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When a helicopter is resting on the ground with its rotor spining, a condition called "ground resonance" can develop. This is a destructive harmonic vibration caused by a dynamic reaction of the rotor blades to the lateral motion of the helicotper. Ground resonance can destroy a helicopter.
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Ground resonance is a divergent oscillation of the helicopter usually in lateral motion, rolling left and right. It is divergent because when ground resonance occurs, the energy of the spinning rotor and the condition of imbalance aggravates the condition and makes it grow in amplitude. The initiation of resonance usually occurs due to landing loads or other touchdown or liftoff condition that causes a shock loading to one of the helicopter's skids or landing wheels. The shock of hitting this wheel (or skid) causes the helicopter to react by rolling slightly towards level.
When a rolling motion (left to right tilting) is imparted to the helicopter and thus the rotor mast, the rotor disk is accelerated laterally to the left or right. Because the rotor blades are gripped only loosely, or at best have a dampener that allows lead-lag motion, when the blades are perpendicular to this motion (i.e. over the nose and tail of the helicopter), they will develop a slight lead or lag orientation to thier normal state. Normally, the blades are slung outward by centripetal force and thus will be at a 90 degree angle to their direction of motion. When this lateral motion occurs, the blade will accelerate or decelerate slightly and no longer be at 90 degrees.
The ground resonant condition occurs because both blades are offset to the same side of the rotor mast, left or right, opposite the direction of initial lateral roll displacement. When this occurs, the center of gravity (CG) of the rotor disk thus shifts to that side of the helicopter. By the time the helicopter has struck the other skid and started to right itself, the blades are causing an in-phase force to further exacerbate the motion.
Ground resonance can occur in model helicopters, full size helicopters and everything in between. Resonance in flight is typically not encountered because the mass of the helicopter does not encounter external forces to excite the oscillation. In flight divergent oscillations are quite possible and have been documented. Prediciting ground resonance is difficult but possible. Reference the NASA Langley Technical Report Server and search for "ground resonance" for further mathematical analysis. Ground resonance is mitigated in full size helicopters by employing the use of damping vibration isolators to attach the landing gear to the airframe. The dampeners are tuned to absorb energy at the proper frequencies of ground resonance and typically can prevent this destructive event.
Model helicopters are plagued by ground resonance because they usually lack lead-lag dampeners in the rotor system and almost exclusively have rigid landing gear structures. Flying a helicopter (full size or model) from a hard landing surface will aggravate the resonance because the harder surface tends to impart more energy to the helicopter. Flying from a soft surface like a grass field can help. For the modelers, the only remedies are to tighten the blade grips a bit, effectively creating more of a lead-lag dampening force, to encompass the helicopter skids with rubber tubing or foam padding, or to fly from a surface with more forgiving characteristics such as soft dirt.
In any case, model helicopter or full size, once resonance begins, the best way to get it to stop is to pull collective and break contact with the ground immediately. In the case of a take-off resonance event, this is the natural tendency. If one is landing the helicopter, this is a frustrating condition because it stops the pilot from accomplishing his initial goal of alighting on the surface of the earth.
The helicopter must be throroughly inspected after such an occurance to preclude the possibility of damage.
Another interesting phenomenon that occurs with helicopters is called "dynamic rollover." This topic will be developed more in the future and include graphics to demonstrate. Dynamic rollover is a state where the helicopter tips onto one skid and despite full cyclic input to roll the helicopter level onto the ground, it continues to roll over onto its side.
Please let us know if this was helpful information or if you have suggestions, comments or other questions you would like answered.