Dynamic Rollover

Dynamic rollover is caused when a wheel or a skid on the aircraft becomes attached to the ground and acts as a pivot point that the helicopter can roll around. It is much more likely to happen on take-off and if it is not handled correctly, the helicopter will roll over on its side and suffer substantial damage. This does occasionally occur to experienced pilots but is much more likely to happen with low time pilots.

Have you ever tried to pull the handle of a door and the door refuses to move? Then, when you push the handle the door opens and takes you by surprise. You were not expecting it to happen. The same thing can happen during a take-off. You raise the collective smoothly and one of the skids lifts of the ground. You expect the other skid to follow but unfortunately it is caught on something and refuses to move. Your natural reaction is to raise the collective further to make the helicopter break free of the ground but all you achieve is a fast roll of the helicopter towards the stuck skid. The momentum of this roll may be sufficient to keep the helicopter rolling over even though you now lower the collective. The inevitable happens and the helicopter rolls over into its side and suffers severe damage.

The results of a dynamic rollover are usually very severe. The helicopter is usually destroyed. The damage from a helicopter rolling over at the end of an autorotation is much less than the damage caused by dynamic rollover. This is because during an autorotation the blades are unpowered and if it rolls over, it will quickly come to rest. During a take-off however, you will be using a lot of power and if the helicopter subsequently rolls over, it will not come to rest so quickly and much more damage will occur.

Static Rollover

Just imagine that for some reason the non-running helicopter was pushed from one side so it rotated around one skid.

If it was let go before 42 degrees was reached the helicopter would fall back to right itself.

But if it were pushed PAST 42 degrees (assuming it was not pulled back).

This would be the result. Look at the arrow that shows the weight acting through the centre of gravity (C of G) through the sequence to see why. When the C of G is outside the skid the aircraft can only fall over (unless it is pulled or pushed back before a point of no return).

More seriously, if the aircraft is allowed to roll around one skid whilst in the hover, the same forces are at work (ignoring any other dynamic force).

At less than 42 degrees, lowering the lever smoothly (as is advised by the section on dynamic rollover) will allow gravity to pull through the C of G to right the aircraft.

But if allowed to roll further than 42 degrees no amount of lowering the lever and certainly no amount of opposite stick can stop the roll.

There simply is no way of making any thrust in the direction necessary to stop the aircraft from falling over. Don't get it to that point.

At less than 42 degrees, the total weight of the aircraft (acting through the C of G) is still inside the skid. Therefore, if total rotor thrust is decreased and its horizontal and vertical components are then reduced, gravity will act to right the aircraft.

If the total weight (acting through the C of G) is outside the skid, even reducing the thrust to zero cannot stop the aircraft falling over. If the aircraft where allowed to get to this angle even putting the lever fully down will not stop the roll. Gravity will pull it to onto its side.

Dynamic Rollover

If the helicopter is allowed to drift whilst close to the ground whatever part of the skid is presented first can be caught on an object sticking up from the ground, a high part of uneven ground or just the level ground itself. The skid is prevented from moving by that contact. The rest of the aircraft will continue to move in its original direction. (This can also happen during a take-off if one skid is stuck to the ground by ice, suction or just obstructed.)

The whole helicopter will rotate around the skid. Dynamic rollover begins to develop at an angle greater than maximum available cyclic disc tilt. That angle is generally accepted to be 15 degrees. However, the figure is not a constant. It may be reduced by increased drift speed, loading, wind and many other factors.

As the roll continues to develop total rotor thrust is increasingly divided into more horizontal and less vertical components. The thrust that is added to the horizontal component can only come from the vertical component (if total thrust remains constant). More total thrust is pulling the aircraft over and less is lifting it vertically.

Opposite cyclic stick input cannot stop fully developed dynamic rollover. The R22B (for instance) has maximum cyclic disc tilt of 10 degrees (left) and 8 degrees (right). If 15 degrees has been reached the cyclic stick has no authority. Even if all available tilt is applied there is still a horizontal component taking the aircraft over onto its side. At best, opposite stick may slow the rate of roll but cannot stop it. With acute disc/mast angles there is also a risk of mast bumping.

An unwary pilot may think it reasonable to raise the lever with the intention of lifting the helicopter off the ground to get out of trouble. Not so. When dynamic roll has developed it is extremely dangerous to raise the lever. Doing so can only add to total rotor thrust which will increase the horizontal component. Raising the lever would pull the helicopter over even faster.

 

If raising the lever increases rate of roll by increasing the horizontal component it can be reduced by lowering the lever. This must be done smoothly as to abruptly 'dump' the lever could cause the helicopter to fall so heavily onto the opposite skid that it might collapse. If not, a roll to the opposite side may simply continue with momentum. (There is also risk of ground resonance being initiated in fully articulated rotor systems.) Lever down smoothly.

Drift to the right is a likely cause of Dynamic Roll. Tail rotor thrust encourages a drift to the right which can easily happen if not prevented with left cyclic stick input. It must be remembered that depending on loading and wind the right skid will be lower in a solo hover, perhaps lower than the left skid.

In roll to the left an additional factor is the couple produced between tail rotor thrust and the horizontal component of main rotor thrust. A couple will always try to straighten out (think of pulling on the ends of a similarly shaped rope). The couple is pulling the aircraft over onto its side. The development of dynamic roll to the left is enhanced by this couple.

Dynamic rollover can also happen to the front or rear of the aircraft. It is not only a risk to the sides.