dbo861
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757 Wake Turbulence

Sat May 22, 2004 5:26 am

Today during my flight lesson, my instructor was talking about wake turbulence, and he said the 757 creates the more wake turbulence than any other plane. Anyone know why this is, and why Boeing hasn't added winglets to combat this wake turbulence? I'd think that something bigger would make the most wake turbulence, but what do I know.
 
CougarAviator
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RE: 757 Wake Turbulence

Sat May 22, 2004 5:35 am

Am told that the 757 has bigger engines than it needs.........
Failure is not an option.....
 
andersjt
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RE: 757 Wake Turbulence

Sat May 22, 2004 5:37 am

Not that I am an engineer, but I think it has something to do with the ratio of wing size to the size of the fuselage? That's my guess.

I don't think the winglets diminish wake turbulence. On a flight from SFO to LAX once, we went through some pretty rough wake turbulence. The captain came on to announce that it was residual from a KAL MD11 that was 30 minutes ahead of us. You could hear the cockpit communications on Channel 9. The MD11 has winglets.
Oh how I long for the day when the skies were truly Friendly!
 
Vorticity
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RE: 757 Wake Turbulence

Sat May 22, 2004 5:39 am

Aerodynamics is by no means a trival study. Size isn't the biggest factor, it has a lot to do with the wing design and so forth when it comes to wake turbulence. Adding winglets isn't trivial either, they have to be carefully designed to actually significantly help the situation, then they have to be certified and so forth. It seems Aviation Partners Boeing has hooked up with CO now for a winglet program on their 757's similar to the ones going on 737NGs. They probably have numbers on what this does to the wake turbulence, but I would guess it would improve. I'm not sure why it wasn't done earlier, maybe the 757 was good enough at the time, and it wasn't worth the money to redesign.
Thermodynamics and english units don't mix...
 
ChiGB1973
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RE: 757 Wake Turbulence

Sat May 22, 2004 5:58 am

They are overpowered; according to our pilots. They must be able to continue T/O if they lose an engine. On a 757-33N coming out of MDW full of customers and cargo, that is one hell of a feat.

One captain mentioned, gear up, flaps to 20 and keep the one engine from turning the plane over on its top! Pilot talk? Who knows? Interesting though!

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avi8tir
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RE: 757 Wake Turbulence

Sat May 22, 2004 6:05 am

I dont think that the 757 is the worst. I think its just exceptionally worse than similar sized aircraft. Dont heavies get 3 miles? I think 757s get the same now. I remember the incident that started all of this years ago. A westwind with the founder of In-n-Out Burger crashed on a 3 mile final to SNA. Crashed into what is now the second half of the Santa Ana Automall at the 55 and Edinger. I think it was determined that it was folowing to close to a 757.
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InnocuousFox
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RE: 757 Wake Turbulence

Sat May 22, 2004 6:45 am

It isn't the worst, but they are given the same wake turbulence warnings that "heavies" do despite the fact that they don't meet the weight limit for a "heavy". It must be bad for their size or something.
Dave Mark - Intrinsic Algorithm - Reducing the world to mathematical equations!
 
greasespot
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RE: 757 Wake Turbulence

Sat May 22, 2004 7:09 am

Engines have nothing to do with wake turbulence. It is caused by the vortices coming off the wing tips. Based on the Pressure differences from the top and bottom of the wing. Therefore it is the wing shape which cause wake turbulence.

GS
Sometimes all you can do is look them in the eye and ask " how much did your mom drink when she was pregnant with you?"
 
FL1TPA
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RE: 757 Wake Turbulence

Sat May 22, 2004 7:34 am

I've read in a flight safety maual that because the 757 has a solid panel flap design it produces a stronger wake vortex than other larger a/c. 747, 777, A340 etc. have slots in the trailing edge flaps - you can see them when they extend for landing. These slot separations help dissipate a forming wake vortex. The 757 has no slots in the flaps which is more conducive to the formation of a wake vortex.

Kinda makes sense to me...
"Looks like I picked the wrong week to stop sniffin' glue."
 
ChiGB1973
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RE: 757 Wake Turbulence

Sat May 22, 2004 1:26 pm

ATA uses "heavy" on occasion for the 757. I don't know what the weight limits are.
 
waketurbulence
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RE: 757 Wake Turbulence

Sat May 22, 2004 2:14 pm

Wake Turbulence, haha just realized that's my SN, can be caused by any commercial jet aircraft. At SNA there are many simultaneous take-offs and landings between commercial traffic and general aviation. Any time commercial traffic is within about 5 miles or less GA a/c are always warned weather it is an A320, 737 or a 757 ect. As far as ATA, they designate their 757-300's as heavies because they surpass the weight standards of normal commercial a/c. I don't think any of their -200's are designated as heavies. I think to be classified as a heavy an a/c needs to be over 255,000 lbs MTOW and the 757-200 is right at that weight. I think it is not classified as a heavy because it doesn't exceed this weight.
-Matt
 
NWA742
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RE: 757 Wake Turbulence

Sat May 22, 2004 2:24 pm

I think to be classified as a heavy an a/c needs to be over 255,000 lbs MTOW and the 757-200 is right at that weight. I think it is not classified as a heavy because it doesn't exceed this weight.

Actually, I've been on a United 752, we were fully loaded to the max, and using Channel 9, we were being given the callsign heavy. So, I think some very loaded 752s are given the heavy callsign.



-NWA742
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waketurbulence
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RE: 757 Wake Turbulence

Sat May 22, 2004 3:12 pm

Yes NWA, I will modify my previous statement. Most times the 757 will not be designated as a heavy a/c. Sometimes these a/c are certified to fly over their usual weight of 250,000 lbs, thus making them a heavy a/c. A lot of charter 757-200 a/c fall into this category. Thanks for pointing that out.
-Matt

Also in the a/c stats page it says "basic max takeoff (220,000 lbs), medium range MTOW (240,000 lbs), and extended range (255,000 lbs or 255,550 lbs).

[Edited 2004-05-22 08:29:32]

[Edited 2004-05-22 08:30:18]
 
steve7e7
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RE: 757 Wake Turbulence

Sat May 22, 2004 4:55 pm

I remember there were a couple of incidents at LHR involving BA 757's at LHR shortly after their introduction, whereby wake turbulence was causing following aircraft to experience uncomfortable approaches.ATC subsequently re-classified the 757 as 'heavy' and increased the seperation.
 
QantasA332
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RE: 757 Wake Turbulence

Sat May 22, 2004 5:24 pm

First of all, wake turbulence is mixed function of the weight of the aircraft in question, its wing loading, and its configuration (a cleaner configuration leads - slightly counter-intuitively - to stronger wake turbulence). Engines play no part.

In the 757's case, it's relatively strong wake can be attributed mostly to its wing-flap geometry. However, it is sometimes only thought of as producing particularly strong wake turbulence simply because it's on the heavy end of the scale in its general weight class...

Cheers,
QantasA332
 
Yikes!
Posts: 284
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RE: 757 Wake Turbulence

Sun May 23, 2004 8:17 am

Greasespot (what kinda handle izzat??) was spot on: engines have nothing to do with wake turbulence.

With the advent of low-drag approaches and associated techniques, the B757 was on the forefront of the "capture the G/S from above" procedure. In brief, from descent from altitude, power is left as close to flight idle as possible. Any slowing is accomplished by low level level-off, speedbrake, flap/gear deployment. More often than not, this results in a g/s capture from above.

Wake turbulence generation is at its maximum when an aircraft is fast and clean. Not to be confused with wingtip vortex generation.

Hence the 757 got tagged as a "heavy" with respects wake turbulence separation standards.

Best Regards,

Yikes!
 
planespotting
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RE: 757 Wake Turbulence

Sun May 23, 2004 9:51 am

wake turbulence is present on any aircraft whether it's a skyhawk a king air or a concorde. The wake turbulence is only present when the wings are producing lift. And i may offer a correction:

The greatest wake turbulence wingtip vortex occurs when the generating aircraft is heavy, slow, in a clean configuration, and operating at a high angle of attack

Right out of my jep commercial book. here is their explanation for WV

whenever an airplane generates lift, air spills over the wingtips from the high pressure areas below the wings to the low pressure areas above them. This flow causes rapidly rotating whirlpools of air called wingtip vortices or wake turbulence

It mentions weight as an important factor in terms of WT, but it doesn't really say what about the wing has an influence. My guess is that because the surface area of the wing on a 757 appears to be smaller than most other commercial airplane wings of similiar aircraft, there is going to be more air distributed over a smaller area, so that means that more air is going to be put through the downwash and behind the wingtips, thus creating a greater amount of wake turbulence. Some kind of ratio of

air density/surface area

or something along those lines. I'm taking my aerodynamics class this fall so perhaps i can better shed light on the subject after that.

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QantasA332
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RE: 757 Wake Turbulence

Sun May 23, 2004 6:43 pm

Wake turbulence generation is at its maximum when an aircraft is fast and clean. Not to be confused with wingtip vortex generation.

In fact, "wake turbulence generation" (i.e. the strength of wake turbulence) is at its maximum not when an aircraft is fast and clean, but when it is slow and clean. And I don't see how there could be a problem of confusion with wingtip vortex generation because wake turbulence is mostly a product of those vortices, and thus whenever those vortices are strongest, an aircraft's wake is.

Cheers,
QantasA332
 
Yikes!
Posts: 284
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RE: 757 Wake Turbulence

Mon May 24, 2004 7:11 pm

I think if you dig into Mechanics of Flight, it will state just what I quoted. Unfortunately I do not have a copy at the hotel here this summer!

If my statement is wrong, then heavy, fast and clean would produce the least wake turbulence. The worst wake turbulence I have ever run into is in trail of a 747 at altitude, 1000' below his altitude. And he'd been gone for 5 or 6 minutes per the ATC reply!

Back to the question, again, the 757 flight path profile at its best, intercepts the glide slope from above in a low-drag approach. If done correctly.

Best Regards,

Yikes!
 
liamksa
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RE: 757 Wake Turbulence

Mon May 24, 2004 7:44 pm

I've always associated the levels of wake turbulence to be proportional to the AoA or pressure differential around the wing. Therefore expect the greatest wake turbulence behind a heavy, clean, slow aircraft.

However i've never had any actual encounters having been taught and read how dangerous it can be to a light aircraft - steer clear!
 
QantasA332
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RE: 757 Wake Turbulence

Mon May 24, 2004 10:08 pm

I think if you dig into Mechanics of Flight, it will state just what I quoted. Unfortunately I do not have a copy at the hotel here this summer... If my statement is wrong, then heavy, fast and clean would produce the least wake turbulence.

None my textbooks think so, and hopefully Mechanics of Flight doesn't really either, as it's otherwise quite a good book!

It does stand to reason anyway, though, that the strongest wake turbulence is definitely generated at slow rather than fast speeds. After all, slow flight is a high-Cl regime, and thus the strongest wingtip vortices (and corresponding wake turbulence) occur(s) there. Accordingly "heavy, fast, and clean" certainly doesn't produce the weakest wake. Rather "light, fast, and dirty" does, pretty intuitively...

Cheers,
QantasA332
 
planespotting
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RE: 757 Wake Turbulence

Tue May 25, 2004 12:16 am

Just because wake turbulence is the strongest when an a/c is slow, clean, heave and at a high aoa, doesn't mean that wake turbulence can just be shrugged off at any other time. Wake turbulence is present whenever the a/c is producing lift, so it's not like it's only noticeable after take off and near the ground. The worst wake i've ever encountered was behind an AA MD-80 @ MLI immediately after passing my private checkride. I acknowledged the caution wake turbulence and was about 400 feet off the ground i was violently jerked to the left and banked about 45 degrees in the same direction.

But just because thats the worst i've experienced doesn't mean it's the worst ever period. And why would heavy fast and clean produce the least amount? The "least" amount of wake turbulence from a heavy jet is like talking about the "smallest" atomic bomb, it's still gonna knock you around.
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nfield
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RE: 757 Wake Turbulence

Tue May 25, 2004 1:15 am

Having spent many a happy hour outside the Green Man pub - only 1/4 mile from touchdown LHR 27L - I can confirm that 757's seem to have the worst wake turbulence. It sounds like a gigantic sheet of paper being ripped up and you can see the surrounding trees getting a battering.
 
QantasA332
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RE: 757 Wake Turbulence

Tue May 25, 2004 6:16 am

Just because wake turbulence is the strongest when an a/c is slow, clean, heave and at a high aoa, doesn't mean that wake turbulence can just be shrugged off at any other time.

Of course! Where there's lift there's a vortex and thus a wake (given a finite wing). Period. It wasn't that I was ignoring that fact, but as I understood things we were just trying to determine what extremes produce the strongest wake...

Cheers,
QantasA332
 
RaginMav
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RE: 757 Wake Turbulence

Tue May 25, 2004 6:26 am

is there any correlation between wake turbulence intensity and the amount of drag that they induce?
 
QantasA332
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RE: 757 Wake Turbulence

Tue May 25, 2004 6:41 am

Is there any correlation between wake turbulence intensity and the amount of drag that they induce?

Sort of. The correlation is really between the wingtip vortices strength/intensity and the amount of drag, which is basically the same comparison as wake turbulence strength versus drag. However, it's the vortices that actually generate the drag, and not the wake per se.

At any rate, the drag in question is of course induced drag. As the vortex strength changes, the amount the lift vector is tilted back changes, correspondingly changing the amount of drag present. Or if you prefer the energy-loss approach, changes in vortex strength change the amount of energy lost at the wingtips, thus increasing or decreasing drag. I'll stop there, but if you'd like me to explain in a bit more depth, just ask.

Cheers,
QantasA332

Edit: typos...

[Edited 2004-05-24 23:44:00]
 
videns
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RE: 757 Wake Turbulence

Tue May 25, 2004 1:43 pm

QuantasA332:
I'd certainly would appreciate it if you'd explain it in more detail!
Travel? Why would i travel if I can watch it on TV?
 
QantasA332
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RE: 757 Wake Turbulence

Tue May 25, 2004 5:24 pm

Okay. I'm not sure what you already know and what you don't, so I'll start from the beginning...

Induced drag is drag due to lift. From the moment wings start generating lift, a pressure differential exists between the top and bottom. That is, lower pressure on top, higher pressure on the bottom. Because of that, air tends to 'leak' around the tips of the wings, from the aforementioned higher-pressure bottom of the wing to the lower-pressure top of the wing, as you would expect it to. That flow creates two counter-rotating vortices at the wingtips. The vortex originating at the left tip rotates clockwise, and that on the left wing rotates anti-clockwise. Those vortices essentially push the departing airstream downward ("downwash"), which effectively tilts the lift vector back (the 'direction' of lift is always perpendicular to the departing airstream). Thus, the lift isn't only acting up, but there's a stream-wise component - by definition, drag. That is induced drag.

The energy-loss approach is just a different way of looking at it. In so many words, vortices obviously contain some lost kinetic energy and something (namely the engine[s]) has to make up for it, in a sense. In that way, it is a sort of drag. (Sorry, hard to explain here...)

So all I was trying to say was that yes, there is a correlation between vortex strength/intensity and drag, simply because a change in vortex strength means a change in the amount of downwash and thus drag.

Having said all of that, I suppose I should have just provided a link instead. So, check out http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html. Sorry to have gone off on a bit of a tangent (and made a fool of myself in the process  Laugh out loud)...

Cheers,
QantasA332
 
UA777Pilot
Posts: 4
Joined: Wed May 26, 2004 9:58 am

RE: 757 Wake Turbulence

Wed May 26, 2004 10:10 am

Hey All!
I was a 757 pilot in a former life. To be classified as a heavy aircraft, an aircraft has to weigh over 225,000 pounds. The 75 is not exactly 225,000 pounds, but when fully loaded will be assigned the call sign of "heavy". The 757 MUST HAVE 5 miles of separation between other aircraft, which is similar to the seperation rules of the 747,767,777, A330,A340, and other Heavies. The reason for this is because of the really big wing span on the aircraft, and the pencil like shape of the fuselage, and also because the aircraft can carry about 200 pax. I hope this helps!
 
QantasA332
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RE: 757 Wake Turbulence

Wed May 26, 2004 11:25 am

The reason for [the 757 having such a strong wake] is because of the really big wing span on the aircraft...

Not really. Rather, longer wing span actually has the effect of weakening wake turbulence, via its weakening of downwash...

Cheers,
QantasA332

Edit: fixed the italics...

[Edited 2004-05-26 04:26:13]
 
goboeing
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RE: 757 Wake Turbulence

Wed May 26, 2004 1:07 pm

UA777Pilot,
You must have meant 255,000 pounds, not 225,000.

Bedford, NH huh? I'm right down the road at ASH most of the year.

Nick

[Edited 2004-05-26 06:13:50]

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