## How A Propeller Generates Thrust

You probably know how an engine starts and runs, but how exactly do propellers generate thrust? The answer is relatively simple, and it all goes back to how lift is created and directed.

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.”

### PROPELLER TWIST

Propellers are airfoils, shaped similarly to wings. But instead of producing lift in a vertical direction, propellers produce lift in a forward direction that we call thrust. (We’ll dig deeper into this below)

Just like wings, propellers have camber and chord lines, in addition to leading and trailing edges. If you look closely at a propeller, you’ll also notice that the blade angle varies from root to tip.

As the propeller spins, the speed of the blade is highest at the tip and slowest at the root. During one full rotation, the tip of the blade has to travel a much further distance than the blade root, all in the same amount of time. That’s why the blade angle is greatest at the root, and the least at the tip. By “twisting” the blade, you get a relatively uniform angle of attack across the entire propeller blade.

If blade angle was uniform across the prop, thrust and pressure would have wide variations from root to tip. There could be a negative angle of attack at the root, and blade stall at the tip. This is why varying blade angle plays such an important role, to prevent large angle of attack and pressure differences across the blade.

### PROPS CREATE PRESSURE DIFFERENTIAL

The simple purpose of a propellor is to convert the engine’s brake horsepower into thrust. Just like wings, propellers accelerate airflow over their cambered surfaces. The high velocity of the air results in a lower static pressure in front of the propeller, pulling the airfoil forward.