Definition
A self-excited, destructive vibration that can occur in an articulated-rotor helicopter while it is in contact with the ground. It happens when the rotor blades, which are free to move slightly fore and aft about their drag hinges, become unevenly spaced around the rotor disc. This unbalanced rotor produces a shaking force whose frequency matches the natural rocking frequency of the helicopter on its landing gear. The two frequencies reinforce each other, and the oscillation grows rapidly until the aircraft can shake itself apart.
Plain English
A violent shaking that can build up in a helicopter while it is sitting on its landing gear. If the rotor blades get out of even spacing, their wobble can match the bouncing of the helicopter on its wheels or skids, and the shaking gets bigger and bigger until something breaks.
Context Anchor
Encountered in helicopter ground operations, especially during landing, liftoff, run-up, or after a hard or uneven touchdown.
Derivation
Resonance comes from the Latin resonare, meaning to sound back or echo. In physics, it describes how a vibration grows when one rhythm reinforces another at the same frequency. Adding ground tells you this particular resonance only occurs while the helicopter is on the ground, where the landing gear allows the airframe to rock at its own natural frequency.
Why Pilots Care
If not corrected immediately by lifting off or shutting down, it can destroy the helicopter within seconds.
Analogy
It is like a washing machine with a heavy load that has shifted to one side. Once it starts thumping in time with its own frame, the shaking grows fast until you stop it.
Grounding Statement
Picture a helicopter sitting on its landing gear while the rotor is turning, and a small uneven shake suddenly grows into a violent wobble instead of fading away.
Intuition Check
Ground resonance is not just normal vibration on the ground. The key idea is a vibration that builds rapidly because the rotor motion and the helicopter body are feeding each other.
Example Sentence 1
During rotor engagement, the pilot felt the airframe begin to rock and immediately recognized the early signs of ground resonance.
Example Sentence 2
Soft turf increased the risk of ground resonance during rotor engagement on the training helicopter.