"Mysteriously, the U.S. Air Force ordered the flying-wing bombers on the assembly line scrapped."
Nothing mysterious about it. The original YB-49 was a bear to control in flight and prone to crashing. It appears to be the nature of flying wings. The modern B-2 solves that by fly-by-wire, with computers between the pilot and control surfaces, keeping the plane stable.
But why order them to be scrapped? That costs the Air Force more than just striking them from inventory, recovering the GFE and letting Northrop deal with them.
Both the UK and Canadian governments issued similar orders ( TSR2 and CF-105 ) but in those cases it was spiteful destruction to prevent a subsequent government from reinstating the programmes.
Secrecy. The USAF routinely orders enough (?) spare parts for secret equipment, then scraps all the tooling. They love shredding/burning amything classified. Then the system gets extended to a longer life, and an expensive new store of spare parts gets rolled into upgrades.
Full dosclosure: old guy who experienced enough of USAF to be a touch cynical about procurement.
Back when Discovery Channel actually had more than just silly "reality" shows, they aired a program called "the Wing will Fly" about exactly this. Someone has put an old VHS copy on youtube[1]. If you are interested in this article, you'll enjoy it.
Even a nautil.us article is not spared from making this classic mistake:
> Spinning in vortices, air could travel more quickly now above the long narrow wing than below it, creating the pressure difference that would lift them into the skies.
What mistake? The classic mistake is to assume air travels faster over the top than the bottom because the top path is longer and they must take equal time. In fact air does travel faster over the top, quite a bit faster than the equal time fallacy would indicate.
Air going over the top of a wing more quickly than the air below is NOT what causes lift.
What causes lift is "entrainment" plus the wing shape redirecting entrained air downwards at the trailing edge. By Newton's third law of motion, the acceleration of air downwards by the wing implies the acceleration of the wing upwards (and with the wing, the airplane).
That's not how it works. It's almost all entrainment and very little Bernoulli effect. The proof lies in symmetric wings, which generate no lift but still entrain air. Einstein famously designed what he thought should be a very good wing based on the Bernoulli principle, and it turned out to not work, and if you look at it[0] you'll see why...
What do you mean, symmetric wings generate no lift? There are plenty of lift-generating symmetric airfoils out there.
Your link indicates that Einstein fell victim to the equal time fallacy. While the equal time fallacy supposes that lift is generated because air moves faster and generates a pressure differential, air moving faster to generate a pressure differential does not necessarily imply the equal time fallacy.
Symmetric wings generate no lift at a neutral angle of attack. At a positive angle of attack they do generate lift, naturally, but the point is that you have entrainment with NO Bernoulli effect pressure difference.
If they’re generating lift, there’s a pressure difference. If there’s a pressure difference, it’s coming from a speed difference.
When you say there’s entrainment with no Bernoulli effect pressure difference, are you talking about zero angle of attack, or non-zero? At zero I don’t see the relevance, and at non-zero it’s just wrong.
> We have seen that several physical principles are involved in producing lift. Each of the following statements is correct as far as it goes:
> * The wing produces lift “because” it is flying at an angle of attack.
> * The wing produces lift “because” of circulation.
> * The wing produces lift “because” of Bernoulli’s principle.
> * The wing produces lift “because” of Newton’s law of action and reaction.
> We now examine the relationship between these physical principles. Do we get a little bit of lift because of Bernoulli, and a little bit more because of Newton? No, the laws of physics are not cumulative in this way.
> There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli’s principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton’s laws.
In other words, it is the Bernoulli effect. It is also circulation, and the rest. That does not mean it is partially Bernoulli and partially other things. Each separate explanation individually explains 100% of lift, they're just different ways of explaining it. It's like calculating the speed of an object dropped from a height by integrating gravitational acceleration over time versus calculating changes in potential and kinetic energy. Does acceleration account for some part of the final speed, and energy accounts for another part? No, each one is individually responsible for 100% of the result. They're just different ways of looking at it.
Ay, I hadn't reread his page, and only (mis)remembered it. I'll have to reread it. It's still the case though that that NASA page says it's not about the air above going faster than the air below. It's at least the case that there is disagreement.
The NASA page says it’s not about the air going faster above due to a requirement to traverse the airfoil in the same amount of time as the air below.
The popular misconception of airfoils states that the upper path is longer than the lower path. This means the air taking the upper path must move faster in order to meet the air taking the lower path. Faster means lower pressure means lift.
This is incorrect. But it’s very important to understand why it’s incorrect. It’s true that air moves faster over the top, that this produces lower pressure on top, and that this produces lift. The incorrect part is thinking that this all happens because the air on top takes a longer path faster in order to meet up with its friends from below. In fact, the air moves over the top much faster than this idea would indicate.
Unfortunately, a lot of people have learned about this fallacy and taken away the wrong lesson, that lift is mostly unrelated to air moving faster over the top of the wing, or to lower pressure on top of the wing. When you’re thinking that way, it’s easy to misread sources debunking the “equal time” theory.
I wonder if there's a word for this; where people learn that one bit of common knowledge is a fallacy and then "overcorrect" by assuming related things are also wrong. Perhaps "meta fallacy?"
In general, birds do not need to flap; they can glide, and flapping is more about propulsion than lift. In the case of hummingbirds hovering or birds launching into the air from the ground, I believe it is correct to say that vortex formation is an essential part of the process. In fact, the analysis of lift generation by airfoils treats the flow as a vortex superimposed on a linear flow, so in that sense, the explanation of lift generation is both inseparable from and dependent on vortex formation, though I am not sure the article's author had this in mind.
"the explanation of lift generation is both inseparable from and dependent on vortex formation"
Are you sure about this? There are of course high lift devices and wingtip vortices are unavoidable. But I'm pretty sure you can (at least theoretically) generate lift with an airfoil in a wind tunnel while having completely laminar flow.
I am referring to the Kutta–Joukowski theorem [1]. I don't pretend to understand it in detail, but here it says "the fluid flow in the presence of the airfoil can be considered to be the superposition of a translational flow and a rotating flow."
Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases.
The wingtip vortices are of course part of the flight, but they are unavoidable side effect.
I think I've seen the Kutta-Joukowski visualization too somewhere. If I recall correctly, it was more about visualization/memorization of pressure distribution than actual airflow.
Due to the superposition of the translational flow on the circulation, the airflow does not wrap around the wing, but the circulation is nonetheless there:
"Circulation is conserved, like energy or momentum. The circulation around a wing cannot simply appear by itself. A circulation vortex must be a closed system, and the two ends of the circulation must meet up. For some time, this seemed just a theoretical issue, but careful investigation demonstrated how this happens. As the wing moves forward, it sheds a "starting" vortex, equal to but opposite in circulation to the wing vortex, called the "bound" vortex, bound as it is to the wing. As the wing moves forward, tip vortices develop, with the result that a "closed" vortex loop develops. If we think about an aircraft taking off, it leaves its starting vortex at the beginning of the runway, and the tip vortices then build up down the length of the runway and into the air as the aircraft climbs. For a large aircraft, these vortices take several minutes to dissipate.
See also my other reply to your earlier post, and its bathtub experiment.
The rotating flow / vortex in that sentence is probably referring to the 'circulation' of the air flow, which has some mapping to the lay idea of vorticity, but isn't fully it. It's basically a measurement of deflection through a closed boundary.
Here's the alternative presentation that I was thinking of. While it is ultimately about boat sails, note that its author was, at the time, a Research Supervisor at Boeing's Aerodynamics Research Department. Take a look at figures 14 through 17, and their description.
> Surely you’ve seen birds flying without flapping?
Isn't that when they just "ride" the winds? The wind pushing against the back of their wings which is hold angled thus pushing it higher (like a boat sail).
Sorry, I don’t really understand what you’re saying here.
Birds and airplanes work on the same principles. Airflow over the wings produces lift. That airflow also produces drag, which must be counteracted with thrust to maintain altitude. The only difference is that birds produce thrust by flapping, and airplanes produce thrust with engines. A non-flapping bird is equivalent to an airplane with the engines shut off.
There is no equivalent, in flying (other than balloons), of a sailboat running before the wind. A gliding bird is always facing into the relative airflow.
Nothing mysterious about it. The original YB-49 was a bear to control in flight and prone to crashing. It appears to be the nature of flying wings. The modern B-2 solves that by fly-by-wire, with computers between the pilot and control surfaces, keeping the plane stable.
Both the UK and Canadian governments issued similar orders ( TSR2 and CF-105 ) but in those cases it was spiteful destruction to prevent a subsequent government from reinstating the programmes.
Full dosclosure: old guy who experienced enough of USAF to be a touch cynical about procurement.
[1] https://www.youtube.com/watch?v=MkhziQF0AiI
> Spinning in vortices, air could travel more quickly now above the long narrow wing than below it, creating the pressure difference that would lift them into the skies.
What causes lift is "entrainment" plus the wing shape redirecting entrained air downwards at the trailing edge. By Newton's third law of motion, the acceleration of air downwards by the wing implies the acceleration of the wing upwards (and with the wing, the airplane).
“Air goes faster on the top of the wing,” “pressure is lower on top of the wing,” and “air is pushed downward” are physically equivalent statements.
[0] https://www.google.com/search?&q=einstein+wing+design
Your link indicates that Einstein fell victim to the equal time fallacy. While the equal time fallacy supposes that lift is generated because air moves faster and generates a pressure differential, air moving faster to generate a pressure differential does not necessarily imply the equal time fallacy.
When you say there’s entrainment with no Bernoulli effect pressure difference, are you talking about zero angle of attack, or non-zero? At zero I don’t see the relevance, and at non-zero it’s just wrong.
Here are a few pages that explain that the Bernoulli effect isn't the full explanation
https://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html https://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html https://www.newscientist.com/blogs/nstv/2012/01/physics-in-a...
Here's a page saying it ain't the Coanda effect, but ultimately that the Bernoulli effect is only a small part of lift: https://www.quora.com/What-are-the-differences-between-Berno...
But perhaps the best resource I've seen so far is John Denker's page on this topic: http://www.av8n.com/how/htm/airfoils.html
You'll trivially find lots of other pages that say it is the Bernoulli effect, but near as I can tell, they are wrong.
> We have seen that several physical principles are involved in producing lift. Each of the following statements is correct as far as it goes:
> * The wing produces lift “because” it is flying at an angle of attack.
> * The wing produces lift “because” of circulation.
> * The wing produces lift “because” of Bernoulli’s principle.
> * The wing produces lift “because” of Newton’s law of action and reaction.
> We now examine the relationship between these physical principles. Do we get a little bit of lift because of Bernoulli, and a little bit more because of Newton? No, the laws of physics are not cumulative in this way.
> There is only one lift-producing process. Each of the explanations itemized above concentrates on a different aspect of this one process. The wing produces circulation in proportion to its angle of attack (and its airspeed). This circulation means the air above the wing is moving faster. This in turn produces low pressure in accordance with Bernoulli’s principle. The low pressure pulls up on the wing and pulls down on the air in accordance with all of Newton’s laws.
In other words, it is the Bernoulli effect. It is also circulation, and the rest. That does not mean it is partially Bernoulli and partially other things. Each separate explanation individually explains 100% of lift, they're just different ways of explaining it. It's like calculating the speed of an object dropped from a height by integrating gravitational acceleration over time versus calculating changes in potential and kinetic energy. Does acceleration account for some part of the final speed, and energy accounts for another part? No, each one is individually responsible for 100% of the result. They're just different ways of looking at it.
The popular misconception of airfoils states that the upper path is longer than the lower path. This means the air taking the upper path must move faster in order to meet the air taking the lower path. Faster means lower pressure means lift.
This is incorrect. But it’s very important to understand why it’s incorrect. It’s true that air moves faster over the top, that this produces lower pressure on top, and that this produces lift. The incorrect part is thinking that this all happens because the air on top takes a longer path faster in order to meet up with its friends from below. In fact, the air moves over the top much faster than this idea would indicate.
Unfortunately, a lot of people have learned about this fallacy and taken away the wrong lesson, that lift is mostly unrelated to air moving faster over the top of the wing, or to lower pressure on top of the wing. When you’re thinking that way, it’s easy to misread sources debunking the “equal time” theory.
[1] https://en.m.wikipedia.org/wiki/Falsus_in_uno,_falsus_in_omn...
If this was so, birds wouldn't need to flap, which is the actual lift generator (pushing air down).
Are you sure about this? There are of course high lift devices and wingtip vortices are unavoidable. But I'm pretty sure you can (at least theoretically) generate lift with an airfoil in a wind tunnel while having completely laminar flow.
Your reference to a wind tunnel might have something to do with extending an airfoil from one side to the other of a wind tunnel, thereby approximating a foil of infinite span, but I don't think it is relevant to free flight. The intensity of wingtip vortices can be reduced by making the span longer, all else being equal, but then the volume of air involved in the circulation increases.
[1] https://en.wikipedia.org/wiki/Kutta–Joukowski_theorem
(I have seen another version that I think offers a more intuitive, though less rigorous, presentation, and if I can find it I will add a link.)
I think I've seen the Kutta-Joukowski visualization too somewhere. If I recall correctly, it was more about visualization/memorization of pressure distribution than actual airflow.
http://www.onemetre.net/Design/Downwash/Circul/Circul.htm
http://www.onemetre.net/Design/Downwash/LiftLine/Liftline.ht...
"Circulation is conserved, like energy or momentum. The circulation around a wing cannot simply appear by itself. A circulation vortex must be a closed system, and the two ends of the circulation must meet up. For some time, this seemed just a theoretical issue, but careful investigation demonstrated how this happens. As the wing moves forward, it sheds a "starting" vortex, equal to but opposite in circulation to the wing vortex, called the "bound" vortex, bound as it is to the wing. As the wing moves forward, tip vortices develop, with the result that a "closed" vortex loop develops. If we think about an aircraft taking off, it leaves its starting vortex at the beginning of the runway, and the tip vortices then build up down the length of the runway and into the air as the aircraft climbs. For a large aircraft, these vortices take several minutes to dissipate.
See also my other reply to your earlier post, and its bathtub experiment.
http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...
Birds flap to create thrust, not lift. Surely you’ve seen birds flying without flapping?
Isn't that when they just "ride" the winds? The wind pushing against the back of their wings which is hold angled thus pushing it higher (like a boat sail).
Birds and airplanes work on the same principles. Airflow over the wings produces lift. That airflow also produces drag, which must be counteracted with thrust to maintain altitude. The only difference is that birds produce thrust by flapping, and airplanes produce thrust with engines. A non-flapping bird is equivalent to an airplane with the engines shut off.
https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
Most interestingly, NASA also calls wrong what you seem to defend, or maybe there are some subtleties that I'm missing:
https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html