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strongest wing tip vortices when slow, clean and heavy. BUT WHY?

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strongest wing tip vortices when slow, clean and heavy. BUT WHY?

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Old 7th Dec 2008, 19:25
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strongest wing tip vortices when slow, clean and heavy. BUT WHY?

Hi all!
as every pilot I learned that the strongest wingtip vortices are generated when the aircraft is slow, clean and heavy. OK, now I was thinking about why it is like this, so i'm going to explain each of this factors and please correct me if wrong or if I missed something.


-SLOW: when flying at slow velocities, angle of attack needs to be increased producing more lift thus producing stronger vorices.

-HEAVY: a heavy aircraft needs more lift thus producing stronger vortices.

-CLEAN: ok here is where I got confused. Aren't we creating more lift when with full flaps? What exactly does flaps has to do with vortices? is it that flaps interfere with whem and don't let them develop? is it because of the drag produced by flaps?


Hope you can help me with this one!

Thanks and best regards
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Old 7th Dec 2008, 20:12
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I'd imagine that the deployment of flaps would allow for the shedding of some of the energy/pressure differential at the outboard edge of the flaps as opposed to the wing tip, thereby reducing the power of the wing tip vortices.

There'd be a reduced AOA with flaps for a given speed as well

I'm going to duck now as someone with a better understanding of this tells me I'm barking up the wrong tree
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Old 7th Dec 2008, 20:29
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Hi Ludo,

Great question! I'm no expert, but here is my take on things:

Imagine that you are travelling in a boat (at 3-4 kts) and trailing your hand in the water (like a wing). You will see a vortex appear behind your hand which will change in intensity as the angle of your hand changes. The speed of the boat is the same. The greater the angle of your hand, the stronger the vortex. I would contend that the strength of the vortex is proportional to the angle and not a lot else.

Your explanations need to be looked at again, bearing this in mind:

-SLOW: when flying at slow velocities, angle of attack needs to be increased producing more lift thus producing stronger vorices.
A given aircraft will be at a constant weight so the lift required will be the same at any speed or angle of attack. But a slower speed will need a higher angle of attack to produce the same lift, hence stronger vortices.

-HEAVY: a heavy aircraft needs more lift thus producing stronger vortices.
A heavy aircraft needs to fly at a higher angle of attack than a light one (assuming they fly at the same speed), hence stronger vortices.

-CLEAN: ok here is where I got confused. Aren't we creating more lift when with full flaps? What exactly does flaps has to do with vortices? is it that flaps interfere with whem and don't let them develop? is it because of the drag produced by flaps?
Again, we are not creating 'more lift' with flaps, (constant weight, constant speed), rather we are increasing the coefficient of lift. This means that with flaps we can fly at a reduced angle of attack. So the vortices reduce in strength due to the lower angle of attack. So a clean aircraft has a higher angle of attack than one with flaps, hence stronger vortices. Cyclone733 makes a good point about flaps creating their own vortices. Just watch a 737 or 757 landing on a misty morning. They produce great white vortices from the outboard edge of the flaps.

Not sure an aerodynamisist would agree with my reasoning, but it makes sense to me!
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Old 7th Dec 2008, 22:37
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Very interesting post.
This would require a look into some fluid dynamics for sure as it has been pointed out.
Flaps would indeed seem to promote the separation of the vortex near the flap tip, where it had lesser opportunity to gain spanwise flow strength. Again I ain't no expert either and i am just speculating.
And here is another question, why does the 757 have the strongest wing vortices? Does it have anything to do with a large wing proportionate to the fuselage which is quite skinny or am i on the wrong track.

cheers
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Old 8th Dec 2008, 04:33
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ludovico - The key thing you need to look into is Induced Drag.

This is the drag created as a by-product of lift when the angle of attack (AoA) increases towards it's maximum.

I think you may be getting confused with where the vortices are coming from... To quote wikipedia... (somebody had to!) The strongest vortices are produced by heavy aircraft, flying slowly, with wing flaps extended.

So while that example gives the strongest vortices, they aren't occurring at the wingtip. Even stronger vortices are created by the flaps with the slow/heavy conditions mentioned, and as previous posters have commented, this happens on the edge of the flaps, not the wingtip.

So the statement as far as WINGTIP vortices are concerned is correct.

Hope that makes some sense...



12/9/08 - And to think I actually believed wikipedia was right..! Ignore the part about having the flaps extended.
I'll stick to little planes in future...

So does this mean that there would be greater wake turbulence encountered behind a clean airliner (heavy, low airspeed & @ high Alpha)...? Or is this a different matter altogether??

Last edited by sapperkenno; 10th Dec 2008 at 00:24.
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Old 8th Dec 2008, 06:04
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The strongest vortices are produced by heavy aircraft, flying slowly, with wing flaps extended
Nope. Wiki got it wrong, clean is the answer. Lowering flap biases the production of lift towards the root (large camber change and greatly increased angle of attack because of the flap)

Quite right about the induced drag however. Induced drag on an airfoil is inversely proportional to the square of the airspeed and is directly related to the amount of induced downwash at the trailing edge of the wing. As the angle of attack is increased (because you're going slower) the induced downwash is increased.

Vortices don't only flow from the end of lowered flap, or the wing tips, but exist along the entire length of the trailing edge of a lift producing airfoil. They can be seen at time also from the tips of the horizontal stabiliser, and tip of the vertical stabiliser on aircraft doing asymetric work.
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Old 8th Dec 2008, 07:17
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thanks for all your responses!

@eckhard - the statements as you re-wrote them now give sense to me as well, thanks for correcting them, good job!

@Brian Abraham - so could we say that the more AoA, the more vortex strength (and the more induced drag of course)?

wikipedia HAS to be wrong, I don't think the AIM could be wrong about the "heavy, clean and slow" thing.
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Old 8th Dec 2008, 23:47
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so could we say that the more AoA, the more vortex strength (and the more induced drag of course)
You've nailed it in one. Induced drag is proportional to the square of the coefficient of lift. As you slow you have to increase the angle of attack which increases the lift coefficient. Have a look at Induced Drag Coefficient

If you have an interest in how things work a good starting point is NASA's index page here Guided Tours of the BGA One would hope, pray and think that NASA may have a bit of a handle on this stuff.
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Old 9th Dec 2008, 01:54
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OK i'll check those links for shure!

And thanks again to all for your responses!
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Old 9th Dec 2008, 09:53
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Airfoil design seems to play a role too.

The 757's wakes are so bad, the plane has been classified as "heavy", even though it's weight does not fit the definition.
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Old 9th Dec 2008, 20:31
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more than one vortex!

Okay, here's what I remeber from wing design 101:

with flaps extended, you have some of the vortcity shed from the outboard edge of the flap, and another one shed from the wingtip, as shown most excellently by the above pic.

Both of those are rotating in the same directions (clockwise / anticlockwise, depending on which wing)

So the upwards component of the vortex from the flap is superposed with the downwards component of the wintip vortex, thus reducing the overall vertical component of the airlfow in the interaction field.

With a clean wing at the same weight and airspeed you get the same lift, but the whole kick of the lift is off of the tip vortex, without the canellation effect.
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Old 9th Dec 2008, 20:45
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Hello ludovico

I asked the exact same thing here at tech log some time ago and couldn't wrap my head around it, but it seems Brian Abraham has a really good point.

I was thinking that flap couldn't affect the vortex since flying an airplane in a given IAS has to produce the same amount of lift, and hence induced drag, whether it is configured with flaps or not. Here is where I guess I was wrong:

A vortex being dependent of the downwash angle from the airfoil to the direction of flight and the pressure differential between the upper- and lower side of the airfoil, is then dependent of the aspect ratio of the wing.
As we read during our studies that's why the gliders have long wings.
But.. lowering flaps shifts the centre of pressure laterally towards the area of the flaps due to the increased camber and downwash angle. When maintaining the same amount of total lift this means less pressure differential toward the wingtips, as if there was a higher aspect ratio.
Meaning smaller vortex when lowering flaps.

As said so many times in this thread.. this is just my theory. If I am corrected I see that as a chance of learning.

I hope this may have been of some help.
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Old 29th Oct 2009, 12:38
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Partial-span flaps increase strength of trailing vortices

This thread was drawn to my attention a few days ago. It is now eleven months since the thread was active, and maybe no-one is watching, but here is my contribution. I am an aeronautical engineer specialising in aerodynamics and airplane performance. I think I was the author of the comment in Wikipedia saying wingtip vortices are strongest with flaps extended.

Most of the comments on this thread focus on angle of attack in an attempt to explain the relationship between trailing vortices and trailing-edge flaps. This is forgivable because it works when explaining the change in strength of vortices when there is a change in airspeed or lift. However, as Brian Abraham pointed out correctly, the strength of vortices and induced drag is a function of the lift coefficient, not the angle of attack. Using angle of attack in an attempt to relate trailing vortices and flap settings leads to the incorrect conclusion because changing flap setting causes a significant change in angle of attack but no significant change in lift coefficient. I will attempt to explain.

Vortices represent kinetic energy and they don’t occur spontaneously in the atmosphere. The law of conservation of energy requires that a force must act on the atmosphere to cause a vortex. In the case of trailing vortices behind an airplane the generating force is the reaction to the induced drag on the airplane. If the induced drag on an airplane increases by one percent, the rate at which energy is added to the trailing vortices also increases by one percent. Therefore the energy in the trailing vortices is known once the induced drag on an airplane and its true airspeed are known.

The lift on a wing is not generated uniformly across the span. Pressure difference between top and bottom surfaces is greatest near the wing root, falling to zero pressure difference at the wingtip. The lift should reduce gradually and smoothly from the center section to the tips. To achieve the minimum induced drag a fixed-wing airplane needs a wing whose spanwise lift distribution is in the shape of an ellipse. That is why R J Mitchell designed the Supermarine Spitfire with an almost elliptical planform so that its spanwise lift distribution with the flaps retracted would always be close to perfect.

The key to unlocking the mystery of partial-span flaps and trailing vortices is neither angle of attack nor lift coefficient. The key is the aspect ratio and the Oswald Efficiency Number (often called Span Efficiency Factor). The Oswald Efficiency Number for typical airplanes with flaps retracted is somewhere between 70% for the least efficient, up to 85% for the most efficient. This means that the effective aspect ratio of the wing of typical airplanes is only 70% to 85% of the actual aspect ratio. An airplane with a perfectly elliptical spanwise lift distribution would have an effective aspect ratio equal to its actual aspect ratio. The higher the aspect ratio of an airplane wing, the lower is the induced drag. The standard formula for aircraft drag coefficient contains the aspect ratio and the Oswald Efficiency Number in the denominator of the induced drag term.

In Daniel P. Raymer’s book "Aircraft Design: A Conceptual Approach" (AIAA Education Series) in Section 12.5 it states "deflection of flaps changes the spanwise lift distribution so that a flap deflection actually increases the induced drag"

When partial-span flaps are extended, extra lift is generated on the part of the wing with the flaps, and less is generated on the part of the wing with the ailerons. In Section 12.6, Raymer states "This extra lift in the vicinity of the flap affects the spanwise lift distribution, and therefore the drag-due-to-lift. Our hard-won elliptical lift distribution is ruined, and we must adjust the drag-due-to-lift upward."

Extending partial-span flaps ruins the gradual and smooth shape of the spanwise lift distribution. This causes a significant fall in the Oswald Efficiency Number, and that in turn causes a significant increase in induced drag coefficient. Even the lift distribution of the Spitfire is severely disrupted when flaps are extended. Induced drag and trailing vortices both increase as the result.

All of the above is in conflict with several websites and FAA Advisory Circular AC 90-23F "Aircraft Wake Turbulence". In section 5 of AC 90-23F it states "The greatest vortex strength occurs when the generating aircraft is heavy-clean-slow." Sadly, the authors of 90-23F appear to have been unaware of the significance of what they have written. Stating that the greatest vortex strength occurs with flaps retracted is contrary to everything known and published about trailing vortices and induced drag, so it demands a careful and detailed explanation of what they meant and why. Section 5 also says "The vortex characteristics of any given aircraft can also be changed by extension of flaps or other wing configuring devices." The authors knew that flaps influence vortex characteristics but they were conspicuously reticent about declaring in which direction the characteristics change, and why.

Perhaps the authors of 90-23F know something that aerodynamicists don’t. Aerodynamicists would be willing to accept the knew knowledge if only the authors would divulge what it is.

If it were true that trailing vortices are strongest in the clean configuration, why wouldn’t airplane designers create airplanes with fixed trailing edge flaps so they would benefit continually from weaker trailing vortices and therefore weaker induced drag? Why wouldn’t pilots fly all the way to their destination with flaps in the takeoff position to benefit from weaker trailing vortices and lower fuel burn? The answer is that trailing vortices and induced drag are stronger with trailing-edge flaps extended, and that is why the designer, the operator and the pilots are all keen for the flaps to be retracted as soon as it is safe to do so.
Classical aerodynamic theory and many decades of airplane design lead us to predict that trailing vortices increase in strength as trailing-edge flaps are extended. If someone knows that the opposite occurs, that someone should explain why the theory and the practice fail.
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Old 29th Oct 2009, 23:34
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i dont now how i came across this post but i did. lol.. i have a frozen atpl and i distinctly remember this in our jaa atpl theory we were thought clean slow and heavy... its so interesting dolphin.. dont get me wrong here because you know more than me but how did the jaa get that one wrong??
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Old 30th Oct 2009, 00:50
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Vortices represent kinetic energy and they don’t occur spontaneously in the atmosphere. The law of conservation of energy requires that a force must act on the atmosphere to cause a vortex. In the case of trailing vortices behind an airplane the generating force is the reaction to the induced drag on the airplane. If the induced drag on an airplane increases by one percent, the rate at which energy is added to the trailing vortices also increases by one percent. Therefore the energy in the trailing vortices is known once the induced drag on an airplane and its true airspeed are known.
But what is dangerous to the trailing aircraft is not the total energy of all the trailing vortices coming off the aircraft, it is the energy that is still organized in a vortex miles behind the aircraft. A quick look at the outboard edge of deployed flaps in moderately high relative humidity conditions will show that a very prominent vortex is generated there. By the same quoted law of conservation of energy, obviously the wing tip vortex is reduced by the addition of flaps to allow for the formation of the second vortex at the flap tip. This last is sarcasm, as obviously the total changes, but I think you can see where this is going. In order for wake turbulence to be dangerous, it has to stay organized as long as possible in time and space.

Perhaps the authors of 90-23F know something that aerodynamicists don’t. Aerodynamicists would be willing to accept the knew knowledge if only the authors would divulge what it is.
It's not that there is any "knew" math knowledge to be had. I think what the misguided FAA authors of AC 90-23F are talking about is the actual ability of observed vortices during several actual passes in different configurations to pose a danger to trailing aircraft. No math regarding the induced drag efficiency involved, just bigger wing tip vortices for the next plane.
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Old 30th Oct 2009, 00:57
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-SLOW: when flying at slow velocities, angle of attack needs to be increased producing more lift thus producing stronger vorices.

-HEAVY: a heavy aircraft needs more lift thus producing stronger vortices.

-CLEAN: ok here is where I got confused. Aren't we creating more lift when with full flaps? What exactly does flaps has to do with vortices? is it that flaps interfere with whem and don't let them develop? is it because of the drag produced by flaps?

SLOW: You are not producing more lift when you slow down, you'r increasing the AOA which increases the pressure differential between the two cambers of the wing, thus producing stronger vortices

CLEAN: If you remember the shape of the Cl vs AOA graph for a wing with and without flaps you can clearly see that, with flaps extended you will need less AOA to obtain a given Cl in comparison to a wing without the flaps deployed. And as I said on the previous paragraph, the lesser the AOA the weaker the wing tip vortices.

Is it possible that 12 answers overlooked this point?

Last edited by palou89; 2nd Nov 2009 at 01:29.
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Old 30th Oct 2009, 02:48
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I think I was the author of the comment in Wikipedia saying wingtip vortices are strongest with flaps extended.

Good old wikipedia. My favourite scientific read!

conventional wisdom I was taught is that clean slow and heavy produces stronger tip vortices.

heavy - increased AOA required for a given speed - stronger circulation at the tip driven by the greater difference in pressure above and below the wing which results from the higher AOA.

slow - the reduced forward speed promotes greater spanwise flow, ergo stronger vortex at the tip.

clean wing vs flaps extended - the span wise lift distribution is alters with flap, the centre of lift moves inwards towards the fuselage. less lift is being produced at the tip, so there’s a smaller vortex at the tip in the dirty config. Palou89's explanation with CL vs AOA is different way of saying the same thing.


its all a bit academic though since the plane will either be going fast and clean.....or very slow and very dirty, so in real life the speed and configuration are not really independent variables - for reasons more important than worrying about vortices. So although flaps out might tend to produce smaller tip vortices, the lower speed you will definitely be flying at with your flaps extended will tend to produce stronger vortices, so which effect wins?? This is why the effect of flap setting on vortex formation could be mis-understood.

The following argument doesn’t stack up..

If it were true that trailing vortices are strongest in the clean configuration, why wouldn’t airplane designers create airplanes with fixed trailing edge flaps so they would benefit continually from weaker trailing vortices and therefore weaker induced drag?

they wouldn’t do that because flaps INCREASE the coefficient of drag - your plane wont get all the way to Ibiza with its flaps out. think in terms of form drag - those big bulky flaps sticking out behind your nice smooth thin wing. or the extra skin friction, resulting from the increased wetted area with your fowler flaps extended. Also, saying that ‘induced drag’ is just the result of whats going on at the wingtip with the vortex is an over-simplification.

changing flap setting causes a significant change in angle of attack but no significant change in lift coefficient.

Extending the flaps will certainly change the lift coefficient. That is the only reason they are there.
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Old 30th Oct 2009, 05:28
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As I understand it in simple terms, the center of lift migrates I/B with speed. Slow and heavy clean would put that lift at the wingtips, flaps extending would bring that lift I/B loading the flaps transfering the vortices to the extreme usefull camber of the wing "flaps".

Great experience of this is on a misty/light rain day (747 fowler flaps my favorite), it is easy to see. You can see the vorteces increase I/B as the aircraft approaches and is on it's landing rollout.
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Old 30th Oct 2009, 18:12
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Old 30th Oct 2009, 23:18
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Reply to Amen Brother

Thanks for your prompt and thoughtful reply. Changing flap setting changes stalling speed because it changes the MAXIMUM lift coefficient. However, with plain and split flaps, changing flap setting doesn't change the instantaneous lift coefficient. Here is why. Lift coefficient is equal to lift divided by dynamic pressure and wing planform area. (Lift is equal to aircraft weight times instantaneous load factor. Dynamic pressure is indicated airspeed squared, times half standard air density.)

With plain flaps and split flaps, changing flap setting changes none of these things - no change in weight, load factor, indicated airspeed or wing planform area. Consequently changing flap setting doesn't change instantaneous lift coefficient. In the minute or two after changing flap setting there is usually a significant change in airspeed that brings about an equally significant change in lift coefficient, but it is the change in airspeed doing that, not the change in flap setting.

With Fowler flaps, there is a change in wing planform area and hence a change in lift coefficient. If extending Fowler flaps increases wing planform area by 10% then lift coefficient will decrease by 10% as the flaps are extending. However, the subsequent reduction in airspeed will cause the lift coefficient to increase to its original value and higher.

My questions asking why designers don't produce airplanes with fixed trailing-edge flaps, and why pilots don't fly all the way to the destination with takeoff flap, to minimise trailing vortices and induced drag were rhetorical questions.
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