What is ground resonance in a helicopter?

• A. A smooth landing technique.
• B. The destructive vibration where the lead-lag hinges of the rotor blades oscillate and, if not corrected, can destroy the helicopter.
• C. A vibrational mode that occurs only at cruise altitude.
• D. A fatigue phenomenon in the landing gear.

Answer: (B)

Ground resonance is a destructive vibration that happens when the helicopter is on the ground with the rotor turning. The rotor blades are mounted on a hub with lead-lag hinges, which allow the blades to lag and advance as they flap. If the fuselage tilts slightly or the landing gear is loaded unevenly, the rotor disk tilts and the lead-lag hinges begin to oscillate. That blade motion couples into the airframe through the rotor–mast system, creating a feedback loop that can grow into a violent, damaging vibration. If not corrected, the forces can overwhelm the structure and destroy the helicopter. This phenomenon is specifically about the dangerous rotor-ground interaction when the rotor is turning on the ground; it’s not a smooth landing technique, not something that only happens at cruise altitude, and it isn’t a normal fatigue issue of the landing gear.