The main rotor gearbox is designed to handle a specific amount of power and torque. So, if the engine generates more power than the main rotor gearbox can handle, it risks failure or damage, which can be catastrophic.

Increasing their speed creates small, but significant physical movement of the contact areas of the teeth. Normal gearing and load transfer is calculated on a certain contact area. As RPM increases, this meshing or contact area can significantly reduce, so smaller contact areas have to transfer much greater loads to the next gear. The teeth will get very hot, particularly at the tips or the reduced contact area. The metal will become brittle and develop fatigue cracking.

Thus, regardless of the engine's capacity, the main rotor gearbox sets an upper limit to the power output, because it determines the maximum power that can be safely transmitted to the rotor without mechanical failure. By reducing the revolutions per minute (RPM) of the engine's output shaft, the power generated by the engine is lowered, making it suitable for helicopter applications where precise power control is critical for maintaining stable flight and managing various mechanical loads.

Thus, manufacturers reduce the power output of their engines for use in helicopters by decreasing the shaft RPM, because the power output of an engine is directly related to its rotational speed.