Dynamic rollover

Dynamic rollover in Helicopter | Dynamic Rollover Causes & Recovery

The lateral rolling tendency in helicopter is called dynamic rollover. A helicopter is more susceptible to a lateral rolling tendency (Dynamic Rollover) when it is in contact with the surface during takeoffs or landings. The helicopter must first roll or pivot around a skid or landing gear wheel until the critical rollover angle is attained for dynamic rollover to happen. Depending on the type of helicopter, dynamic rollover will happen at a different angle. After that, the main rotor power keeps the roll going, making recovery impossible. The cycle lacks the range of control necessary to eliminate the thrust component and convert it to lift once this angle has been reached. No matter what cyclic adjustments are used, the helicopter rolls on its side if the critical rollover angle is surpassed.

Dynamic Rollover Causes

When the helicopter begins to lateral pivot around its skid or wheel, dynamic rollover occurs. The following three conditions must be met for dynamic rollover to occur:

A rolling moment
A pivot point other than the helicopter’s normal CG
Thrust greater than weight

This can happen for a number of reasons, such as failing to remove a tie down or skid-securing device, having the gear caught in ice, soft asphalt, or mud, or having the skid or wheel make contact with a solid object while hovering sideways. Additionally, if you do slope operations or utilize a poor landing or takeoff technique, dynamic rollover could happen. Regardless of the cause, dynamic rollover is conceivable if the right correction approach is not used.

Dynamic Rollover Recovery

Dynamic rollover cannot be stopped once it has started by using only the opposing cyclic control. For instance, the helicopter begins to roll to the right as the right skid makes contact with an item and becomes the pivot point. The main rotor's thrust vector and moment follow the helicopter as it rolls to the right even with full left cyclic applied. The most effective approach to prevent dynamic rollover is to rapidly reduce collective pitch. Any kind of rotor disc and any kind of landing gear can experience dynamic rollover.

It's crucial to keep in mind that rotor blades have a finite range of motion. The controls (cyclic) are no longer able to command a vertical lift component when the tilt or roll of the helicopter beyond that range (5-8°), and the thrust or lift instead becomes a lateral force that flips the chopper over. An already slightly unstable center of gravity is made further riskier by the limited rotor blade movement and the fact that the majority of a helicopter's weight is high in the airframe. Pilots must keep in mind that the sole viable recovery strategy is to lower the collective in order to eliminate thrust.

Helicopter Destroyed after Dynamic Rollover

Static Rollover

When the helicopter's blades are not rotating, they roll over in a static motion. The helicopter will roll if the static rollover critical angle is surpassed when the rotor blades halt, following the same rules as any other item. Every helicopter has a unique critical angle, which results from its center of gravity.

The center of gravity of an object that is rolling has a pivot point and an imaginary line extending upward from the pivot point; as the object rotates around the pivot point, the pivot line gets nearer. The object won't roll to the matching adjacent side of the base until it crosses the pivot line.

When the helicopter's center of gravity is on the upper pivot line, the static rollover critical angle can be calculated by measuring the angle between level ground and an imaginary line drawn from skid to skid.

Automobiles are also subject to static rollover. The static rollover threshold is a crucial metric in the investigation of vehicle roll stability. In terms of gravitational units, it is expressed as a lateral acceleration. Although rollovers are actually dynamic occurrences, there is a significant correlation between roll stability and the likelihood of rollover in accidents.