Ground resonance is an imbalance in the rotation of a
helicopter rotor
A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aero ...
when the blades become bunched up on one side of their rotational plane and cause an oscillation in phase with the frequency of the rocking of the helicopter on its landing gear. The effect is similar to the behavior of a
washing machine
A washing machine (laundry machine, clothes washer, washer, or simply wash) is a home appliance used to wash laundry. The term is mostly applied to machines that use water as opposed to dry cleaning (which uses alternative cleaning fluids and ...
when the clothes are concentrated in one place during the
spin cycle. It occurs when the landing gear is prevented from freely moving about on the horizontal plane, typically when the aircraft is on the ground.
Causes and consequences
Articulated rotor systems with
drag hinges
A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aero ...
allow each individual blade to advance or lag in its rotation to compensate for stress on the blade caused by the acceleration and deceleration of the rotor hub (due to
momentum conservation). When the spacing of the blades becomes irregular, it shifts the rotor's
center of gravity
In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...
from the
axis of rotation
Rotation around a fixed axis is a special case of rotational motion. The fixed- axis hypothesis excludes the possibility of an axis changing its orientation and cannot describe such phenomena as wobbling or precession. According to Euler's r ...
, which causes an oscillation. When the
airframe
The mechanical structure of an aircraft is known as the airframe. This structure is typically considered to include the fuselage, undercarriage, empennage and wings, and excludes the propulsion system.
Airframe design is a field of aerospa ...
begins to rock back and forth from the oscillation, the oscillations can reinforce each other and cause the rotor's center of gravity to spiral away from the axis of rotation to a point beyond the compensating ability of the damping system.
Ground resonance is usually precipitated by a hard landing or an asymmetrical ground contact, and is more likely to occur when components of the landing gear or damping system are improperly maintained, such as the drag hinge dampers,
oleo strut
An oleo strut is a pneumatic air–oil hydraulic shock absorber used on the landing gear of most large aircraft and many smaller ones. This design cushions the impacts of landing and damps out vertical oscillations.
It is undesirable for an airp ...
s, or wheel tire pressure. Under extreme conditions, the initial shock can cause 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 torn away, even at normal rotor speed.
Mitigation
Proper maintenance of the helicopter's damping system components can prevent ground resonance from taking hold. When it does occur, recovery is often possible if action is taken quickly. If sufficient rotor RPM exists, immediate takeoff can restore rotor balance by allowing the airframe to freely move and help damp the oscillation.
[ If rotor RPM is very low during a ground resonance incident, complete shutdown may be sufficient.
]
See also
*Helicopter flight controls
A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor ...
*Helicopter pilotage
A helicopter pilot manipulates the helicopter flight controls to achieve and maintain controlled aerodynamic flight. Changes to the aircraft flight control system transmit mechanically to the rotor, producing aerodynamic effects on the rotor bla ...
*Helicopter rotor
A helicopter main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift force that supports the weight of the helicopter, and the thrust that counteracts aero ...
*Aeronautical engineering
Aerospace engineering is the primary field of engineering concerned with the development of aircraft and spacecraft. It has two major and overlapping branches: aeronautical engineering and astronautical engineering. Avionics engineering is sim ...
References
*''Basic Helicopter Handbook'', US Department of Transportation, Federal Aviation Administration
External links
Video of a Brazilian rescue AS350BA breaking apart after landing due to ground resonance.
Chinook helicopter with wheeled landing gear ground resonance - side view video
Chinook helicopter with wheeled landing gear ground resonance - rear view video
{{Helicopters and rotorcraft
Helicopter aerodynamics
Waves
Aviation risks