Why the slice of bread always falls on the buttered side (and what does this have to do with the America’s Cup)
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The 37th edition of the America’s Cup is approaching. Looking forward to a stadium cheer for our national sailing team, Luna Rossa (hoping they make a far better impression than the Azzurri soccer team at the European Championship), one of the most anticipated events in our “Cup Journal” returns. That is, the lectio magistralis by Italian-Brazilian professor Elio Somaschini* aka Crapun (stubborn, in Lombard), the sailor who has been sailing around the world for 20 years without instruments, using only a watch, and who has the gift of being able to make simple what is not simple .
In this first installment, Somaschini explains, in his Italian “contaminated” by more than 60 years of Brazilian life, Because the slice of bread always falls on the buttered side. And why this has anything to do with America’s Cup boats, the very fast flying AC75s equipped with foils. Happy reading.
Pitch, America’s Cup, slices of bread
Why does the slice of bread always fall on the buttered side? This major problem that plagues humanity has much to do with the America’s Cup.In Figure 1 we have an airplane. Its weight can be seen as acting on that vertical line containing a point of equilibrium that we will call the Center of Mass (or Center of Gravity).
However, the airplane is flying horizontally, in perfect balance, so something has to provide a vertical force up in order to cancel out the Weight force: on the airplane, it is the wings that also create a thrust up, and thereby cancel out the Weight force.
True, but if you have these two forces as in the drawing, this plane will point the nose toward the sky and the tail toward the ground, because these forces do not act in the same vertical!
The solution? It lies in the tail, where those two small horizontal wings also provide a force up and the plane becomes stable (fig.2).
You may be thinking, “But if I apply the force of the wings in exactly the same vertical as the weight, I won’t need the tail.” Unfortunately, the situation would become so unstable that the flight could not be controlled.
“So,” you will say, “if I understand you correctly, if the Center of Mass of my airplane was further forward than the vertical of wing force … the tail should be designed to make force down.” Bravo!
In Figure 3 I have depicted the possible situations.
The importance of “drag”
But I have been a bit of a cheater with you. I don’t know if you noticed, at the beginning I wrote that wings make a force ALSO up. This is where tears flow from the eyes of America’s Cup engineers.
When air or water or a fluid flows over a surface, a force against the movement arises, which they normally call Drag, literally, in Italian.
The reasoning to always do is: if we receive, from the interaction between the wind and the sail, the energy to move the boat, we must reduce all energy-dissipating resistances as much as possible. Drag is one of the most important.
Very high-profile engineers work hard here, looking for solutions that are sometimes unusual or even non-intuitive.
The one who discovered the drag problem was the “father of aerodynamics,” Ludwig Prandtl. He was able to create equations and theorems and a special number (named after him) and, as a result, we have known much more since 1932.
After Prandtl, we know that the best wing shape is elliptical, that single-wing planes fly better than biplanes, and many other things.
It set the stage for the rise of NACA (now NASA), with the mission to study, classify and test all wing profiles, i.e., what we call Foil.
So, the force that the wing provides is not vertical, but rather tilted back, ALWAYS! When the fluid moves, drag arises.
But then if the force is always backward, how does the plane go forward? This is where the engine comes in, which in the case of the AC 75 is called SAIL!
I would just like to devote a second of my thoughts to Otto Lilienthal, a German aviation pioneer who lost his life just trying to be able to change the Center of Gravity during flight, but who left us all studies and hundreds of flights made and inspired humanity.
Small concepts: that vertical wing force is NOT the “lift,” which we will discuss appropriately, but the lift is perpendicular to the centerline of the foil, not the plane of flight…
Talk about slices of bread and AC75 America’s Cup.
Ahhh …. I was forgetting the slice of bread and the America’s Cup!
On AC75s, when the boats are flying and one of the forces shifts, there is a risk of Pitch ( the nose tuck, or gagging!), with dire consequences. Just as with Lilienthal, once the Pitch begins (normally because of a “stool”) there is no more control.
From here you can see the complexity of carrying such a boat. Without computers and dozens of sensors on board, it would be impossible to fly it. Keeping the boat horizontal, at almost 100 km/h, with waves, people moving around and changing the position of the Center of Gravity (CG), sails constantly changing adjustments, the number of variables is very high, I don’t know how much autopilot is behind it…
Now that you know why an AC can stick its nose in the water or all of a sudden turn up its nose and soar like the prancing horse let’s move on to the more important topic.
Take a slice of bread (15 g), spread some butter (30 g), then some jam (20 g). Now lay the slice in on the edge of the table. Slowly push the slice toward the edge of the table. The center of mass of the system is roughly in the middle of the slice. And I push it even more toward the edge, until….
The center of mass leaves the table! Then the slice begins to fall, however at the same time it turns in on itself (It’s nice to do. At home)! What happens is that with only 80 or 90 cm of table height, when the slice falls, it doesn’t have the a time to make a full turn, it only manages to make half a turn! Since at first the butter was on top now it will be underneath. So, as in an AC75, the slice of bread makes a pitch.
I leave you with a question, “If an airplane, which weighs so much, is able to fly, who is doing the power that supports it?“
Don’t say air, air doesn’t even support a pen!
Greetings from the Crapun!
And here’s to the next installment!
*Who is Elio Somaschini
Physicist, sailor, sailor sailed around the world for 20 years without instruments, only with a watch.
Elio Somaschini, discovered by the Sailing Newspaper, has quickly become one of the most followed “popularizers” on the web. His secret? He has a gift for making complex concepts simple and understandable to everyone by applying physical principles to practice. Elio makes you understand why you are doing something. In this lecture series, he explains the secrets of America’s Cup boats, the AC 75
Would you like to personally contact Elio Somaschini to learn more about boating physics? Would you like to reserve a place aboard a Swan 65 with Elio at the America’s Cup in Barcelona? Write to him at crapun@gmail.com
Elio Somaschini’s last lectures:
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