Hard contact with the ground on one corner (and usually  with wheel-type landing gear) can send a shockwave to  the main rotor head, resulting in the blades of a three-blade  rotor disk moving from their normal 120° relationship to  each other. This movement occurs along the drag hinge and  could result in something like 122°, 122°, and 116° between  blades.  [Figure 11-4]  When another part of the landing gear  strikes the surface, the unbalanced condition could be further  aggravated. blade, which reduces lift, and increase on the retreating blade,  which increases lift. At some point as the forward speed  increases, the low blade speed on the retreating blade, and  its high AOA cause a stall and loss of lift. Retreating blade stall is a factor in limiting a helicopter’s  never-exceed speed (V$_{NE}$) and its development can be felt  by a low frequency vibration, pitching up of the nose, and  a roll in the direction of the retreating blade. High weight,  low rotor rpm, high density altitude, turbulence and/or  steep, abrupt turns are all conducive to retreating blade stall  at high forward airspeeds. As altitude is increased, higher  blade angles are required to maintain lift at a given airspeed.  Thus, retreating blade stall is encountered at a lower forward  airspeed at altitude. Most manufacturers publish charts and  graphs showing a V$_{NE}$ decrease with altitude. If the rpm is low, the only corrective action to stop ground  resonance is to close the throttle immediately and fully lower  the collective to place the blades in low pitch. If the rpm is in  the normal operating range, fly the helicopter off the ground,  and allow the blades to rephase themselves automatically.  Then, make a normal touchdown. If a pilot lifts off and allows  the helicopter to firmly re-contact the surface before the  blades are realigned, a second shock could move the blades  again and aggravate the already unbalanced condition. This  could lead to a violent, uncontrollable oscillation.  This situation does not occur in rigid or semi-rigid rotor  disks because there is no drag hinge. In addition, skid-type  landing gear is not as prone to ground resonance as wheel- type landing gear, since the rubber tires' resonant frequency  typically can match that of the spinning rotor, unlike the  condition of a rigid landing gear. When recovering from a retreating blade stall condition  caused by high airspeed, moving the cyclic aft only worsens  the stall as aft cyclic produces a flare effect, thus increasing  the AOA. Pushing forward on the cyclic also deepens  the stall as the AOA on the retreating blade is increased.  While the first step in a proper recovery is usually to reduce  collective, RBS should be evaluated in light of the relevant  factors discussed in the previous paragraph and addressed  accordingly. For example, if a pilot at high weight and high  DA is about to conduct a high reconnaissance prior to a  confined area operation where rolling into a steep turn causes  onset of RBS, the recovery is to roll out of the turn. If the  cause is low rotor rpm, then increase the rpm. A helicopter is susceptible to a lateral rolling tendency,  called dynamic rollover, when it is in contact with the surface  Helicopters with articulating rotors (usually designs with  three or more main rotor blades) are subject to ground  resonance, a destructive vibration phenomenon that occurs  at certain rotor speeds when the helicopter is on the ground.  Ground resonance is a mechanical design issue that results  from the helicopter’s airframe having a natural frequency that  can be intensified by an out-of-balance rotor. The unbalanced  rotor disk vibrates at the same frequency (or multiple thereof)  of the airframe’s resonant frequency, and the harmonic  oscillation increases because the engine is adding power  to the system, increasing the magnitude (amplitude) of the  vibrations until the structure or structures fail. This condition  can cause a helicopter to self-destruct in a matter of seconds.  Figure 11-4.  Ground resonance.  Dynamic Rollover Ground Resonance Common Errors 1.  Failure to recognize the combination of contributing  factors leading to retreating blade stall. 2.  Failure to compute V$_{NE}$ limits for altitudes to be flown. 11-11 116° 122° 122°