Helicopter Emergencies and Hazards (VORTEX RING STATE)

 we can all agree that flying helicopters is an extraordinary hobby.

it is also a fast and efficient means of transport but as with many hobbies and means of transports there are risks and danger.

the best way to avoid accidents is to know the hazardous flight conditions well.

the goal of this article is to study the vortex ring state one of the different hazardous helicopters flight conditions, to learn to recognize and avoid them and most of all ,know how to get out of them.

ok, how do we define the vortex ring states ?

Vortex ring state, in which airflow is upward on the inner blade section, producing a secondary vortex in addition to the normal wingtip vortices. Turbulent airflow results in loss of rotor efficiency. If allowed to continue, uncommanded pitch-and-roll oscillations may occur, with a large descent rate

in normal flight condition, the air flow comes from above passes through the rotor disc and is pushed underneath.in other words, the induced flow at the the rotor tips there is a bit of air that is aerodynamically pushed over the blades creating small vortices just like it does on the wings of airplane

Vortices form because of the difference in pressure between the upper and lower surfaces of a wing that is operating at a positive lift. Since pressure is a continuous function, the pressures must become equal at the wing tips. The tendency is for particles of air to move from the lower wing surface around the wing tip to the upper surface (from the region of high pressure to the region of low pressure) so that the pressure becomes equal above and below the wing

These vortices increase the drag and reduces the lift slightly but it's a normal flight condition 
when the helicopter is going forward, the air flow is pushed behind. But when we slow down and even more so in a hover condition the air flow is pushed underneath the helicopter.

The danger of the vortex ring state is the unexpected increasing descent rate there is also the uncontrollable yawing movement, the reduced effectiveness of the controls, the sudden increasing  loss of altitude , and when the helicopter descends unexpectedly, what is your instinctive reaction ? 
yup, we add collective power and instead of helping it makes it worse.at this stage close to the ground it can turn into a serious accident.
the speed of the helicopter has to be less than that of translational lift.
so in a Robinson helicopter that's about 30 kts the descent rate needs to be more than 300 Ft/minute and the last requirement is to have more than 20% power in use so not in autorotation .
where you really need to be careful are:

• High angled approaches 
• tail wind approaches 
• vertical descent out of ground effect 
• Hovering out of ground effect 
• all flights with very limited power available, meaning, too high or too heavy 

The golden rule to avoid the vortex ring state is to reduce your descent rate to less than 300ft/min 
before reducing your speed to less than 30 kts
of course there is other precautions that we can adopt ,like avoiding hovers out of ground effect ,always know the wind direction or do approaches and landings upwind ,do good flight planning and know your flight limitation.
ok, so we weren't careful and found ourselves in a vortex ring state what do we have to do ?

we need to get out of this turbulent air mass one way of doing it and only if you have enough height is to go into autorotation.
in autorotation we reduce the power and the air flow totally inverts meaning it flows from the bottom to the top so the phenomena is removed immediately.
Another method is to lower the collective slightly and the cyclic slightly forward to fly forward. It's a quite an intuitive maneuver .
the third method is the vuichard method discovered by a  swiss inspector pilot which uses the force of the tail rotor to slide out of the vortex ring state sideways.
finally, a question for you to answer how do you know you're in a vortex ring state ?
You can leave your answers in the comments below 

I hope my explanations were useful.