Air flowing over the top surface of a wing is at a lower pressure than that beneath. The trailing edge and the wing tips are where the airflows interact. The pressure differential modifies the directions of flow, inducing a span-wise vector towards the root on the upper surface and generally, towards the tip on the lower surface, Figure 5.8. “Conventionally”, an aircraft is viewed from the rear. An anti-clockwise vortex will be induced at the right wing-tip and a clock-wise vortex at the left wing-tip, Figures. 5.9, 5.10
& 5.11.
At higher angles of attack (Lower IAS) the decreased chordwise vector will increase the effect of the resultant spanwise flow, making the vortices stronger.
Induced Downwash: (Figure 5.12) Trailing vortices create certain vertical velocity components in the airflow in the vicinity of the wing, both in front of and behind it. These vertical velocities cause a downwash over the wing resulting in a reduction in the effective angle of attack. The stronger the vortices, the greater the reduction in effective angle of attack.
Because of this local reduction in effective angle of attack, the overall lift generated by a wing will be below the value that would be generated if there were no spanwise pressure differential. It is the production of lift itself which reduces the magnitude of the lift force being generated. To replace the lift lost by the increased downwash, the aircraft must be flown at a higher angle of attack. This increases drag. This extra drag is called induced drag. The stronger the vortices, the greater the induced drag.
Upwash Increased
Vertical Velocities in the vicinity of the wing are a function of
tip vortex strength
Downwash Increased
V
Relat iv e Airflow
V
Downwash EFFECTIVE AIRFLOW
Angular deflection of effective airflow is a function of both vortex strength and True Air Speed (TAS).
Induced
e = effective angle of attack
i = induced angle of attack Relative Airflow
Normal Downwash Lift With
e
i
i
Induced Drag ( )Di
Lift Inclined Rearwards because of Decreased Effective Angle of Attack
Effective Airflow
Figure 5.12
WAKE TURBULENCE: (Ref: AIC 17/1999)
Trailing wingtip vortices extend behind aircraft for a considerable distance and can present an extreme hazard to any aircraft unfortunate enough to encounter them. Maximum tangential airspeed in the vortex system may be as high as 90 m/s (300 ft/sec) immediately behind a large aircraft. Wake turbulence cannot be detected, so it is important for pilots to be aware of the potential distribution and duration of trailing vortices, plus modifications made to the “classic”
vortex system by surface wind speed and direction.
Aircraft Wake Vortex Characteristics: Wake vortex generation begins when the nosewheel lifts off the runway on take-off and continues until the nosewheel touches down on landing. Wake vortices exist behind every aircraft, including helicopters, when in flight, but are most severe when generated by heavy aircraft. They present the greatest danger during the take-off, initial climb, final approach and landing phases of flight - in other words, at low altitude where large numbers of aircraft congregate. A wake turbulence encounter is a hazard due to potential loss of control and possible structural damage, and if the experience takes place near the ground there may be insufficient time and/or altitude to recover from an upset.
Touchdown (Wake ends) (Wake begins)
Rotation
Figure 5.13Figure 5.13
The characteristics of trailing vortices are determined by the “generating” aircraft’s:
Gross weight - the higher the weight, the stronger the vortices.
Wingspan - has an influence upon the proximity of the two trailing vortices.
Airspeed - the lower the speed, the stronger the vortices.
Configuration - vortex strength is greatest with aircraft in a “clean” configuration (for a given speed and weight).
Attitude - the higher the angle of attack, the stronger the vortices.
As a general rule, the larger the “generating” aircraft relative to the aircraft encountering the wake turbulence, the greater the hazard. There is also evidence that for a given weight and speed a helicopter produces a stronger vortex than a fixed-wing aircraft.
Distribution of Trailing Vortices: Typically the two trailing vortices remain separated by about three quarters of the aircraft’s wingspan and in still air they tend to drift slowly downwards and level off, usually between 500 and 1000 ft below the flight path of the aircraft. Behind a large aircraft the trailing vortices can extend as much as nine nautical miles.
3 4 Span
Figure 5.14
Figure 5.15 Figure 5.14
Approx. 9 nautical miles behind a large aircraft
500 to 1000 ft
Figure 5.16 Figure 5.15
Vortex Movement near the Ground: Figure 5.17 shows that if the generating aircraft is within 1000 ft of the ground, the two vortices will “touch-down” and move outwards at about 5 kts from the track of the generating aircraft at a height approximately equal to 2 the aircraft’s wingspan.
5 kts
1000 ft
STILL AIR - (viewed from the rear)
Drift 5 kts
Drift
Figure 5.17Figure 5.17
In a crosswind, if the surface wind is light and steady, the wake vortex system “in contact” with the ground will drift with the wind. Figure 5.18 shows the possible effect of a crosswind on the motion of a vortex close to the ground. With parallel runways, wake turbulence from an aircraft operating on one runway can be a potential hazard to aircraft operating from the other.
5 kt Wind
10 kts Drift (5 kts + 5 kts)
Zero Drift (5 kts - 5 kts)
5 kt CROSSWIND - (Viewed from the rear)
Figure 5.18Figure 5.18
The Decay Process of Trailing vortices: Atmospheric turbulence has the greatest influence on the decay of wake vortices; the stronger the wind, the quicker the decay.
Probability of Wake Turbulence Encounter: Certain separation minima are applied by Air Traffic Control (ATC), but this does not guarantee avoidance. ATC applied separation merely reduces the probability of an encounter to a lower level, and may minimise the magnitude of the upset if an encounter does occur. Particular care should be exercised when following any substantially heavier aircraft, especially in conditions of light wind. The majority of serious incidents, close to the ground, occur when winds are light.
Wake Turbulence Avoidance: If the location of wake vortices behind a preceding or crossing aircraft are visualised, appropriate flight path control will minimise the probability of a wake turbulence encounter. Staying above and/or upwind of a preceding or crossing aircraft will usually keep your aircraft out of the generating aircraft’s wake vortex. Unfortunately, deviating from published approach and departure requirements in order to stay above/upwind of the flight path of a preceding aircraft may not be advisable. Maintaining proper separation remains the best advice for avoiding a wake turbulence encounter.