Change the way you think about windsurfing

Using 3D software (Blender) this video will explain the physics behind windsurfing. This will cover the following:
1. Does the wind push or pull the sail?
2. What is the sail angle which provides the greatest lift force
It goes over Bernoulli's theorem and Newton's Laws of motion and how they relate to sailing and flight.

[@Nico_GER7]

This is so good! Thanks for your work.

[@3denlightenment]

Thanks Nico! Love your videos too!

[@MaoZedong2225]

I think it's only useful when you try to go up wind.

[@Nico_GER7]

@@MaoZedong2225 Absolutely not, because the apparent wind always comes at ca. 45°. Even when you go on a reach.

[@treyschwartz3966]

Nico, thanks for finding this video. 3D Enlightenment thanks for making a video that reminds me of my studies in physics. Lately I’ve been thinking of these principles in both sailboarding and flying sailplanes.

[@rokowoju8558]

I am relatively new to windsurfing as an engineer I found this very helpful, interestingly from my learning experience as a beginner I think people that teaching others windsurf should have a better understanding of the processes, I recently I have been teaching two total beginners in the way how i would like to be teached. Windsurfing is something I regret not doing it earlier in my life, BTW Nicos Videos are always educational.

[@3denlightenment]

Thanks. Glad to be helpful. Enjoy your new found sport!!

[@Phractal]

This is exactly what I wanted to know! Thank you very much for this excellent video.

[@3denlightenment]

You are welcome 😊

[@bradleypatterson5706]

This is very interesting and cool to see the conclusions that 45D to the apparent wind generated the most pressure. Will be subscribing to watch for more videos.

[@user-ym5dm9xd5w]

Great explanation. Well done for your research. Definately gives me a new appreciation. I'm just starting to windsurf. This is very helpful. Thank-you.

[@3denlightenment]

You're very welcome. Glad you liked it. There'll be more coming

[@miedwiesport]

great job! thanks for sharing! I will show your video to my windsurf students!

[@3denlightenment]

Thanks Mateusz

[@christianamstutz9308]

Awesome. I'm a beginner windsurfer, and I was on my way to make some sort of visual model for understanding sailing courses. Thank you for saving me the task and providing me with such interesting concepts. Greetings from Buenos Aires

[@3denlightenment]

Thanks, glad you liked it. Greetings from Canada!

[@srenandersen4114]

Brilliant! Keep’em coming!

[@3denlightenment]

Thanks. Will do :-)

[@vyleart4488]

That was truly excellent, thank you!

[@3denlightenment]

You're very welcome!

[@Robbrand89]

Very cool video!

[@Condinou]

Thank you very much for your excellent, extremely interesting and well presented work. well done!

[@3denlightenment]

Thanks very much 😊

[@anthonythm]

damn, easy ti understand. What a master piece. Even so many pilot dont know the relation between them

[@3denlightenment]

Thanks :-)

[@maxagent86]

Great. Please consider similar videos on the physics of windsurf foiling. TIA

[@3denlightenment]

Thanks. Will do!

[@irp4776]

Amazing!

[@windsurf888]

Yes Sir!... geeking out on windsurfing - loving the scientific approach to the kit, designers like Ola (Simmer) and Bouke (Witchcraft) drive their innovation with this kind of thinking and testing (Witchcraft has lots of blog-style posts on their website talking curves, angles, flow and the like).

[@3denlightenment]

Thanks I'll check out the Witchcraft site!

[@AlexanderMorales-wz8ce]

Great work, thank you

[@3denlightenment]

Glad you liked it!

[@southpacific3160]

This is interesting, but also raises a question as to why we bother with a curved (ie camber induced) sail at all? The graph shows that the maximum force is produced at a sail angle from 40 to almost 50 degrees, though at the bottom of this range (40) some of that force is produced by camber, however at 45-50 degrees, ALL of the force is generated by sail angle alone, and none by Bernoulli's principle. Wouldn't it therefore be possible to have just as much lift from a completely flat sail (at the correct angle to the wind), vs a nicely profiled cambered sail? This demonstration of forces can't be the only forces acting on the system to produce power to generate forward momentum, as we all know that cambered sails are faster than non cambered sails.

[@3denlightenment]

Thanks for the comment @South Pacific. Your observations are true. A cambered sail will provide additional lift relative to a flat sail, which is not apparent from the measurements I made in the video as the wind source from the fan was basically just a reflection of the ambient wind. It would seem that the cambered sail gets its benefit from the 'motion wind' moving around it as the sail moves forward. FYI - I am making another video on sailing faster than the wind, which will cover the additional forces resulting from a cambered sail...so stay tuned.

[@rkj7935]

Awesome! I'm looking forward to your series on foiling, speed sailing, and freestyle moves. 🙂 Endless content! 🙂

[@3denlightenment]

Ha.. 🙂Thanks @R Kj ...I'll see what I can do 🙂

[@jc321tube]

@@3denlightenment Please...great stuff here!

[@SupeRails]

thanks for the video! I've just tried windsurfing and I was really lacking the understanding of air/sail physics. Please do more videos!

[@3denlightenment]

Thanks. I will be doing more :-)

[@AlanAllegret]

Beyond just the great video, I admire the calm with which you handle the idiots "debunking" your explanations in the comment section. Nice job on both fronts!

[@3denlightenment]

Thanks Alan! 😊

[@TyphoonPilot1]

Your explanations are sound. Very good.

[@3denlightenment]

Thanks! 😃

[@TigeroL42]

The lift force through static pressure is not enough to lift an airplane alone. That's why the wing always has some AoA, in TOL it has way more. The wing profile forces a huge mass of air downward on every moment, which then causes an opposite reaction on the wing and plane. This effect creates most of the lift force. In high speed flight the drag can be optimized by reducing AoA and relying more on the pressure difference.

[@3denlightenment]

Very true. Thanks.

[@renedekker9806]

@@3denlightenment In airplane wings, science shows that about 2/3 of the lift comes from the lower pressure above the wind, and 1/3 from the higher pressure below the wing. I don't know whether that translates to sails in the same way. By the way, what the video calls dynamic pressure in the video is normally called the total pressure. The dynamic pressure is then the difference between the total and the static pressure.

[@mrdjangofreeman5560]

As an aerodynamics engineer , i feel comfortable to state this tube is BRILLIANT 😊

[@3denlightenment]

Wow! Thanks very much 😊

[@that_doc_main9223]

Thanks for this video 🎉

[@3denlightenment]

You're welcome. There will be more

[@maxi_qu]

Amazing video! Keep doing this and you will have a great career on youtube

[@3denlightenment]

Thanks @Maxi_qu ...we'll see where this goes. :-)

[@wavegurucanada]

Thank you for the effort. Very nice build model. What about the dynamic pressure ?

[@3denlightenment]

The dynamic pressure is the pressure resulting directly from the movement of the fluid (the wind) and its transfer of force upon impact with the sail. The red line on the graph demonstrates that the dynamic pressure is the primary force on the sail and the primary explanation of lift especially beyond, say, 35 degrees. I will be making a video shortly on sailing faster than the wind which may help with the understanding of dynamic pressure.

[@johnryan8645]

Yes! Very nice. I once modeled this in Blender with particle physics turned on. An interesting thing happens when you include Van der Waal forces. Especially for feather structures of bird wings…😉

[@3denlightenment]

Blender is amazing...and sometimes challenging :-)

[@johnryan8645]

@@3denlightenment oh yes! Very challenging😄

[@johnryan8645]

@@3denlightenment for very subtle effects choose ratios of oxygen, nitrogen, and H2O… this also has interesting effects…😎

[@henq]

There's one VITAL thing missing, imho: tha fact that windsurfers, when plaining fast, pull the sail windward. So the mast is NOT in a straight angle towards the zenith, but at a certain tilt. The reason is that windsurfers want to make themselves as light as possible to the board, so the board needs not carry the full wight of the windsurfer's body. The more the board skimms on top of the water instead of navigate _through_ the water (like a sailing ship), the higher the speed attainable. Like a speed boat, nose comes up, the "wet surface" is reduced and only the aft part of the board, touches the water. This way the resistance is reduced and planing becomes possible. The harnass is essential in this. When going fast, the direction of the harnass lines show that the harnass is lower than the boom. This means the pull of the sail is not only directed forward, but also UPward. This allows the surfer to have most of her body NEXT to the board, not ABOVE the board. (When the wind is light, in non-planing mode, the surfer stands vertical, with full weight on the board.). See this clip kzbin.info/www/bejne/nmjFe5iAgbSEfck . But also at 07:07 of this vid, the surfer in front is almost completely weightless (to the board), the sail pulls must of his weight. (See the harness lines from the surfer to the sail go up at an angle of almost 45 degrees to horizontal). To summarize, the force the sail delivers is split between a pull forward for speed, and a pull upward to reduce the body weight. This split is a declicate balance, controled by the lateral angle of the sail, and as soon as the wind becomess less, the surfer has to adjust, changing her stance to a more upright position, putting more weight to the float the board provides because there's not enough force from the sail to keep the weight off-board. I would love the see a @3D Enlightment video vizualizing these essential aero- and hydrodynamic differences between a fast planing board and slow 'displacement mode' windsurfer! With traditional sailing boats, the whole boat, including mast, tilts towards lee side. Totally different dynamics. A displacement boat is speed-limted by its waterline length, so all above considerations are moot.

[@3denlightenment]

Thanks for that contribution to the discussion. Your description is absolutely correct and would make a nice addition to the 3D Enlightenment video collection. Thanks for that! Cheers.

[@michal5869]

Can you measure when the sail is tilted at an angle of 0 to 30 degrees relative to the water surface? I believe that during this time, the crosswind increases the acceleration as it bounces off the water and the sail, causing dynamic pressure to rise. I'm curious about my theory, although it's probably well-known, but it caught my attention

[@3denlightenment]

Hi the wind would mostly be blowing parallel to the water surface but there could be some bouncing off of the waves. If I understand what you are describing I would say that with the sail tilted over at such an angle there would be direct lift vertically allowing less surface pressure and contact with the water...resulting in less friction and drag through the water, resulting in less resistance allowing the board to accelerate . Essentially you and the rig become lighter on the water because of the upward force on the sail.

[@MaoZedong2225]

Correct me if I'm wrong but this only applies when your board is at 90 or less degrees to the wind. when is more you need your sail also nees to be more degres to the wind

[@3denlightenment]

Yes, that is right. I focused on the beam reach as that is a common point of sail, but If you are on a run, for example, you wouldn't have your sail at 45 degrees to the wind. So, if you are sailing downwind, there is a gradual transition from the 45 degrees to 90 degrees. Keeping in mind that the physics of sailing straight down wind are quite different and you cant sail faster than the wind at that point of sail.

[@trexinvert]

Keep in mind "real" windsurfing involves "planing", which is like a motorcycle doing a "wheelie". That is what we see at 0:15. Most beginners and intermediates are just sailing with the board completely in the water. So, the vector mechanics and equilibrium of the motion is somewhat complex.

[@3denlightenment]

Yes, the "center of effort" is shifting back as you begin to plane.

[@rctrix9063]

Bernoulli simply states that the velocity of a gas is independent of it's temperature. Which of the Bernoullis I'm not sure but it's the consequences of this principle that have importance as it relates to pressure. The effect is usually explained wrongly because wings don't provide lift with neutral pitch and symmetrical foils can also provide lift.

[@3denlightenment]

Bernoulli's principle is related to conservation of energy involving fluid flow. Gas is a fluid. The relationship to temperature is a function of a loss of energy as heat for example due to friction. You may be thinking of the relationship between temperature of a gas and its density.

[@Jhoekstra]

Isn’t the fin an important element in the ‘lift’ theory?

[@3denlightenment]

Not as much as you may think. The fin is bilaterally symmetrical so the water flowing over the surface from edge to edge will flow over each side equally resulting in equal pressure on each side. Also, windsurfer boards are relatively flat compared to the water surface (unlike a sail boat which leans (heals) over with the wind) and therefore the fin mostly cuts through the water vertically. Also watch my first video which discusses the influence of the angle of the fin. kzbin.info/www/bejne/mZu3k2RrpdKYY8k

[@Pscribbled]

Even in Bernoulli, isn’t it still technically a pushing force? Just how there’s no actual “sucking” force when you suck a straw, it’s just high pressure pushing to equalize the lower pressure of a vacuum

[@3denlightenment]

True, but there has to be the partial vacuum or sucking force first before there is the pushing force from the positive pressure side. So without the low pressure there is no pressure difference and no movement, that is why it is important to acknowledge it..

[@douglassummers9901]

You should interview the sail designers at NORTH SAILS. They have state of the art computer programs that run millions of airflow experiments for optimization of sail design. They know how sails work. It’s a Billion dollar industry. Your experiment is 1+1 compared to their supercomputers. If you are interested in the truth you should learn from these guys. Call them. They will be happy to help you learn. They know the truth on how a sail works. You won’t have to guess any more.

[@3denlightenment]

Thanks for the input. My objective from the video has been met. If you're speaking to the guys at North Sails, feel free to have them review/comment on the video.

[@GiacomoContri-ji1gc]

Hi, I watched the very nice video but I’m not entirely convinced by your explanation. First, you mention Bernoulli’s theorem as giving a contribution in addition to newton 3rd law. I am not sure this is true, Bernoulli theorem should be equivalent to newton’s laws (+ some approximations and the continuum limit for fluid particles); it can actually be derived from them. Also, you refer to the lift originating from Bernoulli’s principle as only coming from the depressurisation above the wing, but actually a correct application of the principle takes into account both the decrease in pressure above the sail and the increase in pressure below it. For what I know, in this case one could calculate the total time variation of the momentum of the air (which is pushed down by the sail on both sides) and this would be equal to the pressure difference times the wing surface, so that the 3rd law is consistent with Bernoulli theorem as it should. Then, 3rd law and Bernoulli are the same thing, taking both is like overcounting. In addition, the “particle hitting the sail” explanation doesn’t take into account that air particles interact with each other through air pressure, so that air is changing direction in a widespread region around the sail. We experience this for example when we sail close to another boat either to windward or to leeward. This gives a contribution to the lift which in your picture can’t be taken into account. Another thing which I can’t understand is the result of the experiment. In books like Bethwaite’s HPS 2, there are studies of various sail profiles and all have a maximum lift coefficient around 25-30 degrees, after that the sail stalls and drag takes over. Also, if I think about more concrete sailing experience, we can clearly see tell tales already stalled at 45 degree AWA, and indeed one experiences a loss of forward trust. Can it be that maybe the red line in your plot is not describing purely the lift component but maybe a combination of lift and drag? This could originate from the rope measuring the force changing direction with respect to the vertical line when the angle of attack is increased. Then everything would make more sense to me. Of course probably I’m missing something important in my understanding, I would be very happy to hear your thoughts if you have time to answer. Thanks again for the nice video!

[@3denlightenment]

Thanks @Giacomo for your comments. Yes, I think part of the confusion is that Bernoulli's equation is made up of 2 parts (pressure due to gravity being ignored). One part addresses the static pressure which is often spoken as "the pressure" when it comes to aerodynamic discussions. What you are speaking about is the dynamic pressure, which is definitely a derivation of Newtons 2nd Law (F=ma), and so when incorporating the surface area into it, the two are basically reflecting the same thing. This is what I was very briefly getting at at 7:25. I also made a point of highlighting that it is the STATIC pressure differences being discussed in the aerodynamic discussion at about 3:47. Regarding the particles interacting, you are right, and perhaps I could have expanded on the point I was making with Bernoulli's principle being applicable to ideal fluids, with no turbulence, density changes, viscosity, or heat generation. So, when you speak about air interacting with adjacent sails from other boats this would be due to turbulence and Bernoulli's principles would not be directly applicable . You would have to consider Navier-Stokes equations and that gets very complicated. Regarding Berhwaite's HPS 2, I'm not familiar with that book. It could be that this is due to the static lift forces alone, and not the dynamic forces. It is hard to say. I'll have to get my hands on that book at some time. Thanks for your thoughtful comments! :-)

[@GiacomoContri-ji1gc]

@@3denlightenment Thank you for answering and sorry If I get back to you only now, I was a bit busy last week! I think you’re misinterpreting the term dynamic pressure. Dynamic pressure is an old school term to talk about kinetic energy density, which is the other term in the Bernoulli eq p+1/2ρ v^2=const. This term is no pressure at all, in the sense that it doesn’t exert any force perpendicular to any area. If you’re thinking about a collision-like momentum transfer from the change of the velocity AT the boundary between the fluid elements and the wing and you refer to this as a change in the dynamic pressure (which is improper since this would be a change of a scalar quantity), still this can’t be the case. By definition flow lines are parallel to the wing surface, so there’s no velocity variation orthogonal to the surface at the boundary. The resolution of this conondrum is that you’re neglecting air particles interactions under the form of air (static) pressure. Pictorially, the moment the air approaches the sail, it starts pushing the air around since it faces an obstacle, and this interaction results in an equilibrium state where there’s a (static) pressure variation with respect to the atmospheric pressure and a static velocity field which bends around the sail. However, these pressure and velocity fields are varying also far from the sail, since it’s like every air element is pushing the nearby ones down thanks to the pressure difference. This is consistent with Newton’s law, since it’s a rephrasing of the fact that “there’s a force which is causing the air to accelerate”. This force at every point is proportional to the (static) pressure gradient, and consistently there’s a change in what you call dynamic pressure. However, The way the force is transmitted to the sail is through (static) pressure, there’s literally no other interaction between air and sail if we neglect viscosity. To get the total force, you add all the static pressures along the surface and you fine that there’s a net depressurisation above and a net pressurisation below which cause the upward force we call lift (+drag here neglected). To sum this up, it’s not that static pressure is there by itself and in addition you have dynamic pressure from newton second law, it’s the conservation of the sum of the two which is newton second law (in the form of Euler equation under the assumption of irrotational and non viscous fluid). The interaction between air particles is what we call pressure, and it’s there irrespectively of viscosity or turbulence. Even in an idealised perfectly laminar flow you would experience an header when to windward and bow back with another boat. Turbulence adds to this but is a separate effect. This should show the point I’m trying to make. In any case, I think here www.av8n.com/how/htm/airfoils.html#note50 it’s explained very clearly. I was also thinking again about your experiment: since circulation is very important in establishing lift, I’m not sure that separating the flow into two different streamlines creates a negligible perturbation. It may (probably) be that you’re measuring something very different from the real scenario. I read that you wrote that this should at most increase the pressure difference since the air below is now totally still and not “only” slower as in the real case but this is wrong. It’s not velocity by itself which relates to pressure, it’s acceleration which causes/relates to a pressure difference, if one applies B. Correctly. So, at minimum you’re losing the pressure variation coming from the slowing down of the air below the wing. Here en.wikipedia.org/wiki/Bernoulli%27s_principle on the last section is well explained. I would also like to comment on other statements I read by others in various comments. 1. As everyone here says, the “equal time of transit theory”is false. It’s disproven by theory and experiment. Still, this doesn’t invalidate Bernoulli principle. Actually it is experimentally measured that air is faster above the sail, so much faster that reaches the end before the air coming from the bottom. 2. To the ones who are saying that B. Principle is applicable only along a flow line, that’s correct in the general case. However, in the case of an irrotational flow as the one we are considering now, from Euler eq. you obtain B. principle to be valid as a conservation law on every point of the fluid. 3. “But here the flow is not irrotational, there are vortices at the top of the wing”. There are indeed vortices but the flow is still locally irrotational (outside the boundary layer). The tricky point here is that the velocity field is not define on a plane, but topologically speaking on a plane minus a disk. Then, you can have irrotational vector fields which still give finite circulation when integrated along a closed line (for who’s aware, there are closed forms which are not exact). This is in essence the K.J. theorem. 4. The fact that there’s turbulence in the boundary layer is very important to study precisely when the sail stalls, but again it’s not invalidating B. since most of the air which is bent is further from the sail and moves in a laminar flow. Bethwaite books are amazing, strongly recommended:)! Let me know if you agree with all these things:)

[@gokalpozcan]

I really wanted to see the sum of the two lines at 6:30; wouldn't this give a more accurate idea of the combination of the two forces at play at different angles? Thank you for the video.

[@3denlightenment]

Thanks Alp. The red line is a reflection of the air moving over the upper surface and the lower surface so it is in fact a sum of the two forces. The green line is just the air flowing over the upper surface and would reflect the drop in the static pressure from Bernoulli's theorem. There would be a partial vacuum behind the curved surface of the sail causing the sail to be pulled upwards.

[@mattyclare]

Newtownian conservation of momentum - yes. Bernoulli - not at all. Streams don't meet at the back of the sail , only works within a pipe- constantly perpetuated false explanation. Coander effect due to curve of the sail - yes.

[@3denlightenment]

Thanks Matty. The streams meeting at the back of the sail at the same time is a myth and that is why I never mentioned it. I would have thought the air flow would be due primarily to the coanda effect due to the partial vacuum behind the lip of the curve, but the air flow over the upper wing surface has been shown to move faster (see this video @ 2:55 min kzbin.info/www/bejne/q6q1qWVrgriKpMk&ab_channel=FlywithMagnar) and is largely explained by Bernoulli's principle. Either way, it is definitely caused by the partial vacuum.

[@mattyclare]

​@@3denlightenment - kzbin.info/www/bejne/i6KlnpeQYqeBn8k. I like this video for explanations of wing theory , see what you think.

[@3denlightenment]

@@mattyclare Thanks. I'm well aware of that video and I probably should have shown a clip of it in my video. This effect is explained by Bernoulli's principle, and the conservation of energy. So, when the static pressure decreases, the dynamic pressure increases and the air flow moves faster. I tried to indicate this in my video at about 3:25.

[@brantfredrickson4766]

Thank you for your video. I appreiate your effort to compare a sail to an aircraft wing. However an aircraft wing is a device to maximize lift and minimize drag while a sail is a device to maximize drive and minimize heel. This is probably way beyond what simple physics can explain in a short video. Again I appreiate your efforts.

[@3denlightenment]

Thanks Brant. You're welcome. As you know the whole concept of lift is mysteriously controversial. I received so many comments on my first video asking why I didn't relate back to the concepts of aerodynamics. The purpose of showing all of the aerodynamic concepts was to try to clarify that they are similar yet different from sailing. Hopefullly that was conveyed to a certain degree. Also, at least in windsurfing, there is no heeling of the vessel, so that wasn't going to be part of it. Thanks for the appreciation.

[@julianhofman5910]

You forgot an important component, Drag! Lift force is perpendicular to the flow. Drag along with the flow. The sum gives you the total Aerodynamic Force on sail. This can be decomposed in Fr, driving force and component Flat. By following you're theory, you can sail almost against the wind, since there is no drag.... Nevertheless, I like you're measuring setup.

[@3denlightenment]

Thanks for your comment @Julian Hofman. I do discuss drag, but I was not able to measure it in the model. It would be very tricky to set up. I also had two key questions to answer which were focused on lift. In my first video I specifically discuss heading upwind. You can check that out here: kzbin.info/www/bejne/mZu3k2RrpdKYY8k

[@jemand8462]

3:30 is incorrect. The opposite has been shown, the air is flowing at the same speed and actually reaces the end on the downside first. Which makes sense because - where would the air take the energy to suddenly fly faster over one side and why would it do it? It doesn'T "know" it has to be faster to catch up.

[@3denlightenment]

Sorry. Watch this video here kzbin.info/www/bejne/i6KlnpeQYqeBn8ksi=smeaY7Av6tCWbI2H

[@WindsurfingNils]

i disagree with your explanation, while of course the force is the greatest at a 45° angle you completely leave out the force that is not pushing the windsurfer forward but the force to the leeward side, thus the angle at which the sail is held to the apparent wind is far less than 45°. In my estimation the angle at which the sail is held to the apparent wind is between 2° and 20° in which range the pulling forces (Bernoulli) are substantial. America's Cup boats sail the sails at an angle of 2-4° to the apparent wind. Otherwise there would be no point in Giving the sail a wing shape would there?

[@3denlightenment]

Thanks Nils. The experiment was designed to demonstrate that effect if it was occurring at wind speeds typical of windsurfing. The model set up measured the lift forces on the sail as the wind flowed over the leeward and windward side. The red line on the graph was a combination of those two air flows and a direct measurement of the combined lift force. If the angle between 2 to 20 degrees in fact generated the most lift, then that should have been observed in the measurements, but it was not. It is possible that due to the double sail design of the America's cup sails it may make a difference, and at higher wind speeds it may make a difference. Thanks for you comment.

[@WindsurfingNils]

@@3denlightenment Thanks for your answer, what I mean is that yes, the 45° angle will provide the most amount of lift, no question, but it is not about that but rather a question of A: what angle provides the most lift and B: what angle creates the least force leeward (0° of course). You only measure the first component in your experiment. You would have to plot those to forces against each other and the result will be that the optimal angle of the sail to the apparent wind will be somewhere in between 0° (least forces leewards) and 45° (most lift) In your experiment you only measure the lift component, but of course the force to the leeside is super important as with a smaller force in that direction you need a smaller fin to compensate and in turn will go faster.

[@3denlightenment]

@@WindsurfingNils Thanks Nils, just to clarify, when you say "force leeward" are you speaking about drag? If so, that is correct, I didn't measure that. It would be interesting to do that but tricky to set up.

[@Sailorman6996]

Read aboud the coanda effect.

[@3denlightenment]

Thanks. I know all about the coanda effect.

[@klaasvanmanen8214]

You designed an experiment in order to measure the contributions of Bernoulli's principle and Newton's third law to the lift force acting upon an air foil. I'm certain you were able to measure lift force on the airfoil in the experiment. I'm also convinced that you were not able to measure the relative portions of the principle/law mentioned. The idea of Newton's third law here is that a certain mass of air is diverted by the airfoil, which means that it is accelerated (it can only change direction if it is accelerated). The acceleration is exerted on the air by the airfoil (the force exerted is mass of the air times it's acceleration), hence, according to Newton's third law, the accelerated air exerts an equal but opposite force on the airfoil. The dimensions of the airfoil are chosen such that the airflow is only accelerated in one direction, with as little disturbance as possible of the laminar flow, which would after all result in turbulence, which in it's turn does not contribute to the lift force, as the direction of the turbulent air is not uniform, hence the reaction force is also not uniform (but in a wide range of directions). In order to measure the contribution of Bernoulli's principle, you would also need to measure the speed of air over the airfoil, and the same under the airfoil, which you didn't do. Bernoulli's principle as an explanation for lift seems to be an ineradicable misconception, based on the idea that air particles over the airfoil have to travel faster than particles under the same airfoil because the route over the top side is rounded and hence longer, and assuming that the particles take equal times to travel from leading edge to trailing edge. The latter assumption has long been proven incorrect, which makes applying Bernoulli's principle quite impossible. Moreover, Bernoulli's principle was formulated for fluids flowing in one direction, without being diverted, which is certainly not the case when a fluid is flowing over an airfoil.

[@3denlightenment]

Thanks @KLass van manen. When it comes to Bernoulli's principle, I find it interesting that the explanations of lift are often centered around the static pressure differences and dynamic pressure may or may not be discussed at all. In the experiment, I was trying to separate the static pressure differences (air flowing over the top only) from the combined pressure (air flowing over both the top and underside). I totally agree with your discussion of the air flow over the air foil and the that the generation of lift is dependent on laminar flow and also that the generation of turbulence would reduce the lift force. By using the 'air flow diverter' to restrict the air flow to the upper surface, I was able to measure the lift from the static pressure drop between the upper and lower surfaces. Through the conservation of energy, through which Bernoulli's principle applies, the fact that the air flow was essentially motionless on the underside of the airfoil should have accentuated the pressure difference. Given that I was able to measure the lift force from air flowing over the upper surface only, indicated that there was a difference in the static pressures from the air flow. I also agree with your comment that it has long been refuted that air particles travel faster due to the assumption that they take the same amount of time to travel the greater distance on the upper surface. In the end, we are in agreement that the STATIC pressures of Bernoulli's principle have little to no effect on generating force during sailing; however, the DYNAMIC pressures (derivation of Newton's laws) have a significant effect. Thanks for your thoughtful comments 🙂

[@TheStupidcomment]

The whole higher pressure under a wing has been debunked. You can fly with a totally flat wing, what causes lift is air against the angle of the wing at the leading edge.

[@3denlightenment]

True, with a totally flat wing you can fly, but there must be an angle of attack to capture the dynamic pressure.

[@Kogacarlo]

The wind pushes the sail, the sail pulls the boom, the boom pulls the mast and the mast drags the board through the water.

[@3denlightenment]

Pretty much, although it is not a linear chain as the sail is attached to the mast too. Also, unlike in a sail boat, the sailor is holding on to the boom and applies direct pressure on to the board which drives the board forward. So, there are three points of drive from the sail, the mast, and the sailors two feet.

[@Kogacarlo]

@@3denlightenment the sail is attached to the mast in a way that it can not provide forward force. Only when you surf downwind it will and that's almost never. Look at the red line 01:23. The sail can only pull the mast in the direction of the line and that's backwards. And you drive the board forward with your feet?? I never heard that before and I don't think it's true.

[@3denlightenment]

@@Kogacarlo Thanks Carl. For an explanation of foot pressure, check out my first video kzbin.info/www/bejne/mZu3k2RrpdKYY8k on the influence of the sailors feet when steering in lower winds (and when tacking). Also, the foot pressure is what makes windsurfing unique from typical sail boats where the mast is completely held at one point by the vessel. Without the pressure of your feet on the board, the mast would fall over with the wind so foot pressure is definitely involved. I also discuss how the resultant force is always downwind but there are forward components (i.e. across wind), which when combined with the angle of the fin (and dagger board) allow the boat to sail across and upwind.

[@Kogacarlo]

@@3denlightenment "Without the pressure of your feet on the board, the mast would fall over" That must be right. I'll look into it.

[@antoniofetter]

Please, stop using Bernoulli's eq. to explain lift. The Bernoulli's equation is only valide along a streamline. One can not compare two streamlines on the upper and lower parts of the wing, that is wrong!

[@3denlightenment]

Thanks for your comment. The static pressures (not dynamic) are commonly used to explain lift as they demonstrate pressure differences on the upper and lower surfaces of the wing. The dynamic pressure differences come in to play in the discussion of the angle of attack. The experimental model I built and used to measure the lift forces showed that there was measurable lift when the air flowed over the top of the wing (green line on graph) but was much greater when the air flowed over the top AND bottom surfaces (red line on graph). The whole video was also presented not to describe flight but what were the predominant forces when sailing...and Bernoulli's pressures are not very significant. These low pressure forces could also be described through the coanda effect which also results in lower pressures downstream of air flowing over a curved surface, but Bernoulli has derived a mathematical equation to explain pressure differences in ideal fluids through the application of the conservation of energy.

[@antoniofetter]

@@3denlightenment The explanation using Bernoulli's equation relays on the following reasoning: 1 - the fluid particles following along the upper and lower faces of the wing take the same amount of time from the leading edge to the end of the wing; 2 - the curved path of the upper face is longer than the lower path; 3 - because 1 and 2, the particles along the upper face must be faster; 4 - using Bernoulli's equation, because of 3, the pressure on the upper face must be lower than the pressure on the lower face. There is no physical justification for 1 to be true, in fact, numerical experiments clearly show that 1 is not true. One can't justify 3 based on Bernoulli's equation, because Bernoulli's equation is only valid along a SINGLE streamline. Therefore, one can't compare velocities and pressures along a streamline on the upper face and another one on the lower face of the wing. That is wrong!!! That is a violation of the validity of the Bernoulli's equation. This explanation is one of the most widely spreaded hoax of fluid mechanics.

[@3denlightenment]

@@antoniofetter Just to respond to your points. #1 is not true as most people know by now; which is why I never mentioned it. #2 is true. #3 The difference in path distance has nothing to do with why the air travels faster, so we are in agreement that #1 and #2 does not result in #3. #4 By the conservation of energy, the increase in air speed causes a decrease in the static pressure on the upper surface of the airfoil. The increase in speed is likely due to the coanda effect and the sudden compression of air around the airfoil as in Bernoulli's principle. At this point we are talking about a single stream of air on the upper surface only. This lower static air pressure around the upper surface is in contrast to the static pressure on the lower surface, regardless of whether the air is moving on the under surface or not. Because the static air pressure is lower on the upper surface than that below the airfoil , it results in the airfoil rising. So, even though Bernoulli's principles apply only to the upper surface via a single stream of air, the pressure differences created between the two surfaces results in the generation of lift.

[@antoniofetter]

@@3denlightenment One last comment... “So, even though Bernoulli's principles apply only to the upper surface via a single stream of air,” The Bernoulli’s principle applies to ANY streamline, regardless if it follows the upper or the lower part of the wing. A streamline is a line that is tangent to the velocity field, in steady state, it is the path that will be followed be a given fluid parcel. In this context, the Bernoulli’s principle express the conservation of the total energy as the fluid parcel flows along a SINGLE streamline. For instance, if the kinetic energy increases, then the potential energy or the pressure have to change to compensate for it. Therefore, one can not compare the quantities expressed by the Bernoulli’s equation across two or more streamlines. It would be like comparing the total energy of a satellite following an orbit around the Earth, with the total energy of another one stored at a JPL’s warehouse.

[@3denlightenment]

@@antoniofetter Thanks for all the discussion. I agree with what you have said and since I am not comparing quantities expressed by Bernoulli's equation across two streamlines, we are in total agreement.

[@jemand8462]

no, it does not have the profile of a wing. It has the profile of a sail. The air is flowing around it at the same speed, reaching the end of the äquidistant points at the same time, creating no pressure difference and thus no lift. It's all about the reaction force of bending the air into a curve. Basically the same as if the sail was completely straight, just more efficient because it produces less turbulences.

[@3denlightenment]

I agree, but so many explanations center around the curve of the sail and the wind flowing over it. The explanation of lift is often given as a function of air flowing over the leaward side ...which makes it like a wing. Newtons explanation is the air moving and striking the windward side. The windward side of a sail is curved as you state, but the wing has a straight flat bottom, but both provide surfaces for receiving the Newtonian force.

[@jemand8462]

5:00 this is NOT a sail, it's a WING. One side is longer then the other, it has a chamber. A Sail does NOT have a chamber. This whole video is completely false, sorry, as fance as it has been produced.

[@3denlightenment]

The model is a sail. You are right, it is not a wing. There is not a flat side on the underside of the model. It is hollow like a sail. It has a thin membrane over 3 struts simulating the thin shape of a SAIL. There are SO many videos where a sail is described as a wing. I am constantly showing the differences in the video. Your scientific discussion is appreciated.

[@greenstripeypaint]

not really

[@3denlightenment]

Fair enough

[@mm74forums7]

This is completely wrong and confusing! Bernoulli has nothing to do with aerodynamic Lift because Bernoulli's law is about same flow in same pipe with different diameters. There are two flows around wing section, above and below, those flows are disconnected. Bernoulli's law is not applicable to this. Particles of those flows that were on same distance to a leading edge of wing even do not meet at the trailing edge. Top one flies ~1.5 times faster. That is called circulation (Joukovsky - Kutta). If you make a flat profile section and blow air only above it, it will not create Lift, despite that flow speed below the section is 0, and speed difference is large. So it is not a difference in speed of flows creates lift. Very simplistic speaking, Lift is created by bending the flow. "Newtonian" example is also wrong. This mistake caused delay of aviation about 50 years, or even more. Sail is more pulled, not pushed. Sailing with "pushed" sail is big mistake. Holding sail to 45 degrees to apparent wind? Did you ever planed at about 50 km/h? The apparent wind is coming almost from front. If you sheet in 45 you will stop. 45 degrees AoA creates lift. But it also creates lots of Drag. It is main novice's mistake to sheet in too much. Drag will drift you downwind. To sail on beam reach or upwind you should hold sail 15-20 degrees to apparent wind.

[@3denlightenment]

Thanks @mm7 4forums. In my first video, I never spoke about Bernoulli's principle as I didn't think it was applicable. I also never indicate that the air particles meet at the trailing edge as that has been long proven to be wrong. Bernoulli's principle is used, however, to explain lift (even NASA www.grc.nasa.gov/www/k-12/airplane/bern.html) and so I bring it up here. Thanks for the links you provided. I read Babinsky's paper and I have seen the Efficient Engineer video. As both of these articles indicate, there is confusion and misinformation when it comes to describing lift. There are issues discussed in both of these which can be refuted. For example, when Babinsky discounts Bernoulli's theorem by saying that blowing over a piece of paper causes lift but violates the assumption that the pressure is constant above and below is not true. First, Bernoulli's equation has two components Ps (static pressure) and 1/2dv2 (dynamic pressure). Due to the conservation of energy, when the static pressure goes down, the dynamic pressure goes up (increases in velocity) and for ideal fluids, this makes sense, but he doesn't speak about that. When he shows the air blowing over the upper surface, and shows the pressure difference in the manometers in the hair dryer experiment, he is measuring the static pressure in both cases, not the dynamic pressure. The reason the air flow lifts the paper can be explained by the drop in pressure from Bernoulli, but also from the coanda effect and the fact that air is a fluid which, depending on turbulence, etc, can have non-uniform density. If a low density area is created (partial vacuum), the air particles move to that area to retore equilibrium. That low pressure area also would cause the paper to rise. In addition, if he blew air underneath the paper , it would also rise. This would be due to the dynamic pressure and Newton's second and third laws. The dynamic pressure component of Bernoulli's equation can be derived directly from Newton's second law (F = ma) assuming an average particle velocity during acceleration. If is also important to point out that the explanations of lift on a sail cannot be directly compared to lift on an aircraft. As I tried to demonstrate near the last section of my video, the air flow over an aircraft wing is almost always described with the wing slicing through the air essentially parallel to the flow of air. If you change the angle of attack, without changing the direction of the aircraft, the drag component will definitely increase, and the lift component will decrease and the aircraft will not be able to oppose the force of gravity and the aircraft will stall. However, in reality, the angle of attack changes with the change in orientation of the aircraft, since the wings are fixed in position. This change in aircraft angle continues to maintain pressure on the underside of the wing through the momentum and the pressure against the ambient air as it turns. At the end of the turn, when the aircraft is no longer accelerating, the pressure on the underside of the wing will decrease and the aircraft will likely have to readjust its orientation to maintain the pressure on the underside of the wing to maintain lift, depending on the power of the engine. Regarding the apparent wind, it may feel like it is coming from the front (direction of sail), but it is not. If you are on a beam reach and the wind is traveling at 50 km/hr, you may be able to go, lets say 55 km/hr. If you draw a vector diagram, the apparent wind will be just slightly less than 45 deg. Definitely not parallel to the orientation of the board. If you are on a beam reach, and you say you should hold the sail 15 to 20 degrees to the apparent wind, it may appear to work well, but that is only because the determination of the angle of the apparent wind is not correct. I agree, it is a novices mistake to sheet in too much, that is generally because they are not travelling fast and the apparent wind is only slightly off from the ambient wind. All forces on the sail will drift you downwind, and only the fin/dagger board prevents drift. Thanks for your comments.

[@mm74forums7]

@@3denlightenment thanks for the reply, and for the great Blender work too! However, re Bernoulli - the Bernoulli's principle (BP) works, of course. But the condition is - it is one flow, in one single pipe. BP does not define equation between 2 parallel flows. So, it is not applicable to whole airfoil, that goes between 2 flows. Try simple experiment. Place a sheet of paper on a table. Blow over it (not underneath it). It will not lift up despite a difference in air flows speeds above and underneath it. Why? If is very important to point out that the explanations of Lift on an aircraft wing, sail, fin, keel, hydrofoil, ship or aircraft propeller, turbine blade, you name it, CAN be directly compared, because they ALL work same and based on SAME principles. And the true is - the upper surface of airfoil produces much more lift, (2-3 times more) due to negative pressure, than the lower surface does with its positive pressure. In practice it is confirmed by stall effect on aircraft. If upper flow is separated, lower flow can still produce lift (based on Newtonian effect). But this lift will be not enough and aircraft goes down. Same happens when I go on a hydrofoil(HF) and HF wings touch water surface. It causes immediate upper flow separation due to ventilation (catching air bubble). Despite there is still some lift from bottom flow, my board will just fall down, immediately! If your conclusion that a sail works more on inner side pressure, then on outer side suction was true, these wont happen - I would slowly go down, not crushed down. So, an advice to novice windsurfers should be - watch outer sail air flow, do not allow its separation (do not sheet in too much). It is helpful to put several small ribbons on stickers to both sides of sail along a chord somewhere in a mid of sail. They work as indicators - on outer side they will be horizontal and flapping when flow is not separated, and hang vertical when flow is separated. Same applicable to fin. Feel water flow around a fin, do not over-press on it, or do not turn your board too much upwind (increasing AoA to water flow), otherwise you will experience spinout (stall of fin). BTW very often oversheeting of sail causes both - stall on sail, and spinout on fin due to increased side force and decreased thrust. Of course apparent wind is not exactly parallel to course. But it can be much less than 45 dgr. Planing board can go 2-3 times faster than true wind speed. Iceboats can go even 5-6 faster. So apparent wing angle will be smaller. If you are going to go fast, the goal will be to decrease drag component and to increase lift component. If you draw D/L to AoA polar, you will see that there is optimal AoA range where D/L is minimal. So called "Bucket of L/D". So most efficient way will be staying in this bucket while getting most of Lift. I do not remember the bucket ever reaches AoA 45 degrees.