propellers Propellers: Theories, Principles, and Functionality

[FlashbangM17 757]

http://youtu.be/RzlK1SwM_dA

 

Propeller Principles

 

Almost all early aircraft designs used propellers to create thrust. at the end of the 19th century, many unusual and innovative propeller designs included simple fabric-covered wooden paddles and elaborate multiblade wire-braced designs. As aeronautical science developed, propeller designs evolved from flat boards that pushed air backward to airfoils that produced lift to pull aircraft forward.

 

Propeller design developed with new materials that made possible thinner airfoil sections with greater strength. because of structural strength, aluminium alloys have been used widely as a structural material in the majority of aircraft propellers. However, several varieties of propellers still in service are constructed out of wood.

 

Nomenclature

 

Familiarity with some basic terms and component names is critical to understand the principles of how a propeller produces thrust. All modern propellers consist of at least two propellers connected to a central hub. The portion of a propeller blade that is nearest to the hub is referred to as the blade shank, and the portion furthest from the hub is called the blade tip. The propeller hub, or hub assembly, is machined to permit mounting onto an engine crankshaft or a reduction gear assembly.

 

 

Each blade of a propeller functions as a rotating wing that produces lift to move an aircraft through the air. As such, propeller blades share much of the same nomenclature as an aircraft wing. All propeller blades have a leading edge, a trailing edge, and a chord line. A chord line is an imaginary line drawn through an airfoil from the leading edge to the trailing edge. Some terms, however, are unique to propellers. The curved, or cambered, side of a propeller blade is called the blade back and the flat side is called the blade face. A propeller's blade angle is the acute angle formed by a propeller's  plane of rotation and the blade's chord line. A propeller's plane of rotation is always perpendicular to the engine crankshaft.

 

 

Propellers with adjustable blade angles use removable blades that are secured to a hub assembly by a set of clamping rings. Each blade root has a flanged butt, or shoulder, which mates with grooves in the hub assembly. The blade shank on this type of blade is typically round and extends to at least the end of the hub assembly; however, in some cases, the shank can extend beyond the hub assembly and into the airstream. When this is the case, blade cuffs might be installed to improve air flow around the blade shank. A blade cuff is an airfoil-shaped attachment made of thin sheets of metal, plastic or composite material. Blade cuffs increase the flow of cooling air to the engine nacelle. Mechanical clamping devices and adhesive bonding agents attach the cuffs to the blades.

 

 

Propeller Theory

 

A propeller rotating through the air creates an area of low pressure in front of the blade. This low-pressure area, combined with an area of high pressure behind the blade, enables a propeller to produce thrust.  The amount of thrust produced depends on several factors including the propeller blade's angle of attack, speed, and airfoil shape. The angle of attack of a propeller blade is the angle formed by the chord line of the blade and the relative wind. The direction of the relative wind is determined by the speed of the aircraft and the rotational movement of the propeller.

 

 

When the aircraft begins moving forward, the relative wind direction shifts because, in addition to rotating, the propeller now has forward motion.The result is that the relative wind is much closer to the angle of attack. In this case, the angle of attack will always be less than the blade angle.

 

 

Based on the effect that forward motion has on the relative wind of a propeller blade, the faster an aircraft moves, the smaller the angle of attack on the propeller blade. However, if the propeller speed increases, the trailing edge of the propeller the propeller travels a greater distance for the same amount of forward movement. As propeller speed increases, the relative winds strikes the propeller blade at a greater angle and the angle of attack increases.

 

 

The most effective angle of attack for a propeller blade is between 2 and 4 degrees. Any angle of attack exceeding 15 degrees is ineffective because a propeller blade might stall. Typically, propellers with a fixed blade angle are designed to produce an angle of attack between 2 and 4 degrees at either a climb or cruise airspeed with a specific speed setting.

 

Unlike a wing, which moves through the air at a uniform rate, the propeller sections near the tip rotate at a greater velocity that those near the hub. The difference in rotational velocity along a propeller blade segment can be found by first calculating  the circumference of the arc traveled by a point on that segment The circumference of a circle is calculated with the formula: 

 

2πr

 

The circumference is then multiplied by engine speed in r.p.m. to find rotational velocity. For example, to determine blade velocity at a point 18 inches from the hub that is rotating at 1800 r.p.m., use the following formula:

 

Velocity = 2πr x r.p.m.

 

= 2 x π x 18 x 1800

 

= 203,575

 

At this point 18 inches from the hub the blade travels 203,575 inches per minute. To covert this to miles per hour, divide 203,575 by 63,360(the number of inches in 1 mile) and multiply the product by 60, the number of minutes in 1 hour

 

Velocity = (203,575 / 63,360) x 60

 

= 192.7 miles per hour

 

CONTINUED AT NEXT POST

 

 

Here is a rare treat for you this is a U.S. Navy Beechcraft UC-45J "Navigator" it has 2 P&W R-985-AN-14B "Wasp Junior" engines each producing 450hp using Hamilton Standard adjustable-pitch propellers and I removed one of its propellers to show you the basic insides of an adjustable-pitch propeller.  8)s

Civilian model of this aircraft is known as a Beechcraft Model 18 "Twin Beech".

 

 

 

 

 

fun fact about modern turbine engines. a high-pressure turbine blade on your standard commercial jet engine costs from $20000 in older engines to $50000 each in the newer ones and there are hundreds of these in a jet engine

 

this picture is of a modern turbine blade and is up to scale

 

Edited November 3, 2013 by Flashbang

[FlashbangM17 757]

To compensate for the difference in velocity along a propeller blade, the blade angle changes along its length. The gradual decrease in blade angle from the hub to the tip is called pitch distribution, or twist. Blade twist enables a propeller to provide a fairly constant angle of attack along most of the length of the blade.

 

In addition to blade twist, most propellers have a thicker, low speed airfoil near the blade hub and a thinner, high speed airfoil near the tip. This, along with blade twist, enables the propeller to produce a relatively constant amount of thrust along the entire length of the blade.

 

 

Forces Acting On A Propeller

 

A rotating propeller is subjected to many forces that cause tension, twisting, and bending stresses. Of all the forces acting on a propeller, centrifugal causes the greatest stress. Centrifugal force can be described as the force tending to pull the blades out of the hub. The amount of stress created by centrifugal force can be more than 7,500 times the weight of the propeller blade.

 

 

Thrust bending force tanks to bend the propeller blades forward at the tips. Because propeller blades are typically thinner near the tip, thrust produced at the tip tends to flex the blade forward. Thrust bending force opposes centrifugal force to some degree. 

 

 

Torque bending forces occur as air resistance opposes the rotational motion of the propeller blades. This force tends to bend the blades the opposite of the direction of rotation.

 

 

Aerodynamic twisting force tends to increase a propeller's blade angle. When a propeller blade produces thrust, the majority of the thrust is exerted ahead of the blades's axis of rotation. In some cases, aerodynamic twisting force is used to help change the blade angle on a propeller.

 

 

Centrifugal twisting force opposes aerodynamic twisting force. when a propeller rotates, centrifugal force tends to align the propeller's  center of mass with its center of rotation. A propeller's center of mass is typically ahead of its center of rotation; therefore, when a propeller rotates, centrifugal force tends to decrease its blade angle. At operational speeds, centrifugal twisting force is greater than aerodynamic twisting force and is used in some propeller designs to decrease the blade angle.

 

 

Here is a video of a B17 where you can clearly notice the effects of the forces on a propeller that I discussed.

 

http://youtu.be/zm2hMwPdhY0

 

At 4:45 you can already notice torque and thrust bending effects on the propeller even while the engine is idling. 

 

A final force exerted on a spinning propeller is blade vibration. When a propeller produces thrust, blade vibration occurs due to aerodynamic and mechanical forces. These include aerodynamic forces that tend to bend the blades forward at the tips as well as the mechanical power pulses in a reciprocating engine. Of the two, mechanical vibrations are considered to be more destructive than aerodynamic vibrations. The reason for this is that engine power pulses tend to create standing wave patterns in a propeller blades that can lead to material  fatigue and structural failure.

 

The location and number of stress points in a blade depend on the characteristics of the individual combination of propeller and engine. Although concentrations of vibrational stress are detrimental at any point on a blade, the most critical location is the outboard six inches to the blade tips.

 

The design of most airframe-engine-propeller combinations has eliminated the detrimental effects of vibrational stresses. Nevertheless, some engine and propeller combinations do have a critical range in which severe propeller vibration can occur. In this case, critical range is indicated on the tachometer (cockpit instrument that shows engine r.p.m.) by a red arc. Engine operation in the critical range should be limited to a brief passage from one speed setting to another. Engine operation in the critical range for extended periods can lead to structural failure of the propeller or aircraft.

 

Propeller design typically accommodates some degree of vibrational stress. However, in situations in which a propeller has been improperly altered, vibration can cause excessive flexing and work hardening of the metal. In severe instances, the damage can cause sections of a propeller blade to break off in flight.

 

Here is a video on how you would check a propeller to prevent blade vibration through a method known as "Propeller Tracking"

 

http://bcove.me/btz1aa8y

 

Propeller Pitch

 

In the strictest sense, propeller pitch is the theoretical distance a propeller advances longitudinally in one revolution. Although pitch and blade angle describe two different concepts, they are closely related, and the two terms are often used interchangeably. For example, where a propeller is said to have fixed pitch, what is actually meant is that the blades on the propeller are set at a fixed blade angle.

 

A propeller's geometric pitch is defined as the distance, in inches, that a propeller would move forward in one rotation in it were moving though a solid medium and did not encounter any loss in efficiency. Measurement of geometric pitch is based on the propeller blade angle at a point equal to 75 percent of the blade length from the propeller hub.

 

When traveling through air, inefficiencies prevent a propeller from moving forward at a equal rate to its geometric pitch. Therefore, effective pitch is the actual amount a propeller moves forward in one revolution. Effective pitch varies from zero when the aircraft is stationary on the ground to about 90 percent of the geometric pitch in the most efficient flight conditions. The difference from geometric pitch and effective pitch is called slip. Propeller slip represents the total losses caused by inefficiencies 

 

 

If a propeller has a geometric pitch of 50 inches, in theory it should move forward 50 in one revolution. However, if the aircraft actually moves forward only 35 inches in one revolution, the effective pitch is 35 inches and the propeller is 70 percent efficient. In this case, slip represents 15 inches or 30 percent loss of efficiency. In practice, most propellers are 75 to 85 percent efficient.

 

Propeller Classifications

 

​Propellers are typically classified according to their position on the aircraft. For example, tractor propellers are on the front of an engine and pull the aircraft. Pusher propellers are on the aft end if an engine and push the airplane thought the air. Most aircraft engines are equipped a tractor type propellers; however, several seaplanes and a few other aircraft are equipped with pusher propellers.A major advantage of tractor type the propeller is that it experiences lower stresses because it rotates in relatively undisturbed air. 

 

Propellers are commonly classified by the method used to establish pitch. Typical classifications include fixed-pitch ground adjustable, controllable-pitch, constant speed, reversible, and feathering.

 

The simplest type of propeller is a fixed-pitch propeller. Fixed-pitch propellers enable an aircraft to produce optimum efficiency at a specific rotational and forward speed. A fixed-pitch propeller with a low blade angle, often called a climb propeller, provides the best performance for for takeoff and climb. A fixed-pitch propeller with a high blade angle, often called a cruise propeller, is adapted for high speed cruise and high altitude flight.

 

Ground-adjustable propellers are similar to fixed-pitch propellers in that their blade angles cannot be changed in flight. However, the propeller is constructed to permit the blade angle to be changed on the ground. 

 

Here is a video on the basic steps of adjusting a ground-adjustable propeller.

 

http://bcove.me/rcchgwz0

 

Controllable-pitch propellers have an advantage over ground adjustable propellers in that the operator can change blade angle while the propeller is rotating. This enables the operator to change propeller blade angle to achieve improved performance for a particular flight condition. depending on design the number of pitch position might be limited or the pitch might be adjustable to any angle between a minimum and maximum pitch setting.

 

Constant speed propeller, sometimes referred to as an automatic propeller, maintaining  the rotational speed selected by the pilot. Pitch control is provide by a device known as a governor. A typical governor uses oil pressure to control the blade pitch. Constant speed propeller systems provide maximum efficiency by letting the pilot set  the ideal propeller blade angle for most flight conditions

 

 

 

I've finally finished propeller principles and i will be moving on to fixed-pitch and then finally adjustable pitch but for now i need some sleep

 

stay tuned for future updates

 

 

 

Here is another fun fact there is such a thing as a single blade propeller

 

Edited November 2, 2013 by Flashbang

[XtreamerPT 156]

I could not imagine that a single blade prop would make a plane fly.... very good information  8)s

[FlashbangM17 757]

I could not imagine that a single blade prop would make a plane fly.... very good information  8)s

one big issue with a single blade prop is if you damage your blade you have to fix it as a small crack can grow quickly normally you will have to repair by buffing out the damaged area and smoothing it out but as you do this your taking material off of the propeller therefor making 1 side lighter that the other and creating a imbalance you with then remove material on the other blade to correct the imbalance but there is where you run into your issue you only have 1 blade and the other side is only a piece of metal that serves as a counter weight and this makes it really hard to get the correct balance back.

Edited November 2, 2013 by Flashbang

[FlashbangM17 757]

FIXED-PITCH PROPELLERS

 

save ing here

[ColorCopycat 7,171]

Quite the doorstopper, but a good read nonetheless. Is it true that in FRB and HB, you can mess with the pitch of your propellor for better performance provided you enable manual engine control?

[Heliosiah 4,718]

Quite the doorstopper, but a good read nonetheless. Is it true that in FRB and HB, you can mess with the pitch of your propellor for better performance provided you enable manual engine control?

If you set the bindings in the full controls tab, you can switch to manual and use them in the other controls, even while using mouse aim. And control the engine controls the same.

[TheRealSlamShidy 2,298]

 

 

Forces Acting On A Propeller

 

A rotating propeller is subjected to many forces that cause tension, twisting, and bending stresses. Of all the forces acting on a propeller, centrifugal causes the greatest stress. Centrifugal force can be described as the force tending to pull the blades out of the hub. The amount of stress created by centrifugal force can be more than 7,500 times the weight of the propeller blade.

 

 

Thrust bending force tanks to bend the propeller blades forward at the tips. Because propeller blades are typically thinner near the tip, thrust produced at the tip tends to flex the blade forward. Thrust bending force opposes centrifugal force to some degree. 

 

 

 

http://xkcd.com/123/ All I have to say lol ;D

 

Just kidding, very good and enjoyable read :) 

 

Edit: Explaining the joke ;O

 

http://www.explainxkcd.com/wiki/index.php?title=123:_Centrifugal_Force

Edited November 4, 2013 by Facehurt