An aircraft propeller is not just a spinning fan. Each blade is shaped like a small wing, called an airfoil, that moves through the air in a circular path. As the blades spin, they push air backward and create a forward force called thrust.
This matters because thrust is what helps many airplanes speed up, take off, climb, and keep moving through the sky.
A propeller blade is twisted from hub to tip because each part of the blade travels at a different speed. The tip moves much faster than the section near the hub, so the blade angle must change to keep the airflow efficient. Blade pitch controls how much air the propeller tries to move forward with each turn, while angle of attack controls how the blade meets the oncoming air.
Fixed-pitch propellers use one blade angle, while constant-speed propellers adjust their blade pitch to keep the engine and propeller working efficiently.
Key Facts
- A propeller blade is a rotating airfoil that creates thrust by accelerating air backward.
- Newton's third law explains thrust: air is pushed backward, so the aircraft is pushed forward.
- Thrust depends on air mass flow and velocity change: T = m_dot Δv.
- Blade pitch is the distance a propeller would move forward in one turn if it moved through a solid.
- Angle of attack is the angle between the blade chord line and the relative wind.
- Power relates to torque and angular speed: P = τω.
Vocabulary
- Thrust
- Thrust is the forward force that moves an aircraft through the air.
- Airfoil
- An airfoil is a curved shape designed to produce lift or thrust when air flows around it.
- Blade pitch
- Blade pitch is the angle or effective forward distance a propeller blade is set to move air during each rotation.
- Angle of attack
- Angle of attack is the angle between the blade's chord line and the relative airflow it meets.
- Constant-speed propeller
- A constant-speed propeller automatically changes blade pitch to keep a selected rotation rate.
Common Mistakes to Avoid
- Thinking a propeller only blows air backward like a simple fan is wrong because each blade also works as a rotating wing with lift-like pressure differences.
- Confusing blade pitch with angle of attack is wrong because pitch is the blade's geometric setting, while angle of attack depends on both pitch and the incoming airflow.
- Assuming the whole blade moves at the same speed is wrong because the tip travels a much larger circle than the root during each rotation.
- Using a very high blade angle at low speed without considering stall is wrong because a propeller blade can exceed its critical angle of attack and lose efficiency.
Practice Questions
- 1 A propeller accelerates 18 kg of air each second backward by 12 m/s. Use T = m_dot Δv to find the thrust.
- 2 A propeller rotates at 2400 rpm. How many revolutions does it make in 10 seconds?
- 3 Explain why a propeller blade is twisted from root to tip instead of having the same angle along its entire length.