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Old 15-01-2020, 09:40   #1
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How do sails generate lift?

An article in Scientific American explores competing explanations of aerodynamic lift of airplane foils. Like our sails, this matters greatly in understanding how we do what we do. But sailboats are not just like airplanes. We don’t have powerful engines pushing us through the air fast enough to create relative wind speed and pressure differences. And the curvature of our “wings” has to change as we tack. Like most sailors I was taught a very simplified explanation of Bernoulli’s theorem to understand how sails produce lift and how to adjust the angle of the boom and sail shaping to create lift. But this article presents much more complex explanations. It doesn’t mention sailing at all.

https://apple.news/A0Zc9zaiHRXqIyH2c_aHCtQ

Would some of the forum contributors who have vastly greater knowledge of aerodynamics, airplanes or sail shaping care to comment? I have read Ross’s Sail Power along time ago but this article left me with some perhaps simplistic questions.

If the air particles moving around the outside of a sail have to move faster to get around the curve (Bernoulli) why don’t the air particles moving around the inside of the curve move faster to follow the curve of the inside of the sail? And if the air traveling over the curve does begin to separate causing a partial vacuum and then accelerate causing lower pressure does not the air traveling along the inside of the sail not also create a partial vacuum (and turbulence behind the mast) negating the pressure difference? If an airplane with flat airfoil surfaces can fly both right side up and up side down given the proper angle of attack could not flat sails not also work on both tacks given the proper angle of attack? I’ve never heard anyone refer to Newton’s 2nd law to explain sails generating lift. Does that not apply to us?

Lovely sunsets and the wind in my hair are what I will always love about sailing but puzzling about how it all works adds to my enjoyment.
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Old 15-01-2020, 11:32   #2
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Re: How do sails generate lift?

I can't open the attachment without down loading something.

There is a sail manufacturer here in Australia that produces a soft wing. It is two sail cloths each attached to opposing sides of a rotating mast. Each side of the sail is joined at the leech. By rotating the mast you can alter the camber of the wing. They claim significant increases in power over a 2d sail cloth.

My understanding and I am sure someone will point out that I'm not quite right, but it works for me and I think of it this way..... the power of a sail comes from forcing a change in direction of the air flow and the front of the sail causes lift by the outside (leeward) of the sail expanding the air as it pulls the attached airflow around and the inside (windward) of the sail compresses the airflow by restricting is passage producing a pressure differential across the sail.

A completely flat sail will create lift if it is driven or is fixed and the air is moving but it creates more drag than lift.
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Old 15-01-2020, 11:35   #3
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Re: How do sails generate lift?

That is odd. Every "Introduction to Sailing" lecture I have ever heard starts with the parallel of a sail to a wing and discusses the concept of 'lift'.
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Old 15-01-2020, 16:27   #4
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Re: How do sails generate lift?

And the article that is referenced was on, I think, the Apple news feed from Scientific American. The key point was that there are lots of models, simulations and theories, but there are places in each of them that don’t work right or conflict with other data.

Another case of thinking something is "proven, settled science," and then finding out that you didn’t know as much as you thought.
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Old 15-01-2020, 17:05   #5
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Re: How do sails generate lift?

Newton’s 3rd law does apply to us in the form of weather helm. That is part of the lift the sail creates except it is the deflection off the aft of the main which in turn pushes the aft of the boat leeward, causing weather helm. And in my boat with a longer footed old school main, it can be a lot unless the boat is balanced well; the center of effort being only slightly ahead of center of lateral resistance. As far as higher camber wings flying upside down, the only time I have seen it was on stunt planes like a Citabria with plenty of power and applying a very high, upside down, angle of attack, applying more of Newton’s 3rd law I presume. As for the vacuum created on the inside of a sail, it, if it is there, is small. If you have tell-tales on the main you’ll see those on the inside are not hanging there if the sail is trimmed properly. The best wing is a rigid wing with a camber appropriate to the speed desired, as in the one on the Vestas Sailrocket. But of course that is completely impractical. Next best is a rotating wing mast which will minimize that vacuum behind the mast you mentioned. Or perhaps better are the wingsails on free standing masts, which I think look pretty cool, but I’ve only seen them in photos. Anyway that’s my own two cents gleaned from engineering and sailing friends with more lab time than I.
And Newton’s 2nd law certainly applies to us... lighter boats accelerate faster
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Old 15-01-2020, 17:17   #6
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Re: How do sails generate lift?

Let’s see if I can copy some of the article here.

From the recent article in Scientific American by Ed Regis

No One Can Explain Why Planes Stay in the Air
In December 2003, to commemorate the 100th anniversary of the first flight of the Wright brothers, the New York Times ran a story entitled “Staying Aloft; What Does Keep Them Up There?” The point of the piece was a simple question: What keeps planes in the air? To answer it, the Times turned to John D. Anderson, Jr., curator of aerodynamics at the National Air and Space Museum and author of several textbooks in the field.
What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times. People give different answers to the question, some with “religious fervor.” More than 15 years after that pronouncement, there are still different accounts of what generates lift, each with its own substantial rank of zealous defenders.


Two Competing Theories

Bernoulli’s theorem attempts to explain lift as a consequence of the curved upper surface of an airfoil, the technical name for an airplane wing. Because of this curvature, the idea goes, air traveling across the top of the wing moves faster than the air moving along the wing’s bottom surface, which is flat. Bernoulli’s theorem says that the increased speed atop the wing is associated with a region of lower pressure there, which is lift.


There are plenty of bad explanations for the higher velocity. According to the most common one—the “equal transit time” theory—parcels of air that separate at the wing’s leading edge must rejoin simultaneously at the trailing edge. Because the top parcel travels farther than the lower parcel in a given amount of time, it must go faster. The fallacy here is that there is no physical reason that the two parcels must reach the trailing edge simultaneously. And indeed, they do not: the empirical fact is that the air atop moves much faster than the equal transit time theory could account for.


The other theory of lift is based on Newton’s third law of motion, the principle of action and reaction. The theory states that a wing keeps an airplane up by pushing the air down. Air has mass, and from Newton’s third law it follows that the wing’s downward push results in an equal and opposite push back upward, which is lift. The Newtonian account applies to wings of any shape, curved or flat, symmetrical or not. It holds for aircraft flying inverted or right-side up. The forces at work are also familiar from ordinary experience—for example, when you stick your hand out of a moving car and tilt it upward, the air is deflected downward, and your hand rises. For these reasons, Newton’s third law is a more universal and comprehensive explanation of lift than Bernoulli’s theorem.
But taken by itself, the principle of action and reaction also fails to explain the lower pressure atop the wing, which exists in that region irrespective of whether the airfoil is cambered. It is only when an airplane lands and comes to a halt that the region of lower pressure atop the wing disappears, returns to ambient pressure, and becomes the same at both top and bottom. But as long as a plane is flying, that region of lower pressure is an inescapable element of aerodynamic lift, and it must be explained.


Toward a Complete Theory of Lift
Contemporary scientific approaches to aircraft design are the province of computational fluid dynamics (CFD) simulations and the so-called Navier-Stokes equations, which take full account of the actual viscosity of real air. The solutions of those equations and the output of the CFD simulations yield pressure-distribution predictions, airflow patterns and quantitative results that are the basis for today’s highly advanced aircraft designs. Still, they do not by themselves give a physical, qualitative explanation of lift.


McLean’s complex explanation of lift starts with the basic assumption of all ordinary aerodynamics: the air around a wing acts as “a continuous material that deforms to follow the contours of the airfoil.” That deformation exists in the form of a deep swath of fluid flow both above and below the wing. “The airfoil affects the pressure over a wide area in what is called a pressure field,” McLean writes. “When lift is produced, a diffuse cloud of low pressure always forms above the airfoil, and a diffuse cloud of high pressure usually forms below. Where these clouds touch the airfoil they constitute the pressure difference that exerts lift on the airfoil.”


The wing pushes the air down, resulting in a downward turn of the airflow. The air above the wing is sped up in accordance with Bernoulli’s principle. In addition, there is an area of high pressure below the wing and a region of low pressure above. This means that there are four necessary components in McLean’s explanation of lift: a downward turning of the airflow, an increase in the airflow’s speed, an area of low pressure and an area of high pressure.


But it is the interrelation among these four elements that is the most novel and distinctive aspect of McLean’s account. “They support each other in a reciprocal cause-and-effect relationship, and none would exist without the others,” he writes. “The pressure differences exert the lift force on the airfoil, while the downward turning of the flow and the changes in flow speed sustain the pressure differences.” It is this interrelation that constitutes a fifth element of McLean’s explanation: the reciprocity among the other four. It is as if those four components collectively bring themselves into existence, and sustain themselves, by simultaneous acts of mutual creation and causation.
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Old 16-01-2020, 10:00   #7
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Re: How do sails generate lift?

and then there's the Coanda theory.
https://en.wikipedia.org/wiki/Coandă_effect

Clearly lift matters, but so does drag. One benefit of curved wings is attached airflow which results in less drag. That's why we use telltales, we're trying to keep the airflow attached which reduces drag and therefore keeps our speed up.
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Old 16-01-2020, 10:02   #8
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Re: How do sails generate lift?

Many years ago I flew sailplanes (gliders). Several of the club members had backgrounds in aerodynamics and engineering. As a result the subject was often debated over beers at the end of the flying day from the perspective of engine-less aircraft.

The aerodynamics of the Space Shuttle, which was described as "a flying brick", came up as well as symmetrical airfoils common in stunt planes and fighters that have curved or nearly flat surfaces on both the top and bottom of their wings as points of discussion.

At a very memorable moment one of the engineers said: Issac Newton's physics is, and must be, the fundamental principle at work. Any airfoil produces lift by accelerating air in a direction opposite the vector of the generated lift. Or as Newton himself said, "Any action produces an equal and opposite reaction."

He then went on to say that any other description of flow over an airfoil applies to how the airflow is acted upon and how efficiently the wing shape works to produce the acceleration of the moving air.

Time to put Bernoulli and the flying vacuum theory into a secondary role, and trim our sails with Newton's hand on the sheets.
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Old 16-01-2020, 10:09   #9
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Re: How do sails generate lift?

Quote:
Originally Posted by Boatwright View Post
Many years ago I flew sailplanes (gliders). Several of the club members had backgrounds in aerodynamics and engineering. As a result the subject was often debated over beers at the end of the flying day from the perspective of engine-less aircraft.

The aerodynamics of the Space Shuttle, which was described as "a flying brick", came up as well as symmetrical airfoils common in stunt planes and fighters that have curved or nearly flat surfaces on both the top and bottom of their wings as points of discussion.

At a very memorable moment one of the engineers said: Issac Newton's physics is, and must be, the fundamental principle at work. Any airfoil produces lift by accelerating air in a direction opposite the vector of the generated lift. Or as Newton himself said, "Any action produces an equal and opposite reaction."

He then went on to say that any other description of flow over an airfoil applies to how the airflow is acted upon and how efficiently the wing shape works to produce the acceleration of the moving air.

Time to put Bernoulli and the flying vacuum theory into a secondary role, and trim our sails with Newton's hand on the sheets.
well said.
we could build sails from sheets of plywood and our boats would sail, but not as efficiently.
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Old 16-01-2020, 10:21   #10
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Re: How do sails generate lift?

Quote:
Originally Posted by guyrj33 View Post
well said.
we could build sails from sheets of plywood and our boats would sail, but not as efficiently.

that what I was going to say
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Old 16-01-2020, 10:29   #11
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Re: How do sails generate lift?

Stop thinking about lift so much and start thinking about vacuum in both sailboats and aircraft.
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Old 16-01-2020, 10:49   #12
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Re: How do sails generate lift?

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Originally Posted by captlloyd View Post
Stop thinking about lift so much and start thinking about vacuum in both sailboats and aircraft.
Maybe drag more than vacuum, if we care about efficiency. :>)
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Old 16-01-2020, 11:40   #13
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Re: How do sails generate lift?

Wow! Intelligent people replying with very informative answers. May I take a crack at a couple of the concepts? My answers in bold.
-----------------------------
If the air particles moving around the outside of a sail have to move faster to get around the curve (Bernoulli) why don’t the air particles moving around the inside of the curve move faster to follow the curve of the inside of the sail?
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The chord of the wing (sail) tells the tail. Think of the air traveling the greater distance as becoming more thin and travelling faster. Think of the air traveling the shorter distance as remaining mostly static in pressure. An airplane is lifted from above, not pushed from below. You can google images of wing chord to see what I'm saying.
---------------------
And if the air traveling over the curve does begin to separate causing a partial vacuum and then accelerate causing lower pressure does not the air traveling along the inside of the sail not also create a partial vacuum (and turbulence behind the mast) negating the pressure difference?
------------------------
The air traveling the greater distance doesn't separate (that's called a stall), it travels faster and becomes less dense. Telltales are in place to show us laminar flow vs. separation, one is good, the other is not.
------------------------
If an airplane with flat airfoil surfaces can fly both right side up and up side down given the proper angle of attack could not flat sails not also work on both tacks given the proper angle of attack?
---------------------------
What you just described is deflection lift, vs. aerodynamic lift. Deflection lift requires a very high power-to-weight ratio and generates enormous drag.

It's been said, "If you cannot explain it simply, you don't understand it". My training was horrible, but my practical experience has provided some insights. I hope I provided more light than heat. Carry on.

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Old 16-01-2020, 11:42   #14
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Re: How do sails generate lift?

I can suggest a book rather than try and give answers here. Sail Performance by CA Marchaj goes through all the aerodynamic and hydrodynamics related to a sailing. The big difference between planes and boats is that planes only fly through the air. Boats have two wings, one in the air the other in the water and it is the interaction between the two that makes a sailboat work. Lift, as in lift to windward, is primarily a function of the keel. Lift as in creating forward motion is due to the lift produced by the sails balance by the resistance in the keel to create a resultant forward force.
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Old 16-01-2020, 12:12   #15
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Re: How do sails generate lift?

After 50 years flying aircraft and 40 years sailing boats.. I have found out that the same thing that make planes fly makes boat sail....MONEY��
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