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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

#91

Post by di4na »

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…

small point: the imperial vs metric is not an Arianne V thing but a Mars Climate Orbiter one ;)

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

#92

Post by zazen »

carl_dr wrote: post #46
armada651 wrote: post #13

Earlier quoted context omitted.

"We know how heavier-than-air flight works, but I want to be pedantic and nerd about some physics" That title doesn't get quite as many clicks unfortunately.

I think the various discussions in these comments show that we don’t realllllly know, just that we understand what forces are there to allow it, and how to generate them.

Different values of "we" going on here. "We" know how heavier-than-air flight works in the sense that it is known to science. "We" in the comments don't know, because most of us here have at most a Bachelor's in physics, and it takes more than that to really grok aerodynamics, but everyone (including me) can't resist the temptation to show off what partial knowledge they have from that undergrad fluid dynamics class.

The fact that the second "we" doesn't really know aerodynamics, and is going to waste man-days chasing its own half-understandings round in circles in the comments, doesn't contradict that fact that the first "we" does understand aerodynamics. Planes aren't staying in the sky by accident, nor even just by the survivorship of trial-and-error engineering.

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

#93

Post by Cogito »

hamilyon2 wrote: post #51
Cogito wrote: post #40

Earlier quoted context omitted.

It's the exact same principle for planes and helicopters. If a plane isn't producing more lift than weight it will fall, just like a helicopter. Planes work by pushing a wing through the air, helicopters by spinning it. In both cases the wing has to push down enough air to keep the aircraft in flight.

So, people answering on quora for example, are wrong? https://www.quora.com/Can-a-helicopter-having-a-power-to-wei... I am even more confused now.

I'm not sure how you're connecting that quora discussion to what we were saying above, but I think the confusion comes from what 'thrust to weight ratio' means.

Helicopters and planes are both pushing down on air to generate lift. The lift generated has to be equal or greater to the weight for the aircraft to fly.

Thrust can mean many things (at least colloquially). As discussed in the quora you linked, a helicopter will have a defined power to weight ratio that allows it to fly (maintain level flight) in its 'normal' flight envelope. There are a number of things the pilot can do that causes the aircraft to 'push down harder' on the air. One of these is flying close to the ground (the ground effect) which is sort of like pushing against the ground as well as the air, and another is by moving horizontally (usually forwards, like a plane, causing transational lift). Both of these allow the aircraft to maintain height while using less power than if it was hovering, but to do so it is still generating enough lift to counteract gravity.

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

#94

Post by pansa2 »

This relates to one of the reasons I gave up studying only mathematics and became an engineer. I was far more impressed by the achievement of heavier-than-air flight in 1903 than by the mathematicians who proved that it was theoretically possible - years later.

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

#95

Post by afterburner »

zazen wrote: post #87
afterburner wrote: post #83

Earlier quoted context omitted.

Changing the angle of attack effectively changes the shape of the airfoil. No gotcha here either.

I'm not immediately convinced that "effectively changing the shape" is a coherent idea. The lift effect either crucially depends on the actual, unchanging shape of the aerofoil or it doesn't. Flying upside-down proves that it doesn't. Maybe all we're disproving is a straw-man of a "Bernoulli-ist" position, but we're disproving it all right. EDIT: trying to think what you might mean by "effectively changing the shape"…

If you are asserting that the angle of attack does not affect the lift, you are wrong.

A plane flying upside down is most certainly not using the same angle of attack as it does right side up. The real difference in performance is efficiency, the upside down plane is burning more fuel due to the increased drag from sub-optimal operation (a high angle of attack to overcome the optimization for right-side-up flying).

Note that a right-side-up wing can easily plummet by dropping its angle of attack. That is what it's doing while upside down to generate lift.

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

#96

Post by blueblisters »

hamilyon2 wrote: post #35
Cogito wrote: post #7

The continued assertion that "we don't understand heavier-than-air flight" is a weird one. The article even skates around this, saying (essentially) "well maybe we do understand it, but chaos theory!" If you're in the sky and you want to stay there, you have to counteract gravity. Heavier-than-air flight does this by pushing down on air. Want to stay in the sky? Push down on enough air, fast enough, and you will stay…

Not an expert or anything. Never studied aerodynamic or flight in depth. As far as I understand, for helicopter to fly, it definitely has to have thrust to weight ratio greater than one. Flying things that have thrust to weight ratio > 0 are intuitive to me. They generate force and stay in the air indefinitely. Planes obviously don't require that to fly. So, they're different type of beast. They somehow squeeze more…

I am not an expert either but comparing "thrust" from helicopters and airplanes is not meaningful if you are only talking about engine thrust. Thrust is a vector, although in aerodynamics it seems to refer to forward force by convention.

For helicopters in a static hover, the downward "thrust" is actually the lift produced by the spinning blades. The engines produce almost no forward thrust. Whereas, for an aircraft in flight, the engine thrust pushes the plane forward and the wings generate the lift that keeps it in the air.

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

#97

Post by zazen »

afterburner wrote: post #95
zazen wrote: post #87

Earlier quoted context omitted.

I'm not immediately convinced that "effectively changing the shape" is a coherent idea. The lift effect either crucially depends on the actual, unchanging shape of the aerofoil or it doesn't. Flying upside-down proves that it doesn't. Maybe all we're disproving is a straw-man of a "Bernoulli-ist" position, but we're disproving it all right. EDIT: trying to think what you might mean by "effectively changing the shape"…

If you are asserting that the angle of attack does not affect the lift, you are wrong. A plane flying upside down is most certainly not using the same angle of attack as it does right side up. The real difference in performance is efficiency, the upside down plane is burning more fuel due to the increased drag from sub-optimal operation (a high angle of attack to overcome the optimization for right-side-up flying). N…

> If you are asserting that the angle of attack does not affect the lift, you are wrong.

I am certainly not asserting that, and I'm baffled how you could have formed the impression that I was.

You appeared to be attempting to rebut an argument in favour of the significance of angle of attack. We have another pointless internet misunderstanding on our hands.

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

#98

Post by lowkey »

afterburner wrote: post #82
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…

Causing a pressure difference is the same as causing a force vector. There's no gotcha here, it's just two ways of looking at the same thing.

My point about the Bernoulli effect is that there is no physical reason why the upper flow should move faster than the lower flow and in fact testing shows that they do not.

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

#99

Post by slavak »

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…

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 think back to this (https://xkcd.com/895/) comic about teaching how gravity works in general relativity. Only in the case of lift the explanations are actually _correct_, albeit somewhat circular. ("So the air above the wing sticks to the surface, which redirect it downwards. But _why_ does the air stick to the wing?!")

Also in no way do hummingbirds violate any known laws of physics, although they do have a pretty impressive way of harnessing them.[2]

[1] https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

[2] https://phys.org/news/2005-06-hummingbird-flight-evolutionar...

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

#100

Post by lowkey »

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.

That may be the correct answer here. The important point to note is that there is no physical reason why the two separate upper and lower streamlines would collude to arrive at the back of the wing at the same time and in fact they do not.
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