Why does an airplane stay in the air?

X3 Skier said:
Since this a topic that always pops up on POA, It’s good to know everybody is correct.

Not even experts can explain why planes stay in the air
At this point in the history of flight, it shouldn’t be that hard for scientists to say what keeps birds — and aircraft — airborne. But the question remains puzzling.

Read in Scientific American: https://apple.news/A0Zc9zaiHRXqIyH2c_aHCtQ

Cheers
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Looks like that article is behind a paywall.
 
dbahn said:
The problem with the Bernoulli principle is that there is no law of physics that causes two molecules that separate at the leading edge to arrive at the trailing edge at the same instant, which was the basis for claiming that one molecule had to go faster and therefor cause a lower pressure on top of the wing.
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Perhaps the solution to that problem is to look at molecules in the aggregate instead of individually.

That having been said, my impression is that lift is Newtonian, but can be supplemented by Bernoulli, depending on the shape of the wing.
 
FastEddieB said:
It will do so if it meets a constriction in flow, as in a venturi.
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Which has nothing to do with the fundamentals of lift.
FlySince9 said:
Action: Air strikes the bottom of the wing and Reaction: airplane is lifted upward. Incidence in the wing is there for this purpose. Well explained in “Stick and Rudder”
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Also nonsense.

Newton accounts for 100% of the lift - both the high pressure on the bottom, and the low pressure on the top.
Bernoulli accounts for 100% of the lift given the lower velocities on the bottom and the higher velocity on top.
 
asicer said:
If one is having trouble with an airplane staying in the air then I would suggest reducing throttle. Or wait until it runs out of gas.
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That is true. Years ago I was in a 172 when the throttle stuck. I could not advance nor reduce engine speed. It stuck at cruise. I eventually landed on a long runway by getting a about 10 feet off the runway and pulling the mixture, then landed normally.
 
Palmpilot said:
That having been said, my impression is that lift is Newtonian, but can be supplemented by Bernoulli, depending on the shape of the wing.
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They are not separate and distinct concepts, Bernoulli's equation can be derived from Newton's 2nd law. Since NASA seems to be a popular cite, from NASA Glenn:

"From Newton's third law of motion, a turning action of the flow will result in a re-action (aerodynamic force) on the object. So both "Bernoulli" and "Newton" are correct. Integrating the effects of either the pressure or the velocity determines the aerodynamic force on an object. We can use equations developed by each of them to determine the magnitude and direction of the aerodynamic force."

Nauga,
and the endless cycle.
 
Capt. Geoffrey Thorpe said:
Using about 1 1/2 pages of algebra (with diagrams). (Though it's easier to use calculus.)
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Which I guess might be part of the reason the average GA dude thinks they're completely different concepts. The FAA and most instructors definitely don't help either.

Nauga,
and Teen Talk Barbie
 
murphey said:
The US Air Force has always claimed that given the appropriate (translation: huge monster) engine, it can fly anything. The Shuttle is proof.
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The Shuttle is proof that the wings (and Bernoulli) aren't even needed to get into space orbit. ;)
 
Palmpilot said:
Perhaps the solution to that problem is to look at molecules in the aggregate instead of individually.
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Those are the same aggregate molecules that spread further apart with altitude, i.e. "lower pressure". They'll always be individuals unless they chemically bind with other molecules. :)
 
Checkout_my_Six said:
The Shuttle can't fly.....;)
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smv said:
As soon as you think you have figured out how Bernoulli makes an airplane fly, then explain a fully symmetrical wing.

:)
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A symmetrical airfoil doesn't produce any lift at 0° angle of attack. Try again.
 
Palmpilot said:
Looks like that article is behind a paywall.
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The link is free on Apple News. Not sure how that works if you don’t have an iOS device.

What actually matters is lift can be measured quite accurately in a wind tunnel and predicted also quite well using CFD so how it generated is sort of an exercise akin to arguing the number of angels on the head of a pin.

Cheers
 
dmspilot said:
A symmetrical airfoil doesn't produce any lift at 0° angle of attack. Try again.
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What does that have to do with the price of tea in China?
 
X3 Skier said:
What actually matters is lift can be measured quite accurately in a wind tunnel and predicted also quite well using CFD so how it generated is sort of an exercise akin to arguing the number of angels on the head of a pin.
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I was thinking the same thing, and had the exact analogy in mind!
 
X3 Skier said:
What actually matters is lift can be measured quite accurately in a wind tunnel and predicted also quite well using CFD so how it generated is sort of an exercise akin to arguing the number of angels on the head of a pin.
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"I don't need to know how it works, the computer just tells me the answer." ;)

Nauga,
who knows why so many vaporware airplanes have unbelievable performance estimates
 
X3 Skier said:
The link is free on Apple News.
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You're right. I was able to open it in my iPhone just now.

Not sure how that works if you don’t have an iOS device.
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On my Windows notebook, I just got ads for Apple News+, which is a subscription service.
 
dmspilot said:
A symmetrical airfoil doesn't produce any lift at 0° angle of attack. Try again.
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Try what again? It was a challenge, not a statement of fact.

:rolleyes:
 
smv said:
Try what again? It was a challenge, not a statement of fact.

:rolleyes:
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There is no challenge. Bernoulli doesn't have any trouble explaining lift of a symmetrical airfoil since there is none at zero angle of attack. And as soon as you give it angle of attack, it isn't really symmetrical anymore, from the standpoint of airflow. That's why you need to try again. :rolleyes:
 
dmspilot said:
There is no challenge. Bernoulli doesn't have any trouble explaining lift of a symmetrical airfoil since there is none at zero angle of attack. And as soon as you give it angle of attack, it isn't really symmetrical anymore, from the standpoint of airflow. That's why you need to try again. :rolleyes:
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Maybe someday you will understand. I doubt it, but there is hope.
 
smv said:
Maybe someday you will understand. I doubt it, but there is hope.
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Maybe some day you will figure out a way to discuss your argument instead of belittling people and trying to show off how much you think you know.
 
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nauga said:
"I don't need to know how it works, the computer just tells me the answer." ;)

Nauga,
who knows why so many vaporware airplanes have unbelievable performance estimates
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I had a staff guy come to me once with some estimates of engine performance for a new design. I sent him back with some hand calculations I cranked out in 15 minutes that showed how wild his numbers were compared to a 100% efficient cycle. He whined about how the program had given him the answer. I told him next time his program was that far off, he and his program would be leaving “to pursue other opportunities”:cool:

My direction was to get a ballpark estimate by some hand calculations or comparative data before trusting any program.

My rule of thumb was any clean sheet design proposal submitted to the USAF will be optimistic by a minimum of 10-15% in any number in the proposal, except cost which at best was a SWAG. Pretty accurate from the F-111 to the F-22.

Cheers
 
It was stated by a number of people. The truth is that all it takes is money. Lots and lots of money.
 
Lift is 100% about equal and opposite force. Air molecules move down and that pushes or pulls the wing up.

anything past that is just the math.
 
I finally read the article last night. I think it was well done, providing just enough information to explain the conflicting theories to a layman. Summary: It’s more complicated than the simplistic theories we’ve taught pilots over the years. Anyway, worth a read.

That said, we function as pilots using simplified “models” or “constructs” we’ve built in our heads from our studies and experiences. If these models are wrong, they can lead to disastrous results.

But it’s really hard to see where our simplified models of lift can have real world effects, even if we have some of the details wrong. For example, once we see the effects of exceeding the critical angle of attack, and how to deal with those effects, it seems the exact physics of what’s going on as the lift degrades matters little. I’d be curious if anyone can come up with an example of a crash that was caused by a pilot confusing Bernoulli with Newton, let’s say.

In short, it’s an interesting discussion, and one that aeronautical engineers need to continue to investigate in their pursuit of performance. But not one that has any practical effects on us as pilots.
 
Not sure about an airplane....but in A&P school we were taught a helicopter beats the air into submission to stay aloft.

A wise old aviator told me " Birds fly...Men drink ".
 
Zeldman said:
That is true. Years ago I was in a 172 when the throttle stuck. I could not advance nor reduce engine speed. It stuck at cruise. I eventually landed on a long runway by getting a about 10 feet off the runway and pulling the mixture, then landed normally.
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I've thought about that scenario as well. In the Arrow I could use the mixture as a power control lever if I wanted. It will run that lean.
 
Would a complete understanding of "lift" have helped the crew of Air France 447? Or would time have been better spent on learning to fly the airplane and recognize unusual attitudes and how to correct them? Not trying to berate the crew, just trying to focus on what is important to understand about flying and what is best left to discussion by fluid dynamics professors.
 
FastEddieB said:
But it’s really hard to see where our simplified models of lift can have real world effects, even if we have some of the details wrong.
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Pilots seem to manage the typical airplane just fine in spite of being taught fairy tales about lift. But, is that a good reason for continuing to pass along nonsense?
 
Capt. Geoffrey Thorpe said:
Pilots seem to manage the typical airplane just fine in spite of being taught fairy tales about lift. But, is that a good reason for continuing to pass along nonsense?
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Of course not.

But I think it’s enough to present a simplified model, but then to explain “Its not that simple”, with a brief mention of Newton, and maybe a link to the Scientific American article for students so inclined.

But as a practical matter, getting too deep in the weeds on this with a student might even set them up for overthinking a question on aerodynamics on the written test and missing the FAA correct answer. And to what end?
 
FastEddieB said:
But I think it’s enough to present a simplified model, but then to explain “Its not that simple”,
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I agree. But, there is no reason for the "simplified" model to be a fairy tale that contradicts the laws of physics.

I think the explanation laid out by Fidkowski back in post 3 is sufficiently simple as well as being "not wrong". The only thing I didn't like about his explanation is that he used a fairly flat bottom air foil which (IMO) tends to reinforce the fallacy that airfoils have curved tops and flat bottoms and that contrast in shape has something to do with the fundamental cause of lift. That's why I chose a flat plate for the explanation even though it ain't a great airfoil.
 
I truly get a kick out of the conversation anytime folks posit the (wrong) equal transit theory, or that 100% if lift is from the high/ low pressure differential (also incorrect). I listened to another engineer spout something even worse the other day when he said the air wraps around the trailing edge to make the underside airflow slower.

In incompressible flow, lift is created by the airfoil moving air ahead of the airfoil, smoothly over the surface and down behind it. It’s called momentum transfer. Best seen by the classic image of downwash of a plane flying just above a cloud deck. Yes there is some pressure differential but It’s nowhere near enough to generate lift.
 
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