Structural Characteristics of Propeller Blades
The modern propeller traces its origins to the screw. Archimedes was credited to have first devised an early version of the screw propeller with his screw pump, also known as the "Archimedes screw." It was used to efficiently draw water up. There are a number of inventors who are said to have invented the modern propeller. Today, the propeller has many applications, such as propelling planes, boats and helicopters. Although these types have distinct designs and functions, their structural characteristics follow a similar design.
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Basic Parts of the Propeller
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A propeller is made up of a central hub with two or more blades connected to it. The hub is connected to the crankshaft that turns the propeller. The blades are designed similarly to the wings of an airplane. A blade starts with the root, which connects to the hub; the rounded or cambered side is the leading edge, while the tapering side is the trailing edge. The other end of the propeller is the "tip." Looking at the cross-section of the blade, the flat side is the face and the cambered side is the back. This design remains integral in the generation of thrust.
Thrust Generation
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Thrust is the force generated by a spinning propeller and is parallel to the direction of advance. When a propeller spins, it forces air backward. By virtue of Newton's Third Law of Motion, the air that is forced backward pushes the propellers forward. The blades are able to do so because of their design. When moving through the air during rotation, air flows slower on the cambered side while air moves faster on the flat side. The difference in air pressure on both sides of the blade generates thrust.
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Blade Angle
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The blade angle is formed between the plane of rotation and the blade face. To generate thrust, the sections of the blade from the root to the tip are angled differently. This results in a twisted appearance of the blade. This is to ensure that each blade element hits the relative wind at the same angle of attack. This also makes each blade element advance at an equivalent pace and prevents bending.
Types of Propellers
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Propellers can be classified in terms of pitch. Pitch refers to the turning of the blades on the hub to change their angle of attack. Early propellers were fixed pitch; however, today there are several propeller blades that have different types of pitch.
A ground-adjustable pitch allows the blades to be adjusted while the plane is on the ground and not turning. On the other hand, a controllable pitch allowed the pilot to change the angle of attack of the blades while in flight. A full-feathering propeller's blades can be turned in such a way that it is positioned in the line of flight. This is done to reduce air drag when it is necessary to shut the engine in flight or during engine failure. Lastly, a reversing pitch propeller can have negative pitch to produce a reverse thrust.
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References
- Photo Credit propeller image by Artem Lapenko from Fotolia.com
