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How Aircraft Propellers Work



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Aircraft propellers can be analogized as the soul of an aircraft, as without it an aircraft will just be another piece of metal. Simply put, an aircraft needs a propeller to fly.

A propeller is really a type of fan (literally shaped like one too!), which transmits power by converting rotational motion into thrust for propulsion of a vehicle as in an aircraft.

Physics principles behind propellers

The force produced by the blades of a propeller is through the application of both Bernoulli's principle and Newton's third law.

Bernoulli's principle states that for an inviscid flow, an increase in speed of the fluid occurs simultaneously with a decrease in pressure or a decrease in the fluid's gravitational potential energy.

Newton's third law states that to every action, there is an equal and opposite reaction.

Combining the two principles mentioned above, a propeller is powered with through a fluid such as water or air, by rotating two or more twisted blades about a central shaft. The blades of a propeller act as rotating wings, and produce force by generating a difference in pressure between the forward and rear surfaces of the airfoil-shared blades and by accelerating a mass of air rearward.

A propeller's design and its efficiency.
A propeller's efficiency is very much dependent on its design. As such, varying the following factors will change the propeller's efficiency:
the helix angle, the angle between the resultant relative velocity, the blade rotation direction and blade pitch.

Small pitches and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect.
Sound control systems are also required for accurate matching of pitch to flight speed and engine speed. In some aircraft, the pilot can manually override the constant speed mechanism to reverse the blade pitch angle, and thus the thrust of the engine. This is used to help slow the plane down after landing in order to save wear on the brakes and tires, but in some cases also allows the aircraft to back up on its own.

Another part of its design tied with a propeller's performance is the number and the shape of the blades used. Increasing the aspect ratio of the blades reduces drag but the amount of thrust produced depends on blade area, so using high aspect blades can lead to the need for a propeller diameter which is unusable. A further balance is that using a smaller number of blades reduces interference effects between the blades, but to have sufficient blade area to transmit the available power within a set diameter means a compromise is needed. Increasing the number of blades also decreases the amount of work each blade is required to perform.

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