A principle of helicopter aerodynamics, which a helicopter can find itself caught in if its pilot has induced particular flight conditions. Although not always dangerous, past a point (which will become clear) it is certainly not a good thing and if not recovered will eventually cause a crash or at the very least a hard landing.
Anyone who can ride a unicycle, walk the dog with his yo-yo and balance a broomstick in the palm of his hand simultaneously should be able to fly a helicopter with no problem. After 15-20 years of practice, it begins to come as second nature.
The Vortex Ring State is an artefact of the way a helicopter produces its lift and is roughly comparable to... well, nothing. Rotary wing aircraft are unique beasts. They produce their lift by spinning a set of what amounts to slender wings, fast enough to lift themselves and the helicopter off the ground. Simplifying the case, a wing creates lift by forcing the air it passes through downwards. Because of the part of physics outlined by Newton's third law of motion, the wing gets forced upwards with equal, er, force. The amount of lift created is a function of wing surface area, its angle of attack and its airspeed.
However for helicopters it's never that simple. Since a rotor blade rotates around an axis that is at one end of it, no two points along the length of the rotor blade will ever have the same airspeed. The hub of a rotating bicycle wheel turns much more slowly than the tyre. We've already said that lift is a partial function of airspeed - more airspeed basically equals more lift - and the rotor blade is the same width along its length. Therefore it will force more air downwards (this is called 'induced flow' or downwash) and creates more lift at the faster-moving rotor tip, than at the relatively slow-moving root.
Many rotor systems have a mechanism for reducing the effect of this unequal distribution of lift along the rotor blade but that is irrelevant to this writeup. All that is important here is that generally speaking, rotor blade lift increases with the distance from the rotor blade root.
The Vortex Ring State describes the airflow around a helicopter's rotor disc that can precipitate a phenomena called 'Settling with Power' or 'Power Settling' - a situation where a helicopter is descending faster than, and thus enters, the downwash from its main rotor. The prerequisites are a low forward airspeed (less than 10 knots) and a rate of descent of about 300 feet per minute or higher (the minimum RoD will vary from one helicopter to another). The effect itself is a direct result of the dissymmetry of lift between the inner and outer sections of the rotor disc.
We've established that rotor blades force air downwards to create lift, and that outer sections of the blades have higher airspeeds than the inner sections when rotating. The logical conclusion of this is that the outer portions of each rotor blade forces air downwards at a higher velocity than the inner sections. The following is a very rough illustration of the difference in the downwash velocity produced by different portions of the rotor disc:
| | | | | | | | | |
| | | | V V | | | |
| | | V 300 V | | |
| | V 400 V | |
| V 500 V |
V 600 V
*-downwash velocity in feet per minute
Figures used are purely illustrative and should not be used to design a rotor system etc etc...
When a helicopter is descending, ignoring downwash and wind for a moment, all of the rotor disc will meet the air underneath it at the same speed. If the helicopter is descending at 100fpm the air meeting the rotor disc will be travelling upwards at a relative velocity of 100fpm. Now if we bring downwash into it, the velocity (and direction) of air 'leaving' the rotor disc is equal to the difference between the downwash velocity and the relative velocity of the air as it meets the rotors.
If the rotor system in Fig 1 were descending at 100fpm, the velocity of downwash from a rotor blade tip would be 600fpm. From the root it would be 200fpm. If we increased the rate of descent to 300fpm the downwash velocity at the rotor blade root would be zero.
If we kept increasing the rate of descent, eventually we would reach a point at which the relative upward velocity of the air would exceed the downwash velocity. Obviously since the downwash velocity from the inner portions of the rotor disc is lowest, it will occur there first. Using the numbers in Fig 1, once a rate of descent of 400fpm is reached, one third of the length of each rotor blade will be stalled because the velocity of the air meeting it is higher than the velocity of its downwash. We get a situation something like this:
^ | | ^
^ | | | | ^
| | | v v | | |
| | | | | |
| | v v | |
| v v |
Under normal conditions each rotor blade will produce a single vortex that flows around the rotor blade from underneath, around the tip and back into the top again (I'm simplifying, not being at all well-versed in fluid dynamics or aerodynamics). However when the airflow direction is not uniform along the length of the rotor blade (as in Fig 2 - some air is travelling upwards through the rotor system, some is travelling downwards) it will produce vortexes turning in opposite directions due to the interaction of opposing airflows.
If you try to visualize the diagram above in three dimensions (imagine if the right side rotated towards you, the left side away from you, and it left a trail behind it), you may be able to see how there is a ring of downwash surrounding a dome of upwards flow, like an island in the middle of a circular lake. This is where the term 'vortex ring state' comes from. There is a ring of vortexes circulating in one direction, surrounding vortexes circulating in the opposite direction. Of course it's rather more complex than this in reality, as can be seen looking at the first source, which has a number of rather terrifying videos of how rotor wake boundaries change as a helicopter rotor system enters different flight regimes.
It's fairly clear that as rate of descent increases, the rotor blade stall will spread further out from the centre, as the relative velocity of the upward airflow exceeds the downwash velocity of more and more of the rotor disc. The helicopter descends through more and more of its own downwash, turbulence created by the action of its rotors. This 'dirty' air is poor for creating lift, likewise for manoeuvring; the cyclic control effectiveness will reduce in correspondence with the magnitude of the stall. Eventually the main rotor will provide no lift whatsoever; the helicopter will be virtually uncontrollable at this point.
Sources generally use a rather scary diagram (certainly not reproducible here) to show what flight regimes VRS can take place in. Suffice to say that a helicopter is at risk if moving laterally at less than about 10kts and descending at angles greater than 30-40° or at a rate of descent of 300fpm or more, with 20% or more power applied.
As a pilot you'll want to avoid a serious VRS. It does occur in minor form during some fairly normal manoeuvres but does not usually become an impediment to maintaining controlled flight. If it gets serious and the helicopter begins to settle into its own downwash, the symptoms must be recognised as early as possible because past a point, it will be very difficult and eventually impossible to recover. Sources (including accounts of pilots who experienced it) report that as the helicopter enters serious VRS the rotor thrust may fluctuate wildly, the airframe may shudder and cyclic control may become sluggish from the turbulence of the air entering the rotor system.
The instinctive reaction here (if unfamiliar with the symptoms) is to increase the collective pitch of the rotors to increase lift. In fact, doing this exacerbates the situation by creating more disturbance under the rotor disc, stalling it further and actually accelerating the rate of descent. The only way to properly recover is to reduce collective and increase forward airspeed to get out of the rotor wake into 'clear' air. This requires sufficient altitude however; without it, it's time for an unscheduled landing.
I apologise for the unavoidable overuse of the word 'velocity' in this writeup.
Thanks to Fruan, call and tdent for the corrections.
- Rotorcraft Aerodynamics Group; "Vortex Ring State";
- email@example.com; "Helicoptorial AERO-DRY-NAMICS";
- Bloom, Glenn S.; "Helicopters: How They Work";
- Cantrell, Paul; "Settling with Power";
- firstname.lastname@example.org; "Helicopter FAQ";