmly glued together. After the pressing process spar removed from the mold, it is removed from the press and trimmed chamber allowances. This method of production allows to obtain closed loop spar of various reinforcing fillers for various binders in any combination with unlimited possibilities for their placement in the structure. By the jig for manufacturing the spar section specified number of requirements in the appointment of a pressure mode, heating, cooling and heating at curing. These requirements are intended to eliminate the residual strain and warpage due to thermal stress, and the uneven distribution of the mass of material and thickness during the formation of the spar.
Type the source CM for spars selected depending on the flight performance of the helicopter data. For light duty blades of helicopters used cheap fiberglass satin weave. For heavily used hybrid blades KM based on high-strength fiberglass, carbon and technical fabric tape on the epoxy binder.
The use of hybrid QM allows the main power element - spar - produce with virtually any desired distribution of mass and stiffness along the length of the blade.
Due to the requirements for the blades, and taking into account the current load, trailing blade section must meet the following requirements: structural strength, minimum weight, rigidity, sufficient resources (at least share of the spar blades), the smoothness of the airfoil, the ability to manufacture in mass production, ability to repair in the field and others.
The operation worked well the tail section of the blade three-layer honeycomb. This section has a lining, the end of the stringers and ribs of the fabric based on organic fibers and fillers of the cells. The use of the lung structure in QM tail sections enables to reduce the weight of the section as compared to fiberglass, and increase the resource.
Extensive experience gained in the operation of helicopters "Ka", showed that the blade of the CM have the best performance. The most important ones are as follows:
- A large margin of safety with virtually unlimited endurance conditions for share. Bottom blades term service of the Cabinet determined by the degree of their natural wear and tear, which depends on the operating conditions;
- Increasing the life of not only the main rotor blades, but also all of the helicopter by reducing the static and dynamic loads in a carrier system, favorable frequency characteristics and reduce the level of vibration of the helicopter. It is provided a process which allows to produce spar with variable cross-section along the length of the shape and wall thickness, as well as to apply different types of co-reinforcing material with different orientations. These essential qualities provide significant benefits not only to the metal blades, but also to other designs of the blades of the Cabinet;
- A high degree of maintainability. Thanks to valuable properties KM - high resistance to stress concentrators and low rate of destruction of the material - is achieved by simplicity and accessibility repairs, even major damage to the blades in the field;
- Highly resistant blades for almost all types of aggressive substances, fuels, chemicals, oils, etc .;
- Stability of performance of the blade during long operation in any climatic conditions. Long experience in operating helicopters with blades of CM showed that changes in the mechanical properties of the material are so small that they have no impact on aircraft performance or the life of the blades Service.
On the CM characteristics in operation influences humidity.
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