Why are so many pilots wrong about Bernoulli’s Principle?
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- เผยแพร่เมื่อ 14 ต.ค. 2022
- For decades new pilots been taught that lift is created because the air flowing over the wing travels a longer distance than the air flowing under the wing, and therefore, for the two airflows to meet at the end of the wing, the air flowing over the wing must move faster. This hypothesis is not correct and this video explains why.
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- Bernoulli or Newton? • Forget Bernoulli and N...
Links:
- Prof. Babinsky, wing lift: • Wing lift Holger Babinsky
- RC Model Reviews: • How aircraft flaps work - วิทยาศาสตร์และเทคโนโลยี
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I’m an aerospace engineer who graduated from Embry Riddle, the top rated aviation school in the country. And even there, in our early aerodynamics lessons, the equal transit time fallacy was taught. I remember because I asked - Why does the air on the top HAVE TO reach the trailing edge at the same time as the air on the top? And my professor didn’t know… but to his credit, he came back the next week and taught everyone the fallacy of the equal transit time.
That's a great quality of an instructor.
No aniversário de 100 anos da Aviação eu entrei com essa questão "- Por quê um avião voa?" justamente para abolirem essa falácia que todas escolas ensinam sobre a sustentação e com o mesmo argumento do vôo invertido não aceitei tal resposta;
Calm down it's just a math model. Ask any farmer and he will tell you it doesn't have to get there at the same time. It's like permanent magnetism and residual magnetism it's all a personal opinion which one is there. If I like it it's permanent magnetism and I sell it for being a permanent magnet. If I don't like it it's residual magnetism and I don't tell anybody
@jacobstump4414 I have a technical article copied that credits Newton for wing lift, not Bernoulli. Did anyone teach that?
Only the second law of Newton related to the variation of momentum : F=d(mV)/dt, describes correctly the wing lift (vector F for the force on an air particle, scalar m for mass of that air particle, vector dV for the variation of velocity and scalar t for time). This law applies to all air particles moved by the plane flying through the air. The wing lift is equal to the vector sum of the forces exerted by all air particles on the wings. The Bernoulli's equation describes only the aerodynamic behaviour of the air due to the movement of the plane. It has noting to do with the lift.
Being a carpenter I remember being taught about Bernoulli's principle and the reason why roof tiles come off a roof during high winds. I have never forgotten about this phenomenon. Love it.
It is the same phenomenon on why people are unable to stand up in a hurricane.
Did you use your knowledge of this principle to design roof tiles that don't come off in high winds? No? Then why do you love it? So you can tell your clients that it was the wind that knocked off their tiles? Did you _amaze_ them with this revelation?! lol
As an instructor, I've been teaching Bernoulli's for straight and level briefs. I tell them it's not a direct translation from venturi tube to an aero foil, but just explain that there is a similar effect of decreasing static pressure above the wing. Didn't realize people were trying to explain the "reason" for it as an equal transit time...
"I tell them it's not a direct translation from venturi tube"
But isn't it in fact a direct translation to all situations when considered at the parcel level because each partitioned parcel also has to behave according to Bernoulli?
@@davetime5234 Not really because in a venturi tube we're talking about the total pressure of a closed system. An aerofoil is not in a closed system with a certain total pressure.
Thank you for your reply.
I guess what I was getting at: an individual parcel in an open system obeys Bernoulli no less than an individual parcel in a closed system?
Therefore, Bernoulli relationships for conservation of energy, momentum and mass (continuity), should be just as applicable to the open system (which presumably needs to be partitioned in terms of such parcels for analysis)?
I'm wondering about the above implication for those who say Bernoulli isn't applicable to a thin boat sail?
One could say you make a conceptual transition from a closed venturi to an open system, with a wing having a cross section of some thickness?
But, perhaps it is another conceptual jump to a perfectly thin boat sail producing lift? (here there is the challenge of convincing someone it still works without, upon first inspection, any apparent path difference between opposing sides)
But in all 3 of the above cases, Bernoulli still applies, even if the analysis requires more work in the latter two cases?
I'm trying to understand if, in the above, I've outlined the problem correctly?
@@davetime5234 I see what you're saying, but it's a little over my head really lol. I just gotta teach people that wings make lift, then tell them how to take off and land lol.
I figure if the theory of lift was unified, we wouldn't really need to have these discussions. Sadly as far as I know it's not (like the theory of gravity)
The fact that you can make a flat square fly if you put the centre of gravity in the right place and give it control surfaces and enough thrust and a positive angle of attack tells me that everything else including the aerofoil cross section and other twiddly bits like wingtip vortex generators etc are all about improving efficiency. All you need is enough surface area to direct some air downwards, get the basics right, and it'll fly.
Any supersonic aircraft will demonstrate this.
modern wings are mostly working on air pressure differential on top and bottom. the wing moves forward, making high pressure at the bottom, low pressure at the top, sucking the wing upwards while also sucking air over the top
@@FrostCraftedMC , you are funny.: „modern wings“? And don‘t forget, sucking is only an imagination of the real physics. You only can fix that the side which is turned to the incoming airflow (the bottom) produces more pressure and on the side which is a little bit turned away from incoming flow (upside) decreases the pressure. So it´s clear that there is a difference of pressure that can be seen as generating lift.
@@FrostCraftedMC Funny... it's pretty windy below a rotary wing aircraft (ie. helicopter). Pretty sure it's not being 'sucked up'.
@@nitramluap Yep, lift is the opposite reaction to air being pushed down. Helicopter pilots understand that better than their fixed wing counterparts.
I have a private pilot license, but have not flown aircrafts or even being in contact with the world of aviation for about 7 years. Now TH-cam recommends me this video and it reminded me of when I was taking classes, that one of our textbooks warned against these misconceptions regarding aircraft lift. I also remember the book saying that it is still not completely understood, what makes an aircraft fly. I still have the books with me as well as my (expired) private pilot license.
Lift is fully understood by aerodynamic specialists. The problem is to explain it to people without an engineering degree without oversimplifying it.
@@FlywithMagnar As an aeronautical engineer (and pilot) I completely understand why the typical example is taught... It's a "good enough" explanation for almost every human on the planet, even for pilots. As you say it's very difficult to explain the totality of what's taking place, without giving a multi-day class on aerodynamics.
The funny/sobering thing is... even relatively high levels academics (university science classes not focused on aero/fluid dynamic) are still teaching the incorrect science, because it is good enough most of the time. I only know better because I majored in this specific field. That leaves me to question what I think I "know" about other areas of study.
However hard you try you can’t “fly aircrafts”. The plural of aircraft is…….
aircraft.
In the USA, PPL is for life. Just need bi-annual flight review to be current, if I remember correctly.
I know what sentence you're referring to. I remember reading it and laughing / cringing. Most of that chapter's explanation for how an airplane flies is either outright wrong or misleading.
It’s more important for pilots to understand what causes a wing to stop producing lift, so the passengers don’t get upset.
It will also be nice if the pilots understand how a wing produces lift.
"It’s more important for pilots to understand what causes a wing to stop producing lift, so the passengers don’t get upset."
You mean like when the big fan up front stops cooling the pilots?
@@rael5469 You got it…..it’s doesn’t do to overheat when the passengers start screaming 😱
As John Wayne would say... "Stop stalling and spit it out..."
It has nothing to do with Bernouli.
It is action and reaction - it is the deflected downflow of air from under snd over the wing, that provides lift. To make the wing go up, you must deflect molecules of air downwards. No deflection, no lift.
The pressure differentials are a product of molecule deflection, not the cause of lift. (ie: more molecules hitting the bottom of the wing than the top.)
R.
I have been dying to find a good video that actually explains lift for pilots in a correct fashion. As an aerospace engineer it's really hard for me when my student pilots tell me about the Equal Transit Time theory for lift. Thank you for this video!
Show them the wind tunnel video...
Not only is equal transit time shown to be wrong, the air over the top of the wing moves even faster than equal transit time would suggest.
Note that that video is only valid for that profile, angle of attack and wind speed.
Quite right that the equal transit hypothesis is demonstrated as being false. But then it's dismaying to hear him talk about lift almost as though it were purely a function of surface curvature, when even a flat plate tipped at an angle and forced forward at speed will, also very demonstrably, generate lift.
All pilots should look at the book "The Illustrated Guide to Aerodynamics" by Hubert Skip Smith. Excellent conceptual explanations without the math.. Highly recommended..
@@villiamo3861 Good points!
The navy had some great demonstrations on lift. This wing that he is showing has such a high angle of attack that he is getting turbulence above the wing that it symbolizes a stall.
When I used to fly RC models, we took a wing on a 3 channel trainer and put it on backwards. It flew just fine
because wing was in correct aoa. backwards wing didn't make it broken, it's just make it with worse aerodynamic quality
You are 100% correct. When I hear people explain this wrong idea I ask them how is it that aerobatic planes can fly and their wings are symmetrical. And how can they fly upside down?
Aerobatic aircraft use engine power.
The angle of attach of a symmetric wing profile deflects air down, but that flight mechanism creates a lot of drag, which needs more engine power to overcome. Many aircraft can fly upside down, using the control surfaces to deflect the airflow - exactly the same forces as used to change direction, but if the control surface forces exceed the weight of the aircraft, it doesn't fall out of the sky whilst inverted.
So, angle of attack, and control surface inputs, which are completely different flight mechanisms from aerofoils.
@@andyowens5494 yes. Angle of attack is critical. Obviously aerobatic wings are not used on more conventional aircraft for that reason. They are too inefficient. But if the Bernoulli principle was the only thing working, this wouldn't work.
Symmetrical airfoils must always be at a positive angle of attack to produce lift, roughly +4 degrees for unaccelarated flight. Asymmetrical airfoil will still produce significant lift at even zero angle of attack and no lift at roughly -4 degrees under the same condition.
@@kevinbarry71 Thank you for pointing out the obvious issues of angle of attack and symmetrical profile wings. If you have ever "flown" your hand out a car window, angle of attack is readily apparent.
The angle of attack in the smoke demonstration appears to be much more than 4 degrees
Two other phenomena need mention: Vortex around the wing caused by viscosity, and simple flat plate lift. The wing's lift is actually a sum of those two, and the latter is still active even when the wing is stalled as long as its AOA remains positive.
The vortex isn't caused by viscosity, it is caused by the pressure differential, and around the wing tip is the only available route the air can move to try to equalise pressure, it can't move against its own flow upstream to come back around the leading edge, or around the trailing edge to do it.
You're confusing cause and mechanism.
It would be like saying a ball rolls down a hill because it is round, no it rolls down the hill because of gravity, the reason it CAN roll is because it is round.
The cause of the vortex is the pressure differential, the mechanism that allows it is the viscosity because without the viscosity it would immediately collapse, but again, that's not the cause.
@@LeoH3L1 Well, if you want to get really precise, then what's the cause of the pressure differential? The ultimate cause of the wing's lift is the force (thrust) pushing on the wing to accelerate it and then maintain its relative motion with respect to the air. Then there are other co-causes and proximate-intermediary causes-mechanisms. Without the pressure differential along with the air's viscosity (See Prandtl) we would have no vorticity around the wing and it's that vortex's interaction with the air flowing past and through it that's producing the lift, along with a portion (relatively small at low aoa) of lift produced by flat plate drag if angle of attack is positive. We also should mention the shape of the wing, specifically its rounded leading edge and sharp trailing edge. That shape, especially the sharp trailing edge that starts the vortex, also is a cause...can't make lift with a cylinder, unless the cylinder is spinning...Magnus effect used on some "sail" boats. Then there's angle-of-attack...zero angle of attack...zero lift...Want to cause lift, then make the aoa positive but less than 90 degrees. So, we should really say that lift is not caused by any single thing but by a number of factors working in concert, but still the ultimate cause is thrust (from a propulsion unit or from gravity)acting to move the wing relative to the air. So, using your ball rolling downhill analogy: Thrust is gravity. The shape of the airfoil, the vorticity of the air, the aoa of the airfoil...those are the ball's roundness.
@@LeoH3L1 see above response. Recirculation effect on lift was discovered at Boeing Aircraft Research under one Arvel Gentry. The reason for the 'recirculation flow' around a wing/foil/sail is the fundamental viscostiy of the moving fluid. See previous postinjg.
Thank you. This was a good explanation and properly addressed the "equal transit time" assertion, which is STILL incorrectly taught in many flight manuals...
its was good to see that smoke demonstration clearly showing the air over and under do not get to back of the wing at same time...I've always wondered how people just seemed to conclude it does
I’ve had this argument with CFIs and FAA examiners more than once. Myths are hard to overcome.
Yes.. And "equal transit time" is still the explanation written into many flight manuals..
CFI's should not be deficient in this fact! Really.
The angle of attack has an impact on the lift as well. It correlates to the under the wing lift.
There's no such thing as "under the wing lift" The lift a wing generates is the result of the pressure difference between the air over the wing and under the wing. By increasing the angle of attack you increase the pressure difference, which in turn results in more lift. That by the way also explains the wingtip vortices. Because all that is, is air trying to flow from the higher pressure area under the wing to the lower pressure area above the wing. At the inside of the wing generally the fuselage of the aircraft prevents this movement, but on the outside there's nothing that prevents this from happening if you don't add things like winglets.
@@shi01 Have you ever held a piece of flat cardboard (or something similar) against the wind? Tell us you feel nothing pushing against you. Its not "air trying to flow", its literally physical mass of air pushing you. So you can call it "under the wing lift", which it is. Same exact principle if I shot you with a water cannon, you'd go flying yourself momentarily, and not because the water is trying to go around you to reach lower pressure area lol. Air is a fluid too.
@@rykehuss3435 If it would be only reaction force, explain the stall effect. If you increase Aoa, yes the pressure under the wing will increase slightly, but the pressure over the wing drops even further. The the flow over the wing "stalls" the higher pressure under the wing isn't nearly big enough to provide any meaningful lift. The aircraft will drop like a stone regardless of it's speed.
I still don't get it.
There are 12 comments below mine, and they all mirror each other and what I would say. Danke!
I'm not sure there's anything else left to say. Science is a process, and you've done it well. As a person studying to be a CFI I think your material would be helpful to future students who care about HOW AND WHY things work.
Again, thank you.
Ehud Gavron
FAA Commercial Helicopter Pilot, Tucson Arizona US. Future CFI because I love to teach. You have helped me today!
In that particular demonstration, not only was equal transit time wrong, the air over the wing went even faster than equal transit time would suggest. Meaning that lift due to Bernouli's Principal is even greater than equal transit time would predict.
As a CFII and someone with an undergraduate degree in Aerospace engineering, the reason it is taught the way it is, is because the students seeking their pilots license would neither understand nor are they interested in fully learning how a wing generates lift. I know, I have tried. What an airfoil really does is to rotate the air in a clockwise manner using the diagram of the airfoil used in the video. This rotation accelerates the air above the airfoil, and retards the air below the airfoil. As mentioned, the total pressure around the airfoil is constant and the same. But with the higher airspeed above the wing, it has a higher dynamic pressure than below the wing, and therefore has a lower static pressure. Lift is generated due to the differences between these static pressures, multiplied by the surface area of the wing. Anything that rotates air, will generate lift.
Interesting, but incorrect. I am sitting in front of a fan to cool me, which is rotating air...and I assure you it is not producing "lift."
@@royshashibrock3990 - not that form of rotation. But nice try though. FYI, it is the type of air rotation which causes a golf ball to fly, and the Magnus effect.
@@HH-mw4sq Ahahahahahahaha 😂😂😂
Interesting, looks as if you were taught wrongly in your aerospace engineering degree.
@@deang5622 - how so? Please elaborate?
Having never heard that hypothesis that the top air would catch up to the bottom air, it was really easy for me to be skeptical about it. In my mind, the speeding up has always been about squeezing the flow to make a fluid go faster, and that applies to a single flow as well. I have no reason to believe there is much of any interaction between the streams once they split between above and below the wing. Anyway, great exploration of the topic.
Excellent video, Mrl Magnar! I learned a lot including how many misconceptions I had! Thanks!
Thank you for finally stating this. The pressure under the wing is FAR greater than the negative pressure about the wing. If the wing was nothing but a flat surface, it would still work just fine.
Indeed this has been my intuitive thought for 2 decades, but scientists always talking to the wing lift due to lower pressure at the top always bothered me. If there is more pressure at the bottom, its enough to cause lift.
when you hold you hand out of the car slightly tiling up wards, you feel the lift, and the wind pressure on the underside or inside of your hand, a lot more then the any pull force you feel at the top of your hand.
@@Quraishy The shape is more to avoid stall:)
Hi Magnar. Many have an issue with Bernoulli, and they are mistaken. As long as you are outside the boundary layer Bernoulli's principle applies. In fact, when most engineers use the pressure coefficient it is directly related to Bernoulli, as the ratio of the change in static pressure to the dynamic pressure. The ETT was initially a hypothesis of D'Alembert, and is a result of potential flow, the first real attempt to apply Newton's laws of motion to a fluid. In this situation the curvature of the streamlines at the training edge and leading edge are symmetric, and you get no resultant lift force. As such, saying that the curvature is responsible for the acceleration (while true), neglects the resolution to D'Alembert's paradox which resulted because viscosity was not understood until Navier and Stokes 100 years later. So, the asymmetric acceleration around an aerofoil is due to viscosity. There are two specific effects, the Kutta condition, which moves the rear stagnation point to the TE. and the induction of more flow upwards ahead of the wing. The end result is an asymmetric velocity of the flow (circulation) which at the surface of the wing is given as a pressure force, which will also be asymmetric, with lower pressure above and relatively speaking higher pressure below.
Thank you for your contribution!
I'm telling this since decades. Thank you not only to have told, but also to have shown it in practice ❤
The upper surface produces most of it, but "downwash" glancing off the underside also seems to aid/add. Otherwise, helicopter landing zones would not be so breezy. But wait... attach a 25 foot diameter (~8 meter) pan under the helicopter. Is it still able to fly with all the downwash hitting on itself?
@@cosmicraysshotsintothelight If the reaction force off of the blades is greater than the reaction force off of the pan the brick will fly.
I'm not sure what's being said here. It seems to be a criticism of pilot education, but what is the specific criticism? Bernoulli only holds true when there's non-turbulent flow and when laminar flow starts to break down the forces on a wing (or anything else for that matter) become a far more complicated proposition. It's fine to tell pilots about lift generated by laminar flow around an aerofoil but I'd be surprised if they don't cotton on very quickly, maybe around the time of their first stall, that there's a lot more to the fluid dynamics of lift than just what Bernoulli said.
At 2.39 notice how the air is being forced downwards. The lift can can also be determined using this. A wing works in the same as a propeller. Stand behind a propeller at full thrust and you will get the idea.
Static thrust is a special case. I had not seen that slow motion before. I only saw the old spark stop motion. That explained turbulent flow.
I think the explanation you are looking for is Newton's Third Law of motion.
Mark, I was going to make the same point you make, except with a helicopter "Rotary Wing". A helicopter rotor is also shaped the same way as an airplane wing. Its motion with rotor angle of attack create a tremendous movement of air downward. It is F=MA. The Mass of the air times the Acceleration of that air creates the Force upward, which lifts the copter. It is "For every Force, there is an equal and opposite Reaction." Air downward/Helicopter upward. Same with an airplane wing. Thank you for mentioning your point about the propeller. Propellers also have the same airfoil shape. I agree with you 100%.
A wing doesn't work the same way as a propellor but most rotors work the same way as a wing, at least to some extent.
@@mikekelly5869 They all work the same way.. We just view the effect differently. With a fan or propeller stationary we feel the blast of air because the blade is drawing in air one blade at a time in the same place. When the airplane is in motion the blade describes a spiral. The British word for propeller is Aero Screw. Many propellers have Clark Y airfoil. I never worked on helicopters so do not have a full understanding. The larger the rotor "disk" the more it can lift. The same as wing span. It all has to do with Mass Airflow.
Now I’ve retired, it’s fantastic to know after 35 years and 20,000 hours of flying jets (mostly in command) without accident or incident. Is this an example of what they call a ”firm grasp of the non essentials”?
I love that statement. :-) As a 40 year flyer - of meager hours - I honestly have always been in awe of you, the airline pilots, with 20 plus thousands of hours.
As a student pilot in 1980, we students thought that a guy with 250hrs was a GOD! True. :-)
I know you are still flying on the weekends. God bless you - you ARE "da man." Thanks. N6395T (but the Piper Arrow was my favorite - until the wings started falling off. . :-)
Well I'm glad your comment was 99% about how wonderful you are a wonderful your experiences are and kind of 1% of a dig at the contents of the video. Understanding anything is not important for a pilot it's all monkey see monkey do train responses regulated you don't have to understand anything you just kind of point it where the instructor told you and do what the instructor told you to do and that's how it works for you. Then arrogant pilot that you are you call this a non-essential. This is essential and it's a fundamentally simple essential thing for a person who understands airplanes enough to design one for you to fly. I had a pilot friend like you one time I found a book about engineering and airplane design in the thrift store. I told my pilot friend that if an f-15 tomcat had a thousand less horsepower The minimum turn radius at 300 miles an hour went up by 50%. He was sure his pilot experience made him absolutely correct about airplanes and me not owning an airplane made me incorrect. Next time I saw him I got out the textbook from the college and I showed him the f-15 problem and I walked him through those calculations and I said imagine that. I didn't write this book but I understand it do you understand it now?
@@markmcgoveran6811 Was that "put down" comment for me? I was just commenting on the Airline Pilot's exploits. Quite a career, IMHO. My meager manipulation of the controls was given as a "contrast" to his exploits. How do you fit in? Thanks for commenting. p.s. I thought we had just about beat Mr. Bernoulli to death.
@@mmichaeldonavon not really a put down. Everybody needs a different version of an airplane. The pilot needs one thing the engineer needs another. It's a very handy thing to grab as big a piece of knowledge in your version of an airplane or anything else. Bernoulli may have been beat to death for you because you lack mathematical sophistication. A big airplane manufacturer will cut out an airfoil. They fly it in a wind tunnel and they take a lot of measurements wetted area velocity direction I mean they do a lot of measuring. Then these measurements are sealed in a vault and our top secret no one can see them. Then some extremely powerful mathematicians compete at this it's called a benchmark. They use Bernoulli's principle and partial differentiation differential equations and a bunch of other miserable math stuff and they predict the behavior of the air flowing around the airfoil and the forces generated by the airfoil. Of course you just look on a chart and it tells you everything you need to know about launching it at altitude landing in an altitude loads under certain altitudes. That was written by somebody who's very well versed in Bernoulli's principle. Did you use a checklist when you were a pilot? When the first multi engine bombers came out pilots flew up in the air and crashed on takeoff the airplane was worthless, the engineers are idiots they build something that can't fly. The engineer said you guys can't remember everything you need to do to launch this multi-engine aircraft. Here's a checklist. The pilots didn't think they needed a checklist. The general thought they needed a checklist and ordered the pilots to roll down the checklist every time they launched a multi-engine airplane and they quit crashing on takeoff.
@@markmcgoveran6811 Thank you for your in-depth comments. I really liked your comment that: "... because you lack mathematical sophistication." I'll bet you are fun at parties. Thanks. E=Mc2
When I was doing my physics teachers degree I heard my teacher say that the air particles reached the end at the same time. I immediately knew this was wrong and exclamated: "Why, they don't have a date, do they?" I still have much respect for that teacher but that day learned me never trust anybodies word for it.
Thank you sir!
As an Aerospace Enginnering student, in 1976, and a student pilot in 1980 I was tought this principle, exactly the way you explain it and show it.
What has been happening to teaching this principle since those old times?
That is a great demonstration. The 'meets at the tail of the wing at the same time' explanation has always troubled me. Another thing that nobody seems to consider is the change in direction. Clearly the air leaving the tail side is moving downward to some degree and that means a force was applied by the wing.
Why would that mean a force was applied by the wing? Wouldn't that have something to do with the fact that the bottom air is lagging behind the upper air? The downward movement of air is more about induced drag I believe.
@@blusheep2 Well from a simplistic analysis, let's assume at first the air was not moving (assuming no wind at all), and then the air is accelerated downward as the wing slices through it. The underside of the wing is at a slight angle and deflects some air downward (angle of attack). The air over the wing flows over the curved surface and exits the back side in a smooth flow line along that surface (assuming the angle of attack is not so steep that the wing 'stalls'), which is slanting downward even further. Sure, AFTER the wing passes there are all sorts of eddys / whorls and turbulence in the air. But before that the air is being accelerated downward as the wing slices through. Hence, I believe, the wing is acting to push the air downward and an opposite reaction force pushing the wing up.
@@mikefochtman7164 OK, I see. The deflection of the bottom air down demonstrates an earlier force on the bottom of the wing. Its a Newton's 3rd law thing as opposed to a Bernoullie principle thing.
@@mikefochtman7164 Absolutely this!
Treat the wing as a black box. Before the wing, the air is static, after the wing the air is moving downwards. Whatever accelerates that ir must exert a force on it and experience an equal and opposite force.
Only if you need to design a wing and present its characteristics do you need to know HOW it works. Which makes me wonder, why teach pilots this anyway? There are shed loads of systems on aircraft that pilots, aren't taught.
@@blusheep2 Exactly. Coanda effect due to surface resistance slows the layer in immediate contact with the wing and causes the air avove to curve downwards due to velocity transfer to the slowed air below
Hi Magnar, I am a PPL but more important for this I am an Aeronautical Engineer and also was an Aerodynamics teacher at college.
While it is true that Bernoulli's principle applies only to one flow line, it can be also applied to two (or more) flow lines if there is a point where the energy state (speed and pressure) was the same in both flow lines. Sufficiently ahead of the wing, the parcels of air that are going to flow just above the wing and the ones that are going to go just below have the same pressure and speed. So you CAN apply Bernoulli's principle between a point above of the airfoil and another below. Still, transit time is wrong so you can't deduct the speed just by the differences in length. How lift is generated is at the same time more simple, more complicated, and more disappointing (or unsatisfactory explanation) than most people think. Let me know if you want me to expand.
Thank you for your feedback. Lift can be both easy and complicated to explain. I understand and agree with what your wrote. In this video, I just wanted to address a common misconception.
Finally I stumble across some making some sense!
A question or two if you don't mind:
How would you explain to someone who insists that Bernoulli only works for a flow contained by an enclosre such as a pipe, that in fact any parcel within a flow also behaves according to Bernoulli and therefore it also applies to a non-enclosed object such as a wing?
I assume, conceptually it is the interface between parcels that causes the misunderstanding.
And for sails and thin wings which can be considered infinitely thin, how do you deal with the issue of path length of the sold being identical on each side: I assume the effective actual path length is different because the total flow on each side has an average center of flow extended out some distance perpendicular to the surface?
@@davetime5234 ... Let's go point by point:
1) "How would you explain [this] to someone who insists that Bernoulli only works for a flow contained by an enclosure such as a pipe". That's a strange question. Bernoulli is applicable only when several ideal conditions are met, one of which is NOT that the fluid must be contained in an enclosure. The total set of conditions can be encapsulated in 2: "Potential flow" (that is steady flow of an non-viscous fluid, without any sources, sinks or rotors) and "Between any 2 points of the same streamline" (the airflow around a wing, outside of the boundary layer, is a very good approximation to that). So how do you explain Gravity to someone who insists that it only works in Venus? Well, I suppose you tell them they are wrong, that that's not a condition of the theory. That said.... The streamlines of a steady flow, being fixed in space and tangent to the velocity vector on each point, are never "crossed" by parcels of fluid and hence they can be considered as a pipe. But that's more the reason why Bernoulli works in a pipe, rather than why it works around a non-enclosed object.
2) "And for sails and thin wings, how do you deal with the issue of path length being identical on each side". Simple: You don't deal with the issue because there is no issue to be dealt with. To generate lift, there is no requirement that the path on one side is longer than on the other side. What you said sounds like "equal transit time theory", which states that the path along the upper side of the airfoil is longer than over the bottom surface because the path along the upper side is longer. The problem is that theory is totally wrong. Why? 2 reasons: Frist, there is no reason why one would expect it to be right. Imagine the following situation. To cars going on the same highway pass at the same time in front of a Shell gas station. Some points later both cars take different roads, the red car taking a much longer route than the blue car. The roads eventually rejoin, and each car eventually passes in front of a certain Exxon gas station. Would it make any sense to say that the red car had to go faster because it took the longer road? NO! There is no reason to suppose that. UNLESS both cars pass in front of the Exxon gas station AT THE SAME TIME. But why would that be the case? There is an implicit assumption in the equal-transit-time theory, which is that 2 parcels of air that are adjacent ahead of the wing, with one passing above and one below the wing, will rejoin at the trailing edge. But there is no reason why that should be the case. Imagine that I have a hose that forks and after the fork you have one 1 ft hose and one 10 ft hose, and you put the open end of these 2 branches side to side (after the long hose doing a couple of loops). Would you expect that 2 molecules of water separated at the fork will reach their openings together? And this takes us to the second reason why it is false: Because it is demonstrably false! If you calculate the lift based on equal transit time you get a value much lower than the actual lift. And experiments show that the air flowing along the upper side reaches the trailing edge FIRST (i.e. it wins the race against the air flowing along the lower side), DESPITE taking a longer path. It goes much faster than which equal transit would require. Parcels of air separated at the trailing edge never meet again. So why does the air along the top goes faster? Circulation. But I digress.
@@adb012 Thank you so much for your reply!
On the two points:
1)I think the solid enclosure comes up as an issue with Bernoulli, because it is shown and explained in the context of such a solid macroscopic enclosure in nearly every introductory explanation.
So, people are presumably left with the notion that you need some solid constriction to contain the conservation of energy swap between dynamic and static pressure as the flow progresses.
In terms of generalizing the concept beyond the introduction, for a parcel along a streamline, one perhaps worries about the continual transit of mass across the parcel boundaries (random diffusion for example) to adjacent parcels - no rigid pipe walls anymore now that that conceptual device is removed.
I assume the boundary interface states (between parcels) are what carry the analogy forward: defining static, and dynamic pressure differences etc.?
In other words, the concept leap is that all you need is a conceptual enclosure, specifying the appropriate interface variables? (not sure if I explained well what I was thinking..)
2)Equal transit time being wrong seems so obvious, no argument on that at all.
However, isn't the faster transit time due to conservation of momentum and continuity of mass flow rate?
M x V to be conserved, requires V to be as fast as necessary to keep M x V constant (ignoring "momentum leaks").
So, a path length disruption imposed, such as camber, increases V as dictated by the imposed geometry (as a key dependent variable), as momentum seeks be preserved?
And that's why it's not equal transit time, it's momentum conservation and continuity of mass flow rate, that are driving the higher transit speed?
Is the above false? Because if it's not false, then it seems a sail may need different path flow geometries on opposite sides of the thin sail, in order to generate the pressure differential (static pressure drop, due to conservation of energy, in order to service the increased V required to maintain the lateral momentum)?
I mean, after all, a parcel approaching the sail, that gets split into the two different paths around the sail, starts off at a homogenous condition: the split parcels have equal dynamic and static pressure, and temper etc.
So, the sail is, in effect operating on these identical twin parcels, differently? With the differing geometries of the two different paths affecting the identical twin momentums differently, causing one to have a different static/dynamic pressure swapping experience than the other?
@@davetime5234
1) A parcel of air doesn't mix with other parcels of airs by definition. Remember a parcel of air is an infinitesimally small volume of air (that measures let's say dx by dy by dz). It may interact with other particles via action/reaction forces of 2 types: pressure and viscous. But Bernoulli assumes no viscosity (otherwise mechanical energy would not be conserved).
2) You lost me there. Momentum is not conserved. I mean, it is only conserved when there are no external forces applied. But a parcel of air is exposed to external net forces everywhere because it is in a non-homogeneous pressure field. Take a Venturi tube. A parcel of air will have the same mass (and volume if we assume incompressibility) in the wide part and in the narrow part. In the narrow part the parcel will be narrower and longer, but will have the same mass. And it will be going faster. So M x V was not conserved. Which makes sense because it was moving from a zone of high pressure (wide section) to a zone of lower pressure (narrow section) so it was moving along a pressure gradient, which makes work (conservative work, but conservative forces may not change the mechanical energy of a system but absolutely change its momentum).
What I am going to say next doesn't satisfy anybody, but it is what things is: Ina wing, the air flowing above the wing speeds up and reduces its pressure by Bernoulli, and the air flowing under the wing slows down and increases its pressure by Bernoulli too. Note that I said AND and not BECAUSE. It is tempting to say that the reduction in pressure in the top is due to the increase in speed, but when you ask why it increases its speed it's because it moves from a zone of normal pressure (way ahead of the wing) to a zone of low pressure (above the wing). But it cannot be the case that the reduction in pressure is due to the increase in speed which is due to the reduction in pressure. That's circular reasoning. The reduction in pressure and increase ins peed (or the opposite on the lower side) just coexist. Why? Because it is the only thing that they can do to meet the boundary conditions (the flow shall not penetrate the wing) and the Kutta condition (the air shall separate at the trailing edge). Actually it is the circulation, described via the Kutta condition, which is the real "cause" of the lift (if you are desperate to find a "cause"). And don't let the thin sail fool you. The symmetry you try to present between the parcels flowing above and under doesn't exist. The sail will be curved in ONE direction relative to the free airstream, and the angle of attack will be angled in ONE direction relative to the free airstream. Imagine that you and your identical twin are standing in the subway and grabbing one of the vertical poles. Say that the vertical pole is in the middle of the cart, and you are facing forward, just to the left of the pole, grabbing the pole with your right hand. And your twin is just to the right of the pole grabbing it with his left hand. The subway takes right curve. You will apply a pull force on the pole, and your twin will apply a push force on the road. BOTH TO THE LEFT!!! There is no symmetry. A true symmetric condition would be an airfoil at zero AoA, with no camber and with an even distribution of thickness above and below (or a flat straight sail at no AoA). And guess what? That doesn't produce lift.
Nothing can be accelerated instantly. Because it has inertia.
That is why air molecules are literally forced apart at the top faster than it can accelerate toward the wing,
becoming less dense (lower pressure), by a wing at speed.
The air at the bottom of a wing is forced by the wing so fast it becomes compressed faster than it can accelerate away from the wing.
(Higher pressure)
A great demo. It has always bugged me how people parrot this lift principle, as if it's very simple, without really thinking about it, and without understanding Bernouilli's principle.
Great video, but I would like to say that while he does mention the lift being generated by the airflow under the wing, he skips over the reason why. It is because any time a fluid is forced to change direction (the wing's angle of attack causes a downward deflection of the air striking the bottom surface), energy in the fluid is given up, and this energy manifests itself as an increase in pressure on the deflecting surface.
Something interesting to note is that while the low pressure area on top of the airfoil is more or less dependent on the airfoil, and therefore stays in the area with the most curve, this is not true of the pressure being applied on the bottom of the wing, which is quite erratic. This is why an airfoil that stalls due to excessive angle of attack becomes unstable, and why "flat" wings will never work, but symmetrical wings will.
I totally agree!
The most pressure change is on the top of the wing, not under, on a high speed low AOA condition, the button of the wing could be lower pressure than the surrounding as well, just not as low as the top.
Flat wing totally works, I have made dozen of them out of KT board, and you probably have done so with paper.
I'm sorry, but I disagree with you. The most pressure change happens at high AOA where the stagnation point is below the leading edge of the wing. This forces the air to follow a larger curvature, which causes a larger acceleration and hence, a larger pressure drop. Maximum lift coefficient is achieved at critical AOA. For a reference, please read "Aerodynamics" by L. J. Clancy, pages 62-63. It can be downloaded here: www.scribd.com/document/321464060/Aerodynamics-Clancy-pdf
Yes, Newton's Third Law. Air mass displaced downwards is counteracting the weight of the aircraft.
@@FlywithMagnar Maybe I didn't make it clear that when I say the change I mean the absolute value of the difference between ambient pressure and local pressure, and the picture on p63 shows exactly that, on normal AOA, the upper "suction" is more than the under "pushing".
ofc no wing IRL have only upper or under part, but when it comes to what surface is more impotent to keep clean if you have to place things like engines or flaps guide rail or even ice, most of the time is the upper surface to keep clean
I flew jet airliners for 30 years. What maters is dont crash. You dont have to understand any of this to fly safely.
Then, why do you have to learn it? And wouldn't you like to know correct information?
I met an airline pilot who told me that flying a jet is like "monkey sees, monkey does". Now I understand what he means.
Correct, pilot do their jobs, engineer do their jobs. There will be no pilot if no one built airplane.
To hear this is most troubling...since you say you "flew" I assume you no longer do so, which is good. When things go wrong, your knowledge of what you have to work with (the mechanical device you are encased in) may save the day (and the lives of many innocent people). I shudder to think that I am flying on a plane with a pilot that has an attitude such as yours.
@@royshashibrock3990 This particular piece of knowledge has absolutely zero value in problem-solving any normal or emergency situation. A good understanding of the AOA vs CL curve, OTOH...
Thank you. Beautifully demonstrated!
Why can't people get this right?
1. Bernoulli is only valid along a stream tube in an inviscid flow. The inviscid flow part is usually pretty valid for high Reynolds number flows outside of the boundary layer and shear layer trailing the wing. The stream tube is much more restrictive except when you assume that the far field has a uniform velocity and static pressure. Under this assumption (which is usually pretty good), all fluid starts with the same total pressure and thus has the same total pressure around the wing. Your statement about how Bernoulli isn't valid because the wing divides the airflow into two parts is wrong.
2. Curvature in the wing is important for keeping the adverse pressure gradient under control and keeping flow attached at high angles of attack, but it still misses the point. Just as the symmetric airfoil befuddles the equal transit time nonsense, the flat plate airfoil (commonly found on small RC foam models) and the supersonic diamond airfoil (with a sharp leading edge) befuddle curvature as the source of lift.
IMHO, the best conceptual explanations don't even mention Bernoulli, but instead focus on what the forces are doing to individual masses of fluid. Air is under pressure and will expand when given the chance. As it passes over the top of the wing and the upper surface deflects down, air accelerates into what would otherwise be a void behind the wing. This is accomplished through a vertical pressure gradient resulting in reduced pressure on the upper surface. However, pressure is a scalar and the reduced pressure also corresponds to a horizontal pressure gradient which first acts to speed up and then slow down the flow. At the same time, the bottom of the airfoil has to deflect flow away from it which has the opposite effect and produces a high pressure region which slows flow down and then speeds it up. This is why the transit time over the suction side is faster than the transit time over the pressure side.
The exact geometry determines where the peak pressures occur. On most subsonic airfoils, fluid begins to be deflected from the pressure side to the suction side a short distance ahead of the airfoil. Because it already has an "up" component (or down component for negative lift), it has to expand around part of the leading edge resulting in much higher normal accelerations (from the high curvature) and lower pressures which move the location of the minimum pressure forward. In a completely inviscid flow, this perfectly balances out the normal force experienced over most of the wing (which is tilted back towards the trailing edge) and results in no drag. In a real flow, there is some pressure drag, but most drag comes from viscous forces in the boundary layer.
I suspect someone will mention circulation. There are enough people who view circulation as fundamental that I won't completely dismiss it, but I personally view it as a convenient mathematical relationship derived from complex analysis (one derivation literally just uses a conformal map of a complex valued function) that is largely divorced from what is actually happening.
When i was told the faster air stream at the top meet the slower air at the bottom on trailing edge, i was confused, thinking the faster air must be waiting for the slower one at the end....until you come along saying it is just a hypothesis , not a proven theory. I 'm feeling relief. 🤗
As an Instructor Pilot for 20 yrs I couldn't agree more. Very misunderstood concept. Bravo!
Lift is Bernoulli living on top of the wing while Newton lives on the bottom. Two separate operations but both must work together or....
Bernulli, on it's own, would never make enough lift for flight.
Wow, this is one of the most beautiful videos I’ve seen! Thank you.
[Another perspective] Here is how I see it, the upper part of the aerofoil has a longer distance as it is curved, the lower part has a shorter distance. Now let's stream an imaginary 100 molecules of air to the tip of the aerofoil, 50 molecules go up and 50 go down. The shorter part (lower) has more molecules per distance than the upper part where the molecules are less dense. Now we know denser particle have more pressure compared to lower, this is why the molecules below try to push upward, Hence, lift. I completely agree with everything else.
Great explanation, I like the intermittent smoke air flow
Civil Engineer here. Lift has always conceptually made sense from observation. But, being familiar with hydraulic behaviors, the explanations by PPL instructors and videos have always left me incredibly dissatisfied. Thank you for correctly explaining this concept. It's unnerving to see the perpetuation of such a large fallacy!
Using Bernoulli to explain wing lift is only partially correct but is easy for students to understand. A flat plate will develop lift as its incidence is increased, it's just that it's not very efficient and stalls at low incidence. A sailboat sail develops lift even though it has no thickness (between upper and lower surfaces), only camber. It's very easy to do a calculation of the pressure difference created by a notional aerofoil in accordance with Bernoulli but you will soon find out that the lift generated is nowhere near that actually generated by a real wing.
The full description of the generation of lift is best explained by newton's Third Law. Lift is the reaction to the motion of the wing deflecting the air downwards as it passes through it. Lift is equal and opposite to the vertical component of the algebraic sum of the rate of change of momentum of all the air as the aircraft passes through it. In the horizontal direction it is induced drag.
Glad to hear you explain the real science behind lift. In the early70's I was taught in junior high science that lift was not created by increase pressure under the wing but decreased pressure above it. I was always interested in flying but this explanation always left a question in my mind because it didn't make sense that enough negative pressure alone could create enough lift. With my education with internal combustion engines, hydraulics and other machines operating on the laws of physics I began to realize that it is the pressure differential that causes lift. Lower pressure above and higher pressure below do to Bernoulli's law creating differential pressure is the cause of lift.
What is also importent to know though, Bernoulli alone does only explain why the pressure drops over the wing. But another interesting question is why does the air follow the upper wing profile. Why doesn't it simply get pushed aside by the leading edge and create a turbulent void? And that's where the coanda effect comes in.
The decreased pressure on the top side is still of a greater magnitude than the increased pressure on the bottom. I'm no aerodynamicist but I have seen quite a few diagrams of wing profiles showing the pressure gradients along the surfaces (granted it is for racecar wings, so the airfoils are inverted compared to an airplane, but the idea is the same).
Thank you for this video, I like the way you explained this concept, can you please expand upon this and how this applies to exceeding the critical angle of attack which causes a stall, more specifically why does the lift drop so rapidly. Thank you! Waiting for your next video :)
Thank you. Like so many others I had to "learn" the wrong explanation for lift when I learned to fly. It's good to hear someone debunking it.
there nothing wrong with this explanation, it's just not complete
@@einherz In electronics (my field) plumbing analogies are often used as teaching aids. They work fine until they don't. The model of the atom taught in school physics is utterly wrong but conceptually useful - until it isn't. I suspect that the newton / bernoulli lift contribution ratio and their respective real world inticasies are revealed on a need-to-know basis (so to speak).
I have commercial pilot friends who claim that it's 2/3 bernoulli and 1/3 newton, but a quantum physicist would say that it's ultimately neither of these things!:)
@@nikthefix8918 sure it's all about wing form direction aoa. some forms will use more time bernoulli, some forms - newton. but both will used all time, even flat wing with 90* aoa will forced by bernoulli too, same as laminar symmetric wing at 0* aoa will use newton force too. flight is dynamic aircraft is dynamic, air is dynamic environment. engine above wing more bernoulli, engine under wing - less bernoulli, but if imagine all airflows around there newton and bernoulli everywhere:)
This has created great discussion! The wind tunnel test could do with more examples. For example an asymmetric wing and different AOA. More importantly 'zooming out' and visualisation of air much higher and lower from the wing. The example shown seems to result in the upper surface air (close to the wing) having the same speed as the air above. The lower surface in contrast, has markedly slower air (close to the wing) than the air below. In this specific example you could argue that lift is caused by high pressure below the wing and not low pressure above. It does easily disprove the equal transit time hypothesis however.
Good explanation. I knew Bernoulli's didn't explain it all. RC pilots have made flat boards fly (not efficient, but they fly)
Nice experiment! Thanks!
The air at the leading edge is not accelerated directly by the curve, the bottom is also curved and there the air decelerates. It is accelerated by a difference in pressure(force), the pressure gradient is created by the overall wing profile. Even adding a tiny 90 degree flap to the trailing edge will change the gradient and flow near the leading edge.
Tell that Bernoulli bloke I'm fed up seeing his crappy fan heater ads.
Delighted that Magnar clearly lays out that "theory" and "hypothesis" are not the same thing.
The same thing that causes the drag causes the lift, the wing vortex. It produces upwash at the tip, lowering lift, because it blocks the suction surface, but when it reverses direction it produces downwash which then blocks and slows flow on the pressure surface side, and draws air across the suction surface accelerating it. Winglets just move the tip vortex exposing more surface area to air flow, allowing slightly lower AOA, thus improving fuel efficiency. The vortex strength remains the same, ie proportional to lift. Extending flaps just increases and strengthens the vortex sheet. There's a reason Prandtl equated downwash with lift.
The wing tip vortex is an unwanted side-effect of lift. The swirl does not contribute to lift, but is known as induced drag. To reduce the vortex, the designers can increase the aspect ratio of the wing (gliders are good examples.) Another technique is to taper the wing towards the tip. Winglets reduce the vortex, and hence drag, especially at high angles of attack. When Prandtl equated downwash with lift, he ment the downwash inboard of the vortex. He also concluded that the most effective lift distribution is bell-shaped. You can learn more about Prandtl and how NASA developed the Prandtl wing here: Al Bowers - Prandtl wing update: th-cam.com/video/w-dk1NpVNNI/w-d-xo.html
Spot on and well done. Thanks greatly.
I remember going to a Physics teachers workshop and Bernoulli's Principle came up. So, I asked a few Physics Teachers how it worked. I got the standard answer you heard here; pressure is reduced. When I probed further and asked about the actual physical phenomenon causing this drop in pressure, I got one of two answers, "I don't know," or "because Bernoulli's Principle says so."
Teaching Physics is about imparting an understanding about how the physical world operates, not teaching students to memorize laws and equations with no understanding about the underlying phenomena. So, how does Bernoulli's Principle work, in other words, why is the pressure reduced on a surface when air flows over it?
Air pressure is the result of molecules of air molecules impacting the surface. Air pressure decreases for two reasons, fewer molecules strike the surface, and the speed with which they strike the surface is reduced. If air is flowing over the surface, fewer molecules will strike the surface because they are being dragged along the surface by the air stream flowing over the surface. Greater speed of the air stream, results in fewer molecules striking the surface resulting in lower pressure. That's it, it is no more complicated than that.
You are absolutely right!
The Bernoulli effect arguments the lift created by an airfoil. A flat wing, or fully symmetrical airfoil, or semi symmetrical airfoil will all generate lift due to dynamic forces striking the bottom of the wing. The higher the angle of incidence and/or angle of attack (to the point of stall), the higher that dynamic force and subsequent lift.
At an equivalent angle of attack, a semi symmetrical airfoil will be more efficient as it will generate lift both from dynamic forces and Bernoulli effect. Hold a piece of paper by the edges and blow over the top of it. The paper will lift up because of Bernoulli forces….no dynamic forces present.
The wind tunnel test in this video is interesting. Thanks for posting it.
I did watched all your videos all your videos about secret of lift. Great explanation ! My favourite videos which I watched is from professor Alexander Lippisch. Can I post a link for TH-cam video here ?
I believe this misconception comes from the effects of a venturi. In the Pilots Handbook of Aeronautical Knowledge, concerning explaining Bernoulli's principle, it says that for a venturi tube "The mass of the air entering the tube must exactly equal the mass exiting the tube. At the constriction, the speed must also increase to allow the same amount of air to pass in the same amount of time as in all other parts of the tube." In a constricted environment like the tube, this would apply, however in the open space an airfoil operates in, this is not the case and thus two particles flowing over and under an airfoil do not meet at the same time.
I used to be a flying instructor in the late 80's. We would explain this to our students and the easy way to confirm it was the propellor wash you could feel behind an aircraft. If Bernoullie was the reason for lift a propellor would produce no wash and a helicopter would not create any downwash. (That would be nice - I flew rescue helicopters for years and the downwash was always an issue during a winch rescue)
Last century, this was the way FAA taught it. The "FAA correct" answer on the written exam was Bernouli's theory. Even though it was wrong, that was the answer to pass the exam.
As a physicist, I have the impression Bernoulli's principle does not even apply to the problem, it is merely a side effect: That is because it relates a pressure gradient along the flow line with the acceleration along said flow line. However, the problem of lift is about a pressure gradient, and hence acceleration, perpendicular to the flow line. This has to do with he curvature of the flow, not it's speed.
The wings are pulled up mainly because the wing is tilted up in front. The air hitting under the tilted wing is lifting up the wing. Play with your palm pulled out of a running car. Tilt the palm. The lift of the palm is because the air hiting under. The Bernoully applies too, but the main force is from under the wing. Increasing the tilt will increase the loft force until at near 90 degrees there will be no lifting force.
I always thought it was reaction lift when taking off and the Coanda effect during normal flight. Schools still teach nonsense about flight as far as I am aware.
As a trained Aeronautical Engineer, I have always had a problem with this way of describing lift. For one, the air is still until a plane moves through and smacks it. What’s generating lift is a moving baseball bat (wing) hitting ground balls (air molecules), forcing the bat to rise. The angle of attack is what ensures when the air is hit (high pressure on the bottom) it travels down instead of upward. And negative AOA does the opposite. It is that simple. A perfect example is a waterskier. He hits the water forcing it downward and him upward = lift. A fan blade/wing is another obvious example of whacking the air producing a base hit. Once that air molecule is walloped, a lower pressure flow whooshes in to take its place.
I have been an instructor for about 30 years. I have argued your point with aeronautical engineers for years. If each particle at the front had to get back with its respective particle at the end then how would we be able to stir our soup? Each pea that has a neighbor pea would have to get back with its neighbor pea after traveling around the spoon?
Good point!
the air did move faster over the top of the wing and had lower pressure, so the same principle as a sail boat.
nonsense. A sail has no thickness and has exactly equal lengths on each side.
Nice explanation.
3:09 I think it's beautiful to see that the first flowline under the wing gets partially sucked up even ABOVE the wing because of the lower pressure there.
Good explanation. Especially at 3:04 where the velocity of air over the wing is shown.
Someone should make that venturi apparatus with only one side pinched to model air flow over an airfoil. * Of course now you have the problem of showing the tube of liquid which shows the pressure differential. But this isn't difficult to over come. *The demonstrator as it now is shows two wings mirrored with the liquid filled tubes modeling the pressure above the air foil against the pressure in front of the 'wing'. You actually want three measuring tubes. One on the 'airfoil' (at the venturi), one before the venturi and the third under the 'wing'. These could work if you just have a reservoir of liquid feeding all three tubes, then each tube would show the air pressure before the wing, on top of the wing and under the wing (which might be made to show how angle of attack increases air pressure.)
That makes so much sense in contrary what we see always explained the wrong way. This is a great video, thank you.
So why do you believe THIS video but not others? I find it interesting.
@@thomasmaughan4798 Because the other more common explanation never resonated with me and I always felt I did not understand it. With this explanation I feel like I understand the physics behind it. My back ground is in the semiconductor industry, so I may be a layman at fluid-dynamics but from a technical perspective.
@@hugobloemers4425 I admit that the usual explanations of Bernoulli effect are incomplete and don't really explain the region of low pressure directly above the wing; the missing element is *inertia* . Air, being viscous, has inertia. So the leading edge rams into the air in a non-symmetrical way. Under the leading edge is relatively flat; the air gets sliced. The part above the leading edge is rounded, but in a very specific way. A short radius, sharp rounding starts upward and the air has high pressure and low velocity. As the air starts to gain vertical velocity it reduces its pressure in the immediate vicinity, the *venturi effect* but since it is close to the surface of the wing, this venturi effect causes the air to cling to, and follow, the curvature. It has inertia and it is that inertia that is trying to pull the airflow away from the wing; but the venturi effect is creating suction. The air is rapidly gaining velocity, which lowers the pressure even more, and because of this increase in velocity, you simply cannot curve the wing as much or it will detach from the suction with a lot of turbulence. That's called a "stall" if this airflow has such inertia that it overcomes the venturi effect; the suction is not sufficient to keep the airflow attached to the wing.
And finally, you let the airflow gradually rejoin the airflow that went under the wing. This reduces or eliminates turbulence and the energy robbing effect of turbulence.
At the most efficient, you don't need an engine at all; gliders in other words, and they operate entirely on the Bernoulli principle and keep their airfoils straight into the wind. Getting a glider down can sometimes be a challenge.
Summarize: The wing does not know or care what the wind is doing. It knows only that the air pressure under the wing is higher than the air pressure above the wing, and while many approaches exist to make it so, a wing with a rounded upper surface will cause air to try to pull away from it creating a suction, and that's where you get lift. If the wing was simply convex, then you would have an area of pushing down, and an area of lift, more or less cancel. So the abrupt convex area and long gentle curve after that eliminates the down-pushing aspect; the drag vector is to the rear instead of down. The engine is overcoming forward drag and not much needed for staying in the air. (and a glider has no engine).
A helicopter in hover, meanwhile, uses enormous power just to stay in the air, but even then, the blades are shaped as airfoils to increase efficiency and stability.
@@thomasmaughan4798 Thanks for taking the time to write this reply :)
The main reason of lift generation is down deflection of flow due to the angle of attack. There are airfoils with more curvature on the bottom than the top (for instance ) and they provide ample lift provided there is enough AA. Besides, pilots generally don't understand aerodynamics past very basic level. They understand and follow rules and procedures.
You can also take the opposite of a flat wing and use a round rod shaped wing but add a little wingtip votrix by spinning the rod. All you need is to direct more of the air down than up and it will fly.
Note that the wing shown in the TH-cam thumbnail for this clip will not generate lift because it does not direct or pust the air down so the air will not push the wing up.
Very simple and concise explanation, however I would add why the velocity increases over the top surface of the airfoil. This is actually due to curvature of the airfoil, which causes a curvature in the streamline due to the coanda effect. An increase in curvature of a streamline causes an increase in velocity. Hence why the top surface of the airfoil has a larger velocity than the bottom due to it being curved more.
When an aerofoil at a small angle of attack starts from rest, it dumps a starting vortex in the flow which would be anti-clockwise in the example shown. If it is then stopped, it dumps a stopping vortex of clockwise rotation. While the stopping vortex is associated with the aerofoil, it generates lift by the Magnus effect. These vortices can be visualised.
I can explain about non-superfluidity, the Kutta condition and the Kutta-Joukowski circulation theorem if you like, but here I have given the basics which are not difficult to see and to understand. Bernoulli’s Theorem is a secondary explanation. It doesn’t work in liquid helium.
What about inverted flight? How does the aircraft stay up during inverted flight? Why doesn't the wing "lift" right into the ground?
When you fly upside down, you fly with the nose above the horizon. Then, the lift acts towards the sky. Many aerobatic aircraft have symmetrical airfoil, which makes this easier.
@@FlywithMagnar Thanks for replying !
@@FlywithMagnar Very simple: Airfoils can optimice the stall point or airflow. LIFT is created by a moving mass being deflated. End of discussion..:-D This is why planes fly upside down, stones can skip over water and if you hold your hand out of the car window at 60 mph, it will fly...
Wow! So the top air stream actually arrives at the trailing edge *sooner!* This phenomenon is *more* than equal transit time: the lower air stream x velocity is slowed more than the upper air stream (which is perhaps sped up). The problem with equal transit time for pilots is they might assume an inverted (fixed wing) aircraft will inherently produce a lift that pushes them toward the ground. No lift would produce a "zero gravity" experience, so negative Gs indicate that negative angle of attack can overcome airfoil effects to produce negative lift.
Yes, you can fly with air-dams or sails but fuel costs are much higher. Minimize drag by using less angle of attack and more Bernoulli lift.
In 5th grade, I wouldn't let my science teacher off without explaining the Equal Transit Time "theory" -- fallacy in fact -- to my satisfaction. I did not understand where the air molecules on the upper surface of the airfoil got the memo that "You must meet your lower counterparts at the same time at the end of the airfoil. You have a longer distance (being on a curve), so you must run faster."
What would happen, if the air below the wing gets stopped at the end of the wing? Like with full flaps down, does it negate the lift and at how much percentage?
The air doesn't really get "stopped", as it will find a way around just like when a big boxy tractor trailer goes down the highway; but you get a massive amount of drag that can cause you to suddenly drop in speed and stall, not to mention the counteracting motion of a huge drag behind the center of lift is the nose will come down.
Basically pitching up and dropping flaps makes crashing easy and acceleration or climbing difficult.
I'm guessing, this has a similar mechanism as how rolling two similar balls, one rolling down a straight path on a decline, and the other moving through a sine-wave like hills and troughs, both at a similar starting point and going towards a similar end point, would result in the one with the sine-wave hills and troughs finishing sooner than the one at a straight incline, because the sine wave path helps accelerate the ball more efficiently than the straight path.
Just searched for "two balls rolling down hill" and the word brachistochrone comes up.
Wow! I am 71 years old and have two ratings (including glider - sailplane). I learned this in 1967.
I like your video. However another way to demonstrate the increased force on the underside of the wing is to simply rotate the section further clockwise. As we do so becomes clear that the wing will be forced to the right . .
The simple explanation is the shape of the aircraft creates a pressure difference between the upper and lower surfaces; creating lift.
Haha, back in the 90ies I already told my flight students to just forget about Bernoulli and stick with Newton and almost got kicked out of flight school. Of course there are additional fluid mechanical affects on different shaped bodies, but lift is simply caused by Newtons laws by 98 %. The rest if playing around with details.
Boyles law I think also applies i.e. Pressure x volume = a constant. Comparing the still air just before the impact of the leading edge of the wing. The volume of air flowing over the top surface of the wing is larger so the pressure reduces as the leading edge strikes the still air. Conversely the volume of air passing under the wing is lower so the pressure increases. As folk rightly observe this effect (high and lower volumes - a volume differential) is caused by the angle of attack of the wing, and holds true for the aerobatic plane flying upside down, as long as that low pressure is on the heavens side you get to see the stars and live to tell the tale…..else take a dive to the ground.
As you mentioned, the wing's curvature causes the acceleration of the air over it. I'd like to add that this acceleration is further enhanced by the suction effect, drawing air towards the area of lower pressure. This pressure difference is influenced by the angle of attack, not just the air's acceleration due to the wing's curvature.
Yes but before this suction could be possible, the speed had to be increased, hence the curvature, which provoked the acceleration, then the low pressure and from that comes what you just explained.
@@KajolKhan-qj5ne I agree that the wing's curvature is the initial factor that accelerates the air, leading to the subsequent low-pressure area. My point was to highlight the combined effects of this acceleration and the suction effect it creates, along with the role of the angle of attack.
The problem with the video shown at 2:36 is that that is not what happens with the wing at that angle of attack (other than in a headwind situation).
Since it is the wing that is moving through the air.
So in this situation the air should be hitting the wing at the same angle that the wing is at (other than in a head wind situation),
The air is accelerated at the leading edge of the wing, because the air is being pulled down by gravity,
The leading edge pushes the air up causing a squeezing effect of the air, which then accelerates the air relative to the air below the wing.
So, if the air flowing over the top of the wing creates a "low" pressure. And the high pressure is under the wing. |A low pressure will seek a "high" or higher pressure. that would mean the higher pressure under the wing is pushing UP on the wing to help the top of the wing (low pressure) find it's "higher" pressure that is called lift.
a Mooney's wing is asymmetrical (equal top and bottom) gets it lift from the angle of incidence. the way the wing is attached to the body.
Why if you hold a paper downward ( your hand holding the paper is above and the other side of the paper is down below ) and then blow on one side of the paper .. It will not move to the direction of the blow even though the air is faster than the air at the other side ?
If you get tangled in velocity and pressure try the Newton alternative. For an aircraft in level flight and slow, so at high angle of attack, the wing moves a relatively small mass of air with a large downward component of the subsequent velocity. The resulting force has a reaction equal to the mass of the aircraft. At high speed, hence lower angle of attack the mass of air increases and the downward velocity component reduces giving the same effect. It is not rocket science but both flying vehicles use action and reaction. Alas the best demonstration is given by contrails of an approaching aircraft at a close level, preferably as dawn is breaking behind the trails The downward deflection of air is apparent in a fashion that is not as easy to show as blowing over a sheet of paper hanging from one's fingers and watching the trailing edge rise. Propellers and the blades of turbine engines have a strong connection to wings but as devices to provide thrust the reaction connection is apparently more obvious. As indicated at the begining, this is an alternative perspective of the process that makes a wing work and alters none of Bernoulli's work. Blinkers work well on a horse but are a disaster in crowded airspace.
excellent information
It isn’t clear why the velocity of air would positively accelerate over the leading edge. It probably doesn’t. Instead of asking what speeds the airflow over the top, look at what slows it down underneath. Only the difference matters. The angle of attack puts an obstacle into the lower airflow which slows it down. The leading edge also slows the upper airflow initially, but the obstacle is gone quickly. The angle of attack also forces the airflow downward, which adds lift due to momentum. This occurs on the top surface as well, via the Coanda effect, by which a boundary layer of air is guided by the airfoil into a downward departure at the trailing edge.
Instead of equal transit time, think of conservation of mass. You can draw a control volume around the wing and show that mass entering and exiting are equal. From there you can derive L/D from Bernoulli
I thought that lift is achieved by the momentum created by the downward air mass acceleration by the wing. This effect can be visualized by the huge air vortex following a plane. The shape of the wing helps to minimize the disturbances and losses and increase the flow stability. I must be wrong somewhere.
The smoke "bursts" show that the airflow is not speeding up over the top of the wing - it is continuing at the same speed it was before encountering the wing. On the other hand, the airflow under the wing slows down.
A wing causes differential pressure on one side or the other (typically) of its 2 surfaces (if and only if) it is placed in fluid flow of some sort. Without fluid flow a wing produces no lift. The wings of an airplane will not produce lift setting in the hanger with the doors closed and the engine off. Unless there is fluid flow around the wings no lift is produced. So its the fluid flow that provides the energy (does the work) for lift...not the wing. The amount of differential lift (pressure) is directly proportional to the rate of fluid flow around it and the pressure differential between the two sides of the wing. Fluid flow and differential pressure around the wing produces lift. A wing is just a device that allows differential pressure to take place when in a free flowing fluid.
I see MORE misunderstanding! Bernoulli principle is correct and applicable to airfoils. It is not the only aspect and maybe not the most significant aspect but to say everyone is *wrong* is extremely misleading.
Take a sheet of paper and blow across it, just above the paper. it will lift into the airstream. It will also oscillate, like a flag flapping in the wind.
Why does it do this? Bernoulli principle; the movement of air reduces the air pressure within the airstream and the static air under the paper will lift it.
So what about that curved leading edge? If it was just a flat knife edge, then right at the edge you are going to get a disconnection, a turbulence, and the air will change direction suddenly. To be sure, there will be a small area of low pressure directly behind the leading edge, but as the air comes back down, it will press on the wing and that particular lift is canceled. not only that, but you have a narrow region of lift, followed by a wider area of depression, and this is (partly) why flags flap in the wind. It is unstable; lift is not distributed across the wing.
Examine the Coanda Effect. As air gradually changes direction, it produces a low pressure on the curved surface simply because of inertia of the air itself. Might there be a high pressure? Indeed there is; the ram effect of the wing hitting the air compresses the air directly in front of the wing; so what is the vector? To aft (rear). As the air then follows the upper curved surface it accelerates and is compelled to change direction; creating a low pressure along the entire upper surface of the wing since you maintain a curvature but extend the radius as the air is accelerating. This creates a low pressure along most of the upper surface and meanwhile the air under the wing is compressed, somewhat uniformly all along the underside, which creates lift. Some underside wing shapes are concave where the upper surface is convex. The faster the aircraft the less curved are the surfaces.
Anyway, an actual wing is a compromise since the Bernoulli effect will dominate at only one angle of attack (AOA) and is very dependent on airspeed. So good old Newtonian reaction is also part of it particularly at lower speeds and higher angle of attack.
Why might FAA emphasize the Bernoulli aspect over other aspects? Because the low pressure region of air will condense moisture and freeze up, and you lose the Bernoulli effect because the low pressure region is filled with ice. Likewise, the leading edge ceases to be round (invoking the Coanda Effect) and becomes a knife edge. The combination of these effects means the wing can only fly because of Newtonian reaction; inducing way too much drag and your aircraft is going down.
I have had this on my drone (quadcopter) rotor blades in two minutes in cold humid air; the upper surfaces of the blades were coated in ice, also the leading edges, but not the underside of the blades.
The Bernoulli effect is undoubtedly the leading contributor to lift, but not the only contributor. An iced-up wing loses Bernoulli effect and that's all it takes to bring your aircraft down.
I believe your wind tunnel example may be slightly misleading with what is causing the change in airspeed around the airfoil. I looks to me that the AoA of the foil is deflecting air down slowing all of the air in the lower portion of the demonstration making it harder to determine if the air over the top of the airfoil is actually speeding up as you mentioned in the video or is actually remaining constant. Would a more cambered airfoil with a flat bottom parallel with the relative be better conditions to demonstrate how the shape effects the airspeed?
Well explained, however, at 3:28 there is an error: The pressure difference between the lower pressure on the upper side of the wing close to the leading edge and the higher pressure in undisturbed air "higher up" does not directly create lift. The lower pressure on the upper side of the wing close to the leading edge only *contributes* to lift, it does not generate it; lift is always generated by the lower side of the wing. Suction is not a thing, a vaccum cleaner does not actually suck air in but the surrounding static air pressure pushes air into the low pressure region created by the motor. Also, the lower air pressure region on the upper side of the wing close to the leading edge does not "suck" the upper side of the wing upwards.
Put the wing in a mold filled with water until it covers all of the lower side of the wing up to the frontal and aft stagnation points (no air below the wing), there will then be only the airflow above the wing, not below. There will still be a decrease in static pressure on the upper side of the wing close to the leading edge but no lift will be generated and airflow will not be bend downwards once it gets behind the trailing edge.
Lift is generated by the pressure differential between the lower side of the wing and it's upper side and in order to have lift, air must push against the lower side of the wing, pushing the wing upwards (by the combined pressure of static and dynamic pressure on the lower side of the wing versus on the upper side of the wing). The dynamic change of the pressure part is basically Bernoulli.
In addition, the global shape of the wing disrupts the surrounding air in a way that behind the wing, airflow is directed downwards, and at the wingtips, a rotary turbulent airflow is generated in a way that the higher pressure air masses under the wing move in a circular fashion around the wingtip to reach the lower air pressure region above the wing.
The air pressure differential below and above the wing is decisive for creating both lift and wake turbulence. In both cases, local airflow is disrupted and air is deflected downwards behind the wing and clockwise on the left wingtip and counterclockwise on the right wingtip (as viewed from behind).
Newton's third law is insufficient to explain lift as it only looks at the effect, not the cause (the pressure differential).
As for the equal transit time assumption, it's really only a baseless assumption, not a theory, as the idea itself implies unexplained constraints (two molecules joining again at the trailing edge). Air is not butter and the wing not a knife cutting it. In reality, as shown in the video, the air above the wing actually travels much quicker than would be expected if, for some magical reason, two gas molecules split up at the leading edge would have to join again at the trailing edge as postulated by the equal transit time idea.
Whether push or pull, the result is the same; difference in pressure creates lift.
@@Talon19 As long as we don't claim dynamic pressure under the wing is the main cause of lift.
@@davetime5234
Static and dynamic pressure both affect the distribution of pressure around the aircraft.
Sounds nice but is wrong as the equal transit time theory. Arvel Gentry explained lift 50 years ago. No viscosity, no lift. And it is Bernoulli at the end - albeit for different reasons.
I always thought that the wind over the top part of the wing created a vacuum. That caused the bottom part to push up to fill that vacuum. That is what created lift.
I believe you are substantially correct: that the wind travels faster over the top of the wing to cause a lower pressure area there than below the wing where the wind is traveling slower. This very statement negates the 'equal transit time' addendum (I'll call it) which I don't think Bernoulli ever intended to be a part of his theory.