What should Np be at idle?

The correct answer indicates that the Np (propeller speed) should be in the range of 46% to 50% at idle. This range is essential because it ensures the propeller operates within its optimal parameters, providing stability and reliability in flight operations. Idle propeller speed is designed to minimize drag while maintaining sufficient aerodynamic control and fuel efficiency.

Operating the propeller within this specified range also helps in preventing wear on the engine and propeller components, facilitating smoother transitions during engine operations without risking engine stall or inefficient fuel usage. When the Np remains within the 46% to 50% range at idle, it signifies that the propeller is properly managing power, allowing for a safe and stable flight environment.

Other ranges, whether lower or higher, would not meet the operational standards needed for effective propulsion control, potentially leading to performance issues or increased engine strain during critical flight phases, such as takeoff, landing, or maneuvering.