The propeller is the most important integral part power plant, and how it corresponds to the engine and the aircraft depends on the flight performance of the latter.

In addition to the choice of the geometric parameters of the propeller, attention should be paid to the issue of matching the speed of the propeller and the engine, that is, the selection of the gearbox.

The principle of operation of the propeller

The propeller blade makes a complex movement - translational and rotational. The speed of the blade element will be the sum of the circumferential speed and translational (flight speed) - V

In any section of the blade, the velocity component V will be unchanged, and the circumferential speed will depend on the value of the radius on which the section under consideration is located.

Consequently, as the radius decreases, the angle of approach of the jet to the section increases, and the angle of attack of the section decreases and can become equal to zero or negative. Meanwhile, it is known that the wing "works" most effectively at angles of attack close to the angles of maximum lift-to-drag ratio. Therefore, in order to force the blade to create the greatest thrust with the least expenditure of energy, the angle must be variable along the radius: smaller at the end of the blade and larger near the axis of rotation - the blade must be twisted.

The law of distribution of profile thicknesses and twist along the radius of the propeller, as well as the shape of the propeller profile, is determined during the design process of the propeller and subsequently refined on the basis of blowing in wind tunnels. Such studies are usually carried out in specialized design bureaus or institutes equipped with modern equipment and computer facilities. Experimental design bureaus, as well as amateur designers, usually use already developed families of propellers, the geometric and aerodynamic characteristics of which are presented in the form of dimensionless coefficients.

Main characteristics

screw diameter - D called the diameter of the circle that the ends of its blade describe during rotation.

Blade width is the chord of the section at the given radius. The calculations usually use the relative width of the blade

blade thickness on any radius is called the greatest thickness of the section on this radius. The thickness varies along the radius of the blade, decreasing from the center of the propeller to its tip. The relative thickness is understood as the ratio of the absolute thickness to the width of the blade at the same radius: .

The installation angle of the blade section is the angle formed by the chord of this section with the plane of rotation of the propeller.

Blade pitch H called the distance that this section will pass in the axial direction when the screw is rotated one revolution around its axis, screwing into the air as into a solid body.

The step and the installation angle of the section are related by the obvious relationship:

Real propellers have a pitch that varies along the radius according to a certain law. As a characteristic angle of installation of the blade, as a rule, the angle of installation of the section located at 0.75R from the axis of rotation of the propeller, denoted as .

Steep blade is called the change along the radius of the angles between the chord of the section at a given radius and the chord at a radius of 0.75R, that is

For ease of use, all of the listed geometric characteristics are usually presented graphically as a function of the current radius of the screw

As an example, the following figure shows data describing the geometry of a two-bladed fixed pitch propeller:

If the screw, rotating with the number of revolutions, moves forward with a speed V then in one revolution it will cover the path . This value is called the screw pitch, and its ratio to the diameter is called the relative screw pitch:

The aerodynamic properties of propellers are usually characterized by a dimensionless thrust coefficient:

power factor

And the efficiency

Where R- air density, in calculations can be taken equal to 0.125 kgf s 2 / m 4

Angular speed of rotation of the screw r / s

D- screw diameter, m

P And N- respectively thrust and power on the propeller shaft, kgf, l. With.

Theoretical propeller thrust limit

For the designer of an ALS, it is of interest to be able to make approximate estimates of the thrust generated by the power plant without calculations. This problem is quite simply solved using the theory of an ideal propeller, according to which propeller thrust is a function of three parameters: engine power, propeller diameter and flight speed. Practice has shown that the thrust of rationally executed real propellers is only 15 - 25% lower than the theoretical limit values.

The results of calculations according to the theory of an ideal propeller are shown in the following graph, which allows you to determine the ratio of thrust to power depending on the flight speed and parameter N/D 2. It can be seen that, at near-zero speeds, the thrust depends to a large extent on the propeller diameter; however, already at 100 km/h wire speeds, this dependence is less significant. In addition, the graph gives a visual representation of the inevitability of a decrease in propeller thrust with respect to flight speed, which must be taken into account when evaluating the flight data of an ALS.

according to materials:
"Guide for designers of amateur-built aircraft", Volume 1, SibNIIA

Nadezhin Nikita

Propeller theory: 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 design of the propeller.

1.4. Description of the aircraft model.

Conclusion.

List of literature, software.

Applications.

Introduction

Propeller, propeller, propeller, in which radially arranged profiled blades, rotating, discard air and thereby create thrust ("Propeller" is a student newspaper 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 they are the only ones that create thrust.

The idea of ​​a propeller was proposed in 1475 by Leonardo da Vinci, and used it to create thrust for the first time in 1754 by V.M. Lomonosov in the model of an instrument for meteorological research.

M.V. Lomonosov

On the plane A.F. Mozhaisky propellers were used. The Wright brothers used a pusher screw.

Even before the design of the first aircraft, A.F. Mozhaisky made several models of the aircraft, which had a propeller driven by a rubber band. In America, the Wright brothers also first made aircraft models, and only then was the first flying aircraft designed.

From the beginning of the 20th century, all over the world, young people began to design and build model aircraft and hold competitions. In our country, the first competitions were admonished by N.E. Zhukovsky in 1926. Aeromodelling began to be cultivated by the International Aviation Federation FAI, the FAI code was developed, 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, it is necessary to make the best flying model. To do this, it is necessary to increase the take-off height 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. It remains only to increase the coefficient of use of rubber energy, and this is the mechanization of the propeller in flight to change the geometric characteristics. The torque of the rubber motor is variable and has a non-linear characteristic. And the torque required to drive the 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 in flight. In existing designs, the pitch of the propeller is changed, since it is structurally simpler, but this entails an increase in flight speed, and hence the harmful drag of the wing. The gain is small. Increasing the diameter of the propeller with a simultaneous increase in pitch allows the propeller to be used more efficiently. The gain is greater.

Task : designing mechanisms to increase efficiency, reduce fuel consumption for the generation of various types of energy, leading to a decrease in harmful emissions into the atmosphere.

The topic of this work is very relevant for understanding the development of modern technology. The work on increasing 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 modern world this is very important, since the use of mechanisms that increase efficiency on machines, generators leads to a decrease in fuel consumption, and therefore emissions of combustion products into the atmosphere and an improvement in the state of the environment environment and human health.

The purpose of this work : designing a mechanism for increasing the efficiency of using mechanical energy by a propeller of a rubber-engine aircraft model.

The meaning of work : On the example of designing a simple mechanism, the issues of designing more complex mechanisms that can be effectively used in the future when developing new aviation technology are considered.

1. Propeller

In calm air, an aircraft can fly horizontally or climb only when it has a propeller. Such a propeller can be a propeller or a jet engine. The propeller must be driven mechanical motor. 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.

During its movement, the propeller blade describes a helix in space. In its cross section, it has the shape of wing profiles. In a properly designed propeller, all sections of the blade meet the flow at some most favorable angle. In this case, a force develops on the blade, similar to the aerodynamic force on the wing. This force, being decomposed into two components (in the plane of the screw and perpendicular to the plane), gives thrust and resistance to the rotation of this element of the blade. Summing up the forces acting on all elements of the blades, we get the thrust developed by the propeller and the moment required to rotate the propeller (Figure 4). Depending on the amount of power consumed, propellers with a different number of blades are used - two, three and four bladed, as well as coaxial propellers rotating in opposite directions to reduce power losses due to the twisting of the ejected air stream. Such screws are used on Tu-95, An-22, Tu-114 aircraft. The Tu-95 has 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 sports, single-bladed propellers are also used to obtain high results in competitions. The efficiency of the screw depends on the size of the screw coating

(where is the number of blades, is the maximum width of the blade), the smaller the propeller coating, the higher the propeller efficiency can be obtained. The strength of the blade prevents the infinite reduction of the coating. Multi-blade propellers are not profitable, as they reduce efficiency.

Fig.5. Aircraft TU-95 with coaxial propeller.

The first propellers had a fixed pitch in 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 (VSP) are used. In such propellers, the blades rotate in the sleeve relative to the longitudinal axis by a mechanical, hydraulic or electric mechanism.

To increase thrust and efficiency at low translational speed and high power, the propeller is placed in a profiled ring, in which the jet velocity in the plane of rotation is greater than that of an isolated propeller, and the ring itself creates additional thrust due to the speed circulation.

Propeller blades are made of wood, 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, the efficiency drops. Therefore, the propeller is advantageous at subsonic aircraft flight speeds.

The idea of ​​a propeller was proposed in 1475 by Leonardo da Vinci (Figure 1), and it was first used to create thrust in 1754 by M.V. Lomonosov in the model of the device for meteorological research (Figure 2). By the middle of the 19th century, propellers similar to propellers were used on steamships. In the 20th century, propellers began to be used on airships, airplanes, snowmobiles, helicopters, hovercraft, etc.

Rice. 1. Helicopter. Idea proposed by Leonardo da Vinci. Model designed by Leonardo da Vinci.

Fig.2. Instrument model M.V. Lomonosov for meteorological research.

The methods of aerodynamic calculation and design of propellers are based on theoretical and experimental studies. 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 screw and other bladed devices and establishes a mathematical relationship between forces, speeds and geometric parameters in such machines. In the further development of this theory, a significant role belongs to V.P. Vetchinkin. In 1956, the Soviet scientist G.I. Maikoparov extended the vortex theory of the propeller to the main rotor of a helicopter.

NOT. Zhukovsky

At present, the creation of large-sized long-haul aircraft required propulsion systems of greater power and very economical ones. One of the options for such engines are turbofan engines. They have great traction and good economy. Such engines are installed on all foreign aircraft.

The development of the idea of ​​Leonardo da Vinci 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 during their movement. 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 a combustible fuel. After that, 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 fixed straightener, which consists of the same blades, only installed at a certain angle to the swirling air flow. The twist occurs due to the movement of the compressor blades in a circle. The number of compressor stages can be more than 15.

If all the energy received as a result of the burnt fuel is used in the turbine, then an excess of power will be obtained on the engine shaft, which can be used to drive the propeller. You will get a turboprop engine, and the thrust will be created by a propeller. Thrust due to exhaust gases will be minimal.

The next stage of development was dual-circuit engines. In these engines, part of the air does not pass through the compressor (outside), usually after the first two stages of the compressor. Such an engine is called a turbofan. Engine thrust is generated by the fan (the first two stages of the compressor) and the exhaust jet. In this case, the fan, which is essentially a propeller, is located in a profiled housing.

The next stage of development is the turbopropfan 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 turbopropfan engine in the future is the most promising engine for aircraft flying at subsonic speeds.

Double-circuit turbojet engine.

In 1985, the Design Bureau 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 high-pressure engine with a single-stage fan.

In 1990, the design bureau began designing such an engine, which received the designation NK-93. It was intended primarily for the IL-96M, Tu-204P, Tu-214 aircraft, but the Ministry of Defense also showed interest in the new engine (it is planned to be installed on the military transport Tu-330).

IL-76 LL aircraft with NK-93 engine.

Engine NK-93.

NK-93 is made according to a three-shaft scheme with the engine of a smoked 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 (it accounts 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.

Scheme of the NK-93 engine.

The technical characteristics of the new engine have no analogues in the world. According to the parameters of the thermodynamic cycle, the NK-93 is close to the 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 unit is the planetary gearbox and propfan. The angle of installation of the blades can vary within 110 0 when the engine is running. A similar gearbox is used in the NK-12 engines on the Tu-95 aircraft and a similar gearbox is used in gas pumping units on main gas pipelines (NK-38). So we have experience.

During the classes in the aircraft modeling laboratory of the Kostroma Regional Center for Children's (Youth) Technical Creativity, questions of the theory of aircraft flight and flying models are considered. In order to improve the flight characteristics of rubber-engine models, as well as to improve the results of performance in competitions, the work of a propeller-driven installation was considered. Having considered the characteristics of the rubber motor, the energy of which determines the take-off height of the model, it was found that the rubber torque on the propeller shaft has a non-linear characteristic. The maximum torque exceeds the average torque by 5-6 times. The torque required to turn 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 the screw to be sufficiently high, it is necessary to increase the diameter of the screw indefinitely. 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-engine model. It was decided to compensate for the change in torque by changing the diameter of the propeller. Structurally, it is rather difficult to change the screw diameter by an amount proportional to the change in torque, therefore, a change in the screw pitch has also been introduced. The result was a screw of variable diameter and pitch (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, commensurate with 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 there are other working conditions. 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 a variable diameter and pitch, difficulties arise with the counterweight of the propeller blade. Therefore, it was decided to use a two-bladed propeller with a variable diameter and pitch (VDPh) to increase the efficiency of the propeller of a rubber-engine aircraft model.

2. Technical requirements for the class aircraft modelF1 B

A rubber-engine model of the aircraft according to the FAI classification - F1B, made by Nikita Nadezhin under the guidance of Viktor Borisovich Smirnov, was presented for the competition.

With this model, Nadezhin Nikita became the champion in 2013 at the Russian Championship in aviation modeling.

A rubber engine 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 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 participant of the competition has the right to 7 test flights lasting no more than 3 minutes each. The launch of the model must be made at a limited time, announced in advance. The sum of the time of all official 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 at a distance of up to 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 with a directional antenna to detect the model.

The takeoff of the model is carried out due to the energy of the rubber motor, which drives 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 takeoff of the model, the propeller operates in off-design modes, i.e. the propeller is slipping in the air stream. In order to aerodynamically load the propeller and use the available energy of the rubber motor in full, it is necessary to increase the diameter of the propeller and the angle of installation of the propeller blades in the initial period of takeoff. This is well shown in the book by A.A. Bolonkin "Theory of Flight of Flying Models"

3. Description of the design of the propeller

A feature of this model is a propeller (Appendices No. 4,5,6), which changes the diameter and pitch during the 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 the blades. This allows you to significantly increase the efficiency of the propeller and, consequently, the take-off height of the model, and, accordingly, the duration of the flight and the result in competitions increase.

The design of the propeller mechanism is shown on the assembly drawing 10.1000.5200.00 SB VIDSH (variable diameter and pitch propeller, Appendix No. 3) and is a housing in which the propeller shaft made of ZOHGSA steel rotates on 2 bearings. A screw hub is installed on the shaft, also on 2 bearings, then there is a sleeve that can rotate around the shaft. Connecting rods are installed on the bushing, on which propeller blades made of balsa are suspended. The connecting rods are mounted on axes located at a radius R=11 from the shaft axis and at an angle of about 6 degrees to it. The bushing and the hub are interconnected by an elastic element (rubber ring). The hub has a groove that limits the movement of the bushing relative to the hub. This determines the operating angles of rotation of the sleeve 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 sleeve relative to the hub, while the connecting rods extend from 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 a change in the angle of inclination of the axes of the connecting rods, which allows you to adjust the range of pitch changes when adjusting the model. (in the original version, the 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 sleeve, which locks the propeller blades in the desired position after the rubber motor is untwisted. The change in pitch with an increase in diameter by 25 mm is 5 0, which changes the pitch from 670 mm to 815 mm on R blades = 200 mm. For the manufacture of parts, small-sized ball bearings and high-strength materials D16T, ZOHGSA, 65S2VA, 12x18N10T and carbon fiber were used.

4. Description of the aircraft model

The design of the model itself is shown in drawing 10.0000.5000.00SB. (Appendix No. 1.7)

The longitudinal set of the wing consists of two carbon fiber spars of variable section, a carbon fiber box, leading and trailing edges of carbon fiber.

The transverse set consists of ribs made of balsa, covered on top and bottom with carbon fiber plates 0.2 mm thick. The profile "Andryukov" is used on the wing. The center of gravity is located at 54% of the MAR.

The whole set is assembled on epoxy resin. The wing is covered with synthetic paper (polyester) on enamel. For ease of transportation, the wing has a transverse connector with attachment points. The stabilizer and keel are made similar 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 software mechanism (timer) and a transmitter for model search are installed, power frames made of D16T aluminum alloy are glued in front and behind.

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 section there is a platform for mounting 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 section of 1/8 //

The use in this class of models of a mechanism that allows you to simultaneously change 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 the addition of the take-off height of the model by 10-12 meters, the flight duration increased by 35-40 seconds compared with other models, as well as improved flight stability. And as a result - victory in the competition.

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 rotational movement of the aileron. It is rather 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 removal of the aileron from the rocket body.

Thus, using the example of designing the simplest mechanism to increase efficiency, we can consider the creation of 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. .

References, 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 aircraft, ed. DOSAAF 1973

3. Schmitz F.V. Aerodynamics of low speeds, ed. DOSAAF 1961

4. The design was made in the Compass V-11 program

Annex 1.

Appendix 2

Appendix 3

PROPELLER THEORY

Introduction

The propeller converts the rotational power of the engine into forward thrust. The propeller pushes the air mass back, creating a reactive force that pushes the aircraft forward. The thrust of the screw is equal to the product of the mass of air and the acceleration given to it by the screw.

Definitions

propeller blade It is a load-bearing surface similar to an airplane wing. Definitions such as chord, profile curvature, relative profile thickness, relative elongation are similar to the definitions for an aircraft wing.

The angle of installation of the propeller blades ( blade angle or pitch )

This is the angle between the blade chord and the plane of rotation. The installation angle decreases from the root of the blade to the tip, because the circumferential velocity of the blade section increases from the butt to the tip. The angle of installation of the blade is measured in a section located at 75% of its length, counting from the butt.

Screw pitch ( geometric pitch )

This is the distance that the propeller would travel in one complete revolution if it were moving through the air at the angle of the blades. (You can imagine the pitch of a screw as the movement of a bolt twisting along a thread, but we will not use this analogy further)

Blade geometric twist ( blade twist )

The sections of the blade, located closer to its tip, cover a greater distance in one revolution. In order for the pitch of the screw to be the same for all sections of the blade, the angle of installation of the sections gradually decreases from the butt to the tip.

The angle of installation of the blades on many propellers can vary. When the angle of the blades is small, they say that the propeller is in fine pitch mode, and when, on the contrary, it is in large pitch mode (coarse pitch).

tread screws (effective pitch or advance per revolution)

In flight, the propeller does not cover a distance equal to the pitch of the propeller in one revolution. The actual distance traveled by the propeller depends on the speed of the aircraft and is called the propeller pitch.

Screw slip ( slip )

The difference between pitch and lead of a screw is called the slip of the screw.

Helix angle ( helix angle )

This is the angle between the actual trajectory of the propeller section and the plane of rotation.

Angle of attack(α)

The trajectory of the movement of the section of the blade in the air determines the direction of the oncoming air flow. The angle between the chord of the blade section and the direction of the oncoming flow is the angle of attack of the blade section. The angle of attack is affected by the peripheral speed of the section (rotor speed) and the true speed of the aircraft.

Fixed pitch propeller ( fixed pitch propeller )

The figures show the operation of a fixed pitch propeller under changing flight conditions. An increase in the true speed of the aircraft at a constant propeller speed (peripheral sectional speed) reduces the angle of attack of the propeller. Increasing the propeller speed at a constant true airspeed increases the angle of attack of the propeller.

Aerodynamic forces arising on the propeller

A propeller blade is a load-bearing surface similar to an airplane wing. When it moves through the air at a certain angle of attack, then aerodynamic forces are created on it in the same way as on a wing. Between the surfaces of the blade there is a pressure difference. The surface of the blade where more pressure is created is called the working surface of the blade (pressure face or thrust face). When the propeller creates direct thrust, the back (flat) surface of the blade is the working one. The pressure difference creates a total aerodynamic force, which can be decomposed into two components, thrust and rotational resistance.

Propeller thrust

thrust is the component of the total aerodynamic force perpendicular to the plane of rotation. The thrust force is unevenly generated along the length of the blade. It is minimal at the tip of the blade, where the pressure drop between the surfaces disappears, and also decreases in the butt due to the low circumferential velocity. The thrust creates a bending moment on each blade, trying to bend them with their tips forward. (A force equal and opposite in direction to the propeller thrust pushes the air back.)

Torque of resistance to rotation

The force of resistance to rotation of the propeller on the shoulder from the axis of rotation to the point of application of the full aerodynamic force creates a moment of resistance to rotation. A moment equal in magnitude and opposite in direction acts on the aircraft, trying to rotate it about the longitudinal axis. Also, the moment of resistance to rotation creates bending moments on the propeller blades, trying to bend them against the direction of rotation.

The centrifugal twisting moment of the blade ( centrifugal twisting moment )

The lateral components of the centrifugal forces "A" and "B" create a moment relative to the axis of change in the angle of the blade, trying to reduce the pitch of the propeller.

Aerodynamic twisting moment of the blade ( aerodynamic twisting moment )

Since the center of pressure is located ahead of the axis of change in the angle of installation of the blade, the total aerodynamic force creates a moment tending to increase the pitch of the propeller.

The aerodynamic moment counteracts the centrifugal twisting moment, but is weaker than it.

propeller efficiency

The efficiency of the propeller is determined by the ratio of the traction power and the power supplied to the propeller from the engine. The thrust power of the propeller is determined by the product of the propeller thrust by the true speed of the aircraft, and the engine power is determined by the product of the engine torque by the angular velocity of the propeller.

propeller efficiency = propulsion power / engine power

Dependence of propeller efficiency on flight speed

It was shown above that as the flight speed increases, the angle of attack of the fixed-pitch propeller blades decreases. This leads to a decrease in propeller thrust. At some speed, this angle will decrease so much that the propeller thrust will decrease to zero. This means that the efficiency of the screw will also become zero.

For a fixed pitch propeller, there is only one speed at which the blades will flow around at the most favorable angle of attack and the efficiency of the propeller will be maximum. (at constant angular velocity of rotation)

With a further decrease in the speed of the aircraft, the angle of attack of the blades increases. The thrust of the propeller increases, but the product of thrust and speed (traction power) begins to fall. At zero speed, the thrust of the propeller is maximum, but the propeller does not produce useful work, so its efficiency is again equal to zero.

The efficiency of a fixed pitch propeller varies greatly with airspeed.

As can be seen from the figure, using a variable pitch propeller (blade angle), it is possible to achieve its efficient operation in a wide range of flight speeds.

Fixed-pitch propeller with the ability to change the angle of the blades in the hub when servicing on the ground.

A propeller with a choice of three fixed blade angles in flight. The small pitch propeller is set for takeoff, climb and landing. During cruising flight, the propeller is set to the high pitch position. In case of engine failure, the screw is set to the vane position.

Variable pitch propeller (constant speed propellers).

On modern aircraft, propellers are installed that automatically maintain a given speed by changing the angle of the blades. This allows you to maintain high efficiency over a wide range of speeds, improve takeoff and climb performance and ensure fuel economy in cruise flight.

variable pitch propeller

The figure shows a typical propeller and engine control panel on small piston aircraft. All levers are in the takeoff position (far forward).

The propeller speed control is set to maximum speed.

Moving the middle lever back will decrease propeller speed.

Note: An analogy can be drawn between a propeller speed control lever and a gear lever in a car.

The maximum propeller speed is first gear in the car.

The minimum propeller speed is fifth gear in the car.

The figure shows the operating conditions of the propeller at the beginning of the runway runway. The propeller revolutions are maximum, the translational speed is low. The angle of attack of the blades is optimal, the propeller works with maximum efficiency. As the speed increases, the angle of attack of the blades will decrease. This will lead to a decrease in thrust and resistance to rotation. At constant engine power, engine speed will increase. The propeller speed control will begin to increase the pitch of the propeller blades to prevent the propeller speed from increasing. Thus, the angle of attack of the blades will be kept at optimal values ​​all the time.

The figure shows the operating conditions of the propeller when flying at high speed. As true airspeed increases, the propeller speed control constantly increases the pitch of the blades, maintaining a constant angle of attack.

The figure shows the operation of the propeller in cruise flight. Optimum power and propeller speeds are specified in the flight manual. It is generally recommended to first reduce the engine power and then reduce the propeller speed.

Throughout the flight, the constant speed controller controls the pitch of the propeller blades to maintain the desired speed. At least it tries to achieve it.

If the torque from the engine disappears (idle mode or failure), then the regulator, trying to maintain speed, reduces the angle of the blades to a minimum. The angle of attack of the blades becomes negative. Now the total aerodynamic force on the propeller is directed in the opposite direction. It can be decomposed into the negative thrust of the propeller and the force tending to spin the propeller. The propeller will now turn the engine.

On a twin-engine aircraft, if one engine fails, if the propeller of the failed engine autorotates, then the climb characteristics and flight range deteriorate very much and the control of the aircraft becomes difficult due to the additional turning moment. Also, the rotation of a failed engine can lead to its jamming or fire.

feathering

When the propeller blades turn to an angle of attack of zero lift, the force that rotates the propeller disappears and the propeller stops. The drag (negative thrust) of the propeller is reduced to a minimum. This greatly improves climb performance (in case of failure of one of the two engines), since the climb gradient depends on the difference between the thrust of the engines and drag.

Also, feathering the propeller blades reduces the turning moment from the failed engine. This improves the controllability of the aircraft and lowers the minimum evolutionary speed in the event of a V MC engine failure.

On single-engine aircraft, propeller feathering is not provided. However, in the event of an engine failure, it is possible to significantly reduce the negative thrust of the propeller. To do this, the screw speed controller is set to the minimum speed. In this case, the screw will be set to the maximum pitch position.

This allows you to increase the lift-to-drag ratio of the aircraft, which will reduce the altitude loss gradient in gliding with a failed engine. The engine speed will also decrease due to a decrease in the force tending to spin the screw.

If you turn the propeller speed control to increase the speed of rotation, then the effect will be the opposite.

Power take-off from engine to propeller

The propeller must be able to absorb the full power of the engine.

It must also operate at maximum efficiency over the entire operational range of the aircraft. The critical factor is the speed of flow around the tips of the blades. If it approaches the speed of sound, then the phenomena associated with the compressibility of air lead to a decrease in thrust and an increase in the moment of resistance to rotation. This significantly reduces the efficiency of the propeller and increases its noise.

Limiting the speed of flow around the tips of the blades imposes restrictions on the diameter and angular speed of rotation of the propeller, as well as on the true flight speed.

The propeller diameter is also limited by the requirements of a minimum clearance to the surface of the airfield and the fuselage of the aircraft, as well as the need to install the engine as close to the fuselage as possible in order to reduce the turning moment in case of failure. If the engine is located far from the longitudinal axis of the aircraft, then it is necessary to increase the vertical tail to ensure the balance of the aircraft in case of engine failure at low speed. All of the above shows that it is impractical to ensure that the propeller consumes all the available engine power by simply increasing its diameter. Often this is achieved by increasing the fill factor of the propeller.

Propeller fill factor ( solidity )

This is the ratio of the frontal area of ​​all the blades to the area swept by the propeller.

Methods for increasing the fill factor of the propeller:

Increasing the chord of the blades. This leads to a decrease in the relative elongation of the blade, which leads to a decrease in efficiency.

Increasing the number of blades. The power take-off from the engine increases without increasing the speed of the flow around the tips and reducing the relative elongation of the blades. An increase in the number of blades over a certain amount (5 or 6) leads to a decrease in the efficiency of the propeller.

Prop thrust is created by throwing a mass of air back. If the fill factor of the propeller is excessively increased, then the mass of air that can be accelerated as it passes through the propeller will decrease. To effectively increase the number of blades, coaxial screws are used that rotate on the same axis in opposite directions.

Moments and forces generated by the propeller

The screw creates moments in all three axes of the aircraft. The reasons for these moments are different:

screw reaction heeling moment

gyroscopic moment

wake helical moment

moment due to asymmetrical flow around a propeller

Note: Most modern engines are equipped with clockwise rotating propellers (when viewed from the rear). On some twin-engine aircraft, a counter-clockwise rotating propeller is installed on the right engine to eliminate the disadvantages associated with the appearance of a critical engine (see chapter 12).

Heeling moment of propeller reaction

Since the propeller rotates clockwise, an equal and opposite torque acts on the aircraft.

When the aircraft is taking off, the left pneumatic will carry a greater load, which will create more rolling resistance. Therefore, the aircraft will tend to turn to the left. In flight, the aircraft will tend to roll to the left. This moment will be most noticeable at maximum propeller thrust and low flight speed (low efficiency of the rudders).

The heeling torque of the propeller reaction is practically absent for coaxial propellers rotating in opposite directions.

The original text says that twin-engine aircraft with co-rotating propellers have no heeling torque until one of the engines fails. This is not true. In theoretical mechanics it is said that the total moment acting on a rigid body is equal to the algebraic sum of the moments lying in the same plane. That is, the moment of reaction of the propellers will act on the aircraft, regardless of the number of operating engines, and if all the propellers rotate in the same direction, then the moments will add up.

Gyroscopic moment

A rotating propeller has the properties of a gyroscope - it seeks to maintain the position of the axis of rotation in space, and in the case of the application of an external force, a gyroscopic moment appears, tending to turn the axis of the gyroscope in a direction that differs by 90 ° from the direction of forced rotation.

It is convenient to determine the direction of action of the gyroscopic moment using the following mnemonic rule. Imagine yourself sitting in the cockpit of an airplane. The plane of rotation of the engine (propeller) will be depicted as a circle, and the direction of rotation - by arrows along the circle.

If one arrow is drawn from the center of the circle in the direction of movement of the aircraft nose, then the second arrow, directed tangentially to the circle in the direction of rotation of the engine (propeller), will show the direction of the additional (precessional) movement of the aircraft nose, caused by the action of the gyroscopic moment of the engine (propeller).

The gyroscopic moment appears only when the aircraft rotates in pitch and heading.

Coaxial propellers have no gyroscopic moment.

Wake helical moment

The propeller throws back a swirling jet of air, which, rotating around the fuselage, changes the flow around the keel. Since the screw rotates clockwise, the jet flows around the keel at an angle to the left, causing a lateral force on it to the right.

The helical moment from the propeller wake creates a yaw moment to the left. The amount of torque depends on the operating mode of the engine and the speed of the propeller.

You can reduce the helical moment with:

using coaxial screws

installation of a fixed compensator on the rudder

installing the engine with a small lapel of the propeller axis to the right

setting the keel at a slight angle to the left

Moment caused by asymmetrical flow around the propeller

In flight, the propeller axis is deflected from the direction of the oncoming flow by the angle of attack. This leads to the fact that the descending blade flows around at a greater angle of attack than the ascending one. The right side of the propeller will generate more thrust than the left side. Thus, a yaw moment to the left will be created.

This moment will have the greatest value at the maximum engine operation mode and the maximum angle of attack.

Influence of atmospheric conditions

Changes in atmospheric pressure and/or temperature result in a change in air density.

This affects:

engine power at constant throttle position

moment of resistance to rotation of the screw.

An increase in air density leads to an increase in both of these parameters, but the engine power increases to a greater extent.

Influence of air density on the operation of a fixed-pitch engine

An increase in density leads to an increase in propeller speed and vice versa.

Influence of air density on the moment of resistance to rotation (required motor torque) of a fixed pitch propeller

An increase in density leads to an increase in the moment of resistance to the rotation of the screw and vice versa.

This is a separate independent unit, or rather a whole bladed unit. It is the propeller for the apparatus on which it is installed, that is, it turns the engine power into traction and, ultimately, into movement.

The man has long been paying attention to the screw. The first theoretical evidence of this is still in the manuscripts and drawings of Leonardo da Vinci. And practically it was first used (for meteorological instruments) by M. V. Lomonosov. at first it was installed on airships, later and to this day on airplanes and when using engines. It is also used on ground vehicles. These are the so-called hovercraft, as well as snowmobiles and gliders. That is, its history (as well as the history of all aviation :-)) is long and fascinating, and it seems that it is far from over.

As for the theory and principle of action ... I wanted to start drawing vector diagrams, and then I changed my mind :-). Firstly, not that site, and, secondly, I have already described all this, and even :-). Let me just say that the propeller blades have an aerodynamic profile, and when it rotates in the air, the same picture arises as when the wing moves.

Aerodynamic force (picture from previous article :-))

All the same, the same bevel of the flow, only now the lifting force becomes the propeller thrust, forcing the aircraft to move forward.

There are, of course, and their own characteristics. After all (more precisely, its blades) in comparison with makes a more complex movement: rotational plus translational forward movement. And in fact, the theory of the propeller is quite complicated. However, for a fundamental understanding of the issue, all that has been said is quite enough. I will dwell only on some features. I note, by the way, that there are not only pulling propellers, but also pushing propellers (such, by the way, were on the Wright brothers' plane).

The propeller of the German airship SL1 (1911) with a diameter of 4.4 m.

Propeller for A400M transport aircraft.

Transport aircraft A400M.

When the propeller rotates and simultaneously moves forward, each of its points seems to move in a spiral, and the propeller itself seems to be “screwed into the air”, almost like a screw into a nut or a screw into a tree. The analogy is very significant :-). It looks like a thread of a bolt-nut pair. Each thread has a parameter such as a pitch. The larger the pitch, the more stretched the thread, and the bolt is screwed into the nut faster. The concept of pitch also exists for the propeller. In fact, this is such an imaginary distance that a propeller rotating in the air will move when it is turned one revolution. In order for it to “screw in” faster, it is necessary that the force pulling it (the thrust of the screw, the very analogue of the lifting force) be greater. Or all, respectively, vice versa. And this can be achieved by changing the value of the analogue of the angle of attack, which is called the angle of the propeller blade, or simply the pitch of the propeller. The concept of propeller pitch exists for all types of propellers, for airplanes and for helicopters, and the principle of their operation is, in general, the same.

Hercules C-4 transporter of the Krolev Air Force parked with propellers in vane mode.

The first propellers on airplanes had a fixed pitch. But the fact is that any screw has such a parameter as efficiency, which evaluates the efficiency of its work. And it can change depending on the change in flight speed, engine power, and the drag of the propeller affects this. Here, in order to maintain efficiency at a sufficient height, a pitch change system was invented (as early as the 30s of the 20th century) and propellers with variable pitch in flight (VISH) appeared. Now, depending on the flight mode set by the pilot, the propeller pitch can change. In addition, there are usually two more special modes. Reversible - to create when the aircraft is braking on the ground and vane, which is used when turning off (often emergency) the engine in flight. Then the blades are set "downstream" so as not to create unnecessary resistance to flight.

The propeller diameter and pitch are the main technical parameters of a propeller. There is also such a thing as twist. That is, each blade is slightly twisted along its entire length. This is done again so that at the same power the blade creates the most thrust.

American experimental aircraft Bell X-22 with impellers 1966

French experimental aircraft with NORD 500 CADET impellers. 1967

1932 Italy. Experimental aircraft with impeller "Flying Barrel"

Modern screws are generally quite diverse in their design. The number of blades can vary (on average from 2 to 8). can be both pulling and pushing. A screw is also called a propeller. This is an old name and comes from the Latin prōpellere, which means to drive, push forward. Now, however, another word has come into use. This is the word impeller. It means "impeller" and they called it a certain type of propeller enclosed in an annular shell. This allows you to increase the efficiency of its work, reduce losses and increase safety. However, such aircraft are only at the stage of experimental development.

The main speed range for the use of propellers is limited to speeds of 700-750 km / h. But even this is a fairly high speed, and various technical tricks are used to ensure stable and efficient operation throughout the entire range. In particular, multi-blade propellers with saber-shaped blades are being developed, work is underway on supersonic propellers, and the above-mentioned impellers are being used. In addition, the so-called coaxial screws have been used for a long time, when two propellers rotate in different directions on the same axis. An example of an aircraft with such propellers would be the fastest aircraft powered by turboprops, the Russian strategic bomber Tu-95. Its speed (max.) is 920 km/h.

Strategic bomber TU-95.

Unfortunately, , especially in combination with , still has a limited scope. Of course, where short-haul aircraft are so needed, the so-called he will still show himself. But nevertheless, he, together with his companion piston engine, has already lost the height-speed-range competition. But more on that in another post...

Photos are clickable.

The propeller is a unit designed to create a thrust force, which is a reaction rejected by the air flow propeller, creating a thrust force, the propeller converts the mechanical energy of the engine into work performed during the translational movement of the aircraft.

Requirements:

1. high efficiency;

2. automatic change in the angle of installation of the blades, depending on the flight mode and engine operation;

3. The range of blade angles should provide min. positive thrust at idle. The operation of the feathering screw in the negative thrust mode

4. the speed of rotation of the blades with an increase in the installation angle should be at least 10 s / s;

5. there must be automatic protective devices to prevent the occurrence of negative draft;

6. protection of the blades and fairing of the propeller hub (coca) from icing.

Screw classification. The angle of attack of the propeller blades depends on the flight speed at a non-low installation angle. This phenomenon occurs with fixed pitch propellers. The main disadvantage of such propellers is that they can be heavy during takeoff at low flight speed and the takeoff power of the engine is not provided. During level flight at high translational speed, the propeller turns out to be light and the rotation speed can increase to unacceptably high values, at which the reliability of the engine operation is not ensured. In the past, when flight speeds were low, these propellers were used. As the flight speed increased, variable pitch propellers began to be used - VISH (installation range 100) with a further increase in flight speed, i.e. with increasing angles j - installation, they began to use propellers with automatic rotation speed control systems, by changing j from the flight mode. Propellers with such control systems are called automatic propellers - AVISH.

aerodynamic forces.

The point of application of the resulting force is at the center of pressure

Aerodynamic forces appear as a result of the action of the air flow on the blades and distribution over the entire surface. Such a loading scheme of the blade can be considered as a beam, fixed at one end, and subjected to a distributed load, which creates bending and torsional moments. The center of pressure is in front of the plane of rotation. depends on the angles of attack of the blade and the resulting velocities of the oncoming flow. Due to the relatively small shoulders a and b, the magnitude of the moment of aerodynamic forces is small. At negative angles of attack of the blades, the direction changes so that the torques and tend to turn the blade in the direction of decreasing the installation angle.

Screw pitch and pitch. The geometric pitch of the screw H is the distance that the screw would move along the axis of rotation in one revolution when screwed into a nut specially made for it = r is the distance to the section under consideration. The screw is characterized by , R is the radius of the screw. From (1) it follows that the pitch of the screw is given by the rate of change of φ. Air (elastic and compressible) in one revolution the screw moves by an amount much less than H - propeller pitch , - flight speed m / s, n - rev / s.

When calculating, use the relative step , - , is dimensionless and is called the mode characteristic or propeller speed coefficient.

Screw modes

At a constant installation angle, the angle of attack of the blades depends on the magnitude of the flight speed. As the flight speed increases, the angle of attack decreases. In this case, they say, the propeller is “lightened”, since the moment of resistance to the rotation of the propeller decreases, and, consequently, the required engine power decreases. This causes an increase in rotation speed. When the flight speed drops, on the contrary, the angle of attack increases and the propeller becomes “heavier”, the rotation speed decreases.

With a large increase in flight speed or with a small installation angle, the angle of attack can become equal to zero or even negative. In the case of the blades, they meet the air flow not with the working (rear) part, but with the back (front part). In this case, thrust and power can become negative.

Thrust P and thrust coefficient are considered positive if the direction of thrust coincides with the direction of motion of the aircraft, in the opposite direction - negative. In this case, the screw creates resistance.

The propeller power T and the power factor are considered positive when the torque from the aerodynamic forces of the propeller is opposite to the direction of its rotation. If the torque of these forces supports the rotation of the screw, i.e. the rotational resistance force, the power of the screw is considered negative.

When changing and in a wide range, the relative step can vary from zero to infinitely large positive values ​​(when ).

Consider the most characteristic modes of operation of the propeller.

The mode in which the translational speed = 0, and therefore equal to zero, is called propeller operating mode - in place (fig. left). On the graph, this mode corresponds to the point A, where the thrust and power coefficients usually have maximum values. The angle of attack of the blades a when the screw is in place is approximately equal to the installation angle. Since , the screw does not produce any useful work when working in place.

The mode of operation of the screw, when positive thrust is created in the presence of translational speed, is called propeller mode (fig. right). It is the main and most important mode of operation, which is used during taxiing, takeoff, climb, level flight of the aircraft, and partly during gliding and landing. On the graph, this flight mode corresponds to section ab, excluding points a and b. As the relative step increases, the values ​​of the thrust and power coefficients decrease. In this case, the efficiency of the screw first increases, reaching a maximum at point b, and then rapidly decreases. Point b characterizes the optimal mode of operation of the propeller for a given value of the angle of installation of the blades. Thus, the propeller mode of operation of the propeller corresponds to positive values ​​of the coefficients , , .

The mode of operation in which the propeller does not create either positive or negative thrust (resistance) is called zero thrust mode. In this mode, the screw seems to be freely screwed into the air, without throwing it back and without creating thrust. The zero thrust mode on the graph corresponds to point c. Here the thrust coefficient and efficiency screws are zero. The power factor has some positive value. This means that in order to overcome the moment of resistance to rotation of the propeller in this mode, engine power is required.

Zero thrust mode can take place when planning an aircraft. The angle of attack of the blades in this case, as a rule, is somewhat less than zero.

The mode of operation of the screw, when a negative thrust (resistance) is created with positive power on the motor shaft, is commonly called braking mode , or the braking mode of the propeller. In this mode, the angle of inflow of jets is greater than the installation angle , i.e., the angle of attack of the blades is a negative value. In this case, the air flow exerts pressure on the back of the blade, which creates a negative thrust. On the graph, this mode of operation of the screw corresponds to the section between points b and d, where the coefficients and have negative values, and the values ​​of the coefficient change from some positive value to zero. Engine power, as in the previous case, is required to overcome the moment of resistance to rotation of the propeller.

Negative propeller thrust is used to shorten the landing run. To do this, the blades are specially transferred to the minimum installation angle, at which the angle of attack is negative during the run of the aircraft.

The mode of operation, when the power on the motor shaft is zero, and the propeller rotates due to the energy of the oncoming flow (under the action of aerodynamic forces applied to the blades), is called autorotation mode . At the same time, the engine develops the power necessary only to overcome the internal forces and friction moments formed during the rotation of the screw. On the graph, this mode corresponds to the point G. The propeller thrust, as in the braking mode, is negative.

The mode of operation, in which the power on the motor shaft is negative, and the screw rotates due to the energy of the oncoming flow, is called windmill mode . In this mode, the screw not only does not consume engine power, but itself rotates the engine shaft due to the energy of the oncoming flow. On the graph, this mode corresponds to the section to the right of the point G. Windmill mode is used to start a stopped engine in flight. In this case, the motor shaft spins up to the rotation speed necessary for starting, without requiring special starting devices.

The braking of the aircraft during the run also begins in the windmill mode and passes successively through the stages of autorotation and braking until the zero thrust mode.