Wingtip vortices

Wingtip vortices are circular patterns of rotating air left behind a wing as it generates lift.

[1]: 5.14  Indeed, vorticity is trailed at any point on the wing where the lift varies span-wise (a fact described and quantified by the lifting-line theory); it eventually rolls up into large vortices near the wingtip, at the edge of flap devices, or at other abrupt changes in wing planform.

Wingtip vortices are associated with induced drag, the imparting of downwash, and are a fundamental consequence of three-dimensional lift generation.

[1]: 5.17, 8.9  Careful selection of wing geometry (in particular, wingspan), as well as of cruise conditions, are design and operational methods to minimize induced drag.

[3][2]: 8.1.1 [4] Three-dimensional lift and the occurrence of wingtip vortices can be approached with the concept of horseshoe vortex and described accurately with the Lanchester–Prandtl theory.

Wingtip vortices are associated with induced drag, an unavoidable consequence of three-dimensional lift generation.

The lifting-line theory describes the shedding of trailing vortices as span-wise changes in lift distribution.

For a given wing span and surface, minimal induced drag is obtained with an elliptical lift distribution.

For a given lift distribution and wing planform area, induced drag is reduced with increasing aspect ratio.

This primarily involved changing the settings of the outboard flaps, and could theoretically be retrofitted to existing aircraft.

[5] The cores of the vortices can sometimes be visible when the water present in them condenses from gas (vapor) to liquid.

Condensation of water vapor in wing tip vortices is most common on aircraft flying at high angles of attack, such as fighter aircraft in high g maneuvers, or airliners taking off and landing on humid days.

[6] If it drops below the local dew point, there results a condensation of water vapor present in the cores of wingtip vortices, making them visible.

[6] The temperature may even drop below the local freezing point, in which case ice crystals will form inside the cores.

[6] The phase of water (i.e., whether it assumes the form of a solid, liquid, or gas) is determined by its temperature and pressure.

After the vortex core forms, the pressure inside it has decreased from the ambient value, and so the local dew point (

To first approximation, the formation of vortex cores is thermodynamically an adiabatic process, i.e., one with no exchange of heat.

are the absolute temperature and pressure in the vortex core (which is the end result of the process), and the constant

Therefore, this is a marginal case; if the relative humidity of the ambient air were even a bit higher (with the total pressure and temperature remaining as above), then the local dew point inside the vortices would rise, while the local temperature would remain the same.

The water-vapor condensation mechanism in wingtip vortices is thus driven by local changes in air pressure and temperature.

In the case of contrails, the local air pressure and temperature do not change significantly; what matters instead is that the exhaust contains both water vapor (which increases the local water-vapor concentration and so its partial pressure, resulting in elevated dew point and freezing point) as well as aerosols (which provide nucleation centers for the condensation and freezing).

[8][9] Wingtip vortices can pose a hazard to aircraft, especially during the landing and takeoff phases of flight.

The intensity or strength of the vortex is a function of aircraft size, speed, and configuration (flap setting, etc.).

[10] Large jet aircraft can generate vortices that can persist for many minutes, drifting with the wind.

Air traffic controllers attempt to ensure an adequate separation between departing and arriving aircraft by issuing wake turbulence warnings to pilots.

In general, to avoid vortices an aircraft is safer if its takeoff is before the rotation point of the airplane that took off before it.

On landing behind an airplane the aircraft should stay above the earlier one's flight path and touch down further along the runway.

[11] Glider pilots routinely practice flying in wingtip vortices when they do a maneuver called "boxing the wake".

This is followed by making a rectangular figure by holding the glider at high and low points away from the towing plane before coming back up through the vortices.

Given the relatively slow speeds and lightness of both aircraft the procedure is safe but does instill a sense of how strong and where the turbulence is located.