Think you understand Winglets? Think again!!
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Almost everything you thought you knew about winglets is actually wrong. All the other TH-cam channels and even Wikipedia talks about how winglets are there to stop wingtip vortices, this is not correct!
I found this out, to my astonishment, when I was researching for a completely different video and started talking to a great aerodynamics expert and friend of mine called Björn Fehrm.
In this video I will try and explain why winglets has nothing to do with stopping wingtip vortices and everything to do with aspect ratio and wingspan.
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Below you will find the links to videos and sources used in this episode. Enjoy checking them out!
Bjorn´s Corner:
leehamnews.com...
Doug McLean about winglets:
mentourpilot.c...
Airbus: Albatross ONE
• AlbatrossONE achieves ...
PlanesWeekly: Vortices
• SPECTACULAR! Airbus A3...
Stefan Langer: Gliders
• Low Level Flying at th...
Boeing: B777X first flight
• Boeing 777X First Flight
Tamarack Aerospace Group, INC: Active winglets
• Cessna CJ2 with Active...
Boeing: 737MAX and 787 flying together
• Boeing 787-10 Dreamlin...
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I always look at it like this:
The plane uses energy to accelerate through the air. The shape of the wings compared with speed compresses air under the wing. This disturbs the homogeneous condition of the air and creates a status of lower entropy by creating a pressure imbalance.
To counteract the pressure imbalance and recreate a homogeneous high-entropy status the air tries to escape from the low-pressure area under the wing to the high-pressure area. The only way to do so is at the wingtips hence why spanwise flow travels around the tips.
By escaping from under the wing the air is "wasted". The energy used to supress it cannot be used to create lift and is lost (= induced drag).
@@CaptainChrom I think you've got your entropy backwards - entropy is uniformity, so creating "order", of two different zones of pressure, decreases entropy, and entropy will slowly rise again as these two zones merge and become a uniform pressure.
Basically, surface pressure is atoms bouncing on that surface... the faster those atoms are moving across the surface, the less often they are bouncing into the surface... so overall air pressure rises when you speed the air up across the wing (you can measure the temperature increase) but you're changing the direction that the pressure is felt, because atoms are moving across the wing more, they are bouncing into the wing less... by having fewer atoms bumping into the top of the wing than the bottom, you get lift.
@@annoloki yeah I had it backwards
This leads me to a couple of questions. Why with the 777x don’t they have a winglet at the tip of the folding wing section (Even more effective length)
The second question is for the Albatross, if the wing is made free flowing due to conditions, and then the aircraft is back in smooth conditions, how would they bring it down into the locked position again now that it has lift being induced on the winglet.
@@CaptainChrom
Bravo , man!
How's about this approach :
Under mid-wing plates . .🤯
These would obstruct the span-wise flow and thus generate more positive pressure underneath the wing . This would increase lift , thus shortening takeoff/landing distances . It could also increase MTOWs .
*Ekranoplan ruled , Airfish also ! .😃
Great video. Just for the sake of completeness for those who really want to nerd out, it's not the mere presence of spanwise flow and vortices that creates induced drag. Wingtip vortices slow the air behind the wing, deflecting it downward, creating downwash. This downwash deflects the relative wind in front of the wing UP. Since the lift vector is perpendicular to relative wind, the lift vector is now pointed backward a little more compared to the path of the airplane. So now part of the lift vector is actually working against thrust. THIS is induced drag. It's why induced drag is more fully called lift induced drag, because it is a byproduct of lift, unlike mechanical forms of drag like friction, form and interference. It's also why slow, clean aircraft produce more induced drag (and more wingtip vortices) than fast aircraft. A slow wing, especially when not using flaps, has to use a much higher angle of attack to achieve the same lift. When the wing is working that hard, there is a greater pressure differential, therefore more spanwise flow, therefore more wingtip vortices, therefore more change to the relative wind and more induced drag! Finally, low aspect ratio wings always work harder. If you look at a lot of older swept wing designs, you can see the historical precursor to winglets...wing fences!
I agree. for what it’s worth I am a pilot and aerospace engineer. Induced drag is the backward force component from the high angle of attack. At slower airspeeds, the angle of attack must increase. This higher angle increases induced drag due to the higher “back angle”. Wing tip vortices are related, but not entirely. Vortices are best understood as the down wash of air corresponding to a need to backfill that air from the sides at the wig tips. This happens at all airspeeds, perhaps more so at slower speeds.
Overall, I applaud this video and its research. He states at the end the vortices and wingtips have little to do with induced drag. I don’t want to see anyone get confused that somehow vortices are what cause induced drag.
@@timothyconnally2167 "Not entirely related" is wrong. The vortices are where the energy lost by induced drag ends up. And winglets reducing induced drag correlates with reducing the vortices. Not in the big picture, so it's easy to miss, but close to the vortex' axis where the speed of rotation would be highest without winglets. Remember that kinetic energy is proportional to the speed squared.
_"the historical precursor to winglets...wing fences!"_
I thought wing fences were used to improve the stall behaviour of wings rather than trying to reduce drag?
I am still confused what he was saying about how tthe air spirals up and over the wing versus below the wing and why that natters. I think I get the part about how the air pressure is less on the top because the air moves faster up top. Why is there less pressure in moving matter. Do the molecules get spread apart further so it is less dense in relative volume?
@@shakamuni01 You can consider conservation of energy; as kinetic energy is increased (i.e. molecule velocity increases) the pressure (potential energy) drops proportionally to keep the energy level constant. This is Bernoulli's principle, if you search for that you should get plenty of info.
I think the Boeing 787 wing is absolutely beautiful to watch in action. How it flexes between start of takeoff roll and initial climb is a thing of beauty.
YES! to me it's the most beautiful commercial jet for sure
The fact about the winglets on private jets to look nicer reminds me of time I climbed the Sydney Harbour Bridge. The guide said the stone pillars at each end didn’t have any structural function but it was to make it look safer and sexier. Ironically the pillars now serve a function, one used by tourist with a museum and the other has venting chimneys for the Sydney Harbour Tunnel.
I live on Mercer Island, which is 5 minutes from Down Town Seattle WA. My son went to school with the grandson of the key person of the group who developed the integrated winglet. His story is very interesting as he struggled for years to convince airplane manufacturers that integrated winglets will have many positive effects including fuel savings. At first, his co-workers stayed away from him saying negative things about him. But after his winglets made it into the 747-400 these same workers bragged how they knew him well and closely worked with him.
As an aerospace engineer in the 1980s, we understood wingtip vortices. The work, of course, was how to most efficiently deal with them.
there are many reasons why I watch your channel. most have to do with my father being a pilot of large jets and then starting his own cropdusting business later. so he talks about planes and his experiences a lot. in the beginning I simply wanted to be more knowledgeable so I could engage in these conversations better, but somewhere along the line I fell in love with aviation and wanting to learn everything about it. kind of a mental hobby. Ive watched many channels about all the major incidents over the past 50 years and that's when I came across your channel. even though I've seen all of the videos about all the major airline crash investigations, watching yours brings new insight into each one. I was an instructor in the navy for one of my tours and love the way you make these things so understandable. as an engineer, its not always easy to do. you use just enough visual aides and for some reason it really sticks, where the same info from someone else doesn't. that's a gift and I thank you for sharing that with the rest of us.
In our basic aero classes at the Air Force Academy in 1979 we were taught that winglets give you no more advantage than increasing the wingspan. So the aero has been known for decades. Still, interesting to understand why they have become so prominent in civil aviation.
AFA teacher is correct
Span is limited by airport constraints, the Airbus 380 is not permitted to operate on most airports, Remember the collision in Paris where the wing clipped the tail of another waiting airplane .
Winglets are obviously there for style
That’s more correct than you think
True
True
Of course a Zoomie would say that! Go Army! Beat Navy! (Oh, and Air Force)
Couldn't agree more
The only force you can't completely trust is thrust.
That's my kerbal design philosophy ;)
Those airfoils are there to help keep the thing pointed in the right direction... Hopefully.
Good one!
True! Because you could potentially loose the all engines thrust, but you can not loose the 3 other forces, lift (if you keep the speed forward air speed and angle of attack within limit), weight, and drag! And if you in case you lost your full thrust produced by the engines, you can still trade the weight force for forward thrust! In the memory the azure glider, A330. And B767 of Air Canada!
In thrust I trust
The only force you can completely trust is gravity. That's the gravity of the situation.
I was at Cranfield College of Aeronautics in the 80's, before winglets were a thing, but I remember seeing displays of various weird and wonderful winglet designs which I generally assumed were just an exercise in increasing the effective aspect ratio without increasing the wingspan. One of the most interesting designs had multiple winglets like an eagle's tip feathers, but they were ridgid. I think Airbus's flexible wingtips actually mimic the eagle's wing better, because the eagles's tip feathers are flexible, giving them a greater effective aspect ratio without needing stronger wings.
Will it help increase lift on my Ercoupe.
It's called man catching up with nature! That's how the Harrier jump Jet was inspired. He literally saw a Harrier Hawk looking for it's next meal and came up with the Harriet Jump Jet.
- And even in "flapping motion", It is likely that the spanwise flow is being managed (Or is it really "open loop" - unlikely, as modern aircraft have a closed loop for practically everything??)
@@Bolivar2012able That doesn't make sense. The Harrier Jump Jet uses vectored thrust. How is vectored thrust 'inspired' by a bird that is 'hovering' by simply using the wind blowing in the opposite direction of it's travel to stay stationary? Helicopters were a thing, maybe he just wanted an airplane that took off like a helicopter, lol. That makes more sense than looking at a hawk and coming up with vectoring thrust from a jet engine, just like birds do I guess?
Let's see if Boeing and Airbus can start using feathers😄.
As someone who is studying aerodynamics at University, I find content like this more than useful. I hope you keep updating the newest trends in aircraft design, particularly when aerodynamics has a distinct role. Great job! 😎😎😎
If you have ever sat at the arrival end of the runway where the planes are passing overhead at several hundred feet you can hear the vortices as they approach and then hit the ground and disappear. It is a really cool thing to hear, particularly at night. It takes some time for the vortices to travel from altitude to the ground but you can hear it for about as long as it takes for the next flight in the stack to be over you. So not only are the vortices moving down, they are coming at you and then past you.
I live near a pretty busy airport and since my house is lined up with one of the runways, planes are always passing overhead while on short final. For a long time I have wondered what that swooshing sound was after a plane flew over, and I always assumed it was the sound of the airflow from the wings reaching the ground. Interesting to find out that it's actually the vortices that I am hearing.
I used to watch planes land from near the end of a runway and yes, you could hear the vortexes and once, one of these struck a tree and knocked almost all the leaves right of that tree. Really something to see. Heavy, slow and with minimal surface winds make the best, and they do move downward, I guess because the energy they carry makes for more dense air.
Great video overall, but I'd like to clear up one thing. It's true that it doesn't have to do with blocking wingtip vortices, and everything to do with extending wingspan. But the video didn't make it super clear what this accomplishes. It has to do with wingloading. The more you load a wing, the more disturbance you create, and hence the more induced drag you create. Wingloading is a simple equation of weight vs wing surface area. A jet with a smaller wing like a Hawker 900, will create more drag than the same weight plane with longer wings where the loads can be distributed over a larger area. The more wing you have, the less each square inch of the wing has to work. Less work = Less drag.
Also, smaller but higher loaded wing design tends to be easier to control in gusty winds and turbulence. The more glider-like your wings are, the harder the plane becomes to fly. You don't want airliners to behave like gliders when they're trying to land in 40-50kt winds.
And thank you for not mentioning the faulty equal transit time theory that everybody seems to love to teach. As a flight instructor, it's a pet peeve of mine. Great explanation of lift!
Thank you! Excellent points raised!
Forgive my stupid quetion (and my poor english), by why are you saying that the equal transit time is false ? During my glider theoric instructions lessons (and practice), I've been told this thougt 😅 could you clear things up for me ? Thx a lot
@@timotegregoire627 Great question. NASA discovered through extensive testing that this theory is inaccurate. The air on top of the wing reaches the trailing edge of the wing much before the air underneath the wing. Try searching on google "equal transit time theory NASA" and it will come up under "incorrect lift theory" it's a good read!
Unfortunately many instructors teach this theory only because that's what they've been taught and they never researched it themselves. Stay curious, my friend!
agreed!
Newton! not bernoulli!!
@@caprica_13 thx a lot mate !!! Indeed I had never questioned this topic before, thus I will read carefully what I can find online 😌
One of the best high level discussions of wing tip devices I have seen. Thanks!
I have some issue in how the work of the engineers is described.
The wing vortex is energy bleed, regardless of what is creating it. So its not that the engineers don´t understood it.... they didn´t know the source of it. Its not quite the same concept.
Its like seeing a car leaking gasoline, you don´t know where its leaking from, but you know its leaking from somewhere.
Also, the benefit of low aspect-ratio wings was know since at least the 30-tys. The general understanding that the bleed of was reduced with higher aspect-ratio wings was really nothing new. Its worth saying that the wingspan movement is true even for a completely straight wing.
There was experiment as early as in the second world war trying to eliminate this by having a forward angled wing. Junkers Ju 287 is an example of this. This spawned a lot of other problems. This was later repeated with HFB 320 that was ... well somewhat commercially successful. Worth saying that Grumman X-29 also used a forward swep wing, but that was for a completely different reason.
Looking at sail plain where aerodynamics is second to nothing, forward sweped wings are actually pretty common.
For the rotary wing turbine... It intention was not to reduce the amount of drag... I know that, because it say so in the text right below it. "recovery of vortex energy"... Now if it did recovery any signifcant amount of energy... yea, that is a different story, but its not the same as reducing drag.
There is a other factor here to. Engineers often make things that look counter productive for non engineers, making stuff that intentionally don´t work, or even make the situation worse. This is not because they lack knowledge of the subject, but need to tune the mathematical model. Now i don´t work in the aerodynamic field (while my field sort of touch it a bit). But looking at just generally how engineers think, i would say the wing-tip sail and the P feathering is for model calibration. Wingtip tanks.. did exist prior, so its of cause simple just to include them, becasue there already are models of it. The last to second two is really just natural variants. The last one... well i don´t have any clue what they where thinking.
About the wingtip turbine... that make me suspicious. That don´t look like a device an engineer would create.
"this wasn´t clearly understood by Boeing and airbus engineers in the 80-tys"
I would say that is not correct, for the most part. Winglet developments was mostly between 1975 and 1977, between 1979 and 1980 the did full scale testing. Already in 1977 did Learjet role out a prototype with a soft bent winglet. Granted the models got better, and so did computers, making the design more and more optimal. but that is not really the same as the engineers not understanding the core premise of it.
Now for the wingturbine That was filed in 1988. That was several years after the core concept of the winglets was general understood by the community. I would call that a patenttrool invention. That is.. an invention made by a lawyer, not an engineer. This is much more common than people might think.
Then for the more cuved modern wiglets. I would say the reason why we don´t see them on Boeing 747-400 and A310 is probobly due to manufacturing. The ability to do large composite part at that time wasn´t really available. And making a curve part in Aluminum would have other issues. The squarish shape of the winglets on those aircraft probobly not due to a lack of knowledge, but of capacity,
Great extension to Petter's lecture. Would wing tip tanks that you see on jets, defuse wing tip vortex to any benefit for wing efficiency?
My first thought is an increased hazard by storing fuel on the wing tip?
🎈
@@johnfitzpatrick2469 i actually dont have a clue ... it might... but the effect is probobaly very smal.
Fuel are usualy stored in wings... having wingtip tanks really mostly even the load out. Back when the beam.... was an actual beam... that was probaly beneficial
there's a cost-benefit tradeoff to winglets/wingtip extensions - they add weight and as such are only useful on longer sectors. Many shorthaul airlines _deleted_ them on B737/A320 orders as a fuel-saving measure. What's right for one alfying mode isn't necessarily right for another (I'd argue that if removing the wingtips gives you a fuel saving you're better off with turboprops, and not flying up to 30k feet but......)
The original 777 was optioned with folding wingtips. Nobody touched it because of the weight and fuel tankage penalties. It may be that no (or few) 777-Xs fly with them, given they can fit in the existing wingspan established by the 747 and these birds are flying 747-level sector lengths anyway
It's been said a few times that the best solution to spanwise flow was wing fences, but they come with their own disadvantages and passengers don't like the look of them
@@miscbits6399What are the disadvantages of wing fences? As a hobbyist/rc builder I've always believed they reduce spanwise airflow with minimal penalties.
@@Mjr._Kong this applies to wing fences and vortilons
1: They're fugly
2: mass penalty
3: they're fugly
4: potential ice traps (this is a problem with virtually all vortex generators)
5: they're fugly
For the most part aircraft designers have only ever used them to lower stall speed (Mig!) and engine pylons/flap fairing canoes take the function of vortilons.
I don't think there's ever been a large scale study into the effects of fences at civil transport size, at cruising speeds
Wonderful content, Cap'n. I learned to fly in the 90s (single/multi engine land piston engine) and was told that the winglets prevented induced drag as reflected by the reduced vortices. THIS makes a lot more sense. Thanks for the info.
Nice to hear physics and engineering so well described in words rather than equations!
It is also important to keep in mind that the reduction of induced drag also decreases wake turbulence. If wake turbulence can be reduced, then aircraft separation for wake turbulence can potentially be reduced, improving safety and capacity.
Very interesting point on the "look" of winglets being preferred over the practical aspect of wider wings: Cessna had that same issue with their vertical tails being more aerodynamically effective than the 'sexy' swept back tails found on the 150,152, 182's, 206's, 207's. While swept vertical stabilizers were less effective than the more vertical designed stabilizers, the swept back tails simply looked 'cooler' and therefore they sold better, therefore Cessna still has them.
The reason why e.g. Mooney with its weird fin failed?
I think the more we imitate nature the more efficient our designs can be. Watching how birds perform their aerial acrobatic maneuvers, they often move the wing end tip feathers back and forth in upward and downward action. I think this helps in keeping birds' balance without overloading their wings. I wonder what engineers can make out of it.
Flapping wingtips reminds me a little of the early aeroplane called the Christmas Bullet. The concept was that wings could be made much thinner if they were allowed to hinge centrally so the plane would be faster. In practice of course, once the wings had folded there was not much chance of them recovering. It's good to see an old concept utilised in an era when technology has advanced to make it practical and useful.
I always thought of air as being “lazy”, wanting to sneak off the end of the wing to get around to the top without doing its proper share of lifting work. Winglets make it harder, as do STOL kits on top of the wing of some smaller planes.
Well that is kind of what happened. There is a higher pressure under the wing then over it. If it walls of the back, the air is pressed down, and the plain is pressed up. If it falls of the tip... Well it just fall of and makes waste energy
Also interesting is, that the wings aren't formed uniform. Besides from the obvious decreasing wing width to the tip, the angle of attack or cross section might change. Depending on the model some are even going as far as to counteract lift to ease torque forces at the wing root.
Tho whole topic is really interesting.
Someone will correct me if I'm wrong... but I think the "the air laziness" (inertia) is actually the reason why induced drag can be reduced by increasing the aspect ratio of the wing or going faster.
You understood it very correctly then, but now it is not correct.
From a very high time recently retired airline captain, nice work, great description! It’s a very complex topic, and some may even argue that you made a few slightly incorrect statements, but overall a fabulous job. Cheers
Omg; thank you so much for talking this through. I’ve been wondering about the winglets. By even by the 6 minutes mark so many things just clicked. I’m a bit of a nerd, so I understand the aerofoil shape and Bernoulli’s principles, but I couldn’t quite get there without a little help. But all it took was a slight push in the right direction. Thanks for feeding my inner nerd! Fluid dynamics are fascinating!
As you rightly said its not entirely understood.
A lot has been said that the vortices reduce effective wingspan by taking up space on the upper surface that could be producing lift. The counter to that is thin highly swept wings produces lift from similar vortices produced by the leading edge.
A major part of Induced Drag is that in reality the lift vector is tilted back by the AoA and the downward flow of air, which is one of the reasons ground effect makes the wings produce lift more efficiently.
Nasa has been doing studies on the Prandtl Bell Shaped wing loading which cause the wing to shed its vortex inside or before the wingtip causing the wingtip to fly in an "upwash", tilting the lift vector at the tip forward and may produce "thrust". It also causes the 'lifting aileron" to create "proverse yaw" instead of "adverse drag" allowing the designer to drastically reduce the size of the tail and empennage.
Hi! Do you have any reference reading materials on this topic you could point me towards? I'm just a newbie keen to understand the forces around the wings/wingtips :)
@@aiestef me too
A little late to comment, and I fully agree with you, the Lift vector is tilted back by the induced Angle of attack, which is very close to half the downwash angle, called by McCormick:
the ultimate wake angle.
The downwash behind the wing continues until it encounters the ground , as described by Prandtl.
The effective width of this trailing downwash is π÷4
of the effective span
As measured between the cores of the vortices formed by the rolled up shear surface at the trailing edge, around the tip vortex.
The apparent mass flow corresponds to the circular area above the wing , the effect of the winglets is to increase this area by distorting it to an ellipse, of slightly higher vertical dimension, just as is the case on a biplane.
The parasite drag can be expressed as a flat plate area CDA times a drag coefficient of 1 times the 'dynamic pressure' q
The maximum L/D is the square root of the Ratio of the apparent mass area, to the CDA.
Totally independent of the aspect ratio.
Even Anderson misunderstands this, because he applies the tilt back angle of the lift coefficient to compute the induced drag coefficient, but not to the tilted back Lift force to obtain the induced Drag force.
Unfortunately this erroneous belief of the high aspect ratio effect , neglecting the span continues for the people who have not worked in a wind tunnel.
@@arturoeugster7228man o man do I have a headache or what!? Thanks but I'll need to read this a few (many) times ...
One thing I can say about wingspan is that the wind sees so much more wing that it needs to be pushed down a lot less than a narrow wing would have to, to lift the load...more downwash more tilted rearward vector pulling back the wing with all its might.
Seeing the wind kept me safe for 17000+ hours.
@@captarmour of course you are right , for lift the chord determines, together with the span the wing area . Changing the chord alone to fit the design IAS does not affect the induced drag but weakly, very weakly the parasite drag.
So, for climb performance at low IAS, span is essential. For high speed the parasite drag becomes important and determines max speed, cutting the span does not reduce drag much. A common practice in the low altitude Reno races, chop the wing tips.
To the detriment of the sharp turns in formula 1 racers, like Cassut.
we, 3 guys redesigned a formula 1 racer by putting in a new wing with the same area but much larger span. A careful computer optimization of the flight in that oval circuit gave that result. The ability to pull high g's in the turn beat all others.
Except Ray Cote, the man nobody was able to beat, but he asked us to build a wing for him.
(the name of ours: Lucy P) A few years later , every formula 1 racer has the long wings
The production value and content of your videos are consistently outstanding!
About lift: At 5:10 you say "How these two components [of lift] work together ...".
First, you discussed at least three ways of looking at lift (different models) -- air being forced downward, pressure differential (Bernoulli's principle), and circulation -- but the number of models is not really important.
Start Edit -------
Each model is just a simplification of a vary complex phenomenon (lift). The full explanation for lift and drag requires the Navier Stokes equations. Since the Navier Stokes equations are so complicated, we have simpler models that help us understand the complex process. Maybe it's a bit like if you were asked to describe a person. If you are describing a suspect to the police, you might give their height, weight, hair length, eye color, etc. But, if you were a nurse describing a patient to a doctor, you might give their level of consciousness, vital signs, and symptoms. And if you were to try to describe a person to an alien life form ... well, good luck. Anyway, each description of the person, or each model for lift, ATTEMPTS to give a complete depiction while leaving out complexities that are not needed for the given situation.
I retract my previous comment, but include it below so replies made in the first day (before this edit) have context.
End Edit ----
The point I want to make is that these are NOT different components of lift. They are all different ways of describing the same thing.
For example, if you know the pressure distribution across the entire wing, then you don't need any other model to tell you how much lift you have. Likewise, if you know the change in velocity (how much the air is forced down) for every bit of air flowing over the wing, then you can also calculate the lift without any other model.
The different models exist because some of them are easier for engineers to use in some situations and some are easier in other situations. It's a little like using km/h, mph, or knots for speed. Different units are more convenient in different situations. But they aren't different components that need to be put together. Any one of them fully describes your speed without the need for the other.
The vector analysis portion of my structural engineering classes is rather rusty, but based on what I remember -- I have to disagree with you.
@@williamwallace9826 Where do you disagree?
If you fully integrate the pressure over any surface, you get a resultant force. This force can then be broken into lift and drag components. If you have good pressure data, the lift component will generally be accurate. But, admittedly, the drag component doesn't include the friction component of the drag. So, yes, there is something missing with this method. Is this why you disagree?
Now, using the velocity change of the air (or wake field) is really no different in concept than the simple problem of two objects undergoing an elastic collision. If you know the mass of both objects, the starting velocity of both objects, and the final velocity of one object, then you can calculate the final velocity of the second object. Of course since air isn't solid, to use this method you have to integrate a whole lot of little chunks of air, but the idea is the same.
@@andrewsnow7386 I'm sorry, but what you're saying is not true. The resulting forces are not the same, and it's simple to prove. The 'speed difference/bernouilli' calculation only works for wings with a curve at the top. Yet many planes don't have them, and many planes that do have them can still be flown upside down. That means that the 'air forced downwards' component is much larger, not 'just a different calculation to the same result'.
@@Pfooh - didn't Andews mention ?? Fully Integrate Pressure ?? - Maybe he failed to specify; across Both upper and lower surfaces in 3 dimensions - this gives real data - Bernoulli is a very rudimentary simplification, not fully applicable in any open aerodynamic system. - Bernoulli equations can only be properly characterised where there are closed sample volumes. (Lots of silly undergrad simplifications to get kids to understand the basic nature of flow dynamics.)
Velocity In and Out - Circulation characteristics, also give a good indication of how the aerofoil is working - this only needs 2 vectors for every spanwise station - simple summary, without the same detailed understanding of "Why" (chordwise) - that the full Pressure integral provides.
And Don't get anyone started on the stupidity that is "equal transit time"... NASA has a very good "retraction" of that topic.
respectfully.
@@andrewsnow7386 If you indeed FULLY integrate the pressure, that will also include the contribution of friction. The drag and lift components are merely projections of the total aerodynamic force on the gravity vector and the velocity vector, respectively.
Great vid but I think you left out a key bit. The winglet is doing two things: it's making thrust from the forward component of the lift vector, and it's creating an "outwash", or downwash oriented outward, to the extent the winglet is vertical, that opposes the circulation and tends to weaken it (so kind of blocking or inhibiting it) in the zone that the winglet is able to influence. The thrust component is a function of the local AOA which is slightly inboard, tilting the left vector foward slightly. Look carefully at traditional vertical winglets and you can see they are angled leading edge out slightly to achieve the optimum AOA in the flow. Whitcomb originally called them "tip sails" because they make thrust the same as a sailboat that is close hauled running into wind; a lateral lift vector, angled slightly forward to create a forward thrust component. They are used on airliners because they are only worth using at relatively low indicated airspeeds where the wing is working fairly hard, which a jet at 40000 ft is doing. Down below 30000 ft the benefit is not worth the weight, cost and form drag penalty, so you almost never see them on straight wing airplanes.
John, you appear to be the only one here who understands winglets! Like so many aerodynamic explanations, the video fell short of actually explaining the airflow. The winglet must be performing a blocking function, even if that is explained via its own lift reaction (at whatever angle). It would then be counter-intuitive to deflect air purely sideways (with consequent drag) and applying a slight, negative local AoA appears sensible to achieve a little thrust. After all, this still reduces spillage and improves the efficiency of the horizontal portion. With the infinite risk of ambiguity inherent in any such explanation, have I got it right?
Now, do you know a good explanation for auto-rotation on autogyros and helicopters? 2D vector diagrams are useless, particularly because the flow varies so much according to position. You seem to have a knowledge of sailing - applying auto-rotation angular logic to a sailing boat is ridiculous. I've been searching over 30 years for a decent explanation and even the late, great Wing Commander Wallis admitted to me that he didn't understand it.
You do see some winglets on smaller aircraft, though they are usually much smaller drooped winglets rather than full-sized upward ones. What I've seen about them is that they can greatly increase maneuverability at low speed by increasing the effective span of ailerons and might help with STOL. There was some speculation that the reason behind not using them on airliners was ground clearance, so it might be interesting to see what happens as jet diameters get larger and potentially drive changes in dihedral angle to fit them
"Great vid but I think you left out a key bit"
This post by JK seems to suggest that the winglets have a purpose. Yet, the entire point of this video (as I understand it) is that, in fact, they do not have a purpose, other than fitting into aircraft group dimensions, and therefore keeping airport costs lower. If the winglets did have an aerodynamic advantage, then it would not make sense for Airbus and Boeing's latest designs to not have them. Which they do not. So, no, I do not see that this video left out a 'key bit'.
@@alexanderSydneyOz Did you even read my entire post? I explained what the key bit that was left out IS. Winglets make THRUST. They are sails. As I said, Whitcomb originally called them "tip sails" because that's literally what they do. It's a main-sail, close hauled. That new designs no longer use them is irrelevant.
Awesome.
As an aerodynamicist,
I am glad that you explained the equal and opposite reaction first before Bernoulli principle.
Many people will only explain Bernoulli Principle.
In the late 70’s, Learjet Models 28 and 29 became the first production aircraft to employ winglets. The improved wing efficiency enabled the company to certify these aircraft to 51,000 ft, also a first.
Adding winglets to an airplane does add one bit of a complication, the spanwise lift distribution is increased near the tip shifting the center of wing lift outboard which increases the wing bending moment inboard. So structurally, that must be accounted for.
But generally speaking, my recollection is that the benefits from winglets show up in longer flights.
Yes, but further testing showed that it's the same gain as simply increasing the wingspan
Interesting topic and a very nice presentation of the nuances of a complex topic! Your graphics are always wonderful. 👍
Raked wingtips are gorgeous! The 787 has the sexiest wings in commercial aviation -- and that wing flex :)
A380 begs to differ, those monstrously large wings are gorgeous mfs
Takes me back to my engineering days at college. Live the facinating topics covered in aircraft design. Keep up the good work! Enjoying watching these videos.
I think you are very good to explain things Peter. You are a teacher!!!
Thank you! I’m doing my best
Thanks for bringing simplicity to the science of flight & aircraft design to non-scientific minds like mine! And your English pronunciation is so much clearer than many American-born English speakers.
At the end of the day there is no escape from Newton's third law and the conservation of momentum: to make the air lift you up, you need to push the air downwards somehow. Wings basically sweep the air down. And by pushing the air down, air will move, because it's not a solid thing like the ground. And all this movement takes out energy from the aircraft which appear as induced drag.
If you can inflict force to the air such that the air move less, you will have less induced drag. When you fly close to the ground the air has no room to move too much, so it moves less hence you have the ground effect. The kinetic energy lost to this drag is proportional to the mass of the air moved at and unit time, and proportional to the square of velocity change of the air per unit time. On the other hand the lift is proportional to the mass of the air moved per unit time and the velocity change of the air per unit time. So to save energy you should move a lot of air with small velocity change. And exactly that's what gliders do with their disproportionately wide wings so much that they can fly without engine.
Glider have a massive engine , it is called the sun.
Do you know the reason why the SpaceShuttle burned a fuel-mixture with more hydrogen than the stoechiometric ratio? The additional hydrogen by its lower mass per molecule got higher momentum out of a given amount of energy, if you compare mv to 1/2mv² at huge values of v. But if you go to low speeds, mv is actually bigger than 1/2mv², so if you can only accelerate the gas slightly, you go more efficient with more mass accelerated. And this leads us to folding wings and heavily swept wings on fast aircraft, where you switch over to accelerating tiny amounts of air very much on tiny wings.
What if we played CNN on the left side of the isle, and FauxNews on the right side, then vented all the hot air out of the rear APU port?
That's not flying, that's just falling with style.
@Dorian Grey I am talking about things that where related to in the video and that are far beyond your basic 101 understanding of physics 😂
This video contains the most concise, accurate and clear explanation of lift I've ever heard. I'm talking "The Science Asylum" level stuff. Good job!
Now THAT was seriously interesting... I can imagine the Airbus Albatross wings being a bit disconcerting for passengers though but definitely an amazing concept so far!
I already see passengers panicking because of "broken wings". I do not think they will come to production, because it's complicated to explain to the guests over and over again.
Extending flaps and slats are scary enough…
Thank you for an excellent explanation. I am curious about these aerodynamic principles. It was nice to see how the winglet simply extends the spanwise length, effectively reducing drag.
The scimitar wingtips are also used on the P8 Poseidon, which is essentially a highly modified 737. My guess is that if the 777x folding wingtips are successful, they will appear on smaller aircraft.
PS the Albatross shows that we are still learning from birds. The Albatross species of birds are evolved to soar for days while expending as little energy as possible.
They are only folded on the ground to compensate for the long wingspan when arriving at the terminal.
Great video Petter. I am studying this exact topic (Aerodynamics) at the moment in my CPL subject. You've helped me grasp a better understanding. Thanks. Keep up the good work.
At my opinion, the Boeing 787 still looks beautiful, even without winglets. :)
Tbh I like raked wingtips more than massive winglets
Thank you for being clear about the degree to which lift is still mysterious. So much of aerospace is still guess, measure, check; we're just using computers rather than prototypes for the "measure" bit.
I am not at all sorry to say that this is just wrong. There is no mystery whatsoever to people who 'do' aerodynamics for a living. I am an aerospace engineer specializing in subsonic aerodynamics and my day job is in aerodynamic design, and I can tell you that the physical mechanism of lift had been known since at least 1908, when the Kutta-Joukowski circulation theorem was published (Kutta was actually first able to successfully calculate the lift of Otto Lilienthal's airfoils in 1902 - even before the Wrights' first flight).
It is common to see people on youtube who have not studied/worked in this field debate on the physics of lift: is it due to pressure, is it a reaction to deflection of the air. What they fail to realize is that it is the exact same thing.
The TLDR explanation is this:
All of lift is due to a reaction force and all of lift is due to a pressure difference and all of lift is due to a formation of a vortex motion. It is not partly this and partly that. They are all equivalent and so it can be talked about entirely in terms of either a reaction force/pressure difference/circulation.
However, the first of these viewpoints, while true is also useless, when the other two allowed us to advance aircraft from the crude devices they were at the beginning to what we have today.
Now the detailed explanation:
Lift is a force and so at the basis of it, it comes back to Newton's F=ma (and therefore it is a reaction to flow deflection). But what many fail to realize is that the same exact equation can be written using different terms. Pressure is just the force acting on a piece of surface, so F=ma can be restated in terms of pressure (and friction), to eventually become the Navier-Stokes equation(s), which describe all flowing mediums (gas, liquids, plasma, and sometimes even solids). Under some conditions (for example an inviscid, barotropic and irrotational fluid) the complex N-S equations reduce to a well known simple form - Bernoulli's equation. So you see, a Newtonian and Bernoullian approaches are entirely equivalent to each other. But one is practically speaking "better" than the other.
While the Newtonian approach is completely true and valid, it is also useless. With one exception (high hypersonic flows), you can't use it to figure out for example how much lift a certain wing is going to produce. Indeed you can't use it to figure out anything at all, so it is completely and utterly useless as a model to describe reality.
Describing lift in terms of pressure (either using the simplified Bernoulli eq., or the complete N-S eq.) is far better. For starters, you can measure changes in pressure and how it's distributed over, say, a wing. You can also calculate it and therefore predict what a certain wing at a certain speed and a certain angle of attack is going to 'do'. But this can still be a problematic approach, because usually it is either too complex (N-S eq.) or too simple (Bernoulli). What I mean by that is that the N-S are usually impossible to solve analytically, and therefore require methods like CFD simulations, which are tremendously computationally intensive, and are not always desirable. Meanwhile using the simple Bernoulli equation 'as is' is usually only valid for very simple geometries, such as internal flows (flow in pipes, etc..). There is a third option though - circulation theory.
With some mathematical manipulation of Bernoulli's eq. the changes in pressure and speed can be tied to vorticity distribution. That is Kutta-Joukowski's 1908 Circulation theory, which was later expanded by Prandtl's lifting line theory in 1918-1920. What this tells us is that flow around an airfoil forms an effective vortex around it. This is called circulation, or the bound vortex. It also tells us that the amount of lift generated depends on the strength of that circulation. This circulation motion causes airflow to accelerate above and decelerate below an airfoil, thus a pressure differential and lift. It also causes air to be deflected downwards (downwash), by the exact amount that matches the pressure differential, because again a reaction force and a pressure differential are entirely equivalent. Prandtl's theory also predicts precisely the formation and distribution of tip vortices, of down and up wash, wake roll up and of induced drag.
If you want some practical examples of this equivalency between force and pressure differential, consider these examples:
In a wind tunnel, one common method of measuring lift is with a balance that directly measures the force experienced by the model. Another common method is to use pressure taps distributed along the surfaces that measure static pressure. An advantage of the second method is that it gives lift (=pressure) distribution, and not just the total sum. Integrating the pressure taps' reading gives the lift - which in wind tunnel tests always matches the force measured directly by the balance. Integrating the pressure with its moment arm gives the aerodynamic moment - which always matches the moment measured by the balance.
The second practical example is CFD. It is a computational simulation of fluid dynamics. In CFD some version of the Navier-Stokes equations (depending on the type of CFD) are solved numerically to obtain pressure, speed, density and so on at every point in the fluid. Then, the fluid's pressure on top of the model geometry is integrated to obtain lift - thus in CFD lift is calculated through pressure alone, because, again, all of lift is a result of pressure differential, just as all of lift is a reaction to fluid deflection.
This thing about air circulating around the wing and its spanwise-axis is very important with sailboats and wind-turbines. While airplanes mostly keep their altitude while generating lift, i.e. little to no movement in the direction of lift, this is different for boats and turbines. So while a sailboat is going forwards, it experiences the wind angle of attack more and more frontal. With the angle of attack nearing 0° and the sail in a lengthwise attitude to the boat, you no longer produce forwards pull but only bank the boat sideways. Now what happens on fast boats with multiple sails, the aftermost sail is creating this vortex and this gives a better angle of attack for the next forward sail, creating more forward motion and less sideways banking. If you look closely on Americas Cup or Volco Ocean Race footage you will see boats with sails overexpanded into the oncoming wind, which would actually be impossible with overly simplified physics. It is a very interesting thing following these newer findings on airflow dynamics and i am curious how this will affect our harvesting energy and increasing efficiency on things like airplanes.
@Dorian Grey Lol you obviously don't understand the physics behind tacking into the wind. Tacking does not work if the setup of your boat does not support going close to the wind.
Unless if you own a very old square rigger, ya shouldn't be concerned whether a boat can tack into a wind or not. Hell, almost every single modern vessel would find it safer to tack than to jybe
@@dwebster4019 There are still different angles of attack for any type of boat. I am talking finetuning here, not having the possibility at all...
Aerodynamically there is absolutely no advantage in having multiple sails when going to windward. Without exception, the use of two or more sails is only seen when encouraged by or insisted upon by the class rules. This is clearly demonstrated by the A- B- and C-class catamarans where the only limitation is one of total sail area. All modern boats in these classes use single, "cat rigged", sail plans, an arrangement that was quickly proved to be advantageous in the very early days of these classes (I am old enough to have been there to watch).
Man, what a nice explanation for such a complicated and wide subject as lift and induced drag. Really appreciated it!
Don't forget about hangars for the maintenance of these wonderful planes. With a winglet or the new hinged outer long wing on the new 777, it can still get in the hangar.
I tried searching for "what are winglets for?" and despite being subscribed, this video didn't come up. Then, three weeks later, and a year and a half after it was debuted, suddenly the algorithm pushes it at me. Oh, well, at least I finally learned what I wanted to know.
Seriously, there isn't much easily available plain language information available online, on winglets.
Each time I see a Lockheed C-5 Galaxy lift off, I view in amazement. It just doesn't seem possible.
Or An225 mriya (i am from PL not Ukraine) but that one is an achievement
I learned in the 1976-1977 school year in my air force junior rotc, a very interesting formula relating to flight...thrust plus drag equals lift. I never forgot that. Good job peter!!!!
Amazing how vectors work!
For people who watch the vid but don’t “listen”, winglets DO help, not because they block wingtip vortices but because they effectively lengthen the wings without making the wings long enough to cost airlines higher fees at airports. However, you really should watch the video
Thank you Andrew, much appreciated
oh yeah thanks
excellent explanation, always struggled to explain this subject to my students. BTW your aircraft NG at the moment is going thru intense inspection at the wing root for cracks at the fork fittings due to increased bending moments due to the winglets, those fittings are attached to the spar, bulkhead and skin.
Amazing video! Something new I learnt today! Keep making these videos!
Thank you! That’s what I was going for. Please share it on any group you think might appreciate it
Agreed! No doubt
he has some good animation too but idk how he did it
Hi. All of what you say is true and expressed in a beautifully informative way, as usual. Thanks for all your great work. However, ever since I started flying gliders many years ago and reading books about flying, I’ve always had a problem with explanations of induced drag. All that you say is true (as far as I can tell) but what you are describing is an extension of form drag, not induced drag.
When an airfoil hits the oncoming air, it usually does so at an angle (the angle of attack). This means that the force generated is angled back from the vertical. This angled vector can be resolved into two components at 90 degrees to each other. One is the lift, directly opposed to gravity and the second is drag. And that’s it.…. Nothing to do with circular motion around the wing, nor wing tip vortices, nor pressure differentials. They are all form drag. Induced drag is the drag resulting from the fact that a wing does not create a vertical force, but a force angled backward.
Yes, in fact, if you read the PDF I linked to in the description you will find some fun info on that.
Everytime I watch one of these videos I'm amazed at how much thought goes into the smallest things on an aircraft to make it more efficient.
Indeed!
I see what you have done there with the pillows!
Green on starboard, red on port! Well done! 🙂
Wow, winglets as a marketing tool!
Very informative vid, as always. I'm an LSA pilot and always thought winglets were there to help keep the high velocity air that's flowing across the top of the wing, from slipping off of the wing tip., lol. Thanks again, Mentour Pilot
It wasn't all correct or clearly explained. But i've got to appreciate that it's still probably the best explanation attampt of this complex topic, so far.
This was a perfect application video of Petter for the job as physics teacher at a high school. BTW the idea of a loose hanging outer part of the wings, loose at turbulences, is very smart. BTW I studied something with aerodynamics (as part)... and like this topics always.
The fact that you have couch pillows that represent nav lights, and are in the correct position... is strangely satisfying.
All he needs now is a hat with a rotating anti-collision light...
Excellent video I really enjoyed this one. I’ve experimented a lot with winglets on my RC models, very difficult to get any reliable measurements on a small model, but they did look cool
I now that the shape of the wing itself is very important. During World War Two, but not only, the Spitfire used to have elliptical wings. They are said to be the best type of wing in order to reduce induced drag. Have you checked that out? I was surprised not seeing you talking about that. Great video as always, great content!
The principle was that the lift distribution spanwise should be elliptical, bleeding off to zero at the wingtip. Making the wing planform elliptical was the "elegant" solution. And it was esthetically pleasing to boot. The problem is that it is a very difficult shape to mass manufacture.
Elliptical wings were thought to be the most efficient because that was supposed to be the least amount of wing needed. When limited by wingspan, elliptical lift distribution is the most efficient. The whole wing could theoretically (using numerical models known back then) work at the same coefficient of lift (Cy) throughout its whole span. An unwarped rectangular wing will also have elliptical (or very close) distribution of lift, but the ends will fly at a much lower Cy (for reasons I don't understand well enough to describe in a single comment). This is used to keep the wingtips lifting while the root is stalling, which gives a smooth stall, preserves aileron control and prevents inadvertent spins by - in layman's terms - having much more wing at the tip than strictly necessary. An elliptical planform does away with this extra amount of wing, reducing the wetted area and thus the skin drag. Early WW2 fighter planes were all about building the least amount of airplane around the biggest engine available.
BTW: I typically enjoy content of your informative presentations. Note: The other NASA wind tunnel engineers,those not doing the free flight experiments also initially suffered mental blockages to these experiments .
This is an amazing video, thank you! Now I understand more about winglets and about other flight info. Also gotta say, you’ve made very AMAZING animation, how did you do it? What software did you use to animate like that? I like it anyways, thanks again!
It’s all my amazing graphical designer Dom who makes this for me. He is great!
@@MentourPilot I bet he is, tell him we said thanks!
That's a fair way to explain winglet's utility. But if I were to bring another way of explaining it I would say that:
There are 3 types of drag applied on an aircraft:
- Pressure drag (form drag as it is called in this video): implied by the pressure difference between front and back of the shape. The more streamined the profile is, the lower the pressure difference will be, and thereforeso will be the pressure. It represents around 80% of the total drag
- Friction drag: Induced by the no-slip condition. The air particles directly in contact with the wings have no velocity, while the upper layer have a little bit of velocity (the further away from the wing, the higher the speed is). The friction drag is induced by the different velocity among all layers.
- Induced drag: caused by the fact that wings are not infinite. The air under the wing tries to reach the upper part, but can only do that at the edges.
The goal of winglets is mainly to reduce the area where this mix occurs. At the wingtip, the winglets is as wide as the wing but at the edge of the winglet it is significantly smaller. Therefore, the balance of pressure is less brutal.
Without those winglets, a small portion of the wing, at the tip, does not produce lift anymore due to the induce drag. The winglets allow the flow to be effective on the entirety of the wing.
The B787 / B777 for example, don't have winglets but they have raked wingtips. You gain most of the pros of winglets without increasing drag by putting something heavy in the the flows' way. It is mainly efficient on longer routes because you lower both pressure and friction drag even though you don't gain as much in the induced drag.
It’s good to always continue to learn, and that’s why I love these videos!
Same
Thanks! That’s exactly what I was aiming for with this one!
@@MentourPilot and you succeed! And now I don’t have to feel bad that the piper pa28 doesn’t have them! :)
Nice video! My preference for favorite aircraft (essentially most visually appealing aircraft) is very much based on how the winglets look with the fuselage, engines, wingspan, etc. of the plane. The 757-200 and A350-900 are by far my favorites!
10 years from now: "Well, aircraft designers just created detachable wings. Turns out....you don't need any wings at all at the gate!"
You might be on to something 😀😀
Detachable? How? Call the ground crew to carry the wings?
At SeaTac, there is a jet with foldable wings stored with it's wings upward regularly. As you taxi, you see it. I always thought it was pretty cool, but not so sure I'd trust it in commercial aviation. It's labeled "Port of Seattle" and obviously has official uses. Of what, I have no idea.
I mean look at any aircraft carrier, and yes due to hangar space ect, wings that fold for a large portion of their span are the rule, not the exception, but that comes with a very large weight penalty that ground based aircraft don't carry.
Fold them up like they do on an aircraft carrier.
Petter, as you explained before, the wing accelerates the mass flow above and below the wing downwards to produce Lift.
To follow up on your remark at 3:15,
let me try to explain some less understood aerodynamics quantitatively.
First. Some basic wing aerodynamics:
The mass flow accelerated by the wing descends with a vertical velocity component in the plane of the wing near the trailing edge, which is the product of the induced AOA(angle of attack in radians) times the flight velocity.
Further behind in the so called farfield wake, this downward velocity component doubles.
The mass flow is distributed above and below the wing with steadily reducing vertical velocity as the hight above and below the wing increases. The magnitude of the mass flow is the product of the circular area around the wing times the air density and the flight velocity.
mdot=rho×V×(π/4×b^2)
rho...density kg/m^3
V... velocity m/s
b... wingspan meter
mdot...massflow kg/s
This is important:
Conservation of mass requires that the same amount of mass must also flow upwards. This happens with upwards flow just outside the wingtip, very fast at the wingtip and gradually reduced outwards.
The usual, common explanation, that the low pressure on top of a finite wing causes the flow around the wing tip is fundamentally WRONG! ( this can be proven when the 3 - dimensional Euler equations are used and the Bernoulli equation along a streamline).
The vertical velocity distribution u(y) near the trailing edge in the plane of the wing can be expressed as follows:
u(y) = uo × Re[ - 1+|y|÷ √(y^2-(b/2)^2) ]
y... span coordinate from centerline
uo... vertical component as discussed above.
Re[ means real part of[ complex expr.] ( real the -1 and the other term when y larger than b/2) [ recall i=√-1]
When you plot this distribution, then you see the uniform downwash (strictly correct for an elliptically loaded wing) up to the wingtip and a very large upflow just outside. By integration from the center to far starboard, you can verify that the upflow rate equals the downwash flow (at constant density)
uo×b/2= integral u(y)dy
from y=b/2 to y=~inf
the induced angle of attack
AOA = cl ÷ (π × Ar)
uo = AOA × V
cl... lift coeff.
Ar...aspect ratio b^2 ÷A
A...wing area
Second : crossflow around and behind the wing:
There is a spanwise component of the flow above and below the wing. under the wing the flow is outwards, small inboard and faster outboard, then it turns around the wingtip , on the upper side the flow component is inboard.
again very fast near the tip and slower inboard.
this component is proportional to the spanwise slope of the lift distribution ( more accurately: the spanwise circulation distribution around the wing).
For an elliptically loaded wing the circulation Gama(y) is:
with Gamao =max circulation at the center
Gama(y)=Gamao ×[√(1 - (y÷[b/2])^2)]
a semi ellipse, major axis b, minor semi axis, Gamao
Recall that the wing lift for an elliptic wing, according to Kutta-Joukowsky is
L = rho × b × V × π/4 × Gamao
Near the center of the wing, the slope is very small, the flow around the wing is 2- dimensional, no crossflow. Further outboard the combination of the inboard crossflow just behind the trailing edge with the lower wing outboard flow component form a vortex sheet, ( crossing streamlines.)
The strenght of the vortex sheet ( difference in crossflow of upper side and lower side) is proportional to the slope of the circulation distribution. It increases strongly towards the wing tip.
This vortex sheet, further behind the wing rolls up around the tip vortex.(discussed later)
The slope of the elliptic circulation distribution near the tip is high, here in real flow a discrete vortex is formed, due to flow separation just above the wing tip, it forms slightly above the plane of the wing and trails behind surrounded by the rolling up vortex sheet.
If you have a deflected aileron further inboard, then another vortex at the deflected aileron edge will form and rotate around the wing tip vortex.
The tip vortex just behind the wing has the strength of the remaining circulation left over by the outer votex sheet.
The circulation due to the rolling vortex sheet plus the one due to the tip vortex is equal to the max circulation at the center of the wing.
(By the way with laser odometry one can measure the velocity around, but along a single closed path of one of the trailing vorteces, way behind and determine the weight of the airplane, given its speed and span.)
A concentrated vortex will induce a flowfield around it but also laterally ahead of the vortex (according to Biot-Savart).
Now : winglets on the span limited Boeing 737 max:
The flowfield produced by the outer vertical flowfield together with the induced flowfield of both winglet tip vorteces has velocity components perpendicular to the lower winglet (diagonal upwards and sideways) and similar for the upper winglet, this flow pattern creates lateral lift on the winglets with a significant forward component (thrust).
Optimally sizing and orienting them will reduce the induced drag significantly.
In other words the effective wingspan is larger than the geometric span, increasing the maximum
Lift to Drag ratio which is proportional to the wing span:
L/D | max = be÷2 × √[π×e/(cdp×A)]
e ...Oswald efficiency
cdp... parasite drag coefficient
A...wing area
be...effective span as discussed.
Maximum range is achieved when the airplane actually flies at a speed and altitude where the Lift to drag is optimum.
In subsonic flight the optimum dynamic pressure qo ( not really a pressure but the kinetic volummetric energy density of the aproaching air,
courtesy Dr. Burgers) is
qo = L /(b×√[e×π×cdp×A])
and the true airspeed TAS = √[2×qo/rho]
rho ...density at flight altitude.
Drag equations, why?
It is important to understand that the induced drag is high at low speed and is small at high speed, as opposed to the parasite drag ( sum of 'friction drag and pressure drag [flow separation], which is small at low speed and very high at high speed.
Parasite drag:
Dp = cdp × A × (1÷2 rho × V^2) or with
q =1÷2 ×rho×V^2
dynamic pressure
Dp = cdp × Ar × q
its proportional to the reference area Ar, the drag coefficient cdp, the air density and the square of the speed.
The drag coefficient can be defined differently depending on the choice of the rederence Area( mostly wing area but occasionally the wetted area of the fuselage, the wing and the stabilizers, when the surface friction is of interest.)
Induced drag:
Di = (L/b)^2 / (π×e×q)
The induced drag is proportional to the square of the span loading (lift÷span)
and INVERSELY proportional to the air density and the square of the speed.
L...lift
b...wing span
e ...Oswald efficiency
π = 3.141592635...
Note that the induced Drag is NOT dependent of the aspect ratio (span÷mean chord),
another common missconception.
But the INDUCED DRAG COEFFICIENT
cdi = cl^2 / ( π×Asr)
is because of the DEFINITION of this
coefficient:
cdi = Di / (q×A)
the lift coefficient cl
cl = L/(q×A) is mixed into the expression which produces this "contradiction".
An equivalent definition of the aspect ratio is used
Asr = b^2 / A
eliminating the chord.
at subsonic speeds, there is no wave drag.
The total drag in subsonic flow is then
D = Dp + Di
= cdp×A×q
+ (L/b)^2 / (q×e×π)
The total drag has a minimum , that happens at a speed where the induced drag is the same as the parasite drag!
The minimum drag is obtained at a dynamic pressure qo
qo=(L/b)×1/(π×e×cdp×A)
proportional to the span loading.
The understanding of aircraft performance is enourmously increased by actually doing the calculations.
That is the only way in order to have the common misconceptions resolved.
That is the reason I took time to list them here.
I know it is demanding but well worth the effort, especially for pilots who learn aerodynamics mostly qualitatively .
The best looking airplanes (airliners) are the ones that have lower ticket prices and are comfortable to sit in.
Correct
Do you think the Max is gonna come back because it looks sexy, because some people seem to be afraid of it?
@@n3roc Well they can always make the warning light that tells you if the flight computer is about to commit murder a standard feature.
I think low prices & comfort are inversely related, lol.
@Gary Dodgson would you rather have seats or a metal floor
I learned a lot about wingtip vortices when I did a project with NASA and USAF to tow an F-106 Delta Dart behind a C-141 transport aircraft (see NASA Armstrong’s website re the Eclipse Project). I have to disagree about the vortices’ contribution to induced drag. What I learned was that those vortices conserved the angular momentum of the circulation generated by the wing. You can’t get rid of them - the plane wouldn’t fly if you did. The most you can do is change their intensity. A small, high speed wingtip vortex will conserve the circulation angular momentum, but it involves a lot more angular kinetic energy. A larger, slower vortex will conserve momentum, and have a lower angular kinetic energy. It is the angular kinetic energy needed that determines the drag - more being worse. Raptor wings have a familiar feather cascade tip, which allow the birds to soar with low aspect ratio wings. The cascade tightens up when the bird does a fast dive with high-g pullout at the end. In other words, they have the high maneuverability of low aspect ratio wings when needed, but the low drag of a pseudo high aspect ratio wing for extended glide flight.
As an ex-senior structural guy at Boeing/McD, I've seen many design efforts spending in Winglet design in 737MAX and MD-12 on final days of those two types. It appears that time and money have been wasted in useless and pointless for morons and mis-management.
14:49 (airport charges) we see a similar thing in the small yacht (sailing boat) world. Many people look at modern sailboat designs and think it's all about performance, but actually many of them are because they know that almost all Marinas charge on the overall length of the boat (eg per meter) so the hull designs are a compromise of performance, accommodation, and keeping the marina fees down! For the same build cost you could make a better performant boat without compromising , but it would be more expensive in the marinas.
19:26 (looks) we see a similar thing in the sea kayak world! The common image of sea kayaks flare up a lot at the bow and the stern, when they would often be better with decks that point downwards/lower towards the ends, and often with less angle underneath at the tips too. I can't remember the origin now, but I heard/read that when quizzed about why they choose this design despite the arguments of better alternatives the designer said "we want to sell kayaks". This was the shape the customers had become used to and associate with serious sea kayaks, the ones that were designed functionally better didn't sell as well because they just didn't look as good, or at least not how people expect a "proper" sea kayak to look.
I think ranked wing tips are sexier. But this could just be because every time I've noticed them it was from a head-on shot it a 777X or 787 departing worth the wings flexing upwards, creating the effect of a slightly upwards curved wing.
Agreed. There is no sexier commercial aircraft right now (judging by exterior) than the 787. It looks almost like a huge bird of prey in the air.
*Raked
"with"
Yes, these are more 'organic' thus sexier. As bird's feathers they bend upwards. It's a much more analog transition between the normal area of the wing and the raked part, there is actually no clear line but a gradient between them. Air doesn't like sharp edges/corners. I think it's also better structurally as the load changes/vibrations can't easily find a weak point, which is usually around the creases.
I genuinly saw in a book on airplane design a tip to design private jets with a nose tip near the bottom rather than near the middle because it looked better. It's astonishing what private jet customers care about.
They still use it mostly for fuel efficiency since it decreases that drag, looks good, and 💵
Looks and $$ - Marketing, yes..
- when it works, it allows higher density of movements at airports - not a lot of the wingtip "treatments" do anything much at all in cruise (at cruise speeds and attitudes - wing washout and loading is designed to achieve a nearly optimal outcome - this is where airliners spend 80-90% of their time aloft, what they are less optimal at is the descent to Landing and Climb-out parts of the flight), mostly the wingtip "tricks" are to counteract the bad things which happen when Low Slow and heavy.
@@kadmow ohk, thanks
I was taught in the 80 at college that 2D flow over a wing had zero induced drag. We could prove that in a wind tunnel by building the wing from tunnel wall to tunnel wall. This prevented spanwise flow. So we knew then that induced drag was being caused by spanwise flow of air around the wing tips.
In the 90’s, I was part of the team that re-designed the winglets for the Model 60 from the Model 31A Longhorn wing. The winglets for the 31A netted a 0 drag point difference. So they were kept because they were sexy. By adding the ogive trailing edge to the model 60, we corrected the mistakes on the model 31A. We gained 1 to 2 drag point reduction in the various regimes of flight. We then furthered the gains on the mode 45 wing by designing the wing and winglet to work together. We understood enough then how induce drag could be mitigated with a winglet.
Personally, I think the raked wings are much sexier. They look more like large bird wings. Funny how nature seems to have great design features. I've never seen a bird with winglets!
@N. Warner Ya think so? With advances in technology that allow me to control useless appliances with stunning flexibility, I can see how that type of control could be achieved...
I think I saw a documentary once that suggested some of the big eagles use their wingtip feathers as winglets!
Cause birds don't need to meet a specific wingspan
@@kevinshen9391 They fold up nicely, too!
@@TheSteveMol Saves on gate fees!
I thought I knew about winglets. But it happens that I knew nothing about it. I'm glad you made this video. Thank you very much Mentour Pilot.
Amazing video as always
Agreed
Winglets first appeared on sailplanes in the 1970s. They were first used on gliders in the 15 metre competition class where you were not allowed a wingspan of more than 15 metres. Winglets, in effect, gave an increase in span (and thus performance) without breaking the rules! Open class gliders have had ever increasing wingspan which is now up to close to 30 metres (giving a glide angle of around 70:1 ) , sometimes with a small winglet as well, just to , yes, look sexy!
I think I read somewhere that there is a basic difference in wing design philosophy between Boeing and Airbus about wing stiffness. Boeing preferring a more vertically flexible wing and Airbus a stiffer wing. Might the Boeing approach achieve some of the benefits of this Albatross wingtip inherently because their whole wing can "flap" to a higher degree?
Interesting technical and marketing discussion of aircraft design. I'm always amazed with these discussions that even after all these years we don't have a good theoretical understanding of the physics of lift.
Let me try and put your mind at ease. There is no debate whatsoever among people who 'do' aerodynamics for a living. I am an aerospace engineer specializing in subsonic aerodynamics and my day job is in aerodynamic design, and I can tell you that the physical mechanism of lift had been known since at least 1908, when the Kutta-Joukowski circulation theorem was published (Kutta was actually first able to successfully calculate the lift of Otto Lilienthal's airfoils in 1902 - even before the Wrights' first flight).
There is only debate among misguided people who have not studied/worked in this specialized field yet claim to have absolute knowledge of the matter as they argue over the youtube comments.
Back to lift. Before I go into details, if you want just the TLDR version, this is it:
All of lift is due to a reaction force and all of lift is due to a pressure difference and all of lift is due to a formation of a vortex motion. It is not partly this and partly that. They are all equivalent and so it can be talked about entirely in terms of either a reaction force/pressure difference/circulation.
However, the first of these viewpoints, while true is also useless, when the other two allowed us to advance aircraft from the crude devices they were at the beginning to what we have today.
Lift is a force and so at the basis of it, it comes back to Newton's F=ma (and therefore it is a reaction to flow deflection). But what many fail to realize is that the same exact equation can be written using different terms. Pressure is just the force acting on a piece of surface, so F=ma can be restated in terms of pressure (and friction), to eventually become the Navier-Stokes equation(s), which describe all flowing mediums (gas, liquids, plasma, and sometimes even solids). Under some conditions (for example an inviscid, barotropic and irrotational fluid) the complex N-S equations reduce to a well known simple form - Bernoulli's equation. So you see, a Newtonian and Bernoullian approaches are entirely equivalent to each other. But one is practically speaking "better" than the other.
While the Newtonian approach is completely true and valid, it is also useless. With one exception (high hypersonic flows), you can't use it to figure out for example how much lift a certain wing is going to produce. Indeed you can't use it to figure out anything at all, so it is completely and utterly useless as a model to describe reality.
Describing lift in terms of pressure (either using the simplified Bernoulli eq., or the complete N-S eq.) is far better. For starters, you can measure changes in pressure and how it's distributed over, say, a wing. You can also calculate it and therefore predict what a certain wing at a certain speed and a certain angle of attack is going to 'do'. But this can still be a problematic approach, because usually it is either too complex (N-S eq.) or too simple (Bernoulli). What I mean by that is that the N-S are usually impossible to solve analytically, and therefore require methods like CFD simulations, which are tremendously computationally intensive, and are not always desirable. Meanwhile using the simple Bernoulli equation 'as is' is usually only valid for very simple geometries, such as internal flows (flow in pipes, etc..). There is a third option though - circulation theory.
With some mathematical manipulation of Bernoulli's eq. the changes in pressure and speed can be tied to vorticity distribution. That is Kutta-Joukowski's 1908 Circulation theory, which was later expanded by Prandtl's lifting line theory in 1918-1920. What this tells us is that flow around an airfoil forms an effective vortex around it. This is called circulation, or the bound vortex. It also tells us that the amount of lift generated depends on the strength of that circulation. This circulation motion causes airflow to accelerate above and decelerate below an airfoil, thus a pressure differential and lift. It also causes air to be deflected downwards (downwash), by the exact amount that matches the pressure differential, because again a reaction force and a pressure differential are entirely equivalent. Prandtl's theory also predicts precisely the formation and distribution of tip vortices, of down and up wash, wake roll up and of induced drag.
If you want some practical examples of this equivalency between force and pressure differential, consider these examples:
In a wind tunnel, one common method of measuring lift is with a balance that directly measures the force experienced by the model. Another common method is to use pressure taps distributed along the surfaces that measure static pressure. An advantage of the second method is that it gives lift (=pressure) distribution, and not just the total sum. Integrating the pressure taps' reading gives the lift - which in wind tunnel tests always matches the force measured directly by the balance. Integrating the pressure with its moment arm gives the aerodynamic moment - which always matches the moment measured by the balance.
The second practical example is CFD. It is a computational simulation of fluid dynamics. In CFD some version of the Navier-Stokes equations (depending on the type of CFD) are solved numerically to obtain pressure, speed, density and so on at every point in the fluid. Then, the fluid's pressure on top of the model geometry is integrated to obtain lift - thus in CFD lift is calculated through pressure alone, because, again, all of lift is a result of pressure differential, just as all of lift is a reaction to fluid deflection.
Thanks for that theoretical video! Sensei)
Glad you liked it! Did you learn something? 😅
@@MentourPilot Yes, it is one of your most interesting videos as for me. I know that it won't take as many views as usual, but it is very informative even for pilots. 👏
When we had the 787 prototype here in Santiago SCEL Int., Chile, for one of our FIDAEs, one of its raked wingtips scratched a building during a taxi. It was embarassing for Boeing's pilots, but it also shows off the problems brought about by wider wings.
The issue at the root is efficiency obsession, which is an expression of fear-driven scarcity, accompanied by zero-sum thinking or worse (game of musical chairs), i.e. people only using what is already there in the system instead of truly adding something from beyond.
In a way, the private jet anecdote about visual appeal, while arguably not coming from the wisest source, at least not consciously, is still a healthier approach that we should be inspired by and ideally understand properly.
Conclusion: Navier-Stokes is hard.
Sorry that this becomes a long comment, but it was one of the aspects of airplane design, I found most intriguing.
From my textbook on aerodynamics I understood, that the problem was the shortcut happening directly at the end of the wings. Something very similar to shortcuts in acoustics.
Consider a string . As it oscillates back and forth, one side of it compresses the air in front of it, while on the opposite side it leaves some kind of vacuum (not really but "the air gets thinner" (rarefaction)). Now, as all other systems, the air surrounding the string tries to get in balance again as quickly as possible, and so the molecules from the compressed side will try to slip around to fill the void on the opposite side.
Since the string is a rather thin object, the molecules will succeed and thus the string alone is a rather poor sound radiator. Before a wave can form and travel away, most of its efficiency is taken out of it by this short-circuit effect.
However, couple the string to a large soundboard (the body of guitars, violines or the sound board of keyboard instruments) and you get a lot of sound out to the audience. Still, even along their edges, these sound boards would have a short circuit effect, unless they are in a box, which prevents the slip or makes the path so long, that the system already swings again into the opposite direction, before the air could reach a balanced state.
Also, if the boards would be free, the size of this edge region depends on the frequency, since every frequency is associated with a wavelength in air (low frequencies have large waves and high frequencies short). The shorter the wavelength the smaller the edge region (the area where air is compressed or expanded), but also the faster the system will move in the opposite direction. So size and time are related.
And I always understood the wing problem similarly to such a sound board. Across most of its surface, air has no chance to balance until it gets into the slip stream. On a well working wing, all parts of the wing undergo more or less the same flow with a continuous pressure field. Only at the tips is it possible for air to slip around, since the adjacent free air allows that.
But similar to the sound board, the short circuit at the wing tips of the plane, must only be relevant while the airplane is still there / in very close vicinity. And considering the time it takes for air pressure to equalize, this can only affect the wing to a certain depth from the tip towards the fuselage. And so the wing tips are where air can slip around while the wing is at that location and slipping around means equalization and so a portion of the wing is rendered less effective.
By making the wings longer, you basically move the problem further away and proportionally increase the effective wing area. By bending the tips, you turn the problem 90 degrees. The wing forces which are cancelled out are now acting side ways. Two ways to approach the problem.
That was the first explanation I got and it made a lot of sense. But maybe it is just a nice way to think about it.
Still at the very beginning, but I distinctly remember a school textbook telling me the 747-400's winglets prevented vortexes and improved lift.
And that’s incorrect. They are not there to prevent vortices, they are there to prevent the spanwise movement of air, leading to induced drag.. the vortices are just a side product
I heard they alow the air to leave the wing cleaner ruducing vortex's and hence reducing drag.
@@MentourPilot They don't even do that. What they do is redirect the spanwise flow in a slightly different direction. However, spanwise flow is barely changed, and it is basically a nonexistent effect.
@@MentourPilot Depending how much sweep there is in the wing, there may be zero, positive, or negative spanwise flow.
@@MentourPilot Yeah. Believe me, I didn't mean to suggest you were wrong. The number of outright lies my school textbooks taught me ... smh.
Interesting. Thumbs up. The solution appears to be hinged down extremely large winglets. At the gate your airplane's span is small and cheap. But once you start taxiing the huge winglets will automagically lift, forming a raked wing. No controls needed.
Not related to this subject, but I would like to know why in Europe, beards are OK, but verboten with US carriers. Perhaps a joint video with Kelsey on this would be good.
The reasoning used to be that the O2 mask wouldn't seal well enough to make the FAA happy, but that is a load of crap. The probable reality is that US carriers just don't like beards/facial hair.
@@tonyb8066 Thanks, Tony. I figured it was something like that. The Navy used the same dubious rational on us in the '80s when they banned beards. I wonder if it will change sometime soon as beards are now more acceptable... even some Police Departments are allowing them.
Who says it’s a load of crap? Do you have an authority on that? An oxygen mask can not possibly seal as well on a face with a beard as on a clean shaven face.
Thank you for keeping it up my friend.
Always here!
Do winglets reduce the "leaking" of high-pressure air from beside the aircraft into the low-pressure area over the wing? Which reduces lift?
yes
so interesting.. questions I wondered for years... especially which design was better and why there was variation
Mhm, some time ago there was a guy on TH-cam with an aviation channel, who said the winglets are there to reduce vortexes and drag.
The video titel was "Winglets - What are those things on the aircraft wing-tip"
Who is now right and who is wrong? Mentour Pilot or Mentour Pilot?
I was flying this week-end and wondering what these wingtips were made for, and I was sure it had something to do with these vortices, thank you for this real enlightenment.
Now, I have a couple of questions/ideas about how to increase the wing surface without increasing the "apparent" wingspan:
- increase the anhedral/dihedral: like the winglets, putting the wing at a higher angle increases the "real" wingspan. I know some very heavy carriers (Antonov...) have their wings above the fuselage with a somewhat important anhedral, how does that translate into induced drag?
- boxwing: Is there a problem with having multiple wings connected together? Tom Stanton made a model of a "plane" with a circular wing, but I don't remember him talking of drag in his experiment.
- T-Tail: I remember very old commercial planes that had large horizontal surfaces at the top of their back fin, is there a problem (drag-wise) in having an oversized tail?
- canards: you don't see them too often on commercial planes, but it (kinda) supports my previous point: with a nose wing, you're still remaining in the same "square", but you add at least 1/3 of the wing surface. Again, is it bad for the induced drag, or is it that you need regulatory clearance on the front of the plane in the airport (defined by the types you're mentioning)?
- forward sweep: Maybe it's a bit too far, but maybe inverting the wing sweep would simply remove the need for any sort of wingtip, as their wouldn't be vortices any longer due to the inversion of the direction of the airflow?
Thank you again for shedding light on the reason for these (definitely "sexy") winglets.
Have you flown a 737 with split scimitar winglets? Or does your airline not have those?
Not yet
Wow. Just when I thought I had a reasonable, albeit high-level, understanding of wing design, along comes this video. Fascinating. Need to do more reading to build on this info.
Next time on Mentour Pilot:
*"Think you understand toilets? Think again!!"*
Why? Is there something you want me to explain? 😀
Did you know the winglets weren’t there to stop the wingtip vortices?
Captain Joe already did a video on airplane toilets.
@@MentourPilot maybe do a reaction video on captain joes toilet video.
Im Just having fun in the comment section. Love you 😘