Final answer:
The primary governor adjusts the pitch of the propeller blades to maintain the selected RPM during under-speed conditions on a multi-engine aircraft.
Step-by-step explanation:
The primary governor on a multi-engine aircraft works to ensure that the selected RPM (Rotations Per Minute) is maintained even during propeller under-speed conditions. It does this by adjusting the pitch of the propeller blades. When the RPM drops below the desired level, the governor increases the blade angle, which increases the aerodynamic loading on the blades and causes the RPM to increase back to the selected value. Conversely, when the RPM exceeds the desired level, the governor decreases the blade angle to reduce the aerodynamic loading and bring the RPM back down to the selected value.
For example, if the engine's power output decreases due to a temporary loss of power or an engine failure, the RPM of the affected propeller may decrease. The primary governor will detect this decrease and adjust the blade angle to increase the aerodynamic loading and bring the RPM back to the selected level. This helps to maintain balanced thrust and prevent excessive vibration or stress on the aircraft.
Furthermore, the primary governor may also interact with other systems, such as the propeller feathering system, to ensure that the propeller is in the most efficient position during under-speed conditions. It continuously monitors and adjusts the propeller's pitch and RPM to maintain the desired power output and performance.