Dynamic Rollover in a Helicopter: What Makes ‘Copters Flip on Takeoff?

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Takeoffs must be careful to prevent dynamic rollover, image by Helen Krasner

Takeoffs must be careful to prevent dynamic rollover, image by Helen Krasner

Every so often the accident reports include an account of a helicopter that crashed before it had even taken off!

Usually what happens in these cases is that the pilot starts up the machine and then begins to lift the helicopter into a hover as usual. Instead of lifting off, it suddenly and for no apparent reason turns over on to its side and then proceeds to rather expensively thrash itself to bits.

What is Dynamic Rollover?

So how could this happen?   The most common reason – possibly the only one – is dynamic rollover.

This can occur when the helicopter is lifted off at an angle rather than completely vertically. And it doesn’t have to be that large an angle, for the helicopter’s centre of gravity is not the only factor that needs to be considered.  Power is being applied, and this makes the whole situation very different.

If you look carefully at a helicopter, it would seem to be quite difficult to turn it over.   Helicopters sitting on the ground appear to be very stable: their skids are normally spaced fairly far apart, forming a wide and apparently solid base. In fact, without the engine turning, a helicopter can only turn over if on a very steep slope – a situation known as ‘Static Rollover’. However, if power is being applied, a sideways force is created. This does not normally matter, as it is very small, but if the helicopter is lifted off at an angle, so that one skid comes off the ground first, the remaining skid acts as a pivot point about which the force acts.

This force increases as more power is applied and/or the angle of bank becomes steeper, causing the helicopter to tend to roll about the skid which is in contact with the ground. Beyond a certain critical angle it is impossible to stop the helicopter entering dynamic rollover … and turning over. The angle of bank at which this can happen is fairly small, often less than 10 degrees, and far, far less than the angle required for static rollover.

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