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    Wake Turbulence & Wingtip Vortices
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    Wake Turbulence & Wingtip Vortices

    Definitions

    Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes various components, the most important of which are wingtip vortices (see below) and jetwash. Jetwash refers simply to the rapidly moving gasses expelled from a jet engine; it is extremely turbulent, but of short duration. Wingtip vortices, on the other hand, are much more stable and can remain in the air for up to two minutes after the passage of an aircraft. Wingtip vortices make up the primary and most dangerous component of wake turbulence.

    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.

    Topics of Interest

    Fixed wing - Level flight: At altitude, vortices sink at a rate of 91 to 152 m per minute and stabilize about 152 to 274 m below the flight level of the generating aircraft. For this reason, aircraft operating greater than 610 m above the terrain are not considered at risk.

    Helicopters also produce wake turbulence. Helicopter wakes may be of significantly greater strength than those from a fixed wing aircraft of the same weight. The strongest wake can occur when the helicopter is operating at lower speeds (20 - 50 knots). Some mid-size or executive class helicopters produce wake as strong as that of heavier helicopters. This is because two blade main rotor systems, typical of lighter helicopters, produce stronger wake than rotor systems with more blades.

    During takeoff and landing, an aircraft's wake sinks toward the ground and moves laterally away from the runway when the wind is calm. A 3 to 5 knot crosswind will tend to keep the upwind side of the wake in the runway area and may cause the downwind side to drift toward another runway. Since the wingtip vortices exist at the outer edge of an airplane's wake, this can be dangerous.

    Incident data shows that the greatest potential for a wake vortex incident occurs when a light aircraft is turning from base to final behind a heavy aircraft flying a straight-in approach. Light aircraft pilots must use extreme caution and intercept their final approach path above or well behind the heavier aircraft's path. When a visual approach following a preceding aircraft is issued and accepted, the pilot is required to establish a safe landing interval behind the aircraft s/he was instructed to follow. The pilot is responsible for wake turbulence separation. Pilots must not decrease the separation that existed when the visual approach was issued unless they can remain on or above the flight path of the preceding aircraft.

    Warning signs: Any uncommanded aircraft movements (e.g., wing rocking) may be caused by wake. This is why maintaining situation awareness is so critical. Ordinary turbulence is not unusual, particularly in the approach phase. A pilot who suspects wake turbulence is affecting his or her aircraft should get away from the wake, execute a missed approach or go-around and be prepared for a stronger wake encounter. The onset of wake can be insidious and even surprisingly gentle. There have been serious accidents where pilots have attempted to salvage a landing after encountering moderate wake only to encounter severe wake turbulence that they were unable to overcome. Pilots should not depend on any aerodynamic warning, but if the onset of wake is occurring, immediate evasive action is vital.

    Wake turbulence can be measured using several techniques. A high-resolution technique is doppler lidar, a solution now commercially available. Techniques using optics can use the effect of turbulence on refractive index (optical turbulence) to measure the distortion of light that passes through the turbulent area and indicate the strength of that turbulence.

    Audibility: Wake turbulence can occasionally, under the right conditions, be heard by ground observers. On a still day, heavy jets flying low and slow on landing approach may produce wake turbulence that is heard as a dull roar/whistle. Often, it is first noticed some seconds after the direct noise of the passing aircraft has diminished. The sound then gets louder, sometimes becoming as loud as was the original direct sound of the aircraft. Nevertheless, being highly directional, wake turbulence sound is easily perceived as originating a considerable distance behind the aircraft, its apparent source moving across the sky just as the aircraft did. It can persist for 30 seconds or more, continually changing timbre, sometimes with swishing and cracking notes, until it finally dies away.


    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 other's wingtip vortices by flying in a V formation so all but the leader are flying in the upwash from the wing of the bird ahead. A little upwash makes it a little easier for the bird to support its own weight.

    Many technical writers use the alternative expression "trailing vortices" because these vortices do not trail only from the wing tips. They also trail from the outboard tip of the wing flaps and other abrupt changes in wing planform.

    A wing generates aerodynamic lift by creating a region of lower air pressure above the wing 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: A NASA study on wingtip vortices produced these pictures of smoke in the wake of an aircraft, clearly illustrating the size and power of the vortices produced.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.

    Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License and Creative Commons Attribution-ShareAlike License.)

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