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Propellers

A Propeller “Lifts” an Airplane Forward

Think of a propeller as a spinning wing. Like a wing, it produces lift, but in a forward direction—a force we refer to as thrust. Its rotary motion through the air creates a difference in air pressure between the front and back surfaces of its blades. In order for a propeller blade to spin, it usually needs the help of an engine.

A propeller is a wing with a twist.  In cross section, a propeller is shaped like a wing to produce higher air pressure on one surface and lower air pressure on the other. This is the Excalibur III, a North American P-51C Mustang, on display at the National Air and Space Museum’s Steven F. Udvar-Hazy Center.
Credit: National Air and Space Museum, Smithsonian Institution, Archives Reference No.: 2004-40594

A Propeller Is a Wing with a Twist

In cross section, a propeller is shaped like a wing to produce higher air pressure on one surface and lower air pressure on the other.

Propellers and Pitch

Angle of attack is the angle a wing makes with the oncoming airflow. Pitch angle is the angle a propeller blade makes with its plane of rotation. A wing has nearly the same angle of attack across its entire length. But a propeller blade has a twist, so its pitch angle varies along its length.

On a controllable-pitch propeller, the pitch of the entire blade can be altered during flight to give the best performance at different air speeds. This is similar to changing gears with a car or bicycle.

A ship’s propellers create thrust in water in much the same way an airplane’s propellers create thrust in air.

In order for a propeller blade to spin, it usually needs the help of an engine.

Related Images

Spirit of St. Louis

Boeing 247-D

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Q:

Why do propellers have two, three, or four blades?

A:

To handle the power supplied to the propeller from the engine. For small engines with relatively small horsepower, only two blades are necessary (i.e. small airplanes, and most airplanes through the middle 1930s, with engine power less than 500 horsepower). For larger engines with more horsepower, there must be more blades to maintain a reasonable power distribution to each blade (i.e. ... more

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Thrust: Propellers

Learn how propellers generate thrust. 

Pop Quiz

Which of the following objects are similar to an airplane propeller?

A) a boat prop
B) a ceiling fan
C) a windmill
D) all of the above

All of the above.  A boat’s prop (propeller or screw), ceiling fan, and windmill all work in much the same way an airplane’s propeller.  All three create a pressure difference between the front side and back side of the blades.