I think we also have a good understanding of the theory about why air molecules travel faster in constricted spaces (this is pretty clear when the flow is in a tube) - although the theory requires refining when it comes to aerofoils or solid bodies in fluid. The whole mechanism is due to the conservation of mass, momentum and energy that are being simultaneous preserved as the airflow interacts with solid bodies - to understand more of this you might want to revisit Navier Stokes equations.
Dear Alyia, I found your video extremely well presented and informative. I did watch several lectures including one by Doug McLean and really like the fact that you are clearly explaining the state of knowledge with all its limitations. You deserve a lot of credit for this great work, thank you.
The first thing a moving aerofoil does is to dump a starting vortex, but vorticity gets no mention in this video. Have you heard of the Kutta-Joukowski circulation theorem?
Aerodynamics is a fascinating area to study and one day you will learn that lift theory is not debatable or a mystery, but explained well by Navier-Stokes equation. Computational aerodynamics works well in many cases, but may fail because the software fails or the hardware is inadequate, but do not mistakenly interpret this as a failure of theory. Please learn from physics and math sources and avoid incorrect ideas: "vacuum theory," "equal transit time," "detachment" is not understood or "no simple explanation." Even if you understand only half of what you read in physics or math, keep your amazing positive attitude, anything you learn is more than you knew before, keep reading, do not give up, and you will learn a lot!
Excellent! Perfect speed, well-explained, a little bit of humour, learned a lot. As I used to sell CFD software, I was waiting for the word "laminar". But all in all, very good explanations of the current thinking on the whole subject. Well done.
As an aeronautical engineer who used to teach aerodynamics in college, I profoundly dislike these explanations. There are wrong theories for lift, like equal transit time, which is in fact a naive theory, it was never a real one used by real aerodynamicists. Naiver-Stokes is everything. It can handle any situation in fluid dynamics for which the continuum approximation is good (i.e. taking the fluid as a continuum rather than individual molecules). It can handle viscosity, compressibility, vorticity, temperature changes.... everything. (Naiver-Stokes is basically Newton's law applied to a parcel of air, plus mass conservation plus energy conservation). It can handle lift too. The problem with Naiver-Stokes is not that it cannot handle lift, but that we cannot handle Naiver-Stokes. It gets too complicated and there is no known closed solution for the general case, although numerical treatment can get arbitrarily close (given enough computing power, time, and knowledge of the properties of the fluid). Of course that the 3rd law needs to apply. If the air makes an upwards force on the wing (which pushes the airplane up) , the wing needs to make a downward force on the air (which deflects the air down). This is known as the action-and-reaction law, but which is the action and which is the reaction is totally arbitrary in the best case or, even better, meaningless. To explain lift, it sound nicer to say "the wing deflects the air down so the air pushes up on the wing", but the reality is not that one comes because of the other or the other way around. They are concurrent effects. Both happen together and need each other. Forces ALWAYS come in action-reaction pairs. This is enough to explain lift in the macroscopic view, but how is the force of the air over the wing manifested? It is due to the pressure distribution around the airfoil where, overall, the pressure is higher in the lower side than it is on the upper side. Why? Well, Naiver-Stokes. But the extremely simplified version of Naviere-Stokes, that is Bernoulli, is an EXCELLENT predictor of lift (predictor is not the best way). As Aliya explained, you can put pressure probes around the airfoil. With the measured pressures, you can calculate what the speed of the air should be in each spot around the airfoil. Then you measure the speed at those spots and voila, almost perfect match. No equal-transit required. You can also use the potential flow theory (which is the condition under which Bernoulli applies), with the Kutta condition (the assumption that each airfoil will produce enough circulation to ensure that separation happens at the trailing edge) to calculate IN ADVANCE what would be the pressure and speeds distribution and, again, very you get very good matches with actual measurements. This theory even permits to predict that the slope of the coefficient of lift vs angle of attack is 2*pi (with the angle of attack in radians), which is a perfect match for not too thick airfoils and not too low Reynolds numbers (at too low Reynolds numbers the viscosity effect takes more importance and the approximation of potential flow is less valid). Even then, Bernoulli itself is not enough. The Bernoulli explanation would be "The air moves faster on the upper side so the pressure is lower". But why does the air move faster on the upper side? Because the pressure is lower, so the air accelerates from a region of higher pressure (normal atmospheric pressure) to a region of lower pressure (on the upper surface. Wait a second.... this sounds like what was first, the chicken or the egg. Does the pressure go down because the air moves faster, or does it move faster because the pressure goes down? And the answer is YES. I know this may sound not satisfactory, but both things happen at the same time and are concurrent, they need each other. Similar to what we said earlier about action and reaction. At the endo of the day, it is very simple in principle (the math makes it complicated). You start with a uniform airflow far away ahead from the wing. You put an airfoil in the middle, with a certain geometry and a certain angle of attack, and impose 2 conditions: 1) The airfoil is impenetrable (air cannot flow from the outside of the airfoil to the inside or vice versa). 2) The airflow must separate at the trailing edge (Kutta condition). These are called "boundary conditions". The apply the math (be it the simplified potential airflow model or more complicated models) and come up with the only solution that meets these 2 criteria. The fields of velocities and pressures is an output of these calculations, the exchange of forces between the wing and the air is an output of these pressures, and lift and the downward deflection of the air are an output of these forces. IT IS NOT DIFFEENT THEORIES, in the same way that if you have a mass with a spring that collides with another mass you don't have to choose between 3rd law, conservation of momentum, conservation of energy or spring theory. It is all part of the same phenomena and you will use all the equations at the same time, solving a system with many unknowns and many equations (in the case of Naiver-Stokes you are solving differential equations, but still). A the end of the day, if you start to dig deeper and deeper you will always end with General Relativity, Quantum Mechanics or a combination of both (which sucks because we don't have a theory that combines both). The rest is either approximations or emerging properties (like temperature is in fact an average of the kinetic energy of the molecules). That calculating lift is complicated doesn't mean that "there is no agreement", "nobody understands it", "there are competing theories" or anything like that. That is not the case. We understand lift VERY well and there are no competing theories.
Great discussion on the some of the theories of lift, I'd also like to see you add circulation theory to this discussion as it can definitely be a major contribution towards lift as well.
For me : air viscosity creates a virtual wall above the wing (at some point above the wing the air is undisturbed by the passage of the airplane) so the shape of the top of the airfoil creates a Venturi effect : meaning more speed, meaning less pressure. I agree that the 3rd law is better for the bottom part of the airfoil. I like very much the PRANDTL theory where the 3rd dimention is needed to explain all, because air also moves sideways to let the airplane pass though it. So it create turbulences and you can push on those side turbulences to increase lift or speed of the aircraft. (Wild Geese flying in a V formation, the side ones take a nap, 8% less effort needed) like a water skier can surf on the boat wake without end.
I find it so interesting how there are many different theories out there. Being a pilot myself many of the books set by the FAA teaches all pilots Bernoullis Principle only.
In my opinion... Newton's laws must be obeyed - if there is a lift force, then some surrounding air must be given momentum (unless the force is applied to some other object in the sky??). ie. the wing must grasp some air and throw it downward. How does it grasp that air and move it? This is where Dr. Bernoulli joins us, along with some of his friends.
IMHO, As with many other phenomena, we make our explanation way more complicated than it deserves. Can't we simply say that the angle of attack causes the pressure on the underside of the wing to exceed the pressure on the top of the wing? Isn't that sufficient to explain lift?
I agree that lift is due to a combination of multiple components but predominately the turning of momentum. Consider supersonic speed. Bernoulli's principle doesn't have the time to take effect. A rocket tricks air by employing an aerospike. Pretty much the shape of the rocket behind the aerospike is immaterial to the aerodynamics. For a bug to take off, it claps its wings and by pulling them apart, creates a vacuum to suck them up. Buzzards take advantage of updrafts. Some insects can see turbulence and time their wings to react with them for the best outcome. Finally, big passenger planes drag behind them huge vortices. That is why landing planes are separated by a minimum distance and also why military planes no longer take off in pairs in case these flip the rear plane upside down. You haven't mentioned these at all. The Concorde made use of them for lift.
That's the point. Bernoulli's principal requires that the air traveling above the wing travels faster than the air below the wing. However this is not the case! There is no speed differentiation between the air above or below the wing. The air that separates at the tip of the wing has no requirement or inclination to re-unite at the tail of the wing. All these air molecules are for ever separated and have no magical effect to race each other and join back up. The air above the wing is completely cable of traveling at what ever speed is pertinent to it's resistance and it's inclination to return to zero movement. Let's be extra specific and state the direction of the air in Bernoulli's principal is horizontal to the wing. This effect can be shown in contrived experiment by creating two different speed air streams above and below the wing. And this of course does create lift. However this is not what is occurring in reality. In reality the air above the wing is pushed vertically UP further than the air below the wing is pushed vertically down. Because the air above the wing is pushed further there is higher resistance for air to flow over the wing than under the wing. Any air flow will take the path of least resistance which is under the wing. Now the whole model is turned upside down from traditional thinking. (And) From that new model paradigm you can now easily visualize why the wing must be lifted. The problem with Bernoulii's principal is it is about horizontal air movement and fails to take vertical air movement into consideration.
Hi Aliya, I just came across this video, I've always been interested in flight, but untrained; can I share some of my thoughts. I'm sure lift theory has advanced so much since your video, so maybe some of my concepts have already been proven or disproved. Both the Bernoulli and Newtonian principles seem to have gaps that I can't see anyone addressing, or even recognising. Bernoulli's principle doesn't have to assume an equal transit time. Wind tunnel smoke tests clearly demonstrate differences in time for air particles travelling over a wing vs under a wing. Smoke tests also demonstrate that the air over the wing slows down at the start, so it’s not all faster; likewise, air under the wing speeds up in some places. But overall, there is a net increase in air speed; therefore, there is a net decrease in pressure. At the same time, there is a net decrease in "relative" air speed under the wing; therefore, a net increase in "relative" air pressure under the wing. It's these pressure variables along the wing, and increase under the wing that aren't discussed. Another aspect of Bernoulli's principle that isn't properly addressed is the criticism that Bernoulli's principle only applies to fluids in a tube. Some assume that as a wing in flight isn't in a tube, then Bernoulli is irrelevant. However, in a wind tunnel, it is in a tube. Even so, an aircraft in open skys can be considered as if flying in a tube. Bernoulli's principle applies in large and small tubes. A very small wing in a very large tube has no discernable interaction with the walls of the tube; instead, that small wing is contained within a sleeve of ambient/normal airflow and pressure. A high or low pressure wave/differential presses or is restrained by that "normal" air pressure, as if in a tube. At this point there's a transition to Newtonian motion. I find it easier to think of these as displacement, deflection and thrust vectoring. When the pressure wave/differential occurs, it is resisted by the "relatively static" mass and motion of the surrounding "normal" air. This pressure differential tries to displace/move a static mass of air by either pushing against that mass, or drawing in closer. This means pressure differentials can also be co sidered as mass (floatation displacement) differentials. A mass being displaced then means forces and acceleration. As we add movement to these masses of the wing with its associated pressure/mass differentials against the mass of the normal airflow and pressure, we now have principles of momentum to consider, thereby we have momentum differentials. A consequence of non-colinear momentum and displacement is the deflection of those associated mass bodies. These deflections thereby have resultant vectors, for both mass bodies. So far these differentials have been direct wing interaction with the surrounding air, but after the trailing edge there is no more direct interaction. The higher air flow on the top edge is now coplaner with the downward dloping trailing edge profile, resulting in a downward flow of air/mass with inertia, which the surrounding aft airflow has to overcome to normalise. Although this mass flow normalisation is behind the wing, it is reacting against other air masses that have already been normalised, similar to the principles of a jet or rocket exhaust in air vs a vacuum. Two other aspects that are typically overlooked and not discussed are U-wings and uplift and bracing loads on a house roof. Both of these are outside of any Newtonian discussions I've heard, yet the capacity for high wind loads to lift a roof off the walls is very significant. Now to explore some of your other aeronautical videos.
3:48 The fact that the equal transit time assumption (it's so arbitrarily baseless, it's not even a theory in my opinion, only an assumption) is wrong is actually *not* proof that Bernoullis principle won't apply as the two ideas are unrelated. It's therefore not a limitation of Bernoullis principle.
4:52 Inverted flight is also not a limitation on Bernoullis principle, nor are symmetrical airfoils. In those cases, lift is generated by the AoA which lowers the forward stagnation point under the inverted wing, airspeed above the inverted wing will still be faster than airspeed below it. Bernoulli principle still explains the reduction of the static air pressure on the upper airstream hitting the inverted wing. It really isn't a limitation of the lift theory based on Bernoulli.
5:50 The "people or cars" example really hasn't anything to do with Bernoulli or lift. There is no directed outside pressure driving cars forwards against the will of their driver. If the faster cars would physically push the cars ahead through the narrowing by using brute force against their bumpers, those cars in the narrow part would not slow down (they would rather try to hit the gas because of the fear of getting rear-ended). The diffrence is that gas and water molecules are in contact with each other behaving therefore like a stream or a fluid, whereas pedestrians and cars behave like independent particles, they don't push each other around. At least as long as they're not engaging in POGO dancing. In the case of the wing of a spaceship flying well beyond the atmosphere, if that wing is occasionnally hit by particles, there would not be lift because there would be no "stream", only collisions which will obey Newtons law of impulse and force and counterforce (action-reaction). Because in that case we talk about particles and not about fluids or pressurized gases forming a stream and nobody in their right mind would try and use Bernoullis principle in order to predict the reaction of the wing to such a collision with a particle. Finally both Bernoulli and Newton need to be combined, Bernoulli can explain the lower pressure above the wing whereas Newton can explain the "ram air" effect creating higher pressure on the lower wing. Newton also explains why air masses are directed downwards behind the wing.
Thank you!, finally someone else said it: "The fact that the equal transit time assumption (it's so arbitrarily baseless, it's not even a theory in my opinion, only an assumption) is wrong is actually not proof that Bernoullis principle won't apply as the two ideas are unrelated. It's therefore not a limitation of Bernoullis principle."
Bernoulli's law doesn't apply to lift generation. Bernoulli's law states the relationship between pressure and flow speed along streamlines, while lift requires a relationship between pressure across streamlines. The lift is generated due to the curvature of streamlines along the aerofoil. Holger Babinksy has a great paper with the mathematical derivation of this relationship. Also, recent research has claimed that lift exist because of nature's tendency to seek a minimum flow curvature state.
Actually, the Bernoulli's principle can explain - its contributions to the lift - why some aircraft can fly inverted. Without going to much into the mechanics; consider a symmetrical aerofoil. If we invert it, it would still fly because we just change the pitch -applying it in the opposite direction of our intended pitch and it would behave the same way as the one not inverted. If we are inverted, we can pitch down, creating a positive angle of attack and hence lift in the direction of positive lift considering an inertial reference frame. The same is true for the unsymmetrical aerofoil although the behaviour maybe different to the special case of the symmetrical aerofoil.
In the same breath - Bernoulli's principle can still explain its contributions to lift for a flat plate, all we need to do is change the angle of attack - this allows us to change the behaviour of flow as it traverses the top surface (going faster) compared to the bottom surface.
At aeronautical engineering school we used to say that Bernoulli's Principal sucks and Newton's goes down. We were typical, crude, libidinous college students, now we are typical, crude, libidinous engineers. Ah, Progress.
The cause of lift is the acceleration of air downward. The is no other cause. You can not accelerate air downward without getting an upward force, nor can you have an upward force without accellerating air downward. A pressure differential does NOT cause lift. The bernulli effect does NOT cause lift. Rather, a pressure differential IS lift, caused by the deflection of air. The equal transit time hypothesis simply confuses the cause with the effect. The bernulli effect can help you understand HOW a force is transmitted to a wing surface, but does NOT explain the cause of the force.
Your English is great but yeah there's no shame in speeding up or slowing down the video to suit personal preferences. Can't wait to learn all the things and who knows maybe I'll finally scratch design and build* a RC plane! By build* I mean let a 3D printer do all the work ;) Let the bingeing begin!
You have some good points, but the traffic example is not difficult to understand. You are discussing lift in the incompressinle flow regime. Air molecules at normal temperatures and pressures act essentially like water and do not compress. Traffic flow is compressible. That is the distance between cars reduces, or compresses, as it comes to an obstacle or a reduction in the number of lanes. This phenomenon happens when aircraft approach the speed of sound. This is why aircraft design for supersonic flight is different than for subsonic flight. As far as inverted flight, yes, angle of attack and alot of thrust are your friends.
I've read another version of Bernoulli's principal based explanation. It is from the book University Physics by Zemansky. It says: " The flow lines crowd together above the wing, correspon- ding to increased flow speed and reduced pressure, just as in the Venturi throat." What I understand is that the air flow above the wing is 'squeezed' (similar to what would happen to a flow in a constricted region of a pipe). Thus the flow speed increased and pressure is lowered. The author also adds that this is the very reason why the umbrella seems to generate lift in windy days (and a strong wind can turn it inside out). My observation: Early biplanes like Wright brother's plane had a simple curved 2D wing cross-section, (inverted curve). It was nothing like modern airfoil.
Change in pressure causes fluid to move not the opposite. Air is faster above the wing because of a favourable pressure gradient along the flow direction on the suction side and an adverse pressure gradient on the pressure side. This is due to the curvature in the flow as the wing passes through the air due to the way a wing is designed. Newton's second law. Wings are also designed in a way to produce downwash. Newton's third law. Newton's second and third Laws explain lift completely and correctly.
It is explained by 2nd law. Consider two particles in the stagnation region. At the suction side, the streamlines are curved in a way that there is low pressure on the suction side as compared to the stagnation pressure. Therefore the prticle at the suction side get accelerated. At the pressure side, the stream line curvature is in a the way that pressure gradient is adverse as compared to the suction side. Apply 2nd law to both fluid particles or to a small control volume of particles at suction or pressure sides. This is why speeds re different.
@@AliyaBurkit It is well understood, specially in the case of flow through pipes and cyclones etc. It happens due to centrifugal force. Whenever there is a bend in a pipe i.e. streamlines change direction/bend etc. there is change in pressure gradient across the streamlines. Same is the case with typhoons, cyclones etc. Also, when there is high curvature of the stremline, there is high pressure gradient between successive streamlines and low absolute pressure like in the center of the typhoon has high speed and low pressure. There is a lecture of Dr. Holger Babinsky in which he explains it very clearly what I am trying to say.
Can't you prove all different mechanisms by making "infinite" length wings with the perfect shape for every principle, and calculate the amount of lift they produce? And then give them all relative weights so you know when you combine them how much each contributes to the lift. Droplet horizontal wing, angle of attack flat surface. Differentiate the shapes of the first to seperate third law effects and Bernoulli effects Man really feel bad I didn't study maritime tech as a major, would have loved it
do you work at nasa mam? if yes, i was at my right lecture... i am just 7th but i want to be an astronaut from now... suppose if ur doing space craft mechanics, can u send me the portions? also thank you for teaching explainatory and i will be an absolute astronaut :) Edit: are u doing 1 to 1 online class teaching mam. If yes, i would love to join it :3
I don't work at NASA but that was my dream. Now it's just on the back burner of my mind. If you want to do an astronaut, you need to know a lot of things, so this topic is on my video list!
Your explanation of Newton's theory is not quite right. The third law is just a secondary effect, the primary effect if the second law of motion. F=ma. When we accelerate a mass of air, we are essentially creating a force. that force is directed downwards at the trailing edge and hence create lift due to Newton's third law of motion. However, that theory still gives me some doubts because if we closely look at the angle at which the flow leaves the trailing edge - the wing should also create thrust which is far fetched :/
@@AliyaBurkit nyc..I like ur way of teaching n I liked that t-shirt also..😄 I have learnt compressible n incompressible flow from u...that was outstanding
and from where does the velocity of the air OVER the wing get this extra energy to speed up ... THAT is the incorrect portion of the statement ... it is a direct violation of thermodynamic law ... energy was created out of nowhere NOT converted ... . technically the velocity does NOT change the the pressure does because it is filling a "vacuum" ... actually it's replacing the air that once was at the same slice in space as the air under the wing ... which is essentially a virtual vacuum ... . molecule A of air and B of air are equidistant from the wing Leading edge and in direct line above each other ... once the reach the leading edge they change velocity and direction and follow the wing ... one over and one under .... the one under has a shorter path normally and so carries on at almost the same speed it came to the wing at and in the same relation to where A should be ... while A not only has to change velocity but also has to move up and away from B it therefore no longer is above B because it takes TIME to change ... so as B leaves the trailing edge of the wing A will be some distance behind it ... THAT is where the lift comes from it is the direct difference between location A and B ... it would be a pressure ratio and either add or subtract from the air pressure around ... and for lift it subtracts making it a lower pressure above and a higher pressure below causing the bottom air to push the wing UP ... while if A leaves the trailing edge before B the it is sucked down as A is a higher pressure than B .... also known as stall or stagnation ... . this is where most call the basic lift theory wrong ... as it needs to speed a velocity up with NO input in violation of thermodynamics ... energy can neither be created nor destroyed only changed.
and if you measure the velocity of the air over the wing and under the wing ... they will be the same ... so it holds BUT it is not an accurate theory ... as it is the pressure relationship between the two parts of the air over and under that causes lift and yes it still agrees with bernoulli ... and Thermodynamics ... as the fluid (air) isnt created and sent as a sheet or wave at the wing it is all encompassing and the wing is either passing through it or the air is being passed over it ... (or both) so even though A and B are at different locations at the trailing edge from where they started before the wing ... the air before they arrived is still there at a different pressure ... as it went from a compacted zone to an expanded zone and actually LOST pressure to fill the less dense space ...
the turbulent flow is caused when the air cant be replaced by the existing volume in that location fast enough causing the lower air to back flow ... increasing drag and reducing lift and thrust .... the pressure change is just a ratio of compression over and under the wing ... at some point for an airfoil that ratio degrades to a point where the lifting portion of the wing is smaller than the drag portion of the wing ... also the laminar flow is at some point lower than the turbulent airflow ... . the speed change measured in a wind tunnel ISN'T comparing the two particles it is comparing how fast the air is being replaced on one surface over another ... if you put that wing in the tunnel and have it run to create lift and then add smoke and SLOW MOTION capture the smoke as it passes over the wing ... the smoke on top is slower than smoke on bottom by a small fraction ... as the angle of attack increases the "stickyness" of the wing surface is not as strong as the normal volume of air ... so it leaves the wing surface causing turbulent flow from under the wing to backflow ... THIS is where the coriollis effect plays its role in lift and why is should never be ignored or back burnered as it is what makes lift happen and yes it is a small affect in MOST situations ... BUT not all .. and it's minimal influence in some aspects can be ignored temporarily once it can be shown that the effect of the affect is too negligible to consider above the error ratio ... . whats so hard abut that ... nothing and it does cover all the ranges of flight ... and all the wings shapes .. and angles of attack ...
and John Anderson is right ... the closest one can get to a simple explanation is - Lift is a multi science discipline that no one has been able to fully implement so far ... because we dont properly understand how nature actually works ...
yeahh you finally made a new video, i wonder where have you been since i learn a lot from your channel 😥
thank you! it's going to be better now :)
I think we also have a good understanding of the theory about why air molecules travel faster in constricted spaces (this is pretty clear when the flow is in a tube) - although the theory requires refining when it comes to aerofoils or solid bodies in fluid. The whole mechanism is due to the conservation of mass, momentum and energy that are being simultaneous preserved as the airflow interacts with solid bodies - to understand more of this you might want to revisit Navier Stokes equations.
Welcome back Dr. Aliya, i miss you❤️
Thanks
Dear Alyia,
I found your video extremely well presented and informative. I did watch several lectures including one by Doug McLean and really like the fact that you are clearly explaining the state of knowledge with all its limitations. You deserve a lot of credit for this great work, thank you.
Thanks, really upset that u stopped making videos, really ur are worth watching for me... Finally video out....feeling good
thanks!
The first thing a moving aerofoil does is to dump a starting vortex, but vorticity gets no mention in this video. Have you heard of the Kutta-Joukowski circulation theorem?
Aerodynamics is a fascinating area to study and one day you will learn that lift theory is not debatable or a mystery, but explained well by Navier-Stokes equation. Computational aerodynamics works well in many cases, but may fail because the software fails or the hardware is inadequate, but do not mistakenly interpret this as a failure of theory. Please learn from physics and math sources and avoid incorrect ideas: "vacuum theory," "equal transit time," "detachment" is not understood or "no simple explanation." Even if you understand only half of what you read in physics or math, keep your amazing positive attitude, anything you learn is more than you knew before, keep reading, do not give up, and you will learn a lot!
Its good to see you humble Sister. Great going. God bless
Thank you 🙏
Excellent! Perfect speed, well-explained, a little bit of humour, learned a lot. As I used to sell CFD software, I was waiting for the word "laminar". But all in all, very good explanations of the current thinking on the whole subject. Well done.
As an aeronautical engineer who used to teach aerodynamics in college, I profoundly dislike these explanations. There are wrong theories for lift, like equal transit time, which is in fact a naive theory, it was never a real one used by real aerodynamicists. Naiver-Stokes is everything. It can handle any situation in fluid dynamics for which the continuum approximation is good (i.e. taking the fluid as a continuum rather than individual molecules). It can handle viscosity, compressibility, vorticity, temperature changes.... everything. (Naiver-Stokes is basically Newton's law applied to a parcel of air, plus mass conservation plus energy conservation). It can handle lift too. The problem with Naiver-Stokes is not that it cannot handle lift, but that we cannot handle Naiver-Stokes. It gets too complicated and there is no known closed solution for the general case, although numerical treatment can get arbitrarily close (given enough computing power, time, and knowledge of the properties of the fluid). Of course that the 3rd law needs to apply. If the air makes an upwards force on the wing (which pushes the airplane up) , the wing needs to make a downward force on the air (which deflects the air down). This is known as the action-and-reaction law, but which is the action and which is the reaction is totally arbitrary in the best case or, even better, meaningless. To explain lift, it sound nicer to say "the wing deflects the air down so the air pushes up on the wing", but the reality is not that one comes because of the other or the other way around. They are concurrent effects. Both happen together and need each other. Forces ALWAYS come in action-reaction pairs. This is enough to explain lift in the macroscopic view, but how is the force of the air over the wing manifested? It is due to the pressure distribution around the airfoil where, overall, the pressure is higher in the lower side than it is on the upper side. Why? Well, Naiver-Stokes. But the extremely simplified version of Naviere-Stokes, that is Bernoulli, is an EXCELLENT predictor of lift (predictor is not the best way). As Aliya explained, you can put pressure probes around the airfoil. With the measured pressures, you can calculate what the speed of the air should be in each spot around the airfoil. Then you measure the speed at those spots and voila, almost perfect match. No equal-transit required. You can also use the potential flow theory (which is the condition under which Bernoulli applies), with the Kutta condition (the assumption that each airfoil will produce enough circulation to ensure that separation happens at the trailing edge) to calculate IN ADVANCE what would be the pressure and speeds distribution and, again, very you get very good matches with actual measurements. This theory even permits to predict that the slope of the coefficient of lift vs angle of attack is 2*pi (with the angle of attack in radians), which is a perfect match for not too thick airfoils and not too low Reynolds numbers (at too low Reynolds numbers the viscosity effect takes more importance and the approximation of potential flow is less valid). Even then, Bernoulli itself is not enough. The Bernoulli explanation would be "The air moves faster on the upper side so the pressure is lower". But why does the air move faster on the upper side? Because the pressure is lower, so the air accelerates from a region of higher pressure (normal atmospheric pressure) to a region of lower pressure (on the upper surface. Wait a second.... this sounds like what was first, the chicken or the egg. Does the pressure go down because the air moves faster, or does it move faster because the pressure goes down? And the answer is YES. I know this may sound not satisfactory, but both things happen at the same time and are concurrent, they need each other. Similar to what we said earlier about action and reaction. At the endo of the day, it is very simple in principle (the math makes it complicated). You start with a uniform airflow far away ahead from the wing. You put an airfoil in the middle, with a certain geometry and a certain angle of attack, and impose 2 conditions: 1) The airfoil is impenetrable (air cannot flow from the outside of the airfoil to the inside or vice versa). 2) The airflow must separate at the trailing edge (Kutta condition). These are called "boundary conditions". The apply the math (be it the simplified potential airflow model or more complicated models) and come up with the only solution that meets these 2 criteria. The fields of velocities and pressures is an output of these calculations, the exchange of forces between the wing and the air is an output of these pressures, and lift and the downward deflection of the air are an output of these forces. IT IS NOT DIFFEENT THEORIES, in the same way that if you have a mass with a spring that collides with another mass you don't have to choose between 3rd law, conservation of momentum, conservation of energy or spring theory. It is all part of the same phenomena and you will use all the equations at the same time, solving a system with many unknowns and many equations (in the case of Naiver-Stokes you are solving differential equations, but still). A the end of the day, if you start to dig deeper and deeper you will always end with General Relativity, Quantum Mechanics or a combination of both (which sucks because we don't have a theory that combines both). The rest is either approximations or emerging properties (like temperature is in fact an average of the kinetic energy of the molecules). That calculating lift is complicated doesn't mean that "there is no agreement", "nobody understands it", "there are competing theories" or anything like that. That is not the case. We understand lift VERY well and there are no competing theories.
Thank you, Ms. Burkit. Coming across this channel reminds me why I love RUclips. Keep being real :)
Thank you for being part of this channel!
Great discussion on the some of the theories of lift, I'd also like to see you add circulation theory to this discussion as it can definitely be a major contribution towards lift as well.
For me : air viscosity creates a virtual wall above the wing (at some point above the wing the air is undisturbed by the passage of the airplane) so the shape of the top of the airfoil creates a Venturi effect : meaning more speed, meaning less pressure. I agree that the 3rd law is better for the bottom part of the airfoil. I like very much the PRANDTL theory where the 3rd dimention is needed to explain all, because air also moves sideways to let the airplane pass though it. So it create turbulences and you can push on those side turbulences to increase lift or speed of the aircraft. (Wild Geese flying in a V formation, the side ones take a nap, 8% less effort needed) like a water skier can surf on the boat wake without end.
I find it so interesting how there are many different theories out there. Being a pilot myself many of the books set by the FAA teaches all pilots Bernoullis Principle only.
Actually, there are not different theories. Ms. Alyia is in the process of learning so she makes some mistakes.
Finally...video out❤️
yes, I'm happy myself))
4months late...but finally 🎉
Yeah..
In my opinion... Newton's laws must be obeyed - if there is a lift force, then some surrounding air must be given momentum (unless the force is applied to some other object in the sky??). ie. the wing must grasp some air and throw it downward.
How does it grasp that air and move it? This is where Dr. Bernoulli joins us, along with some of his friends.
finally , its been ages BTW you got the potential u can grow fast here
Thanks :)
This is the best video on the topic, thank you so much :D
Thank you for everything that you provide for us!
You're welcome :)
Please make more videos, they are Great.
Thank you!
IMHO, As with many other phenomena, we make our explanation way more complicated than it deserves. Can't we simply say that the angle of attack causes the pressure on the underside of the wing to exceed the pressure on the top of the wing? Isn't that sufficient to explain lift?
Excellent!
I agree that lift is due to a combination of multiple components but predominately the turning of momentum. Consider supersonic speed. Bernoulli's principle doesn't have the time to take effect. A rocket tricks air by employing an aerospike. Pretty much the shape of the rocket behind the aerospike is immaterial to the aerodynamics. For a bug to take off, it claps its wings and by pulling them apart, creates a vacuum to suck them up. Buzzards take advantage of updrafts. Some insects can see turbulence and time their wings to react with them for the best outcome. Finally, big passenger planes drag behind them huge vortices. That is why landing planes are separated by a minimum distance and also why military planes no longer take off in pairs in case these flip the rear plane upside down. You haven't mentioned these at all. The Concorde made use of them for lift.
That's the point. Bernoulli's principal requires that the air traveling above the wing travels faster than the air below the wing. However this is not the case! There is no speed differentiation between the air above or below the wing. The air that separates at the tip of the wing has no requirement or inclination to re-unite at the tail of the wing. All these air molecules are for ever separated and have no magical effect to race each other and join back up. The air above the wing is completely cable of traveling at what ever speed is pertinent to it's resistance and it's inclination to return to zero movement. Let's be extra specific and state the direction of the air in Bernoulli's principal is horizontal to the wing. This effect can be shown in contrived experiment by creating two different speed air streams above and below the wing. And this of course does create lift. However this is not what is occurring in reality. In reality the air above the wing is pushed vertically UP further than the air below the wing is pushed vertically down. Because the air above the wing is pushed further there is higher resistance for air to flow over the wing than under the wing. Any air flow will take the path of least resistance which is under the wing. Now the whole model is turned upside down from traditional thinking. (And) From that new model paradigm you can now easily visualize why the wing must be lifted. The problem with Bernoulii's principal is it is about horizontal air movement and fails to take vertical air movement into consideration.
Your teaching skill is as beautiful as you.
Thank you so much 😊
Hi Aliya,
I just came across this video, I've always been interested in flight, but untrained; can I share some of my thoughts. I'm sure lift theory has advanced so much since your video, so maybe some of my concepts have already been proven or disproved.
Both the Bernoulli and Newtonian principles seem to have gaps that I can't see anyone addressing, or even recognising.
Bernoulli's principle doesn't have to assume an equal transit time. Wind tunnel smoke tests clearly demonstrate differences in time for air particles travelling over a wing vs under a wing. Smoke tests also demonstrate that the air over the wing slows down at the start, so it’s not all faster; likewise, air under the wing speeds up in some places. But overall, there is a net increase in air speed; therefore, there is a net decrease in pressure. At the same time, there is a net decrease in "relative" air speed under the wing; therefore, a net increase in "relative" air pressure under the wing. It's these pressure variables along the wing, and increase under the wing that aren't discussed.
Another aspect of Bernoulli's principle that isn't properly addressed is the criticism that Bernoulli's principle only applies to fluids in a tube. Some assume that as a wing in flight isn't in a tube, then Bernoulli is irrelevant. However, in a wind tunnel, it is in a tube. Even so, an aircraft in open skys can be considered as if flying in a tube. Bernoulli's principle applies in large and small tubes. A very small wing in a very large tube has no discernable interaction with the walls of the tube; instead, that small wing is contained within a sleeve of ambient/normal airflow and pressure. A high or low pressure wave/differential presses or is restrained by that "normal" air pressure, as if in a tube.
At this point there's a transition to Newtonian motion. I find it easier to think of these as displacement, deflection and thrust vectoring. When the pressure wave/differential occurs, it is resisted by the "relatively static" mass and motion of the surrounding "normal" air. This pressure differential tries to displace/move a static mass of air by either pushing against that mass, or drawing in closer. This means pressure differentials can also be co sidered as mass (floatation displacement) differentials. A mass being displaced then means forces and acceleration.
As we add movement to these masses of the wing with its associated pressure/mass differentials against the mass of the normal airflow and pressure, we now have principles of momentum to consider, thereby we have momentum differentials. A consequence of non-colinear momentum and displacement is the deflection of those associated mass bodies. These deflections thereby have resultant vectors, for both mass bodies.
So far these differentials have been direct wing interaction with the surrounding air, but after the trailing edge there is no more direct interaction. The higher air flow on the top edge is now coplaner with the downward dloping trailing edge profile, resulting in a downward flow of air/mass with inertia, which the surrounding aft airflow has to overcome to normalise. Although this mass flow normalisation is behind the wing, it is reacting against other air masses that have already been normalised, similar to the principles of a jet or rocket exhaust in air vs a vacuum.
Two other aspects that are typically overlooked and not discussed are U-wings and uplift and bracing loads on a house roof.
Both of these are outside of any Newtonian discussions I've heard, yet the capacity for high wind loads to lift a roof off the walls is very significant.
Now to explore some of your other aeronautical videos.
I love your videos! You are so informative and I have learnt a lot from you. Thank you!
3:48 The fact that the equal transit time assumption (it's so arbitrarily baseless, it's not even a theory in my opinion, only an assumption) is wrong is actually *not* proof that Bernoullis principle won't apply as the two ideas are unrelated. It's therefore not a limitation of Bernoullis principle.
4:52 Inverted flight is also not a limitation on Bernoullis principle, nor are symmetrical airfoils. In those cases, lift is generated by the AoA which lowers the forward stagnation point under the inverted wing, airspeed above the inverted wing will still be faster than airspeed below it. Bernoulli principle still explains the reduction of the static air pressure on the upper airstream hitting the inverted wing. It really isn't a limitation of the lift theory based on Bernoulli.
5:50 The "people or cars" example really hasn't anything to do with Bernoulli or lift. There is no directed outside pressure driving cars forwards against the will of their driver. If the faster cars would physically push the cars ahead through the narrowing by using brute force against their bumpers, those cars in the narrow part would not slow down (they would rather try to hit the gas because of the fear of getting rear-ended). The diffrence is that gas and water molecules are in contact with each other behaving therefore like a stream or a fluid, whereas pedestrians and cars behave like independent particles, they don't push each other around. At least as long as they're not engaging in POGO dancing.
In the case of the wing of a spaceship flying well beyond the atmosphere, if that wing is occasionnally hit by particles, there would not be lift because there would be no "stream", only collisions which will obey Newtons law of impulse and force and counterforce (action-reaction). Because in that case we talk about particles and not about fluids or pressurized gases forming a stream and nobody in their right mind would try and use Bernoullis principle in order to predict the reaction of the wing to such a collision with a particle.
Finally both Bernoulli and Newton need to be combined, Bernoulli can explain the lower pressure above the wing whereas Newton can explain the "ram air" effect creating higher pressure on the lower wing. Newton also explains why air masses are directed downwards behind the wing.
Thank you!, finally someone else said it:
"The fact that the equal transit time assumption (it's so arbitrarily baseless, it's not even a theory in my opinion, only an assumption) is wrong is actually not proof that Bernoullis principle won't apply as the two ideas are unrelated. It's therefore not a limitation of Bernoullis principle."
Look at the fan blade in engine from 1944 to 2000
Again, super cool content, thank you so much ! :)
Your Videos are very effective ❤️
thank you!
Bernoulli's law doesn't apply to lift generation. Bernoulli's law states the relationship between pressure and flow speed along streamlines, while lift requires a relationship between pressure across streamlines. The lift is generated due to the curvature of streamlines along the aerofoil. Holger Babinksy has a great paper with the mathematical derivation of this relationship. Also, recent research has claimed that lift exist because of nature's tendency to seek a minimum flow curvature state.
Actually, the Bernoulli's principle can explain - its contributions to the lift - why some aircraft can fly inverted. Without going to much into the mechanics; consider a symmetrical aerofoil. If we invert it, it would still fly because we just change the pitch -applying it in the opposite direction of our intended pitch and it would behave the same way as the one not inverted. If we are inverted, we can pitch down, creating a positive angle of attack and hence lift in the direction of positive lift considering an inertial reference frame. The same is true for the unsymmetrical aerofoil although the behaviour maybe different to the special case of the symmetrical aerofoil.
In the same breath - Bernoulli's principle can still explain its contributions to lift for a flat plate, all we need to do is change the angle of attack - this allows us to change the behaviour of flow as it traverses the top surface (going faster) compared to the bottom surface.
Good job 👌
Upper particle can be faster or slower, there is no guarantee for example at high angles of attack.
yes, but the point here is that they don't have to meet up together at the end
@@AliyaBurkit exactly.
Wooow New video
Yep, haha
At aeronautical engineering school we used to say that Bernoulli's Principal sucks and Newton's goes down. We were typical, crude, libidinous college students, now we are typical, crude, libidinous engineers.
Ah, Progress.
Nice explain thanks
MAKE A VIDEO ABOUT THRUST FORCE ...
yes, it's coming up
when will you upload newtons third law detailed about lift please i can't wait. do it fast
It's already out there! ruclips.net/video/RwN00nYzzvM/видео.html
If you don't mind can you upload introduction to aerospace: propulsion please
Yes I will very soon
The cause of lift is the acceleration of air downward. The is no other cause. You can not accelerate air downward without getting an upward force, nor can you have an upward force without accellerating air downward.
A pressure differential does NOT cause lift. The bernulli effect does NOT cause lift.
Rather, a pressure differential IS lift, caused by the deflection of air. The equal transit time hypothesis simply confuses the cause with the effect. The bernulli effect can help you understand HOW a force is transmitted to a wing surface, but does NOT explain the cause of the force.
Your English is great but yeah there's no shame in speeding up or slowing down the video to suit personal preferences. Can't wait to learn all the things and who knows maybe I'll finally scratch design and build* a RC plane! By build* I mean let a 3D printer do all the work ;) Let the bingeing begin!
Thank you 🙂
You’re welcome 😊
Can you continue to make lectures about aerodynamics? It would be so helpful and your subscribers would rocket :) .
I will do a little bit of all fields, starting with introductory topics first, then go deeper on each one
Look at turboprop on TU -95 this is German made prop
Lift is provided by God 🤗
for long time you are far. you are welcome !!
thanks!
You have some good points, but the traffic example is not difficult to understand. You are discussing lift in the incompressinle flow regime. Air molecules at normal temperatures and pressures act essentially like water and do not compress. Traffic flow is compressible. That is the distance between cars reduces, or compresses, as it comes to an obstacle or a reduction in the number of lanes. This phenomenon happens when aircraft approach the speed of sound. This is why aircraft design for supersonic flight is different than for subsonic flight. As far as inverted flight, yes, angle of attack and alot of thrust are your friends.
i can use a flat piece of plywood as a wing with an angle of attack-
Wow you come back
I did :)
Coanda
Circulation
I've read another version of Bernoulli's principal based explanation.
It is from the book University Physics by Zemansky. It says: " The flow lines crowd together above the wing, correspon-
ding to increased flow speed and reduced pressure, just as in the
Venturi throat."
What I understand is that the air flow above the wing is 'squeezed' (similar to what would happen to a flow in a constricted region of a pipe). Thus the flow speed increased and pressure is lowered.
The author also adds that this is the very reason why the umbrella seems to generate lift in windy days (and a strong wind can turn it inside out).
My observation: Early biplanes like Wright brother's plane had a simple curved 2D wing cross-section, (inverted curve). It was nothing like modern airfoil.
Btw excited for more videos!
Every time I discuss with my brother he gets pissed.
Change in pressure causes fluid to move not the opposite.
Air is faster above the wing because of a favourable pressure gradient along the flow direction on the suction side and an adverse pressure gradient on the pressure side. This is due to the curvature in the flow as the wing passes through the air due to the way a wing is designed. Newton's second law.
Wings are also designed in a way to produce downwash. Newton's third law.
Newton's second and third Laws explain lift completely and correctly.
the suction effect is not explained by Newton's laws
It is explained by 2nd law.
Consider two particles in the stagnation region.
At the suction side, the streamlines are curved in a way that there is low pressure on the suction side as compared to the stagnation pressure. Therefore the prticle at the suction side get accelerated.
At the pressure side, the stream line curvature is in a the way that pressure gradient is adverse as compared to the suction side.
Apply 2nd law to both fluid particles or to a small control volume of particles at suction or pressure sides. This is why speeds re different.
@@fadoobaba exactly this phenomenon is not fully understood yet - "the streamlines are curved in a way that there is low pressure on the suction side"
@@AliyaBurkit It is well understood, specially in the case of flow through pipes and cyclones etc. It happens due to centrifugal force.
Whenever there is a bend in a pipe i.e. streamlines change direction/bend etc. there is change in pressure gradient across the streamlines. Same is the case with typhoons, cyclones etc. Also, when there is high curvature of the stremline, there is high pressure gradient between successive streamlines and low absolute pressure like in the center of the typhoon has high speed and low pressure.
There is a lecture of Dr. Holger Babinsky in which he explains it very clearly what I am trying to say.
Can't you prove all different mechanisms by making "infinite" length wings with the perfect shape for every principle, and calculate the amount of lift they produce? And then give them all relative weights so you know when you combine them how much each contributes to the lift. Droplet horizontal wing, angle of attack flat surface. Differentiate the shapes of the first to seperate third law effects and Bernoulli effects
Man really feel bad I didn't study maritime tech as a major, would have loved it
Where does your name originate from?
do you work at nasa mam? if yes, i was at my right lecture... i am just 7th but i want to be an astronaut from now... suppose if ur doing space craft mechanics, can u send me the portions? also thank you for teaching explainatory and i will be an absolute astronaut :)
Edit: are u doing 1 to 1 online class teaching mam. If yes, i would love to join it :3
I don't work at NASA but that was my dream. Now it's just on the back burner of my mind. If you want to do an astronaut, you need to know a lot of things, so this topic is on my video list!
@@AliyaBurkit ok thanks mam
You're welcome
Your explanation of Newton's theory is not quite right. The third law is just a secondary effect, the primary effect if the second law of motion. F=ma. When we accelerate a mass of air, we are essentially creating a force. that force is directed downwards at the trailing edge and hence create lift due to Newton's third law of motion. However, that theory still gives me some doubts because if we closely look at the angle at which the flow leaves the trailing edge - the wing should also create thrust which is far fetched :/
When flying my Helicopter I always assumed I'm beating the air into submission. That's why we call them Choppers. Otherwise great video.
Bottom line = Air is accelerated downwards; airplane is accelerated upwards (against gravity). Everything else is detail.
R u a nasa scientist
No, haha, just striving to be
t-shirt is from H&M
@@AliyaBurkit nyc..I like ur way of teaching n I liked that t-shirt also..😄 I have learnt compressible n incompressible flow from u...that was outstanding
and from where does the velocity of the air OVER the wing get this extra energy to speed up ... THAT is the incorrect portion of the statement ... it is a direct violation of thermodynamic law ... energy was created out of nowhere NOT converted ...
.
technically the velocity does NOT change the the pressure does because it is filling a "vacuum" ... actually it's replacing the air that once was at the same slice in space as the air under the wing ... which is essentially a virtual vacuum ...
.
molecule A of air and B of air are equidistant from the wing Leading edge and in direct line above each other ... once the reach the leading edge they change velocity and direction and follow the wing ... one over and one under .... the one under has a shorter path normally and so carries on at almost the same speed it came to the wing at and in the same relation to where A should be ... while A not only has to change velocity but also has to move up and away from B it therefore no longer is above B because it takes TIME to change ... so as B leaves the trailing edge of the wing A will be some distance behind it ... THAT is where the lift comes from it is the direct difference between location A and B ... it would be a pressure ratio and either add or subtract from the air pressure around ... and for lift it subtracts making it a lower pressure above and a higher pressure below causing the bottom air to push the wing UP ... while if A leaves the trailing edge before B the it is sucked down as A is a higher pressure than B .... also known as stall or stagnation ...
.
this is where most call the basic lift theory wrong ... as it needs to speed a velocity up with NO input in violation of thermodynamics ... energy can neither be created nor destroyed only changed.
and if you measure the velocity of the air over the wing and under the wing ... they will be the same ... so it holds BUT it is not an accurate theory ... as it is the pressure relationship between the two parts of the air over and under that causes lift and yes it still agrees with bernoulli ... and Thermodynamics ... as the fluid (air) isnt created and sent as a sheet or wave at the wing it is all encompassing and the wing is either passing through it or the air is being passed over it ... (or both) so even though A and B are at different locations at the trailing edge from where they started before the wing ... the air before they arrived is still there at a different pressure ... as it went from a compacted zone to an expanded zone and actually LOST pressure to fill the less dense space ...
the turbulent flow is caused when the air cant be replaced by the existing volume in that location fast enough causing the lower air to back flow ... increasing drag and reducing lift and thrust .... the pressure change is just a ratio of compression over and under the wing ... at some point for an airfoil that ratio degrades to a point where the lifting portion of the wing is smaller than the drag portion of the wing ... also the laminar flow is at some point lower than the turbulent airflow ...
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the speed change measured in a wind tunnel ISN'T comparing the two particles it is comparing how fast the air is being replaced on one surface over another ... if you put that wing in the tunnel and have it run to create lift and then add smoke and SLOW MOTION capture the smoke as it passes over the wing ... the smoke on top is slower than smoke on bottom by a small fraction ... as the angle of attack increases the "stickyness" of the wing surface is not as strong as the normal volume of air ... so it leaves the wing surface causing turbulent flow from under the wing to backflow ... THIS is where the coriollis effect plays its role in lift and why is should never be ignored or back burnered as it is what makes lift happen and yes it is a small affect in MOST situations ... BUT not all .. and it's minimal influence in some aspects can be ignored temporarily once it can be shown that the effect of the affect is too negligible to consider above the error ratio ...
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whats so hard abut that ... nothing and it does cover all the ranges of flight ... and all the wings shapes .. and angles of attack ...
and John Anderson is right ... the closest one can get to a simple explanation is - Lift is a multi science discipline that no one has been able to fully implement so far ... because we dont properly understand how nature actually works ...
U look young WATS ur age
I'm not that young 😂
All these theories are wrong.
Sweet girl but for that NASA T-shirt.
NASA equates with stupidity.
🙉No explanation for lift but aircrafts are allowed to fly ?🙊
🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒🐒
you look cute 😍
😄
this is just sad...