Final answer:
The blade angle on a propeller that allows for reverse thrust is called the pitch angle. In helicopters, the tail rotor rotates in the opposite direction to counteract the torque of the main blades. The pitch angle allows the propeller blades to change the direction of the airflow.
Step-by-step explanation:
The blade angle on a propeller that allows for reverse thrust is called the pitch angle. When the pitch angle is changed to negative values, it allows the propeller blades to change the direction of the airflow, resulting in reverse thrust. This is important for aircrafts when they need to slow down or move backward.
For example, in a helicopter, the tail rotor is used to counteract the torque produced by the main propeller blades. By rotating in the opposite direction, the tail rotor creates a force that prevents the helicopter body from rotating in the opposite direction of the main blades.