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Helicopter Vortex Ring State

Issue No 26, 8 May 2023

By: Anthony O. Ives

Vortex ring state is a dangerous flight condition which a helicopter can experience in a fully powered, low speed and high descent rate scenario. Vortex ring state reduces the efficiency of the helicopter's rotor to produce thrust resulting in an increasing descent rate, the natural response of the pilot to apply more power will worsen the condition causing higher rates of descent. Which situations vortex ring state can be encountered can often be confusing for pilots as it is often associated with allowing the helicopter to descent too rapidly however autorotations with high rate descents do not generate the vortex ring state. Autorotations are a power off helicopter descent, vortex ring state is only possible in power on descents. 'Settling with Power' is another term used to refer to vortex ring state but the term can also refer to any condition where increasing helicopter engine power does not reduce the helicopter's descent such the engine not being able to generate enought power due to helicopter weight or environmental conditions such as temperature, altitude or humidity, etc.

There are three conditions that must all occur at the same time to produce a vortex ring scenario, similar to the fire triangle. A fire can only start if you have fuel, oxygen and heat, if one is missing then a fire is not possible and if one is removed then the fire will extinguish. The three conditions for helicopter vortex ring state are as follows:

Power Applied: The engine must be turning the rotor blades, in the case of autorotation where aerodynamics forces are turning the rotor blades, vortex ring state is not possible.
Low Forward Speed: The helicopter has to have no or little forward speed typically less than 30 Knots, but the exact speed would depend on the type helicopter so best to consult the RFM (Rotorcraft Flight Manual) for the type of helicopter you are flying.
High Descent Rate: The helicopter has to be descenting rapidly typically greater than 300 feet per minute, but the exact descent would depend on the type helicopter so again its best to consult the RFM (Rotorcraft Flight Manual) for the type of helicopter you are flying.

This is a general description of vortex ring state, vortex ring state scenarios can varying from helicopter to helicopter so you should always refer to the RFM (Rotorcraft Flight Manual) for a full understanding of the vortex ring characteristics of the helicopter you are flying.

Like the fire triangle you have to find a way to elimate one of the three contributing factors in order to recover from vortex ring state. Most flight training references [1] and [2] recommend increasing forward speed using a typical procedure as follows:

Increase Forward Speed: Using forward cyclic to increase forward speed do not increase power until forward speed is sufficient.
Increase Power to Regain Lost Altitude: Only increase power once sufficent forward speed is achieved, increasing power with insufficent forward speed can deepen the vortex ring condition.

Vortex ring state is the result of the helicopter descenting into its own downwash and creating large vortices at the rotor blade tips. Increasing rotor power will increase the downwash resulting in large vortices its a sort of vicious circle senario as increasing power is your natural response when loosing height. Reference [3] gives details of how to calculate inflow for vortex ring state and also demostrate how the massive inflow rates produced in vortex ring state would reduce the lift produced by the helicopter rotor blades. See picture below for graphic description of vortex ring state.

Helicopter Vortex Ring State

The vortices at the rotor blade tips require more power but the rotor is generating little thrust due to the downwash reducing the lift produced by the rotor blades. Vortex state ring can be hard detect typical indications:

Increase in Vibrations.
Random Uncommanded change in Pitch, Roll or Yaw.
Vertical Speed Indicator (VSI) showing High Descent Rate.

The high descent rate is usually the best way of detecting vortex ring state in a high hover scenario, however it best to avoid getting into vortex ring state, this is usually by training pilots to be aware when they are likely to encounter vortex ring state. Typical scenarios where vortex ring state may be encountered are:

Aerial photography (High Hover): This is the common scenario where particularly a novice pilot can find themselves in vortex ring state, as they are distracted by their friend wanting to take a photograph they end up in a high hover with descent rate increasing.
Quick Stops: This is a maneuver to bring the helicopter to a stop from a high forward speed, however allowing the helicopter to descend could lead it to encounter vortex ring state as the pilot has to apply power towards the end of the maneuver.
Power Recovery Autorotations: Vortex ring state cannot be encountered in an autorotation but in power recovery autorotations because power is applied at a point where there is little forward and potentially a high descend rate then vortex ring state could very likely be encountered.
Approach to Land: is another less likely flight condition where you may encounter vortex ring state, however this is normally avoided by using a constant slope approach.

The graphic below shows a typical approach path of helicopter which is about a constant angle of 10 to 15 degrees, this keeps the ratio of forward speed to descent rate constant and prevents the helicopter entering vortex ring state.

Helicopter Approach to Land

Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:

Mathematics
Helicopters
VTOL UAVs (RC Helicopters)
Sailing and Sailboat Design

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References:

[1] The Helicopter Flying Handbook, FAA-H-8083-21B, 2019, United States Department of Transportation, Federal Aviation Administration, https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/helicopter_flying_handbook

[2] Learning to Fly Helicopters, R. Randall Padfield, 1992, McGraw Hill

[3] http://www.eiteog.com/EiteogBLOG/No11EiteogBlogThrust.html

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