Converting engine power (torque) into thrust necessary for the forward motion of aircraft, snowmobiles, gliders, and hovercraft. There are pulling propellers - installed on an airplane, etc. in front of the engine (in the direction of movement) and pusher propellers - placed behind the engine. The screws can be single or double coaxial, when two screws are located one above the other, the shaft of the upper screw passes through the hollow shaft of the lower screw and they rotate in opposite directions. According to the method of attaching the blades to the hub, there are screws: fixed pitch, the blades of which are integral with the hub; variable pitch - the most common type, the blades of which in flight can be rotated in the hub around the axis by a certain angle, called the pitch of the propeller; reversible, in which in flight the blades can be installed at a negative angle to create thrust directed in the direction opposite to the movement (such blades are used, for example, for effective braking and reducing the length of the aircraft's run when landing). A feature of a feathered propeller is the ability to install the blades in flight along the air flow, so that when the engine stops in flight, it does not increase the drag of the aircraft from the propeller. The number of propeller blades ranges from 2 to 6 for single propellers and up to 12 for coaxial propellers.
The types of propellers are main rotor And tail rotor, applied on helicopters, rotorcraft, gyroplanes.
Encyclopedia "Technology". - M.: Rosman. 2006 .
Blade propellers for converting engine torque into propeller thrust. Installed on airplanes, rotorcraft, snowmobiles, hovercraft, ekranoplanes, etc.
V. in. are subdivided; according to the method of installing the blades - on fixed-pitch, fixed-pitch and variable-pitch propellers (can be feathered or feathered-reversible); according to the pitch change mechanism - with a mechanical, electric or hydraulic drive; according to the work scheme - direct or reverse scheme; by design - single, coaxial, double-row, V. in. in the ring.
V. in. consists of blades ( cm. Propeller blade), bushing and may also include changes in propeller pitch. V. in. differ in diameter D (0.5-6.2 m) and number of blades k (2-12). The bushing serves to secure the blades and transmit torque from the engine shaft. The pitch change mechanism ensures a change in the angle of the blades in flight.
1)
At V. v. With a constant pitch, the blades do not rotate around their axes.
2)
V. blades fixed pitch can be set at the required angle before flight, but they do not rotate during operation.
3)
At V. v. variable pitch, you can change the angle of the blades using a manual control system or automatically using a speed controller. The regulator maintains a given engine speed by controlling the pitch by supplying oil through a system of channels into the corresponding cavities of the V. control mechanism. with hydraulic drive.
4)
In weathervane V. century. the blades can be installed downstream to reduce aerodynamic drag when the engine is forced to stop in flight ( cm. Feathering the propeller).
5)
Vane-reversible wind blades can also be installed in a position where, during its rotation, negative thrust is created, which is used during landing to reduce the length of the run and maneuver on the ground ( cm. Reversing the screw).
Mechanical and electrical mechanisms for changing pitch have a large inertia and therefore are practically not used. The most common are V. century. with hydraulic drive.
1)
At V. v. with a hydraulic drive of a direct design, the blades are set to a small pitch using forces created by oil pressure, and to a large pitch by the centrifugal forces of counterweights. Such V. v. used for engine powers up to 2000 kW.
2)
At powers above 2000 kW, the mass of counterweights increases significantly, which is why V.V. reverse scheme, in which the blades are installed at a large pitch using forces created by oil pressure, and at a small pitch - by the centrifugal forces of the blades themselves.
- A single propeller has one row of blades,
- coaxial V.v. consists of two single screws mounted on coaxial shafts and rotating in opposite directions ( cm. Coaxial screw),
- two-row V. v. consists of two single screws located one behind the other and rotating in the same direction.
- V. V. the ring has a profiled ring, thanks to which additional traction will be created; effective at low speeds (up to 200 km/h).
To reduce aerodynamic drag and power losses at the input to the V.V. install fairings (elliptical, conical, etc.) covering the hub and butt parts of the blades. On the E. century. anti-icing systems can be installed.
To V. in. new generation include V. v. of reduced diameter with a large number of wide thin saber-shaped blades, which are unreasonably called propfans.
In the initial period of the development of military aviation. were made mainly from wood, and in subsequent years others found application (steel, titanium, aluminum alloys, composite materials, etc.).
To assess the quality of V. century. and comparing them with each other, mainly dimensionless α and power are used
(β) = N/(ρ)n3D5
(N - , (ρ) - air density, n - propeller rotation speed)
and propeller efficiency
(η) = (αλ)/(β)((λ) = V/nD - relative, V - flight speed). Characteristics of V. v. determined in flight tests, from V. century research. and their models in wind tunnels, as well as theoretically. When calculating, two cases are distinguished; determination of the shape, size and number of blades from given values (α), (β) and (η) (direct problem) and determination of (α), (β), and (η) from the known geometry of the air flow. (inverse problem).
For the first time, consider the V. blade. as proposed by the Russian engineer S.K. Dzhevetsky in 1892, he also put forward the hypothesis of flat sections in 1910 (each section of the blade is considered as). By decomposing the lifting force of the profile dY and its aerodynamic drag dX, the thrust dP and the force dQ of the resistance to rotation of the blade element under consideration are determined, and the total thrust of the blade and the resistance force to its rotation (hence the engine power required for rotation of the airspeed) are obtained by integration along the blade. Basically, the forces acting on the blade element are determined by the relative speed W of the oncoming flow and its geometric angle of attack
(α)r = (φ)-arctg(V/(ω)r),
(φ) - installation angle of the blade element.
Ideally, the free-stream velocity
W = (ω)Xr + V,
where (ω) is the angular velocity of the blade, r is the radius vector of the section under consideration, V is the flight speed vector. During its movement, the blade carries along with it, giving it an additional, inductive speed w. As a result, the true speed Wн,. flow around an element and true ((α)н differ from ideal ones. Calculation of w and (α)н are the main task of propeller theory.
In 1910-1911 G. X. Sabinin and B. N. Yuryev developed Dzhevetsky’s theory, including in it, in particular, some provisions of the theory of an ideal propeller. Calculations V. v. According to the formulas they obtained, they agreed quite satisfactorily with the experimental results. In 1912, N. E. proposed the vortex theory, which gives an accurate physical representation of the operation of the propeller, and almost all calculations of V.V. began to be carried out on the basis of this theory.
According to Zhukovsky's theory, the propeller is replaced by a system of attached and free vortices. In this case, the blades will be replaced by attached vortices, which transform into a vortex running along the axis of the propeller, and free vortices descend from the trailing edge of the blade, forming in the general case a helical vortex sheet. Assuming that (ω) the relationship (ω) with the circulation of speed around the blade section. The hypothesis of flat sections during continuous flow around a blade was confirmed experimentally by the coincidence of pressure distributions over the sections of the blade of a rotating air flow. and wings with the same section profiles. It turned out, however, that rotation affects the propagation of flow separation along the surface of the blade and, in particular, the vacuum in the separation region. The flow separation region starting at the end of the blade is similar to a rotating pipe; the vacuum in it is controlled by centrifugal force and is much greater on the inside of the blade than on the wing.
At (λ) 1, the difference between the true (ω) and the average becomes noticeable, and the calculation of the V.V. with true (ω) becomes similar to the calculation of a wing of finite span ( cm. Wing theory). When calculating heavily loaded V. v. (with a large ratio of power to the surface swept by the propeller), it is necessary to take into account the deformation of the vortices.
Due to the fact that to the peripheral speed of the V.V. translational is added, the influence of air compressibility affects primarily the air pressure. (leads to a decrease in efficiency). At subsonic peripheral speeds of the blade tip, forward speed of the aircraft and subsonic speed W, the influence of air compressibility on (ω) is weak and affects only the flow around the blade. In the case of subsonic flight speed and supersonic speed W at the end of the blade (when it is necessary to take into account the compressibility of the medium), the theory of airborne vortices, based on the scheme of attached (carrying) vortices, becomes practically inapplicable, and a transition to the scheme of the load-bearing surface is necessary. Such a transition is also necessary at subsonic speed of the blade tip, if its width is large enough. V. century obtained experimentally in the USSR. and corrections due to air compressibility were widely used when choosing the diameters and number of air propeller blades. and together with the choice of the shape of the blades (especially the profiles of their sections) made it possible to improve the flight characteristics of domestic aircraft, including those that participated in the Great Patriotic War.
During the first period of mastering high subsonic speeds, the main task of designing V.V. considered the creation of large-diameter propellers (up to 6 m) with a high efficiency (85% propeller) at maximum flight speed. The characteristics of profiles at high transonic speeds of the airfoil were first obtained experimentally on propellers with so-called drained blades, and one of the profiles had the properties of a supercritical profile (1949). The second period (since the 60s) was characterized by an additional requirement - increased thrust of the V.V. during takeoff. For this purpose, blades with profiles of increased curvature were developed. Further development of V. century. associated with the development of propellers with a large number of wide thin saber-shaped blades. With an increase in the number and width of the blades, the flow around their butt parts, where the effect of the profile lattice is significant, becomes of great importance. A means of reducing wave drag can be the choice of the shape of the spinner. Calculations and experiments show that at flight speeds corresponding to the flight Mach number M in the USSR, a great contribution was made to the development of the theory, calculation methods, and design of aircraft. contributed by S. Sh. Bas-Dubov, B. P. Blyakhman, V. P. Vetchinkin, K. I. Zhdanov, G. M. Zaslavsky, V. V. Keldysh, A. N. Kishalov, G. I. Kuzmin , A. M. Lepilkin, G. I. Maikapar, I. V. Ostoslavsky, N. N. Polyakov, D. V. Khalezov.
Aviation: Encyclopedia. - M.: Great Russian Encyclopedia. Editor-in-Chief G.P. Svishchev. 1994 .
air propeller Encyclopedia "Aviation"
air propeller- Rice. 1. Propeller diagrams. propeller blade propulsion device for converting engine torque into propeller thrust. Installed on airplanes, rotorcraft, snowmobiles, hovercraft, ekranoplanes, etc.V. V … Encyclopedia "Aviation"
air propeller- Rice. 1. Propeller diagrams. propeller blade propulsion device for converting engine torque into propeller thrust. Installed on airplanes, rotorcraft, snowmobiles, hovercraft, ekranoplanes, etc.V. V … Encyclopedia "Aviation"
air propeller- Rice. 1. Propeller diagrams. propeller blade propulsion device for converting engine torque into propeller thrust. Installed on airplanes, rotorcraft, snowmobiles, hovercraft, ekranoplanes, etc.V. V … Encyclopedia "Aviation"
AIR PROPELLER- a bladed mover, the working medium of which is air. The propeller is a common aircraft propulsion device. In terms of blade geometry and hydrodynamic characteristics, a marine propeller differs significantly from aircraft and... ... Marine encyclopedic reference book
A propeller, a propulsion device in which radially arranged profiled blades, rotating, throw out air and thereby create traction. V. in. consists of a bushing located on the engine shaft and blades with a span along the... ... Great Soviet Encyclopedia
air propeller- orasraigtis statusas T sritis fizika atitikmenys: engl. impeller airscrew; propeller vok. Luftschraube, f; Propeller, m; Saugschraube, f rus. propeller, m; propeller, m pranc. aéro propulseur, m; helice aérienne, f; hélice propulsive, f… Fizikos terminų žodynas
Nadezhin Nikita
The theory of the propeller: from the first propellers to the efficient units of the future.
PLAN:
Introduction.
1.1. Air propeller.
1.2.Technical requirements for the F1B class aircraft model.
3. Description of the propeller design.
1.4. Description of the aircraft model.
Conclusion.
List of references, software.
Applications.
Introduction
A propeller, a propeller, a propulsion device in which radially arranged profiled blades, rotating, throw out air and thereby create a traction force (“Propeller” - a student publication in large circulation at the Moscow Aviation Institute). A propeller consists of one, two or more blades connected to each other by a hub. The main part of the propeller is the blades, since only they create thrust.
The idea of a propeller was proposed in 1475 by Leonardo da Vinci, and was used to create thrust for the first time in 1754 by V.M. Lomonosov in the model of a device for meteorological research.
M.V. Lomonosov
On the plane A.F. Mozhaisky used propellers. The Wright brothers used a pusher propeller.
Even before the design of the first aircraft began, A.F. Mozhaisky made several aircraft models in which the propeller was a propeller driven by a rubber band. In America, the Wright brothers also first made airplane models, and only then the first flying airplane was designed.
Since the beginning of the 20th century, young people all over the world began to design and build model airplanes and hold competitions. In our country, the first competitions were instructed by N.E. Zhukovsky in 1926. Aeromodelling sport began to be cultivated by the International Aeronautical Federation FAI, the FAI Code was developed, and All-Russian and international competitions are held.
According to the rules of the competition, all models of participants must meet certain requirements, and in order to win the competition, you must make a model that flies the best. To do this, it is necessary to increase the take-off altitude of the model, but this is difficult to do, since the energy reserve on the model is limited by the weight of the rubber motor, which is checked during the competition. All that remains is to increase the rubber energy utilization coefficient, and this is mechanization of the propeller in flight by changing the geometric characteristics. The torque of the rubber motor is variable and has a nonlinear characteristic. And the torque required to drive a propeller is proportional to the diameter of the propeller to the fifth power. To realize the available torque and increase the efficiency of the propeller, it is necessary to change the diameter and pitch during flight. In existing designs, the propeller pitch is changed, since it is structurally simpler, but this entails an increase in flight speed, and therefore harmful wing drag. The gain is small. Increasing the diameter of the propeller while simultaneously increasing the pitch allows the propeller to be used more efficiently. The winnings are greater.
Task : design of mechanisms to increase efficiency, reduce fuel consumption for the production of various types of energy, leading to a reduction in harmful emissions into the atmosphere.
The topic of this work is very relevant for understanding the development of modern technology. Work to increase the efficiency of the propeller makes it possible in the future to design more complex mechanisms aimed at increasing the efficiency of other products that consume thermal and electrical energy and are associated with improving the ecology of the surrounding space. In the modern world, this is very important since the use of mechanisms that increase efficiency on machines and generators leads to a reduction in fuel consumption, and therefore emissions of combustion products into the atmosphere and improve the state of the environment and human health.
The purpose of this work : design of a mechanism that increases the efficiency of using mechanical energy by the propeller of a rubber-engined model aircraft.
Meaning of work : Using the example of designing a simple mechanism, issues of designing more complex mechanisms that can be effectively used in the future when developing new aircraft are discussed.
1. Propeller
In calm air, an airplane can fly horizontally or climb only when it has propulsion. Such a propulsion device can be a propeller or a jet engine. The propeller must be driven by a mechanical engine. In both cases, thrust is created due to the fact that a certain mass of air or exhaust gases is thrown in the direction opposite to the movement.
Fig.4. Diagram of forces acting on a propeller.
As it moves, the propeller blade describes a helical line in space. In its cross section it has the shape of wing profiles. In a properly designed propeller, all blade sections meet the flow at some favorable angle. In this case, a force similar to the aerodynamic force on the wing develops on the blade. This force, being decomposed into two components (in the plane of the propeller and perpendicular to the plane), gives thrust and resistance to the growth of a given blade element. By summing up the forces acting on all elements of the blades, we obtain the thrust developed by the propeller and the torque required to rotate the propeller (Figure 4). Depending on the amount of power consumed, propellers with different numbers of blades are used - two, three and four blades, as well as coaxial propellers rotating in opposite directions to reduce power losses due to twisting of the thrown air stream. Such propellers are used on Tu-95, An-22, Tu-114 aircraft. The Tu-95 is equipped with 4 NK-12 engines designed by Nikolai Kuznetsov (Figure 5). The ends of the blades of these propellers rotate at supersonic speed, creating a lot of noise (the NATO name for the Tu-95 aircraft is “Bear”, adopted in 1956 and the Russian Air Force uses this aircraft to this day). In aircraft modeling, single-bladed propellers are also used to achieve high results in competitions. The efficiency of the screw depends on the amount of coating on the screw
(where is the number of blades, is the maximum blade width), the smaller the propeller coating, the higher the propeller efficiency can be obtained. The strength of the blade prevents an infinite reduction in coverage. Multi-bladed propellers are not beneficial, as they reduce efficiency.
Fig.5. Airplane TU-95 with a coaxial propeller.
The first propellers had a fixed pitch during flight, determined by a constant angle of installation of the propeller blades. To maintain a sufficiently high efficiency over the entire range of flight speeds and engine power, as well as for feathering and changing the thrust vector during landing, variable pitch propellers (VPR) are used. In such propellers, the blades are rotated in the hub relative to the longitudinal axis by a mechanical, hydraulic or electrical mechanism.
To increase thrust and efficiency at low forward speed and high power, the propeller is placed in a profiled ring, in which the jet speed in the plane of rotation is greater than that of an isolated propeller, and the ring itself, due to the circulation of speed, creates additional thrust.
The propeller blades are made of wood and duralumin. Steel, magnesium, composite materials. At flight speeds of 600-800 km/h, the propeller efficiency reaches 0.8-0.9. At high speeds, under the influence of air compressibility, efficiency decreases. Therefore, a propeller is beneficial at subsonic aircraft speeds.
The idea of a propeller was proposed in 1475 by Leonardo da Vinci (Figure 1), and was used to create thrust for the first time in 1754 by M.V. Lomonosov in a model of an instrument for meteorological research (Figure 2). By the mid-19th century, steamships were using propellers similar to a propeller. In the 20th century, propellers began to be used on airships, airplanes, snowmobiles, helicopters, hovercraft, etc.
Rice. 1. Helicopter. An idea proposed by Leonardo da Vinci. Model based on a sketch by Leonardo da Vinci.
Fig.2. Device model M.V. Lomonosov for meteorological research.
Methods for aerodynamic calculation and design of propellers are based on theoretical and experimental research. In 1892-1910, Russian research engineer and inventor S.K. Dzhevetsky developed the theory of an isolated blade element, and in 1910-1911 Russian scientists B.N. Yuriev and G.Kh. Sabinin developed this theory. In 1912-1915 N.E. Zhukovsky created the vortex theory, which gives a visual physical representation of the operation of the propeller and other blade devices and establishes a mathematical connection between forces, velocities and geometric parameters in such machines. In the further development of this theory, a significant role belongs to V.P. Vetchinkin. In 1956, Soviet scientist G.I. Maikoparov extended the vortex theory of the propeller to the rotor of a helicopter.
NOT. Zhukovsky
Currently, to create large-sized long-haul aircraft, propulsion systems of greater power and very economical ones were required. One of the options for such engines is turbofan engines. They have great traction and good efficiency. These are the engines that are installed on all foreign aircraft.
The development of Leonardo da Vinci's idea was embodied in the creation of gas turbine engines with an axial compressor. The blades of an axial compressor create an increase in air pressure as they move. Each stage increases the pressure by a certain amount and at the end the air compressed by the compressor enters the combustion chamber, where heat is supplied to it in the form of burning fuel. After which the hot gas enters the turbine, which can be either axial or radial. The turbine, in turn, turns the compressor, and the gases that have lost some of their energy enter the nozzle and create jet thrust.
Compressor blades are part of a propeller blade. There can be several dozen such blades in each stage. Between the stages there is a stationary straightening apparatus, which consists of the same blades, only installed at a certain angle to the swirling air flow. Spinning occurs due to the movement of the compressor blades around the circumference. The number of compressor stages can be more than 15.
If all the energy obtained as a result of the burned fuel is worked on the turbine, then there will be excess power at the engine shaft, which can be used to drive the propeller. The result will be a turboprop engine, and the thrust will be generated by the propeller. Thrust due to exhaust gases will be minimal.
The next stage of development was dual-circuit engines. In these engines, some of the air does not pass through the compressor (from the outside), usually after the first two stages of the compressor. This type of engine is called a turbofan engine. Engine thrust is created by the fan (the first two stages of the compressor) and the jet stream of exhaust gases. In this case, the fan, which is essentially a propeller, is located in a profiled housing.
The next stage of development is a turbofan engine (NK-93). Why did they start making such engines? Yes, because the efficiency of the propeller at subsonic flight speeds can approach 0.9, and the efficiency of the jet stream is much less. The turbofan engine is the most promising engine for aircraft flying at subsonic speeds in the future.
Double-circuit turbojet engine.
In 1985, OKB named after N.D. Kuznetsov began studying the concept of a propfan engine with a high bypass ratio. It was determined that a hooded engine with coaxial propellers would provide 7% more thrust than an unhooded turboprop engine with a single-stage fan.
In 1990, the design bureau began designing such an engine, designated NK-93. It was intended primarily for IL-96M, Tu-204P, Tu-214 aircraft, but the Ministry of Defense also showed interest in the new engine (it is planned to install it on the military transport Tu-330).
IL-76 LL aircraft with NK-93 engine.
Engine NK-93.
The NK-93 is made according to a three-shaft design with the engine of a blackened double-row counter-rotating propfan SV-92 through a gearbox. Planetary gearbox with 7 satellites. The first stage of the propfan is 8-bladed, the second (accounting for 60% of the power) is 10-bladed. All saber-shaped blades with a sweep angle of 30 0 on the first 5 engines were made of magnesium alloy. Now they are made of carbon fiber.
NK-93 engine diagram.
The technical characteristics of the new engine have no analogues in the world. In terms of thermodynamic cycle parameters, the NK-93 is close to engines currently being developed abroad, but has better efficiency (by 5%). Flight tests are carried out on the IL-76LL aircraft. The highlight of this propeller installation is the planetary gearbox and propfan. The installation angle of the blades can vary within 110 0 during engine operation. A similar gearbox is used in NK-12 engines on the Tu-95 aircraft, and a similar gearbox is used in gas pumping installations on main gas pipelines (NK-38). So we have experience.
During classes in the aircraft modeling laboratory of the Kostroma Regional Center for Children's (Youth) Technical Creativity, issues of the theory of flight of aircraft and flying models are discussed. In order to improve the flight characteristics of rubber-engine models, as well as improve performance results at competitions, the operation of a propeller-driven installation was examined. Having examined the characteristics of the rubber motor, the energy of which determines the take-off altitude of the model, it was found that the torque of the rubber on the propeller shaft has a nonlinear characteristic. The maximum torque exceeds the average torque by 5-6 times. The torque required to rotate the screw is
Where
Aerodynamic coefficient
Air density
Screw diameter
Propeller revolutions per second
It is known from theory that in order for the efficiency of a screw to be sufficiently high, it is necessary to increase the diameter of the screw without limit. As is known, this condition cannot be fulfilled constructively. But, knowing this, we see one of the possible ways to increase the flight duration of a rubber-motor model. It was decided to compensate for the change in torque by changing the diameter of the screw. Structurally, it is quite difficult to change the diameter of the screw by an amount proportional to the change in torque, so a change in the pitch of the screw has also been introduced. The result was a variable diameter and pitch propeller (VIDSH). In large aviation, changing the diameter of the propeller is not used due to the complexity of the design and high speeds at the ends of the blades, comparable to the speed of sound, which reduce the efficiency of the propeller.
It is possible to increase the efficiency of a propeller by reducing the propeller coating. This means making the propeller single-bladed. Such screws are now used on high-speed cord models. The results are very positive. The speed increases by 10-15 km/h, but the working conditions are different there. The engine runs at constant speed and constant maximum power. On rubber motor models, the energy of the rubber motor is variable and not linear. When using a single-blade propeller with variable diameter and pitch, difficulties arise with the counterweight of the propeller blade. Therefore, it was decided to use a two-bladed propeller with variable diameter and pitch (VIDSP) to increase the efficiency of the propeller of a rubber-engined aircraft model.
2. Technical requirements for a class aircraft modelF1 B
A rubber-engine model of an aircraft according to the FAI classification - F1B, made by Nikita Nadezhin under the leadership of Viktor Borisovich Smirnov, was presented for the competition.
With this model, Nikita Nadezhin became the champion at the Russian Aviation Modeling Championship in 2013.
A rubber-motor model is a model of an aircraft that is driven by a rubber engine; the lifting force of the model arises due to aerodynamic forces acting on the load-bearing surfaces of the model.
Technical characteristics of rubber-motor models must comply with FAI requirements:
bearing surface area - 17-19 dm 2
minimum weight of the model without rubber motor - 200 g
The maximum weight of a lubricated rubber motor is 30 g.
Each competition participant has the right to 7 qualifying flights lasting no more than 3 minutes each. The launch of the model must be carried out within a limited time, announced in advance. The sum of the times of all qualifying flights of each participant is used for the final distribution of places among the participants.
During the flight, the model can fly away from the launch site to a distance of 2.5-3 km. To search for a model, a radio transmitter weighing 4 grams is installed on it with power for several days. The competitor has a radio receiver with a directional antenna to detect the model.
The model takes off using the energy of a rubber motor, which rotates the propeller. The change in the torque of the rubber motor during its spin-up occurs unevenly and its maximum value exceeds the average value by 4-5 times. Therefore, at the initial moment of take-off of the model, the propeller operates in off-design modes, i.e. the propeller is slipping in the air flow. In order to aerodynamically load the propeller and use the available energy of the rubber motor to its full extent, it is necessary to increase the diameter of the propeller and the angle of installation of the propeller blades during the initial take-off period. This is well shown in the book by A.A. Bolonkin “The Theory of Flight of Flying Models”
3. Description of the propeller design
A special feature of this model is the propeller (Appendices No. 4,5,6), which changes the diameter and pitch during takeoff of the model. The propeller mechanism, when changing the torque of the rubber motor, allows you to change the diameter of the propeller and the angle of installation of the blades. This allows you to significantly increase the efficiency of the propeller and, consequently, the take-off altitude of the model, and, accordingly, the flight duration and results in competitions increase.
The design of the screw mechanism is presented in the assembly drawing 10.1000.5200.00 SB VIDSH (screw of variable diameter and pitch, Appendix No. 3) and is a housing in which the screw shaft made of ZOKHGSA steel rotates on 2 bearings. A screw hub is installed on the shaft, also on 2 bearings, followed by a bushing that can rotate around the shaft. The bushing has connecting rods on which the propeller blades, made of balsa, are suspended. The connecting rods are installed on axes located at a radius R=11 from the shaft axis and at an angle to it of approximately 6 degrees. The bushing and hub are connected to each other by an elastic element (rubber ring). There is a groove in the hub that limits the movement of the bushing relative to the hub. This determines the operating angles of rotation of the bushing and the amount of extension of the connecting rods. When a torque is applied to the propeller shaft relative to the propeller blades, a force arises that rotates the bushing relative to the hub, and the connecting rods move out of the hub and rotate around the transverse axis of the shaft due to the movement of the connecting rod axes along the generatrix of a single-cavity hyperboloid around the shaft. The design provides for changing the angle of inclination of the connecting rod axes, which allows you to adjust the range of pitch changes when adjusting the model. (in the original version, adjustment of the pitch change limits was not provided, drawing 10.0000.5100.00 SB, Appendix No. 2). The movement of the connecting rods is proportional to the torque applied to the propeller shaft relative to the blades. A standard stopper is installed on the hub, locking the propeller blades in the desired position after spinning the rubber motor. The change in pitch with an increase in diameter by 25 mm is 5 0, which at R blade = 200 mm changes the pitch from 670 mm to 815 mm. For the manufacture of parts, small-sized ball bearings and high-strength materials D16T, ZOKHGSA, 65S2VA, 12x18N10T and carbon fiber were used.
4. Description of the aircraft model
The design of the model itself is presented in drawing 10.0000.5000.00СБ. (Appendix No. 1.7)
The longitudinal wing assembly consists of two carbon fiber spars of variable cross-section, a carbon fiber caisson, and leading and trailing edges made of carbon fiber.
The transverse set consists of ribs made of balsa, covered on top and bottom with carbon fiber overlays 0.2 mm thick. The Andryukov profile is used on the wing. The center of gravity is located at 54% of the MAR.
The entire set is assembled using epoxy resin. The wing is covered with synthetic paper (polyester) on enamel. For ease of transportation, the wing has a transverse connector with fastening points. The stabilizer and fin are designed similarly to the wing.
The fuselage consists of two parts. The front power part is made of a tube made of SVM (Kevlar) and a carbon fiber pylon, in which a program mechanism (timer) and a transmitter for searching for a model are installed; power frames made of aluminum alloy D16T are glued in front and back.
The tail part is a cone and consists of 2 layers of high-strength aluminum foil D16T 0.03 mm thick, between which a layer of carbon fabric on epoxy resin is glued. At the end of the tail there is a platform for attaching the stabilizer and a mechanism for rebalancing and landing the model.
The model uses rubber motors made of FAI “Super sport” rubber, consisting of 14 rings with a cross-section of 1/8 //
The use in this class of models of a mechanism that allows simultaneously changing the diameter and pitch of the propeller depending on the torque of the rubber motor, made it possible to increase the efficiency of the propeller, which resulted in an increase in the take-off altitude of the model by 10-12 meters, the flight duration increased by 35-40 seconds. compared to other models, and flight stability has also improved. And as a result - victory in competitions.
Conclusion
Conclusion: The principle of converting translational motion into rotational motion inherent in this design can be used in cases where simple lever mechanisms cannot be used.
Practical recommendations: A similar mechanism can be used to drive the ailerons of a cruise missile. The translational movement of the thrust inside the wing, along the trailing edge, is converted into the rotational movement of the aileron. It is quite difficult to use other mechanisms due to the low construction height of the wing profile in the area where the aileron is located and the distance of the aileron from the rocket body.
Thus, using the example of designing a simple mechanism to increase efficiency, we can consider issues of creating more advanced mechanisms for converting hydrocarbon energy into mechanical thermal and electrical energy, which in modern conditions will reduce the level of emissions of harmful substances into the atmosphere and improve the state of the environment and human health .
List of literature, software
1.A.A.Bolonkin. Theory of flight of flying models, ed. DOSAAF 1962
2.E.P.Smirnov, How to design and build a flying model of an airplane, ed. DOSAAF 1973
3. Schmitz F.W. Aerodynamics of low speeds, ed. DOSAAF 1961
4. Design was carried out in the Compass V-11 program
Annex 1.
Appendix 2.
Appendix 3.
Propeller in a ring
Amateur designers of snowmobiles, airboats, airplanes and other vehicles using propellers often solve the dilemma of obtaining acceptable thrust with the small dimensions of a propeller-engine installation. One way to increase thrust without increasing the diameter of the propeller is to increase the number of blades. Thus, increasing the number of blades from 2 to 4 leads to an increase in propeller thrust by 70-80%. But in this case, the efficiency of the propeller decreases, so an engine with twice the power is required. One way to increase the static thrust of a propeller without increasing engine power is to use a ring attachment. In this case, the static thrust increases by 1.2 times, which is equivalent to an increase in the diameter of the propeller by 30%.
The rotor blades, rotating, capture air and throw it in the direction opposite to the movement. A zone of low pressure is created in front of the screw, and high pressure behind the screw. The rotation of the propeller blades leads to the fact that the air masses thrown out by it acquire circumferential and radial directions and this consumes part of the energy supplied to the propeller.
The propeller-guide nozzle complex has a number of specific advantages associated with the action of the nozzle:
1. The circulation of the oncoming flow that occurs around the profile of the nozzle unloads the screw, shifting part of the stop of the complex onto the nozzle.
2. When the complex operates in an oblique flow, the nozzle forms a velocity field in front of the propeller, aligning it almost coaxially with the propeller, maintaining the value of the inflow velocity. As a result, the bevel of the incoming flow has little effect on the propeller.
3. The pressure difference on the discharge and suction sides of the propeller blades without a nozzle, which determines the useful action of the propeller, decreases due to flow at the ends of the blades (as on an airplane wing). The presence of a nozzle prevents such overflow, practically eliminates end losses and thus increases the efficiency of the complex.
In general, the efficiency of the complex can be 20% higher than the efficiency of a screw without an attachment.
The nozzle is a ring covering the propeller. The section of the nozzle along the propeller axis is given a wing profile, with its convex surface facing the propeller (Fig. 1).
Due to the bevel of the air flow, the profile of the nozzle flows around at a certain angle of attack. As a result, a lift force Cy and a thrust force P arise. The efficiency of the nozzle significantly depends on the operating mode of the propulsion complex. Thus, during the take-off run, when the propeller creates a large thrust at a low speed of the aircraft, the flow bevel at the inlet of the nozzle is quite large, which leads to unloading of the blades. The profile resistance of the nozzle at low speed is small. However, at high speeds, the flow slope decreases, and the profile resistance increases sharply. The effectiveness of the nozzle decreases.
The gap between the tip of the propeller blade and the nozzle is 1-2% of the propeller radius. With a larger gap, the efficiency of the complex approximately corresponds to the efficiency of a screw without a nozzle. With a smaller gap, it is difficult to ensure unhindered rotation of the screw due to vibrations and temperature deformations of the complex parts.
The nozzle creates a more uniform load on the engine. By reducing the harmful effects of oblique flow on the propeller, the nozzle reduces variable loads on the blades and propeller shaft, and serves as a kind of damper during lateral gusts of wind. The attachment also serves to protect the propeller from damage and makes the operation of the vessel safer.
The calculation of the nozzle is quite complicated. Just like the propeller calculation, it often does not give the calculated results in practice. Therefore, it is easier to select the nozzle experimentally.
Below are the parameters of a four-blade propulsion system “propeller in a ring” in comparison with two and four bladed propellers without attachments.
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F (ring) |
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Screws can be pulling or pushing. The first type of propellers are installed in front of the fuselage and wing, the second type of propellers are installed in their tail section. For layout reasons, traction screws were the predominant use. When choosing the type of propeller, you also have to take into account the fact that flying pieces of ice when the aircraft becomes icy can damage the propeller blades located behind the wing and fuselage.
On high-power engines it is advantageous to install two propellers rotating in different directions. Such screws are called coaxial.
The use of coaxial propellers allows not only to transfer more power from the engine shaft, but by reducing losses due to the twisting of the air flow, to obtain a slightly higher efficiency compared to a single propeller.
In addition, coaxial propellers, rotating in different directions, create almost no reaction torque, which is very important for ensuring the lateral balance of the aircraft.
The simplest type is a fixed pitch propeller (FPH), in which the hub and blades are organically intact. The material for making such screws is most often wood. Such propellers are currently used only on light aircraft. Since the installation angle of a fixed propeller does not change during flight, such a propeller will be beneficial only when flying at a very limited speed range. In other cases, the efficiency of the propeller is low.
Propellers in which the angle of installation of the blades can be changed in flight are called variable pitch propellers (VPS). The blades of such propellers can rotate relative to their longitudinal axes automatically or at the will of the pilot, changing the angle of installation.
To reduce drag in the event of an engine failure in flight, variable-pitch weathervane propellers are used, the blades of which, using a special drive, at the will of the pilot, are set to the position of least resistance when the propeller is stopped. This is achieved with a blade installation angle of 83-85°.
Braking or reversing screws have been widely used in recent years. Reversible propellers are high-pressure propellers with devices that allow the blades to be installed in such a way that the propeller develops negative thrust when rotating. The presence of negative thrust makes it possible to reduce the length of the post-landing run, increase the glide angle, and increase the maneuverability of the aircraft when moving on the ground.
The angle of installation of the blades of the VPSH can be changed by mechanical, hydraulic and electric drives.
A mechanical propeller is a propeller in which the rotation of the blades to a particular angle is carried out either by the pilot or by the forces that arise during operation of the propeller and change when the operating mode changes. Sometimes such propellers are called aeromechanical propellers. They are widely used on light aircraft.
With variable pitch hydraulic propellers, the angle of the blades is changed by a hydraulic motor under the influence of oil pressure. The pressure is created by a pump driven by an aircraft engine. To power the pump, oil is used to lubricate the engine (non-autonomous propeller), as well as oil not included in the engine lubrication system (autonomous propeller).
Changing the angle of installation of the blades can be done by a piston or gear hydraulic motor. There can be one gear motor per propeller or one for each blade.
In both cases, the rotational movement of the hydraulic engine with the help of a mechanical transmission rotates the blades.
Transmission from the moving element of the piston engine to the blade is carried out in two ways:
the piston transmits movement to a holder - a traverse or a leash, connected to an eccentrically mounted finger on the blade or glass in which the blade is attached (Fig. 114). Sometimes the piston and blade cup are connected using connecting rods;
The piston, moving progressively, moves the pin installed in the screw cutout of the cage. The finger, moving along the cutout in the clip, turns it. This movement is transmitted to the blades through a bevel gear.
Hydraulic screws can be made using reverse, direct and double patterns.
A reverse design propeller is a propeller in which the blades rotate by a small pitch under the influence of the moment of the transverse components of the centrifugal forces of the Mtsb blades, and by a large pitch - under the influence of the moment Mmech created by the hydraulic mechanism (Fig. 114, a). When the oil supply stops or the system's tightness is broken, the propeller blades rotate to a minimum pitch under the influence of the specified centrifugal forces. As a consequence of this, during flight the engine will spin up, i.e. the number of revolutions will sharply increase above the maximum permissible. The pilot will have to turn off the engine to avoid engine destruction.
A direct propeller is a propeller in which the blades rotate by a small pitch under the action of the moment M mech created by the hydraulic mechanism, and by a large pitch - under the influence of the difference in the moments of the centrifugal forces of the counterweights M pr of the centrifugal forces of the blades M cb (Fig. 114, b). When the oil supply stops, the blades of such a propeller are set to the maximum (working) pitch. For straight screws, unwinding is not dangerous.
The weight of such screws is greater than the weight of the reverse design screws, but its advantage is the ability to obtain some power (up to 70% of the maximum) when the oil supply to the screw is stopped.
A double circuit propeller is a propeller whose blades are set to a small pitch under the influence of the moment M mech created by a hydraulic mechanism and the moment of centrifugal forces of the blades M cb, and to a large pitch - only with the help of a hydraulic mechanism (Fig. 114, c).
To prevent the double-pattern propeller blades from rotating to a small pitch in the event of a failure of the oil supply system, a mechanism called a pitch lock is provided. If the oil supply is interrupted, the pitch lock locks the oil in the large pitch cavity of the propeller cylinder group, fixing the blades at the pitch at which the blade was at the time of the accident. The pitch lock can also be installed on a reverse circuit screw, but only with a two-channel oil supply to the screw.
Electric variable pitch propellers. The blades of these propellers are rotated to the desired angle using electric motors. One electric motor or several (according to the number of blades) can be installed on one propeller; in the latter case, the blades are mechanically linked to synchronize rotation. Some propellers have an electric motor mounted on the aircraft engine, and the movement of the blades is transmitted using a differential gear train. Electric motors are always reversible, since the blades must turn in both directions. The engines receive electrical power from the aircraft's general network. The electric motors that drive the propeller blades are equipped with limit switches that turn off the motors at the moment when the blades rotate to the maximum small or large pitch.
Literature used: "Fundamentals of Aviation" authors: G.A. Nikitin, E.A. Bakanov
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Before jet engines were developed, all airplanes had propellers, that is, propellers driven by internal combustion engines like automobiles.
All propeller blades have a cross-sectional shape resembling that of an airplane wing. As the propeller rotates, the air flows around the front surface of each blade faster than the rear. And it turns out that there is less pressure in front of the propeller than behind it. This creates a traction force directed forward. And the magnitude of this force is greater, the higher the speed of rotation of the propeller.
(Image above) Air flow moves faster along the leading surface of a rotating propeller blade. This reduces the air pressure at the front and causes the plane to move forward.
A propeller-driven aircraft takes off into the air thanks to the thrust generated by the rotation of the propeller blades.
The ends of the rotating propeller blades describe a spiral in the air. The amount of air that a propeller pushes through itself depends on the size of the blades and the speed of rotation. Additional blades and more powerful motors can increase the useful performance of a propeller.
Why do propeller blades have a twisted shape?
If these blades were flat, the air would be evenly distributed over their surface, causing only resistance to the rotation of the propeller. But when the blades are curved, the air flow in contact with their surface acquires its own direction at each point on the surface of the blade. This shape of the blade allows it to cut through the air more efficiently and maintain the most favorable ratio between traction force and air resistance.
Variable-angle propellers. The angle at which the blade is installed in the rotor hub is called the initial cone angle. On some aircraft, this angle can be changed and thus make the most useful operation of the propeller under different flight conditions, that is, during takeoff, climb or in cruising flight.
