Ground resonance

As ground resonance is called a dynamic interaction between the rotor system for helicopters, and the helicopter cell on the landing device. This condition is dangerous because it can destroy the helicopter in no time.

Bottom resonance is often triggered by the landing impact acting on the landing gear, thereby causing a temporary deflection of the rotor system. The rotor mast is accelerated because of the relative ground force to the rotor circuit in one direction. Swivel joints or elastic mounting of the rotor blades at hingeless rotor heads lead to a displacement of the rotor blades in the rotor plane, creating a balance of the blade. Exceeds caused by this imbalance vibration now, the natural frequency of the helicopter on the landing gear, so the whole helicopter begins to oscillate.

Structurally it is attempted to suppress the ground resonance by damping systems in the rotor and the landing gear. Also the design of the helicopter airframe and landing device, the natural frequency can be shifted to less critical areas. Completely rule to ground resonance can not be, however. To escape an incipient ground resonance, the pilot must immediately take off again to eliminate the reaction forces of landing device (see video).

A well-known example is the prototype V1 of the Bo 105, which was completely destroyed before the actual start of flight tests due to ground resonance.