The design of the helicopter main rotor blade
Helicopter rotor blades must be built so that they create the necessary lifting force at all arising withstand load them. And not just maintained, and would have more margin for all sorts of emergencies that may arise during the flight, and in the maintenance of the helicopter on the ground (for example, a sharp gust of wind updraft, sharp maneuver, icing of blades, inept promotion screws after launch engine, etc.. d.).
One of the calculated modes for selection of a helicopter rotor is a vertical mode is set to any chosen to calculate the height. In this mode, due to lack of speed in the translational plane of rotation of the screw the required power is large.
Knowing about the weight of the helicopter and wondering constructed value of the payload, which would raise the helicopter, proceed to the selection of the screw. Selection of screw comes down to, to choose a diameter of the screw and a number of its revolutions per minute at which the goods could be estimated to be raised vertically screw up with the least expenditure of power.
It is known that the thrust of the rotor is proportional to the fourth power of its diameter, and only the second power of speed, ie. E. The thrust developed by the rotor is more dependent on the diameter, than the number of revolutions. Therefore, a predetermined thrust is easier to obtain an increase in diameter than the increase in the number of revolutions. For example, increasing the diameter 2 times to obtain traction = 24 16 times larger, and increasing the number of revolutions twice obtain only traction = 22 4 times greater.
Knowing the power of the engine, which will be installed on the helicopter for driving the rotor, first picked up the diameter of the rotor. To do this, use the following relationship:
The blade rotor is working in very difficult conditions. On it there are aerodynamic forces that are bent, twisted, torn, eager to tear off her trim. In order to "confront" such action of aerodynamic forces, the blade must be strong enough.
When flying in the rain, in the snow or in the clouds under conditions conducive to icing, blade work even more complicated. Raindrops falling on the blade with a huge "speed, knocked her paint. When icing pas blades are formed of ice build-up that distorts its profile, preventing its swinging motion, aggravate it. When storing the helicopter on the ground to the blade harmful effect sharp changes in temperature, humidity, solar radiation.
Hence, the blade must not only be strong, but it must still be resistant to external influences. But if only it! Then the blade could be made of all-metal, covering it with a layer of anti-corrosive, and the problem would be solved.
But there is one requirement: the blade, in addition, should be even and easy. Therefore, it is manufactured as a basis of a hollow blade design take a metal spar, most of all - steel tube of variable cross section, an area gradually or stepwise reduced from the root to the end of the blade.
The spar is the main longitudinal strength member blade takes shear force and bending moment. In this regard, the work of the spar of the blade similar to the operation of an aircraft wing spar. However, the blade spar operate by rotating the screw yet the centrifugal forces is not present at the spar of an aircraft wing. Under the influence of these forces the blade spar is subjected to tension.
By spar welded or riveted steel flanges for mounting the cross-power set - rib blade. Each rib, which may be metal or wood, consisting of walls and shelves. For metal shelves glued or welded metal paneling and a wooden shelf is glued plywood or fabric covering is sewn or glued to toe plywood paneling, and a ponytail sewn linen, as illustrated. In the fore part of the profile ribs are attached to the front flange stringer and in the tail section - to a rear stringer. Stringers are auxiliary longitudinal strength members.
Sheathing covering shelf ribs, forming a blade profile in any of its sections. The easiest is the fabric covering. However, to avoid distortion of the profile as a result of deflection of fabric covering the areas between the ribs, rib blades have to put very often, approximately 5-6 cm from one another, that the heavier blade. The surface of the blade with a poorly stretched fabric covering looks ribbed and has low aerodynamic qualities, as its drag large. In the course of one revolution of the profile of a blade change, which contributes to the appearance of additional vibration of the helicopter. Therefore, fabric covering impregnated with dope, which as its drying fabric pulls strongly.
In the manufacture of plywood sheathing stiffness of the blade increases and the distance between the ribs may be increased in 2,5 times in comparison with blades covered with cloth. In order to reduce the resistance, smooth surface of plywood is processed and polished.
Good aerodynamic shapes and high strength can be achieved if manufacture hollow-metal blade. The difficulty of its production consists in producing variable cross-section along the spar, which forms the nose of the profile. The tail part of the profile of the blade is made of sheet metal cladding, which front edges flush welded to the side member and the rear edge of the rivet together.
Profile of helicopter rotor blades is selected so that by increasing the angle of attack stall flow arose at the greatest possible angle of attack. This is necessary in order to avoid flow retreating blade stall, where the angle of attack is especially high. In addition, in order to avoid vibration profile must be chosen such that at least when you change the angle of attack has not changed the position of the center of pressure.
A very important factor for durability and operation of the blade is the relative position of the center of pressure and the center of gravity of the profile. The fact that the joint action of bending and torsion, the blade is exposed to self-excited vibrations, that is. e. vibrations with increasing amplitude (flutter). To avoid vibration vane must be balanced with respect to the chord, that is. e. should ensure that the center of gravity on the chord, which would exclude the self-expansion of the vibration. balancing task is to ensure that the blades are built in the center of gravity of the profile was ahead of the center of pressure.
Continuing with the severe operating conditions of the main rotor blade, it should be noted that damage to the blade paneling raindrops can be prevented if along its front edge to strengthen the sheet metal edging.
Fighting the de-icing of the blades is a more difficult task. If these types of icing in flight as hoarfrost and rime, a great danger to the helicopter is not present, then the glassy ice, gradually and imperceptibly, but very solidly build up on the blade, resulting in blade weighting, distortion of its profile and, ultimately, to reduce the lift that leads to a sharp loss of controllability and stability of the helicopter.
There was one time, the theory that the flapping of the blades due to ice movement in flight will shear, was untenable. Icing blade begins first of all at the root part where the blades bend during its flapping motion is small. Subsequently, a layer of ice begins to spread further by the end of the blades, gradually come to naught. There are cases when the ice thickness at the root portion reaches 6 mm, and at the end of the blade - 2 mm.
To prevent icing is possible in two ways.
The first way- A careful study of the weather forecast in the area of flight, bypassing encountered along the way clouds and change altitude to overcome the Won icing, flight termination, etc...
The second way- A blade equipment anti-icing devices.
Known all glad of these devices for the helicopter blades. To remove the ice from the rotor blades can
protivoobledenitel alcohol be used, which sprays the front edge of the screw alcohol. Last, mixing with water, it lowers its freezing point and prevents the formation of ice.
Spalling ice with the propeller blades can be effected by air, which is blown into the rubber bladder sandwiched along the leading edge of the rotor. Inflationary camera nadkalyvaet icy crust, some pieces which then swept propeller blades with the counter air flow.
If the front edge of the rotor blade is made of metal, it is possible to heat or electricity, or hot air, is passed through a pipeline laid along the front edge of the rotor.
The future will show which of these ways to find a wider application.
are very important number of rotor blades, and the specific load on the rotor swept area for aerodynamic characteristics of the rotor. Theoretically, the number of propeller blades can be any one of an infinitely large number of them, are so large that they will eventually merge into a spiral surface, as anticipated in the project Leonardo da Vinci or helicopter-bike I. Bykov.
However, there are some most profitable number of blades. The number of blades should be no less than three, since two blades having larger unbalanced thrust forces and oscillation of the screw. The change of the main rotor thrust around its average value during one turn of the screw in single-vane and two-bladed propellers. Three-bladed propeller is almost saves the average value of the thrust during the entire turnover.
The number of rotor blades also must be very large, since in this case, each blade runs in the flow disturbed by a previous blade, which reduces the efficiency of the rotor.
The larger the rotor blades, the greater part of the disc swept area they occupy. In the theory of the main rotor of the helicopter introduced the concept of the duty ratio of which is calculated as the ratio of the total area
For the calculation mode helicopter rotor (vertical rise) the most favorable value of the fill factor is the amount of 0,05-0,08 (average value 0,065).
This is the average load. Minor load called a load within 9-12 kg / m2. Helicopters with this load, maneuverability and have a high cruising speed.
utility helicopters have an average load in the range of up to 12 20 kg / m2. Finally, a large load is rarely used, it is a load of up to 20 30 kg / m2.
The fact is that although high specific load on the swept area and provides more useful load the helicopter, but the engine failure on a helicopter autorotation mode will decrease quickly, which is unacceptable, since in this case violated reduce safety.
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