Vortex ring state

As a vortex ring state or vortex ring state ( english vortex ring state ) refers to a dangerous condition of a helicopter in hover or at low horizontal speed in which this generated by the main rotor downwash own ( downwash ) decreases.

• 2.1 vortex formation at the main rotor blade
• 2.2 genesis of the vortex ring stage of the main rotor

• 3.1 Hazardous environment
• 3.2 Impact of the vortex ring stage
• 3.3 Measures of helicopter pilots

General Explanations of

Induced velocity

The offset of an engine or a turbine in rotation the main rotor of a helicopter produced immediately above the rotor plane (assumed surface on which the blades rotate ) and a negative pressure below a pressure. The air mass is drawn through the plane of the rotor and accelerated by the main rotor in dependence on the angle of attack of the rotor blades to the bottom. This is also referred to as technical downdraft downwash.

The speed of the main rotor downward accelerated enforced air mass is called induced velocity (derived from the Latin inductio " ( Her-/Hin ) Introduction"). It is abbreviated with vi and given in meters per second ():. XXI and 48 It is for helicopters in hover ( vi0 ) usually about 6 ( = 21,5 ). 62

The air flow rate in hover is not evenly distributed over the rotor blade. In the center it is higher than at the blade tip and the end which is the rotor head facing toward. 201 is referred to as the non-uniform induced velocity. 64 The distribution of the induced velocity over the rotor blade is provided with vir, where r running the rotor radius indicates in meters. XX f, 64

Perpendicular ( vertical ) airspeed

Vertical is defined herein as directed perpendicular to the surface or to the center of the earth.

As vertical or vertical airspeed (also called descent or descent speed ) directed to the ground settling velocity of a helicopter is called. It is with vz: XXI abbreviated and given in meters per second. It is located in helicopters typically at 2.5. 53

The vertical air speed air flow is generated in hover, which counteracts the downwash. Since in the area of the rotor head of the air flow rate is lower than in the rotor blade center, here is the acceleration of the air mass as a function of the rate of descent of the helicopter by the opposing air flow is partially offset or even reversed. 203

Horizontal (horizontal ) air velocity

As a horizontal or horizontal airspeed reached the parallel to the ground airspeed is called. It is stated inter alia as true airspeed or actual speed over ground in knots ( kn).

Flow behavior of the helicopter

Vortex formation at the main rotor blade

In fluid dynamics circular flows of a fluid ( gas or liquid) are called eddy or vortex. Vortices are formed when a sufficiently large velocity difference arises within a fluid, such that a portion of the gas or liquid flows much faster than the rest

Of the rotating rotor blades results in a laminar flow such that the air flows in layers along the surface of the rotor blades, which do not mix with each other. The blades have, depending on its angle of incidence but also a frictional resistance, which causes the formation of vortices on the rotor blades. A larger angle results in a larger friction and thereby also larger in vertebrae.

The vortex escape the flow as a function of their size the energy they convert, for example, into heat energy. At the hover power losses are recorded by vortex at the rotor blade tips in the order of 2 to 4%. 51

Formation of the vortex ring stage at the main rotor

Also at the rotor blade tips are formed especially in hover vertebrae. At high sink rate ( vertical flight speeds ) - in hover in the range of to of the induced velocity - flowing a part of the accelerated already down air mass outside by the main rotor, is drawn from this re- up and accelerated down, so that the induced rate is further increased will. This process is also called recirculation. 53

The vortices at the blade tips interact with each other when the sink rate (vertical flight speed ) reaches about the induced velocity. Increases the vertical air speed at the induced rate of the interactions between the vertebrae to be so large that is formed on the rotor plane, a closed vortex ring around the rotor blade tips. 63 This increases along the rotor blade from the outside inwards and has the shape of a torus.

Thereby, the lift generated by the main rotor is reduced and the vertical air speed. The helicopter is caught in a vicious circle of reduced buoyancy, resulting in increased vertical airspeed and resulting amplification of recirculation in connection with the enlargement of the vortex ring. This can lead to almost complete loss of buoyancy and therefore to the loss of the helicopter.

The vortex ring does not affect the interior of the rotor, that is, around the rotor head around from. Here, however, the inflow speed of the rotor blades is increased from below due to the larger vertical flight speed, this can lead to a further reduction of the buoyancy at a stall in the interior of the rotor blades and. This area of ​​the stall increases in expansion of the fluidized ring along the rotor blade from the inside to the outside until the two portions extend over the entire rotor blade. 70

Causes, Effects and Countermeasures

Hazardous conditions

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

• The horizontal velocity is less than 10 knots: 59 ( = 18,5 ) or the helicopter is in strong tailwind in rapid descent and
• The vertical speed is greater than the induced velocity ( about 300 ft / min = 1.5 ) 59:70 or
• The vertical velocity ( vz ) is between and the induced velocity ( vi0 ); So we have:

• The Motor-/Turbinenleistung is greater than 20% of the maximum power. 59

Effects of vortex ring stage

If a helicopter is flying in the vortex ring state, strong vibrations, especially in the area of the rotor head, determine. In addition, the helicopter takes due to the recirculation caused by the improper flow to the rotor from above and irregular eccentric outflow passages beneath the rotor difficult to control movements.

The engine or turbine performance of the helicopter is almost completely converted to the increasing acceleration of the air masses in the vortex ring, without producing significant boost. The vertical speed may be up to about 10 ( = 36 ), ie four times the normal descent rate increases. 70

Measures of helicopter pilots

The helicopter pilot has to react very quickly if they experience signs of vortex ring stage to prevent a crash. This is even more so on the landing approach.

Provided sufficient controllability of the helicopter - - only increasing the horizontal speed (usually the front, but since the increase in the rotor speed and the pitch angle of the rotor blades to increase the buoyancy, the swirl ring stage only exacerbated (see O. A. cycle ), the pilot remains also at the side or rear), in order to stabilize the helicopter.

Due to the (horizontal ) movement forward, backward or to the side recirculation is reduced at the rotor blade tips and resolved in the most favorable case, the vortex ring. This results in an increase in buoyancy and reduction in the vertical speed.

The helicopter is still at a sufficient height above the ground, the pilot can go into autorotation. The free flow of air through the bottom of rotating main rotor (without driving by the motor or turbine ) is no longer accelerating the air downward, so that the recirculation is stopped. 204

Accidents caused by a vortex ring stage

A tiltrotor aircraft Bell - Boeing V-22 crashed due to the vortex ring stage on 8 April 2000.

A helicopter from the Robinson R44 type crashed on October 24, 2010 in Altenbeken from from about ten meters above a road. On board was among others the German pop singer Anna -Maria Zimmermann, who was seriously injured by the crash. The Federal Bureau of Aircraft Accidents Investigation of an analysis of the accident to the conclusion that several factors, including a vortex ring state, led to the crash.

One of the two modified UH -60 helicopter, 'm in the attempted arrest of Osama bin Laden were used during Operation Neptune 's Spear on 2 May 2011, fell during the hovering flight also in a vortex ring state. In the subsequent forced landing, the helicopter collided with a wall of the property and crashed. He was blown up and abandoned by the employed Special Forces, the United States Navy SEALs.

Vortex ring stage of the tail rotor

Under certain unfavorable conditions, the tail rotor can fall into a vortex ring state. This is particularly the case if by the tail rotor to site the accelerated masses of air ( air jet ) as a result

• Faster rotation of the helicopter in hover about the vertical axis in the direction of the air jet of the tail rotor,
• Rapid lateral floating of the helicopter in the direction of the air jet of the tail rotor, or
• The strong crosswind, against the air jet of the tail rotor

Hit counter-flowing air masses. 63 The tail rotor becomes ineffective and at worst the helicopter uncontrollable.

This is also a way to increase the horizontal speed forward. By weathervanes effect of stabilizing fins ( tendency of an aircraft to rotate about its center and align in the wind to minimize air resistance) the helicopter is stabilized. With the modified flow of the tail rotor, the recirculation is reduced to the tail rotor blade tips and resolved in the most favorable case, the vortex ring. 63

Helicopter in which a fenestron compensates the yaw moment are due to the large-scale covering of the tail rotor blades largely insensitive to the formation of a vortex ring stage of the tail rotor. 63