The Aerodynamics of Vortex Ring State and the Vuichard Recovery Technique

Vortex Ring State (VRS) is one of the most hazardous conditions a helicopter pilot can be in. While it’s part of all the helicopter pilot training syllabuses in the world, it is still present in (fatal) accident reports and discussed widely in the helicopter industry. Lots of pilots have requested for it to be covered on Pilots Who Ask Why, so let’s get into it!

One development that has actually started a lot earlier than commonly appreciated within the industry, is called the Vuichard Recovery Technique (VRT), which could prove a better way of recovering from VRS than the conventional technique taught to most students – even today.

It’s still very new though and regulators are still figuring out whether or not it should be implemented worldwide or not. There are still unknowns that will need to get addressed, but in the meantime, let’s talk about what it is all about!

The conditions, symptoms, and recovery techniques might be very obvious to some, but the nitty gritty details of Vortex Ring State are often overlooked or poorly understood.

The aim of this article is to fix that once and for all, in the hope that it will contribute to the industry wide move to raise awareness for this dangerous state and increase the acceptance and understanding of the newly proposed technique, currently being embraced by some regulators across the globe.

We will go about this here by asking 6 important questions:

  • What are the aerodynamic principles we need to understand first?
  • How exactly does Vortex Ring State work from an aerodynamics perspective?
  • What are the considerations for pilots?
  • What does a conventional VRS recovery look like?
  • What does the Vuichard Recovery Technique have to offer compared to the conventional recovery technique, and what are its benefits?
  • Has this new technique actually been adopted by the industry and its regulators so far?

By answering these questions we can build a crystal clear picture of what VRS is exactly, how it works, and what to do about it! You ready?

WHAT ARE THE AERODYNAMIC PRINCIPLES WE NEED TO UNDERSTAND FIRST?

One thing that needs to be discussed beforehand in order to understand VRS properly is wash-out. Wash-out is the design principle featuring a reducing blade pitch angle as you move towards the tip of the blade, with the aim of equalising the amount of produced lift along a blade. To fully understand this, let’s have a quick look at the lift equation:

Where:

  • CL is the Lift Coefficient (determined by the shape of the blade and the angle of attack)
  • ρ is the air density
  • V is the true airspeed of the blade
  • S is the surface of the blade

The way wash-out works is by changing Cl. To zoom in further on CL, have a look at the graph below to see how CL increases as α (angle of attack) increases, until the blade stalls and loses all lift producing capabilities.

Based on this, we can derive that if the angle of attack reduces, lift reduces, and vice versa. We can now move on to what the vector diagram looks like for the tip and root of a particular blade during normal flight vs in VRS.

Relative (air)flow is made up of Induced Flow (flow going downwards through the disc) and Rotational Flow (airflow hitting the blade due to the blade’s rotational velocity). The angle of attack is the angle between the chord (purple line in the picture below) and the relative flow:

The angle at which the relative flow acts in is roughly the same at the tip and root, as both rotational flow and induced flow increase almost evenly as you get closer towards the tip. This is obviously a simplification, but we’ll use this model going forwards. So what is the main difference?

At the root, due to wash-out, the blade will now be angled up more (a higher pitch angle), and will therefore have a higher angle of attack.

At the tip however, the pitch angle is smaller, and will therefore have a smaller angle of attack:

Washout reduces angle of attack at the tips (and therefore Cl) to make up for the higher airspeed of the blade around the tip. The result is a more equal lift distribution along a blade and therefore across the disc. Otherwise the tis would produce a lot more lift due to their higher velocity (as we saw in the lift equation).

Keep these basic principles in mind for now, we will refer back to these in a few minutes as they are relevant for VRS.

WHAT DO THE VRS AERODYNAMICS LOOK LIKE?

To zoom in to what is actually going on during VRS, let’s first look at a rotor disc from the side that is in the hover:

As you can see, the induced flow is slightly higher around the tip areas due to the higher true airspeed (TAS) (as well as the naturally occurring vortices). This is all normal, and nothing to worry about. The root and tip vector diagrams still look like what we have discussed earlier:

But now let’s introduce a fairly big RoD at low speed and observe what will happen if we enter VRS:

The RoD is obviously the same across the disc, which means that it will be larger compared to the induced flow at the root (which was smaller, remember?). Meanwhile at the tips, the induced flow is sucked back into the disc and goes all the way around the edge of the disc, making the induced flow even larger than it already was! Neither the rooto or the tip of the blades are now actually contributing to keeping our helicopter in the air: We are now in VRS.

The rate of descent has been demonstrated to reach up to 6000’ per minute, and can increase rapidly, but these factors depend on a lot of other variables.

The higher the rate of descent, the more air will get sucked back into the disc. You could describe this as the helicopter descending through its own downwash and the disc is sucking its own wake back into itself! Not a fun time.

Let’s have a look at the root and tip of the blades and put them in a vector diagram as we did earlier, but this time with the new conditions:

The root now has completely stalled due to the large flow from below and its naturally higher blade angle (due to wash-out, remember?) The angle is too large and therefore stalls (as per the Cl – α graph shown before).

The tip however is not stalled. Instead, the induced flow is now so large that the angle of attack (α) is almost non-existent and therefore no lift is produced here either (again, wash-out reduced the pitch angle as well).

Notice that neither situations produce any lift, hence the large RoD during VRS and the reason it is so hazardous.

Now that we have discussed the flow dynamics, and you are actually still awake, let’s move on the most important reason you are here:

SO WHAT ARE THE CONSIDERATIONS FOR PILOTS?

One of the main reasons Vortex Ring State is such a hazardous condition, is because it will cause the helicopter to descent rapidly if uncorrected, with a less responsive control output.

What makes this worse is that the conventional recovery technique requires a lot of height to be completed successfully, which is why most training schools teach pilots to adhere the popular ‘prevention is better than cure’ perspective.

When the Rate of Descent (RoD) of a helicopter approaches the speed of the induced flow going downwards through the rotor disc, the tip vortices will not dissipate anymore, and instead will make themselves stronger and stronger. Sounds a little creepy right? Well it is, especially considering it can be a very deadly flight condition.

VRS is often confused with Power Settling or Settling With Power (SWP), the terminology is different depending on which side of the Atlantic you are based. To highlight the origins of it, in the 1950’s the US Navy called VRS ‘Power Settling’ and used ‘Settling with Power’ to describe the situation where power required exceeds power available at any point (NOT VRS!).

The US Army reversed this in the 1960’s and caused most of the Vietnam pilots to refer to VRS as ‘Settling With Power’ and the running out of power on a tight landing site in hot / high conditions ‘Power Settling’ (PS). Around the 1980’s, the US Navy adopted this terminology.

The take-away here is that neither SWP nor PS is VRS. VRS is what happens to a rotor disc when it is descending with a high RoD, a low forward speed, with power applied. ‘Power applied’ means that the vortices at the blade tips are being forced into the induced flow, you would not have this in an autorotation as there is no induced flow, which IS a requirement!

VRS can develop as a result of SWP, but these 2 terms are not the same. So to summarise, the requirements for VRS to develop, as taught in more modern literature, are:

  • A low or zero true airspeed and below translational lift
  • Some sort of collective input (power applied), creating induced flow
  • A RoD ranging from 300 – 800’ per minute, depending on disc size and aircraft type

When this happens, the main clues (symptoms) to detect VRS are:

  • Random uncontrolled Pitch, Roll and Yaw
  • Aircraft vibrations
  • Stick shake
  • Increasing RoD
  • Less control authority

So technically speaking, you could recover from VRS by simply entering autorotation. In most cases though, this won’t be ‘simple’ as you will highly likely be lower to the ground and not have enough vertical separation from the surface or obstacles to make this work, due to the nature of the phases of flight that are prone to VRS. The most common situation in which VRS is a big risk are:

  • A downwind approach
  • A downwind quickstop
  • A flare in combination with height loss
  • A normal powered descend with a high RoD and low IAS
  • The final phase of an autorotative landing to power recovery

It’s unfortunately not as simple as stating these ‘static’ requirements or flight phases. It really is all about whether or not the blade vortices are being reintroduced into the induced flow that is going through the disc or not.

This depends on the combination of airspeed, rate of descend, as well as the amount of power in use (and therefore pitch angle / induced flow). The graph below shows what this looks like (this is type specific information and will not translate to all helicopter types, it’s used to illustrate the principle):

To summarise this weird looking graph: the wake location of the vortex will change depending on the 3 variables discussed earlier. VRS will be worse when the wake lies directly on the disc. If the RoD is too little, the wake lies below the disc, if the RoD is too large, the wake is located above the disc. And of course, if IAS is too high, the wake lies behind the rotor disc, which brings us to the recovery:

WHAT DOES A CONVENTIONAL VRS RECOVERY LOOK LIKE?

To perform a ‘conventional’ recovery from VRS, we need to get rid of one of the earlier discussed requirements. You could get rid of power, but this means entering autorotation, which let’s be honest, is usually not an option, especially during low altitude operations.

The other more realistic option is to increase TAS by using forward cyclic, although this also results in height loss, just in a less dramatic way. As the TAS reaches a point where fresh airflow blows the wake to a location behind the disc, we have successfully recovered from VRS and we can safely increase power to climb away.

In addition to this, there is also the newer Vuichard Recovery Technique:

THE VUICHARD RECOVERY TECHNIQUE

So you might be asking “what is this other recovery technique all about then?” Well, unfortunately, helicopter aerodynamics are less clear-cut and researched compared to the fixed wing side of things. There are more grey areas and quite often, research that does get done yields multiple results, rather than one obvious thing. Which is why we are still learning loads about helicopter aerodynamics and how to interact with it.

With this in mind, the VRS technicalities and therefore recovery techniques, have been somewhat of a grey area as well, with various pilots, industry branches or parts of the world differing in views on how to define and attack it (as discussed earlier). The main problem with the conventional way of recovery is the critical amount of height lost in the process. This is where the Vuichard Recovery Technique (VRT) comes in.

Developed by senior Swiss pilot and examiner Claude Vuichard, the VRT is a method that promises (and has been proven to) recover from VRS with only around 50’ height loss or less, which is substantially better than what we initially had to deal with during the ‘old’ recovery technique taught in most syllabuses.

The goal of the VRT is to use the upwards flowing part of the vortex to our advantage. Remember that we said one of the main problems at the tips was the excessive amounts of induced flow because of the vortices, resulting in a very small angle of attack and therefore a breakdown in lift?

Well, by pushing the entire disc to the side horizontally using tail rotor thrust and lateral cyclic, the blade tips at one side of the disc will enter the upwards flow of the vortices, increasing the angle of attack, and restoring lift. But how?

This is achieved in helicopters with counter clockwise rotating blades by increasing collective power and keeping the nose straight with power pedal, while using right cyclic to achieve 10 – 20 degrees of bank towards the advancing side of the disc. For helicopters with clockwise rotating blades, pedal and cyclic use will be required in the opposite direction. The aim is to make the outer advancing side of the disc (utilising the tail rotor thrust and cyclic) to hit the upwards flowing air of the vortex.

This up-flow of air interacting with the disc will then massively reduce the amount of induced flow discussed earlier (which is the root cause of all our VRS issues), increase angle of attack, and therefore restore lift generating capabilities. Effectiveness does slightly differ depending on helicopter type. The blade tip vector diagram will look more like this:

As described by Claude, the recovery is complete as soon as one side of the disk edge hits this upwards flow properly, which can be achieved (and is demonstrated) with less than 50’ height loss! This procedure, despite it’s smaller amount of height loss, is still only recommended to perform above 1000’ AGL for training: apply common sense.

Please keep in mind though, that all of these aerodynamic diagrams represent a simplified ‘model’ of reality. In order to fully describe the interaction between the disc and the flow dynamics of the vortex, multiple combined models will have to be used that are beyond the scope of this article.

To visualise this recovery technique, have a look at Claude’s video on his youtube channel below, which visualises the actual vortices and makes the whole technique a little bit more tangible to understand (you can skip to 2:06 to see the visualisation straight away):

Claude’s initiative, the Vuichard Recovery Aviation Safety Foundation, has been pushing regulators for years to bring this technique into the hands of training schools and make it legally required. His goal is to eliminate VRS as the cause of any fatal accident by 2025. If you want to know more or are interested in more details, Claude has safety courses that you can attend by booking via his website, which is linked below.

SO HAS THE INDUSTRY ADOPTED THIS NEWER TECHNIQUE, OR NOT?

Not really. While most regulators are starting to catch on to the fact this offers a much more efficient way of recovering from VRS, most syllabuses remain unaffected, which is a shame. EASA is currently doing lots of research into VRS, which will hopefully speed up the process of officially recognising Claude’s breakthrough discovery. To summarise the industry developments:

CONCLUSION

It is going to be very interesting to see if and when EASA fully adopts this, and whether or not their 2 year research program that started in 2021 will yield any significant points. Either way, the VRT is on the rise, and will hopefully save many lives in the industry!

Please only practice this if your flight instructor is fully qualified and endorsed by The Vuichard Recovery Aviation Safety Foundation, on a type that has been demonstrated to be safe for this practice.

Full credit to Claude Vuichard for developing this technique and for doing such amazing work to improve flight safety across the industry.

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