Ground resonance is an imbalance in the rotation of a

helicopter rotor On a helicopter, the main rotor or rotor system is the combination of several rotary wings (rotor blades) with a control system, that generates the aerodynamic lift (force), lift force that supports the weight of the helicopter, and the thrust ...

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 machine designed to laundry, launder clothing. The term is mostly applied to machines that use water. Other ways of doing laundry include dry cleaning (which uses ...

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 allow each blade to advance or lag in its rotation to compensate for the stress on the blade caused by the acceleration and deceleration of the rotor hub (due to

momentum conservation In Newtonian mechanics, momentum (: momenta or momentums; more specifically linear momentum or translational momentum) is the Multiplication, product of the mass and velocity of an object. It is a Euclidean vector, vector quantity, possessi ...

). When the spacing of the blades becomes irregular, it shifts the rotor's

center of mass In physics, the center of mass of a distribution of mass in space (sometimes referred to as the barycenter or balance point) is the unique point at any given time where the weight function, weighted relative position (vector), position of the d ...

from the

axis of rotation Rotation or rotational/rotary motion is the circular movement of an object around a central line, known as an ''axis of rotation''. A plane figure can rotate in either a clockwise or counterclockwise sense around a perpendicular axis intersect ...

, 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 aeros ...

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 air ...

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, 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 move and help dampen the oscillation freely. Complete shutdown may be sufficient if rotor RPM is very low during a ground resonance incident.

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

Causes and consequences

Mitigation

See also

References

External links