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

[ludovico]

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 :}

[Cyclone733]

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

[eckhard]

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!:8

[Mt-Fuji Turb]

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

[sapperkenno]

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...

http://farm4.static.flickr.com/3019/2880032106_968d9dcb61.jpg?v=0

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??

[Brian Abraham]

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.

[ludovico]

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.

[Brian Abraham]

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 (http://www.grc.nasa.gov/WWW/K-12/airplane/induced.html)

If you have an interest in how things work a good starting point is NASA's index page here Guided Tours of the BGA (http://www.grc.nasa.gov/WWW/K-12/airplane/guided.htm) One would hope, pray and think that NASA may have a bit of a handle on this stuff. :p

[ludovico]

OK i'll check those links for shure!

And thanks again to all for your responses!:}

[Admiral346]

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.

[aidey_f]

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.

[KristianNorway]

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.

[Dolphin51]

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.

[Airbusfreak]

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??:8

[ahramin]

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.

[palou89]

-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?

[Amen_Brother]

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.

[muduckace]

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.

[Pugilistic Animus]

Everybody's correct:ok:

PA:}

[Dolphin51]

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.

[Brian Abraham]

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
Aaaaaaaaaah. But it does. Let us assume we are flying at a given speed with no flap extended. The wing will have an angle of attack of X° and be operating at a lift coefficient of Y. If you maintain the same speed and extend flap the angle of attack of that portion of the wing that is flapped is now (X+A)° with a corresponding increase in the lift coefficient. The unflapped portion of the wing, because the flapped portion is now producing a greater percentage of the required lift, has a reduced angle of attack and a lower lift coefficient and produces a reduced percentage of the total lift. As a pilot you notice the effect that when you lower flap you have to lower the nose to maintain the status quo. A diagram of the span loading of a flapped wing will show a discontinuity at the outboard end of the flap, compared to the smooth elliptical shape of an unflapped wing.

[Dolphin51]

G'day Brian! I'm sure you are aware that two different lift coefficients are defined. There is the aircraft lift coefficient C_L (capital C) based on the lift on the aircraft and the wing planform area; and there is also the section lift coefficient c_L (lower case c) based on two-dimensional flow around an airfoil section.

On 9 December on this thread you correctly stated that induced drag is proportional to the square of the coefficient of lift. That is a correct reference to the aircraft lift coefficient C_L. In your latest post you comment about the lift coefficient on the flapped part of the wing, and a different lift coefficient on the unflapped part of the wing. That is correct, but in your latest post you are referring to section lift coefficients.

In the well-known mathematical equation for induced drag coefficient, the reference to the square of the coefficient of lift is a reference to the aircraft lift coefficient.

Aircraft lift coefficient is defined by a mathematical equation which says C_L is equal to the lift divided by dynamic pressure and wing planform area. In my previous post I addressed this mathematical equation explicitly and in detail. You must admit your latest post comments about lift coefficient, but makes no attempt to address a mathematical equation. It is all subjective. We don't tackle mathematical equations by subjective prose.

(Section lift coefficient c_L is also defined by a mathematical equation.)

We are straying from the matter in question. My point is that the change in induced drag when trailing-edge flaps are extended can't be explained by a change in lift coefficient because changing flap setting (but changing nothing else) doesn't alter the aircraft lift coefficient (unless the flaps are area-changing Fowler flaps). What changes is the Oswald Efficiency Number (or Span Efficiency Factor) and this changes in such a way that extending partial-span flaps increases induced drag.

In your 9 December post you gave a web link to a NASA site that contains the formula for induced drag coefficient. There, in the denominator, is e, called the efficiency factor. That is the one I am calling Oswald Efficiency Number.

I am very happy to discuss mathematical formulae for induced drag, and lift coefficients, but we need to remember that mathematical formulae can't be solved by intuition or subjective assessments. Best regards.

[chrisN]

None of my business really, but it seems to this non-expert that some writers are confusing transient effects with a new stable configuration.

When lowering the flaps, instantaneously the momentum of the aircraft is maintained (no infinite force, so no immediate change in velocity – just accelerations that start). If the aircraft is kept pointing in the same direction, at the moment that the flaps are lowered:

1. Alpha at the inboard/flap section is immediately increased, if alpha is defined as the angle between the airflow and the chord from leading edge to (now lower) trailing edge. Hence inboard contribution of lift is transiently increased. If I do this in my glider, the immediate effect is to bounce it higher than it was, or reduce the rate of descent, very briefly – I can feel it happen. A sensitive accelerometer would display it as a transient effect.

2. Alpha at the outer/tip section is not changed instantly – airflow, attitude and wing section are not changed instantly.

What happens next depends on the pilot’s actions. If the nose is lowered to arrive at the same stable speed as before, after things settle down, alpha is higher inboard, lower outboard; overall Oswald efficiency (a term I had not heard before – thanks, D.) is lower as Dolphin says, and more energy is transferred to the atmosphere generally. The energy comes from more power expended or a greater rate of descent/expending more potential energy. Ultimately, that manifests itself as the vortices, as I understand it – the ultimate effect of a lift-producing wing passing through air is to add rotation, which appears as the vortices. More energy expended, stronger vortices. Unless someone has found a way round the laws of physics.

If the nose is not lowered, after things settle down a new phase of flight is entered with possibly different speed, certainly different alphas at tip and inboard, and the maths is too complicated for me to summarise. (But anyway, when flying, I can’t tell the induced drag – it is total drag that produces effects that the pilot sees.)

Chris N.

[Brian Abraham]

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
G'day D. Call me a dullard but I'm having trouble getting my head around this little bit. On flap retraction the immediate aircraft reaction is to reduce its rate of climb - read a loss of lift, on flap extension the immediate reaction of the aircraft is to balloon - read increase in lift (as I see it). In the lift formula the only thing that is changing to bring about these effects (once again as I see it) is a change in CL.

As I understood it CL = lift coefficient and cl (both lower case) the section coefficient. Should the section l be a capital as per your last?

While we have you D as an expert to hand, can you give a definitive explanation as to the reasons why a swept wing (eg Boeing 707 or like) stalls at much higher angles of attack than a straight wing.

Thanks D, and standing by to be educated. :8

[Joetom]

Slow, clean and heavy. Make it easy I will try.

Wing needs to produce required lift, which it will do, I think Slow holds the simple key, wing moving slow allows more time for higher pressure air under the wing to find lower pressure air on top of the wing.

An easy to think of it could be, keep slowing the wing to zero speed and have magic higher pressure air under the wing, this hi pressure air would have all the time in the world to slip over the wing tip and find the low px air.

Well at least that's what I though I read many many years ago.

Anyone have an easy way to explaine why heavy aircraft glide further than lighter aircraft ? (engines switched off)

[Pugilistic Animus]

and even when you have equations it still requires verification I've seen the most elegant math make fools of engineering teams when the actual data was in :}

Dolphin51 obviously knows what the hell he's talking about:ok:,...:D

but let me say this again:rolleyes: aerodynamics is an EMPIRICAL/EXPERIMENTAL science based upon experiment,... real life data on airfoil sections for designers is available in 'Theory of wing sections' by Abbott and Von Doenhoff--this is a BASIC [foundational] text on the subject it has much data and some theoretical treament on high lift devices

also I don't remember the exact name of the authors but there's ananother text ' Aircraft performance, stability and control' the further expands on the subject---:8

not too bad to get in the right mind set with Hurt either;)

the reason I like Davies so much is that although he was a TP and spoke engineering' the reason why his text was so effecdtive was because he could translate the abstractions of certification into plain pilot 'Horse Hooey',...I mean he spoke just enough horse hooey to get the stuff that matters WRT to stick and rudder,...pilots are not supposed to be too smart anyway:ooh:

PA

[Microburst2002]

This question always puzzled me.

The engineer has to know what he says, of course. But let me explain my "theory".

I always thought that induced drag (the one responsible for the wake turbulence) for a given lift depended on how this lift was was distributed on the wing planform. If most of it was in the root, like in airplanes with long span, or with eliptical planform, or with some taper ratio, then induced drag was less. Because the "leakage" of air at the wingtips was less.

All wings (or all I know, at least) have flaps in the root area and ailerons near the tips. If we maintaing the flight path after extending flaps, lift will be the same (after the ballooning) but lift distribution will be now more concentrated in the root, therefore reducing induced drag. This makes sense, Doesn't it?

But then, why don't we keep the flaps down all the way as he suggests?

Because when flying at high speeds parasite drag is the problem, not induced drag. And with the flaps down we have lots of parasite drag. This makes sense, too.

What do you think, guys?

[Pugilistic Animus]

boundary layer control and aspect ratio is also important WRT to your question,...much too complex for detail on pprune :\

or wiki :}

[Dolphin51]

I agree that tackling a practical problem with a mathematical approach can be challenging. The following might clarify things.
We want to determine the effect on aircraft lift coefficient when trailing edge flaps are moved, but nothing else changes except those things that have to change such as angle of attack and engine thrust. So we imagine this thought experiment. An aircraft is flying straight and level at 150 knots IAS with flaps retracted. Aircraft weight is 10,000 pounds and wing area is 250 square feet. To find aircraft lift coefficient we divide 10,000 by half the standard density of air and the square of 150 and the wing area, 250. (The result is 0.525)
Next we imagine plain flaps have been extended but the aircraft is still flying straight and level at 150 knots. To find the aircraft lift coefficient we divide 10,000 by half the standard density of air and the square of 150 and the wing area, 250. (Again, the result is 0.525 so we conclude that with plain flaps, changing flap setting has no effect on aircraft lift coefficient.)
I agree that in real life, as the flaps are running angle of attack will change and there will be a departure from level flight, the pilot will have to increase thrust to maintain 150, and he will have to re-trim. But most importantly, the above calculations show that after all those transient effects have died away and the aircraft has returned to level flight at 150 knots, the aircraft lift coefficient will be the same as it was prior to initiating the change in flap setting.
Changing flap setting might change induced drag and the strength of trailing vortices but that won’t be because of any significant change in aircraft lift coefficient because we have convinced ourselves that aircraft lift coefficient is independent, or almost independent, of flap setting. We need to look elsewhere to find an explanation for the change in induced drag.
Your question about stalling angles of swept-wing versus straight-wing aircraft is a good one. I will put my thoughts in a separate post on this thread.

[Yankee Whisky]

I fly simple aircraft such as gliders and L-19 towplanes.
It is my experience that more lift needed to be provided by slower airflow, using a clean wing, is one source for vortex increase at the tips.
When lowering flaps, the wing's AOA decreases at a given airspeed, hence the wings are at less AOA producing less vortex. However, the airflow around the flap tips will now become the large vortex generators.
I follow other L-19's closely on final and have experience strong roll tendencies of my aircraft when flying through another L-19's vortex.( created by its use of full landing flap.)
Mind; the lowering of flaps at a given airspeed and AOA requires an increase in power setting to overcome the added drag OR the aircraft nose needs to be lowered to overcome the added drag i.e. steepen the glide path, which is exactly what the pilot needs to land in a given space at the slowest safe speed.
I am not very scientific with this explanation, but it helps me understand the operation of aircraft.

[Pugilistic Animus]

just to add the chordwise load distribution over flapped wing sections was given special treatment by Julian Allen in a paper titled " calculation of chordwise load distribution over airfoil sections with plain split or serially hinged trailing edged flaps". NACA report 634,1938


just as an aside in engineering the way certain comparisons in terms of 'efficiency are made' is to compare an ideal [or simplified] theoretical condition to the real condition as mentioned above this simpliedied section is basically a simple flap compared to the real flap this is the flap chord ratio and it is this ration that is the 'efficiency factor mentioned in one of the above post,...so how does one do so?

well the equations are too cumbersome for here but basically when design ing the section,...data from other designs is used,...so that makes these questions well within the realm of experiemtnal engineering would not everyone agree:)

Munk's integrals have also proven useful in this area:8

[Brian Abraham]

G'day D, I awoke this morning with an inspired "I know what he's talking about". Remarkable how we (I) can become fixated on looking at something from one particular view point - and be wrong. Sorry to have put you to an extensive post when a good slap about the ears would have sufficed. Thanks.

[Sir Richard]

Joetom

Heavier aircraft (of the same type) glide further than lighter aircraft as they start with a higher total energy. (potential due to mass and altitude, kinetic due to mass and velocity....assuming different masses but identical starting altitudes and speeds)

I leave the experts to demonstrate the mathematics.:8

[Dolphin51]

G’day Brian. You asked why a swept-wing aircraft stalls at a higher angle of attack than one with a straight wing. I think the best answer is that an aircraft with a lower aspect ratio appears to stall with a higher angle of attack than an aircraft with a higher aspect ratio.

Every fixed-wing aircraft flies in the downwash induced by its own trailing vortices. If the downwash in the vicinity of an aircraft is 10 knots and the aircraft is flying level at 100 knots true airspeed that shows the atmosphere appears to be descending towards the aircraft at a gradient of one in one ten. Ten percent or six degrees. If the wing of this aircraft needs an angle of attack of twelve degrees the angle between the wing and the horizon will be eighteen degrees, not twelve. The effective angle of attack is twelve degrees, but the induced angle of attack is six degrees, so the geometric angle of attack is eighteen degrees.

The downwash and induced angle of attack are what tilts the lift vector backwards so that part of the lift is actually drag – that part of the drag called induced drag. The higher the induced drag the higher the induced angle of attack. Induced drag is highest at slow speeds and on aircraft with a low aspect ratio.

Perhaps the lowest aspect ratio of any manned aircraft is seen on the Anglo-French Concorde with an AR of only 1.5. At low speeds the induced angle of attack on the Concorde causes it to fly so nose-high that the pilots have an inadequate view of the airspace ahead. This is especially true when approaching to land and the pilots need to have the runway in view. The designers of the Concorde gave it a drooping nose so that the pilots had an adequate view in spite of the extreme nose-high attitude. It was the same in the Fairey Delta 2 research aircraft which had a drooping nose.

Delta-wing combat aircraft have an exaggerated nose-high attitude during takeoff and landing because of their low aspect ratio wing.

The spanwise lift distribution of a swept-wing aircraft is far from elliptical, especially at low speeds so its effective aspect ratio is significantly less than its geometric aspect ratio. (Oswald Efficiency Number is much less than one.) As a result, swept wings also appear to have an unusually high angle of attack when flying slowly. In fact, their angle of attack is not unusual but the strong downwash means the angle between the wing and the horizon is significantly greater than the angle between the wing and the approaching air.

[HarryMann]

My goodness... phew... :)

If we're sticking to the original question, I think ahramin (http://www.pprune.org/members/164119-ahramin) had it best covered quite a while back.. my answer would be much the same as his. The Slow and heavy bit is easy of course, vorticity function of Cl ^2

Somebody said this of those authors quoting also CLEAN as a condition..
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.This is matter of QUALITY Vs QUANTITY as ahramin (http://www.pprune.org/members/164119-ahramin) said I think...

Due to non-elliptic distribution, the TOTAL vorticity will almost certianly be highest with flaps extended (perhaps even with Fowlers extending without deflection, which also upsets the elliptical distribution)

BUT, that DIRTY voriticity will not all be wound up fractionally inboard of the wingtip, as it would for a nominally elliptical distribution*

So the upset(ting) potential may well be less, when in a dirty configuration, with strong trailing vortices but several of them across the span, dumping at points where there are significant steps in chordwise circulation (which is what creates lift).

* A straight taper wing of sensible (>7) AR and of nominally constant section and a wee bit of washout, does as someone stated, produce a spanwise lift-distribution quite close to elliptical, maybe close to 0.9 efficiency factor.

- The Spitfire Wing -

The story goes that this was not made elliptical due to the so-called 'ideal' lift distribution (an expensive production challenge anyway, as Vickers found out). It was originally sketched by RJ Mitchell, around the constraints he had to work with...
Span
Area
Thickness (Spit was always a thin wing, )
Guns- 8 of them within the above three constraints...meant that he had to keep the chord wide well outboard

The consequence was that he opted for a nominally elliptical planform to fit everything in, was accused of copying the Heinkel He70, certainly admired its aerodynamic smoothness, but his colleagues denied it had significantly influenced the Spitfire's layout.
Physical constraints had determined the Spirfire's wing (Mitchell was known as a good down to earth practical, not a fanciful or too theoretical engineer). Of course, he was aware that this planform wouldn't do any harm to it's maneouvring drag and so it also proved, whatever came near or bettered it at various stages in it's fighting life, it always could turn inside them, and at a similar power output, outclimb them in steady state (non-zoom) conditions.
Even the last of the line, the Griffon powered Seafires could climb to 40,000 feet faster than almost anything piston powered, certainly getting up there quicker than a Sea Fury (10 minutes from memory). In fact it could go on to about 50,000 & 51,500 wasn't unknown in the tropics! That was 10 years after it first flew too...quite a sustained development programme that that original elliptical wing (strengthened immensely torsional stiffness) made possible. The main spar was an absolutely unique design... (thin wing, had to be a bit special)

Thus, however highly thelater P51 is rated, the Spit was always to its last operational days, the better pure interceptor, exactly what it had been designed to do in 1935/6... and the 'accident' of the elliptical wing may have helped a bit.

Oops! Have I strayed somewhat :O

[BOAC]

D - I think you have left out the effect of vortex lift? If you are straying into Concorde/FD2/HP17 territory then is it not established that a very large part of the lift generation on very low aspect ratio wings is from vortex rather than 'classical' aerodynamics? I was always taught that it was vortex lift which continued flow attachment and 'lift' well past normal angles.

[Pugilistic Animus]

c=wing chord

cl = section lift coefficent

alpha [i]=downwash angle in degrees at span wise position 'y' measured from the center along dy

AoA e = effective aoa [as described by in HTBJ]:} =Alpha-alphi

effective Aoa at is a function of cl

skipping a few setps we finally have [to just scratch the surface]

alphae =alpha -180/pi*b/2pi*int[d/dy(cl*c?4b)dy/y1-y],b/2,-b/2]


solving this equation for your chosen sections will determine both down wash and span wise distribution

a mathmatical explaination of Dolphin1's eloquent post

as far a the vortex effect on general circulation [as BOAC mentions] I'm not gerttin on that bus:\:\:\

[Pugilistic Animus]

I'm trying to edit slightly the above post, but the computer wont let me:ugh: nor can I delete it as a copy:*

c=wing chord

cl = section lift coefficent

alpha [i]=downwash angle in degrees at span wise position 'y' measured from the center along dy

AoA e = effective aoa [as described by in HTBJ]http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/badteeth.gif =Alpha-alphi

effective Aoa at is a function of cl

skipping a few steps we finally have [to just scratch the surface]

alphae =alpha -180/pi*b/2pi*int[d/dy(cl*c/4b)dy/y1-y],b/2,-b/2]


solving this equation for your chosen sections will determine both down wash and span wise distribution

a mathematical explaination of Dolphin51's eloquent post

as far as the vortex effect on general circulation, in supersonic/non-ideal flows [as BOAC mentions] well I'm not gettin on that bushttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wibble.gifhttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wibble.gifhttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wibble.gif

PA:)

[HarryMann]

D - I think you have left out the effect of vortex lift? If you are straying into Concorde/FD2/HP17 territory then is it not established that a very large part of the lift generation on very low aspect ratio wings is from vortex rather than 'classical' aerodynamics? I was always taught that it was vortex lift which continued flow attachment and 'lift' well past normal angles.It is, due to the low AR and sweep, and 'controlled' in Concorde's case by a very clever Ogive planform and cunningly cambered leading edge (tricks learnt on the Fairey delta, Vulcan and Lightning, and TSR2 (of course!)

The LCS (Lift Curve Slope), normally 2 Pi (radians) for an infinite AR (2D) wing, deteriorates in a consistent manner with reduction of AR. Due to large amounts of 3D flow, the stall becomes mushy, very draggy and indeterminate as top-surface flow and thus overall circulation breaks down, Clmax usually suffers.. think Lockheed Starfighter (the widow-maker!)

But with sweep, lots of it, and the appropriate l.e shapes, that 3D flow can be controlled and utilised..Think of the lift-line being rotated backwards so the ciurculation takes place alomg a more fore-aft than spanwise axis... massive vortices develop as incidence increases, and whilst drag goes up massively too, Clmax's are maintained without flaps, once the vortex is stabilised...

http://www.efluids.com/efluids/gallery/gallery_pages/HW012/Werle_12.jpg

http://www.efluids.com/efluids/gallery/gallery_pages/HW011/Werle_11.jpg
One of the bonuses of the delta wing is the vortex sheet
which attaches itself at low speeds and high angles of attack.
It increases lift, it is claimed, by about 30 per cent and by
as much as 60 per cent in the ground cushion on landing.
By rounding the wing tips and extending the root fillets (thus
producing the "gothic" plan-form) this vortex sheet can be
made to stay attached up to and beyond the stall.

One downside is being on the back-side of the drag-curve, induced drag is high, and when slowing even more, increases dramatically! Of course, cart down helps.

[BOAC]

I just love that lower picture, HM! So illustrative. Any idea what alpha that is, and what the 'white' ?vortex? is which appears to start 'nowhere'?

Being well out of touch with modern aerodynamics like the ogive, has the 'stall' been re-defined for these shapes? Obviously there is no clear point where flow 'separates' since it is pretty well 'separated' at most angles and the classic 'nose-drop' and sudden onset of sink rate are no longer there. Do you know what the trigger is for the ultimate breakdown of the 'attached' vortex?

[Lightning Mate]

Sir Richard,

"Heavier aircraft (of the same type) glide further than lighter aircraft as they start with a higher total energy."

Not true I'm afraid.

Best glide range is achieved at the optimum lift/drag ratio. The heavier aeroplane will maintain exactly the same lift/drag ratio as the lighter one provided that speed is increased.

Therefore, providing the aeroplane is flown at the correct speed for the weight, glide range will not be affected by weight.

[John Farley]

Lightning Mate

I would agree with your comments providing you prefaced them with "in still air".

Competition glider pilots stuff in loads of ballast when flying into a head wind to up their glide speed and increase their range (think headwind case equal to normal light weight glide speed when you would have zero range)

JF

[Lightning Mate]

Good call John - maybe I should have gone to Specsavers....

[Pugilistic Animus]

Beautiful:D:D:D
There'll never be another Concorde:(
PA

[HarryMann]

I just love that lower picture, HM! So illustrative. Any idea what alpha that is, and what the 'white' ?vortex? is which appears to start 'nowhere'?

Being well out of touch with modern aerodynamics like the ogive, has the 'stall' been re-defined for these shapes? Obviously there is no clear point where flow 'separates' since it is pretty well 'separated' at most angles and the classic 'nose-drop' and sudden onset of sink rate are no longer there. Do you know what the trigger is for the ultimate breakdown of the 'attached' vortex?BOAC

It looks like the white is a trace started from a smoke cannister or similar, set some distance above the wing surface, presumably to show the flow in that plane, which if so, looks interesting (caused by the vortex picking it up being much expanded by then)

I think one way of defining a 'stall ' for these types of wings, could be simply maximum Cl, attainable, regardless of drag and thus thrust required for stable unaccelerated flight.i.e. A lot!

Another 'limit' might be buffeting 'G' - Concorde would shake about at lower speeds noticeably, I believe at anything much below 250 kts, and buffet badly during landing flare - have never flown on it, but think that pax were told/warned not to worry :)

PS. Green smoke is indicating fuselage vortices, as these can upset things quite a bit, apparently.

The ideal shape for M 2.0 cruise is a straight taper leading edge, with span roughly half the root chord. This is not ideal for subsonic flow, flaring the wing into the fuselage and reducing sweep at the tip creating that ogive or gothic shape, and then some further wing/body refinements made enormous difference, and much work went into the engine nacelle interaction with the wing's flow too, let alone the marvel of the whole intake system (26 feet long?). Reducing trim drag was a very importnat engieering goal, and together with fore/aft fuel management, gave the range required.

[Pugilistic Animus]

does anybody have some stats on the photo of the streamline ilike BOAC had orginally asked i.e AoA, Reynold's number, mach number

any more photo's like that?

Harrymann I did hear that the ogival shape improved low speed performanc; anyway inersting stuff:ok:

my belief however is that is was not planned I'll bet they got a serrendipitouslu good planform and then wrote equations for it afterwards:}---it's a shame we're not looking ahead into the hypersonic region too much wrt passenger aircraft,...the new stuff being submitted looks too unreliable to ever get through FAR 25:uhoh:

PA

[HarryMann]

This would be low speed subsonic stuff, so take a guess, Re could be anything typical of fullsize flight, since model could be in a pressurised tunnel. Alpha. It may be alpha typical of rotation or flare, doesn't look too drastic though

Yes the variation on straight swept l.e. was for subsonic improvements, principally approach and landing speeds

[FullOppositeRudder]

I would agree with your comments providing you prefaced them with "in still air".

Competition glider pilots stuff in loads of ballast when flying into a head wind to up their glide speed and increase their range (think headwind case equal to normal light weight glide speed when you would have zero range)Mostly so, but it goes a little further. Here is a more detailed explanation of why we often use water ballast on days of strong lift:

Re: [GBSCstudents] Water ballast (http://home.comcast.net/%7Everhulst/GBSC/student/ballast.html)

As elsewhere in this discussion, it can get quite technical. Most weekend pilots go without the complications of ballast, and simply enjoy the pleasure of pure flight using solar power and genuine renewable energy - all free from the sun :)