When using a universal propeller protractor, how should it be zeroed?

To properly zero a universal propeller protractor, it is crucial to do so with the aileron in the neutral position. This ensures that the measurement accurately reflects the true attitude of the aircraft without the influence of any roll that could be caused by aileron input. By having the ailerons neutral, the wings are held level, allowing the protractor to provide a true reference for propeller angle measurements.

Zeroing the protractor in this manner is essential for ensuring that any adjustments made to the propeller's pitch will be based on reliable and precise measurements. This practice is fundamental to maintaining proper aircraft performance and control, especially during flight ops where accurate propeller pitch settings can significantly affect airspeed and climb performance.

Other options, such as using full throttle or adjusting in-flight, can introduce variables that make the readings unreliable, which could lead to incorrect adjustments being made. Hence, zeroing with the ailerons neutral provides the best reference point for accurate propeller measurement and control.