Is it defying the law of Archimedes?

The fact that aircraft don't fall out of the sky always seems to me to defy the law of gravity.
Now， the same is true for electric foiling workboats.
Artemis Technologies - Maritime Design https://www.artemistechnologies.co.uk/

 

no.

"Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially, is equal to the weight of the fluid that the body displaces."

just that the boyancy is not the only force lifting the craft. Just like in the case of a an aircraft.

 

Aerofoils and hydrofoils are both based on dynamics, buoyancy is based on statics and you also can have it in the air for example a balloon is "flying" due to buoyancy.
Edit: With buoyancy = Archimedes law

 

In fact, just as kerosene states, even an object sitting on the ground or in the water is experiencing a buoyant force. The plane on the tarmac is experiencing more buoyant force than the plane flying at ten thousand feet. This is because the weight of the air above compresses the air on the ground to be heavier than air at ten thousand feet, and therefore more buoyant force is experienced on the ground.

Interestingly, it can seem the reverse in water. Because water doesn't compress easily, a diver's BC filled with air will float due to the relative difference in weight of water per volume than the weight of air. However, force that BC down until the pressure of the water compresses the air on the BC, less and less buoyant force is felt on the BC, until the water pressure compresses the air enough that the air in the BC becomes equal to the weight of the water around it. Then, no buoyant force will push it upward. It may begin to sink because the materials of the BC itself may weigh more than water.

A foil works on a gradient of compressed fluid under the wing to decompressing fluid over the wing. This is caused by the movement of that fluid across the wing. Water still compresses a little and the difference in that fluid across the top from that fluid across the bottom can have a great deal of force; especially in a fluid that resists compression/decompression.

Forward movement through a fluid also is effected by Lateral Resistance vs forward resistance. The broad plane of a foil will resist moving against the air much more than the narrower cross sections fore and aft, exactly the way a keel on a sailboat keeps a boat moving forward while resisting moving sideways with the wind. In the case of a wing, sideways becomes down.

 

There's no need for the fluid to compress at all. Whether you're dealing with water or air, the speeds involved in sailing are low enough that compressibility can be totally ignored.

 

If we are in agreement that the pressure on the top of the wing or lee side of a sail is lower than the fluid across the underside or windward side of the foil, how is that pressure variant achieved without some compressibility in the fluid?

 

If the fluid density is nearly constant, the speed of sound is very high and pressure signals propagate through the flow almost instantly. But you only need to consider that if you're trying to explain how a particular flow pattern develops (or calculate an unsteady flow).

Given a steady flow pattern (or in calculating it via incompressible flow equations), the flow speeds and pressures are related by Bernoulli's equation.

 

Bernoulli and Newton https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html
Here is NASA's explanation that agrees with what you are saying.
Bernoulli's Equation https://www.grc.nasa.gov/www/k-12/airplane/bern.html

I've always had difficulty not making assumption. Thanks for the correction.

-Will

 

Another comparison with an airplane would be a dynamic diving submarine with a slightly difference.
The airplane is overcoming the gravity force with it's dynamic lift (and with a little help of, but negligible, buoyant force). The dynamic diving submarine is overcoming it's buoyant foce also with dynamic (downwards) force (and a little help of gravity force). Both cases work (nearly) without the law of archimedes.
Examples for cases according to the law of archimedes would be the balloon which lifts up by decreasing the density of the air inside the balloon and for water it would be the static diving submarine which increases it's weight by pumping water and thus increasing it's weight.

 

It reminds me the subject about circulation in this forum...

I shared the feeling of Sun recently, when being grounded during a airplane exhibition. The airbus a350 xwb making a low speed passage. With the help of the fly-by-wire navigation system, the aircraft can handle such a small air speed that we almost saw the airplane hanging into the air, defying gravity !!



Thus, I cannot miss the opportunity to recall, that Bernoulli's principle apply on ideal fluids, viscosity less and adiabatics fluids. That is why there is hardly no plumbers that are used to design aircrafts ! Although both are struggling with gravity !!!

As for the satellites in geo-stationnaries orbit, they all are, from a pure Newtonian point of view, in infinite free fall in the gravitational earth's field. Defying gravity, struggling with gravity, winning over gravity are equivalent terms that do not underline any contradiction with the Newtonian theory.

Still, can the Newtonian theory explain all the gravitational effects ? The answer is well known, and is negative. Our approach of the gravity is very practical. In other words, we made the gravitational theory to explain some effects, but if more and more effects are discovered, that cannot be explained by the classical theory, shall we put in question the theory itself.

In science history, one should remember of the "Celestial spheres" theory of Ptolemée. This theory explained the planets and stars movements, with such a precision that it has been used for centuries, although the earth should have been placed in the center of the Universe (known Universe... Of course... ).

To me, this is the proof that, even if a theory is wrong, from the start, it may be used to explain some phenomena, as long as we do not question too deeply its underlying principle.

In other words, our knowledge is very relative....

 

Newton's theory of gravity doesn't even attempt to explain gravity. It only describes it mathematically so we all see it's effects from the same perspective. I have never read anything about Newton explaining why gravity works, only how it behaves.

In that, it is similar to bernoulli's formulas. It describes an observed effect. Modern science has used his formula to explain why.

Yes. In fact, the only reason it doesn't work perfectly is for the same reason the Copernican theory of a helio-centric system using ellipsis is imprecise. Our planets don't orbit in perfect ellipses. There are just too many variables that have effects on all the other variables. It isn't just the Sun and each orbiting planet. All the planets effect each other and their moons have their on contributions, as well. We have just recently learned that the inner core of our own planet spins at a different rate than the surface. Another new discovery is that both the Earth's surface and the inner core wobble in relationship to one another. Does that effect our smooth, mathematically described orbit around the Sun? Absolutely. New discoveries, New information to plug into the models, new evidence to the effective accuracy of our working theories.

I suspect, even Ptolemy may have been taught that the Sun was the center of elliptical orbiting planets. It was the Platonic perfect geometry and the Church's insistence that Man was the center of all divine thought, that dictated the structure.

The Mathematics of Conics (ellipses) goes way back to early Greek thinkers like Apollonius of Tyre.

 

It is indeed a point of view that is worth sharing, because it will certainly cut boring and long debates occuring right here, right now, on many scientific subjects. I'm also afraid to see that some scientists, themselves, forget the root basis of their work, and don't quit making "tail bitting" reasoning with no end in details and technical words.....

Regarding the mathematics of Conics and the Gravity itself, I must add that the Greek have not found the Conics buried in the sand of their islands.

You will find here a quote from the Veda (Indian mythologie) that states, ages before the Greeks, the conical path of some celestial bodies.


The same goes from Gravity, which is an ancient Indian word. I suspect that Newton knew it, but never spoke a word on it. I let you find and discover this very interesting part of the history.

 

What nonsense. Your attempt at cultural appropriation is ridiculous.

 

Probably with "indian" Alan Cattelliot means indo-european where the wort gravity actually comes from.

 

When one of the oldest scriptures in the world spoke of gravity https://www.artofliving.org/in-en/culture/amazing-india/oldest-scriptures-spoke-of-gravity