When airborne, a helicopter can cope with large cyclic control inputs; however, on the ground, rather serious problems can easily occur from slight control inputs that cause small angles of tilt on the main rotor disc. What is also rather dangerous is that the human physiology does not easily pick up small motion cues, and an unaccelerated angular motion disturbance can go unnoticed for quite some time until it has reached the point where it is potentially perilous. Because movement is often picked up through visual cues before the other senses, if you are operating at night and possibly using optical night vision devices, much of the normal movement prompts are degraded. If you are distracted or not fully concentrating when you lift off, you can easily be caught out.
Most American designed single rotor helicopters have their rotor-blades turning in an anti-clockwise direction when viewed from above, and these helicopters tend to hover with the left landing gear low. This is caused by the main rotor being tilted to the left to counteract the rightward thrust being generated by the tail rotor. The degree of fuselage tilt depends on the hinge offset in the main rotor hub and the vertical position of the tail rotor in relation to the main rotor, as well as the aircraft’s centre of gravity. When the helicopter is in the process of lifting off the ground and into the hover, the total rotor thrust is increased. This also increases the tail rotor thrust, amplifying the turning moment. In our case, the rotor flapping will roll the helicopter to the left if no cyclic input is made to counter this effect. The more you raise the collective lever, the more you will increase the rolling moment. Normally, the pilot subconsciously corrects for the roll, centring the rotor disc so that the aircraft rises vertically away from the surface, albeit with the fuselage hanging left side low. However, some pilots may inadvertently input too much right cyclic on the ground in order to correct for this tendency, setting up a rolling moment in the opposite direction. If the helicopter is allowed to continue to roll on its landing gear, the stabilizing moment caused by the centre of gravity, which is normally acting down through the fuselage between the undercarriage, goes rapidly to zero as soon as the centre of gravity moves over that wheel or skid.
Of course, increasing the all up mass will increase the overall torque required, consequently increasing the tail rotor thrust and thereby increasing the propensity for the aircraft to roll. If the centre of gravity is allowed to get close to the lateral limit, you can clearly exacerbate the rolling moment. A high centre of gravity makes the helicopter less stable in this condition.
As pitch is increased, the induced flow from the main rotors is dispersed outwards when it comes into contact with the surface. Should there be a cross wind, the vortex created from the into-wind blade can cause the downwash to recirculate, reducing the lift in that part of the disc. This loss of lift requires an increase in power to the main rotor, which also increases the tail rotor thrust, amplifying the existing problem.
Onboard ship, the movement of the vessel or the disturbed air flowing across the deck can exacerbate the conditions required to produce dynamic rollover. Lifting off from sloping ground is also problematical. In this situation, the rolling moment of the fuselage has already begun as the aircraft lifts the down-slope gear from the ground. If the movement is sharp or poorly controlled, the pilot is already inducing the danger but if, as a result of this mishandling, the fuel in the tank sloshes across from one side of the aircraft to the other, the lateral centre of gravity can shift markedly and aggravate the roll. Naturally, an aircraft with a narrow track landing gear is much more prone to dynamic rollover than one with a widely spaced undercarriage. Furthermore, if the pilot inadvertently tilts the rotor prior to lift off against firm oleos or a springy undercarriage, he is setting up an adverse condition from the moment he unsticks the helicopter from the ground.
As every student knows, the classic situation that causes rollover is when one wheel or skid is either trapped or stuck to the ground by mud, ice, tie-downs or anything else which can manage to lash one side of the undercarriage to the surface. In this case, the helicopter pivots around the fixed anchor point in the manner of a hinge until the rotor blades come into contact with the surface.
An unfortunate instinctual action by most pilots when they perceive the roll starting is to raise the collective faster in an attempt to climb into the hover. However, all they achieve is to accelerate the rollover, since they are increasing the total rotor thrust that is causing the problem in the first place. Applying opposite cyclic may also seem like a good idea, but the slowing and reversing of the rolling motion takes time, and the speed that the rollover occurs after onset is faster than most pilots can react. The crucial time between a safe take off and rollover can be measured in milliseconds, and once it starts, the process tends to become inevitable. That said, the only sensible corrective action is to lower the collective lever as soon as possible when the condition is first perceived.
Dynamic rollover can catch any helicopter pilot out. If you are about to get airborne in a narrow track, wheeled helicopter with firm oleos, which has a high centre of gravity, and you are taking off from sloping ground with a crosswind, at night using optical devices, please concentrate very hard on your actions.