How aircraft propellers work



How aircraft propellers work

A propeller is a marvel of engineering that, in exception of the invention of the internal combustion engine, is probably the most important breakthrough to accomplish powered flight. Yet a propeller is nothing more than wings rotating around a hub. If you look at the cross section of a propeller, you will notice it is shaped like a wing. On a wing the bottom side is flat, and the top is curved with the trailing edge tapered and the leading edge thicker. As a wing moves forward the flow of air is divided into the air that flows over the wing, and the air that flows under. Because the top side of a wing is curved, the air traveling over the top of the wing must travel farther to reach the rear of the wing. As the airflow over the top is traveling farther than the bottom airflow in the same amount of time, it must be traveling faster.

Bernoulli’s Principle says that a rise in speed of a fluid, in our case air, occurs simultaneously with a drop in pressure, causing a low pressure in the fast moving air on the top of the wing. With a lower pressure on the top relative to the bottom, lift is created and the wing rises to try to equalize the pressure difference.

Now take two wings and attach them to a hub and you have a propeller. The rear side of a propeller is flat, the front surface curved. As the propeller rotates, the airflow over the front of the blade travels further relative to the airflow over the back, and Bernoulli’s Principle pulls the aircraft ahead.

A propeller also has some twist built into it, due to the fact that the tips of a propeller are traveling much faster than near the hub. The formula c=pi*d, or the circumference equals pi times the diameter. Because the diameter at the tips is double the diameter halfway out, so is the distance it must travel, and correspondingly its speed.

You will notice at propeller tips, the blades are nearly straight up and down, because of the high speed of the tips need less of a bite on the oncoming air to generate lift. Closer to the hub more of a bite is required to generate lift, so the blade is gradually twisted to achieve the thrust over the length of the blade.

I’m sure many have heard the loud crack of a Harvard’s propeller at an air show. When this happens the tips are actually traveling supersonic, and you’re hearing sonic booms. On higher performance aircraft they will add more shorter blades to a propeller rather than longer blades to utilize the engine power, as supersonic blade tips are not only noisy but lose their thrusting efficiency.

Propellers are simply twisted wings, attached to a rotating hub, simple in concept but highly specialized to most efficiently turn engine power into thrust.

Like many advances in aviation, the advent of the aircraft propeller came from experimentation in fluid dynamics and naval innovations. Propellers were originally pioneered as a variation on water screws and used to propel watercraft. Though other inventors experimented with propellers on airships and gliders, the Wright Brothers were the first to modernize that propeller for use in the air. Their observation that propellers could exhibit the same principle of lift as aircraft wings marked the invention of the modern aircraft propeller.

While aircraft wings are primarily designed to maximize lift (the force keeping the aircraft aloft), the primary functions of aircraft propellers are to maximize lift and thrust (the force moving the aircraft forward). The propeller is driven by the aircraft’s engine and is frequently connected directly to the crankshaft. The amount of lift and thrust provided by a propeller is based on the pitch of the propeller blades, the rotation speed, the blade shape, and the number and placement of blades. The efficiency of a propeller is measured by the amount of thrust it generates multiplied by the axial speed (the speed of forward movement), divided by the resistance of the propeller multiplied by the speed at which the propeller rotates.

Blade pitch is vital to a propeller’s ability to generate lift and thrust. If a blade is pitched parallel to the direction of airflow, it is in the «feathered» position. When feathered, the propeller is generating the maximum amount of lift as each blade is pushing air downward (increasing air pressure below the blade), but this position also creates little to no thrust. In contrast, if the blades are pitched nearly perpendicular to the direction of airflow, they will push air back and generate a maximum amount of thrust while generating little to no lift. (This degree of blade pitch is found most commonly in marine propellers, whose main goal is thrust.) Based on the type and speed of the aircraft, expected weight, and other considerations, blade pitch varies to maximize lift and thrust.

The speed of rotation is also critical to generating a sufficient amount of lift and thrust. At the most basic level, a still propeller generates no lift or thrust because it is not creating any airflow over the blades. Faster moving blades move more air, pushing the aircraft forward. However, because the blade tips move faster than the blade bases, the blade tips reach sonic speeds sooner than the rest of the aircraft. At sonic speeds, propellers tend to exhibit sharp decreases in efficiency as resistance sharply increases. Thus, while a critical speed of rotation must be reached to generate sufficient lift and thrust for an aircraft, traditional propellers can rarely approach the speed of sound.

Along with blade pitch and rotation speed, the shape of the blade is instrumental in gaining maximum efficiency from a propeller. As the Wright Brothers discovered, blades shaped like aircraft wings would operate under the same principles of flight, and would thus generate the greatest amount of lift. However, unlike a wing, in which the angle of attack is relatively similar along all points, the angle of attack for a propeller differs at all points along the blade. For this reason they devised the twisted shape of a blade, which greatly improved efficiency. Blades are shaped to maximize efficiency of airflow at each point.

Lastly, the number and placement of propeller blades is essential to understanding how propellers generate thrust and lift. In the abstract, more blades would equal more force, which would generate greater lift and thrust. However, in reality the movement of each blade and the resultant airflow impacts the movement and airflow of every other blade in the propeller. Thus, airplanes and helicopters differ in the number of blades based on aircraft design. Use of more blades can divide the energy required to generate lift and thrust, thus reducing the burden on any individual blade, but the resultant airflow of more blades encountering drag resistance can decrease efficiency.

Though the engineering of an efficient propeller is a highly specialized task today, the Wright Brothers’ realization, that propellers can act like aircraft wings, is a fairly simple concept: using a high speed of rotation, propellers spin carefully shaped blades in a way that pushes air below and behind the aircraft, generating lift and thrust.

The principle behind a propeller is just like the forces that work in an airfoil. The plane undergoes different aerodynamic forces that directly act on it including lift, drag, weight and thrust. Thrusting is mainly provided by the propeller’s action that is also affected by the same aerodynamic forces. The efficiency of propellers can be obtained by making sure that it is positioned in its optimal blade angle of attack.

Propellers can be affected by the blade angle or the pitch angle such that at low pitch, the number of revolutions per minute is increased, thus would be beneficial during takeoff. Propeller design also affect its efficiency that is why a twist in the blade is usually created to provide a more balanced angle of attack or optimal lift to drag ratio on each blade assembly.

PROPELLER’S THRUSTING MOTION

One general function of propellers is that it can either slow down or increase thrusts of an airplane through the change in the angle of pitch of the blades. As air particles hit the propellers, they will be accelerated and will be pushed opposite the direction of flight. This in turn would induce a force to create forward movement of the plane, thus, thrusting occurs.

In aircrafts, thrust would be dependent on the volume and density of air that is accelerated as it comes in contact with the propellers at a given time.

The functions of propellers would also depend on their types. Fixed pitch propellers, for example, have only one pitch settings. They are widely used in early aircrafts and are usually composed of two blades which can be made from wood or metal. The metal propellers, which was intended in the military aircrafts, is now widely used in aircraft propeller construction since it can be made thinner with the use of high end materials with optimum mechanical properties such as aluminum alloys.

However, variable pitch propellers, which can either be adjustable or controllable, would work depending on the need for a situation such that a pilot has control on them. These types of propellers will be discussed below.

Ground adjustable pitch propellers, as the name implies, can be manipulated only before flight when the aircraft is still in the ground. This type has a split hub and the advantage in using this type is that it can be adjusted depending on the field of flight, altitude as well as airplane characteristics.

Two-position propellers work even in flight unlike the ground adjustable ones. The pilot can change the

A propeller is a marvel of engineering that, with the exception of the invention of the internal combustion engine, is probably the most important breakthrough in allowing us to accomplish powered flight. Yet a propeller is nothing more than wings rotating around a hub. If you look at the cross section of a propeller, you will notice it is shaped like a wing. On a wing the bottom side is flat, and the top is curved with the trailing edge tapered thin and the leading edge thicker. As a wing moves forward the flow of air is divided into the air that flows over the wing, and the air that flows under. The top side of a wing is curved, thus the air traveling over the top of the wing must travel farther to reach the rear of the wing. As the airflow over the top is traveling farther than the bottom airflow in the same amount of time, it must be traveling faster.

Bernoulli’s Principle says that a rise in speed of a fluid, in our case air, occurs simultaneously with a drop in pressure, causing a low pressure in the fast moving air on the top of the wing. With a lower pressure on the top relative to the bottom, lift is created and the wing rises to try to equalize the pressure difference.

Now take two or more wings and attach them to a hub, rotate the hub, and you have a propeller. The rear side of a propeller is flat, the front surface curved. As the propeller rotates, the airflow over the front of the blade travels further relative to the airflow over the back, creating a low pressure in front of the propeller, and Bernoulli’s Principle pulls the aircraft ahead.

A propeller also has some twist built into it, due to the fact that the tips of a propeller are traveling much faster than near the hub. The formula c=pi*d, or the circumference equals pi times the diameter. Because the diameter at the tips is double the diameter halfway out, so is the distance it must travel, and correspondingly its speed.

You will notice while looking at propeller tips from the side, the blades are nearly straight up and down, because of the high speed of the tips need less of a bite on the oncoming air to generate thrust. Closer to the hub more of a bite is required to generate thrust due to a reduced velocity, so the blade is gradually twisted to achieve the thrust more evenly over the length of the blade.

I’m sure many have heard the loud crack of a Harvard’s propeller at an air show. When this happens the propeller tips are actually traveling supersonic due to blade length, and what you are hearing is sonic booms. On higher performance aircraft, designers will add more and shorter blades to a propeller rather than longer blades, to utilize the engine power, as supersonic blade tips are not only noisy but lose their thrusting efficiency.

Propellers are simply twisted wings, attached to a rotating hub, simple in concept but highly specialized to most efficiently turn engine power into thrust.

How aircraft propellers work

Aircraft propellers work on the same principle as the aerodynamic lift created by air passing over a wing. The cross section of a wing will show that the upper surface creates a drop in air pressure allowing air to pass quicker on the upper side as compared to the lower side. Thus causing lift to occur.

If you take a propeller and cut a cross section from it, you will find it is similar in shape to a cross section cut from a wing. This lift that occurs is translated into thrust as the direction of movement is horizontal rather then the vertical.

On smaller airplanes the pitch of the propeller may be fixed, or non-adjustable. On larger airplanes the pitch, or angle of attack, of the propeller can be adjusted to determine the most effective bite into the air. A blade that is in a neutral position is called a «feathered prop» indicating it’s in a position that provides no thrust. If the engine driving a propeller is damaged, a feathered prop will prevent the propeller from turning the engine by force of the air passing through it. The engine can be shut down and the propeller will remain motionless.

Adjustable pitch propellers are useful for stopping an airplane. The propellers can be placed in «Reverse» pitch which will change the flow of air, or thrust from rearward to forward. This in effect acts like brakes, slowing and actually stopping the airplane as it lands. This maneuver takes place after the airplane has touched down and is rolling down the runway. It can also be used for maneuvering around the airport.

Helicopter blades also follow the same theory of lift, and as they are considered a vertical flight it’s referred to as lift rather then thrust. Their propeller is tilted which in turns provides the thrust for horizontal movement.

Aircraft such as the V-22 Ospery the propeller transitions from lift to thrust as it moves from vertical lift to forward thrust when the propellers change position.

If you were able to skewer an airplane on a vertical pole, and spin the whole aircraft fast enough, the plane would lift vertically. The lift from the wings of the airplane would create similar lift as that produced by a propeller.

In jet passenger aircraft the thrust is developed by the jet engines, without the lift created by the wings they would be a very inefficient flying machine.

With jet fighter planes, the overwhelming quantity of thrust compensates for the general lack of lift created by the wings. Aerodynamic lift is limited in this instance to allow for speed and strength in maneuvering. Lack of lift is the reason they glide like a rock after engine failure.

The more lift created in a wing will allow it to glide further. Gliders have no engine and rely completely on lift generated by the wing to keep them aloft.

The rotors of a helicopter will generate enough lift that when allowed to free wheel, (auto-rotate) the decent can be controlled well enough to land the craft. The faster the blades spin, the slower the decent. Changing the pitch of the blade will allow the helicopter to flare or slow before landing to make a safe touch down.

When all is considered a propeller and a rotating wing are the same thing. Propellers are constructed to withstand the stress of high speed revolutions. If a wing was constructed in a similar way, it would create the same lift as a propeller. As is the case with helicopters, basically a wing used as a propeller.

We all know the Wright brothers built the first successful airplane. But their advances in propeller design are less known. They realized a propeller is really just a rotating airfoil (wing). By applying knowledge gained from their wind tunnel testing on wings, they greatly improved propeller design. Incredibly, their first hand carved propellers were 95% as efficient as the best designs we have today!

Propellers produce thrust in precisely the same way a wing produces lift. A cross section of a propeller is indistinguishable from a wing. They often have a slightly longer upper surface known as camber but this is not essential.

Bernoulli’s principle describes an increase in speed as a fluid (air) passes through a constriction in a tube. With this acceleration comes a drop in pressure. Propellers and wings make use of this pressure differential to generate thrust and lift. As air accelerates over the top of the propeller blade it’s pressure becomes lower than the airflow beneath. The shape and camber of the blade is not as important as commonly believed.

The angle of attack of the propeller blades is the prime influence to thrust generation, much more than shape does. This is where Newton’s laws come into play. The propeller blades act on the mass, air, which is accelerated rearward. The equal and opposite reaction generates thrust to pull the aircraft forward. Small aircraft must strike the best compromise between low and high angles. Where weight and complexity are less critical, variable pitch propellers are used. Multiple engined aircraft can «feather» the blades when an engine fails. This decreases the angle of attack to reduce drag as much as possible. Complex aircraft can reverse the pitch to produce reverse thrust and shorten landing runs. They can also vary thrust to aid in steering, especially handy for float planes.

As engine rpm increases, the higher propeller speed generates more thrust. Some aircraft have a gearbox, which can adjust propeller speed. Float plane pilots must be careful of blade speed on the water. A blade damaged by excessive speed on the water looks like it was damaged by coarse sand blasting. Blade speed increases with the distance from the hub. The «twist» in the blades are required so the amount of thrust generation will be the same all the way out to the tip. Because the speed nearest the hub is much slower, the angle of attack can be much higher. The higher angles provide more «bite» in the air.

Aircraft speed and blade speed are the limiting factors in propeller designs. A propeller is at maximum speed when the blade tips approach the speed of sound. Although propellers are more efficient than turbojets, drag, inefficiency and noise increase dramatically if the tips go supersonic. For this reason, propeller aircraft are limited to speeds below Mach 0.7

Propellers are vulnerable to icing just as wings are. To combat icing, blades have heated leading edges. They also share a similar system that sprays glycol antifreeze onto the blades.

The spinning action of propellers impart asymmetrical torque onto the aircraft. Some aircraft use two contra rotating propellers per engine. This zeroes out the torque and is more efficient as the blade tip vortexes cancel each other out. Cost and complexity limit these to military aircraft.

Jet engines seemed destined to replace the propeller driven aircraft. The economy of piston engines and the advantages of gas turbine engines guarantee propellers are not a thing of the past. The trend to replace them with jets has actually reversed. Increased efficiency and decreased noise have allowed aircraft such as the Bombardier Q400 to compete head on with similar jet powered aircraft.

What goes up must eventually come down, and without a propeller it might be sooner than you think. Actually the propeller is one of many different types of mechanics used in order to perpetuate flight in aircraft today. The propeller itself acts much like a household fan operates, converting the rotational energy into a kind of motion called «thrust». So how and what makes a propeller work to generate flight? Actually to understand the science behind the propeller one would have to take into consideration a variable degree of mathematical equations. However, to simplify the whole process I will try to break down the components of how propellers work.

Nuts and Bolts of the propeller:

-Propeller blades-
The propeller blades can come in various shapes and sizes, ranging from single double, and multiple blade types. They essentially require a motor to cause the rotation, and then air which moves through the blades causing «forward thrust». The cross-section of a majority of propeller blades is slightly or severely angled, which creates an airfoil type of shape. This angle is also known as the «pitch» or «angle of attack», which can determine the amount of thrust or the torque that can cause more or less directed airflow. The curvature in the propeller blades acts much like a corkscrew pulling inward, which is why the angle of a blade changes along the length, becoming greater toward the hub, where the air has less thrust.

Another more complex blade system known as an «adjustable pitch» can be found upon larger aircraft, and even inverted propeller systems such as in helicopters. This type of blade system can have three or more elaborate blades that pivot or adjust the angle of attack, which can be used by the pilot during flight.

-Motors and the mechanics-

Like all machines used today there has to be a mechanical component to deliver the rotation to the propeller blades. Most planes require a degree of horsepower to provide sufficient rotation, which is what drives almost any aircraft. Common motor companies include Bombardier, Rollsroyce, Bentley, De Havilland, are just a few among the many manufactures that design plane engines. They come in varying types of designs that I will list below.

* Opposed Engines have two banks of cylinders that are opposite of each other. The crankshaft is located around the center and both cylinder banks power it. The motor itself can be either liquid or air cooled, but liquid cooled engines are most common among aviation.

* In-line engine types are quite common in aviation. This engine type has cylinders that are lined up in one row. Although it is less common to have an even number of cylinders such as 4 cylinders or 8 cylinders, there are some that have had three or five cylinder engines. This engine also can be both liquid and air cooled, but since the crankshaft is located above the cylinders the motor is mounted in an inverted fashion.

* Radial engines were quite common with older planes and single prop aircraft. The radial engine has a row of cylinder heads that are arranged in a circle around the base of the crankshaft, which sits in the center of the engine.

* V-type engines can be liquid or air-cooled and they have two in-line banks of cylinders that sit apart from each other at an angle. This radial angle can be anywhere from 30-60 degrees, but still forms a notable V formation. Some of the most famous motors used in airplanes include this specific design.

-Physics and all that Science mumbo jumbo-

If you introduce a flat or rounded blade and introduce a circular motion, you will be generating a field of pressure. This may be felt as a breeze or displacement of air, which is the basic principle of a propeller. Propellers from the earliest examples of air flight by the legendary Wright brothers, to modern day aircraft, have utilized a specific design called the airfoil shape. Using steel, wood or other material, a propeller must be spun at a very high rate of speed, which creates a surmountable amount of pressure across the propeller blades. The variance of pressure upon the blades is what causes a vacuous form of wind pressure, which as it passes through the rotating blades it creates forward thrust.

Now thrust is only a component in which aircraft are dependent upon, because this continuous thrust also now requires lift. The thrust or velocity that is created by a propeller will keep an aircraft moving forward or upward, but it still requires lift to hold the aircraft aloft in the air. There are four major forces that play for or against flight.

) Lift is the upward force that is generated when air pushes across the wings that act like an airfoil.
2.) Drag is the opposing force, which is caused by air resistance, which actually slows a plane down. Most airplane designs are built to aerodynamically combat the drag effect, but it still requires acceleration to prevent loss of altitude.) Gravity is the one of the most commonly known forces upon earth. It holds us firmly to the


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