Ground Resonance in Helicopter, Ground Resonance Causes & Recovery

Ground Resonance in Helicopter, Ground Resonance Causes and Mitigation

Ground Resonance

Ground resonance is a dangerous situation in a helicopter in which an imbalance in the rotation of the main rotor occurs when the blades become bunched up on one side of their rotational plane and causes an unwanted oscillation in phase with the frequency of the rocking of the helicopter on its landing gear. This effect is similar to the behavior of a washing machine when the clothes are concentrated in one place during the spin cycle and the washing machine enters into vibrating mode. Ground resonance occurs when the landing gear is prevented from freely moving about on the horizontal plane, typically when the aircraft is on the ground.

Ground Resonance Causes & Consequences

The main rotor head of a fully articulated rotor system has drag hinges that allow each individual blade to advance or lag in rotation as a means of reducing stress on the blade brought on by the rotor hub's acceleration and deceleration (due to momentum conservation). The rotor oscillates as the blade spacing varies because it causes the center of gravity to be offset from the axis of rotation. When oscillations cause the airframe to start rocking back and forth, they may reinforce one another and push the rotor's center of gravity farther away from the axis of rotation than the damping system can compensate for.

The risk of ground resonance increases when parts of the landing gear or damping system, such as the drag hinge dampers, oleo struts, or wheel tyre pressure, are not correctly maintained. Ground resonance is typically brought on by a hard landing or an unequal ground contact. Under really harsh circumstances, the initial shock can set off powerful oscillations that quickly intensify and severely damage the entire airframe. When this happens, even at regular rotor speed, body panels, fuel tanks, and engines can be completely destroyed.

Ground Resonance Mitigation

The only way to stop ground resonance if the main rotor's speed is low is to instantly close the throttle and fully lower the collective so that the blades are at a low pitch.

Fly the helicopter off the ground and let the blades re-phase naturally if the rpm is within the acceptable operating range. Make a typical touchdown after that. A second shock could move the blades again and worsen the already unbalanced state if a pilot rises off and permits the helicopter to firmly re-contact the surface before doing so. This might cause an erratic, violent oscillation.

Because there is no drag hinge in rigid or semi-rigid rotor discs, this condition does not exist. Additionally, because the resonant frequency of the rubber tyres may often match that of the rotating rotor, as opposed to a rigid landing gear, skid-type landing gear is less susceptible to ground resonance than wheel-type landing gear.

Ground resonance can be avoided with proper maintenance of the helicopter's dampening system's component parts. When it does happen, recovery is frequently feasible provided early action is done. If enough rotor RPM is available, a quick takeoff can help restore rotor balance by allowing the airframe to move freely and reducing oscillation. Complete shutdown can be sufficient if, during a ground resonance incidence, the rotor RPM is relatively low.