Propellers

A propeller is a type of fan that transmits power by converting rotational motion into thrust. A pressure difference is produced between the forward and rear surfaces of the airfoil-shaped blade, and a fluid (such as air or water) is accelerated behind the blade. Propeller dynamics, like those of aircraft wings, can be modelled by either or both Bernoulli's principle and Newton's third law. A marine propeller of this type is sometimes colloquially known as a screw propeller or screw, however there is a different class of propellers known ascycloidal propellers – they are characterized by the higher propulsive efficiency averaging 0.72 compared to the screw propeller's average of 0.6 and the ability to throw thrust in any direction at any time. Their disadvantages are higher mechanical complexity and higher cost.

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.