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Why we don’t understand heavier-than-air flight

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Re: Why we don’t understand heavier-than-air flight

#161

Post by axiolite »

jadyoyster wrote: post #141
hattar wrote: post #138

Earlier quoted context omitted.

> The Bernoulli effect explanation is bogus. I hear this but never seem to get any further info. Why are wings shaped with a curved top and flat bottom? Is there a good summary I can go read to understand this all?

This page explain things well: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... AFAIK the wing shape thing is about reducing drag (turbulence?)–not essential but hard to fly well without a nicely shaped wing.

That's a terribly oversimplified explanation. May I recommend this one instead:

https://www.nasa.gov/sites/default/files/atoms/files/foam_wi...

When learning about flight, keep asking yourself "Then how do planes fly upside down?" Any explanation which does not mesh with sustained, inverted flight is oversimplified to the point of uselessness and inaccuracy.

Re: Why we don’t understand heavier-than-air flight

#162

Post by mr_toad »

formercoder wrote: post #85
lowkey wrote: post #56

Can confirm. I worked at Pratt & Whitney testing jet engines early in my career. At the time I read a similar article and spread it amongst my colleagues - the cognitive dissonance was palpable. As engineers we had been taught that lift was due to air above the wing traveling faster than air below the wing and thus creating lift by way of a pressure differential. The more accurate answer as seen in the article is tha…

Same as sailing, does the airplane have an equivalent of the keel?

Yes, the engine.

Speaking of sailing, the angle and shape of the sails are both important to maintaining velocity. Racing boats adjust (trim) the shape of the sails all the time.

Merely deflecting the wind obeys conservation of momentum, but conservation of energy in an unpowered sailboat (overcoming losses due to friction) also means extracting energy from the airflow.

Re: Why we don’t understand heavier-than-air flight

#163

Post by slipframe »

Retric wrote: post #80
Anarch157a wrote: post #75

Earlier quoted context omitted.

Kelly Johnson caused a stir in the engineering community when he came up with the F104 Starfighter, with it's thin and almost flat wings.

Yea, start throwing around enough thrust and you can fly just about anything. The F-15 got to the point where one wing was optional: https://theaviationist.com/2014/09/15/f-15-lands-with-one-wi...

There is more to that F-15 incident than just incredible thrust; the F-15's wide fuselage provides significant body lift.

https://en.wikipedia.org/wiki/Lifting_body#Body_lift

Re: Why we don’t understand heavier-than-air flight

#164

Post by slavak »

lowkey wrote: post #103
slavak wrote: post #99

Earlier quoted context omitted.

The experts weren't _wrong_ in their understanding. Bernoulli (creating lift by way of pressure differential) and Newton (reaction to redirection of the flow downwards) are different ways of describing the same thing; integrating either the pressure or velocity vector of the airflow around the wing will give you the correct results for lift.[1] Whenever people argue about which interpretation of lift is correct I thi…

I commented elsewhere that there is no physical reason why the Bernoulli effect would cause the upper streamline and the lower streamline to reach the back of the wing at the same time - and to my knowledge there is no experimental evidence that it does. I may be wrong about that but I have never seen an adequate rebuttal.

You're right- there isn't, and it doesn't. In fact a parcel of air moving over the top of the wing will beat its counterpart moving below it to the trailing edge. This[1] video has a very good demonstration of this. (Relevant part begins around 0:25.)

So saying that air on top of the wing moves faster, creating a pressure differential and thus lift, is absolutely correct. The problem begins when some people try to come up with an intuitive explanation for _why_ the air would need to speed up. "Because this is the lowest energy state that conserves energy, momentum, and mass" isn't a very satisfying answer; neither is "because this system of PDEs say so"; so they came up with the "equal transit time" explanation, which is simple, intuitive, and completely wrong.

Hopefully aeronautical engineers at P&W didn't actually believe that last bit?

[1] https://www.youtube.com/watch?v=UqBmdZ-BNig

Re: Why we don’t understand heavier-than-air flight

#165

Post by Cogito »

lowkey wrote: post #103
slavak wrote: post #99

Earlier quoted context omitted.

The experts weren't _wrong_ in their understanding. Bernoulli (creating lift by way of pressure differential) and Newton (reaction to redirection of the flow downwards) are different ways of describing the same thing; integrating either the pressure or velocity vector of the airflow around the wing will give you the correct results for lift.[1] Whenever people argue about which interpretation of lift is correct I thi…

I commented elsewhere that there is no physical reason why the Bernoulli effect would cause the upper streamline and the lower streamline to reach the back of the wing at the same time - and to my knowledge there is no experimental evidence that it does. I may be wrong about that but I have never seen an adequate rebuttal.

The streamlines don't need to meet for Bernoulli to apply.

That statement, that the upper airstream flows faster because it has to meet up with the lower airstream, is wrong and easy to disprove experimentally.

So, putting aside that specific statement, re-read the post you're replying to.

The airstream above the aerofoil does travel faster, and there is a lower pressure region there (commensurate with the effect described by Bernoulli), and it turns out that this is a valid way to model the forces involved just as it is valid to model them as a redirection of the airstream.

Re: Why we don’t understand heavier-than-air flight

#166

Post by tsimionescu »

exporectomy wrote: post #152
tsimionescu wrote: post #18

Earlier quoted context omitted.

This was the video in question: https://m.youtube.com/watch?v=HABNe7_D22k?t=1m52s . It's not about predicting the exact movement of tides, he's arguing science fundamentally can't explain the regularity of tides...

Oh, well that's different. Funny though, the guy he's interviewing said Islam is a scam and Muslims are suckers who have fallen for it. Funny what you can get away with if you couch it right.

To be fair, he claims that ALL religions are a scam, not specifically Islam. I can tell you for sure that Bill O'Reilly wasn't grilling him for his bad treatment of Muslims!

Re: Why we don’t understand heavier-than-air flight

#167

Post by jadyoyster »

axiolite wrote: post #161
jadyoyster wrote: post #141

Earlier quoted context omitted.

This page explain things well: https://www.grc.nasa.gov/www/k-12/UEET/StudentSite/dynamicso... AFAIK the wing shape thing is about reducing drag (turbulence?)–not essential but hard to fly well without a nicely shaped wing.

That's a terribly oversimplified explanation. May I recommend this one instead: https://www.nasa.gov/sites/default/files/atoms/files/foam_wi... When learning about flight, keep asking yourself "Then how do planes fly upside down?" Any explanation which does not mesh with sustained, inverted flight is oversimplified to the point of uselessness and inaccuracy.

Actually meant to post this link, sorry! https://www.grc.nasa.gov/www/k-12/airplane/wrong1.html

Re: Why we don’t understand heavier-than-air flight

#168

Post by boublepop »

A great but completely wrong explanations of how a wing works is: The cuved shape of the wing means air has to move a longer distance over the top of the wing to connect back together with the air moving under it. So since it’s moving faster it has a lower pressure and the pressure difference between the bottom and top leads to the upwards force we call lift.

It sounds like a great eli5, and it gets repeated in serious educational contexts again and again but it’s just absolutely completely wrong.

Re: Why we don’t understand heavier-than-air flight

#169

Post by boublepop »

naves wrote: post #78
anvandare wrote: post #60

Earlier quoted context omitted.

I usually just answer Socratically: "So how can (some) planes fly upside-down?" whenever I encounter the Bernoulli-adherents.

Because there is not up or down for the wing when its cutting through a fluid. It is not that we have seen planes flying intercontinental flights upside down. And those upside down events do not happen at 10 feet above ground. There is plenty of fluid (air) above and below the aircraft and power (fighters jet engines are the most powerful ones on aircrafts) to be able to correct any up-downward force with flaps (basi…

> Because there is not up or down for the wing when its cutting through a fluid.

Except that there obviously is. Unless you ignore gravity.

Re: Why we don’t understand heavier-than-air flight

#170

Post by boublepop »

jjk166 wrote: post #124
lowkey wrote: post #106

Earlier quoted context omitted.

See my sister comment. The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline. The problem with this theory is that there is no…

> The Bernoulli effect explains the pressure differential by way of an above-wing streamline reaching the back of the wing at the same time as the below wing streamline. Since the upper wing is curved and therefore a longer path the theory claims the pressure differential is caused by the upper streamline traveling faster than the lower streamline. That's not how the Bernoulli effect explains the pressure differentia…

That is sadly literally how it is explained both in classes and in textbooks. It’s incorrect but it’s how it’s taught which is the root of a lot of misconceptions and why this “explanation” which is not correct keeps living on.
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