Wingtip vortices

Wingtip vortices are tubes of circulating air which are left behind by the wing as it generates lift. One wingtip vortex trails from the tip of each wing. The cores of vortices spin at very high speed and they are regions of very low pressure. The cores of wingtip vortices are sometimes visible due to condensation of water vapour in the very low pressure.

Wingtip vortices are associated with induced drag, an essentially unavoidable side-effect of the wing generating lift. Managing induced drag and wingtip vortices by selecting the best wing planform for the mission is critically important in aerospace engineering.

Wingtip vortices form the major component of wake turbulence.

Migratory birds take advantage of each others' wingtip vortices by flying in a V formation so that all but the leader are flying in the upwash from the wing of the bird ahead. This upwash makes it a bit easier for the bird to support its own weight, reducing fatigue on migration flights.

Some technical writers use the alternative expression "trailing vortices" because these vortices also occur at points other than at the wing tips. They are induced at the outboard tip of the wing flaps and other abrupt changes in wing planform.

Cause and effects

A wing generates aerodynamic lift by creating a region of lower air pressure above the wing of an aircraft than beneath it. Fluids are forced to flow from high to low pressure and the air below the wing tends to migrate towards the top of the wing, via the wingtips. The air does not escape around the leading or trailing edge of the wing due to airspeed, but it can flow around the tip. Consequently, air flows from below the wing and out around the tip to the top of the wing in a circular fashion. This leakage will raise the pressure on top of the wing and reduce the lift that the wing can generate. It also produces an emergent flow pattern with low pressure in the center surrounded by fast moving air with curved streamlines.

Wingtip vortices only affect the portion of the wing closest to the tip. Thus, the longer the wing, the smaller the affected fraction of it will be. As well, the shorter the chord of the wing, the less opportunity air will have to form vortices. This means that for an aircraft to be most efficient, it should have a very high aspect ratio. This is evident in the design of gliders. It is also evident in long-range airliners where fuel efficiency is of critical importance. However, increasing the wingspan reduces the maneuverability of the aircraft, which is why combat and aerobatic planes usually feature short, stubby wings despite the efficiency losses this causes.

Another method of reducing fuel consumption is use of winglets, as seen on a number of modern airliners such as the Airbus A340. Winglets work by forcing the vortex to move to the very tip of the wing and allowing the entire span to produce lift, thereby effectively increasing the aspect ratio of the wing. Winglets also change the pattern of vorticity in the core of the vortex pattern; spreading it out and reducing the kinetic energy in the circular air flow, which reduces the amount of fuel expended to perform work by the wing upon the spinning air. Winglets can yield very worthwhile economy improvements on long distance flights.

Since the cores of vortices have a very low pressure, when the air is of high humidity, water vapour condenses to form cloud in the vortex cores, allowing wingtip vortices to be seen. This 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.

Hazards

Wingtip vortices can also pose a severe hazard to light aircraft, especially during the landing and take off phases of flight. The intensity or strength of the vortex is a function of aircraft size, speed, and configuration (flap setting, etc.). The strongest vortices are produced by heavy aircraft, flying slowly, with wing flaps extended. Large jet aircraft can generate vortices which are larger than an entire light aircraft. These vortices can persist for several minutes, drifting with the wind. The hazardous aspects of wingtip vortices are most often discussed in the context of wake turbulence. If a light aircraft is immediately preceded by a heavy aircraft, wake turbulence from the heavy aircraft can roll the light aircraft faster than can be resisted by use of ailerons. At low altitudes, particularly during takeoff and landing, this can lead to an upset from which recovery is not possible. Air traffic controllers ensure an adequate separate between departing and arriving aircraft, particularly where a heavy aircraft is preceding a light aircraft.

Gallery

See also

* Aspect ratio
* Helmholtz's theorems
* Horseshoe vortex
* V formation
* Von Kármán vortex street
* Vortex
* Vortex shedding

External links

*Video from NASA's Dryden Flight Research Center tests on wingtip vortices:
**C-5 Galaxy: [http://www1.dfrc.nasa.gov/Gallery/Movie/C-5A/HTML/EM-0085-01.html]
**Lockheed L-1011: [http://www1.dfrc.nasa.gov/Gallery/Movie/L-1011/index.html]
* [http://www.ll.mit.edu/AviationWeather/WW-11077_WindPrediction.pdf Wind prediction for analysis of vortex drift]
* [http://antwrp.gsfc.nasa.gov/apod/ap060822.html Flares released by an air force jet form a "smoke angel"]


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