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The mysteries of aerodynamic lift

scientificamerican.com

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Re: The mysteries of aerodynamic lift

#91

Post by j0e1 »

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

Is it just me or are the description of 'Burnoulli's Theorem' and 'Co-dependency of Lift's Four Elements' separately in the two images identical erroneous? Just agreeing that that is confusing.

Re: The mysteries of aerodynamic lift

#92

Post by lmilcin »

minblaster wrote: post #79
lmilcin wrote: post #77

I don't think there is anything mysterious about lift. We can model it very well, we have precise equations that predict the exact results (though we can't solve them) and we know where these equations come from. The fact that I do not understand the equations doesn't mean there is anything mysterious behind it. There might be some artistry with regards to actually designing the aerodynamic shapes. We have no way of…

Wouldn’t you say something that has an equation that works but no theory of why is mysterious? Mysterious doesn’t mean magical, it just means there’s an unexplained gap in what we understand, which is the case.

But there is no unexplained gap. This might only be "mysterious" to somebody, but not in general. When you decide to title your article telling something is "mysterious" it suggests it is mysterious in general.

Anything could be said to be mysterious to somebody, there will always be somebody that has no knowledge to somebody. This way you can title anything as "mysterious" but it is not very useful (unless you count to bait clicks).

Re: The mysteries of aerodynamic lift

#93

Post by H8crilA »

lutorm wrote: post #85
jvanderbot wrote: post #49

Earlier quoted context omitted.

Yes, and in rotating-wing configurations (e.g., helicopters), the lift is basically calculated using the momentum of the column of air being forced downward + the momentum of the chassis. By accelerating air downward in a column below the rotating wingspan, the column of air gains a net negative momentum (downward), necessitating a net positive (upward) momentum to the chassis to keep the system's momentum conserved.…

Sure, that's just momentum conservation. But how do you know how much downward momentum the column of air has?

Airplane/helicopter mass * g * time

Assuming the plane/helicopter didn't accelerate up or down and assuming there's no wind.

Re: The mysteries of aerodynamic lift

#94

Post by scooble »

tgarv wrote: post #67
munificent wrote: post #60

Earlier quoted context omitted.

> There's the big square sailed ones which always have to move downwind. Actually, that's not true. Square rigged "pirate-style" ships are more efficient when sailing downwind compared to fore-and-aft style sailboats, but they can still sail just fine close-hauled (into the wind) or on a reach (perpendicular). There are some efficiency pros and cons to both styles, but they all have the same aerodynamic capabilities.…

Well said, and I just want to add some detail about how square-rigged ships are able to sail close-hauled: They typically (or maybe always?) have "staysails", which run fore-and-aft between the masts, which works mostly like the main triangular sail you think of with a simple sailboat (with the exception that, as far as I know, these staysails aren't on a boom that can change its angle relative to the ship). These ar…

> A single huge fore-and-aft sail would be too hard to handle

This isn't quite right, as split fore and aft rigs existed that solved this problem. I suspect it is more that power replaced sail for crossing oceans and square sails make little sense for coastal work so they had already switched to fore and aft rigs (with a few notable exceptions, such as Humberkeels in the north of England).

Re: The mysteries of aerodynamic lift

#95

Post by mike_ivanov »

munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

> the fundamental reason airfoils in boats and places work the way they do requires viscosity

There is no viscosity in solar wind, yet the solar sail is expected to work.

EDIT: s/solar wind/solar radiation/

Re: The mysteries of aerodynamic lift

#96

Post by mannykannot »

"The third problem provides the most decisive argument against regarding Bernoulli’s theorem as a complete account of lift: An airplane with a curved upper surface is capable of flying inverted."

Unfortunately, the author has fallen into the very trap that he is trying to explain. If you were to measure the velocity and pressure fields around the wing of an airplane flying inverted, you would find that they conform to Bernoulli (so long as the airplane is flying slowly enough that compressibility is not an issue, which is another source of complication.)

What the author is doing here is to accept some bogus, hand-waving arguments for why the airflow velocity changes around the wing, such as the equal transit-time 'theory', which is, quite simply, false. To answer that question, you need the Navier-Stokes equations (which are the application of Newton's laws to a viscous fluid), or some realistic approximation.

You may find this more informative:

https://fermatslibrary.com/s/how-airplanes-fly-a-physical-de...

Re: The mysteries of aerodynamic lift

#97

Post by scooble »

BerislavLopac wrote: post #78
munificent wrote: post #60

Earlier quoted context omitted.

> There's the big square sailed ones which always have to move downwind. Actually, that's not true. Square rigged "pirate-style" ships are more efficient when sailing downwind compared to fore-and-aft style sailboats, but they can still sail just fine close-hauled (into the wind) or on a reach (perpendicular). There are some efficiency pros and cons to both styles, but they all have the same aerodynamic capabilities.…

> The main reasons square-rigged ships fell out of fashion, as far as I know, are mostly logistically. True, but it gets more complicated than that. For fast long-distance commercial travel they were replaced by the steam and later diesel propulsion, which both a) were faster and b) required less crew (so were logistically superior). On the other hand, where sailing is still used (and that's mainly sport and leisure)…

> preferred because it requires far less crew

I think that only really applies when the sails were set from aloft. I'm familiar with square-rigged cargo vessels of around 60ft in the UK that were crewed by 2.

Re: The mysteries of aerodynamic lift

#98

Post by phdp »

mike_ivanov wrote: post #95
munificent wrote: post #24

This is a somewhat confusingly written article about a famously confusing topic. It directly parallels arguments about how sailboats are able to sail. Sails are also airfoils so similar mechanics come into play. Interestingly, because a sail has effectively no thickness, both sides of the sail always have the same length, which immediately calls the Bernoulli argument into question. Sailboats are also interesting bec…

> the fundamental reason airfoils in boats and places work the way they do requires viscosity There is no viscosity in solar wind, yet the solar sail is expected to work. EDIT: s/solar wind/solar radiation/

A solar sail doesn't work via aerodynamic lift. It works on conservation of momentum. Also, the biggest contributor to force on a solar sail is not solar wind, but radiation.

Re: The mysteries of aerodynamic lift

#99

Post by usrusr »

munificent wrote: post #60
Tyr42 wrote: post #51

Earlier quoted context omitted.

Yeah. There are two kinds of sailboats. There's the big square sailed ones which always have to move downwind. Then there are the triangular sailed ones, which can move in any direction except for right into the wind. (Maybe a 30 degree on each side dead zone). The square sailed boats can't move faster than the wind, as they just rely on the air pushing on the sail to move with the wind. That's also how the triangula…

> There's the big square sailed ones which always have to move downwind. Actually, that's not true. Square rigged "pirate-style" ships are more efficient when sailing downwind compared to fore-and-aft style sailboats, but they can still sail just fine close-hauled (into the wind) or on a reach (perpendicular). There are some efficiency pros and cons to both styles, but they all have the same aerodynamic capabilities.…

> The main reasons square-rigged ships fell out of fashion, as far as I know, are mostly logistically.

Some numbers from Wikipedia:

In 1902, the Preussen, famously huge square rigged ship launched with a crew complement of 45-49 while the contemporary Thomas W. Lawson, schooner rigged and even bigger, got by with 16-18.

https://en.wikipedia.org/wiki/Preussen_(ship) https://en.wikipedia.org/wiki/Thomas_W._Lawson_(ship)

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