How does the blade Centre of Gravity (CG) move during ground resonance?
Refer to figure.
Ground resonance is a critical issue that can occur when a helicopter is in contact with the ground. It involves an escalating oscillation caused by the interaction between vibrations in the rotor system and fuselage vibrations of similar frequency. This phenomenon typically begins with a slow rocking of the helicopter's fuselage but can rapidly intensify. If left unaddressed, it has the potential to result in the destruction of the helicopter.
Four specific conditions must be met for ground resonance to occur:
- Self-excited vibration must be present in the rotor system.
- Fuselage vibrations must also be present.
- The vibrations in the rotor and fuselage must have similar frequencies, allowing them to reinforce each other.
- The helicopter must be in contact with the ground.
Notably, ground resonance can be preceded by a phenomenon known as "undercarriage paddling." This involves lateral oscillations, often felt in the landing gear or skids of the helicopter. While some degree of lateral oscillation is expected in skidded helicopters, it becomes problematic if its frequency or severity increases to match that of ground resonance. In such cases, it can evolve into a full-fledged ground resonance situation.
Articulated rotor systems featuring drag hinges offer the capability for each individual rotor blade to adjust its position, either advancing or lagging in rotation. This adjustment serves to counterbalance the stress placed on the blades due to the rotor hub's acceleration and deceleration, a consequence of momentum conservation principles.
However, when the spacing between the rotor blades becomes irregular, it shifts the rotor's center of gravity away (outboard) from the axis of rotation. This shift sets the stage for an oscillatory motion to occur. As the airframe starts to sway back and forth in response to this oscillation, these movements can interact and mutually reinforce one another. Consequently, the rotor's center of gravity spirals away from the axis of rotation, reaching a point that exceeds the compensating capacity of the damping system. This process can result in instability and potential control issues for the aircraft.
Recovery from the onset of ground resoance can be achieved be removing the vibrations in two ways:
- Helicopter in ground idle, immediate shutdown.
- Helicopter in flight idle, take off into hover.
The rotor's centre of gravity (sometimes referred to as the centre of mass) is not the helicopters centre of gravity.
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