Define “ground resonance” in helicopters.

Ground resonance is a phenomenon that occurs when there is a dynamic interaction between the rotor system of a helicopter and the ground. This situation typically arises in helicopters with a fully articulated rotor system and is characterized by a coupling of the rotor's vibrations with the natural frequencies of the aircraft or the ground surface. Under certain conditions, such as when the helicopter is on a soft surface or when it experiences a loss of support due to the wheels losing contact, these vibrations can grow exponentially rather than dampening out, leading to significant oscillations that can cause structural damage to the helicopter or even lead to a rollover.

The correct understanding of ground resonance involves recognizing how factors like rotor speed, damping effects, and the helicopter’s landing gear interact with the ground. The potential for ground resonance is a critical consideration for pilots and engineers in helicopter operations and design, highlighting the importance of stability and control during takeoff and landing phases.

The other options do not accurately capture the essence of ground resonance:

• A vibration caused by engine misalignment refers to mechanical issues unrelated to the dynamics of the rotor system interacting with the ground.
• A phenomenon that improves hover stability mischaracterizes ground resonance; it poses a danger rather than enhancing stability.
• A method to control rotor speed does not