At what percentage of Nr will constant frequent generators trip off?

The correct answer is 67%. Constant-frequency generators, which are often used in helicopter systems, are designed to operate within specific performance parameters, including rotor speed (Nr). When the rotor speed (Nr) drops to 67% of its normal operating range, the frequency and stability of the electrical output from the generators can no longer be maintained effectively. This is a critical threshold for maintaining the reliability and efficiency of the system, and at this point, the generators will automatically trip off to prevent damage or malfunction.

This design feature ensures that the generators do not operate under conditions that could lead to electrical instability or possible failure. Therefore, the tripping of constant-frequency generators at 67% of Nr is a safety mechanism built into the system to ensure overall operational integrity.