When G loading the rotor disk, a ______ per revolution vibration is the indicator the stall region is large enough to require the retreating blade to compensate for lack of lift.

• A. One
• B. Two
• C. Three
• D. Four

Answer: (D)

When the rotor disk is heavily g-loaded, the retreating blade tends to stall more, enlarging the stalled region. The vibration pattern you monitor per rotor revolution reflects how large that stall area is. A four-per-revolution vibration is the point at which the stall region is big enough that the retreating blade must be compensated for the lift shortfall to keep the rotor balanced and controllable. Lower per-rev counts (one through three) indicate increasing stall is present but not to the level that requires retreating blade compensation. So, four per revolution is the threshold indicating you need to apply retreating blade lift compensation.