Resource Page : the Vortex Ring State

The Vortex Ring State (VRS) is a potentially hazardous regime of operation that a rotorcraft can enter during the critical descent and landing phases of flight, as well as potentially in manoeuvres at low forward speeds.

The page below contains a set of resources that you might find useful in helping you to understand

  • the physical origins of the Vortex Ring state,
  • how and why the VRS is such a dangerous phenomenon,
  • how the possibility of entering into the VRS might influence the design and operation of future air vehicles, and indeed
  • how certain design decisions might make an aircraft more or less susceptible to the VRS.



We would be very happy to discuss our work with you. If you would like to talk to us about our research, please feel free either to contact us directly or to access our contact form through the tab below.


Click on one of the buttons below to download a copy of Dr Brown's paper, delivered on 8 September 2022 at the 48th European Rotorcraft Forum in Winterthur, Switzerland.

There is currently extensive interest worldwide in developing small, lightweight, electrically-powered, multi-rotor vertical takeoff and landing aircraft. The aim is to use these new vehicles to carry a small number of passengers on short-range intra-urban missions. The concern is that these aircraft might have certain characteristic design features that, in combination with the environmental conditions that they will experience, will render them particularly susceptible to the VRS. This paper extends our classical understanding of the basic physics that underpins the VRS in order to assess the likely impact of this phenomenon on the safety and operational characteristics of this new class of vehicle.


Two versions of the paper are available for download :


  • The version of the paper that has been archived as part of the proceedings of the 48th European Rotorcraft Forum.
  • A version of the paper that has been modified to correct some typographical errors and to improve the clarity of some of the figures. This version of the paper should be considered as fluid - it may be updated in future to incorporate additional data and information as it becomes available.
Understanding the Vortex Ring State
The tab below will take you to a re-recording of Dr Brown's online lecture to the Bedford branch of the Royal Aeronautical Society, delivered on 9 March 2022.

In his lecture, Dr Brown examines our current understanding of the VRS.

He tells the story of how data from laboratory and numerical experiments was combined in the mid-'00s to change our understanding of the basic mechanisms at the heart of the phenomenon.

As an illustration of how mathematical abstraction can lend deep insights into practical engineering problems, he then shows how a theory was developed that is now used throughout the world to help aircraft designers to understand the susceptibility of their creations to this potentially dangerous aerodynamic phenomenon.

Dr Brown ends the lecture by discussing the implications of his research for the next generation of helicopters and eVTOL aircraft that are about to come into service.


Research Papers
Dr Brown and his collaborators have produced a series of technical papers that have changed the way that we understand, model and analyse the Vortex Ring State.

The following publications are available in the open literature :

  • BROWN, R.E., 'Are eVTOL Aircraft Inherently more susceptible to the Vortex Ring State than Conventional Helicopters?', 48th European Rotorcraft Forum, Winterthur, Switzerland, 6-8 September 2022.
  • AHLIN, G.A, and BROWN, R.E., 'Wake Structure and Kinematics in the Vortex Ring State', Journal of the American Helicopter Society, Vol.54, No.3, 2009, pp. 032003-1 - 032003-18.
  • AHLIN, G.A, and BROWN, R.E., 'The Vortex Dynamics of the Rotor Vortex Ring Phenomenon', 63rd American Helicopter Society Annual Forum, Virginia Beach VA, USA, 1-3 May 2007.
  • PERRY, F.J., CHAN, W.Y.F., SIMONS, I.A., BROWN, R.E., AHLIN, G.A., KHELIFA, B.M., and NEWMAN, S.J., 'Modeling the Mean Flow through a Rotor in Axial Flight including Vortex Ring Conditions', Journal of the American Helicopter Society, Vol.52, No.3, 2007, pp. 224-238.
  • BROWN, R.E. and AHLIN, G.A., 'Predicting the Onset of the Vortex Ring State Under Accelerated Flight Conditions', 61st American Helicopter Society Annual National Forum, Gaylord TX, USA, 1-3 June 2005.
  • BROWN, R.E. and AHLIN, G.A., 'Investigating the Physics of Rotor Vortex-Ring State Using the Vorticity Transport Model', 31st European Rotorcraft Forum, Florence, Italy, 13-15 September 2005.
  • BROWN, R.E., SIMONS, I., CHAN, W.Y.F., PERRY, F.J., and NEWMAN, S., 'Modelling the Mean Flow Through a Rotor in Axial Flight Including Vortex Ring Conditions', 2nd International Basic Research Conference on Rotorcraft Technology, Nanjing, China, 7-9 November 2005.
  • GREEN, R.B., GILLIES, E.A., and BROWN, R.E., 'The Flow Around a Rotor in Axial Descent', Journal of Fluid Mechanics, Vol.534, 2005, pp. 237-261.
  • BROWN, R.E., LINE, A.J., and AHLIN, G.A., 'Fuselage and Tail-Rotor Interference Effects on Helicopter Wake Development in Descending Flight', 60th American Helicopter Society Annual National Forum, Baltimore MD, USA, 7-10 June 2004.
  • BROWN, R.E., NEWMAN, S.J., PERRY, F.J., AHLIN, G.A., MODHA, A., and KHELIFA, M., 'Revisiting the Effects of Blade Geometry on Rotor Behaviour in Descending Flight', 30th European Rotorcraft Forum, Marseilles, France, 14-16 September 2004.
  • BROWN, R.E., LEISHMAN, J.G., NEWMAN, S.J., and PERRY, F.J., 'Twist Effects on Rotor Behaviour in the Vortex Ring State', 59th American Helicopter Society Annual National Forum, Phoenix AZ, USA, 6-8 May 2003.
  • BROWN, R.E., NEWMAN, S.J., PERRY, F.J., ORCHARD, M., MODHA, A., and LEWIS, S., 'Predicting the Onset of Wake Breakdown for Rotors in Descending Flight', Journal of the American Helicopter Society, Vol.48, No.1, 2003, pp. 28-38.
  • BROWN, R.E., LEISHMAN, J.G., NEWMAN, S.J., and PERRY, F.J., 'Blade Twist Effects on Rotor Behaviour in the Vortex Ring State', 28th European Rotorcraft Forum, Bristol, England, 17-20 September 2002.
  • BROWN, R.E., NEWMAN, S.J., PERRY, F.J., ORCHARD, M., MODHA, A., and LEWIS, S., 'Comparative Numerical and Experimental Investigations of the Vortex Ring Phenomenon in Rotorcraft', 57th American Helicopter Society Annual National Forum, Washington DC, USA, 9-11 May 2001.



We would be very happy to discuss our work with you. If you would like to talk to us about our research, please feel free either to contact us directly or to access our contact form through the tab below.


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