Wake turbulence

In wake vortices (English wake turbulence ), also called vortex braids or edge vortex is zopfartige, counter-rotating air turbulence behind flying aircraft. Their intensity is the weight of the aircraft dependent. Life is affected by wind and atmosphere. In the center of the vortex, the air pressure is reduced. With high humidity condensation may there produce a visible stripe (English wing tip trails ).

There are vortices behind other non- flying vehicles, where they at the bottom but very much have less of an impact due to the lower intensity and the air friction.

Emergence and impact

Wake vortices are a by-product of the dynamic lift. They occur inevitably in each aircraft as soon as the air flows quickly to the wing along. Their intensity is influenced by the geometry of the wings and their angle of attack. Indirectly, therefore size and mass of an airplane connected, the larger the first two values ​​, the stronger the vortices. One can distinguish two different formation causes of wake vortices, the first is dominant:

• This aircraft can generate lift, air on the wings must be accelerated downward. Since this acceleration does not take place outside the aircraft range, a rotation impulse, and are formed behind the aircraft two counter-rotating vortices. Between the two vertebrae is formed a strong, vertically descending air stream, which is dangerous for the following aircraft.

• Since there is negative pressure on the upper wing surface in the generation of buoyancy, it comes at the wingtips to pressure equalization, in which air flows from the bottom wing to the top side. In this case, the air has the tendency to move from the higher to lower pressure to equalize the pressure difference. Because of the high speed it is however not possible via the front or rear edge of the wing. At the end of the wing there is a possibility for pressure equalization between upper and lower surfaces of the wing. This speeds up the air around the wing tip around from top to bottom and thus contributes to the edge vortex.

In extended buoyancy aids, so off or landing, intensified depending on the weight of the aircraft, the intensity of remaining behind the aircraft tubular vortex. In unfavorable cases, the vortices can persist for so long and achieve such strength that they cover, for example at the bottom of all the house roofs. Especially in the field of an airport to aircraft bound to come so close that must be taken already in the queue, but especially during the landing approach and at the start of the distances of the machines in order to avoid turbulence and control problems caused by wake turbulence for various machines. Here is considered a target time of two to three minutes. This scale determines the maximum capacity of the airport.

Vortices can generate such strong currents or turbulence across the flight path of an aircraft below that the load limits of components are exceeded. So it is not unheard of that a started with insufficient distance aircraft in which the pilot of this disorder to be counteracted, the rudder was torn down. Vortices can be so strong that they endanger the flight of a following aircraft.

The air cools down in the center of the swirl at the wingtip adiabatic, because there is an area particularly low pressure here. The air often reaches temperatures below the dew point, causing condensation of the water contained in the air to form water vapor / mist and a vortex is visible. The questions of buoyancy are crucial for the construction of aircraft, the wake turbulence caused by loss of lift with increasing wing length decreases. Thus a decrease in the tear-off speed and fuel consumption, but also a restriction of the maneuverability is connected. Especially when you take combat aircraft but for the benefit of maneuverability shorter wings and relatively strong vortices in purchasing. From this special case apart, a high aspect ratio is generally preferred and / or the wing tips are fitted with winglets.

Categories

By separating the aircraft weight classes, the required distances (English wake turbulence separation minima) defined in order to avoid the dangers of wake turbulence. The following table shows the classification in accordance with ICAO

The Boeing 757 is due to increased incidence wake vortices after near misses, despite its weight of less than 136 tonnes almost always sorted into the heavy category. In the U.S. there specifically for the 757, another class, called the class MH (Medium - Heavy). In the UK, the Civil Aviation Authority the classes Heavy, Medium Upper, Lower Medium, Light and Small; continues to enjoy the Airbus A380 in some aspects, a special treatment. Continental Europe uses the ICAO classes.

In order to maintain the capacity benefits (passengers / time) part, Airbus proposes for the A380 at the airstrip use a drop below the minimum clearances to aircraft flying ahead, this increases the time required for any subsequent aircraft extended.

DLR led in 2006 by extensive studies on this and came to the conclusion that distinguish the wake of an A380 in cruise flight is not significantly different from those of a Boeing 747. For takeoff and landing, however, an enlarged separation was issued as a recommendation to the ICAO, the spacing for the A380 as following aircraft at 3 NM:

Despite equal or greater weight than the A380 C-5 Galaxy and An-124 have been treated like a B747.

Behavior in case of danger of wake turbulence

The air traffic control traffic controller in the tower are usually at risk of turbulence and wake vortices a warning. Nevertheless, there is ultimately the responsibility of the pilot to land safely and avoid an accident. For this reason have proved among others the following procedures as appropriate:

• Landing behind a large aircraft landing on the same runway: Remain above the glide path of the preceding aircraft and land after its touchdown point.
• Landing behind a large aircraft taking off on the same runway: "short" segment, ie at the beginning of the runway.
• Departure behind a large aircraft taking off on the same runway: Lift ( rotated ) before the point of rotation of the previous plane is reached, and to remain on the climbing path.

Scientific research

Airbus had to develop new technologies that hold the wake at this great aircraft within limits for the Airbus A380 because of the great weight and thus very intense vortex.

The research on this topic can be divided into three areas:

First vortex detection and prediction

The development of methods for the estimation of the vortex behavior, eg as a function of meteorological parameters, have a theoretical vortex forecast, eg in to computer models. The physical processes of transport and decay of vortices in Earth's atmosphere are understood. Vortices and their meteorological parameters can be predicted and observed using a pulsed LIDAR.

Second vortex avoidance

Through development of aircraft with a favorable eddy characteristic is trying to reduce the vortex strength. It is also demonstrated that aircraft wake vortices can be mitigated by the creation of multi -vortex systems.

To constructively to reduce on-wing, the wake vortex formation, there are the following considerations:

• On the right and left wings of different rotary engines are installed. The turbines rotate in opposite directions, respectively, so that opposite, both internally and externally rotating vortices arise. Thus, the vortex is disturbed and sounds from the past. However, this approach is rather theoretical nature: the cost, quasi develop engines twice, once for the right and the left wing are hard to finance. The entire supply chain for spare parts of small blades up to complete engines would almost double.
• A specially -clad gear is extended at an early stage. Also weakens the problematic vertebrae.
• The flaps are not quite reach out to the fuselage. The result at this point is also a counter-rotating vortex, which weakens the vortex.

3 vertebrae Compatibility

The third part of the research relates to the development of methods for increasing the safety of ingress into a vortex to make it eg when entering, within such vortex does not come to flap breaks, what has happened before.

Actual accidents

On 12 November 2001 against 9:15 clock local time, crashed shortly after takeoff from John F. Kennedy Airport, New York, an Airbus A300 on the flight 587 to Santo Domingo American Airlines near Rockaway Beach, Queens, NY in a densely populated area. Cause was the reaction of the copilot to wake a previously flown Boeing 747 with excessive rudder movements.

Audibility

Vortices may be audible under certain conditions. Especially on windless days vortices behind heavy aircraft can be perceived as dull roar and hiss. Stable vortices are audible as a broadband low-frequency noise. Is the wake weaker, they can break with a sound like ripping paper. The audible sound of the wake turbulence usually occurs when the aircraft is already over, and then increases in intensity. In this case, the noise is clearly to locate behind the aircraft in the sky, sometimes it seems to follow the plane even along its flight line. The noise may last thirty Seconds or Greater, with its timbre is constantly changing, sometimes with rustling and rushing shares until it finally dies.