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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

#131

Post by Serow225 »

adtac wrote: post #79
lowkey wrote: post #68

Earlier quoted context omitted.

I suspect, like many other things that didn’t make sense - the reason was that it wasn’t actually true. The Bernoulli effect explains that lift is due to the design of the wing such that the path above the wing is longer than the path below the wing. This coupled with the fact that due to the Bernoulli effect an air particle just above the wing would reach the back of the wing at the same time as an air particle just…

Isn't Bernoulli's principle only applicable when talking about the same flow? I've always found the "above path is longer than the lower path" explanation to be unintuitive because we're not talking about the same flow. They're separate flows.

Bernoulli's principle, the actual thing, has very strict criteria* to be applicable. People usually neglect this entirely in casually throwing the term around.

- points 1 and 2 lie on a streamline,

- the fluid has constant density (note effects of height difference > gravitational potential energy between point 1 and 2),

- the flow is steady, and

- there is no friction.

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

#132

Post by jeffreyrogers »

This is silly. We know why: the wing pushes air down and as Newton taught us, every action has an equal and opposite reaction. To push air down the wing must be feeling a force up on it, which causes lift.

You can also see the same thing with a helicopter flying over water: the water is affected in a circular region fairly close to the rotor itself, indicating that there is a large downward force being exerted on the air and a corresponding upward force being exerted on the rotor.

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

#133

Post by jjk166 »

IshKebab wrote: post #130
jjk166 wrote: post #122

Earlier quoted context omitted.

Another common and wrong explanation. The air is pushed down by induced rotation. An inclined wing is one way to do it, but is not necessary.

I don't know what you are trying to say.

Sorry. An inclined wing just means the wing is at an angle relative to the airflow. An induced rotation means that the wing causes the airstream to turn, so the airflow around the wing has a circular, rotating component.

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

#134

Post by PaulHoule »

garmaine wrote: post #16
Sniffnoy wrote: post #6

Earlier quoted context omitted.

Hm, I was hoping this article would explain in what sense we don't understand flight, or in what sense people think we don't understand flight, but it didn't seem to answer that question...

We understand flight perfectly well. Although the common explanations are often BS.

We understand flight well enough to make highly optimized airplanes.

There are some old controversies that are largely settled. The Microsoft Flight Simulator manual in 1980 "teached the controversy" but it was really settled decades before that. People still remember the controversy from back then and keep repeating it and probably will still do it when people are living in space colonies.

The Bernoulli effect explanation is bogus.

An alternate (correct) explanation is that if you just took a piece of cardboard, held it sideways, and moved it laterally it would push the air down and thus the cardboard would be pushed up.

If you like vector fields you can show that there is a topological defect (vortex ring) that is threaded through the wings and comes around to the other side. If you do an integral around the ring you can show the vortex holds the plane up.

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

#135

Post by PaulHoule »

laurent92 wrote: post #64
k__ wrote: post #52

Earlier quoted context omitted.

Reminds me of learning what electricity is. I learned it multiple times. In middle school, in high school, in university, on youtube explained by a quantum physicist. Everytime I understood less of it.

> I learned multiple times. Everytime I understood less of it. Isn’t that how to spot seniority? The junior says “I know ReactJS and SpringBoot!” The senior says: “I don’t know much…” Unrelated, but that reminds me how my Masters Degree teachers touted the importance of their subject in their introduction course, all explaining the Ariane V explosion from a completely different angle: - The measurements professor: “A…

https://en.wikipedia.org/wiki/Gimli_Glider

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

#136

Post by Ericson2314 »

HPsquared wrote: post #62
handrous wrote: post #57

Earlier quoted context omitted.

I once read a book called "There Are No Electrons". I'm not sure I'd recommend it, but its approach was interesting: the author reasons that unless you're in grad school studying physics (and perhaps even then), everything you've been taught about electricity is a lie anyway, so the author attempts to present a framework of easier to understand lies intended to make the reader able to better reason about and predict…

For everyone other than the aforementioned physics grad student, it's a lie. For the physics grad student, it's a mystery.

The hydraulic analogy is at least honest!

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

#137

Post by Ericson2314 »

belter wrote: post #25
jobigoud wrote: post #4

Earlier quoted context omitted.

Air moves faster on the upper side , creating a pressure differential.

Its complex...The example normally given is, the wing is shaped a little flat in the under side and curved on the top. So that would explain the flow as you mentioned. However when an airplane flies upside down, its not sucked into the ground ;-) It seems nobody really knows: "No One Can Explain Why Planes Stay in the Air" https://www.scientificamerican.com/article/no-one-can-explai... Edit: Added brief from article…

When I first saw the Bernoulli's principle demo of the floating disk at the science museum as a kid it made me mad. And when I actually learned about it in high school physics I still didn't like it. Reading that article now is very satisfying :).

In seriousness though, there is a big difference between "bottom up" causality-focused theories and these derived principles based on complicated notions of steady states. Even when the student is too junior not to have any choice but use the latter, I think the difference needs more emphasis.

Also the 3rd law model of flight is so much easier to understand they should teach it first.

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

#138

Post by hattar »

PaulHoule wrote: post #134
garmaine wrote: post #16

Earlier quoted context omitted.

We understand flight perfectly well. Although the common explanations are often BS.

We understand flight well enough to make highly optimized airplanes. There are some old controversies that are largely settled. The Microsoft Flight Simulator manual in 1980 "teached the controversy" but it was really settled decades before that. People still remember the controversy from back then and keep repeating it and probably will still do it when people are living in space colonies. The Bernoulli effect expla…

> 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?

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

#139

Post by AnotherGoodName »

jeffreyrogers wrote: post #132

This is silly. We know why: the wing pushes air down and as Newton taught us, every action has an equal and opposite reaction. To push air down the wing must be feeling a force up on it, which causes lift. You can also see the same thing with a helicopter flying over water: the water is affected in a circular region fairly close to the rotor itself, indicating that there is a large downward force being exerted on the…

One thing to add is that you can push air down in numerous ways but some ways are less efficient.

So an 45degree angled blade absolutely will give you lift but a tapered aerofoil will do the same with less energy spent pushing the air in unwanted directions (specifically fewer swirling vortexes immediately behind the wing causing drag).

So yes push air down to stay up. Don't push air sideways or in circles. The aerofoil shape and the equations that simplify the 'don't push air the wrong way' into a simple term of drag are all about doing this.

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

#140

Post by Ericson2314 »

NickNameNick wrote: post #12
jobigoud wrote: post #4

Earlier quoted context omitted.

Air moves faster on the upper side , creating a pressure differential.

Why does the air move faster on the upper side of the wing? It's not because there's a magic force that requires air particles parted be the leading edge to rejoin thier partner at the trailing edge. The air particles on the upper surface reach the trailing edge much sooner than the ones under the wing.

My Newton's-laws-only bullshit:

bottom air:

"bounces" down, simple enough. Force on wing up and back.

top air:

bounces up off front of wing (because it's not infinitely thin), but then is unimpeded by wing. It get's slightly more compressed at the very front, but then as the wing goes down this big gap is left. The air isn't going to bounce on the air above significantly because air compressed and this is laminar flow to boot: Viscosity > internia-ness.

The about-to-be-vacuum means the bottom air pushes the wing up more easily, usually to the point where there is no more vacuum, just low pressure. But if you go really fast (or are a hydrofoil?) then there might be an actual vacuum.

The vacuum "initially" just accelerates the air vertically, but once things get going since the airfoil "carves out a triangle", the air might speed up horizontally too. There is air behind it (front re aircraft heading) pushing on it but not air in front which is getting "untraffic jammed" away.

There we go, I think this accounts for everything in the article without any Bernoulli. Screw Bernoulli.

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