Vortex Ring State

Vortex Ring State (VRS) is a hazardous aerodynamic condition in which a rotor becomes engulfed in its own wake. It impacts rotors at low airspeeds and moderate descent rates, causing a loss of thrust and control authority.

In this article, we focus on the main rotor. Tail rotors also experience VRS, that’s discussed in our article Loss of Tail Rotor Thrust.

Aerodynamic Principles

Helicopter rotors create thrust by accelerating air in the opposite direction (downwash). This creates vortices on the outer portion of blades—regions in which the airflow is circular as shown in red in the figure below.

Vortices shed from the rotor can persist for over a minute before dissipating. In this time, they move under the influence of wind and other vortices while the rotor continues shedding new vortices. In normal conditions, vortices move away from a helicopter and are only a danger to other aircraft flying too close. This is shown in the figure below.

At low air speeds, there’s no airflow to push vortices aft of the helicopter. The vortices travel almost straight down. When the helicopter also descends at about the same rate as these vortices, the vortices begin to pile up around the rotor, as shown in the 3rd snapshot of the figure below.

When the helicopter descends at this rate, the piled-up vortices push each other around in a chaotic fashion. The air velocities they induce on the rotor are likewise chaotic, and large (due to the extra vortices lingering close by). This is the infamous vortex ring state.

At higher descent rates, including autorotation, the helicopter descends faster than the vortices. In this condition, old vortices are safely above the helicopter, mitigating adverse aerodynamic effects.

Conditions for Entry

For a helicopter to enter vortex ring state, it must be at low airspeed with a significant descent rate. It must also hold this condition for a few seconds, while the vortices pile-up around the rotor, as described in the prior section.

Low Forward Airspeed

The aircraft must be flying below effective translational lift, typically less than 30 knots. At larger airspeeds the vortices are pushed aft and cannot pile up around the rotor. The range of descent rates depends on the airspeed, and a larger range of descent rates cause VRS at lower airspeeds.

Descent Rate

There’s a range of descent rates at which VRS occurs. There are two descent rates to consider here: (1) the smaller descent rate at which VRS begins when (very) slowly accelerating down from hover (V1) and (2) the larger descent rate at which VRS subsides, before reaching autorotational descent (V2). Both V1 and V2 depend on the helicopter model, weight, atmospheric conditions, and horizontal airspeed.

The range is typically between 300 and 4,000 feet per minute (FPM). Prouty, in his famous book Helicopter Aerodynamics, has characterized the range based on both flight and wind tunnel tests, as 0.25v to 1.25v, where v is the hover induced velocity. Some industry experts use 0.5v as a typical VRS condition.

An example curve of V1 and V2, as a function of airspeed and descent rate, follows. In this example V1 is about 1000 FPM without horizontal airspeed, and VRS apparently doesn’t occur above roughly 25 KT.

Measuring V1 and V2 precisely in flight test has proven challenging. The results are stochastic: repeating the same flight profile, with the same helicopter, at the same time of day, often results in +/- 25% scatter in V1. This is likely due to a combination of (1) the chaotic nature of the bunching up vortices and (2) the lack of a clear criteria for exactly when VRS started. Measurement of V2 is even more difficult.

For more details about flight test measurements, check out EASA’s VRS research project.

Symptoms and Hazards

VRS is particularly dangerous because, upon sensing a thrust loss, a pilot may instinctively try to increase collective. This can increase the strength of the vortices and worsen the condition.

Key symptoms experienced by the pilot are listed below.

• Increased vibration and buffet with associated rumbling sounds.
• Loss of thrust / a feeling of "lightness in the seat".
• Reduced control effectiveness, particularly in collective and cyclic.
• Uncommanded pitch, roll, and/or yaw oscillations.

To make matters worse, VRS conditions are most likely to be encountered at lower altitudes in approach, when a sudden loss of altitude can quickly become deadly.

Avoidance

Avoidance is the primary safety strategy. Pilots are trained to maintain small descent rates at low airspeeds. This must be closely monitored during low-speed approaches, particularly when a tail wind is present.

Recovery

There are three often-cited techniques to pull a helicopter out of VRS. The most common maneuver is to apply forward cyclic to gain airspeed and then raise collective. Forward cyclic accelerates the helicopter forward and distances it from the vortices. Once in cleaner air, the collective becomes responsive, and can arrest the descent.

The second technique is called the "Vuichard Recovery." The pilot moves the cyclic laterally (in the direction of tail rotor thrust) before increasing collective. The point of lateral cyclic is again to move the rotor away from the vortices. Because helicopters can typically roll faster than pitch, lateral cyclic can potentially exit VRS quicker than longitudinal cyclic. The downside is that it is considered less safe and requires the pilot to recall (under pressure) which direction of lateral cyclic to use.

The last technique is to lower collective, then apply forward cyclic, and finally increase collective. While lowering collective may promote faster VRS exit, this method is rarely taught because it results in a greater altitude loss, increasing the likelihood of a crash.