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

In addition to the choice of geometric parameters of the propeller, the issue of coordinating the speed of the propeller and the engine, that is, the selection of a gearbox, deserves attention.

Operating principle of a propeller

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

In any section of the blade, the velocity component V will remain unchanged, and the peripheral speed will depend on the size of the radius at 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 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 amount 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 helical profile, are determined during the design process of the propeller and subsequently refined based on purging in wind tunnels. Such research is 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 is the diameter of the circle that the ends of its blade describe during rotation.

Blade width is the chord of the section at a given radius. Calculations usually use the relative width of the blade

Blade thickness at any radius the largest thickness of the section at that radius is called. 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 a given section with the plane of rotation of the propeller.

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

The pitch and angle of installation 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 rule, the characteristic angle of installation of the blade is taken to be the angle of installation of the section located at 0.75R from the axis of rotation of the propeller, denoted as .

Twisted blade is called the change in 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 represented graphically as a function relative to the current radius of the screw

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

If the screw, rotating with the number of revolutions, moves forward with the speed V then in one revolution it will travel the distance . This value is called the propeller advance, and its ratio to the diameter is called the relative propeller advance:

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

Power factor

And 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 propeller r/s

D- screw diameter, m

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

Theoretical propeller thrust limit

Of interest to the designer of the UAV is the ability 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 the propeller thrust is represented as a function of three parameters: engine power, propeller diameter and flight speed. Practice has shown that the thrust of rationally designed real propellers is only 15 - 25% lower than the maximum theoretical values.

The results of calculations based on the theory of an ideal propeller are shown in next schedule, which allows you to distribute 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 strongly depends on the diameter of the propeller, but already at speeds of about 100 km/h this dependence is less significant. In addition, the graph gives a clear idea of ​​the inevitability of a decrease in propeller thrust based on flight speed, which must be taken into account when assessing the flight data of the UAV.

based on materials:
"Guide for designers of amateur-built aircraft", Volume 1, SibNIIA

A bladed propeller of an aircraft, also known as a propeller or a blade machine, which is driven into rotation by the operation of the engine. With the help of a screw, torque from the engine is converted into thrust.

The propeller acts as a propulsion device in such aircraft as airplanes, gyroplanes, gyroplanes, snowmobiles, hovercraft, ekranoplanes, as well as helicopters with turboprop and piston engines. For each of these machines, the screw can perform different functions. In airplanes it is used as a main rotor, which creates thrust, and in helicopters it provides lifting and taxiing.

All aircraft propellers are divided into two main types: variable-pitch and fixed-pitch propellers. Depending on the design of the aircraft, the propellers can provide push or pull thrust.

As the propeller blades rotate, they capture air and throw it in the opposite direction of flight. A low pressure is created in the front of the propeller, and a high pressure area behind. The rejected air acquires a radial and circumferential direction, due to this, part of the energy that is supplied to the propeller is lost. The very swirl of the air flow reduces the streamlining of the device. Agricultural aircraft operating in fields have poor uniformity of chemical dispersion due to the flow from the propeller. A similar problem is solved in devices that have a coaxial screw arrangement; in this case, compensation occurs through the operation of the rear screw, which rotates in the opposite direction. Similar propellers are installed on aircraft such as the An-22, Tu-142 and Tu-95.

Technical parameters of blade propellers

The most significant characteristics of the propellers, on which the thrust force and the flight itself depend, are, of course, the pitch of the propeller and its diameter. Pitch is the distance a propeller can move when screwed into the air in one full revolution. Until the 1930s, propellers with a constant pitch of rotation were used. Only in the late 1930s were almost all aircraft equipped with propellers with variable pitch rotation

Screw parameters:

The diameter of the propeller circumference is the size that the tips of the blades describe as they rotate.

The propeller's gait is the actual distance covered by the propeller in one revolution. This characteristic depends on the speed and revolutions.

The geometric pitch of a propeller is the distance that the propeller could travel in a solid environment in one revolution. It differs from the movement of a propeller in the air by the sliding of the blades in the air.

The angle of location and installation of the propeller blades is the inclination of the blade section to the real plane of rotation. Due to the presence of twist of the blades, the angle of rotation is measured across the section, in most cases this is 2/3 of the entire length of the blade.

The propeller blades have a leading - cutting - and trailing edge. The cross-section of the blades has a wing-type profile. The profile of the blades has a chord, which has a relative curvature and thickness. To increase the strength of the propeller blades, a chord is used, which has a thickening towards the propeller root. The section chords are in different planes, since the blade is made twisted.

The propeller pitch is the main characteristic of the propeller; it primarily depends on the angle of the blades. Pitch is measured in units of distance traveled per revolution. The larger the pitch the propeller makes per revolution, the greater the volume discarded by the blade. In turn, an increase in pitch leads to additional loads on the power plant, and accordingly, the number of revolutions decreases. Modern aircraft have the ability to change the pitch of the blades without stopping the engine.

Advantages and disadvantages of propellers

The efficiency of propellers on modern aircraft reaches 86%, which makes them in demand in the aircraft industry. It should also be noted that turboprops are much more economical than jet aircraft. Still, screws have some limitations both in operation and in design.

One of these limitations is the “locking effect”, which occurs when the diameter of the screw increases or when the number of revolutions is added, and the thrust, in turn, remains at the same level. This is explained by the fact that areas with supersonic or transonic air flows appear on the propeller blades. It is this effect that does not allow aircraft with propellers to reach speeds higher than 700 km/h. At the moment, the fastest vehicle with propellers is the domestic model of the Tu-95 long-range bomber, which can reach a speed of 920 km/h.

Another disadvantage of propellers is their high noise level, which is regulated by global ICAO standards. The noise from the propellers does not meet noise standards.

Modern developments and the future of aircraft propellers

Technology and experience allow designers to overcome some noise problems and increase thrust beyond the limitations.

Thus, it was possible to avoid the locking effect due to the use of a powerful turboprop engine of the NK-12 type, which transmits power to two coaxial propellers. Their rotation in different directions made it possible to bypass locking and increase traction.

Thin saber-shaped blades are also used on the propellers, which have the ability to prolong the crisis. This allows you to achieve higher speeds. This type of propeller is installed on the An-70 type aircraft.

Currently, development is underway to create supersonic propellers. Despite the fact that the design has been going on for a very long time with considerable cash injections, it has not been possible to achieve a positive result. They have a very complex and precise shape, which greatly complicates the calculations of designers. Some off-the-shelf supersonic propellers have been shown to be very noisy.

Enclosing the propeller in a ring - an impeller - is a promising direction of development, since it reduces the tip flow around the blades and the noise level. This also improved safety. There are some aircraft with fans that have the same design as the impeller, but are additionally equipped with an air flow direction device. This significantly increases the efficiency of the propeller and engine.

PROPELLER THEORY

Introduction

The propeller converts the rotational power of the engine into forward thrust. The propeller throws back the air mass, which creates a reaction force that pushes the plane forward. The thrust of the propeller is equal to the product of the mass of air and the acceleration imparted to it by the propeller.

Definitions

Propeller blade- This 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 airplane wing.

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

This is the angle between the chord of the blade and the plane of rotation. The installation angle decreases from the root of the blade to the tip, because the peripheral speed of the blade section increases from the butt to the tip. The installation angle 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 full 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)

Geometric blade twist ( blade twist )

The sections of the blade located closer to its tip travel a longer distance in one revolution. To ensure that the propeller pitch is the same for all sections of the blade, the installation angle 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 blade angle is small, the propeller is said to be in fine pitch mode, and when, on the contrary, it is said to be in coarse pitch mode.

gait screw effective pitch or advance per revolution)

In flight, the propeller does not travel 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 propeller pitch.

Screw slip ( slip )

The difference between the pitch and the advance of the propeller is called the slip of the propeller.

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 blade section 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 circumferential velocity (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 when flight conditions change. An increase in the true speed of the aircraft at a constant speed of rotation of the propeller (peripheral speed of the section) reduces the angle of attack of the propeller. Increasing the propeller rotation speed at a constant true flight speed increases the angle of attack of the propeller.

Aerodynamic forces arising on a propeller

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

Propeller thrust

Traction is the component of the total aerodynamic force perpendicular to the plane of rotation. The traction force is created unevenly along the length of the blade. It is minimal at the tip of the blade, where the pressure difference between the surfaces disappears, and also decreases at the butt due to the low peripheral speed. The thrust creates a bending moment on each blade, tending to bend their tips forward. (A force equal and opposite in direction to the thrust of the propeller throws the air back.)

Moment 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 of equal magnitude and opposite direction acts on the plane, tending to rotate it relative to the longitudinal axis. Also, the moment of resistance to rotation creates bending moments on the propeller blades, tending to bend them against the direction of rotation.

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 blade installation angle, tending to reduce the propeller pitch.

Aerodynamic twisting moment of the blade ( aerodynamic twisting moment )

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

The aerodynamic moment counteracts the centrifugal torsional 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 propeller thrust power is determined by the product of the propeller thrust and the true speed of the aircraft, and the engine power is determined by the product of the engine torque and the angular speed of rotation 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 propeller 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)

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

The efficiency of a fixed pitch propeller varies greatly with changes in flight speed.

As can be seen from the figure, using a variable pitch propeller (the angle of installation of the blades), you can achieve it efficient work over a wide range of flight speeds.

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

A propeller with a choice of three fixed blade angles in flight. A small propeller pitch is set for takeoff, climb and landing. During cruising flight, the propeller is set to the high pitch position. If the engine fails, the propeller is set to the feathered position.

Variable pitch propeller (constant speed propellers).

Modern aircraft are equipped with propellers that automatically maintain a given rotation speed by changing the angle of the blades. This allows you to maintain high efficiency over a wide range of speeds, improve take-off and climb characteristics and ensure fuel economy in cruising flight.

Variable pitch propeller

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

The propeller speed control is set to maximum speed.

Moving the middle lever back will reduce the speed of the propeller.

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

The maximum propeller speed is the 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 take-off run on the runway. The propeller speed is maximum, the forward speed is low. The angle of attack of the blades is optimal, the propeller operates with maximum efficiency. As the speed increases, the angle of attack of the blades will decrease. This will reduce traction and rotational resistance. With constant engine power, the engine speed will begin to increase. The regulator for maintaining a constant speed of rotation of the propeller will begin to increase the angle of installation of the propeller blades in order to prevent an increase in the speed of the propeller. Thus, the angle of attack of the blades will always be kept at optimal values.

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

The figure shows the operation of the propeller in cruising flight. Optimal power and propeller speed settings are specified in the flight manual. It is usually recommended to first reduce engine power and then reduce propeller speed.

Throughout the flight, the constant speed controller controls the angle of the propeller blades to maintain the set speed. At least he's trying to achieve that.

If the torque from the engine disappears (idle throttle 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 screw and the force tending to unwind the screw. 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 are greatly deteriorated and control of the aircraft becomes difficult due to the additional turning torque. Also, rotation of a failed engine can lead to jamming or fire.

Feathering

When the propeller blades are rotated to an angle of attack of zero lift, the force rotating the propeller disappears and the propeller stops. The drag (negative thrust) of the propeller is reduced to a minimum. This significantly improves climb performance (if one of the two engines fails), since the climb gradient depends on the difference between the engines' thrust and drag.

Also, feathering the propeller blades reduces the turning torque from a failed engine. This improves the aircraft's controllability and reduces the minimum evolutionary speed in case of engine failure V MC.

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

This allows you to increase the aerodynamic quality of the aircraft, which will reduce the gradient of altitude loss during gliding with a failed engine. The engine speed will also decrease due to a decrease in the force tending to untwist the propeller.

If you turn the propeller speed controller to increase the rotation speed, the effect will be the opposite.

Power take-off from the engine to the propeller

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

It must also operate at maximum efficiency throughout the aircraft's entire operating range. The critical factor is the speed of flow around the blade tips. If it approaches the speed of sound, then phenomena associated with air compressibility 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 diameter of the propeller is also limited by the requirements for 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 torque in the event of its 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 balancing of the aircraft in the event 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. This is often achieved by increasing the fill factor of the propeller.

Propeller fill factor ( solidity )

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

Methods for increasing propeller fill factor:

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 flow speed around the tips and reducing the relative elongation of the blades. Increasing the number of blades beyond a certain number (5 or 6) leads to a decrease in the efficiency of the propeller.

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

Moments and forces created by a propeller

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

propeller heeling moment

gyroscopic moment

spiral moment from the wake

moment caused by asymmetrical flow around the propeller

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

Propeller heeling moment

Since the propeller rotates clockwise, a moment of equal magnitude and opposite direction acts on the airplane.

During the take-off run of the aircraft, the left tire will carry a greater load, which will create greater rolling resistance. Therefore, the plane will tend to turn left. In flight, the plane will tend to bank to the left. This moment will be most noticeable at maximum propeller thrust and low flight speed (low efficiency of the rudders).

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

The original text says that for twin-engine aircraft with propellers rotating in the same direction, there is no heeling moment of the propeller reaction until one of the engines fails. This is not true. In theoretical mechanics it is said that the total moment acting on solid, is equal to the algebraic sum of the moments lying in one plane. That is, the reaction moment of the propellers will act on the plane, 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 strives to maintain the position of the rotation axis in space, and if an external force is applied, a gyroscopic moment appears, tending to rotate the gyroscope axis 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) is represented by a circle, and the direction of rotation is represented by arrows around the circle.

If you draw one arrow from the center of the circle in the direction of movement of the nose of the aircraft, then the second arrow, directed tangentially to the circle in the direction of rotation of the engine (propeller), will show the direction of additional (precessional) movement of the nose of the aircraft 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.

Spiral moment from the wake

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

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

You can reduce the helical moment using:

using coaxial screws

installing a fixed compensator on the rudder

installing the engine with the propeller axis slightly turned 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 deviated from the direction of the oncoming flow by an angle of attack. This leads to the fact that the descending blade flows at a greater angle of attack than the ascending blade. The right side of the propeller will produce more thrust than the left side. This will create a yaw moment to the left.

This moment will be greatest at maximum engine operating mode and maximum angle of attack.

Influence of atmospheric conditions

Changes in atmospheric pressure and/or temperature cause changes 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 engine power increases to a greater extent.

The influence of air density on the operation of an engine with a fixed pitch propeller

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

The influence of air density on the moment of resistance to rotation (required engine torque) of a fixed pitch propeller

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

Operating principle of a propeller

The propeller creates thrust in the air, acting on it like a wing. An airplane wing typically moves translationally, while a propeller blade moves both translationally and rotationally. The propeller blade is an elongated rectangle in shape, one size of which is significantly smaller than the other, rotating at an angular velocity W about the axis x - x(Fig. 4.1), passing at one edge of this rectangle. The plane of a rectangle leaving some angle j with the plane of rotation, also moves translationally in the direction of the axis of rotation at a speed V. Cutting the blade with a radius cylinder r, whose axis coincides with the axis X; we get an elongated rectangle in cross-section. Since the width of the blade is usually small compared to its length, the section by the cylinder is replaced by a section close to them, but convenient for drawing, by a section of the tangent plane to the cylinder and perpendicular to the axis of the blade (Fig. 4.1).

Since the blade makes a complex movement - translational and rotational, it is necessary to add these two movements. Geometric sum peripheral rotation speed U = Wr, and forward speed (flight speed) V,(Fig. 4.2) gives the vector W(speed of air flow relative to the section profile). If we take another section by a plane tangent to a cylinder of smaller or larger radius, then the component velocity V remains the same, and the peripheral speed Wr will be less or more; the latter changes according to a linear law, becoming equal to zero on the screw axis.

Since the blade is taken flat, the angle j at all radii will be the same, and the angle β , called the angle of flow inflow to the section, will be different at different radii due to the variable circumferential rotation speed W r. Therefore, with decreasing radius r corner β increases and the angle a=φ-β decreases and may become zero or even negative.

Propellers are divided into fixed-pitch propellers (FPP) and variable-pitch propellers (VPP).

The propeller converts the torque of the turboprop or propeller into thrust. In this case, losses occur, estimated by the coefficient of efficiency (efficiency) of the propeller.

The fixed propeller is characterized by a constant blade installation angle. Structurally, this propeller has a sleeve in which the blades are rigidly attached, which transmit thrust to it, and it also receives torque from the engine shaft to the propeller.

The VIS consists of blades, a bushing with a mechanism for rotating the blades and devices that ensure its reliable operation. To control the propeller there is automatic and manual equipment.

The following requirements apply to propellers:

High efficiency;

For VIS - changing the angle of installation of the blades in a range that ensures easy engine starting; minimum positive propeller thrust in idle mode; maximum negative thrust during the run and minimum drag of the blades in the feathered position; automatic change in the angle of installation of the blades depending on the flight mode of the aircraft and engine operation with a rotation speed of at least 10 °/s;

Minimum values ​​of reactive and gyroscopic moments;

The design of the propeller and the speed controller must have automatic protective devices that limit the arbitrary transition of the propeller blades to small installation angles and prevent the occurrence of negative thrust in flight;

Protection of blades and propeller hub fairing from icing;

Sufficient strength with low weight, balance and minimal noise.

The main characteristics of the propeller are usually divided into geometric, kinematic and aerodynamic.

4.2. GEOMETRICAL CHARACTERISTICS OF THE SCREW

Geometric characteristics include: diameter D propeller, number of blades, blade shape in plan, thickness c, section chord b and installation angles of blade sections. Screw diameter (D=2R) determines the circle described by the ends of the blades when the propeller rotates relative to its axis (Fig. 4.3). Diameter is the most important characteristic screw, since it primarily determines its traction characteristics.

The diameter is selected from aerodynamic considerations and is consistent with the possibility of placing the propeller on the aircraft. The diameters of modern screws range from 3m to 6m.

Large screw diameters lead to low efficiency. due to the possibility of supersonic speeds appearing at the end sections of the blades, and also complicate the layout of the engine on the aircraft. Small diameters do not allow the specified engine torque to be converted into the required thrust.

If you cut the blade at a certain radius r cylindrical surface having a longitudinal axis coinciding with the axis of rotation of the propeller, then the cut imprint is called the cross-section of the blade. This section has a wing-shaped profile. The part of the blade located between two radii ( r And r+Δ r), is a blade element with area ΔS =bΔr. Here and below, instead of arc-shaped sections, flat ones are considered.

Ratio of current section radius r to the radius of the screw R called relative radius =r/R. The radius of the idle part of the blade occupied by the hub is denoted by r 0. And 0 = r 0 /R.

To convert engine torque into thrust with a minimum diameter, the propeller has several blades. Modern theater engines usually have four-bladed propellers. More blades reduce efficiency. On powerful theater engines, instead of increasing the number of blades, coaxial propellers are used, located one behind the other and rotating in opposite directions around one axis.

The characteristic dimensions of the blade section are the maximum width b and thickness- With blades, as well as their relative sizes

= And =

For modern screws m ax = 8...10% (Fig. 4.4).

Line 0V(see Fig. 4.3), passing through the middle of the sections of the blade, is called its axis. The type of blade axis (straight or curved) and the distribution of the blade width along this axis characterize the planform of the blade. Approaching m ax to the end of the blade increases the thrust of the propeller, but increases the bending moment due to the movement of the center of pressure towards the end of the blade.

The maximum thickness of the blade section decreases towards its end (at high flow velocities, a smaller relative profile thickness is required). For a comparative assessment of this thickness, consider its relative value on 0 =0.9 and denote 0,9 . For modern screws 0,9 =4…5% (Fig. 4.4).

4.3. KINEMATIC CHARACTERISTICS OF WINE

The plane perpendicular to the axis of rotation of the propeller and passing through any point of the blade is called the plane of rotation of the propeller. There are countless such parallel planes. Typically, the plane of rotation of the propeller is understood as a plane passing through the middle or end of the profile chord (Fig. 4.5).

The blade sections are inclined to the plane of rotation. Blade section angle φ measured between the plane of rotation of the propeller and the chord of the profile. Magnitude φ determines the pitch value for a given screw radius h as the distance a propeller would move in a rigid medium in one revolution

h=2r tanφ n s ,

Where ns- the number of revolutions of the screw per second.

When operating propellers, the pitch value is not measured, but the term “screw pitch” has become widespread.

Kinematic characteristics propeller are the circumferential, translational and resulting speeds of the blade cross-section, angles of attack and inflow of flow, speed coefficient. In flight, the cross-section of the propeller blade rotates at a peripheral speed U=ωr=2πл s r and moves forward at flight speed V. In addition to these basic

speeds, inductive suction and twisting speeds arise in the plane of rotation, which for simplicity are not considered here. In this case, the resulting speed W determined by the formula

Speed ​​direction W forms an angle of attack α with the profile chord, and with speed U jet inflow angle β. Then

φ=a+β,

β=arc tg =arc tg .

At constant values forward speed V and installation angle φ with increasing cross-sectional radius of the blade, angle β decreases, and angle a increases.

To ensure that each section of the blade is at the same best angle of attack a naive (at which the aerodynamic quality is maximum), it is necessary with a decrease in angle β reduce the installation angle φ . Therefore, the installation angles of the propeller blade in the root part (at the butt) are greatest, and decrease towards the end of the blade (Fig. 4.6). This distribution of blade cross-section angles is called geometric twist. The twist must provide the condition a=φ-β=const=a naive.

To determine the amount of twist of the blade, use the concept of relative twist of the blade section (Fig. 4.7), comparing the angle φ installation of any blade section with a section installation angle located at =0.75 and designated as φ 0.75: =φ - φ 0.75. The total twist of the blade is determined by the difference in installation angles at the beginning of the working part of the blade φ ro and at the end of the blade φ R. Since the blade installation angle changes along the radius of the propeller, it is measured at the nominal radius r nom. Meaning r nom usually taken equal to 1000 mm for screws with D<4 м и 1600 мм для винтов с D>4 m.

At constant values ​​of the angle of installation of the blade section ( β and circumferential flight blade U) the angle of attack changes depending on the flight speed. As speed increases V attack angle a decreases, and when decreasing V- increases. In order for the angle of attack to change when the flight speed changes a remained constant, it is necessary to change the blade installation angle (Fig. 4.8).

This is possible by rotating the blade in the propeller hub relative to the propeller's own axis. In the case of a fixed propeller, this is achieved by increasing the peripheral speed U(increasing propeller speed).

4.4. AERODYNAMIC CHARACTERISTICS OF THE PROPELLER

The aerodynamic characteristics of the propeller include thrust R, moment of resistance M and power N required to rotate the propeller, and efficiency η in

As mentioned above, the propeller blades, which are in rotational and translational motion, have different speeds of movement in relation to the oncoming air flow. Considering two sections of the blade (see Fig. 4.9) at radii r And r+Δ r and the part of the blade obtained between these sections is called element of the blade at radius r. The area of ​​this blade element will be dS=bdr.

In reverse motion, a flow flows onto the indicated element of the blade at a speed V parallel to the axis of the screw, and, secondly, the flow at a speed U in a direction perpendicular to the speed V, giving the resulting speed W- the speed of flow on the blade element. Angle between vector W and the chord of the section is the angle of attack of the section α .

Corner φ between the section chord and the vector U(or, which is also the plane of rotation of the propeller) is the installation angle of the blade section, and the angle β between velocity vectors U And W- inflow angle. Such a blade element can be considered as a wing and general aerodynamic formulas can be applied to it.

Lift force for blade element:

dY=C y d S ,(4.1)

and drag

dX=C x dS. (4.2)

As is known from aerodynamics, the drag coefficient C x depends on the relative wing span. What relative scale should we take in this case? At first glance, it seems that an infinite scope should be assumed; but, as is known from aerodynamics, such a wing will not have induced drag. Therefore, it will not cause induced velocities, which is the opposite of what an ideal propeller's jet should have. Thus, if we take the element of the blade to be a wing of infinite span, then we should find the speed caused by the propeller in some other way, and then the triangle of speeds in the section of the blade should be taken as shown in Fig. 4.5. In order to be able to use these formulas to determine the thrust and power of a blade element, one should take into account With y And C x for some fictitious relative span, and assume that the element works in isolation in the blade - without any influence of neighboring elements. Further, it should be assumed that the effect of the flow on such an element, despite the fact that it moves along a helical trajectory, is similar to the effect of the flow on a wing moving translationally. This last assumption is usually called the plane section hypothesis.

dY= С y b dr(4.3)

dX= C x b dr(4.4)

The absolute values ​​of the linear dimensions of the blade will be expressed in relative form:

b= D, r= And dr=d

Let's express W through U And β.

U=ώr=2πn s r= πn s(4.5)

W 2 = =(4.6)

Elementary lift values dY and resistance forces dX taking into account (4.6) will be expressed:

dY=Cy =C y (4.7)

dX=C x =C x (4.8)

Let us project the lift force and drag of the element into two mutually perpendicular directions - the direction parallel to the propeller axis, and the direction coinciding with the plane of rotation of the propeller (Fig. 4.10).

Projection dY gives thrust to the propeller axis dP blade element:

dP=dYcosβ-dXsinβ= ()(4.9)

Projection dX on the plane of rotation of the screw gives the force of resistance to the rotation of this element:

dT=dYsinβ+dXcosβ= () (4.10)

Moment of resistance to rotation dM blade element:

dM=dT r=dT = ( ) . (4.11)

Required rotation power dN blade element:

dN=dM ω= dM 2πn s = ( ) (4.12)

General traction R and power N for screw with i blades will be expressed by the corresponding integral dependencies of expressions (4.9) and (4.12):

P= ( ) . (4.13)

N= () . (4.14)

In formulas (4.13) and (4.14), the integrands are variable functions, depending on the geometric and aerodynamic characteristics of the propeller blade, and designating them accordingly S R– thrust coefficient and With N– power factor, we obtain the final expression for thrust and power:

P= C P ρn 2 D 4 ,(4.15)

N= C N ρn 3 D 5 ,(4.16)

Propeller efficiency η in can be written as:

η in = = = = λ= π (4.17)

Relative speed is the ratio of the oncoming flow speed to the peripheral speed at the tip of the blade:

Rice. 4.11a. Aerodynamic characteristics of the propeller

Here the relationship is called the gait of the screw (the forward movement of the screw in a flexible medium), and =λ is the relative gait, then: λ=π.

When selecting a propeller and during aerodynamic calculations of an aircraft, the power transmitted by the engine to the propeller is specified, and only knowledge of the efficiency of the propeller is required; propeller thrust is usually not used in aerodynamic calculations. It is convenient to combine the curves C N and η so that the corresponding values ​​are plotted on the curves C N – η, then we get the diagram shown in Fig. 4.11a.

On it, λ is plotted along the abscissa axis, and C N along the ordinate axis; curves C N are located according to the screw installation angle parameter φ; On the C N curves there are points corresponding to the efficiency of the screw, when connected, curves of the same efficiency are formed. As can be seen, the curves of identical efficiencies are closed and intersected by the corresponding curves C N twice. The kernel of these closed curves corresponds to highest value Efficiency This diagram is called the aerodynamic characteristic of the propeller. The diagram should indicate the test conditions, i.e., the type of screw device, the diameter of the screw tested, the type of screw or its geometric characteristic, the shapes and dimensions of the body behind the screw, the flow rate and the number of revolutions during testing. The diagram shown in Fig. 197, is the main one for selecting screws.

4.5. OPERATING MODES

Rice. 4.12. Screw operation in place

At a constant blade angle j her angle of attack α depends on the flight speed (see Fig. 4.10). As flight speed increases, the angle of attack decreases. In this case, they say that the screw “lightens”, since the moment of resistance to rotation of the screw decreases, which causes an increase in its rotation frequency. When the flight speed decreases, on the contrary, the angle of attack increases and the propeller becomes “heavier”, its rotation frequency decreases.

Screw power N and power factor C N 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 is directed in the direction of rotation of the screw, i.e. the rotation resistance force T<0, мощность винта считается отрицательной.

The most typical operating modes of the propeller are discussed below.

The mode in which the forward speed V=0, hence, λ And h in are equal to zero, called mode work the screw into place(Fig. 4.12). In Fig. 4.11 this mode corresponds to the point A, where are the thrust coefficients Wed and power C N usually have maximum values. Blade angle of attack ά when the screw is in place, approximately equal to the installation angle φ. Because h in =o, then the screw does not produce any useful work when working in place.

The mode of operation of the propeller, when positive thrust is created in the presence of forward speed, is called propeller mode(Fig. 4.13). It is the main and most important operating mode, which is used during taxiing, takeoff, climb, horizontal flight of the aircraft and partially during descent and landing. In Fig. 4.11 this flight mode corresponds to the section ab. As the relative feed λ increases, the values ​​of the thrust and power coefficients decrease. The efficiency of the screw first increases, reaching a maximum at a certain point b, and then falls.

Dot b characterizes the optimal operating mode of the propeller for a given blade angle j. Thus, the propeller mode of operation of the propeller corresponds to positive values ​​of the coefficients With P, C N And h c. Such flight conditions typically occur when the aircraft is descending. In power plants with fixed pitch propellers, the propeller can spin up.

Fig.4.15. Propeller operation in braking mode

The operating mode in which the propeller does not create either positive or negative thrust (resistance) is called zero thrust mode. In this mode, the propeller seems to be freely screwed into the air, without throwing it back and without creating thrust (Fig. 4.14). Zero thrust mode in Fig. 4.11 corresponds to point V. Resultant force dR appears in the third quadrant. Here the thrust coefficient S p and propeller efficiency h in are equal to zero. Power factor C N has some positive value corresponding to the energy expenditure to overcome the rotation of the screw. In this case, the angle of attack of the blades is, as a rule, somewhat less than zero.

The mode of operation of the propeller, when negative thrust (resistance) is created with positive power on the engine shaft, is called braking mode, or the braking mode of the propeller (Fig. 4.15). In this mode, the angle of inflow of jets β greater installation angle φ , i.e. blade angle of attack α- the value is negative. In this case, the air flow puts pressure on the back of the blade, which creates negative thrust, because resultant force dR ends up in the third quadrant. In Fig. 4.11 this mode of operation of the screw corresponds to the area enclosed between the points V And G, on which the coefficients Wed And η in have negative values, and the coefficient values With N vary from some positive value to - zero.

Fig. 4.16 Propeller operation in autorotation mode

As in the previous case, to overcome the moment of resistance to rotation of the propeller, a certain engine power is required. Negative propeller thrust is used to reduce the length of the post-landing run. For this purpose, the blades are specially moved to the minimum installation angle φmin, at which during the aircraft's run the angle of attack α negative.

The operating mode when the power on the engine shaft is zero and the propeller rotates due to the energy of the oncoming flow (under the influence of aerodynamic forces applied to the blades) is called autorotation mode(Fig. 4.16). The engine develops power N, necessary only to overcome the internal forces and moments of resistance generated during rotation of the screw.

Resultant force dR= - dP oriented strictly along the axis of rotation of the propeller and directed against the flight of the aircraft. In Fig. 4.11 this mode corresponds to the point G. The propeller thrust, as in the braking mode, is negative.

Rice. 4.17. Propeller operation in windmill mode

The operating mode in which the power on the engine shaft is negative and the propeller rotates due to the energy of the oncoming flow is called windmill mode(Fig. 4.17). In this mode, the propeller not only does not consume engine power, but itself rotates the engine shaft due to the energy of the oncoming flow. In Fig. 4.11 this mode corresponds to the area to the right of the point G and then, considering the propeller as a source of energy, h in> 0

The windmill mode is used to start a stopped engine in flight. In this case, the motor shaft spins up to the rotation speed required for starting, without requiring special starting devices.

Braking of the aircraft during the run is carried out by moving the propeller blades to the minimum installation angle and begins in windmill mode, successively passing through the stages of autorotation, braking, and zero thrust mode. As the travel speed decreases, the propeller begins to operate in minimum thrust mode

4.6. CLASSIFICATION OF VARIABLE PITCH PROPELLERS

It was previously shown that the angle of attack of the blades at a constant installation angle φ depends on the flight speed. In a fixed propeller at low flight speeds (take-off), the angles of attack of the blade sections are close to the installation angles of the blades, which causes “heaviness” of the propeller. In this case, the engine power is insufficient to spin the propeller to takeoff (maximum) speed. In horizontal flight at high forward speed, the angle of attack of the blades can significantly decrease, which will create excess engine power (compared to the propeller), which will lead to an increase in speed to unacceptably high values, at which the reliability of engine operation is not ensured.

In the past, when the speed range of aircraft was limited, fixed pitch propellers were used. As aircraft improved and the range of flight speeds increased, the need for variable pitch propellers arose. The first VIS had a relatively small range of changes in blade angles, which usually did not exceed 10°. These were, as a rule, two-pitch propellers. Take-off and climb in this case were carried out at a small installation angle (small pitch), which made it possible to obtain the take-off speed of the engine rotor when operating in place. When switching to horizontal flight, the blades were transferred to a large pitch using special mechanisms.

With a further increase in the range of aircraft flight speeds and, consequently, with an increase in the range of changes in blade installation angles, propellers with automatic speed control systems began to be used by changing the installation angle depending on the flight mode.

Depending on the source of energy for forced movement of the blades relative to their longitudinal axes, VIS are divided into:

Mechanical (energy is taken from the engine using a differential gear mechanism or from the effort of the pilot);

Electric, in which the movement of the blades is carried out using an electric motor located in the spinner of the propeller and connected to the butts of the blades by a bevel gear transmission;

Hydraulic, in which the power element is a hydraulic piston in the propeller spinner, the translational movement of which is converted using a crank mechanism into rotational movement of the blades.

The basis of VIS regulation is to maintain constant propeller (engine) speed, regardless of the developed engine power, by changing the angle of the blades using a centrifugal regulator.

When the engine deviates from the equilibrium mode towards greater developed power, an attempt to increase its speed is countered by setting the blades to a larger angle. In this case, the propeller rotation speed remains at the same level (within the tolerance limit) with a simultaneous increase in thrust. When the mode deviates downward, the regulation process goes in the opposite direction.

Propellers with such speed control systems are called automatic air propellers. Structurally, automatic propellers are very complex units, the successful operation and maintenance of which is possible only with an in-depth study of the principles of their operation and the rules of technical operation.

4.7. FORCES AND MOMENTS ACTING ON THE BLADES

Centrifugal forces of blades and their moments

On the cross section of an arbitrary radius of the blade, we select the end elementary masses. When the propeller rotates, these elements of the blade are acted upon by centrifugal forces directed radially from the axis of rotation and lying in the plane of rotation of these elements.

Vectors of centrifugal forces dP ts1 And dP c2 the extreme parts of the blade element (Fig. 4.18) are directed from the axis of rotation and perpendicular to it. They can be decomposed in the corresponding planes of rotation into axial and normal components dK 1 ,dK 2 And df 1, df 2. Last strength are also shown in the cross section of the blade.

The decomposition of the vectors of centrifugal forces for other similar parts of the section located between the leading and trailing edges within the same section of the blade gives a diagram of the transverse components of the centrifugal forces (Fig. 4.19). The transverse components of the centrifugal forces (Fig. 4.18) change their direction when passing through blade axis. Replacing forces in one direction with corresponding resultants dF 1 And dF2, we get the moment Mts from the transverse components of centrifugal forces, which tends to rotate the blade to reduce the installation angle.

In variable pitch propellers, the rotation of the blades to the required installation angle occurs relative to the axes coinciding with the axes of the butt (cylindrical) parts of the blades.

Magnitude of moment Mts, depends on the rotor speed, material, geometric dimensions, installation angles and blade twist.

Aerodynamic forces and their moments

Aerodynamic forces appear as a result of the action of air flow on the blade and are distributed over its entire surface. This blade loading scheme can be considered as a beam rigidly fixed at one end, subject to the action of a distributed aerodynamic load, which creates bending and torque moments.

The resultant of the aerodynamic forces of the blade element is applied at the center of pressure, which is usually located in front of the axis of rotation of the blade (see Fig. 4.5) and tends to rotate the latter in the direction of increasing the installation angle. The magnitude of the total moment of the aerodynamic forces of the blade for a given propeller depends on the angle of attack of the blade and the magnitude of the resulting oncoming flow velocity. The value of the moment of aerodynamic forces is small.

At negative angles of attack of the blades, the direction of the resultant force changes so that the torques of the aerodynamic forces in this case tend to turn the blades in the direction of decreasing the installation angle.

Centrifugal forces of counterweights and their moments

Typically, the amount of torque from aerodynamic forces is small, so it cannot be used as an independent source of energy to rotate the blades towards increasing the installation angle. In this regard, on some variable-pitch propellers, special counterweights (weights) are additionally installed, which are secured to the butt parts of the blades using brackets (Fig. 4.20).

When the screw rotates, centrifugal forces of counterweights arise R p, directed from the axis of rotation. Counterweights relative to the blades are placed in such a way that the components P n on the shoulder h created blade torque M c = P nf h, tending to rotate the blade towards increasing the installation angle. Counterweight torque value Mts depends on their mass, distance from the axis of rotation, arm h and propeller speed. All these parameters are chosen in such a way that the combined action of two torques from the centrifugal forces of the counterweight and aerodynamic forces ensures rotation of the blade towards increasing the installation angle with the required rotation intensity. Component R pk counterweight directed along the blade causes a bending moment, which is perceived by the counterweight bracket.

4.8. DIAGRAMS OF OPERATION OF PROPELLERS WITH HYDRAULIC BLADE ROTARY MECHANISMS

Currently, in propeller-driven aviation, hydraulic propellers are most widely used, in which the blade angles are changed under oil pressure. According to the principle of operation, they are divided into two-sided and one-sided screws. In hydraulic one-way screws, oil (from the engine cooling system) from a special pump under high pressure is supplied to one of the cavities of the hydraulic cylinder through the spool of the centrifugal regulator. The other cavity is permanently connected to the drain line, which serves as the engine power supply system ( R m)

Single-sided reverse action screw

The kinematic diagram of the propeller (see Fig. 4.21) is designed in such a way that the installation angle of the blades increases when piston 2 moves to the right, when the pressure in cavity A exceeds the pressure in cavity B. The installation angle decreases under the influence of the moment from the transverse components of the centrifugal forces of the blade M c/w by draining oil from cavity A of the hydraulic cylinder.

In general, the following moments act on the blade: M c/w– moment from the transverse components of centrifugal forces, aimed at reducing the angle of installation of the blade j; the moment from aerodynamic forces is directed counter to it M a/d and the moment acting in the same direction from the pressure in cavity A on the piston - M A.

In equilibrium mode, when the spring 7 balances the force from the centrifugal weights 6, the collar of the spool 5 covers the cavity A of the cylinder 1 and creates a hydraulic stop in it, which receives the force from M c\b and the blade is in a fixed position.

If the engine power increases (the fuel supply increases) while maintaining the same power consumption by the propeller, the engine speed will increase. This will cause an increase in the centrifugal forces of weights 6 and spool 5 will open access to oil into cavity A. In this case M A+M a\d > M c\b, which will cause the blade to move to a larger angle j. With an increase in power consumption by the propeller, its rotation frequency decreases to a given value and an equilibrium mode is established.

With a decrease in engine power (reduction in fuel supply), the process occurs in the reverse order. A special feature of such screws is their relative simplicity of design. The disadvantages include the possibility of unwinding the screw if the tightness of cavity A of the hydraulic cylinder is broken. Under the influence M c\b the blades can move to the minimum installation angle. For this purpose, it is necessary to provide special stops in the screw design to prevent the piston from moving when cavity A depressurizes.

Single sided screw direct action has a mechanism for rotating the blades with a one-way oil supply. In it, the force of oil pressure is used only to move the blades to reduce installation angles (Fig. 4.22).

To move the blades to increase installation angles, counterweights are used so that the moment from the transverse components of the centrifugal forces M g directed in the opposite direction M c/b. Thus, in the direction of decreasing the installation angle, the blades rotate when the following inequality is satisfied: M A + M c/b >M gr. + M a/d.

In this case, oil is supplied to the cavity A through the spool channel of the centrifugal regulator.

The blades rotate in the direction of increasing the installation angle provided: M gr. + M a/d > M A + M c/b what happens when oil is drained from the cavity A into the engine crankcase due to the upward movement of the spool due to the increased centrifugal forces of the regulator weights. The use of counterweights in the blade rotation mechanism is of great importance in ensuring flight safety when the pressure in the oil system decreases. In this case, the possibility of rotation of the propeller blades towards small installation angles is eliminated, and, consequently, the spinning of the propeller and the appearance of negative thrust. However, the presence of counterweights increases the weight of the propeller.

IN double-acting screws oil pressure is used to both increase and decrease the angle of installation of the blades (Fig. 4.23), depending on the position of spool 5, oil from the pump can enter both cavity A and cavity B of the cylinder. The piston is connected to the blade in such a way that when it moves forward, the blade will perform a rotational movement relative to its axis.

If oil from the pump flows into the cavity A, then from the cavity B it will merge. Then the moment ratio:

M A + M a/d >M B + M c/b,

Where M A - A.

In this case, the angle of installation of the blades will increase. When oil is supplied to cavity B from cavity A, the oil will drain and the angle of installation of the blades will decrease. The moment ratio in this case will be

M A + M a/d,< М Б + М ц/б ,

Where m B - moment created by the force of oil pressure in the cavity B.

From an examination of the operation of double-acting screws, it is clear that the moments created by the oil pressure force are controllable. They are determined by the position of spool 5 . Moments M a/d, And M c/w, permanently operating and cannot be controlled.

4.9. COMBINED OPERATION OF THE SCREW AND REGULATOR

On modern aircraft with turboprop engines, only automatic propellers are used, for which purpose, in the control systems discussed above, speed controllers with a centrifugal type sensor are installed (Fig. 4.21). The purpose of the regulators is to, working in conjunction with the VIS, automatically maintain a given engine rotor speed constant. It is set by the degree of compression of the regulator spring using the adjustment mechanism 7 .

Let's assume that the controller has already been set to a certain rotation speed. It is automatically maintained by a constant screw-regulator system as follows. During engine operation, two forces continuously act on the regulator spool 5: the elastic force of the spring 7, which tends to lower the spool down, and the centrifugal forces of the weights 6 , trying to lift the spool up. If the engine operates in steady state, when the rotation speed is maintained constant, spool 5 is in the neutral position (the channels for the passage of oil are blocked by the spool flanges), and equilibrium is established between the elastic force of the spring and the centrifugal forces of the weights. The engine rotor speed corresponding to this position is called equilibrium or specified. Obviously, the more the spring is compressed, the greater the centrifugal forces of the weights will be required, and, consequently, the greater the engine rotor speed to keep the spool in the neutral position and vice versa.

Let us now assume that the engine rotor speed has changed for some reason, for example, increased. Obviously, this is possible either by increasing the power developed by the engine, or by reducing the power absorbed by the propeller.

Let's consider the simplest case - increasing engine power by increasing the fuel supply (by moving the engine control lever (EC) forward). In this case, the equality of engine and propeller power is violated, as a result of which the engine rotor speed increases. The centrifugal speed controller reacts to this and must maintain it constant. As the rotation speed increases, the centrifugal forces of the weights increase 6 , which, overcoming the elastic force of the spring, lift the spool 5 upward. In this case, high pressure oil will flow into the cavity A, and from the cavity B it will drain into the engine.

By the moments of oil pressure and aerodynamic forces, the blades will rotate in the direction of increasing the installation angle, while overcoming the moment of the transverse components of the centrifugal forces of the blades. Thus, the screw will become “heavier”, its moment of resistance to rotation will increase, and, consequently, the power it consumes will increase. The process of tightening the screw will continue until the set rotation speed is restored, when, as the centrifugal forces of the weights decrease, the regulator spool will be returned by the spring to the neutral position and will block the oil channels.

When engine power decreases (due to reduced fuel supply), the opposite picture will be observed. The engine rotor speed will begin to decrease, causing the elastic force of the spring, overcoming the centrifugal forces of the weights, to lower the spool down. In this case, oil from the pump enters the cavity B, and from the cavity A it drains into the engine. The propeller blades under the action of the oil pressure moment (in the cavity B) and moments of transverse centrifugal forces, overcoming moments of aerodynamic forces, will turn in the direction of decreasing installation angles. In this case, the screw is made lighter, since the power it consumes is reduced. The process of lightening the screw will end when the set rotation speed is restored and the spool returns to the neutral position.

Throttle characteristics of the propeller.

The described process of regulating the rotation speed when changing the fuel supply is presented in graphs (Fig. 4.24), which show the dependences of the engine and propeller power on the rotation speed at different fuel consumptions.

Developed engine power N doors has (with a certain error) a power-law dependence on the rotation speed: N dv ~ n (2…3) While the power consumption of the screw N in has a higher dependence on its speed: N in ~ n 5. The initial operating mode of the power plant is the intersection point of the engine power curve corresponding to fuel consumption Q T 0, with the power curve of a propeller whose blades are set at an angle φ 0 . This steady state of operation of the power plant corresponds to the rotation speed p 0 . With an increase in fuel supply, the engine power characteristic will be located above the initial one (shown by the dotted line Q T 1>Q T 0) due to more high temperature gases in front of the turbine. As can be seen from the graph, the intersection of the propeller power curves at φ 0 and engine power at Q T 1>Q T 0 corresponds to a rotation speed that is greater p 0 . In this case, the centrifugal regulator, ensuring a constant rotation speed, will move the blades to a larger installation angle φ 1(dashed power curve, screw at φ 1>φ 0 ), which will cause a decrease in speed to previously set n 0.

Thus, with an increase in fuel supply, and, consequently, with an increase in engine power, the propeller will become heavier, i.e., the angle of installation of the blades increases and the thrust increases. When the fuel supply decreases, on the contrary, the regulator, maintaining a given rotation speed, moves the blades to smaller installation angles, thereby reducing engine thrust. Qualitative nature of the change in the angle of installation of the blades φ from the fuel supply Q T into the engine is shown in Fig. 4.25.

Speed ​​characteristics of the propeller.

Let us now consider the operation of the propeller-regulator system when the flight speed changes and there is a constant supply of fuel to the engine. Let's assume that an airplane is switching from a climb mode to a level flight mode or from a level flight mode to a descent mode. In both cases, the flight speed will increase with a constant fuel supply.

In Fig. 4.26 shows graphs of changes in the available power of the gas turbine engine - N doors and power consumed by the propeller N in depending on flight speed V. In the region of subsonic flight speeds, the power (as well as thrust) of the engine N doors with increasing flight speed it decreases slightly at the same time N in falls more intensely. At speed V 0 The engine-propeller system operates in equilibrium mode ( N doors=N in). With an increase in flight speed to V 1 there is excess power ( N dv > N c), causing an increase in propeller speed. In an effort to maintain the speed at a given value, the centrifugal speed controller will move the blades to large installation angles φ 1 This will cause a decrease in speed due to the greater power consumption of the propeller N in (φ 1) and the equilibrium regime is restored, but when large values blade installation angles.

Nature of change φ=f(V) shown on the graph in Fig. 4.27.

When the flight speed decreases, the control process proceeds in the reverse order. As the flight speed decreases, the angle of attack of the blades increases, and, consequently, the propeller becomes “heavier”. At the same time, the rotation speed decreases, and the regulator, trying to maintain the set value, moves the blades to smaller installation angles.

Altitude characteristics

The propeller-regulator system will also respond to changes in flight altitude, since the altitude characteristics of the engine and propeller change differently.

High-altitude characteristics of the theater N dv =f(h), presented on the graph in Fig. 4.28 (upper broken curve) has two characteristic breaks. On the ground, engine power is determined by the minimum fuel supply to the engine, which corresponds to the required takeoff power. In the altitude range (0…h 1) maintaining constant power (N door=const) by increasing the gas temperature in front of the turbine to the maximum permissible (increasing fuel supply) T g max. At altitudes from h 1 before h=11km engine power drops. In this altitude range, the decrease in atmospheric air density is partially compensated by an increase in the degree of air compression in the compressor, associated with a decrease in atmospheric temperature ( N dv ~ρ (0.8...0.9)).

At altitudes above 11 km, where the ambient temperature is constant, engine power decreases in proportion to the decrease in air density ρ .

The propeller power, as follows from Fig. 4.28 (a series of curves at different φ), decreases with increasing altitude in proportion to the change in air density ρ .

If we assume that the angle of installation of the propeller blades φ 0 on earth met the condition N doors=N in., then with increasing flight altitude N doors >N in. Such a discrepancy N doors And N in causes an increase in rotation speed, but the regulator, maintaining its set value, moves the propeller blades to large installation angles.

Thus, with an increase in flight altitude to h 1 there is an intensive increase in the installation angles of the blades; at the heights (h 1…11)km the angles continue to increase, but with less intensity; at altitudes above 11 km, the installation angle remains constant, since the change in engine and propeller power is equally proportional to the change in air density.

When the flight altitude decreases, the process of changing the installation angle will be reversed, i.e., the propeller blades will be moved to smaller installation angles. The nature of the blade angle change is shown in Fig. 4.29.

4.10. AEROMECHANICAL PROPELLERS

On airplanes with low-power engines, aeromechanical propellers are used, in which the blades turn automatically, without the use of extraneous energy sources and a speed controller. Thus, aeromechanical propellers are autonomous and automatic. Automatic rotation of the blades is achieved by changing the magnitude of the torques acting on the propeller blades in flight.

For conventional propellers, the magnitude of the moments of aerodynamic forces is small, and the direction of their action is determined by the magnitude of the angles of attack. If the blades are given a special shape or bent at an angle γ (Fig. 4.30) relative to the axis of rotation of the blade, then by changing the position of the center of pressure, the moments of aerodynamic forces will ensure rotation of the blade in the hub towards a decrease in the installation angle. Counterweights are installed on the blades of aeromechanical propellers, which create torques directed towards increasing the installation angle (tightening the propeller).

Counterweights are installed on the blades of aeromechanical propellers, which create torques directed towards increasing the angle of installation (tightening the propeller). Moments of the transverse components of the centrifugal forces of the blades Mts tend to rotate the blades in the direction of decreasing the angle of installation of the blade. Moments Mts, created by counterweights, are greater than the moments created by the transverse components of the centrifugal forces of the blades. In steady-state modes, the torque ratio should ensure the condition

M p = M c + M a.

However, the values ​​of the above moments vary depending on the flight mode, so choosing the correct ratio of torques acting on the propeller blades over a wide range of angle changes is very important and challenging task. This ratio of moments should ensure that the propeller becomes “heavier” when the flight speed increases, and, conversely, when the flight speed decreases, the propeller should “lighten.” The rotation speed at a constant engine operating mode must remain constant.

In accordance with this, when the engine is operating in place, when the propeller thrust is maximum, and, therefore, the torque from aerodynamic forces is maximum, the propeller blades are set to the minimum angle stop. This ensures that the take-off (maximum) engine rotor speed is obtained and the most favorable conditions plane takeoff.

In flight, as the speed increases, the propeller thrust decreases, and the torques also decrease M a, and the moments of the centrifugal forces of the counterweights and blades, independent of the flight speed, retain the same values ​​(at n=const). As a result, the torque ratio will change and the blades will gradually rotate towards increasing the installation angle, preventing the propeller from unwinding. Obviously, when the flight speed decreases, the picture will be the opposite. Thus, the blades of the aeromechanical propeller automatically change the installation angle depending on the flight speed. The rotational speed of the screw changes, but within relatively small limits.

Due to the lack of reasonable alternatives, almost all aircraft in the first half of the last century were equipped with piston engines and propellers. To improve the technical and flight characteristics of the equipment, new propeller designs were proposed that had certain features. In the mid-thirties, a completely new design was proposed that provided the desired capabilities. Its author was the Dutch designer A.Ya. Dekker.

Adriaan Jan Dekker began working in the field of screw systems back in the twenties. Then he developed a new impeller design for windmills. To improve the basic characteristics, the inventor proposed using planes resembling an airplane wing. In 1927, such an impeller was installed in one of the mills in the Netherlands and was soon tested. By the beginning of the next decade, three dozen such impellers were put into operation, and in 1935, 75 mills were already equipped with them.

Experimental aircraft with propeller A.Ya. Dekker. Photo Oldmachinepress.com

In the early thirties, after testing and introducing a new design at the mills, A.Ya. Dekker proposed using similar units in aviation. According to his calculations, a specially designed impeller could be used as an aircraft propeller. Soon this idea was formalized in the form of the necessary documentation. In addition, the designer was concerned about obtaining a patent.

The use of a non-standard propeller design, as conceived by the inventor, should have provided some advantages over existing systems. In particular, it became possible to reduce the speed of the propellers while obtaining sufficient thrust. In this regard, the invention of A.Ya. Dekker is often called a “Low rotation speed propeller”. This design was also named in patents.

The first patent application was filed in 1934. At the end of July 1936, A.Ya. Dekker received British patent number 450990, confirming his priority in creating the original screw propeller. Shortly before the first patent was issued, another application appeared. The second patent was issued in December 1937. A few months earlier, the Dutch designer sent documents to the patent offices of France and the United States. The latter issued document US 2186064 at the beginning of 1940.

Screw design of the second version. Patent drawing

British patent No. 450990 described an unusual propeller design that could provide sufficient performance with a certain reduction in negative factors. The designer proposed using a large ogive-shaped propeller hub, which smoothly transitions into the nose of the aircraft fuselage. Large blades had to be rigidly attached to it unusual shape. It was the original contours of the blades, as A.Ya. believed. Dekker, could lead to the desired result.

The blades of a “low-speed” propeller were supposed to have low aspect ratio with a large chord length. They should have been mounted at an angle to the longitudinal axis of the hub. The blade received an aerodynamic profile with a thickened nose. It was proposed to make the tip of the blade arrow-shaped. The tip was located almost parallel to the axis of rotation of the propeller, and it was proposed to make the trailing edge curved with a protruding end part.

Internal structure of the screw and gearbox. Patent drawing

The first project in 1934 involved the use of four blades. A screw of this design had to be mounted on a shaft extending from a gearbox with the required characteristics. The significant area of ​​the propeller blades in combination with the aerodynamic profile was supposed to provide an increase in thrust. Thus, it became possible to obtain sufficient thrust at lower speeds in comparison with a propeller of a traditional design.

After filing the application for the first patent A.Ya. Dekker tested an experimental propeller and made certain conclusions. During the inspection, it was found that the proposed design has certain disadvantages. Thus, the air flow behind the propeller diverged to the sides, and only a small part of it passed along the fuselage. This led to a sharp deterioration in the efficiency of the tail rudders. Thus, in its existing form, the Decker screw could not be used in practice.

Further development of the original propeller led to the appearance of an updated design with a number of important differences. It was she who became the subject of the second British and first American patent. It is interesting that the document from the USA, unlike the English one, described not only the propeller, but also the design of its drives.

The Fokker C.I aircraft - a similar machine became a flying laboratory for testing the ideas of A.Ya. Dekker. Photo Airwar.ru

The updated product Low rotation speed propeller was supposed to have two coaxial counter-rotating propellers. It was still proposed to build the front propeller on the basis of a large streamlined hub. The rear propeller blades should have been attached to a cylindrical unit of comparable dimensions. As in the previous project, the front propeller spinner and the rear propeller ring could serve as the nose fairing of the aircraft.

Both propellers were to receive blades of a similar design, which was a development of the developments of the first project. Again, it was necessary to use significantly curved blades of low aspect ratio, having a developed aerodynamic profile. Despite the swept leading edge, the length of the profile increased from the root to the tip, forming a characteristic curve of the trailing edge.

According to the patent description, the front propeller was supposed to rotate counterclockwise (when viewed from the pilot's side), the rear propeller - clockwise. The propeller blades had to be mounted accordingly. The number of blades depended on the required characteristics of the propeller. The patent showed a design with four blades on each propeller, while a later prototype received larger number planes.

The process of assembling the original screws, you can examine the internal elements of the product. Photo Oldmachinepress.com

The American patent described the design of the original gearbox, which made it possible to transmit torque from one engine to two counter-rotating propellers. It was proposed to connect the engine shaft to the sun gear of the first (rear) planetary circuit of the gearbox. Using a ring gear fixed in place, power was transmitted to the satellite gears. Their carrier was connected to the front propeller shaft. This shaft was also connected to the sun gear of the second planetary gear. The rotating carrier of its satellites was connected to the hollow shaft of the rear propeller. This design of the gearbox made it possible to synchronously regulate the speed of rotation of the screws, as well as ensure their rotation in opposite directions.

According to the inventor's idea, the main thrust was to be created by the front propeller blades. The rear one, in turn, was responsible for the correct redirection of air flows and made it possible to get rid of the negative effects observed in the basic project. After two coaxial propellers, the air flow passed along the fuselage and should have normally blown the tail unit with rudders. To obtain such results, the rear propeller could have a reduced rotation speed - about a third of the front one.

The original propeller propulsion was created taking into account the possible implementation in new aircraft projects, and therefore it was necessary to conduct full tests. At the beginning of 1936, Adriaan Jan Dekker founded own company Syndicaat Dekker Octrooien, which was to test the original propeller and, if positive results were obtained, begin promoting this invention in the aviation industry.

Finished propeller on an airplane. Photo Oldmachinepress.com

At the end of March of the same year, the Dekker Syndicate acquired a Dutch-built Fokker C.I multi-role biplane aircraft. This machine, with a maximum take-off weight of only 1255 kg, was equipped with a BMW IIIa gasoline engine producing 185 hp. With a standard two-bladed wooden propeller, it could reach speeds of up to 175 km/h and rise to a height of up to 4 km. After some restructuring and installation of a new propeller, the biplane was supposed to become a flying laboratory. In April 1937, the company A.Ya. Dekkera registered the modernized aircraft; he received the PH-APL number.

During the restructuring, the prototype lost its standard cowling and some other parts. Instead, an original gearbox and a pair of “low-speed propellers” were placed in the forward part of the fuselage. The front propeller received six blades, the rear – seven. The basis of the new propeller was a pair of hubs assembled from an aluminum frame with casing made of the same material. The blades had a similar design. Due to the installation of screws, the nose of the car most noticeably changed its shape. At the same time, the cylindrical fairing of the rear rotor did not protrude beyond the fuselage skin.

Tests of the flying laboratory with the original propeller started in the same 1937. The Ipenberg airfield became the site for them. Already at the early stages of testing, it was established that coaxial propellers with low aspect ratio blades can indeed create the required thrust. With their help, the car could perform taxiing and jogging. In addition, from a certain time, testers tried to lift the car into the air. It is known that the experienced Fokker C.I was able to perform several approaches, but there was no talk of a full takeoff.

Front view. Photo Oldmachinepress.com

Tests of the prototype aircraft revealed both the pros and cons of the original project. It was found that a pair of counter-rotating propellers was indeed capable of producing the required thrust. At the same time, the propeller-motor group assembly was distinguished by its relatively small size. Another advantage of the design was the reduced noise produced by the low aspect ratio blades.

However, it was not without problems. Air propeller A.Ya. Dekker and the gearbox he needed differed from existing samples in that they were excessively complex in manufacturing and maintenance. In addition, the experimental propeller installed on the Fokker C.I showed insufficient thrust characteristics. It allowed the plane to move on the ground and reach a fairly high speed, but its thrust was insufficient for flight.

Apparently, the tests continued until the very beginning of the forties, but for several years they did not lead to real results. Further work was prevented by the war. In May 1940, Nazi Germany attacked the Netherlands, and just a few days later an experimental aircraft with unusual propellers became a trophy of the aggressor. German experts expectedly showed interest in this development. Soon the flying laboratory was sent to one of the airfields near Berlin.

The engine started, the propellers began to rotate. Still from newsreel

There is information about some tests being carried out by German scientists, but these tests ended fairly quickly. According to some reports, the first attempt by the Germans to lift the plane into the air ended in an accident. The car was not restored, and this is where the story of the bold project ended. The only aircraft equipped with Low rotation speed propellers could not show its best side, and therefore original idea refused. Subsequently, only traditional propellers were used in large quantities.

According to the ideas underlying the original design, a special “Low Speed ​​Propeller” was supposed to be a full-fledged alternative to traditionally designed systems. Differing from them in some complexity, it could have advantages in the form of smaller dimensions, reduced speed and reduced noise. However, competition did not work out. Development by A.Ya. Deckera couldn't even complete the entire test cycle.

Perhaps, with further development, the original propellers could show the desired characteristics and find application in certain aircraft projects. However, the continuation of work was slowed down due to various problems and circumstances, and in May 1940 the project was stopped due to a German attack. After this, the unusual idea was completely left without a future. Subsequently, promising designs of propellers were again worked out in different countries, but direct analogues of the Adriaan Jan Dekker system were not created.

Based on materials:
https://oldmachinepress.com/
http://anyskin.tumblr.com/
http://hdekker.info/
http://strangernn.livejournal.com/
https://google.com/patents/US2186064