FAA Powerplant Written Practice Test

How does the aerodynamic twisting force affect operating propeller blades?

1. It increases the blade angle

2. It tends to turn the blade into a high blade angle

3. It decreases the blade angle

4. It has no effect on the blades

The correct answer is: It tends to turn the blade into a high blade angle

The aerodynamic twisting force indeed tends to turn the blade into a high blade angle. This is due to the way propeller blades interact with the airflow when they are rotating. As the blade moves through the air, the lift generated causes the blade to want to twist or change its pitch angle. This aerodynamic twisting force is a result of the variation in lift along the blade's length—particularly the difference between the angle of attack at the root and tip of the blade. For most propeller designs, the blades are constructed with a certain amount of "washout," meaning the angle of attack is higher at the root than at the tip during normal operation. When the propeller is subjected to aerodynamic forces, the lift exerted on the blades causes them to naturally twist towards a higher angle of attack, which can increase the blade angle. This design allows for more efficient operation and helps to prevent stalling at higher thrust conditions. Understanding this twisting force is crucial in analyzing propeller performance and ensuring optimal operation across different flight conditions and power settings, contributing positively to the aircraft’s efficiency and control.