Hi Jim, circulation is not a strange mathematical condition, although most engineers tend to express it that way. The circulation is the manifestation of the viscosity. The use of the term “inviscid” lift is why engineers think that; and another justification for the existence of circulation is needed, KJ theory, the Kutta condition. But if people accept that D’Alembert’s paradox, as confirmed by Euler, which predicated no drag and no lift, is the result of no viscosity, then it become much clearer. Fundamental Eulerian flow is inviscid and hence lacks any rotation. Now, in CFD you can have what is called mathematical dissipation which emulates viscosity, which is a dissipative force, it is diffusion, and is non conservative, but that is a mathematical construct that comes from errors in numerically solving Euler’s equations. But, the potential flow solutions you have shown, without circulation, are analytic solutions, and no numerical uncertain is involved, hence there is no rotation (no turbulence). Potential flows, analytic Eulerian flows, are irrotational. And with all due respect to Prof. Fidkowski, while an infinite number of solutions are possible, the uniform flow direction and relative position of the aerofoil dictate the single solution that is allowed. This is the symmetric solution. That symmetric solution, BTW, gives symmetric pressure and velocity, and hence equal transit... Also, with all due respects to my colleagues who are physicists and mathematicians, while an equation can have a singularity in it, reality does not. So, even a wing on your aircraft, when it starts moving, has a symmetric potential flow solution, and the acceleration is not infinite at the TE, but it is significant. It is this with viscosity that starts the formation of a vortex at the trailing edge; this grows, and that centre of rotation is a low pressure. This then sucks the stagnation point down from that symmetric point to the trailing edge. That is, the Kutta condition is caused by viscosity. This is accompanied by a vortex being shed from the trailing edge in a counter clockwise direction. Noting you have an appreciation of physics, it will be easy to accept that conservation of angular momentum is also required, hence some other part of the fluid must rotate in the clockwise direction. This is the circulation, or the bound vortex around the aerofoil. Effectively, the viscosity of the fluid has made it such that fluid more easily flows over the upper surface (it moved the stagnation point, so this should be intutive), so it has a higher velocity, and the converse is true for the bottom surface. That is, the viscosity term in the Navier Stokes equation, the correct form of Newton’s laws for fluids, alters the balance between the pressure and acceleration on the upper surface relative to the bottom surface, and this persists as an asymmetry in the boundary layer. So, viscosity causes the circulation, which results in a difference in flow speed, which through Bernoulli we can show as a pressure difference, which is the actual force pushing up and down on the aerofoil. Yes, if you take away or try to simplify one of those steps, “the wheels come off the car”. So, don’t, and if people try, tell them not to. There are really only 3 terms in Navier Stokes, acceleration, pressure, and viscosity, so no more or no less is needed to explain lift.
I thought I´d never get it, so, thanks for this video. Finally I understand the Kutta-Joukoski-Transformation, and its actual role !!! Also, about the named reason of how "myths" around lift rise: If u let the theorem out, and the math involved with it, u are left back with a mere "skelett" of an argument, the Bernoulli, and that´s too little to avoid myths from rising. Also, I noticed that the Kutta-Joukowski-Theorem gives a better approximation for the overpressure under the airfoil, than for the vacuum over the airfoil 11:35
The manner in which I see circulation in wings is through a series of steps starting with a helicopter while hovering. In order to hover and so lift itself, a helicopter needs to converge the fluid air mass above it and pull it down and accelerate air mass through its main rotor. The downflow through the rotor has a high velocity "ring" which after it passes below the main rotor it will diverge,. decelerating to a lower air velocity and slowly it will move outwards to circulate upwards to converge back and re-accelerate through the main rotor. Basically, the process causes a big doughnut ring shape that circulates within itself, similar to the bubble rings made by dolphins in the water. We may call this a horizontal doughnut shape circulating within itself. Interestingly enough, if one cuts the horizontal doughnut ring with a vertical plane across any large diameter, what one sees are two vortices similar to those generated on the wingtips of a wing of an aircraft, moving forward. If we imagine a tighter horizontal circulating doughnut ring brought about by the hovering helicopter, this doughnut ring needs to circulate to provide lift for the helicopter where the velocity in the circulation needs to vary to be able to be reaccelerated to create the lifting force. Assume that the vortex velocity on the outer surface of the doughnut/ring circulation, is 10 units, circulating as necessary to lift the helicopter. If this circulation in the horizontal doughnut ring is made to travel horizontally with a velocity of 100 units, then the upper surface of the leading part would be traveling at 110 units while the lower surface would be traveling at 90 units,. hence this will cause a lower pressure on top and higher pressure below the lower surface, hence an uplift ( Magnus effect) Interestingly enough the back trailing part of the moving, doughnut with circulation, will have its top surface moving at 90 units while the lower surface moving at 110 units causing a down lift. The side parts of the forward-moving doughnut ring with a circulation, moving parallel with the direction of motion of the doughnut, will be exactly the same as the two wingtip vortices on the wing of an aircraft. th-cam.com/video/dfY5ZQDzC5s/w-d-xo.html If the doughnut/ ring keeps its circulation but the doughnut shape is changed into a toilet seat shape with a flat surface, then we may consider the circulation to be that around a wing traveling forwards at a velocity of 100 units and a circulation of 10 units. At such ratios of forwarding speed to circulation, one would not be aware of the circulation but it exists if the lower velocity below the lower surface of a wing is slower than that on the upper surface. One can retain the forward half of the toilet seat which could be said to be closing up on resembling an aircraft wing with a forward velocity and a circulation around it complete with two wingtip vortices. It is interesting that a horizontally moving doughnut with circulation, will have an uplift in its leading part acting as a wing and a down lift in its trailing part acting as a down elevator, and this uplift and downlight on the leading and trailing part would finish up with the circulating doughnut ring moving in a vertical plane whose normal is in the direction of travel. This analogy seems to fit well the circulation around the wing which causes it to lift, and also includes the wingtip vortices around the sides moving parallel with the forward motion of the toilet seat with circulation. The circulation around the toilet seat or the doughnut ring will always come in pairs!!!!! front and back circulation and left and right circulation as in the wingtips of an aircraft. Realy there is nothing new here as the downflow behind the wings of an aircraft is in fact the downflow in the center of the doughnut ring with a circulation. dolphin bubble rings circulating as helicopter rotor stramlines th-cam.com/video/bT-fctr32pE/w-d-xo.html Note how they seem to travel forward with their plane at a right angle to the direction of motion.
The easiest manner to see the circulation around the airfoil of a high aspect ratio wing is to first study the helicopter in hover mode, where the fast accelerated central downstream of the main rotor and the slower decelerated upstream, outside the main rotor, forms a circulating doughnut ring., It is, in fact, an amplified extension of the wingtip vortices of the main rotor. If the helicopter, from a hovering mode, goes into a forward motion, this doughnut with circulation will still exist and the front leading part and the rear trailing part will be subjected to the Magnus effect where the leading circulation will uplift, while the back circulation will down lift. The side circulations will be exactly the same as a pair of wing tip vortices on a conventional wing, with their axis in line with the forward motion. Since the rear Magus effect is producing a down lift, the back part of the circulating doughnut can be discarded while the opposite rotating pair of side vortices with their axis moving in the direction of forwarding motion would still dump the air into the center of the modified forward-moving doughnut, where the front circulation can be obtained and sustained by doing away with the main rotor and replacing it with a forward-moving plate having a large angle of attack, where the lower surface producing a compression zone will slow down the moving air while the top surface will create a suction to speed up the airflow and the circulation around the airfoil will remain sustained, existing to complement the pair of vortices at the wingtips. It is the dumping of circulating air behind the wing or the central downstream of the air by a forward-moving, or stationary helicopter doughnut shape with circulation, that creates lift. What is accelerated and centrally dumped down under the main rotor of a hovering or a forward moving helicopter, is exactly the same Newtonian philosophy as what is dumped behind the trailing edge of a moving wing and those pair of vortices at the wingtips. That inconspicuous doughnut shape, with a circulation, exists in a hovering or forward-moving helicopter as much as it exists in a moving wing, where due to its forward motion becomes phased with the moving doughnut with circulation. The wing leads the central part of the doughnut. The back part of the doughnut shape is left far behind and is replaced by the back stabilizer of an aircraft.
Here is one practical interesting concept of circulation around a wing. Let us assume that we consider the WW2 Lightning aircraft with two booms connecting the main leading wing to the horizontal rear stabilizer. With imagination, this squarish " flying frame" with a rectangular aperture in its centre can be compared with either a toilet seat or a picture frame with wide strippings. If a helicopter rotating its main rotor was structured inside the inner rectangle, then the WW2 Lightning twin boomed aircraft, could achieve VERTICAL TAKE OFF, due to acceleration of air from above to down below it, which will circulate at different circularly vectored accelerations and velocities, enclosing the individual parts of the frame of the rectangular shape, picture frame aircraft. This procedure will form a horizontal rectangular doughnut shape with circles spinning around all four frames parts that make the shape of the twin boomed aircraft. If the aircraft had to be propelled forwards with additional horizontal propulsion, the rectangular horizontal doughnut with internal circulation will have a Magnus uplifting effect on the leading wing and a down lifting effect on the trailing wing, with no lifting effect on the side boom wings moving parallel with the direction of motion, Assuming that the rectangular doughnut circulation obtained from the helicopter rotating rotor is 10 units and the rectangular shape boomed aircraft is moving forward at 100 units. Then above the front leading part of the rectangle ( a front wing), the velocity will be 110 units while that below it will be 90 units. causing a Magnus effect uplift. The rear trailing part( a rear wing) will have a velocity of 90 units above it and 110 below it and so the differential pressure will cause it to downshift, while the circulation around the side booms in conjunction with the forward velocity will cause the circulation to become an elongated helical shape, rather than the circular flat " circles" remaining at the leading and trailing parts of the rectangular twin flat boomed aircraft. The down-lift at the rear end is undesirable, and so the following procedure can be taken. The rear straight part of the rectangle with a circulation around it can be moved further back or lifted up higher as in high tailed aircraft or else the main rotor of the helicopter could be replaced with the down lift effect of many smaller fans placed closer to the trailing edge of the front leading wing of the rectangle. We can go even further than that, and totally eliminate the rotor or the fans in the central rectangle to sustain the downdraft by other means. Giving a high angle of attack to a cambered curve making the front wing of the rectangular-shaped aircraft, will slow the air particles underneath it and speed up those above it and this will sustain the circulation around the front leading wing, still dumping its downgoing airstreams circulation behind its trailing edge ahead of the rear part of the rectangle, which now is away behind or above, from the effect of the trailing edge down dumping of the circulation around the front wing,. This rear part of the rectangle becomes a horizontal stabiliser of the WW2 Lightning aircraft. The circulation around the twin booms will still exist as the elongated wingtip vortex is now shaped like a helix. We shall assume that the WW2 twin flat boomed Lightning has switched its engine off to eliminate the swirl of those two propellers driven by Allison engines !!!!! and that the twin booms have been shifted to the wingtips!! it is to be noted that all Energy Forces for the acceleration forces to produce the front motion and the circulations and varying accelerations of air-mass particles around this rectangular flying frame are all obtained solely from the energy and power of the engines or the rate of change of lost altitude or thermals etc. In a wind tunnel, all the energy for circulation is obtained from the energy of the motors driving the air particles wind tunnel For a forward-moving aircraft, the atmosphere particles have the following Bernoulli equation C= Pa + 0 while all the circulations and other linear velocities that the air mass particles can gain is Varying = Pa + energy from the engines, which will result in accelerating and moving the air particles to varying amounts of P1+ (V2)^2/2, which later on lose all the gained energy from the engine to finish up with the original C= Pa, a long way behind the aircraft. It is important to note that around a wing, the air particles are constantly gaining energy from the engines through the effect of suctions and compressions around the wing. If the suctions and compressions maintained by the ENGINES accelerate the mass particles in a downward direction then that is an UPLIFT on the wing, If the suctions and compressions maintained by the ENGINES accelerate the air mass particles in a forward direction then that is DRAG. Those particles of air in a Bernoulli streamline are continuously robbing energy from the ENGINES and losing it to other issues and hence around a wing one cannot say that Constant C= P + (V^2)/2.
Let us consider the lift to drag ratio of a wing is 50: 1 as in a modern fast glider. Let us consider the drag as being a horizontal force while the lift is a vertical force Force = Pressure multiplied by area. Let the area of the driving propeller be Ap and area of wing be Aw while pressure Pp is the differential pressure on the blades of a prop and Pw that on the upper and lower surface of a wing. Force generated by propeller = Ap*Pp and force on the wing = Aw*Pw Using the lift to drag ratio mentioned then 50*Ap*Pp= Aw*Pw ....... equation 1 Assume that the differential pressure across the surfaces of the blades of a Propeller is twice that of the wing, hence Pp= 2*Pw Substituting in equation 1. 50 *Ap * 2* Pw= Aw*Pw Hence 100 *Ap= Aw Which is not a bad amplifier of forces and I look upon this as an operation containing A limited CIRCULATION (orientation) of 90 DEGREE with an AMPLIFICATION OF 100. Hence a wing is no more than a vector amplifier and a ( 90 degree) operator of drag forces into lift forces. In this case, 1 unit of Drag is operated upon to produce 100 units of lift oriented with respect to the Drag by 90 degrees. in this case, wing lift = wing Drag multiplied by the operator (100 oriented by pi/2) Let j be a 90-degree operator. or Lift = Drag* 100*e^(j.pi/2) where 100 is the resulting product of, (lift/ drag ratio) multipled by (ratio of propeller/ wing differntial pressure). No wonder we do our best to change the shape of a flat plate to one where the down angle of the leading edge and the slope after that, are designed to cause the flight path of the air particles to contain ACCELERATIONS all the way above and below the wing and not to lengthen the upper surface!! That wing is like an amplifier and pi/2 phase changer, which operates on the magnitude of the drag ( hence related to engine power) to amplify it and orient it by 90 degrees, it contains all the findings of Newton, Bernoulli, Venture, Coanda, Navier Stoke, the Wright Brothers, Bleriot, up to 1940 when Sir Frank Whittle at a young age stopped thinking of Bernoulli equation as simply, A Constant = P+ V^2/ 2 and saw it all as *****continuously varying contents of ( gravitational forces)+ ( static pressure forces) + ( velocity forces) + (internal energy forces due to explosive chemical or atomic energy) + ( external energy contributed by external heat, all types of suctions, and compressions around a pair of curved surfaces on a propeller, fan, compressor, diffusers, turbine, or a pole in the wind shedding vortices to swing it about. Contained in all that is a modern aircraft taking off and flying and landing and occasionally crashing. I wonder how many pilots see flying as explained in the last paragraph ***** I see One aircraft as being no more than one single big complex particle, traversing the streamline of the Bernoulli equation applied to a giant Venturi, or should I include Newton with that!!!
Circulation isn't an intuitive concept, so thank you for trying to help people understand it more. However, I don't think that your comparison to a helicopter is useful here. First, you've added a dimension to the problem. In the video, circulation is presented as a 2D problem, but this helicopter model requires 3D. I recommend keeping the dimensions as low as possible when trying to understand a new concept. Second, while the airflow in the helicopter example may be circulating, that is not the circulation the video talks about. Sadly, the same word is appropriate for both contexts, but it's not the same event. The circulation in the video must always be paired with a free-stream flow to describe lift. It's also important to note that the free-stream + circulatory flow is in the same plane as the airfoil cross section. However, in the helicopter example, the circulation described is not in the same plane. In fact, the helicopter circulation plane is orthogonal to the airfoil cross section plane. Your points might very well be valid, but not for this type of circulation. I just wanted to leave this here so that others who are trying to grasp this concept don't get confused by this.
@@tristanc6967 Yes, you are correct, and in fact, before I wrote my comment I did wonder if I would submit it but this is the manner in which I thought. If one looks from ahead or from the back of an aircraft and sees wings coming or going away from him, then the two " circulations " he sees at the wingtips are exactly the same as a helicopter but the circulation is in fact a helix as the aircraft is moving forward always accelerating stationary mass from above and under the wing DOWNWARDS. which is the reason for lift. Now if you look from the side of the aircraft to see the normal streamlines these too are part of the streamlines on the much larger picture. Unfortunately looking at the streamlines around the airfoil section does not normally show the bigger picture. In a helicopter that is hovering, if we go around it we would see the same overall picture. but not with a winged aircraft moving forward. Now if we had to set the helicopter in a forward motion or a gyrocopter the whole picture will change and there will be an additional circulation due to the forward movement. So I thought that with a wing moving forward and a helicopter moving forward in the process of "ACCELERATING THE AIR MASS ALONG THE "CIRCULATING MASS" we see in a hovering helicopter and then the " additional circulating mass " along with the circulation when the helicopter is now moving forwards at speed the circulation will be somewhat helical as in the case of a wing moving forward. No matter how we think about the wings in a helicopter or a winged aircraft be it Bernoulli or Circulation or anything else THE REAL TRUTH ABOUT LIFT is that mass of air is accelerated continuously hence continuous lift. Here we must discuss a much larger picture than what is happening in the atmosphere even up to a kilometer away from helicopters or wing aircraft. The acceleration of masses of air from above a forward moving helicopter and a forward-moving wing is the TRANSIENT effect that causes lift, but the accumulation of air below the helicopter or the wing must eventually circulate from down below from where it was thrown to cause lift and then it will make its way up again to regain the distribution of air solely due to gravitational forces where other following aircraft will again accelerate the stationary air particles through a transient fast localized acceleration and the air will redistribute itself at a slower rate of change of state to set by gravitational forces. I feel that if one looks upon a hovering helicopter, yes the flow is in quadrature but only because the helicopter is hovering but if the helicopter moves forwards then there must be a HELIX flow as when the winged aircraft moves forwards. I thought that hidden in all this acceleration of masses of air in the immediate vicinity of the flying craft will circulate again to resettle due to gravitational forces. The circulation that is in 2 dimensions and to which you refer and you are correct, must exist in the whole picture I tried to describe. Again please note that to see this circulation in a helicopter then the helicopter is not hoveri9ng but it must be moving forwards and I do think that the WHOLE BIG PICTURE of these circulations be it a winged aircraft or a helicopter moving forwards or an F-35 or a Harrier hovering and translating forwards, what Bernoulli and the circulation theory that many people consider ARE ALL CONTAINED IN THE BIGGER PICTURE OF ACCELERATING THE MASS OF AIR DOWNWARDS close to the surface of the wings and then this accumulation of air below the flying craft will slowly circulate back to the steady-state distribution where only the force of gravity or the blowing wind exist. Again please consider the helicopter moving forwards and not only hovering to compare it to the wing and its very big circulation in that large helix contained in a volume away from the wings. I thought that concept would add some mind processes to what we conventionally consider. Thank you for your comment.
At 1:13 you mention "Friction free, inviscid fluid" but ignore the third vital component of Circulation Theory which is that it applies in a fluid Flo in conservative field: i.e. one in which there is no net change in energy. This is clearly not the case in an aerofoil where there are energy changes through turbulence and friction. Circulation Theory adds nothing but confusion to any discussion about aerodynamic lift.
Yes, it does muddy things from a number of angles. But it does still pop up here and there, so I thought it would be worth touching on. Thank you for the comment.
Hi, thank you for this video. What do you think about the new theory of lift by Gonzalez and Taha (variational theory of lift)? it proves, among other interesting stuff, that Kutta condition is a special case of their theory of curve minimization.
Don't know much. But it appears that It does address some of the arm waving required to apply the Kutta Condition to a not 0 radius for real airfoil (or just accepting that you get a smooth flow off the trailing edge). But I wouldn't say that it is a very intuitive, feel good, explanation for non-math people.
Here is an interesting manner of how to think about circulation and associated effects including countermeasures. Using the Cambridge Video, th-cam.com/video/6UlsArvbTeo/w-d-xo.html please refer to time 0:04 to 0:10 Note how "two Bernoulli related VENTURI situations" are shown in the video, one is above the leading edge and one below it. They both start well ahead of the leading edge. (Note I mentioned Bernoulli but that goes with Newtonian and viscous effects) * The one above starts with a slow wide high-pressure zone converging to a fast low-pressure restriction above the leading edge then diverging again to a slow wide higher pressure zone above the trailing edge. * The one below starts with a " faster" lower pressure zone diverging to a slow high-pressure zone under the leading edge then converging again to a fast narrower low-pressure zone below the trailing edge. I find it fascinating how those two " Bernoulli combinations VENTURI surrounding a wing " one above, the other below, AND SKEWED OR DELAYED OR PHASED out ( because the AOA) with respect to one another is what makes a wing work as it does, where the energy in those fluid particles is changing all the time due to the power sources in use engines and wind tunnels flow sources and the losses associated with the surroundings. Let us now discuss a very interesting matter. Looking at " Bernouilli flow lines" one set above and one set below the wing with phasing in between. Of interest is the fact that in the low-pressure areas, one is over the leading edge and the other under the trailing edge, while the high-pressure zones are situated one under the leading edge and the other over the trailing edge. There is absolutely no doubt that the pressure distribution would provide LIFT to that airfoil. but if we take THE CENTRE OF THE CHORD AS REFERENCE POINT THEN, THE PRESSURE DISTRIBUTION WOULD ALSO PROVIDE THAT WING WITH A TORQUE. which would try to SPIN and " circulate" the wing about the centre of the chord to gain a higher angle of attack. Hence, investigating carefully those two-phased out " Bernoulli shaped VENTURI flow lines" one above and one below the wing, the result is A LIFT AND A TORQUE OR A CIRCULATION OF THE WING. This torque and lift are counteracted upon by shifting the weight of the aircraft to the location of the centre of lift to produce a counter torque. To ensure full control of the lift direction and AOA is to add a horizontal stabiliser to counteract the TORQUE WHICH THE PHASED OUT TWO BERNOULLI VENTURI FLOWS on the wing always bring about. Seeing that hardly anyone talks about circulation I do not think that anyone would mind if we standardise this aspect of how a wing operates in saying. " Taking the centre of a plate ( airfoil) as a reference, then due to the angle of attack and the forward velocity of the plate in a fluid, the resulting pressure distribution would always produce a LIFT and a TORQUE on a plate, trying to spin the plate in addition to lifting it. Perhaps one may say that a moving plate with an angle of attack has both LIFT and TORQUE and CIRCULATION. This Torque/circulation of the wing is counteracted by applying a counter Torque, through moving the centre of gravity to the location of the centre of the lift which is known to be about 1/4 to 1/3 of the wing and not at its 1/2 point as referenced for torquing purposes. Perhaps one can also say that the horizontal stabiliser/ elevator is also a factor that counter opposes the torque/ circulation in a wing!" Note that this is a philosophy open to discussion. It is however interesting that if one drops a long strip of paper, it will spin fast as it drops and also moves sideways. Hence the LIFT AND THE TORQUE leading to spinning and circulation and lift. Now we come to a point where all the physics have to join in to stabilise a wing with weights and other control surfaces. It is no wonder that all the flying wings through flying history ( including some seeds from trees) were so dangerous to fly, not due to their: LIFT but due to their circulation obtained through this inherent torque that is always associated with LIFT in a plate/ airfoil, moving forward with an angle of attack, in a fluid.
People who find some difficult to comprehend Circulation and its contribution to lifting should start with considering the lift in a helicopter while hovering. The downdraft from the main rotor normally circulates air by first accelerating it down where it diverges out to decelerate and then goes up to be reaccelerated through the rotor. This creates a doughnut shape with a circulation in it. As the helicopter inclines to move into forwarding flight, the circulating air added to the horizontal forward motion will cause an extra lift on the front circulation which tends to BALOON the helicopter and a pitch control needs to be applied, where the cyclic pitch would reduce the circulation of the front vortex, hence less lift, till the helicopter has enough forward speed that the downdraft in the rotor is the same as a normal winged aircraft, including what would be regarded as the wingtip vortices, by treating the whole rotor as the area of a fixed-perforated wing with its own wingtips!. An interesting experiment to see circulation is to take a long strip of stiff paper about 2.5 inches by 10 inches and fold it such that it would have a strong straight spine retaining its flatness as a plate wing. Raise the plate to head high and drop it at an angle. The plate will fly and then increase its angle of attack till it climbs again to form a U flight and then it reverses to fly again a series of U flights rotating and spinning in the same lateral direction while dropping to the ground. If one investigates the streamlines around a flat plate with an angle of attack to the flow, one finds that there are two venturi effects phased out with respect to each other between the leading edge and the trailing edge, whose combination caused both LIFT AND TORQUE on the plate. Hence when the plate is dropped it will suffer both lift and torque to spin it and so it does, circulating freely as gravity acts as propulsion produced lateral lift that pulls it sideways while the torque spins it while dropping in a series of U shapes short flights. The interesting issue about a flat plate with an AOA, is that as it starts it leaves a vortex behind it, and so it retains the other as they come in pairs, and what the plate retains seem to spin it or circulates it as this plate wing or stip of paper does so beautifully, becoming a lifting rotating cylinder in fact! It has to fly freely to see this spinning effect or circulation. It is interesting that when a plate is held in a wind tunnel to test it we are applying torque and a force to hold it stationary as we do when in a wing we move the weight forward to coincide with the center of lift or put a horizontal stabilizer at the back of an aircraft............ which in fact is counter torquing the inherent natural lift and spin/circulation of a plate wing.
@@carmelpule1 Please note that to start with one should look at a helicopter in a hover, but to compare the circulation to which we normally a wing moving forward, then the helicopter must be considered to be moving forwards. It certainly adds to the complication of the concept of trying to visualize the whole picture. Really the circulation would also be seen if one considers the streamlines around the immediate upper and lower surfaces of the main rotor, and what happens to that air that is accelerated below the rotor, it will certainly make its way up again away from the rotor to distribute itself to that " pressure" zone decided by gravitational forces. Also, the circulation in a helicopter in hover is what it is because of the pressure built up under the axis of the rotor which will push the overall circulation in quadrature with the blades of the rotor, but to the ahead movement of the rotor blade, there will still be a circulation if 2 Dimensions. To be able to see the " conventional circulation" around the blade of a helicopter one needs to home in close to the surfaces of the blades but when the helicopter is moving forward there is another style of circulation which is what we refer to in a moving wing. If in a moving wing we look ahead or back on the moving wing the two circulations at the tips of the wing will be similar to that of a hovering helicopter. One suggests that in addition to seeing the functions close to the surfaces of a moving wing or a translating helicopter one needs to see the whole picture by looking at the resulting paths of the moving masses from saying one kilometer away to see the whole block and the " various" types of circulations that are occurring while one must not stay in the same location but look at the whole picture orienting oneself 360 degrees around the craft moving in air particles with mass. With rockets moving in space the exhausted mass from the rear or side thruster of a rocket does not circulate to redistribute themselves as the mass particles in our atmosphere needs to do due to gravitational effect, neglecting the winds of course.
What you believe is wrong. Proponents of circulation theory like myself would just say total lift is proportional to total circulation, and nonsense about equal transit time just doesn't figure. The Kutta condition fixes the circulation at a definite value. If the Kutta condition were not complied with, then a starting or stopping vortex would be shed from the trailing edge of the wing until it was complied with. If a large aeroplane takes off, the starting vortex is left behind on the runway, and may take a few minutes to disperse. I would not climb aboard an aeroplane with a pilot who was not aware of this, or of the vortex system of a wing in general. Both the starting and the stopping vortex can actually be visualised, and total lift is proportional to total circulation. Modern computational fluid dynamics can just as well use vortex methods as finite difference or finite element methods. We can compare them with each other and with solutions derived from conformal transformations to build up confidence.
I'm not sure what you think that I believe... The Circulation theory has been around since the very early 1900's. And, if you assume and ideal inviscid fluid, Circulation plus the Kutta condition give you a reasonable model of the airflow and lift. There is no question about that. Also, most pilots are aware of wake turbulence. Also, as you point, out equal transit etc. is the product of the south end of a northbound horse. But, the reality is that trying to explain the concept of a virtual vortex that flows "upstream" under the wing while there isn't any actual flow in that direction seems to fall pretty flat with non-technical pilots or the general public. And, as I mentioned, I believe that attempts to "simplify" circulation is what has lead to some of the fairy tales that you find in the popular media. So, while circulation isn't "wrong" I prefer to not go there. In any case, thank you for the interest and comment.
Hi Jim, circulation is not a strange mathematical condition, although most engineers tend to express it that way. The circulation is the manifestation of the viscosity. The use of the term “inviscid” lift is why engineers think that; and another justification for the existence of circulation is needed, KJ theory, the Kutta condition. But if people accept that D’Alembert’s paradox, as confirmed by Euler, which predicated no drag and no lift, is the result of no viscosity, then it become much clearer.
Fundamental Eulerian flow is inviscid and hence lacks any rotation. Now, in CFD you can have what is called mathematical dissipation which emulates viscosity, which is a dissipative force, it is diffusion, and is non conservative, but that is a mathematical construct that comes from errors in numerically solving Euler’s equations. But, the potential flow solutions you have shown, without circulation, are analytic solutions, and no numerical uncertain is involved, hence there is no rotation (no turbulence). Potential flows, analytic Eulerian flows, are irrotational. And with all due respect to Prof. Fidkowski, while an infinite number of solutions are possible, the uniform flow direction and relative position of the aerofoil dictate the single solution that is allowed. This is the symmetric solution. That symmetric solution, BTW, gives symmetric pressure and velocity, and hence equal transit...
Also, with all due respects to my colleagues who are physicists and mathematicians, while an equation can have a singularity in it, reality does not. So, even a wing on your aircraft, when it starts moving, has a symmetric potential flow solution, and the acceleration is not infinite at the TE, but it is significant. It is this with viscosity that starts the formation of a vortex at the trailing edge; this grows, and that centre of rotation is a low pressure. This then sucks the stagnation point down from that symmetric point to the trailing edge. That is, the Kutta condition is caused by viscosity. This is accompanied by a vortex being shed from the trailing edge in a counter clockwise direction. Noting you have an appreciation of physics, it will be easy to accept that conservation of angular momentum is also required, hence some other part of the fluid must rotate in the clockwise direction. This is the circulation, or the bound vortex around the aerofoil. Effectively, the viscosity of the fluid has made it such that fluid more easily flows over the upper surface (it moved the stagnation point, so this should be intutive), so it has a higher velocity, and the converse is true for the bottom surface. That is, the viscosity term in the Navier Stokes equation, the correct form of Newton’s laws for fluids, alters the balance between the pressure and acceleration on the upper surface relative to the bottom surface, and this persists as an asymmetry in the boundary layer.
So, viscosity causes the circulation, which results in a difference in flow speed, which through Bernoulli we can show as a pressure difference, which is the actual force pushing up and down on the aerofoil. Yes, if you take away or try to simplify one of those steps, “the wheels come off the car”. So, don’t, and if people try, tell them not to. There are really only 3 terms in Navier Stokes, acceleration, pressure, and viscosity, so no more or no less is needed to explain lift.
I thought I´d never get it, so, thanks for this video. Finally I understand the Kutta-Joukoski-Transformation, and its actual role !!! Also, about the named reason of how "myths" around lift rise: If u let the theorem out, and the math involved with it, u are left back with a mere "skelett" of an argument, the Bernoulli, and that´s too little to avoid myths from rising.
Also, I noticed that the Kutta-Joukowski-Theorem gives a better approximation for the overpressure under the airfoil, than for the vacuum over the airfoil 11:35
The manner in which I see circulation in wings is through a series of steps starting with a helicopter while hovering. In order to hover and so lift itself, a helicopter needs to converge the fluid air mass above it and pull it down and accelerate air mass through its main rotor. The downflow through the rotor has a high velocity "ring" which after it passes below the main rotor it will diverge,. decelerating to a lower air velocity and slowly it will move outwards to circulate upwards to converge back and re-accelerate through the main rotor. Basically, the process causes a big doughnut ring shape that circulates within itself, similar to the bubble rings made by dolphins in the water. We may call this a horizontal doughnut shape circulating within itself. Interestingly enough, if one cuts the horizontal doughnut ring with a vertical plane across any large diameter, what one sees are two vortices similar to those generated on the wingtips of a wing of an aircraft, moving forward.
If we imagine a tighter horizontal circulating doughnut ring brought about by the hovering helicopter, this doughnut ring needs to circulate to provide lift for the helicopter where the velocity in the circulation needs to vary to be able to be reaccelerated to create the lifting force.
Assume that the vortex velocity on the outer surface of the doughnut/ring circulation, is 10 units, circulating as necessary to lift the helicopter. If this circulation in the horizontal doughnut ring is made to travel horizontally with a velocity of 100 units, then the upper surface of the leading part would be traveling at 110 units while the lower surface would be traveling at 90 units,. hence this will cause a lower pressure on top and higher pressure below the lower surface, hence an uplift ( Magnus effect)
Interestingly enough the back trailing part of the moving, doughnut with circulation, will have its top surface moving at 90 units while the lower surface moving at 110 units causing a down lift. The side parts of the forward-moving doughnut ring with a circulation, moving parallel with the direction of motion of the doughnut, will be exactly the same as the two wingtip vortices on the wing of an aircraft. th-cam.com/video/dfY5ZQDzC5s/w-d-xo.html
If the doughnut/ ring keeps its circulation but the doughnut shape is changed into a toilet seat shape with a flat surface, then we may consider the circulation to be that around a wing traveling forwards at a velocity of 100 units and a circulation of 10 units. At such ratios of forwarding speed to circulation, one would not be aware of the circulation but it exists if the lower velocity below the lower surface of a wing is slower than that on the upper surface. One can retain the forward half of the toilet seat which could be said to be closing up on resembling an aircraft wing with a forward velocity and a circulation around it complete with two wingtip vortices.
It is interesting that a horizontally moving doughnut with circulation, will have an uplift in its leading part acting as a wing and a down lift in its trailing part acting as a down elevator, and this uplift and downlight on the leading and trailing part would finish up with the circulating doughnut ring moving in a vertical plane whose normal is in the direction of travel.
This analogy seems to fit well the circulation around the wing which causes it to lift, and also includes the wingtip vortices around the sides moving parallel with the forward motion of the toilet seat with circulation. The circulation around the toilet seat or the doughnut ring will always come in pairs!!!!! front and back circulation and left and right circulation as in the wingtips of an aircraft. Realy there is nothing new here as the downflow behind the wings of an aircraft is in fact the downflow in the center of the doughnut ring with a circulation.
dolphin bubble rings circulating as helicopter rotor stramlines th-cam.com/video/bT-fctr32pE/w-d-xo.html Note how they seem to travel forward with their plane at a right angle to the direction of motion.
The easiest manner to see the circulation around the airfoil of a high aspect ratio wing is to first study the helicopter in hover mode, where the fast accelerated central downstream of the main rotor and the slower decelerated upstream, outside the main rotor, forms a circulating doughnut ring., It is, in fact, an amplified extension of the wingtip vortices of the main rotor.
If the helicopter, from a hovering mode, goes into a forward motion, this doughnut with circulation will still exist and the front leading part and the rear trailing part will be subjected to the Magnus effect where the leading circulation will uplift, while the back circulation will down lift. The side circulations will be exactly the same as a pair of wing tip vortices on a conventional wing, with their axis in line with the forward motion.
Since the rear Magus effect is producing a down lift, the back part of the circulating doughnut can be discarded while the opposite rotating pair of side vortices with their axis moving in the direction of forwarding motion would still dump the air into the center of the modified forward-moving doughnut, where the front circulation can be obtained and sustained by doing away with the main rotor and replacing it with a forward-moving plate having a large angle of attack, where the lower surface producing a compression zone will slow down the moving air while the top surface will create a suction to speed up the airflow and the circulation around the airfoil will remain sustained, existing to complement the pair of vortices at the wingtips. It is the dumping of circulating air behind the wing or the central downstream of the air by a forward-moving, or stationary helicopter doughnut shape with circulation, that creates lift.
What is accelerated and centrally dumped down under the main rotor of a hovering or a forward moving helicopter, is exactly the same Newtonian philosophy as what is dumped behind the trailing edge of a moving wing and those pair of vortices at the wingtips. That inconspicuous doughnut shape, with a circulation, exists in a hovering or forward-moving helicopter as much as it exists in a moving wing, where due to its forward motion becomes phased with the moving doughnut with circulation. The wing leads the central part of the doughnut. The back part of the doughnut shape is left far behind and is replaced by the back stabilizer of an aircraft.
Here is one practical interesting concept of circulation around a wing. Let us assume that we consider the WW2 Lightning aircraft with two booms connecting the main leading wing to the horizontal rear stabilizer. With imagination, this squarish " flying frame" with a rectangular aperture in its centre can be compared with either a toilet seat or a picture frame with wide strippings. If a helicopter rotating its main rotor was structured inside the inner rectangle, then the WW2 Lightning twin boomed aircraft, could achieve VERTICAL TAKE OFF, due to acceleration of air from above to down below it, which will circulate at different circularly vectored accelerations and velocities, enclosing the individual parts of the frame of the rectangular shape, picture frame aircraft. This procedure will form a horizontal rectangular doughnut shape with circles spinning around all four frames parts that make the shape of the twin boomed aircraft. If the aircraft had to be propelled forwards with additional horizontal propulsion, the rectangular horizontal doughnut with internal circulation will have a Magnus uplifting effect on the leading wing and a down lifting effect on the trailing wing, with no lifting effect on the side boom wings moving parallel with the direction of motion,
Assuming that the rectangular doughnut circulation obtained from the helicopter rotating rotor is
10 units and the rectangular shape boomed aircraft is moving forward at 100 units. Then above the front leading part of the rectangle ( a front wing), the velocity will be 110 units while that below it will be 90 units. causing a Magnus effect uplift. The rear trailing part( a rear wing) will have a velocity of 90 units above it and 110 below it and so the differential pressure will cause it to downshift, while the circulation around the side booms in conjunction with the forward velocity will cause the circulation to become an elongated helical shape, rather than the circular flat " circles" remaining at the leading and trailing parts of the rectangular twin flat boomed aircraft.
The down-lift at the rear end is undesirable, and so the following procedure can be taken. The rear straight part of the rectangle with a circulation around it can be moved further back or lifted up higher as in high tailed aircraft or else the main rotor of the helicopter could be replaced with the down lift effect of many smaller fans placed closer to the trailing edge of the front leading wing of the rectangle. We can go even further than that, and totally eliminate the rotor or the fans in the central rectangle to sustain the downdraft by other means.
Giving a high angle of attack to a cambered curve making the front wing of the rectangular-shaped aircraft, will slow the air particles underneath it and speed up those above it and this will sustain the circulation around the front leading wing, still dumping its downgoing airstreams circulation behind its trailing edge ahead of the rear part of the rectangle, which now is away behind or above, from the effect of the trailing edge down dumping of the circulation around the front wing,. This rear part of the rectangle becomes a horizontal stabiliser of the WW2 Lightning aircraft. The circulation around the twin booms will still exist as the elongated wingtip vortex is now shaped like a helix. We shall assume that the WW2 twin flat boomed Lightning has switched its engine off to eliminate the swirl of those two propellers driven by Allison engines !!!!! and that the twin booms have been shifted to the wingtips!!
it is to be noted that all Energy Forces for the acceleration forces to produce the front motion and the circulations and varying accelerations of air-mass particles around this rectangular flying frame are all obtained solely from the energy and power of the engines or the rate of change of lost altitude or thermals etc. In a wind tunnel, all the energy for circulation is obtained from the energy of the motors driving the air particles wind tunnel
For a forward-moving aircraft, the atmosphere particles have the following Bernoulli equation
C= Pa + 0 while all the circulations and other linear velocities that the air mass particles can gain is
Varying = Pa + energy from the engines, which will result in accelerating and moving the air particles to varying amounts of P1+ (V2)^2/2, which later on lose all the gained energy from the engine to finish up with the original C= Pa, a long way behind the aircraft. It is important to note that around a wing, the air particles are constantly gaining energy from the engines through the effect of suctions and compressions around the wing.
If the suctions and compressions maintained by the ENGINES accelerate the mass particles in a downward direction then that is an UPLIFT on the wing, If the suctions and compressions maintained by the ENGINES accelerate the air mass particles in a forward direction then that is DRAG. Those particles of air in a Bernoulli streamline are continuously robbing energy from the ENGINES and losing it to other issues and hence around a wing one cannot say that Constant C= P + (V^2)/2.
Let us consider the lift to drag ratio of a wing is 50: 1 as in a modern fast glider. Let us consider the drag as being a horizontal force while the lift is a vertical force
Force = Pressure multiplied by area.
Let the area of the driving propeller be Ap and area of wing be Aw while pressure Pp is the differential pressure on the blades of a prop and Pw that on the upper and lower surface of a wing.
Force generated by propeller = Ap*Pp and force on the wing = Aw*Pw
Using the lift to drag ratio mentioned then
50*Ap*Pp= Aw*Pw ....... equation 1
Assume that the differential pressure across the surfaces of the blades of a Propeller is twice that of the wing, hence Pp= 2*Pw
Substituting in equation 1.
50 *Ap * 2* Pw= Aw*Pw Hence
100 *Ap= Aw Which is not a bad amplifier of forces and I look upon this as an operation containing
A limited CIRCULATION (orientation) of 90 DEGREE with an AMPLIFICATION OF 100.
Hence a wing is no more than a vector amplifier and a ( 90 degree) operator of drag forces into lift forces.
In this case, 1 unit of Drag is operated upon to produce 100 units of lift oriented with respect to the Drag by 90 degrees.
in this case, wing lift = wing Drag multiplied by the operator (100 oriented by pi/2) Let j be a 90-degree operator.
or Lift = Drag* 100*e^(j.pi/2) where 100 is the resulting product of, (lift/ drag ratio) multipled by (ratio of propeller/ wing differntial pressure).
No wonder we do our best to change the shape of a flat plate to one where the down angle of the leading edge and the slope after that, are designed to cause the flight path of the air particles to contain ACCELERATIONS all the way above and below the wing and not to lengthen the upper surface!!
That wing is like an amplifier and pi/2 phase changer, which operates on the magnitude of the drag ( hence related to engine power) to amplify it and orient it by 90 degrees, it contains all the findings of Newton, Bernoulli, Venture, Coanda, Navier Stoke, the Wright Brothers, Bleriot, up to 1940 when Sir Frank Whittle at a young age stopped thinking of Bernoulli equation as simply, A Constant = P+ V^2/ 2 and saw it all as
*****continuously varying contents of ( gravitational forces)+ ( static pressure forces) + ( velocity forces) + (internal energy forces due to explosive chemical or atomic energy) + ( external energy contributed by external heat, all types of suctions, and compressions around a pair of curved surfaces on a propeller, fan, compressor, diffusers, turbine, or a pole in the wind shedding vortices to swing it about.
Contained in all that is a modern aircraft taking off and flying and landing and occasionally crashing. I wonder how many pilots see flying as explained in the last paragraph ***** I see One aircraft as being no more than one single big complex particle, traversing the streamline of the Bernoulli equation applied to a giant Venturi, or should I include Newton with that!!!
Circulation isn't an intuitive concept, so thank you for trying to help people understand it more. However, I don't think that your comparison to a helicopter is useful here.
First, you've added a dimension to the problem. In the video, circulation is presented as a 2D problem, but this helicopter model requires 3D. I recommend keeping the dimensions as low as possible when trying to understand a new concept.
Second, while the airflow in the helicopter example may be circulating, that is not the circulation the video talks about. Sadly, the same word is appropriate for both contexts, but it's not the same event. The circulation in the video must always be paired with a free-stream flow to describe lift. It's also important to note that the free-stream + circulatory flow is in the same plane as the airfoil cross section. However, in the helicopter example, the circulation described is not in the same plane. In fact, the helicopter circulation plane is orthogonal to the airfoil cross section plane.
Your points might very well be valid, but not for this type of circulation. I just wanted to leave this here so that others who are trying to grasp this concept don't get confused by this.
@@tristanc6967 Yes, you are correct, and in fact, before I wrote my comment I did wonder if I would submit it but this is the manner in which I thought. If one looks from ahead or from the back of an aircraft and sees wings coming or going away from him, then the two " circulations " he sees at the wingtips are exactly the same as a helicopter but the circulation is in fact a helix as the aircraft is moving forward always accelerating stationary mass from above and under the wing DOWNWARDS. which is the reason for lift.
Now if you look from the side of the aircraft to see the normal streamlines these too are part of the streamlines on the much larger picture. Unfortunately looking at the streamlines around the airfoil section does not normally show the bigger picture. In a helicopter that is hovering, if we go around it we would see the same overall picture. but not with a winged aircraft moving forward.
Now if we had to set the helicopter in a forward motion or a gyrocopter the whole picture will change and there will be an additional circulation due to the forward movement.
So I thought that with a wing moving forward and a helicopter moving forward in the process of "ACCELERATING THE AIR MASS ALONG THE "CIRCULATING MASS" we see in a hovering helicopter and then the " additional circulating mass " along with the circulation when the helicopter is now moving forwards at
speed the circulation will be somewhat helical as in the case of a wing moving forward.
No matter how we think about the wings in a helicopter or a winged aircraft be it Bernoulli or Circulation or anything else THE REAL TRUTH ABOUT LIFT is that mass of air is accelerated continuously hence continuous lift. Here we must discuss a much larger picture than what is happening in the atmosphere even up to a kilometer away from helicopters or wing aircraft.
The acceleration of masses of air from above a forward moving helicopter and a forward-moving wing is the TRANSIENT effect that causes lift, but the accumulation of air below the helicopter or the wing must eventually circulate from down below from where it was thrown to cause lift and then it will make its way up again to regain the distribution of air solely due to gravitational forces where other following aircraft will again accelerate the stationary air particles through a transient fast localized acceleration and the air will redistribute itself at a slower rate of change of state to set by gravitational forces.
I feel that if one looks upon a hovering helicopter, yes the flow is in quadrature but only because the helicopter is hovering but if the helicopter moves forwards then there must be a HELIX flow as when the winged aircraft moves forwards.
I thought that hidden in all this acceleration of masses of air in the immediate vicinity of the flying craft will circulate again to resettle due to gravitational forces. The circulation that is in 2 dimensions and to which you refer and you are correct, must exist in the whole picture I tried to describe. Again please note that to see this circulation in a helicopter then the helicopter is not hoveri9ng but it must be moving forwards and I do think that the WHOLE BIG PICTURE of these circulations be it a winged aircraft or a helicopter moving forwards or an F-35 or a Harrier hovering and translating forwards, what Bernoulli and the circulation theory that many people consider ARE ALL CONTAINED IN THE BIGGER PICTURE OF ACCELERATING THE MASS OF AIR DOWNWARDS close to the surface of the wings and then this accumulation of air below the flying craft will slowly circulate back to the steady-state distribution where only the force of gravity or the blowing wind exist.
Again please consider the helicopter moving forwards and not only hovering to compare it to the wing and its very big circulation in that large helix contained in a volume away from the wings.
I thought that concept would add some mind processes to what we conventionally consider.
Thank you for your comment.
At 1:13 you mention "Friction free, inviscid fluid" but ignore the third vital component of Circulation Theory which is that it applies in a fluid Flo in conservative field: i.e. one in which there is no net change in energy. This is clearly not the case in an aerofoil where there are energy changes through turbulence and friction. Circulation Theory adds nothing but confusion to any discussion about aerodynamic lift.
Yes, it does muddy things from a number of angles. But it does still pop up here and there, so I thought it would be worth touching on. Thank you for the comment.
Hi, thank you for this video. What do you think about the new theory of lift by Gonzalez and Taha (variational theory of lift)? it proves, among other interesting stuff, that Kutta condition is a special case of their theory of curve minimization.
Don't know much. But it appears that It does address some of the arm waving required to apply the Kutta Condition to a not 0 radius for real airfoil (or just accepting that you get a smooth flow off the trailing edge). But I wouldn't say that it is a very intuitive, feel good, explanation for non-math people.
Here is an interesting manner of how to think about circulation and associated effects including countermeasures.
Using the Cambridge Video, th-cam.com/video/6UlsArvbTeo/w-d-xo.html please refer to time 0:04 to 0:10
Note how "two Bernoulli related VENTURI situations" are shown in the video, one is above the leading edge and one below it. They both start well ahead of the leading edge. (Note I mentioned Bernoulli but that goes with Newtonian and viscous effects)
* The one above starts with a slow wide high-pressure zone converging to a fast low-pressure restriction above the leading edge then diverging again to a slow wide higher pressure zone above the trailing edge.
* The one below starts with a " faster" lower pressure zone diverging to a slow high-pressure zone under the leading edge then converging again to a fast narrower low-pressure zone below the trailing edge.
I find it fascinating how those two " Bernoulli combinations VENTURI surrounding a wing " one above, the other below, AND SKEWED OR DELAYED OR PHASED out ( because the AOA) with respect to one another is what makes a wing work as it does, where the energy in those fluid particles is changing all the time due to the power sources in use engines and wind tunnels flow sources and the losses associated with the surroundings.
Let us now discuss a very interesting matter. Looking at " Bernouilli flow lines" one set above and one set below the wing with phasing in between. Of interest is the fact that in the low-pressure areas, one is over the leading edge and the other under the trailing edge, while the high-pressure zones are situated one under the leading edge and the other over the trailing edge.
There is absolutely no doubt that the pressure distribution would provide LIFT to that airfoil. but if we take THE CENTRE OF THE CHORD AS REFERENCE POINT THEN, THE PRESSURE DISTRIBUTION WOULD ALSO PROVIDE THAT WING WITH A TORQUE. which would try to SPIN and " circulate" the wing about the centre of the chord to gain a higher angle of attack.
Hence, investigating carefully those two-phased out " Bernoulli shaped VENTURI flow lines" one above and one below the wing, the result is A LIFT AND A TORQUE OR A CIRCULATION OF THE WING.
This torque and lift are counteracted upon by shifting the weight of the aircraft to the location of the centre of lift to produce a counter torque. To ensure full control of the lift direction and AOA is to add a horizontal stabiliser to counteract the TORQUE WHICH THE PHASED OUT TWO BERNOULLI VENTURI FLOWS on the wing always bring about.
Seeing that hardly anyone talks about circulation I do not think that anyone would mind if we standardise this aspect of how a wing operates in saying.
" Taking the centre of a plate ( airfoil) as a reference, then due to the angle of attack and the forward velocity of the plate in a fluid, the resulting pressure distribution would always produce a LIFT and a TORQUE on a plate, trying to spin the plate in addition to lifting it. Perhaps one may say that a moving plate with an angle of attack has both LIFT and TORQUE and CIRCULATION. This Torque/circulation of the wing is counteracted by applying a counter Torque, through moving the centre of gravity to the location of the centre of the lift which is known to be about 1/4 to 1/3 of the wing and not at its 1/2 point as referenced for torquing purposes. Perhaps one can also say that the horizontal stabiliser/ elevator is also a factor that counter opposes the torque/ circulation in a wing!"
Note that this is a philosophy open to discussion. It is however interesting that if one drops a long strip of paper, it will spin fast as it drops and also moves sideways. Hence the LIFT AND THE TORQUE leading to spinning and circulation and lift.
Now we come to a point where all the physics have to join in to stabilise a wing with weights and other control surfaces.
It is no wonder that all the flying wings through flying history ( including some seeds from trees) were so dangerous to fly, not due to their: LIFT but due to their circulation obtained through this inherent torque that is always associated with LIFT in a plate/ airfoil, moving forward with an angle of attack, in a fluid.
People who find some difficult to comprehend Circulation and its contribution to lifting should start with considering the lift in a helicopter while hovering. The downdraft from the main rotor normally circulates air by first accelerating it down where it diverges out to decelerate and then goes up to be reaccelerated through the rotor. This creates a doughnut shape with a circulation in it. As the helicopter inclines to move into forwarding flight, the circulating air added to the horizontal forward motion will cause an extra lift on the front circulation which tends to BALOON the helicopter and a pitch control needs to be applied, where the cyclic pitch would reduce the circulation of the front vortex, hence less lift, till the helicopter has enough forward speed that the downdraft in the rotor is the same as a normal winged aircraft, including what would be regarded as the wingtip vortices, by treating the whole rotor as the area of a fixed-perforated wing with its own wingtips!.
An interesting experiment to see circulation is to take a long strip of stiff paper about 2.5 inches by 10 inches and fold it such that it would have a strong straight spine retaining its flatness as a plate wing. Raise the plate to head high and drop it at an angle. The plate will fly and then increase its angle of attack till it climbs again to form a U flight and then it reverses to fly again a series of U flights rotating and spinning in the same lateral direction while dropping to the ground.
If one investigates the streamlines around a flat plate with an angle of attack to the flow, one finds that there are two venturi effects phased out with respect to each other between the leading edge and the trailing edge, whose combination caused both LIFT AND TORQUE on the plate.
Hence when the plate is dropped it will suffer both lift and torque to spin it and so it does, circulating freely as gravity acts as propulsion produced lateral lift that pulls it sideways while the torque spins it while dropping in a series of U shapes short flights.
The interesting issue about a flat plate with an AOA, is that as it starts it leaves a vortex behind it, and so it retains the other as they come in pairs, and what the plate retains seem to spin it or circulates it as this plate wing or stip of paper does so beautifully, becoming a lifting rotating cylinder in fact!
It has to fly freely to see this spinning effect or circulation. It is interesting that when a plate is held in a wind tunnel to test it we are applying torque and a force to hold it stationary as we do when in a wing we move the weight forward to coincide with the center of lift or put a horizontal stabilizer at the back of an aircraft............ which in fact is counter torquing the inherent natural lift and spin/circulation of a plate wing.
@@carmelpule1 Please note that to start with one should look at a helicopter in a hover, but to compare the circulation to which we normally a wing moving forward, then the helicopter must be considered to be moving forwards. It certainly adds to the complication of the concept of trying to visualize the whole
picture. Really the circulation would also be seen if one considers the streamlines around the immediate upper and lower surfaces of the main rotor, and what happens to that air that is accelerated below the rotor, it will certainly make its way up again away from the rotor to distribute itself to that " pressure" zone decided by gravitational forces. Also, the circulation in a helicopter in hover is what it is because of the pressure built up under the axis of the rotor which will push the overall circulation in quadrature with the blades of the
rotor, but to the ahead movement of the rotor blade, there will still be a circulation if 2 Dimensions.
To be able to see the " conventional circulation" around the blade of a helicopter one needs to home in close to the surfaces of the blades but when the helicopter is moving forward there is another style of circulation which is what we refer to in a moving wing.
If in a moving wing we look ahead or back on the moving wing the two circulations at the tips of the wing will be similar to that of a hovering helicopter.
One suggests that in addition to seeing the functions close to the surfaces of a moving wing or a translating helicopter one needs to see the whole picture by looking at the resulting paths of the moving masses from saying one kilometer away to see the whole block and the " various" types of circulations that are occurring while one must not stay in the same location but look at the whole picture orienting oneself 360 degrees around the craft moving in air particles with mass.
With rockets moving in space the exhausted mass from the rear or side thruster of a rocket does not circulate to redistribute themselves as the mass particles in our atmosphere needs to do due to gravitational effect, neglecting the winds of course.
What you believe is wrong. Proponents of circulation theory like myself would just say total lift is proportional to total circulation, and nonsense about equal transit time just doesn't figure.
The Kutta condition fixes the circulation at a definite value. If the Kutta condition were not complied with, then a starting or stopping vortex would be shed from the trailing edge of the wing until it was complied with.
If a large aeroplane takes off, the starting vortex is left behind on the runway, and may take a few minutes to disperse. I would not climb aboard an aeroplane with a pilot who was not aware of this, or of the vortex system of a wing in general.
Both the starting and the stopping vortex can actually be visualised, and total lift is proportional to total circulation. Modern computational fluid dynamics can just as well use vortex methods as finite difference or finite element methods. We can compare them with each other and with solutions derived from conformal transformations to build up confidence.
I'm not sure what you think that I believe... The Circulation theory has been around since the very early 1900's. And, if you assume and ideal inviscid fluid, Circulation plus the Kutta condition give you a reasonable model of the airflow and lift. There is no question about that.
Also, most pilots are aware of wake turbulence.
Also, as you point, out equal transit etc. is the product of the south end of a northbound horse.
But, the reality is that trying to explain the concept of a virtual vortex that flows "upstream" under the wing while there isn't any actual flow in that direction seems to fall pretty flat with non-technical pilots or the general public. And, as I mentioned, I believe that attempts to "simplify" circulation is what has lead to some of the fairy tales that you find in the popular media.
So, while circulation isn't "wrong" I prefer to not go there.
In any case, thank you for the interest and comment.