Why we don’t understand heavier-than-air flight • Hacker News Board

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

#141

Post by jadyoyster » Sat, Jun 26, 2021, 8:41 PM UTC

hattar wrote: post #138

PaulHoule wrote: post #134

Earlier quoted context omitted.

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?

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.

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

#142

Post by genewitch » Sat, Jun 26, 2021, 8:43 PM UTC

handrous wrote: post #57

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 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…

If this is the book with "Greenies" in it, i've read that, and it was interesting to read but i don't know that it gave me any better idea of how to build a circuit. I lost all ability to design any circuit when it was explained that transistors work because the places for the charges to go moved, not the charges themselves.

I look at "quantum computer" components and go "what does a grid of wires have to do with 'computing'? And then you realize the big secret - There's regular computers that take the 'output' of the qubits/QC stuff and 'decide' what the results are, since it's all just a blob of probability anyhow...

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

#143

Post by marvin » Sat, Jun 26, 2021, 8:48 PM UTC

jobigoud wrote: post #4

tempestn wrote: post #3

Earlier quoted context omitted.

In this case I took it to be poking some fun at the two conflicting 'intuitive' explanations for a wing producing lift: one being that air strikes the bottom of the wing as it moves forward, pushing upward on it, and the other being that air moves faster under the flat underside of the wing than over the curved upper side, causing a pressure differential. Of course reality is more complex than either simple answer, a…

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

I'm confused. I was in a plane that flew upside-down, and it didn't fall down. Am also a pilot.

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

#144

Post by marvin » Sat, Jun 26, 2021, 8:55 PM UTC

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…

This is nonsensical. Glider pilots (no engine!) fly aerobatics programs all the time, in planes with curved wings. If the pilot is a bit of a masochist, they could fly upside down for half an hour in still air given enough altitude. The various forms of air resistance will be greater and energy/altitude loss hence higher, but the wing will still be generating lift equal to the weight of the airplane.

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

#145

Post by speedgoose » Sat, Jun 26, 2021, 8:59 PM UTC

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…

Actually the first flight of Ariane 5 exploded automatically when it started to fall apart, because the flight computer read an Ada exception as flight data and from this, it decided to turn as fast as possible.

They reused the launch code of Ariane 4 in Ariane 5, but Ariane 5 was much faster to take off. It was an overflow on the acceleration and bad testing because reading an Ada exception as flight data is not great.

We learn that at school in France many years later.

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

#146

Post by civilized » Sat, Jun 26, 2021, 9:05 PM UTC

S_A_P wrote: post #66

tempestn wrote: post #3

Earlier quoted context omitted.

In this case I took it to be poking some fun at the two conflicting 'intuitive' explanations for a wing producing lift: one being that air strikes the bottom of the wing as it moves forward, pushing upward on it, and the other being that air moves faster under the flat underside of the wing than over the curved upper side, causing a pressure differential. Of course reality is more complex than either simple answer, a…

As others have said it’s not really the shape of the wing that matters. Some shapes work better but I’ve always thought of it as more of a fluid density problem. As you increase speed the wing is in contact with a larger mass of air which at some critical point becomes large enough to support the weight of the aircraft. After you hit that speed then you are just manipulating the air flow to steer the craft. Holding y…

When you hold your hand outside the window of a car in motion, your hand is only pushed upwards if you incline it upwards. If you incline it downwards, it will be pushed down. This is the angle-of-attack effect and simply relies on the normal force of the air striking the hand. If the hand is inclined upwards, the normal force has an upward component, creating lift.

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

#147

Post by Judgmentality » Sat, Jun 26, 2021, 9:06 PM UTC

handrous wrote: post #57

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 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…

Solid state physics was my favorite class in college (and it was a senior course that people regularly flunked so not really a beginner-friendly tutorial). It was also very hard, and one of only 2 courses I actually attended while studying because I couldn't just show up for the test and ace it. It was fascinating to finally understand the physics behind circuits I'd been building for years (I'd understood RLC circuits since high school, but transistors, op-amps, diodes, and all that stuff I knew how to use but the "how the fuck does this amplify current?" mystery remained).

Unfortunately I don't remember what book I used. But yeah, this definitely opened my eyes to how electricity works.

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

#148

Post by marvin » Sat, Jun 26, 2021, 9:09 PM UTC

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 didn't read your link, but the "thrust" in thrust to weight ratio generally refers to the force that the engine produces. Therefore, it isn't a brilliant analogy to compare the thrust of a helicopter rotor (which does get all its power from the engine) to the thrust of an airplane propeller.

The propeller doesn't contribute materially to the lifting force on an aircraft, while the rotor of the helicopter provides practically all the lifting force on the helicopter.

Both machines need to (somehow) generate lift equal to their weight to stay airborne. The airplane just does this by moving a static wing through the air, which is a much more efficient way of doing it. Its engine/propeller isn't even immediately required to move the wing through the air; once airborne, the aircraft can fly downwards at an angle without engine power to maintain its speed.

You'd say that a helicopter uses "powered lift" while an aircraft does not.

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

#149

Post by azalemeth » Sat, Jun 26, 2021, 9:12 PM UTC

Someone1234 wrote: post #34

belter wrote: post #25

Earlier quoted context omitted.

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…

If an aircraft flies level upside down it will lose altitude towards the ground (as opposed to right side up wherein given adequate thrust it should keep its current altitude). In order to stay at a fixed altitude upside down you have to bring the nose of the aircraft up several degrees (increasing based on air speed).

I think it's easier to think of inverted flight as normal flight for a negative AoA. If the airfoil is symmetric -- as almost all aerobatic aircraft's are -- then it's functionally identical and inverted flight becomes a coordinate system "trick".

My favourite two "explanations" of flight are 1) dP/dt for air is greater down than up; and 2) Kelvin's circulation theorem, but alas that one is not very pub-friendly...

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

#150

Post by a1369209993 » Sat, Jun 26, 2021, 9:19 PM UTC

beckingz wrote: post #70

HPsquared wrote: post #62

Earlier quoted context omitted.

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

Turns out, the world is really really complicated. So it's better to say that our models are simplified, not lies.

> So it's better to say that our models are simplified, not lies.

To be fair, for most purposes (atoms, molecules, metals) there usually (very-) technically aren't any electrons, just configurations of the relevant quantum fields (eg in the form of electron orbitals) whose asociated conserved quantities would allow them to convert into a certain number of free-flying electron particles if you dumped in enough energy to make up the difference.

You see this a bit more obviously with ('virtual'[0]) photons, where some non-particulate field configurations simply can't be thought of as particles at all (eg attactive electromagnetic forces).

0: https://profmattstrassler.com/articles-and-posts/particle-ph...