What Is a Wind Propeller?

A propeller is a device that converts the engine's turning power into propulsive force by rotating the blades in air or water. There can be two or more leaves connected to the hub. A thruster. There are many types of propellers, and they are also widely used, such as propellers for airplanes and ships.

A propeller is a device that converts the engine's turning power into propulsive force by rotating the blades in air or water. There can be two or more leaves connected to the hub. A thruster. There are many types of propellers, and they are also widely used, such as propellers for airplanes and ships.

Chinese name

propeller

Foreign name

propeller

Principle

Reverse thrust after rotation

Ancestor

Archimedes Screw Pump, Bamboo Dragonfly

Propeller historical origins

1. The ancient wheels, the so-called "paddle wheels" in Europe, cooperated with modern steam engines to install a row of straight blades of the original paddle wheels diagonally on a rotating hub. Make up a spiral

A prototype of an oar.

2. Ancient windmills can output torque when rotating with the wind. Conversely, in the water, inputting torque to rotate the windmill can cause the windmill to move the boat.

3 At that time, Archimedes screw pumps of ancient Greece have been used for more than ten centuries. ^[1] It can lift water horizontally or vertically, and the fact that the spiral structure can pump water is important as a propeller Enlightenment.

The great British scientist Hooker successfully adopted the principle of wind speed meter to measure water flow in 1683. At the same time, he proposed a new propeller-propulsion of ships, and made a significant contribution to the propulsion of ships.

Propeller geometry

(D) Propeller diameter (D)

One of the important parameters affecting the propeller performance. In general, as the diameter increases, the tensile force increases and the efficiency increases. Therefore, if the structure allows it, try to choose a larger diameter propeller. In addition, the airflow speed of the propeller tip should not be too large (<0.7 sonic speed), otherwise shock waves may occur, resulting in reduced efficiency.

(B) Number of propeller blades (B)

It can be considered that the coefficient of pull and power of the propeller is proportional to the number of blades. Ultralight aircraft generally use simple two-bladed propellers. Only when the diameter of the propeller is limited, the method of increasing the number of blades is used to obtain a good cooperation between the propeller and the engine.

() Propeller degree ()

The ratio of the blade area to the propeller rotation area (R2). Its effect is similar to that of the number of blades. As the degree of realism increases, the tensile coefficient and power coefficient increase.

() Propeller blade angle ()

The blade angle varies with the radius, and its variation law is the most important factor affecting the performance of the blade. Conventionally, the value of the blade angle at 70% diameter is the name value of the blade angle. Pitch: It is another way to express the blade angle.

(H) Propeller Geometric Pitch (H)

When the angle of attack of the blade profile is zero, the distance that the blade advances after one revolution. It reflects the size of the blade angle and more directly indicates the operating characteristics of the propeller. The geometric screw moments of the blade sections may not be equal. It is customary to use the geometric screw moment at 70% diameter as the name value. Propellers can be ordered abroad according to diameter and pitch. Such as 64/34, it means that the paddle diameter is 60 inches, and the geometric screw moment is 34 inches.

(Hg) Propeller actual pitch (Hg)

The distance the airplane has traveled once the blades have rotated. Hg = v / n can be used to calculate the actual propeller torque. According to H = 1.1 1.3Hg, we can roughly estimate the value of the geometrical screw moment of the propeller.

(HT) Propeller theoretical torque (HT)

When designing the propeller, it is necessary to take into account that the speed of the air flowing through the propeller increases, and the speed of the airflow passing through the plane of propeller rotation is greater than the flight speed. Therefore, the distance the propeller advances relative to the air, the theoretical screw torque will be greater than the actual screw torque.

Propeller ship propeller

Propeller was born

In 1836, the British Archimedes used a screw propeller, a long wooden screw-like screw. At the beginning of the test, it sailed at a speed of 4 knots. Suddenly, the obstacle in the water broke the screw, leaving only a small part. While Smith, the shipbuilding engineer, was overwhelmed, the ship unexpectedly increased speed to 13 knots. This incident inspired shipbuilding engineers, who turned the long screw into a short screw and the short screw into a blade shape, and the propeller was born.

Propeller development

1

In 752, the Swiss physicist Bernoulli first reported that the propeller was superior to the various thrusters that existed before it. He designed a thruster with a double-lead screw installed in front of the stern rudder. In 1764, the Swiss mathematician Euler studied other thrusters that could replace sails, such as paddle wheels (paddle wheels), including propellers.

Submarines and submarines move under the water, traditional paddles and sails cannot be used, and heavy and heavy paddle wheels are difficult to adapt. So the first manual propeller, however, was not used on the ship, but as a propulsion tool for submersibles.

The advent of the steam engine provided new good power for the ship's propulsion. The propeller conformed to the development of the steam engine and became the latest subject of ship propulsion.

The first experimental power-driven propeller was American Stephen. He built a 7.6-meter-long boat in 1804, which was directly driven by a steam engine. The first experimental voyage on the Hudson River was found to be ineffective. So put on a watt steam engine, the experimental speed was 4 knots, and the maximum speed reached 8 knots.

Stephen propeller has 4 pinwheel-type blades, forged. Compared with ordinary pinwheels, it increases the radial width of the blade. In order to choose a better combination of pitch and speed in the experiment, the pitch of the blade can be adjusted. structure. During the two-week test voyage on the Hudson River, the propeller changed several pitch values, but the results of the experiment were not ideal, and the performance was far worse than that of a paddle wheel. This experiment made him understand that under the condition of low speed of the steam engine, the superiority of the paddle wheel has been fully exerted, and its propulsive efficiency is higher than that of the propeller is an inevitable conclusion.

The introduction of the Archimedes screw was first seen in 1803, and in 1829 it was patented by the British Archimedes. And on this basis in 1840-1841

Archimedes screw pump schematic

Some civilian propellers were built. In 1843, the British Navy replaced the paddle wheels with propellers for the first time on the "Letterer" ship. Subsequently, Smith designed 20 propeller ships and participated in the war against Russia. Smith became a famous figure.

In 1843, the U.S. Navy built the first propeller ship "Pullinsiden", which was designed by Captain Alesund. With the active promotion of Alesund, the United States successively built 41 civilian propeller ships. The maximum displacement is 2,000 tons.

Although Britain and the United States have achieved some successes, there are still many problems with propellers used for ship propulsion, such as terrible vibrations on wooden shell ships, wear of propeller shaft bearings below the waterline, paddle shaft seals, thrust bearings, etc.

With the advancement of technology, the above defects of the propellers are overcome one by one, and the speed of the steam engine is increased, more and more propellers are replacing paddle wheels on ships. By 1858, the Great Orient was equipped with the largest propeller in the world at that time. It had a diameter of 7.3 meters, weighed 36 tons, and had a speed of 50 revolutions per minute. At that time, the propeller standard was no longer authoritative. Its propulsion efficiency is close to that of paddle wheels, but it has many advantages that paddle wheels cannot compete with, and paddle wheels are gradually disappearing on sea vessels.

During the development of science and technology,

propeller

The performance of many mechanical devices has been widely used before people knew it. But before people fully understand its physical laws and complete theoretical analysis, it is difficult for these devices to reach its best performance. The propeller is no exception. Until 1860, although it had become a standout on the sea, its achievements were all dependent on years of accumulated experience. The advancement of propellers, relying on the intuitive reasoning of experts, has not been able to meet the development needs of ship technology. It requires scientists to make a complete explanation of their hydrodynamic characteristics, which has led to the development of propeller theory.

The theoretical research of propellers has done much more than any professional field in the development of ship technology. It has transitioned from empirical methods to digital design, and then applied computer technology to optimize the propellers. The design of a good propeller is very important, and model tests also play a major role.

Propeller China Development

Since our country has become a semi-colonial colony since the middle of the 19th century, very few contributions have been made. After liberation, China's shipbuilding industry has undergone new development. A lot of design and research work has been carried out on propeller technology, and a large number of propellers designed and manufactured by various types of ships have been equipped. The most memorable thing is the advent of the "off-blade", which is a great creation in the development of propeller technology in China. That was in the 1960s. There was a master named Zhou Ting at the Wenchong Shipyard in Guangzhou. Based on his decades of experience in making propellers, he made the blades of the propellers into a 82-kilogram heavy knife style in the Romance of the Three Kingdoms. He vividly called it "off blade" (Figure 4).

The "Off Knife and Paddle" has tested sailing on some ships, increasing the speed of the ships, and even more surprisingly, the spiral vibration has been greatly reduced. It was used on the Yangtze River 2000-horsepower tugboat and the Chinese landing craft, and both achieved good results. This achievement has attracted many people in the shipbuilding industry. In 1973, the "closed-knife" open water test was first conducted in Shanghai, and a design map was also provided. Interestingly, the "high-slanted" propellers developed today in world-famous shipbuilding countries, such as (Figure 5) the latest naval large-slanted propellers, have a diameter of 6.3 meters, a shaft power of 35660 kilowatts, and a ship speed of 32.8 knots; Figure 6 shows This is the latest large-scale inclined propeller used on passenger ferries. The propeller has a diameter of 5.1 meters, a shaft power of 15,640 dry watts, and a ship speed of 23.2 knots. Figure 7 shows the large side-screw propeller used on the latest chemical tanker. The propeller has a diameter of 6.2 meters, a shaft power of 10400 kilowatts, and a ship speed of 16.7 knots. They are very similar to "off-knife paddles". Their important characteristics are vibration and low noise, which are also the characteristics of "off-knife paddles". Changzhou Zhonghai Ship Propeller Co., Ltd. has built the largest marine propeller in our state-owned enterprise, which can provide the best off-blade.

How Propellers Work

The propeller can be considered as a wing that rotates and advances.

The airflow flowing through each section of the blade is synthesized by the forward speed in the direction of the rotation axis and the tangential speed generated by the rotation. Take a small section at each of the propeller radius r1 and r2 (r1 <r2) to discuss the air flow on the blade. Vaxial speed; npropeller speed; airflow angle, that is, the angle between the airflow and the plane of rotation of the propeller; the angle of attack of the blade section; the angle of the blade, that is, the angle between the blade profile chord line and the rotation plane . Obviously + = .

When the air flows through the small sections of the blade, aerodynamic force, resistance D and lift L are generated, and the total aerodynamic force after synthesis is R. The component force of R in the flight direction is the pulling force T, and the force P opposite to the rotation direction of the propeller prevents the propeller from rotating. Add the pulling force of each small segment on the whole blade and the force to prevent rotation to form the pulling force of the propeller and the torque to prevent the propeller from rotating ^[2] .

It is necessary to make the sections of the propeller work at a high angle of attack with a large lift and drag in order to obtain a larger pulling force and a smaller drag torque, which means higher efficiency. When the propeller is working. The axial speed does not change with the radius, while the tangent speed changes with the radius. Therefore, near the blade tip and the larger radius, the airflow angle is smaller, and the corresponding blade angle should be smaller. In the vicinity of the blade root, where the radius is smaller, the airflow angle is larger, and the corresponding blade angle should also be larger. The blade angle of the propeller should gradually increase from the blade tip to the blade root according to a certain rule. So it is more accurate to say that the propeller is a twisted wing.

The airflow angle actually reflects the ratio of the forward speed to the tangential speed. For a certain profile of a propeller, the angle of attack of the profile changes with the ratio. As the angle of attack changes, the pull and drag moments also change. Reflect the airflow angle at the tip of the propeller with the ratio of advance moment "J", J = V / nD. Where Dpropeller diameter. The theory and test prove that the pulling force (T) of the propeller and the power (P) and efficiency () required to overcome the propeller torque can be calculated by the following formula:

= J · Ct / Cp

In the formula: Ct-pull coefficient; Cp-power coefficient; -air density; n-propeller speed; D-propeller diameter. Where Ct and Cp depend on the geometrical parameters of the propeller, and their values vary with J for each propeller. The characteristic curve shows the relationship between the propeller pull coefficient, power coefficient and efficiency with the forward ratio. It is one of the main basis for design and selection of propeller and calculation of aircraft performance.

Air propeller

Propeller overview

A device that converts the engine's rotational power into propulsion or lift by rotating the blades in the air, referred to as a propeller. It consists of multiple blades and a central hub. The blades are like a twisted slender wing mounted on the hub. The engine shaft is connected to the hub and drives it to rotate. Before the advent of jet engines, all powered aircraft used propellers as a means of generating propulsion. Propellers are still used in subsonic aircraft with piston and turboprop engines. Helicopter rotors and tail rotors are also a type of propeller.

Propeller principle

When the propeller rotates, the blade continuously pushes a large amount of air (propulsion medium) backward, and a forward force is generated on the blade, that is, the propulsive force. In general, the propeller has a forward speed in addition to rotation. Seen from a small section of blades, it looks like a small section of wing, and its relative air velocity is composed of forward speed and rotational speed. The component of the aerodynamic force on the blade in the forward direction constitutes the pulling force. The component in the rotating surface forms a torque that prevents the propeller from rotating and is balanced by the torque of the engine. The angle between the blade section chord (equivalent to the wing chord) and the plane of rotation is called the blade installation angle. The distance that the propeller rotates once and propels forward with the blade installation angle as the guide is called the pitch. In fact, the forward speed of each section on the blade is the same, but the peripheral speed is proportional to the distance (radius) of the section from the axis of rotation, so the angle between the relative airflow of each section and the plane of rotation varies with the distance from the axis Increasing and gradually decreasing, in order to keep each section of the blade and the relative airflow within a favorable angle of attack, the installation angle of each section also decreases as the distance from the shaft increases. This is why every blade has a twist.

Propeller efficiency is expressed as the ratio of propeller output power to input power. The output power is the product of the propeller's pulling force and flight speed. The input power is the power that the engine uses to rotate the propeller. Before the airplane took off and took off, the propeller efficiency was zero because the forward speed was zero, and the engine's power was all used to increase the kinetic energy of the air. As the forward speed increases, the propeller efficiency continues to increase, and the efficiency is higher in the range of 200 to 700 km / h, and the flight speed increases again. Due to the compression effect, the wave resistance appears at the blade tip, and the efficiency drops sharply. The maximum efficiency of the propeller in flight can reach 85% to 90%. The diameter of the propeller is much larger than that of the jet engine, and the air flow as a propulsion medium is large. When the engine power is the same, the air velocity behind the propeller is low and the thrust generated is large, which is very beneficial for takeoff (requires large thrust).

Propeller structural characteristics

Propellers have 2, 3 or 4 blades. Generally, the larger the number of blades, the greater the power absorption. Sometimes a high-power turboprop aircraft also uses a sleeve propeller, which is actually two counter-rotating propellers that can counteract the reaction torque. In light aircraft with engine power below 100 kW, two-bladed wooden propellers are commonly used. It uses a spliced piece of wood to twist the blades on both sides, and the middle opening is connected to the engine shaft. The propeller must withstand the centrifugal inertia force and aerodynamic load of the blade itself when rotating at high speed. The centrifugal force of the high-power propeller at the root of the blade can reach 200 kN (20 ton force). There are also vibrations caused by engines and aerodynamics. High-power engines generally use 3- and 4-blade propellers, and mostly use aluminum alloy and steel to make the blades. Aluminum and steel blades can be made thinner due to their solid material, which helps improve the efficiency of the propeller at high speeds. After the 1970s, blades were also made from composite materials to reduce weight.

Propeller rotation

When the engine stops in the air, the propeller will continue to rotate in the original direction like a windmill. This phenomenon is called propeller rotation. The propeller's rotation is not driven by the engine, but is "pushed" by the oncoming airflow of the blades. Instead of generating tension, it increased the resistance of the aircraft. When the propeller rotates, a large negative angle of attack is formed. The total aerodynamic direction and effect of the blades have changed qualitatively. One of its component forces (Q) is in the same direction as the tangential velocity (U) and becomes the driving force for the automatic rotation of the blade, forcing the blade to continue to rotate in the original direction: the other component (-P) is opposite to the speed direction , It acts as a resistance to flight. After the engine of some ultralight aircrafts is stopped in the air, due to the low flight speed, the spin torque generated by the propeller cannot overcome the resistance of the propeller. At this time, the blade resistance is large, and the lift-drag ratio (or glide ratio) of the aircraft will be greatly reduced.

Propeller classification

Propellers are divided into fixed (propeller) pitch and variable pitch propellers.

Fixed-pitch propeller

Wooden propellers are generally fixed-pitch. Its pitch (or blade mounting angle) is fixed.

The blade installation angle suitable for low speed is too small when flying at high speed; similarly, the installation angle suitable for high speed flying is too large at low speed. Therefore, the fixed-pitch propeller is more efficient only in the selected speed range, and is less efficient in other states. The fixed-pitch propeller has a simple structure and light weight, and is widely used in light aircraft and ultralight aircraft with very small power.

Variable pitch propeller

In order to solve the contradiction between high-speed and low-speed performance of fixed-pitch propellers, variable-pitch propellers have appeared in flight. The propeller pitch changing mechanism is driven by hydraulic or electric power. Initially, a double-pitch propeller was used. Use high distances at high speeds and low distances at low speeds (such as take-off and climb conditions), and then gradually increase the number of pitches to accommodate more flight conditions. The most complete variable pitch propeller is a constant speed propeller with a speed regulator. The speed regulator is actually a device that can automatically adjust the pitch and maintain a constant speed. The driver can change the speed of the engine and the propeller by controlling the regulator and the throttle. On the one hand, the drag of the propeller is adjusted while the propeller is in the best working state. In a multi-engine aircraft, when one engine fails to stop, the propeller rotates like a windmill under the action of oncoming airflow, which increases flight resistance on the one hand, causes a large unbalanced torque, and may further damage the engine. The variable pitch propeller can also be feathered automatically for this purpose.

That is, the propeller is turned to the direction substantially along the airflow and the propeller is kept still to reduce the resistance. The variable-pitch propeller can also reduce the pitch and generate negative tension to increase resistance and shorten the landing run distance. This state is called reverse paddle.

In order to improve the economics of subsonic civilian aircraft and reduce the fuel consumption of aircraft, the United States began to study a multi-blade propeller called a fan propeller in the late 1970s. It has 8 to 10 scimitar-shaped paddles, with thin blades and small diameters. The scimitar shape can act as a swept-wing (see swept-wing aircraft), and thin blades help increase the propeller speed. It is suitable for higher flying Mach numbers (M = 0.8). Due to the large number of blades, the power absorbed by the propeller per unit propulsion area can be increased to 300 kW / m2 (generally the propeller is 80 to 120 kW / m2).

Propeller pull change

Change with speed

Under the condition that the flying speed is constant, the tangential speed (U) increases as the rotation speed increases, the pitch ratio decreases, and the blade angle of attack increases, and the propeller pull coefficient increases. Because the pull force is proportional to the square of the speed, the speed increases. When the throttle is large, the pulling force can be increased.

Change with speed

In the case of constant speed, the flight speed increases, the pitch ratio increases, the blade angle of attack decreases, and the propeller pull coefficient decreases. , The pulling force decreases accordingly. When the flying speed is equal to zero, the tangential speed is the resultant speed, and the blade angle of attack is equal to the blade angle. When the aircraft is interviewed on the ground, the flight speed (V) is equal to zero, and the blade angle of attack is the largest. Some sections have aerodynamic performance that is worse than the stall angle of attack because the angle of attack is too large, so the propeller may not produce the maximum pull force.

Propeller tension curve

According to the law that the propeller pulling force decreases with increasing flight speed, the available propeller pulling force curve can be drawn.

Propeller effective power

The propeller generates a pulling force, pulling the aircraft forward and doing work to the aircraft. The work done by the propeller per unit time is the effective power of the propeller.

Comprehensive propeller situation

In flight, increase the throttle and fix it. The change of the propeller's pulling force with the rotation speed and flight speed is as follows: As the engine output power increases, the propeller rotating speed (tangential speed) is rapidly increased to a certain value, and the propeller pulling force is increased. As flight speed increases, due to the increase in flight speed, the angle of attack of the blades begins to gradually decrease, the pull force also gradually decreases, and the aircraft resistance gradually increases, so that the speed increase trend gradually decreases. When the pulling force is reduced to a certain degree (that is, the pulling force is equal to the drag force), the speed of the aircraft is no longer increased. At this time, the flight speed, rotation speed, blade angle of attack, and propeller pulling force are all unchanged, and the aircraft maintains flight at a new speed.

Propeller plane

The structure of a propeller aircraft is more complicated. In order to reduce speed and increase propeller efficiency, most engines are equipped with a retarder. The engines of these aircraft are equipped with oil radiators. Liquid-cooled piston engines are also equipped with a coolant radiator. Both the hub and engine have streamlined covers to reduce drag. The engine and propeller at the front of the fuselage often affect the pilot's line of sight. Some aircraft arrange the engine under the cockpit and use a long axis to connect the propeller at the nose, such as the American P-39 fighter. Some planes have the cockpit offset to the side of the wing to improve forward vision and become a special asymmetric plane, such as the German BV-141. A pull-in fighter with a machine gun on the head needs a coordination mechanism to ensure that the bullet is fired from the middle of the rotating propeller blades. In some aircraft, the cannon barrel is mounted in the propeller shaft, and the shell is fired from the barrel inside the propeller shaft. When the propeller rotates, a reaction torque is generated. High-powered aircraft usually use a large vertical tail or offset the torque generated by the vertical tail to balance the torque. The reverse rotating coaxial propeller can also be used to counteract the reaction torque. 22 aircraft.

Modern propeller aircraft mostly use variable pitch propellers with adjustable blade angle. This type of propeller can adjust the blade angle according to flight needs and improve the working efficiency of the propeller. As the propeller rotates, the peripheral speeds of the blade root and the blade tip are different. In order to maintain the optimal aerodynamic state of each part of the blade, the blade angle of the blade root is designed to be the largest, and the blades of the blade tip are gradually decreased. The blade with the minimum working angle is a force situation of a cantilever beam. In order to increase the strength of the blade root, the sectional area of the blade root is designed to be the largest.

The number of blades on an aircraft depends on the power of the engine. There are two, three, and four blades, and there are five and six blades. The propellers mounted on the head of the aircraft are pull-type propellers, the propellers mounted on the rear of the aircraft are thrust-type propellers, and there are also aircraft equipped with both pull-type and thrust-type propellers. ^[3]