How Airplanes Fly: A Physical Description of Lift (1999)(allstar.fiu.edu)
allstar.fiu.edu
How Airplanes Fly: A Physical Description of Lift (1999)
http://www.allstar.fiu.edu/aero/airflylvl3.htm
53 comments
Frankly I've never understood how the Bernoulli explanation became so widespread. Anyone who has held her hand out a car window has learned everything qualitative one can know about airfoils. It's almost as though someone were trying so hard to find a non-intuitive model that he fooled himself that his model actually explained anything.
It's fairly well accepted that the Bernoulli principle is not the only mechanism of lift today, but it IS a real mechanism, and part of what creates lift in most airfoils, is entirely responsible for technologies such as Bernoulli disk drives, and a minuscule factor in things like kites and paper planes (and your hand out the window).
The thing is a lot of people have taken "Bernoulli" to mean "equal time of passage" (a completely made-up, non-physical phenomenon) rather than the quite valid Bernoulli equation: https://en.wikipedia.org/wiki/Bernoulli%27s_principle#Incomp.... In reality Bernoulli, Newton, and Kutta are just multiple correct abstractions for turning the flow.
You're right, of course. However, the various explanations are suited to different audiences. An aerospace engineer knows everything she needs to know about all of these tools, so she just uses whatever is best suited to a particular situation. For explanation to a general audience who know about force and momentum because they've already seen Newton in other situations, why even bring up "pressure"? I suspect this came about when some dumbass said "well everyone knows about pressure because car tires".
After bootcamp I went through a US Navy course that covered some basics of aviation related stuff. We were given the Bernoulli explanation - so somehow it made it into official Navy training materials. And I repeated that explanation, because I believed it to be true, until I read this article a few years ago. I have no idea if the training is still the same, it wouldn't surprise me one bit if it is. So that source alone would be producing thousands of people a year who are misinformed.
Thank you for mentioning this. I don't know how many times I've argues against the classical definition of lift in the last couple of years and finally reverted to the hand out the window explanation. That's usually when it dons on people that lift isn't what they were taught it was.
[deleted]
I think John Denkar also provides a nice explanation here:
https://www.av8n.com/how/
Specifically in ch 3 he talks about airfoils, airflow, and circulation:
https://www.av8n.com/how/htm/airfoils.html
https://www.av8n.com/how/
Specifically in ch 3 he talks about airfoils, airflow, and circulation:
https://www.av8n.com/how/htm/airfoils.html
Thanks, this is an interesting and detailed explanation.
It should be pointed out that your link contradicts the op in places, in particular Denkar says that the role of the Coanda effect in producing lift is a "fairytale" where the op calls it out to explain the lift on the upper airfoil surface (and I have seen this elsewhere as well.)
I hadn't known until recently just how religiously contentious this topic could get.
It should be pointed out that your link contradicts the op in places, in particular Denkar says that the role of the Coanda effect in producing lift is a "fairytale" where the op calls it out to explain the lift on the upper airfoil surface (and I have seen this elsewhere as well.)
I hadn't known until recently just how religiously contentious this topic could get.
The article is bunk. Everyone knows that lift is generated by gripping the armrests. Proof you can try yourself: when you're in an airplane and it suddenly drops, a good solid squeeze on the armrests will bring it back up.
Be careful when performing that experiment - if you grip the armrests incorrectly, you may pull up the ground instead. Last time I tried it the ground suddenly appeared and hit the plane. Fortunately, it was a low-speed collision; the plane stopped, and the crew let us out like nothing ever happened.
A bit off topic: this page is completely robust, the design is responsive and it works on every browser. And yet it has been build on 1999 :)
> this page is completely robust... And yet it has been build on 1999
Maybe it's robust _because_ it was built in 1999, where nobody felt an obligation to add fancy scripts to a text document ;-)
Maybe it's robust _because_ it was built in 1999, where nobody felt an obligation to add fancy scripts to a text document ;-)
Web design in the 2000s made a lot of sites less readable rather than more. See also http://motherfuckingwebsite.com/ .
(The current trend of font size inflation is incredibly frustrating because it makes it impossible to configure a browser such that old and new sites are both readable; I have to blame it partly on designers with large screens not setting their DPI appropriately and partly on web browsers not adjusting their default stylesheets for modern screen sizes)
(The current trend of font size inflation is incredibly frustrating because it makes it impossible to configure a browser such that old and new sites are both readable; I have to blame it partly on designers with large screens not setting their DPI appropriately and partly on web browsers not adjusting their default stylesheets for modern screen sizes)
And yet it's frustrating to read because the fonts are tiny on mobile. https://m.imgur.com/zbR84D0
The aviation community appreciates the importance of robust/utilitarian/minimal transmission of information. Look at the format of TAFs and METARs for instance, the raw format of how the weather data is actually transmitted.
It's incomprehensible to the layman because there is an extreme economy of transmitted data. To decode it you have to know a fairly large vocabulary of domain specific symbols and do some computations to retrieve the correct wind speed and direction, which are encoded.
This is so you can always get your weather, one byte at a time if needed.
KDEN 101353Z 33009KT 10SM FEW080 FEW180 FEW220 04/M09 A2985 RMK AO2 SLP079 T00441089
It's incomprehensible to the layman because there is an extreme economy of transmitted data. To decode it you have to know a fairly large vocabulary of domain specific symbols and do some computations to retrieve the correct wind speed and direction, which are encoded.
This is so you can always get your weather, one byte at a time if needed.
KDEN 101353Z 33009KT 10SM FEW080 FEW180 FEW220 04/M09 A2985 RMK AO2 SLP079 T00441089
I think part of the problem is that the classroom demonstrations give a false impression of how quickly airplanes actually move. 200kph is not something that can be visualized on a 20-inch wide TV screen, or even a 20-foot wide projection. With slow-moving animations it is hard to imagine wings pushing enough air downward to keep a 747 airborne. So observers latch onto quazi-mystical explanations that sound good enough to be put on a test.
A good discussion from 2011: https://news.ycombinator.com/item?id=3379928.
Another recent thread that touched on same subject:
https://news.ycombinator.com/item?id=10218029
https://news.ycombinator.com/item?id=10218029
It is great atrticle - but one thing it has backwards - Bernoulli principle is harder to understand than the second Newton law.
Also I don't buy that viscosity argument - viscosity diverts air not down - but vertically. The move down is simple dynamics:
.\
the dot is th air particle - the \ is the wing - when the wing moves the dot is forced down under the wing. The particle moves - and so it gets momentum down. The particle touching the wing is moved by the wing (the wing pushes it down - and the particle pushes it up according to the third law - generating the lift), the other particles are pushed by the particles toughing the lift.
Also I don't buy that viscosity argument - viscosity diverts air not down - but vertically. The move down is simple dynamics:
.\
the dot is th air particle - the \ is the wing - when the wing moves the dot is forced down under the wing. The particle moves - and so it gets momentum down. The particle touching the wing is moved by the wing (the wing pushes it down - and the particle pushes it up according to the third law - generating the lift), the other particles are pushed by the particles toughing the lift.
NASA calls this "The Skipping Stone" theory of lift[1], and it is incorrect. What you're neglecting is that not only the bottom part of the wing diverts air downwards, but indeed the top part does as well; this contributes a large part of the lift generated by the wing.
[1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
[1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
Not quite. If you had a zero width wing you get a void behind the wing. Aka you pushed air down so it's not there.
This creates a low pressure area behind the wing. But, that low pressure is still pushing down. It just pushes less than the normal air pressure under a wing.
You can see this by dropping a single sheet of paper vs a stack of paper. In your model the low pressure above the wing pulls the paper up slowing it's decent. But, if you drop a stack of loose printer paper the top piece falls as fast as the bottom and is not pulled from the stack.
This creates a low pressure area behind the wing. But, that low pressure is still pushing down. It just pushes less than the normal air pressure under a wing.
You can see this by dropping a single sheet of paper vs a stack of paper. In your model the low pressure above the wing pulls the paper up slowing it's decent. But, if you drop a stack of loose printer paper the top piece falls as fast as the bottom and is not pulled from the stack.
It's interesting that that page links directly to a "right" page that says basically the same thing. Apparently it's very bad to simplify the explanation so much that one neglects the momentum of the laminar flow over the top of the foil. When I'm explaining it to an 8yo, however, I do neglect that.
Viscosity is fundamental. A two dimensional airfoil inside inviscid fluid has zero drag (maybe you expected that) and indeterminate lift (that for me was a surprise), unless you force the stagnation point to be on the trailing edge. This mathematical condition is known as Kutta condition, and the physical process that allows it is viscosity. I know the explanation is somewhat cryptic but it's a starting point.
tl;dr
The wings are angled so that air is forced slightly downwards when it collides into the wing. This collision is sustained by engines which force the plane into the air in front of it.
Just one more bit: "Air has viscosity". Without naming viscosity, the explication doesn't hold. If the air didn't have any viscosity, like it is natural to imagine, planes could not fly.
This is incorrect. A wing can impart momentum to air even in the absence of viscosity. However, the optimal wing shape is different. For an example of a wing that moves air "down" without viscosity, look at turbomolecular vacuum pumps:
http://www.physics.rutgers.edu/ugrad/387/388s06/UHV_LEED/UHV...
http://www.physics.rutgers.edu/ugrad/387/388s06/UHV_LEED/UHV...
Not really - if a wing moves through fluid without viscosity it still has to move the particles down.
Is there such a thing as non-viscous fluid?
Yes, one example is superfluid helium.
https://en.wikipedia.org/wiki/Superfluid_helium-4
Viscosity is weird.
https://en.wikipedia.org/wiki/Non-Newtonian_fluido
https://en.wikipedia.org/wiki/Superfluid_helium-4
Viscosity is weird.
https://en.wikipedia.org/wiki/Non-Newtonian_fluido
tl;dr The wings pump air down from above, causing lift not through pressure effects but rather from the volume of displacement.
(I got a little bit lost in the discussion of upwash. It seemed to me that the idea was upwash was less efficient at moving air than downwash, and therefore more air was moved down.)
(I got a little bit lost in the discussion of upwash. It seemed to me that the idea was upwash was less efficient at moving air than downwash, and therefore more air was moved down.)
Pressure effects still take place. Pressure is just force divided by area. If a net force (lift) exists, then a pressure differential must exist per newton's third law. A flat plate at a positive alpha has both a pressure and speed differential across the upper/lower surfaces, see http://www.windsofkansas.com/AZ2Fig5.jpg
That's not correct, then the upper surface of the wing would not have effect on lift, when in fact it contributes over half of it.
All the low pressure above a wing does is reduce a downward force it does not provide lift.
Drop a piece of paper and it drifts slowly. Drop a book and it falls fast. In your model the paper is supported by the air below and pulled up by the void behind it.
However, if you put a piece of paper behind a book it sticks to it and is not pulled up. Thus, all lift comes from the bottom. Edit: You can do the same thing with a piece of paper on top of a wing.
PS: The same is true if you pick up a piece of paper with a vacuum. Even if it seems like the suction is providing lift all the force is from outside air.
Drop a piece of paper and it drifts slowly. Drop a book and it falls fast. In your model the paper is supported by the air below and pulled up by the void behind it.
However, if you put a piece of paper behind a book it sticks to it and is not pulled up. Thus, all lift comes from the bottom. Edit: You can do the same thing with a piece of paper on top of a wing.
PS: The same is true if you pick up a piece of paper with a vacuum. Even if it seems like the suction is providing lift all the force is from outside air.
> All the low pressure above a wing does is reduce a downward force it does not provide lift.
NASA explains why the "skipping stone" theory of lift is incorrect well [1]. In summary, the upper surface does generate lift.
[1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
NASA explains why the "skipping stone" theory of lift is incorrect well [1]. In summary, the upper surface does generate lift.
[1] https://www.grc.nasa.gov/www/k-12/airplane/wrong2.html
I did say the top of the wing reduced a downward force, unlike the skipping stone theory. But, that's not lift.
Do you think individual air molecules are going to magically hold the top of the wing and pull?
Now, you can use an inaccurate model of what's going on. And mathematically it looks like the top of a wing provides lift. But, physically that's not what is happening.
PS: This is only an issue, because we think an aircraft as zero forces acting on it when it's sitting still on a runway. Instead of being under ~2100 pounds of force per square foot on all sides from air.
Do you think individual air molecules are going to magically hold the top of the wing and pull?
Now, you can use an inaccurate model of what's going on. And mathematically it looks like the top of a wing provides lift. But, physically that's not what is happening.
PS: This is only an issue, because we think an aircraft as zero forces acting on it when it's sitting still on a runway. Instead of being under ~2100 pounds of force per square foot on all sides from air.
I think you've changed definitions on the fly. Both the higher pressure on the bottom and the lower pressure on the top contribute to the lift.
If you use a suction cup to lift a piece of horizontal glass, well, I think most reasonable people would say it contributes a force. Maybe you'd say no it doesn't, you don't actually lift it, you just provide a lower pressure at the top allowing the bottom air pressure to push it up... Well duh, of course.
If you use a suction cup to lift a piece of horizontal glass, well, I think most reasonable people would say it contributes a force. Maybe you'd say no it doesn't, you don't actually lift it, you just provide a lower pressure at the top allowing the bottom air pressure to push it up... Well duh, of course.
I don't think science should care what 'reasonable people' believe, because reasonable people are often wrong. The whole point is to build accurate models of the world and perpetuating a myth is harmful in the long term.
PS: Can you point to where you think I changed definitions? I may have been less than clear.
PS: Can you point to where you think I changed definitions? I may have been less than clear.
Your definition of lift is very different from the commonly accepted one.
If an aircraft that weighs 1000 kg flies level, then the lift has to be 9800 newtons.
That means the lift is the sum of the effect from both the bottom overpressure and the top underpressure.
You can check it from any dictionary.
https://en.wikipedia.org/wiki/Lift_%28force%29
If an aircraft that weighs 1000 kg flies level, then the lift has to be 9800 newtons.
That means the lift is the sum of the effect from both the bottom overpressure and the top underpressure.
You can check it from any dictionary.
https://en.wikipedia.org/wiki/Lift_%28force%29
In level flight every actual force vector above the wing is lowering the magnatude of lift.
If you owe 2000$ and have 2200$ your net worth is positive 200$. But, people don't talk about debt as adding to your net worth even if it's part of the equation debt is reducing the result not increasing it.
PS: I agree you don't need to model the 2.1 tons per square foot of force in most situations. But, just because you ignore something does not mean it goes away.
If you owe 2000$ and have 2200$ your net worth is positive 200$. But, people don't talk about debt as adding to your net worth even if it's part of the equation debt is reducing the result not increasing it.
PS: I agree you don't need to model the 2.1 tons per square foot of force in most situations. But, just because you ignore something does not mean it goes away.
If that was it, then the wings would have been symmetrical or flat. The angle of attack probably accounts for less than 10% of the lift.
Aerobatic wings are symmetrical. Lift is proportional to CL, which is related to angle of attack:
https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/lift...
Most non-aerobatic wings are asymmetrical because it produces lift at zero angle of attack, and also because it results in a lower stall speed.
https://en.wikipedia.org/wiki/Camber_(aerodynamics)
https://www.grc.nasa.gov/www/k-12/WindTunnel/Activities/lift...
Most non-aerobatic wings are asymmetrical because it produces lift at zero angle of attack, and also because it results in a lower stall speed.
https://en.wikipedia.org/wiki/Camber_(aerodynamics)
[deleted]
Source? The whole article basically argues against this view. What is your evidence to say AOA only contributes 10%?
This is common misconception probably coming from level flight situation. Commercial aircraft wings are made for adequate lift in takeoff and minimal drag in level flight. Which encourages building wings that generate all necessary lift in level flight while being relatively "flush".
> As one can see, the popular explanation, which fixates on the shape of the wing, may satisfy many but it does not give one the tools to really understand flight.
Well, apparently that is it! That also explains why planes don't slam into the ground when flying upside down.
Well, apparently that is it! That also explains why planes don't slam into the ground when flying upside down.
Consider how flaps are shaped. When extended they increase the angle of attack; not the ratio of air travel distance.
Generally, when flaps and slats are extended, they increase the camber of the airfoil, increasing the Coefficient of Lift for the same AoA. Also, the air gaps introduced at the junctions between slats/flaps and the main wing re-energize the boundary layer on the top surface of the wing to retard flow separation on the upper surface (becomes a concern because of the increased camber).
They actually do both, depending on their deployment.
For me the best 5 year old explanation of lift has always been such:
Body, gas including, temperature equals energy, total energy is the sum of kinetic and potential energy and potential energy is what bumps into other objects and pushes that away. The faster the gas flows, the more kinetic energy there is, the less potential is left, thus the object is pushed with lower force. If an object shaped such that gas around opposing faces flows in different speeds moves through the gas, opposing faces are pushed with different force and you get lift. The higher the speed, the more total energy is converted to kinetic energy, the less potential energy is left, the higher the force differential. Thus, the faster the object goes, the more lift it generates. That explains minimal take-off speed of an airplane.
This is by no means scientifically rock solid, but gives really good layman explanation.
Body, gas including, temperature equals energy, total energy is the sum of kinetic and potential energy and potential energy is what bumps into other objects and pushes that away. The faster the gas flows, the more kinetic energy there is, the less potential is left, thus the object is pushed with lower force. If an object shaped such that gas around opposing faces flows in different speeds moves through the gas, opposing faces are pushed with different force and you get lift. The higher the speed, the more total energy is converted to kinetic energy, the less potential energy is left, the higher the force differential. Thus, the faster the object goes, the more lift it generates. That explains minimal take-off speed of an airplane.
This is by no means scientifically rock solid, but gives really good layman explanation.
5 year olds don't really have a good conceptualization of kinetic or potential energy.