LASoctoberB6
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If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 2:57 pm

A 757 for example, if it is equipped with winglets, then when a smaller aircraft like a 737 is holding position after the 757 takes off, does it have to wait as long for takeoff clearance as it would if the 757 wasn't equipped with winglets?
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2H4
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:09 pm

Winglets don't necessarily reduce an aircraft's wake turbulence. Much (most?) of the 757s wake turbulence is a result of vorticies spilling off the outside trailing edges of the flaps.

2H4
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LASoctoberB6
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:15 pm



Quoting 2H4 (Reply 1):
Winglets don't necessarily reduce an aircraft's wake turbulence. Much (most?) of the 757s wake turbulence is a result of vorticies spilling off the outside trailing edges of the flaps.

Ahh, okay. I thought it was the edge of the wing that created the vorticies..
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2H4
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:19 pm



Quoting LASOctoberB6 (Reply 2):
Ahh, okay. I thought it was the edge of the wing that created the vorticies..

The wingtips do indeed create vorticies, but as I understand it (and I'll gladly stand corrected), those vorticies are not necessarily the ones that create the biggest hazard for following aircraft.

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2H4
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:27 pm

Here are some visual examples of what I'm talking about:

757:

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Click here for bigger photo!

Photo © Steve Morris - AirTeamImages
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Photo © Steve Morris - AirTeamImages





MD-11:

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Photo © Tim de Groot - AirTeamImages



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legoguy
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:37 pm

Theoretically would little end plates on the edge of the flaps help reduce the vortices such as the wing tip fences on the a320?
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saab2000
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:38 pm

Those are impressive pics!! I have on a couple of occasions flown through some wakes and I can assure anyone who cares that I don't mind that waiting period one bit. I take off in PHL a lot after heavies and know that these can be hazardous. ATC sometimes try to rush things, but I will normally add a few seconds after T/O clearance just for safety's sake.

Wake turbulence can be a real hazard, more than most people recognise.
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2H4
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RE: If A Heavy Aircraft Has Winglets..

Wed Apr 30, 2008 3:44 pm



Quoting Legoguy (Reply 5):
Theoretically would little end plates on the edge of the flaps help reduce the vortices such as the wing tip fences on the a320?

I suspect so, but given the extra weight, expense, and (presumably) drag in flight, I don't think it warrants reducing wake for such a tiny fraction of the flight. I could very well be wrong, but that's my gut feeling.

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tdscanuck
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RE: If A Heavy Aircraft Has Winglets..

Thu May 01, 2008 3:18 am



Quoting Legoguy (Reply 5):
Theoretically would little end plates on the edge of the flaps help reduce the vortices such as the wing tip fences on the a320?

Yes, but little end plates will have a little effect. To have any real benefit, winglets need to be relatively large (a 737 winglet is about 6 feet tall).

On top of that, a flap is essentially a very low aspect ratio wing. It's got horrible induced drag characteristics but that's OK because it's main purpose in life is high lift, not high lift:drag ratio. I agree with 2H4...I can't see how the benefit would outweigh the cost.

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LASoctoberB6
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RE: If A Heavy Aircraft Has Winglets..

Fri May 02, 2008 11:07 pm

Why is it that the edge of the T/E flap creates the vortex and not so much as the wingtip?

Quoting 2H4 (Reply 4):
Here are some visual examples of what I'm talking about:

I knew what you were talking about from a memory of:

Quoting Saab2000 (Reply 6):
I take off in PHL

in line up for takeoff on US Airways in 2000. That was all I can really remember from that flight was seeing the vorticies from the landing aircraft. But thanks, 2H4, for your detailed answer and research for the pictures.
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speedbird128
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RE: If A Heavy Aircraft Has Winglets..

Sun May 04, 2008 2:17 pm



Quoting LASOctoberB6 (Reply 9):
Why is it that the edge of the T/E flap creates the vortex and not so much as the wingtip?

The flaps are what increase the lift of the wing, so it's only expected they also cause the vortices.
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prebennorholm
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RE: If A Heavy Aircraft Has Winglets..

Tue May 06, 2008 11:24 pm



Quoting LASOctoberB6 (Thread starter):
A 757 for example, if it is equipped with winglets, then when a smaller aircraft like a 737 is holding position after the 757 takes off, does it have to wait as long for takeoff clearance as it would if the 757 wasn't equipped with winglets?

In this thread two different things have been confused:

1. Wake turbulence

2. Wing tip (and flap end) vortices.

When a few hundred thousand lbs of a/c is airborne, then it is the result of enormous amounts of air being accelerated downwards over the entire wing span. Equal amounts of air will rise outside the wing span creating two enormous rotors in opposite direction. That is wake turbulence.

Wake turbulence is a function of weight and wing span. More weight = more powerful wake turbulence. Greater span = reduced wake intensity by spreading the energy over more air masses.

Tip vortices, or lift spills over the wing tip, are far less powerful and do not influence clearance distances. They are tiny rotors inside the huge wake rotor.

In addition: Winglets do not necessarily reduce the intensity of the tip vortices at take-off or landing. They are carefully shaped and angled to intercept the tip vortices at cruising speed and convert some of the vortex energy into forward thrust much like a sail on a sailboat. Their influence on the danger of wake turbulence at slow speed is negligible.

Incidentally, the best thing to reduce the intensity of wake turbulence is drooped ailerons. The reason is that they create a more even distribution of the lift over the entire wing span. It isn't much, but it is measurable. Even if drooped ailerons INCREASE the intensity of tip vortices.

Drooped ailerons also reshape the tip vortices in such a way that winglets, optimized for cruise conditions, become even more useless at slow speed.

Please notice that in the text above I wrote about winglets, not tip fences as seen on A320 and A380 planes. Tip fences are a slightly different story, even if the purpose is the same. They are way less efficient at cruising speed, but on the other hand they work at all speeds, also with drooped ailerons. But like winglets they don't reduce wake turbulence in any measurable way.
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