Doug McLean | Common Misconceptions in Aerodynamics
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- เผยแพร่เมื่อ 16 มิ.ย. 2024
- Doug McLean, retired Boeing Technical Fellow, discusses several examples of erroneous ways of looking at phenomena in aerodynamics, that have either taken hold in parts of the aerodynamics community or have been expressed in books or papers by other authors. These examples are mostly about interpreting the basic physics of the phenomenon in question.
Most are from his book Understanding Aerodynamics - Arguing from the Real Physics, but a couple of them are new. Examples will include ways of explaining the lift of a wing or the thrust of a rocket in intuitive physical terms, interpretations of the induced drag of a wing and how tip devices such as winglets work, widespread misunderstandings of how lift is manifested in the global flowfield around a wing, the common pitfalls of discussing pressure drag and thrust, and common misunderstandings of the accuracy of CFD.
Although these topics involve a wide variety of physical phenomena, Dr. McLean will attempt to identify the common threads. An appropriate subtitle for this talk would be An Argumentative Aerodynamicist Gets Old and Cranky and Takes Issue with Just About Everyone.
Sponsored by the Aerospace Engineering Department (www.engin.umich.edu/aero) as part of the 585 Lecture Series.
Speaker Bio:
Doug McLean is a retired Boeing Technical Fellow. At Boeing, he worked on CFD codes for transonic wing design, codes for airplane spanload optimization including the effect of structural weight, novel wingtip devices to reduce induced drag, transonic airfoil technology, swept-wing laminar flow, turbulent skin-friction reduction, and pressure-sensitive paint. He received a B.A. in physics from the University of California at Riverside in 1965 and a Ph.D. in Aerospace and Mechanical Sciences from Princeton University in 1970. He is the author of Understanding Aerodynamics - Arguing from the Real Physics (Wiley, 2012), which is intended to promote greater physical understanding of aerodynamics. He has designed his own model airplanes since he was a youngster and held a national record in the Pennyplane class of indoor rubber-powered models. - วิทยาศาสตร์และเทคโนโลยี
What an intelligent human being. These are the real unsung heroes of our modern word I swear.
>Intelligent
>hero
Pick one
@@RakedLeaf Why do I have to "pick one" when I think he is both intelligent and a hero? A person can be both in my opinion.
@@r3ttgaming177 he's not a hero
@@RakedLeaf well, certainly he is for r3ttgaming177, stop being so charismatic
@@RakedLeaf Maybe he's not a hero for you, and that's fine... However, he is a hero in my eyes.
This is the best title for a presentation ever.
"Equal transit time is wrong" good never made sense to me
equal transit plays a part in particle physics to explain why photons won't accelerate in a gravitational field. they spin to compensate for the field effect, and this distorts momentum to a perceived constant. the particle can't handle anymore acceleration, so to allow this energy it spins. how cute is that?! or it spins in reverse to the curvature of the field..but we don't know if there is gravitational waves, so i can't say if thats right.
@@treatb09 Why would photons accelerate, even in a gravitational field? That makes no sense. Speed of light is constant.
why wouldn't they? . how much do you know about physics? all curves are an acceleration, and to maintain a velocity, it must accelerate around a curve, since a photon is a "constant" velocity, it enters a curve but never slows down or accelerates..it contradicts the laws of physics otherwise...so thus it spins to still capacitate energy, but since it can't accelerate or decelerate in an energy field, thus spins in reverse of said field (like a car going too fast, putting its tires in reverse on a nearly frictionless surface per say.) . gravity isn't a resistance force either. and your gravitational field or curvature can be calculated this way, by the projected curvature of a photon from two lengths, one straight and one curved and spin of the photon being proportional to the energy of the field. minus a projected deceleration and acceleration something or another. i'm still working on the theory, but i'd love for you to steal it so i can prove you wrong too. love seeing my stolen work and how off it gets by needy cunts. like my center of mass concept for the shape of a planet. you tools really botched that one
@@treatb09 Wow, you have serious issues, boy. Have you tried seeking professional help?
Curves result in acceleration, sure, but going in a straight line through non-euclidean space is still travelling in a straight line.
@@treatb09 irrelevant to the topic here innit ?
Definitely one of best talks on aerodynamics on TH-cam. Thank you!!!!
One of my favorite “EXPLAINER” videos of all time.
Good stuff to feed my brain. The art of understanding and not merely regurgitating answers while learning.
Hear, hear! I just started re-taking most of the math classes I took in college almost 40 years ago (primarily to re-learn Linear Algebra for various applications). I'm shocked, disappointed, saddened, and (in a way) scared by how math education seems to have degraded since I first learned these topics.
There is no time built into the schedule to allow the teacher time to delve deeply into a single topic. The teacher only has enough time to give enough information for us to answer the questions on the homework and exams administered by the computerized learning management system (LMS) we all have to pay an exorbitant price for. More time is devoted to explaining how to properly format answers to the questions in the LMS than lecturing about the WHYs and HOWs (or even the history) of what we're "learning." Everything seems tailored to completing the homework and exams rather than learning how to think like a mathematician.
Even worse, there is not enough time for the teacher to stop speaking for even 10 seconds, to allow us to swallow the information that we was just forced down our throats -- let alone process it in our minds. It seems that the teacher is there just to ram as much "stuff" down our throats rather than actually teach. The scariest thing is that nobody asks any questions, either, beyond asking the teacher to repeat something they missed, or can't read on the board.
This is all very sad, and makes me worry about the quality of our scientists and mathematicians coming out of school these days and in the future. If this is "education," I'm not sure I want to continue with the traditional lecture format any longer.
Absolutely fantastic lecture. Thank you so much!!!
Mr. McLean would be interesting to have a discussion about common misconceptions about presentations. His presentation is nicely done.
I have seen the common misunderstanding of Bernoulli's equation many times, that velocity causes low pressure; It just causes it for some unknown mysterious reason. That misunderstanding is even written in books!
I'm glad to hear Doug come out and say, "it's not a one-way street".
Delta pressure cause acceleration, acceleration causes delta velocity, etc. The differential equation is intertwined.
The air on the upper wing surface is actually at a lower pressure than the bottom and it's not "just because it's going fast".
At 19:40 and 26:00 in the video he describes this pretty clearly.
There’s even a MIT video on aerodynamic with this misunderstanding.
"The air on the upper wing surface is actually at a lower pressure than the bottom and it's not "just because it's going fast"."
Then why don't they fly into the ground when flying upside down? Does the bottom of the wing then have low pressure?
Or is it more likely that the wing is just "planing" off of the passing air?
@@rael5469 Your first guess is correct. When flying upside down, you'll have to adjust the angle of attack to still generate lift in the other direction relative to the wing. Changing the angle of attack changes the flow of air on the wing. If you just invert the wing without changing the angle the aircraft will actually fly into the ground.
@@jonasdaverio9369 Then explain how a T-38 (Thunderbirds) flew upside down? It did not generate lift on the "bottom" surface of the wing....it simply skipped off the air on the new-bottom (top) of the wing. No lift on the upper surface, just air piling up on the lower surface. ....like the old Douglas X-3 Stiletto.
en.wikipedia.org/wiki/Douglas_X-3_Stiletto
Actually....I stand corrected. Even the X-3 Stiletto had full span leading edge devices which changed the curvature of the wing for lower speed flight.
@@rael5469 I only talked about angle of attack. The angle of attack can be changed very easily by pushing on the stick. No need to modify the wing dynamically
Amazing level of detail and insight.
Thank you Mr. McLean for your passion and expertise.
He has doctor rate. So His title is Dr. McLean.
Ex industry professionals make the best lecturers. They have got their hands dirty and have the battle scars to prove it.
He’s dedicated his entire life to understanding airfoils in fluid systems. Saying an academic from Princeton got his “hands dirty” in the field is an ironic metaphor that is insulting to actual technicians. Any scars are probably from being hassled for obsessing over the minutiae of fundamentals while the necessary applicable science is well-established.
Though, his reverence of engineers’ intuitive understanding of the fundamentals of physics and Boomer-like iconoclasm give me hope that I’ll meet other similar forces in the industry.
Hey Doug McLean
you are the man !
That is exactly I have been dealing and feeling too !
Thank you for the explanation
Thank you for filming this class. 😊
Fascinating lecture! It helps me understand (or question) the experiences I have had sailing a little Laser sail boat. Parcels of air movement (wind) are key to success in racing small boats - but no time to calculate even if we could sense the forces - unless an America's Cup competitor.
I can't believe I stumbled across this video. I am pretty sure I was there live when he spoke - that's the main lecture hall in the FXB. I enjoyed the talk then, and now I get to enjoy it again!
Thanks to my old friend, Al Zheimer!
@@robbmain3030 oof
11:35 - Thank you, thank you, THANK YOU for giving F=ma, the most beautiful equation in the world, the top position it deserves also here.
Nice presentation. Need to read book. I see some gaps in explaining lift.. great that author mention two ways relationship between pressure and velocity.
I always attributes lift to coanda effect… thank you so much for clearing the concepts!!!
Great lecture for details in between theory and practice....
I think this is the single best engineering lecture I've ever seen. Powerhouse.
A problem though. At about 18:09 on the last line of his presentation cell is the item "mass conservation not a direct reason for acceleration." He says according to Newton there has to be a force associated with the acceleration. And claims no one can tell him what the force would be.
That kind of ruined it for me. The restriction the asymmetrical geometry presents to the mass flow rate is a hurdle to the free flow of mass supply, thus changing the density profile. This is synonymous with the formation of a pressure gradient. And that pressure gradient is the force he is claiming can't be explained.
So, the "law" of mass conservation is itself directly responsible for the force that results in the acceleration. The total mass in must equal total mass out, and any move toward an imbalance creates the pressure gradient force necessary to accomplish sufficient acceleration.
I mean, for gosh sakes, the (mass) continuity equation is one of the fundamental Navier-Stokes rules formally describing the interconnected process.
He needs to go back to his previous view cell at 16:09 and rethink the "equal transit time" fallacy to bring that dead end into the proper significance of what it was no doubt aiming for in the "collective subconscious" (or whatever), in terms of his "missing force" from the continuity equation that Navier-Stokes take dead aim at.
@@davetime5234 Mass conservation would indeed be a direct reason for acceleration if you could rigorously establish the cause-and-effect for stream tube pinching. As McLean points out, he can't find a validly argued mechanism for stream tube pinching. I agree with you about the pressure gradient. But I can't, for example, explain the upflow well ahead of the wing leading edge, so this whole problem space is riddled with the danger of oversimplification.
I would say that McLean is talking and thinking at a level above what we normally see. My brother taught Aeronautical Engineering at the USAF Academy, they relied on F=mA and mass conservation, and that's indeed a much more rigorous way to treat the subject than that equal transit time nonsense. If a section of the flow were actually constrained by a tube, and that tube constricted, then mass conservation would give rise to the exact force we're looking for. But where there is no tube, the problem is more subtle, it seems to me.
@@Tordvergar "danger of oversimplification"
NS equations are able to handle extraordinary complexity (to the point of useful solutions being a challenge). But the driving laws of NS are simple in concept: conservation of mass, energy and momentum, each to be enforced separately but with interconnecting consequence.
We don't actually need to worry about a constraining tube if we know the entire extended relative flow has to exhibit continuity (without mass flow interruption taken across the entire area of influence), resulting in logical consequences for the other basic properties, of energy and momentum conservation, when that continuity is challenged.
"the upflow well ahead of the wing leading edge"
The pressure gradient due to the restrictive path to the mass flow, has to settle in some kind of 3D configuration to keep the mass transit rate logjam from stagnating, which would continue to build static pressure (due to the mass accumulation). This obviously has to form in "cooperation" with a flow rate deficit tendency to form at the exit end: the pressure will continue to drop from mass depletion until the acceleration is high enough to maintain the thruput rate.
I think it's reasonable to relate that up-flow ahead of the wing as analogous to backed up traffic due to an incident downstream at an intersection, and with vehicles exiting by alternative routes, preserving some of the vehicle thruput rate.
But in the case of the airfoil, the continuity equation says, that for the whole system, the traffic rate cannot be impeded after reaching a steady state situation (pathways naturally self-facilitate via the formed pressure distribution to keep the traffic flowing).
So, the backed-up traffic of air mass is facilitated by its self-imposed pressure gradient profile, to continuously fully clear according to the continuity rule (a scenario also consistent with conservation of momentum: the quantity of motion cannot be destroyed).
edit: The high transport speed at lower pressure (the Bernoulli conservation of energy notion of higher dynamic pressure at the expense of static pressure) has to result from the pressure gradient effectively filtering out the molecular velocities at the higher end of the thermal velocity distribution, in producing an average higher directed speed of flow (consistent with conservation of energy).
Bravo! Been an aero engineer and a pilot, including lots of flight test for the Navy and contractors over a long career. I've been looking for this guy for decades.
How did you become a pilot, that’s my dream
@@Valkyrie12124 Thanks. Am 80 years old, done flying after 63 active years as a pilot. Started as a kid at the local airport. Later became a Navy pilot and carried on a good career in the Navy and, later, as a civilian, airlines, etc. Most of career in flight test, Navy and others. Loved it all but don't miss it now. Getting started these days is VERY different from things in my day.
very very educational for those who want a much more in depth understand as to the physical phenomenon behind lift and flow. I feel he could be a bit more thorough about what constitutes a good explanation as opposed to everything that doesn't, but that's just me.
It's because he's a prior Physics major and knows a ton of the advanced computations and principles of general mechanics. I liked how he spoke about biot savart law and understood that the velocity ("Current" in terms of Electro Magnetism) at varying distances contribute to the point in question r-hat, with distance r. Its hard to calculate but he explained the "intuitive" aspect very well. Unfortunately, you need at least advanced vector calculus with various example problems solved by yourself by hand in order to understand what he is talking about to the professional level he is speaking. I'd recommend a good physics program with competent professors to explain Biot-Savart law well (in advanced integral form) to be able to understand the 2nd half of this lecture.
Around @26:17 he did explain lift more or less correctly: it's both Bernoulli flow plus Newton II & III (or vector N-II & III and Euler). But note, that assumes the continuum approximation, which if you want to be pedantic is false. He was also clear about that at the beginning, he is doing the physics correctly for the continuum approximation, which is a fiction, but "good enough". Air (water) is a gas (liquid) of discrete particles. So the true _ultimate_ explanation (of lift) for a purist is molecular collisions. Molecular collision theory (with some adjustments for inter-molecular forces in a real fluid, stronger in liquids, which give more turbulence and drag) explains it all, and there the Bernoulli effect is pure idealization, and it's really all just Newton II & III. That's why the *angle of attack,* not the wing shape, is the dominant factor, and is why you can fly a plane upside down without inverting the wing foil profile.
@@Achrononmaster yesss....!!
@@Achrononmaster Can you recommend any books which explains this topic in right way.
@bijou smith.
Its really true what you have said. The generation of lift is Misconception from beginning well saying pressure difference would actually generate lift but in reality it's much more complicated. Simple Bernoulli equation can't just the lift the sooner you remove Idealisation the clear the picture will be. It's viscous property of flow which generates lift, fluid is one hell of thing, they are freaking intelligent just required a space they shall compress or expand to occupy it. Continues mechanics comes handy here because they explain why we are assuming no slip conditions or neglecting normal velocity for solving navier stokes equations. Molecular Molecular interaction is will give significant understand in unsteady lift distribution.
I'm just a mum staying at home but I found this caught my interest and I understood. I wish though you had explained Newton's second with all the equations and its relation to lift, pressure fields and velocity fields. I really wanted to hear that. Anyway thank you for making this simple to understand. Yeah this covid time broadened my interest. Lol.
Dudes face even looks aerodynamic
🤣🤣
Yeah but that's kinda rude :(
Lmao
@@briancaguirre tu
😂
I am not an aeronautical engineer or expert in aerodynamics. However, I have always been interested in aviation in one form or another including model design and building, skydiving, paragliding, hang gliding etc. When I was a kid, for a science project I explained the thrust from a propeller using the "newtonian approach" which the teacher basically scolded me for and said that it was not the way that the thrust was generated. When the Bernoulli explanation was told to me, I didn't understand why the two points of air had to come back together at the same location. I was afraid to ask and I figured I was just missing something. This always bothered me and it is good to hear that I wasn't the only one who didn't get this "explanation".
After watching this video, I started to think back to my skydiving days. Skydivers call the low pressure zone behind them the burble. One thing you didn't want to do is get your pilot chute stuck in the burble. It can flop right onto your back and stay there which can be highly inconvenient if you don't know how to deal with the situation. This is just a demonstration of how strong the low pressure can be.
Tracking is when you change your body position with your hands and legs pointed back which gives you a significant horizontal velocity. You are changing the position of the burble, which shifts further back towards your legs. The change in the burble position changes the airflow and air pressures around your body.
I have been thinking about the burble and how it affects the airflow. When objects are dragged through the air, there is a high pressure zone in the front and a low pressure zone in the back. There is always a loss in energy with time (thrust times velocity, drag times velocity, or for a glider, vertical velocity times weight). Gliders always lose altitude within an airmass, and planes have to be pulled through the air to generate lift. This energy goes into generating the differential air pressure between the front and back of an object. It is the position of the "burble" or low pressure zone that induces the flow pattern around the wing. The flow pattern and burble generate the differential pressure. I don't know if this is correct or not but it appears from this argument that drag induces the lift and not the other way around. You can have drag without lift but you can't have lift without drag. It also seems to me to be a better way of thinking about lift than the old grade-school Bernoulli explanation.
I would be interested to hear what people think about this. Thanks.
Two flows of air do not get back together. Difference is drag.
Top flow gets to trailing edge first.
I can relate to the first paragraph of this comment.
can you make vedio on your topic some points are really intresting
How rare on TH-cam to hear someone burble coherently.
I fear that I won't be able to do my job in my later years due to mental decline, but these senior engineers with insanely good intuition give me hope.
Apparently the brain is the only organ in the human body that doesn’t age, if kept exercise. Amazing creation
Hey, I’m in school now for mechanical after working coast guard(have experience). Do you have any advice as to what to expect?
This lecture is very impressive, it should recommended for all new pilots..
These are not for pilots. They can't understand these theoretical facts. These are for scientists and engineers. Cheers !
He is using a Dell, not Apple. A sign of a solid engineer!
You are judging an engineer from the laptop he uses, not a sign of a solid engineer
@Thomas Jones in the making, hopefully
@@morcogbr True, I am not an engineer, I'm a scientist by profession. My comment was based on my solid scientific study, where I found that the engineers I respect the most use Dells. Both of them.
That is not a personal computer, it was given by his employer.
Great, and interesting insights. Is there another video where he gets into the next parts he announced?
What an incredible lecture
Funnily enough I happened to have this video downloaded so I could watch it while on a flight, where I happened to have the window seat above the wing of a Boeing 737-900. Hearing the introduction and realizing I was in that specific situation was a bit of a trip.
It's easy to understand what's going on with a 'lift generating surface' when you look at rotary wings - propellor, helicopters. It sure is windy behind/under them...
He addressed this reason @20:40
Amazing:-) Wish Doug had another hour.
Stokes's theorem (and Green's functions/theorems in general) are the bit of mathematical physics that most physics/engineering 'enthusiast's would do well to become familiar with. A good reference that I find myself going back to is 'The mathematics of Classical and Quantum Physics' by Byron and Fuller. It builds everything on core theorems of vector space, which after all is the world in which all of physics from hydrodynamics to QED lives. If you are not formally trained, but looking for that extra level of mathematical capability, look no further if your goal is to detect hand-waving in textbooks or lectures.
Any recommendations to what one should be familiar with before reading Byron and Fuller's book? I've got it lying around and am still not sure what the prerequisites are. Maybe single and multi variable real analysis, linear algebra and a bit of ODEs and a bit of complex analysis?
@@OttzelTV Just look up the pre-req's for a 4th year Modern Physics course at any major university.
This dude is the real deal. You can only learn from people who have been there and done it.
You can learn from anyone, don't get caught up in that elitist mentality. I've met plenty of people with decades of experience, that make amateur mistakes. This guy probably isn't the case, but it would be wise of you to take a more humble approach and try to learn from anyone who makes sense.
@@acruzp Theorists with little or no application experience used to be a dime a dozen. With the advent of the Internet, they are now half a cent a hundred, and less on sale. Real humility is in recognizing the difference that practical experience has made in someone else's conceptual understandings relative to one's own.
Very good. I would listen to industry experts. Their theories and analyses have to stand the test of actually flying.
Thank you Dr McClean. Thank you Michigan Engineering.
Reading book. One of the best book about fluid dynamics ( and I already read a lot). Strong points ( so far ) pressure-velocity two way relationship, critics of Reynold number meaning and applicability.
Andrew Pawhat book ?
Thanks I will check it out. I'm sure it will be helpful and ofcourse interesting
Hello, can you suggest a couple of other good books on lift?
I'm feeling like I dont have clear concepts regading lift
What is the critique of the meaning of the Reynold's number?
The Maxwell X-57 aircraft, currently in development, is about to relearn some stuff talked about at 37:30.
It’s crazy there are so many misunderstood videos on TH-cam regarding lift. I always felt there was something wrong with just saying air travelling faster = lower pressure.
But air is NOT travelling faster: th-cam.com/video/rHidaQgBb-Y/w-d-xo.html
Sick video, I wish I was into this more.
fuck me ... an actual professor , thank you sir , we have been waiting for you for a long time
I've always intuitively thought that lift was generated from the Newtonian explanation of thrust: particles colliding with the wing at a positive angle of attack, being pushed down, and subsequently the wing being pushed up. (I knew this understanding was incomplete because it couldn't explain how flat bottomed airfoils generated lift at zero angle of attack) I never understood people who only teach the Bernoulli way like it's always been taught to me. Glad to know lift is more complex than what's been taught to me by people who think they are right. I still don't understand lift, but I'm glad I found this video.
The thing is; that particles above the wing are ALSO being 'pushed down' because of the lower pressure above the wing allowing higher pressure far above the wing to push it down. In fact, MORE AIR IS "pushed down" from above the wing than below it.
What happens above the wing contributes more to the difference in pressure than the small increase below the wing.
Towed gliders must stay above tow plane ! to avoid the down wash ( displacement of air ) and it is considerable ! a typical C172 displaces 7 tons per second! Bernoulli contributes a lift component , but if you do the arithmetic Bernoulli alone is not sufficient to 'suck' an airplane into the sky !
Air interacting with a space capsule re-entering the atmosphere (aka very thin air at high mach numbers) basically DOSE act like the simple newtonian stream of bullets under thouse conditions, and it's why such vehicles can only generate lift by being tilted to the incoming airstream by dispacing their center of mass.
I think the best way to demonstrate incompleteness of each approach would be to show 2 airfoils: one would be driven solely by Newton's deflection, the other one would be driven only by Bernoulli's pressure difference. otherwise it's unclear how much each of these effects contributes.
As a retired naval aviator and graduate of NPGS, it's all done with mirror farms with a sprinkling of Bernoulli's thrown in. Also occasional magic from McBeth.
What do they use for helo aerodynamics, Señor Shmuckatelli? All that and a dose of Icarus?
Lift demons and thrust pixies.
Great lecture!
Why I am watching this at 3 AM in the morning in my bed? And I like it.
I have my BSc in physics as well and I'm also about to get my private pilots license and every time the explanation of lift came up during ground school it never made any sense to me. Why do air particles HAVE to meet at the other side of the airfoil? They don't. Why is the fluid speed faster over the top surface etc. It's always explained in the pilot community with these naive explanations but this and a couple other videos are gems when it comes to gaining a true understanding of things. This reminds me why I chose physics as my degree!
One thing I'm surprised no one touches on: the contribution of inertia of the contiguous volume of air being accelerated by the angle of attack, the underside of the wing.
It's not clear why it's never mentioned that inertia of the air contributes to lift for airplane wings. It is only hinted at when Newton's 3rd law is mentioned. An airplane wing traveling at constant velocity (ie. plane not accelerating) encounters air at neutral (isotropic) pressure and accelerates air below the wing at a magnitude dependent on the angle of attack of the wing.
(Extreme example: if the wing's angle of attack were made by turning the wing vertical, the inertia of the air would prevent lift, and the airplane wing would be acting only as an air compressor for the air immediately adjacent to it.)
If the air could be accelerated instantly - with no inertia - it would not create lift. The delay in response, caused by inertia, for the air under the wing to 'get out of the way' of the wing's angle of attack is a contribution to lift. The air cannot 'get out of the way' instantly, partly due to inertia (air being matter, and all matter experiences inertia under acceleration). The angle of attack is therefore partially compressing the air immediately adjacent to the (underside of) wing, and the compressed air pushes back against that, creating lift (Newton's 3rd law).
This is also why a wing does not 'stall' if its constant velocity is great enough: the angle of attack of the wing accelerates the air underneath, and at great enough velocity, the acceleration of the air under the wing is great enough so that its inertia (inability to instantly accelerate 'out of the way' of the lower part of the wing) is great enough to contribute to lift.
If the wing is moving too slowly, inertia of air being accelerated by the angle of attack can no longer contribute to lift.
.
The false "equal transit time" is a misstatement of a related requirement: in steady state, rate of mass in has to equal rate of mass out. We know intuitively this has to be true. This more carefully considered truth drives the acceleration of airflow due to the path restriction of the airfoil's asymmetry to be faster than "equal transit time."
For the "equal transit time" fallacy people are focusing on too tight a location in space.
he is the only other person I have ever heard describe the wing shadowgraph at cruise. The only other person I ever knew who knew about it before i told them was Dr. J Polve Col USAF Ret. Who was my Aero instructor in university. So he is making a good start.
To clarify - Dr Polve was my Aero instructor. he was a brilliant man and I learned a ton from him. He told me about the standing shock wave on the top of a wing, and the possibility of seeing the shadow.
The equivalence with electric and magnetic fields clicked it a lot for me, because we have the same exact confusions, to the point that he claims “in electromagnetism that is a causal relation, the induction law”, but that’s mostly a historical artifact of how we discovered these phenomena.
Many years ago in the UK I gained a bachelors degree in aeronautical engineering and followed that up with a master's degree program in aircraft design. I was never satisfied with the explanations given about aerodynamic lift by my teachers so I was keen to watch this video. While I did gain an understanding about why the explanations given by my teachers were so unsatisfying, I still cannot claim to have a complete understanding of the phenomenon. Is it just me?
Indeed no, you aren't alone. And McLean's explanation doesn't help. On the one hand he demolishes the arguments using Bernoulli and turning down momentum, but then at 23:22 he says both are partly right and then at 26:45 asks his audience for assistance to reconcile them.
Thanks for letting me reply to comments here, I had to go to TH-cam itself because I couldn't comment, I was in Facebook 😂. Blah blah I know, I understand everything being said but need more of this! It's fun.
incredibly interesting.. though I keep hoping the next point would come quicker
Many people don’t realize that there are symmetric airfoils, such as in RC helicopter blades. This is so they can generate the same amount of lift when flying inverted (upside down). The Bernoulli/Equal Time explanation falls flat for these from the start, as lift is generated only when the blade pitch is changed, and thus there has to be a different explanation.
YEP ! 26:00 to 28:00 are the deepest keys to apprehend this fascinating result (instinct make me think of a sort of succion, caused by gradient and sustained by fluid's inertia , countr acting gravity for gliders, thrust etc...) . He should have made some // with swim too...
to help understand the problem with wingtip vortex recution... you're basically not trying to reduce vortices you're looking for more air you can affect so you can push yourself up more efficiently
the vortex is just another persepctive to look at that
I found this discussion very interesting and informative. I wish he had actually RESOLVED some of the mystery! I would even gladly buy the book (assuming it's available in electronic format - I'm not a Luddite... ;-), if I had ANY confidence it might help to actually EXPLAIN lift, but I have the sneaking suspicion that it would just provide greater detail on what is WRONG with the two prevailing 'pieces of the puzzle' without properly integrating them into a comprehensive all-encompassing DEFINITIVE picture!
For now, I'll just continue to use airplanes - though maybe a little more suspiciously...
AFAIK There is NO COMPLETE theory of lift . Aerodynamists use various theories depending on what they are looking for .
@@Cheezsoup well there is navier-stokes which is pretty much complete, but yeah maybe that's just renaming the problem
@@screwhalunderhill885
If you know Navier-Stokes to be complete and can show same there is a MILLIOM dollars waiting for you.
@@Cheezsoup lol
@@Cheezsoup The problem isn't with the "completeness" of Navier Stokes, it's the existence and smoothness of its solutions, which is inherently a mathematical problem, not a physical one.
Why dont we see swept up wingtips on rear horizontal stabilizers? Only on maing wings?
Wouldnt adding this also reduce drag?
Vortex as a function aspect ratio would be interesting. Compare vortices of sailplane with cargo plane.
10:08 and 11:04
Well what you said is right, I dont see how what they said is "wrong", but rather is incomplete.
Its still true that newtons third law (together with his second) implies conservation of momentum, and therefore if you want to design an engine to produce thrust, you must "throw stuff out the back" so to speak. And if you do "throw stuff out the back", then the conservation of momentum requires your rocket move forward.
I think the problem is people are confusing the strict statement of the Third law in newtons original formulation (which as you said only talks about forces) with the Conservation of Momentum.
Interesting discussion and agreeable Theoretical explanations. Especially ground effect. Often misused words have caused common misunderstandings and debate. If we correctly describe lift and induced drag as components of a total reaction vector ( up & back if you like ) it becomes easier to talk about each component as parts of a whole rather than commonly labelled individual phenomena dependant on the other. Moving and airfoil through relatively still air compared to the inverse wind tunnel or modelling ( moving fluid ) environment does not get much consideration but perhaps it should. Vortices change the velocity of airflow over a certain area of the airfoil in increasing or decreasing amounts. Think angle of attack speed and the trade off effects on the total reaction. Useful for work airfoil area vs not useful etc.
I think this could be simplified for a lay audience (people like me) by talking about how a wing interacts with the double vortex in 3D (a helical field), and how wing shapes and velocity affects the interaction
- long skinny wings
- short fat wings
- faster aircraft have swept wings,
- winglets
- super-critical foil
I never thought about the fact that the 3D flow field must also extend forward. I tended to think that the air was still until it met the wing. Another error due to 2D thinking and forgetting about how fast pressure is propagated.
The brilliance of McLean's talk is in how simplified a context subject.
I suggest a class in physics for non-science majors. Or, some ground schools in flight training qualify as the aforementioned. Of
course, there are probably 'physics for dummies' type books written for the layman.
Rather than ask he simplify fuether, you lift your knowledge. I guess that's what torqued me, people wanting to be spoon fed all the while they hardly move a muscle.
The pressure changes in air move at the speed of sound. Therefore the effect of the aircraft on the surrounding air reaches out in front of the plane by a distance equal to the time it takes for the plane to get there. As the speed increases the distance to air in front of the plane that is undisturbed is reduced. When the plane is flying at the speed of sound, then air in front of the plane is undisturbed because the change in pressure cannot propagate faster than the plane is flying.
My God how video examples would have helped with this lecture.
Im seeing this 10 years after the fact. Be interesting to hear his perspectives on Boeing now, especially since the Max series was probably developed during his time with the company.
A "no-slip condition at the surface." What is the effect of this assumption? What is "slip."
An extremely valuable perspective from the real side of math and physics. Doug McLean is assuredly an aerospace scientist.
On the Basis of Messrs Dunning and Kruger, I know enough aerodynamics to know, with some certainty that I don't understand it and that all those simplified explanations you find in bools are just "lies we tell to children" or Just-so-stories and in the long run do more harm than good.
Having said that this lecture made a lot of sense.
See also how transistors work...
I hate pop-science and 10 minutes videos
34:38 This thing’s pretty funny. Unless the holes in the flow straightener are very small, the vortex is just going to split into a bunch of vortices that are just going to recombine behind it. If the openings in the straightener are small enough, that’s a lot of parasitic drag for one, but he’s essentially built a Dyson vacuum cleaner and the vortices (I think) going to exert a huge amount of skin drag from being de-spun.
And I haven’t got through his entire explanation of why it doesn’t work, but if he didn’t mention it, I also imagine whatever vortification it removes is just going to be generated again, meaning even more drag.
Yep, basically what you said.
Great talk. The main thing I wish more teachers would point out is that the reason an airfoil affects the atmosphere ahead of itself is because pressure differences propagate at the speed of sound.
I especially like your point that the inertia of the fluid is essential to maintaining the pressure difference. i.e. inertia provides the time delay between propagation of the pressure, and mass actually being accelerated along the pressure gradient.
I think it's a bit trivial to state that inertia is needed. If it doesn't have inertia it doesn't exert forces (momentum is conserved).
one of the biggest problems explaining lift starts with using molecules with momentum instead of at rest with the object having momentum. A wing moves out from under molecules that are weighted down from above and as long as the pressure differential is great enough, the weight of the object is negated. Take scales on to an elevator and weigh yourself as the floor moves down as opposed to being stationary. The top of the wing is constantly moving out from under the pressure of the molecules directly in contact with it.
Jon Miller as Galeleo pointed out the it doesn't matter if the air is moving over a stationary body or the body is moving through stationary air.
The wind tunnel is the most practical way to study lift. The reality is of course that the air is still and the wing comes along and slices through it. The wind tunnel encourages concentration on near field effects while seeing the aircraft approach and fly past gives you an appreciation of the far field effects, which also exist. This is especially striking when there is smoke to help you visualize the effects on the air, such as in the NASA vortex studies.
I studied aerodynamics at Hermann Schlichting in the 60th in Germany. Never lift was explained by a longer path. The lift was explained by pressure difference and circulation. All foil measurements and calculations are based on pressure difference until nowadays. As a comment: lift is the force vertical to the flow direction. The total force provided by the foil is the vector sum of lift and drag.
All these "alternate theories" are an amateur scientist phenomenon.
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Yes. The lift force is the upper/lower pressure difference and using the two components described by circulation is one way to make calculating the values easier.
As a 40 year Boeing aerodynamics engineer, I'm happy to be known as an engineer and not a scientist. Engineers have to make decisions with incomplete data. Scientists generally draw conclusions only after enough data are available that the conclusion is irrefutable.
I, too, am a 40 year Boeing aero engineer and I like the differentiation: a scientist strives to understand the world as it is, an engineer strives to make the world the way we want it to be. (Doug McLean and I worked together for 36 years)
@@dougball328 I assume that you have an autographed copy of his book.
@@dougball328 I recall that you retired to Greater Phoenix. Hope you are enjoying retirement.
@@engineeringoyster6243 I do not have a copy of his book. Not sure how that happened. Retirement in AZ is nice, but would love to see some rain here !
Nice work
So, vorticity is the "Failure mode", the way we see or observer the "failure". In that case, the phenomenon of tip vortices?
Doug, you mentioned "sun shadow graph"? at the start of your talk. An observation I made a couple of times whilst acting as a flight test acceptance engineer, - I have observed "pressure lines" or shadow lines on top of the wing surface during flight (my best description of them was "ghosts" - do you have an explanation for this? I've never been able to work it out. The aircraft in question was an A320 at cruise altitudes. These shadow lines were definite enough to photogrph.
Yes! You're seeing the light bend as it moves through a density jump, also known as the shock wave. It dances around, moving forward and backward, casting a shadow on the wing.
I suspect that the overpressure due to aerodynamic lift never makes it to the ground due to the dissipative action of viscous effects in the atmosphere; this is the same reason why the trailing vortices don't last indefinitely. It seems most plausible that, at least a large distances, a net temperature increase would be only lasting manifestation of lift in the atmosphere.
That addresses the energy portion but not really the force portion of the question. At least I would need to see a more thorough formulation of the concept to convince me.
Easily shown to be false asymptotically. On a sufficiently large planet, with a sufficiently thin atmosphere (so that elevation can be safely ignored in modeling gravitational force) and looking at the equilibrium case, every parcel of mass in the atmosphere has the same downward force no matter where you put it, and the total mass of the atmosphere must be exactly supported by the whole of the planet's surface.
When a plane takes off, mass of "atmosphere" increases, hence total projected force onto Earth's surface must also increase, in the static limit.
Back in the world of 1/r^2, you can elevate a parcel of mass to reduce its net downward force, thus cancelling out some of the mass of the plane by reshaping the entire atmosphere (possible but implausible). This will be tricky to do while maintaining the criteria of static equilibrium. My own math is not up to the challenge of making any insightful observation about this more complicated problem.
Using ballpark parameters for a 747 wing at MTOW, via Google calc:
400 tonne / 6000 sq ft in lb/sq inch = 1.02 pound / (sq inch)
The ground support will be _much_ larger than the wing area once you get out of ground effect, and it won't be a pressure we would much notice on the ground.
I know this also because many tanks are designed with treads that put no more force on the ground than a man walking, so that either can follow the other onto soft surfaces.
What is 6000 sq ft in shoe size? Huge. Not going to leave deep track marks in the sand if the animal is only 400 tonnes.
Another example, vector thrust can only be explained by inertia of air because the exhaust pipe is turned at 90 degrees angle to the primary jet exhaust and the produced lift occurring near the aircraft; same as rocket in space near 0 atmosphere.
All bow to the voice of reason. Finally.
Love his book
I've found the best way to remove assumptions you have about lift generation is to literally draw airfoils in flow fields.
ahhhhhhhhhh I need him to finish his talk, too important to cut for time.
15:54 "Longer path length" is what I was taught in the 1970s.
@conacal rubdur Who knows what evil lurks in the hearts of YT? The Shadow knows!
@conacal rubdur no. Why do you ask?
@conacal rubdur I'm not an engineer, just an interested middle-aged geek.
I do, though, share your concerns regarding the aerospace industry. How much of aerospace engineering is transferable to other engineering disciplines in the job market? (That's something to ask your adviser or the website of some Society of Engineers.) And how much would your graduation be delayed if you changed majors in a couple of years? (Also something to ask your adviser.)
Good luck with this important choice!!
@conacal rubdur Go for It! Otherwise, as an old mechanical engineer you will ask yourself why didn't make aerospace. You will find jobs in both of them, but you will love the job if you do what you like the most. Regards
Who ever said that the laminar flow above and below the wing separates and re-join after the wing. It does not and obviously doesn't have too. A flat airfoil will still fly as long as you have AofA. Lift is best described with similarity to a parachute /Paraglider... simply put, the wing is catching and deflecting air the same way your hand will move with deflection outside a moving car window.
Is it true that the vorticity on that capping surface is zero? It seems like we're just assuming that the flow field ahead of the propeller is irrotational, but there must be some because the flow is continuous across the propeller plane.
I am sure that the Pressure field sustained by fluid innertia and acceleration can be explained with some simple test setup with a piston and gas. In fact must be able to do that together with combustion engine engineers who look into detail of intake manifolds and piston movement. I am sure a Technical Physicist can build a model and explain. Quite surprised that Boeing would not have done that R&D considering the scramjet technology out there. Rather important to understand pressure waves?
Outline at 4:20. Nice.
You can see that the Bernoulli explanation of lift is a minor effect because planes can fly upside down, and also that a flat sheet can act as a wing. A bit of cheating there as at any angle of attack the flow over the upper surface forms a curve but with more drag.
The real reason wings have a more highly curved upper surface is that it allows the wing to fly more slowly before stalling, aiding take off and landing, accentuated by devices like flaps on wing trailing edges during take off. Stunt aircraft have symmetrical wings, and they fly rather well.
It's interesting how the "stall" concept is never stated in lectures like these. (Calling a stall a concept, #humor )
My `simplified intuitive model` is that the bottom is for lift, the top is for control. When the flow over the top is spoiled control is lost by degree. A plane can fly upside down, but not as well (less control.) *Malcom Connah,* slats and flaps strengthen your argument.
45:56 So...a layman's understanding here...He's saying that an airliner weighing 65 tonnes produces a downward force equivalent to the lift of 65 tonnes, but it doesn't crush everything it flies over, because that airliner is hanging in a volume of air far larger than the aircraft. The change in PSI you experience from an airliner flying over your head is around the same as a cold front going through.
Question - this is always spoken to in terms of the air, or fluid is “flowing” over or around the foil. But, isn’t it true to say the foil is moving through a fluid not in motion? Is it the same, but different point of view (perspective of foil, or perspective of fluid particle)?
Great lecture!
Yes, it is the same. What matters is the relative motion between the fluid and the body. Hence why wind tunnel tests (static body, moving fluid) are representative of real flight scenarios (moving body, static** fluid).
Amazing video! Gotta appreciate an expert who debunks mistakes and concludes with "it's not simple, I'm still working on it". In the interest of challenging misconceptions, there is the common one at th-cam.com/video/QKCK4lJLQHU/w-d-xo.html also in most fluid dynamics textbooks, saying internal stresses are due to molecules with high momentum moving to regions with lower momentum. Actually, in dense fluids (e.g. water) and near walls, the major contribution is due to the intermolecular forces (higher than 80%) and fluids have a lot more internal structure than expected, closer to a deforming solid lattice than balls bouncing around.
When a clockwise rotating cylinder starts from rest, it is initially surrounded by anti-clockwise vorticity. This diffuses outwards under the action of viscosity. If the cylinder is in a crossflow then the diffuse anti-clockwise vorticity will be convected away as a starting vortex, leaving the cylinder as a naked clockwise vortex.
We can solve for the velocity distribution around the cylinder either by using a conformal transformation, or by doing an easy solution of a Fredholm integral equation of the second kind. We can then work out the pressure distribution by Bernoulli’s principle, and integrate this to work out the transverse force or lift. We will find that total lift is proportional to total vorticity, and given fixed total vorticity, it is independent of cylinder radius. We can shrink the radius to zero, and deduce that for a naked singular vortex, lift is proportional to vorticity. This is the Magnus effect.
For two vortices, the mutual interactions cancel so total lift remains proportional to total vorticity. We can go on adding vortices at selected strengths in order to build up the profile of an aerofoil, and we will still find that total lift is proportional to total vorticity. This is the Kutta-Joukowski circulation theorem, but Magnus effect is easier to say.
Remember that starting vortex? A slightly-inclined aerofoil also generates a starting vortex which is left behind on the runway by an aeroplane. We can solve another Fredholm integral equation for the aerofoil but we need a supplementary condition of either known total vorticity or equal flow speed above and below the trailing edge. The latter is known as the Kutta condition and whenever it is violated a starting or stopping vortex will be shed until it is restored. With the Kutta condition our solution will normally have nonzero total vorticity and to work out the lift we can either add up total vorticity, or do an integral of the pressure distribution around the aerofoil worked out from Bernoulli’s principle. The answer will be the same.
Liquid helium is a superfluid which means that for a slow-moving hydrofoil there is no flow separation at the trailing edge, no starting vortex and no net total vorticity associated with the hydrofoil and no lift. Equal transit time also applies. Many “explanations” of lift fail to deal with this.
Generally to solve that Fredholm integral equation we need a numerical method. The matrix we get has a dominant leading diagonal so matrix inversion is easy and only takes say 13 lines of code which we could pass around in Excel VBA. This is not a difficult subject when we know what to do.
Can the Q & A be posted please?
Amazing
I remember I had found the same explanation about inertia and centrifugal forces about lift in a turbomachinery book looked the most legit
This could be made into a whole class
Along the leading edge there is very thin plane normal to the direction of travel. Above that long, thin plane the air travels over the wing; below that line air travels under the wing. Hence the angle of attack effect. Air directly in front of that thin plane is pushed "forward" by the wing: Surely there is drag there and there is air compression with the "power/energy" to push the air up and backward over the upper and down and backward over lower surfaces. Surely this pressure on the lower surface can compress and lift the lower surface. Looking at the leading edge (with two, (splitting) packets of compressed air is where calculations should start.
I've put a splitter on the back of my car that resembles the engine cowling of a Boeing Dreamliner. It seems to work exceptionally well at reducing the vacuum behind the car, and has a significant impact on fuel economy. (Partially seen in my profile photo)
If you are referring to the random sized shark-teeth edges, weren't those supposed to lower the sound (noise) from the engines? You have used the words "exceptionally" and "significant" to describe the performance of your design, so I am assuming a patent is in the offing? If not, at least a video for the rest of us, is required.
Do you mean a rear diffuser? en.wikipedia.org/wiki/Diffuser_(automotive)
My Aerodynamics class in college was disappointingly BORING as hell.... this dude continues the tradition.
Isn't the drag and subsequent vortices caused by a difference in the proportions of air moving by imbalanced times of arrival at the back side of the wings, along the edge path? In other words, the flow at 10 feet along the wing is flowing faster than than the airflow at 20 feet, and still faster at 30 feet at the very tip? (I might have gotten that reversed)
"people with inventive minds" - love the euphemism for "crackpots" ;)
You don't need a viscous effect for air to follow an upper surface. All you need is the realization that air is pressurized and therefore simply colliding with it will not create anything resembling a perfect vacuum.