"When is a helicopter like a Patsy Cline song? When it falls to pieces." That's the darkly comedic subhed in a new Air & Space Magazine about ground resonance, a condition when a sitting helicopter's rotors become imbalanced while spinning. If the frequency of the now-vibrating rotor is close to the body of the chopper's normal vibration frequency, the oscillations increase. In seconds, the whole helicopter can just fall apart. (Ground resonance tore the helicopter above apart in just four seconds.) From Air & Space:
“I was standing right next to it,” says Frank Robinson, founder of the world’s leading helicopter company, describing a close call he had during a 1961 test of a gyroplane. “I had to grab hold of it and hang on and ride the damn thing down. You don’t want to be standing out there when it starts to jump around – it can jump on you. And there’s not a good way to get out of it. Just cut everything, hang on and hope..."How Things Work: Ground Resonance
The destruction is wrought by the considerable energy stored in the rotor blades. The shaking rapidly grows in violence, exceeding the strength of the mast, transmission mounts, and landing gear. The cyclic control in the cockpit flails about so violently that the pilot cannot hold it, the rotor blades strike the tail boom or the cockpit, parts begin falling off, and moments later the helicopter may be a heap of scrap.