Ground Resonance In Helicopters

13061937_10209519007471523_6199836277029420823_nBack in the mid-fifties, when I was an EM instructor in the aircraft maintenance course at Ft. Rucker Alabama, I was charged with writing a lesson plan about “ground resonance in helicopters.” This was a condition where a helicopter, while on the ground, could set up a vibration sequence that could, within seconds, totally destroy the helicopter. As I started looking for research material in the post library, I found very little on the subject. BUT a friend of mine, a fellow instructor named Pounders, had just bought a new set of Encyclopedias Britannica, and one of their promotional programs was to research any question.

Pounders wrote them a letter, asking them to research “Ground Resonance in helicopters” and I went on to other things, waiting for the research material to come back from such a prestigious information organization. About two weeks later I was called to the office of Colonel Knox.

“Vaughan, you’re doing a lesson plan on ground resonance, aren’t you?”

“Yes sir.”

“I’ve got a letter here from Encyclopedia Britannica.”

“Good!” I said, thinking this was what I had been waiting for.

“They want us to research ground resonance, and since you are working on that, I’m assigning this project to you.”

I was a little taken aback, but then I figured why wouldn’t they contact us? We are teaching the subject. They have probably gone to half a dozen places. So, I sent them what I had.

A helicopter with a fully articulated rotor system has drag hinges that allow each individual blade to advance or retreat during rotation, to accommodate stress on the blade which is caused by changes in rotor speed of the hub. Because the individual blades are allowed accelerate or decelerate, the spacing between the blades can become irregular, shifting the rotor’s center of gravity away from the axis of rotation. This condition is caused ground resonance” because it occurs only when the aircraft is on the ground so that the airframe is not free to ride out the asymmetric forces against it. If the oleo struts and air pressure in the tires are not properly maintained, complete destruction of the aircraft is possible. If there is sufficient RPM in the rotor blades, the sequence can be stopped by lifting the aircraft from the ground.

Two weeks later, Pounders got the reply, from Britannica, for his research question.

A helicopter with a fully articulated rotor system has drag hinges that allow each individual blade to advance or retreat during rotation, to accommodate stress on the blade which is caused by changes in rotor speed of the hub . . .

The response was, word for word, what I had sent them.

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