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 :

Two presentations of the work are also available to watch :

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.

Dr Brown's work on the Vortex Ring State has been covered in a number of articles in the popular aviation press. Click on the links below to read these articles.

• Pamir Sevincel, 'Are eVTOL Aircraft Inherently more susceptible to the Vortex Ring State than Conventional Helicopters?', Clear Prop, online, April 2023.
• Alex Scerri, "Sophrodyne's Richard Brown on the Susceptibility of eVTOL Aircraft to Vortex Ring State", Vertical Magazine, online, January 2023.
• Jens Rosenow, 'The Invisible Threat - are eVTOLs Especially Endangered?', Rotors Magazine (subscription required), online, December 2022.
• Elan Head, 'An old Aerodynamic Problem could pose a New Challenge for eVTOL Aircraft', The Air Current, online, September 2022.

The website of the Vuichard Recovery Aviation Safety Foundation, is also very much worth visiting for its very graphic representations of the behaviour of helicopters within the Vortex Ring State.

Dr Brown has been working with Claude Vuichard, the founder of this organisation, to help create training materials that will educate pilots to the characteristics of the VRS as well as to provide them with workable strategies to avoid, or escape from, the phenomenon should they encounter it in flight.

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?', Cheeseman Award Paper, 79th Annual Vertical Fight Society Forum and Technology Display, West Palm Beach FL, USA, 16-18 May 2023.
• 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.

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