What does the alpha range indicate in turboprop propellers?

The alpha range in turboprop propellers refers to the portion of the propeller's operational envelope where it is maintaining a constant speed during flight. In this range, the propeller blades are set at an optimal pitch angle to ensure smooth operation and efficient performance at various flight regimes. This constant speed setting is crucial for maintaining engine efficiency, fuel economy, and overall performance of the aircraft.

When the propeller is in the alpha range, it is commonly adjusted via a governor that regulates the pitch of the blades to keep the RPM within required limits, particularly under dynamic conditions like climb, cruise, and descent. This allows the powerplant to operate effectively, matching the thrust required by the flight conditions while also preventing fluctuations that could adversely impact performance.

The other scenarios mentioned in the options, involving reverse thrust, zero thrust, and feathering, represent different operational states that do not align with what is represented in the alpha range. Reverse thrust, for instance, is a specific condition used during landing to help slow down the aircraft and is not part of the constant speed operation during standard flight. Likewise, zero thrust and feathering are adjustments made under specific circumstances outside the normal operational range.