As opposed to jet airplanes, which are propelled via jet engines, many aircraft make use of propellers, which slice through the air to create thrust. Within an air mass, airplane propellers move in a motion that is similar to that of a corkscrew – what is called a helical motion.
Aircraft such as the ATR 72, Tupolev Tu-95, and Bombardier Dash 8 use a system that incorporates propellers powered by engines that are equipped with turbochargers (i.e exhaust gas-driven superchargers), and are hence often referred to as “turboprops.” Other aircraft, such as the common single-engine Cessna – 172 use a simple propeller assembly. In this case, the airplane propeller is mounted onto a crankshaft connected to the engine.
Function of Airplane Propellers
Coordinated balance between the four forces acting on an airplane; lift, weight, thrust and drag, enable an aircraft to fly. Propellers are primarily concerned with the force of thrust – engines deliver their energy output through airplane propellers.
The internal combustion process in the piston engine of aircraft generates power and develops force. In simpler words, the airplane engine provides the propeller with power; the propeller uses this power to create thrust.
How do Airplane Propellers Work?
To generate thrust, airplane propellers must rotate. Power delivered from the engine helps in achieving this rotation. The engine, a crankshaft, and the propeller, make up the whole thrust-generating assembly. These three work in precise coordination to create thrust as follows.
- The airplane engine uses the fuel energy to create mechanical motion of the pistons, moving up and down repeatedly.
- This mechanical input impacts the engine crankshaft, as a result of which the crankshaft rotates.
- Connected to the engine crankshaft is the airplane propeller (directly, in most general aviation aircraft). This rotational energy of the crankshaft is then transferred to the propeller and the propeller begins to rotate.
Airplane propellers, due to this rotation, are able to produce a propelling force termed as thrust. The connected engine-propeller assembly can be viewed in the following video of a C 130 propeller assembly.
How do Propellers Produce Thrust?
Blades of an airplane propeller are designed in a similar fashion to the wings of an aircraft. The wings are often referred to as “aerofoils,” as are the propeller blades. Aerofoils are surfaces designed especially to manipulate the airflow around them to generate a directional force.
Just as wings produce the force of lift (perpendicular to the wing) to lift the airplane through the air, airplane propellers produce a force of thrust (perpendicular to the propeller blades) to propel the aircraft through the air. Airplane propellers produce thrust in the following step-by step manner.
- The rotating propeller of the aircraft is cambered like a typical aerofoil.
- This camber affects the airflow in front of the propeller by speeding it up.
- The high velocity of the air results in a lower static pressure in front of the propeller (Bernoulli’s principle).
The developed low pressure creates a forward force of thrust: as explained in Aeroplane General Knowledge and Aerodynamics, published by Aviation Theory Centre: “As the propeller blade rotates through the air, the acceleration of the airflow over the front cambered surface of the blade causes a reduced static pressure ahead of the blade (Bernoulli). The result is a forward thrust force on the propeller blade which pulls the aeroplane along.”
In addition, in the words of NASA, “A spinning propeller sets up a pressure lower than free stream in front of the propeller and higher than free stream behind the propeller. Downstream of the disk the pressure eventually returns to free stream conditions. But at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli’s equation to the air in front of the propeller and to the air behind the propeller.”
Thrust & Acceleration
Thrust is required to accelerate an airplane in a horizontal manner, and drag is produced as a reaction force, resisting the aircraft’s forward motion.
To accelerate, the thrust produced by airplane propellers must necessarily be greater than the drag produced as a consequence. If these two were to be equal, the airplane would be in a state of equilibrium, with no forward motion.
Federal Aviation Administration, Flight standards Service. Airplane Flying Handbook. (2004).
Federal Aviation Administration, Flight standards Service. Pilot’s Handbook of Aeronautical Knowledge. (2008).
Federal Aviation Administration. FAA Regulations. Accessed on 11th November, 2011.
Trevor, T. Aeroplane General Knowledge and Aerodynamics. Aviation Theory Centre. (2004).
Lund University School of Aviation and Nordic Aviation Resources. Performance. (2001).
Editorial Note: Updated April 2, 2012, for clarity.