Ground resonance
Ground resonance, in multi-bladed helicopters, is a hazardous condition during touchdown. A series of shocks to the landing gear can cause an imbalance in the rotor system. Under extreme conditions, the imbalance causes violent oscillations that quickly build and result in catastrophic damage of the entire airframe. In some cases, complete destruction occurs, e.g. body panels, fuel tanks, and engines are all ripped from their mountings. The imbalance is possible because multi-bladed helicopters include lag-lead hinges at the rotor hub to reduce stresses in flight. Under normal conditions, all blades are spaced at equal angles. Shocks to the rotor shaft and hub can cause an imbalance if they are sufficiently violent. Note that two bladed helicopters are not susceptible to ground resonance because they do not require lag-lead hinges. Two-bladed rotors remain balanced through all flight conditions. Recovery is possible in some cases. If sufficient rotor RPM exists, immediate takeoff can restore rotor balance. If rotor RPM is low, complete shutdown might be sufficient.
References - Basic Helicopter Handbook, US Department of Transportation, Federal Aviation Administration
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