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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

#41

Post by tomsto »

bbojan wrote: post #38
tomsto wrote: post #33

Earlier quoted context omitted.

This is not how it works, no falling or positive angle of attack is required for an asymmetrical aerofoil. Imagine swinging a bucket of water over your head - the force your arm feels is similar to what the top surface of the wing feels.

Is that so? I thought that for asymmetric airfoil, zero angle of attack is by definition the angle where it creates no lift. So, tautologically, if it's creating lift a (positive) angle of attack is required.

For a symmetric aerofoil, 0 angle of attack is 0 lift. For an asymmetric - there is (some) lift, which comes entirely from the suction side.

https://www.wolframalpha.com/input/?i=NACA+6409+airfoil&assu...

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

#42

Post by Cogito »

parineum wrote: post #14
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…

Knowing Newtonian physics doesn't mean we understand all things that move. The point about "not understanding" flight is that, if we truly understood it, we could design the optimal aircraft from first principles before it ever entered a wind tunnel. Instead, we work based on incrementally improving tribal knowledge of what has worked in the past and try to make something similar to fit our desired flight envelope. C…

I'm not sure if this view undersells aero or rocket engineers more.

Rocket engineering uses an immense amount of both modelling and physical testing. No-one says "Well I've got the Tsiolkovsky rocket equation, so let's go to the moon!"

I'm not even sure what the bar of 'understanding' is here - the fact that we have iterated and improved on powered flight as much as we have necessarily means that we understand it on a deep level, let alone the fact that we can create excellent models that predict what will happen to a wing in different situations.

At what point would you say we do understand flight?

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

#43

Post by GuB-42 »

oolonthegreat wrote: post #31

I didn't understand what we don't understand about hta flight but maybe that's just me.

- Most of the popular explanations are misleading, the others are incomplete.

- The real answer is so hard to compute that there is a million dollar prize attached to it.

- Today, we design planes using approximations and trial-and-error. It works well because we are very experienced in designing planes, sometimes at the cost of many lives, but it is not exactly a "first principles" approach.

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

#44

Post by varispeed »

bcaa7f3a8bbc wrote: post #2

Note: The author is not entirely serious. It's part of a series called Mystifications: A short series of semi-satirical pop science articles, called "Here's why we don't understand". The science presented is mostly accurate. The first article was "we don’t understand electricity" and now it's "we don’t understand flight". You'll find the articles more enjoyable if you think of it as a thought experiment about the dep…

Well, I secretly knew this video is true: https://youtu.be/JYAq-7sOzXQ?t=98

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

#45

Post by [deleted] »

armada651 wrote: post #13
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…

"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.

[deleted]

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

#46

Post by carl_dr »

armada651 wrote: post #13
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…

"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.

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

#47

Post by sigmoid10 »

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).

If wings only generated lift in one direction (i.e. towards the curved side), then even flying with your nose up would pull you down if you are inverted. What people here are missing is that curved wings in level flight generate lift, but any shape of wing can generate lift with a positive angle of attack. Just stick your hand out the window while driving on the highway and tilt it slightly, you'll see.

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

#48

Post by Retric »

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…

It’s a common misunderstanding that the underside of a wing is flat and the top part curves. A paper airplane with thin flat wings still gets lift though there are several issues trying to scale this up. Similarly many aircraft will happily fly upside down.

Wings need to support the weight of your aircraft while being light this means they need to be reasonably thick especially using the obvious choice of storing fuel inside them. The first obvious choice is a teardrop shape which gets lift from being angled up similarly to the way a flat wing does.

Real wings don’t quite use a teardrop shape, but if you look at the front most part of a wing you see it curves both down and up. https://en.wikipedia.org/wiki/Angle_of_attack#/media/File:Ai...

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

#49

Post by krisoft »

parineum wrote: post #14
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…

Knowing Newtonian physics doesn't mean we understand all things that move. The point about "not understanding" flight is that, if we truly understood it, we could design the optimal aircraft from first principles before it ever entered a wind tunnel. Instead, we work based on incrementally improving tribal knowledge of what has worked in the past and try to make something similar to fit our desired flight envelope. C…

You have an unduly rosy picture of rocket science. The turbopump used in most liqued fueled rockets to presurize the fuel is notoriously complicated. Small changes to the design can result in a dramatic loss of efficiency, or even worse if it starts cavitating the pump can eat itself. For this reason nearly nobody designs turbopumps from scratch and first principles, they take a well understood design and maybe tweek it a bit. And you can bet that they then test those tweeks on a bench a lot.

Similarly devilishly complicated is the injector design. Obviously you want to mix the oxidizer and fuel in the optimal ratio for the highest efficiency. That’s the easy part. But then you also want to offset from this optimum near the edges to produce a colder flow near the nozzle wall to protect it from melting. Of course nowadays people do a lot of computer simulations to save on testing time, but it is still not uncommon to discover combustion instabilities or hot-spots in the engine tests.

So no, nobody can, let alone did, design a rocket entirely in a computer and then send it to the moon without many many tests, and incrementally improved tribal knowledge.

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

#50

Post by technothrasher »

LinAGKar wrote: post #11
trhway wrote: post #10

Earlier quoted context omitted.

rockets and bullets don't fly. They [free]fall.

Rockets fly, although they use thrust rather than lift.

Bullets fly too, ballistically. As do balloons, bouyantly. This is all kind of a pointless semantic eddy though.
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