Vortex ring stage

As a vortex ring state or vortex ring stage ( English vortex ring state ) is called a dangerous condition of a helicopter in hover or low horizontal speed in which the latter is lowered in its own generated by the main rotor downwash (downwash).

General explanations for understanding

Induced speed

The main rotor of a helicopter, which is set in rotation by a motor or a turbine , generates a negative pressure directly above the rotor plane (assumed surface on which the rotor blades rotate) and an overpressure below it. The air mass is sucked through the rotor plane and accelerated downwards by the main rotor depending on the angle of attack of the rotor blades. This technical downdraft is also known as downwash.

The speed of the air mass accelerated downwards by the main rotor is called the induced speed (derived from the Latin inductio "(Her- / Hin) introduction"). It is abbreviated as v _i and is given in meters per second ( ). For helicopters in hovering flight (v _i0 ) it is usually around 6 (= around 21.5 ). ${\ displaystyle {\ tfrac {m} {s}}}$ ${\ displaystyle {\ tfrac {m} {s}}}$ ${\ displaystyle {\ tfrac {km} {h}}}$

Air throughput in hover

The air throughput when hovering is not evenly distributed over the rotor blade. In the middle it is higher than at the blade tip and the end that faces the rotor head . One speaks here of the non-uniform induced speed . The distribution of the induced speed over the rotor blade is given by v _ir , where r is the current rotor radius in meters.

Vertical (vertical) airspeed

Vertical is defined here as perpendicular to the earth's surface or directed towards the center of the earth.

The vertical or vertical airspeed (also called the rate of descent or rate of descent) is the rate at which a helicopter descends towards the ground. It is abbreviated _as v _z and given in meters per second. For helicopters it is usually 2.5 . ${\ displaystyle {\ tfrac {m} {s}}}$

Air flow in descent

Due to the vertical airspeed, an air flow is generated in the hover, which counteracts the downwash. Since the air throughput is lower in the area of the rotor head than in the center of the rotor blade, the acceleration of the air mass is partially compensated for or even reversed by the counteracting air flow depending on the rate of descent of the helicopter.

Horizontal (horizontal) airspeed

Horizontal or horizontal airspeed is the airspeed reached parallel to the ground. It is given, among other things, as true airspeed or actual ground speed in knots (kn).

Flow behavior on the helicopter

Vortex formation on the main rotor blade

Laminar flow around a rotor profile

In fluid mechanics , circular flows of a fluid (gas or liquid) are referred to as eddies or vortex. Vortices form when there is a sufficiently large difference in speed within a fluid, so that part of the gas or liquid flows significantly faster than the rest.

A laminar flow is created on rotating rotor blades , so that the air flows in layers along the surface of the rotor blades that do not mix with one another. However, depending on their angle of attack, the rotor blades also have a frictional resistance which causes vortex formation on the rotor blades. A larger angle of attack results in greater frictional resistance and thus also in larger eddies.

Depending on their size , the eddies withdraw energy from the flow . B. convert it into thermal energy. In hover, performance losses due to eddies at the rotor blade tips are in the order of 2 to 4%.

Development of the vortex ring stage on the main rotor

Three-dimensional representation of the vortex ring

Vortices also form at the rotor blade tips, particularly when hovering. At high rates of descent (vertical flight speeds) - when hovering in the range between to and the induced speed - part of the air masses that have already been accelerated downwards flows past the main rotor, is sucked in again from the top and accelerated downwards, so that the induced speed increases further becomes. This process is also called recirculation . ${\ displaystyle {\ tfrac {1} {4}}}$ ${\ displaystyle {\ tfrac {5} {4}}}$

The vortices at the rotor blade tips interact with each other when the rate of descent (vertical airspeed) reaches approximately the induced speed. If the vertical airspeed increases to the induced speed, the interactions between the vortices are so great that a closed vortex ring is formed around the rotor blade tips on the rotor plane. This grows along the rotor blade from the outside to the inside and has the shape of a torus . ${\ displaystyle {\ tfrac {1} {4}}}$ ${\ displaystyle {\ tfrac {3} {4}}}$

Schematic representation of the vortex ring stage

This reduces the lift generated by the main rotor and increases the vertical flight speed. The helicopter gets into a vicious circle of reduced lift, resulting in increased vertical airspeed and the resulting increase in recirculation in connection with the enlargement of the vortex ring. This can lead to an almost complete loss of lift and thus to the helicopter crashing.

The vortex ring does not affect the interior of the rotor, i. H. around the rotor head. Here, however, due to the greater vertical airspeed, the flow velocity of the rotor blades is increased from below, which can lead to a flow separation in the interior of the rotor blades and thus to a further reduction in lift. This area of flow separation grows as the vortex ring increases along the rotor blade from the inside outwards until both areas extend over the entire rotor blade.

Causes, consequences and countermeasures

Dangerous framework conditions

The development of the vortex ring stage is favored by certain framework conditions. These are:

The horizontal relative speed to the air is low (flight speed less than 10 knots , i.e. approx. 18.5 or the helicopter has a strong tailwind when descending rapidly) ${\ displaystyle {\ tfrac {km} {h}}}$

and

the rate of descent is greater than the induced speed (approx. 300 feet / minute = approx. 1.5 )

{\ displaystyle {\ tfrac {1} {4}}}

{\ displaystyle {\ tfrac {m} {s}}}

or

The rate of descent (v _z ) lies between and the induced velocity (v _i0 ); the following applies: ${\ displaystyle {\ tfrac {1} {4}}}$ ${\ displaystyle {\ tfrac {5} {4}}}$

{\ displaystyle \ mathrm {{\ tfrac {1} {4}} \, {v} _ {i0} \, {<} \, {v} _ {z} \, {<} \, {{\ tfrac {5} {4}} \, {v} _ {i0}}}}

and

the current engine / turbine output is greater than 20% of the maximum output.

Effects of the vortex ring stage

When a helicopter flies in the vortex ring stage, strong vibrations can be detected , especially in the area of the rotor head. In addition, due to the unclean flow onto the rotor from above and irregular eccentric outflows below the rotor, the helicopter executes movements that are difficult to control due to the recirculation .

The engine or turbine power of the helicopter is almost completely converted into the ever stronger acceleration of the air masses in the vortex ring without generating any significant lift. The vertical speed can be increased to over 10 (= 36 ), i.e. four times the normal rate of descent. ${\ displaystyle {\ tfrac {m} {s}}}$ ${\ displaystyle {\ tfrac {km} {h}}}$

Actions by the helicopter pilot

The helicopter pilot must react very quickly to any signs of the vortex ring stage in order to prevent a crash. This applies even more to the landing approach .

Since increasing the rotor speed or the angle of attack of the rotor blades to increase lift only aggravates the vortex ring stage (see above vicious circle), the only thing left to the pilot - provided that the helicopter is sufficiently controllable - is to increase the horizontal speed (mostly forward, but also to the side or backwards) to stabilize the helicopter.

The (horizontal) movement forwards, backwards or to the side reduces the recirculation at the rotor blade tips and, in the best case scenario, dissolves the vortex ring. This results in an increase in lift and a reduction in vertical speed.

If the helicopter is still at a sufficient height above the ground, the pilot can switch to autorotation . The main rotor, which rotates freely from below due to the air flow (without drive from the motor or turbine), no longer accelerates the air downwards, so that the recirculation is interrupted.

Accidents caused by a vortex ring stage

A V-22 Osprey on approach for landing

A Bell Boeing V-22 tilt rotor aircraft crashed on April 8, 2000 because of the vortex ring stage.

A Robinson R44 helicopter crashed on October 24, 2010 in Altenbeken from a height of about ten meters above a street. On board was the German pop singer Anna-Maria Zimmermann , who was seriously injured in the crash. In an analysis of the accident, the Federal Bureau of Aircraft Accident Investigation came to the conclusion that several factors, including a vortex ring stage, led to the crash.

One of the two modified UH-60 helicopters used in the attempted arrest of Osama bin Laden during Operation Neptune's Spear on May 2, 2011, also got into a vortex ring stage while hovering. During the subsequent emergency landing, the helicopter collided with a wall of the property and crashed. It was blown up and left behind by the special forces deployed, the United States Navy SEALs .

Vortex ring stage on the tail rotor

Under certain unfavorable conditions the tail rotor can also enter a vortex ring stage. This is particularly the case when the air masses accelerated to the side by the tail rotor (air jet) are due to

rapid rotation of the helicopter in hover around the vertical axis in the direction of the air jet of the tail rotor,
rapid sideways hovering of the helicopter in the direction of the air jet of the tail rotor or
strong cross wind against the tail rotor air jet

encounter air masses flowing in the opposite direction. The tail rotor becomes ineffective and in the worst case the helicopter becomes uncontrollable.

Here, too, there is a way out by increasing the horizontal speed forward. By the weather vanes effect of the rudder unit (tendency of an aircraft to rotate about its center of gravity to rotate and extending into the wind align to the air resistance ) to minimize the helicopter is stabilized. Due to the changed flow to the tail rotor, the recirculation at the tail rotor blade tips is reduced and, in the best case, the vortex ring is dissolved.

Helicopters, in which a Fenestron compensates for the yaw moment, are largely insusceptible to the formation of a vortex ring stage on the tail rotor due to the large-area covering of the tail rotor blades.

literature

Walter Bittner: Flight mechanics of the helicopter. Technology, the flight dynamics system helicopters, flight stability, controllability. 3rd, updated edition. Springer, Berlin et al. 2009, ISBN 978-3-540-88971-7 .
Helmut Mauch: The helicopter flight school. GeraMond Verlag GmbH, Munich 2010, ISBN 978-3-7654-7349-4 .

Web links

Scientific Comic Books, Volume 17 - "Float like in seventh heaven"
Case studies on how to behave in and overcome the Vortex Ring State for helicopters and tilt rotor aircraft, University of Maryland website , on umd.edu

Individual evidence

↑ ^a ^b ^c ^d ^e ^f ^g ^h Walter Bittner: Flight mechanics of the helicopter. Technology, the flight dynamics system helicopters, flight stability, controllability. 3rd, updated edition. Springer, Berlin et al. 2009, ISBN 978-3-540-88971-7 .
↑ ^a ^b Jean-Pierre Petit, "Floating like in seventh heaven", www.savoir-sans-frontieres.com
↑ ^a ^b ^c Norbert Grüntjens: RC electric helicopter . The guide. Big steps to success. Ikarus, Schramberg-Waldmössingen 2006, ISBN 3-00-020372-9 .

↑ ^a ^b ^c vortex ring stage. In: rc-heli-action. Born in 2012, No. 2, ISSN 1869-9219 .

↑ ^a ^b ^c ^d ^e ^f Helmut Mauch: The helicopter flight school. GeraMond Verlag GmbH, Munich 2010, ISBN 978-3-7654-7349-4 .

↑ Kevin, Lieutenant Colonel US Marine Corpsv Gross, Tom Macdonald, MV-22 test pilot and Ray Dagenhart, MV-22 lead government engineer: Dispelling the Myth of the MV-22 . In: The Naval Institute (ed.): Proceedings . No. September 2004, September 2004. Retrieved April 9, 2009.

↑ Saving the Pentagon's Killer Chopper Plane . In: Wired, July 2005 . Retrieved March 9, 2011.

↑ Ex- "DSDS" singer in an artificial coma . focus.de. Retrieved October 25, 2010.

^ The (Stealth) Blackhawk Crash. avweb.com, accessed May 2, 2011 .

↑ Niels Klußmann, Arnim Malik: Lexicon of aviation. 3rd, updated edition. Springer, Berlin et al. 2012, ISBN 978-3-642-22499-7 , p. 311.

[Bittner-1] ↑ ^a ^b ^c ^d ^e ^f ^g ^h Walter Bittner: Flight mechanics of the helicopter. Technology, the flight dynamics system helicopters, flight stability, controllability. 3rd, updated edition. Springer, Berlin et al. 2009, ISBN 978-3-540-88971-7 .

[Petit-2] Jean-Pierre Petit, "Floating like in seventh heaven", www.savoir-sans-frontieres.com

[Grüntjens-3] Norbert Grüntjens: RC electric helicopter . The guide. Big steps to success. Ikarus, Schramberg-Waldmössingen 2006, ISBN 3-00-020372-9 .

[RCHeli-4] vortex ring stage. In: rc-heli-action. Born in 2012, No. 2, ISSN 1869-9219 .

[Mauch-5] ↑ ^a ^b ^c ^d ^e ^f Helmut Mauch: The helicopter flight school. GeraMond Verlag GmbH, Munich 2010, ISBN 978-3-7654-7349-4 .

[Proceedings_Sept2004-6] Kevin, Lieutenant Colonel US Marine Corpsv Gross, Tom Macdonald, MV-22 test pilot and Ray Dagenhart, MV-22 lead government engineer: Dispelling the Myth of the MV-22 . In: The Naval Institute (ed.): Proceedings . No. September 2004, September 2004. Retrieved April 9, 2009.

[wired_200507-7] Saving the Pentagon's Killer Chopper Plane . In: Wired, July 2005 . Retrieved March 9, 2011.

[8] Ex- "DSDS" singer in an artificial coma . focus.de. Retrieved October 25, 2010.

[9] The (Stealth) Blackhawk Crash. avweb.com, accessed May 2, 2011 .

[10] Niels Klußmann, Arnim Malik: Lexicon of aviation. 3rd, updated edition. Springer, Berlin et al. 2012, ISBN 978-3-642-22499-7 , p. 311.