Turbotrent
Topic Author
Posts: 146
Joined: Tue Jan 01, 2002 8:43 pm

Wake Turbulence

Hello everybody,

A few days ago I was reading an article and the author mencioned a few times 'wake turbulence'. It is probably a stupid question, but what is wake turbulence? Has it something to do with an other airplane nearby? I hope somebody can help me.

Greetings,

Tom
Man's flight through life is sustained by the power of his knowledge.

Mr Spaceman
Posts: 2723
Joined: Thu Mar 15, 2001 5:09 am

RE: Wake Turbulence

Hello Tom.

Simply stated..."Wake Turbulence"...is the turbulent air caused by an aircraft as it passes through air. An airliner leaves a wake through air just like a ship does through water, (waves).

This turbulent air is left behind an aircraft (in it's wake) and is mostly caused by a combination of Wingtip Vortices and other forms of Parasite Drag. Some part of this wake could be considered Jet Blast.

This air comprises of swirling eddies and twisting vortexes left behind by an aircraft's wing & body as it disturbs the air it's flying through. (while producing both Lift & Drag).

It is well known that airliners leave the most powerful / dangerous wake turbulence while they are flying "Low & Slow"...such as during final approach, when their gear and flaps are down.

Lighter aircraft should always be spaced accordingly behind larger aircraft while on final approach (2 to 4 minutes) to ensure that lighter aircraft do not suddenly encounter Wake Turbulance and experience uncommanded Roll, Pitch & Yaw of the aircraft, caused by invisible eddies of air.

I hope this starts to help. I'm sure you'll get more replies.

Chris

"Just a minute while I re-invent myself"

Guest

RE: Wake Turbulence

Wake turbulence specifically refers to wingtip vortices. Here is an explanation of what a wingtip vortex is: One of the main ways that a wing produces lift is by differential pressure (as opposed to newtonian lift, but that's a whole 'nother thread). The upper curvature, or camber of a wing causes a low pressure area to form above the wing due to accelleration of the airflow (remember, as the speed of a fluid increases, the pressure decreases), and a relatively higher pressure area below the wing. A fluid, in this case air, tends to flow from an area of high pressure to an area of low pressure. Now, as we examine this phenomenon near the wingtip, the higher pressure air tends to curl over the wingtip, flowing towards the area of low pressure, "energizing" the air and forming a vortex trailing behind the aircraft.

As was previously stated, large, aircraft produce large vortices. Small aircraft produce small vortices. The configuration that produces the most pronounced vortex is "heavy, clean, and slow," (altitude has nothing to do with it) "clean" meaning that the flaps/slats are retracted. Large aircraft are usually in this condition directly after departure, however, they have been known to produce strong vortices during cruise flight.

If my memory serves me correctly, there is a mandatory 5 minute waiting period for small aircraft departing or arriving behind a heavy aircraft/Boeing 757 - The 757 produces a notoriously strong vortex.
But this does not mean that you are safe with any other large aircraft.

Maybe someone else can describe the techniques used to avoid wake turbulence.

surv1
Posts: 14
Joined: Wed Feb 07, 2001 9:23 pm

RE: Wake Turbulence

Wake turbulence and wingtip vortices are produced any time a wing is producing lift, which means they are generated from the time the aircraft leaves the ground until it returns to the ground, and generally consists of 2 counterrotating cylindrical vortices produced at the wingtips. The strength of the vortices is governed by the weight, speed and shape of the wing of the generating aircraft. The greatest vortex strength occurs when the generating aircraft is operating HEAVY-CLEAN, and SLOW.

Wake turbulence encounters can cause in-flight structural damage of catastrophic proportions, especially in general aviation aircraft. The most common hazard of wake turbulence is it's ability to induce severe induced rolling moments which can exceed the roll control capability of the encountering aircraft-and pilot. Vortices from larger aircraft sink at a rate of several hundred feet per minute, and generally remain a bit less than a wingspan apart, unless the aircraft is near the ground, and then the vortices tend to move outward laterally over the ground at 2-3 knots.

Be aware of wind effects. A crosswind could result in the upwind vortex remaining in the touchdown zone for a period of time, and push the downwind vortex onto a parallel runway. A tailwind can move the vortices forward down the runway into your touchdown zone. A light quartering tailwind requires the maximum caution, as it combines both of the above effects.

Some of the ways to avoid an encounter:
1. Fly at or above the generating aircrafts flightpath, altering course as necessary to avoid the area behind and below the generating aircraft.
2. When landing behind a larger aircraft on the same runway, stay at or above the larger aircrafts approach flightpath, note his touchdown point, and land BEYOND it.
3. When landing behind a larger aircraft on a parallel runway, consider vortex drift onto your runway. If you can see his touchdown point, land BEYOND it.
4. When landing behind a larger departing aircraft on the same runway, note his rotation point and land well PRIOR to his rotation point.
5. When departing behind a larger aircraft, note the larger aircrafts rotation point and rotate PRIOR to his rotation point. Climb above the larger aircrafts climb path until turning clear of his wake.
6. Avoid headings which will take your aircraft below or behind generating aircraft. A 1000 foot below is considered safe.
7. Helicopters generate similar trailing vortices when in forward flight. Since helicopters airspeed is much slower, vortices are stronger. Also avoid downwash from hovering helicopters.
8. Jet Engine Exhaust- This is pretty self-explanatory. Just use common sense and give yourself lots of room.

The FAA's viewpoint is pretty simple: Vortex visualization and avoidance procedures should be exercised by the pilot, using the same degree of concern as in collision avoidance since vortex encounters can be as dangerous as collisions.

NOTE: For VFR departures behind heavy aircraft, Air Traffic Controllers are required to use at least a 2-minute seperation interval, unless a pilot requests a deviation from the 2-minute interval and accepts responsibility for maneuvering his aircraft so as to avoid the wake turbulence.

Turbotrent
Topic Author
Posts: 146
Joined: Tue Jan 01, 2002 8:43 pm

RE: Wake Turbulence

Thanks guys !!!! Very usefull information!!

Greetings,

Tom
Man's flight through life is sustained by the power of his knowledge.

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