resonance

The energetic vibration of a body produced by application of a periodic force of nearly the same frequency as that of the free vibration of the affected body. It is the condition of two bodies adjusted to have the same frequency of vibration. There are two types of resonance: sympathetic and ground. Sympathetic resonance (sympathetic resonance) is a harmonic beat that develops when the natural vibration frequency of one mechanism is in phase with another mechanism’s vibrational frequency. Ground resonance (ground resonance) is a self-excited vibration that develops when the landing gear of a rotorcraft repeatedly strikes the ground or deck of a ship, thus unseating the center of mass of the main rotor system. The pounding effect of the landing gear is prone to occur during start-up, ground taxi, rolling take-off and landing, engine shutdown, and operation from the deck of a rolling or pitching ship. When a helicopter begins to bounce from one wheel to the other in rapid succession, a pendular oscillation of the fuselage sets in. The succession of shocks is transmitted to the main rotor system, causing the main rotor blades to change their angular relationship with each other. This unbalances the main rotor system, which in turn transmits the shock back to the landing gear. This condition, if allowed to continue, can lead to the complete destruction of the helicopter. See also ground resonance.