Ground resonance

Ground resonance, in fully articulated multi-bladed helicopters, is a hazardous condition during touchdown or at other times when the helicopter is running while sitting on the ground. A series of shocks to the landing gear can pass through to the rotor disk and 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 mast 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.

ee also

* Anatomy of a helicopter
*Helicopter flight controls
*Helicopter pilotage
*Helicopter rotor
* Aeronautical engineering

References

*"Basic Helicopter Handbook", US Department of Transportation, Federal Aviation Administration

Dampeners are used on fully articulated blade systems to prevent dissymetry of lift, not to reduce stress on the rotor systems.

External links

* [http://www.alexisparkinn.com/photogallery/Videos/2006-2-10_Ground_Resonance_Side_View.mpg Video of a CH-47 Chinook being forced into ground resonance and self destructing.]
* [http://www.alexisparkinn.com/photogallery/Videos/2006-2-10_Ground_Resonance_Rear_View.mpg Video of the same Chinook but from behind and closer.]