Pascal just posted this in comments to a post which is not even on the main page anymore! I think the guy is even more compulsive than I am, but you may as well profit.
It's about how an airplane wing forms the pressure difference which gives it lift.
[More here.]
"How this pressure differential is formed is quite frankly a very complex topic"
Not really, honestly. It's related to the physics of fluids flow, and even though the equations (Bernouilli theorem) are a little complicated (high school level), the principle is very simple: "the faster a fluid moves in a circuit, the lower the pressure". When a given circuit has varying sections, in the narrowest ones the linear speed of the fluid increases, and hence its pressure drops. Just take my word for it and the rest becomes very simple.
Air is the fluid that is in relative motion to the plane. Actually it is the plane that's moving, but it makes no difference in the local interactions. The wing profile, in birds as well as airplanes, is more convex on the top side, meaning that the air sliding OVER it has a longer way to travel from the front of the wing to the back in the same amount of time, compared to the air passing UNDER the wing, on the flatter side. A curve between two points is longer than a straight line.
As a result, the air right above the wing travels faster, its pressure decreases relatively to that under the wing, and that difference in pressure is the force which pushes the plane upward and durably lifts it in the air.
The other specificities of the typical wing profile are just for minimizing parasitic turbulences, which would disrupt the regularity of that pressure ratio.
When a plane drops inside a turbulence zone, it's because sudden winds from behind decrease the relative speed of the air flow against the wings. It's just as if the air flow was trying to catch up with the plane. This decreases the pressure differencial, and the airplane drops. If it is too close to the ground, or doesn't reach a zone where this reverse wind ends, sometimes the plane may crash, and you hear about it in the TV news. Or, more often, "airports were closed for safety reasons due to strong winds". Strong enough to compromise the sustentation of the heavier-than-air craft.
One main problem with this pressure difference, is the wing tip. The one place where the two areas of different pressure come in free contact, causing the air to escape laterally toward the top side. Because the wing tip is moving, that imbalance of the system is maintained, and the turbulences are constantly left behind the plane. Their vortex shape is a universal physical curve in nature, you'll find it in the shape of galaxies and solar flares as well as in the microscopic motions of molecules. This tells us that the Universe is fractal in shape.
Every time you notice a wing tip with an odd shape, it's probably a system designed to decrease this loss in sustentation, which can significantly increase fuel consumption. A small, double vertical wing, for instance, becomes an obstacle to top and bottom air simply meeting. In many military jet planes, the delta-shaped wings reduce the actual tip to little more than a point. Eagles, vultures and other ace glider birds have those separated feathers at the end of their wings, resembling fingers: these result in the tip becoming like a group of much smaller wings, aerodynamically speaking, and it not only divides the "parasitic vortex" phenomenon, it also greatly decreases its global scale. Making the glider bird that much more efficient at saving energy in flight. It helps owls be near-totally silent in their night hunts, not even a faint "woosh" to warn the rodents.
Bert replied:
When I wrote that this was a very complex topic, I really meant it!
Although Pascal's explanation is seldom true (not enough lift is created in that manner for most aircraft to fly), I do give him a "A" for effort. :-)
Entire books have been written on this topic, and I can't even remember enough of the mathematics of fluid dynamics to read those. One (and maybe the most important) reason for all this complexity being that not all wings operate on the same principles. Never compare a fighter wing to an airliner wing, and much less to a glider wing. They are all different in form, function and operation.
Beware of too much knowledge. :)
22 comments:
When I wrote that this was a very complex topic, I really meant it!
Although Pascal's explanation is seldom true (not enough lift is created in that manner for most aircraft to fly), I do give him a "A" for effort. :-)
Entire books have been written on this topic, and I can't even remember enough of the mathematics of fluid dynamics to read those. One (and maybe the most important) reason for all this complexity being that not all wings operate on the same principles. Never compare a fighter wing to an airliner wing, and much less to a glider wing. They are all different in form, function and operation.
An airplane only really flies because that is how the machines who wrote the Matrix code wanted it to be.
"Beware of too much knowledge. :)
"
No such thing! BOOOOO!!! :-))
Joe: yes indeed.
Bert: A few years ago I'd have agreed with you wholeheartedly, but becoming more and more in doubt. Beyond a certain minimum, is ever-increasing collecting of data really the way to wisdom?
"Beyond a certain minimum, is ever-increasing collecting of data really the way to wisdom?"
What's the minimum that will bring you peace? If we trust creationists, all we need are Bibles...
In a different mindset: don't you think it preferable that we really know why airplanes fly? The contrary would really scare the shit outta me...
And there's a huge difference between collecting data and understanding. The latter is highly desirable, and actively sought by science.
Collecting data (oft to perform pseudo science) is widely misused by idiots apparently intent in ruining life for everyone (do consider watching the lecture I recently recommended). I know of the feeling that you are referring to, and am genuinely scared that it may be used to bring about another dark age.
Both the obsessive avoidance of science and the obsessive collecting of it are based on Fear.
There's a happy medium, which I guess can only be found by the individual in his own trek.
We do disagree, then. For I fear little but fear itself, and seek knowledge for the good it allows me to do. And of course for the kick I get from understanding the world around me. Why is it almost always the hot water tap that's dripping?
Google: "angle of attack" for the real cause of lift in an aircraft wing.
(Or, stick your hand out the window of a car at speed, palm-down, and twist it a little. You'll see.)
Jeff R.
Flying makes me go a big rubbery one.
Goggles down, chocks away!
As far as I can remember, roughly 2/3 of lift are produced by the upper wing suction effect, and 1/3 of lift by the pressure under the wing.
And so-called winglets at the tip of each wing will reduce drag produced by those vortexes. And they make a plane look so much more beautiful ;-)
High winds during take-off and landing are only a problem when they come from abreast while they try to shift the plane from the runway centreline. This can be tolerated by the plane only to a certain wind velocity, and often leads to screeching tyres and some blue smoke at touch-down, and a smaller number of passengers clapping their hands for a good landing.
A strong headwind shortens the required amount of runway both for take-off and landing.
And now it's up up and away to the clouds!
"As far as I can remember, roughly 2/3 of lift are produced by the upper wing suction effect, and 1/3 of lift by the pressure under the wing."
Good approximation for many fighter wings (very inefficient energy-wise). For an airliner-type of wing, most of the lift is actually produced by deflecting the large mass of air that flows on top of the wing downwards. Hence the very large drop towards the back of the wing.
Nexialism
Or here
Thank you Alex, I didn't know there was a name for my disease... ;-)
As an erstwhile fan of 50's SF I discovered "It! The Terror from Beyond Space." was based on "Voyage of the Space Beagle". On finding said book I found a philosophy for life.
I wish to be a nexialist myself. I have found a cross discipline position has been instrumental in surviving RIFs at work, a jack of all trades and master at some! It also helps talk to other engineering teams and manage projects.
As a freelancer, I find it invaluable when it comes to smelling impossible tasks from a mile away. :-))
Bert,
Hope you don't mind me asking. Freelancing in what? I'm assuming mech eng, possibly aeronautics/avionics.
Alex
"Hope you don't mind me asking. Freelancing in what? I'm assuming mech eng, possibly aeronautics/avionics."
Tsk, tsk. Now, didn't your Mom tell you about curiosity? ;-) The base trade is EE, but I would rather call it small equipment design. I mostly do the mech stuff nowadays. Started with lab toys, moving up (down?) towards consumer products.
Nothing wrong with consumer products. Make it sorta work with a reset button and crank out million making a penny on each.
I've spent a lot of time on costlier systems. My two biggest deployments were 1500 roadside callboxes and 1000 TTY interfaces for roadside callboxes. I was also involved in ship board black box recorders and sonar systems, with quantities in the dozens.
I'm onto consumer systems myself now.
CS - doing everything from small Windows apps down to ASIC verification.
Well, I reckoned, probably not many people would see it, but it would interest Eolake, who gets and reads every post, no matter how old. Good enough for me.
Guess he WAS interested... :-)
"I'll have you know", Mister Bert, that it's none of MY fault if the basics of aerodynamics I was taught were a bunch of "seldom truths". Whatever THAT means, beats me, I flunked English. But it sounds un-nice. ("And you're seldom yourself, so there!")
I was just reciting my lesson there. Therefore I feel that "A" was perfectly deserved indeed. :o)
Besides, if all I said was nonsense, how come stuff ceases to fly when you remove the wings, huh? I do believe this stands in favor of my theory, that it is wings indeed that carry the plane/bird in the air.
Or, it's just that Joe Dick revealed the truth. "I'll take my Viagra red, please."
Eolake questioned the System...
"is ever-increasing collecting of data really the way to wisdom?"
Ever-increasing collecting of data keeps you safely busy so you won't ask such insidious questions, mister dangerous anarchist. Perhaps I should report you to the Queen of Hearts.
"If we trust creationists, all we need are Bibles..."
Yeah, ignorance is bliss. No WAIT, I meant, um... ah... "GOD is bliss"! Yeah, that's it. (Phew! That was close. Nearly got myself lapidated.)
"don't you think it preferable that we really know why airplanes fly? The contrary would really scare the shit outta me..."
We still don't really know how we WALK. Are you afraid to take a step on a hard surface?
It's lucky we reproduced for the last 100,000 years even though we had no idea how it works. (It works quite pleasantly though, doesn't it?)
Similarly, we went milleniae breathing without knowing what Oxygen was. Good thing nobody ever got scared of breathing. ;-)
Bert boldly asserted...
"For I fear little but fear itself"
I am only afraid of fear itself when it is in others, and makes them act without thinking like some stampeding cattle herd or a lynching mob.
Me, I'm too cerebral now to fear being afraid. I'm more focused on dealing with dangers in the promptest and most efficient way. Which sometimes is as easy as just keeping quiet in the presence of a primitive angry character eager for trouble. :-)
People hate it so much when you don't push back, sometimes it ruins their mood for getting in a fight with you. "Bah. That guy's no fun."
"Why is it almost always the hot water tap that's dripping?"
Hot damn! I had never noticed that!
Now I can't sleep until I know. I must know now, no? Fortunately, that leaves me a whole day.
Jeff R.,
"Angle of attack" is not THE answer, but I'll give you and Bert that it IS a component of wing lift. The pressure differential of aerodynamic air flow is another one. And, of course, other manahmanah, I mean phenomena, can be involved.
But here's an experience that's so simple, I'll describe it without illustrations:
Take a piece of paper. Fold its long side in half to mark it. Open the fold, then fold the top half ON ITSELF twice (giving you 4 layers of paper), before redoing the original halfway fold. This'll give you a rectangular sheet of paper half as big as the original, with a thick part along its new long end (formerly the width of your sheet).
Fold that rectangle in the middle, similarly to your very first fold, but now in a line that's perpendicular to all the previous ones. Open said fold. Mark four similar parallel folds, two on each side, so that your rectangle becomes a curved surface of six same-width sections with one thick curved border.
Voilà. You've made a paper plane which is SOLELY A WING PROFILE, with no angle of attack. The way it is folded, the thick part makes its section convex on the top side (while its front view is convex downwards, for lateral stability). Go ahead, fly it, and dare tell me it's not impressive.
Of course, it'll never fly as far as Beep's B.R.O. (Big Rubbery One), but even advanced science has its limits. We can never equal the raw gushing energy or our reproductive instinct, which is the force of life itself.
..."and a smaller number of passengers clapping their hands for a good landing."
A very observing scientific report, Mr Beep. I say, well done! Impeccable. Nice job, old chap, we're good for publishing.
Winglets, shminglets. The physical formulation of passengers tensing is the kind of research travel companies will pay big money for!
"As a freelancer, I find it invaluable when it comes to smelling impossible tasks from a mile away."
Me, I just use my big bloodhound nose, for a same result.
But we're not all helped by our genetics, so I guess your method is fine too.
Alex "was also involved in ship board black box recorders"
Say, tell me something: if these black boxes are capable of surviving a plane crash, why on Earth don't you make the WHOLE craft in the same darned material?
Dear Pascal, I'm sorry if I sounded un-nice, I really tried not to. But sometimes, it just feels like facts need a little help.
As a physician, how would you react if I stated that "cancer is no big deal, really, since it can be cured"? Wouldn't you feel an urge to straighten things up? And perhaps not so nicely, huh? ;-)
And you still got an A.
Yep, still got the A. :-)
Say, Bert, was I once again betrayed by my special brand of humour? Either you didn't realize I was clearly joking there, or *I* didn't realize you were joking back. I don't know whether you knew that I knew that you knew that I... ah, whatever! I'm giving myself a headache.
About fluid flow, a news report I saw yesterday reminded me of an important corollary: when a large body is moving fast, it also creates a pressure drop near its lateral sides. As a result, when you get passed by a speeding 18-wheeler truck or a train, if you're too close to it you might get sucked against it and badly hurt. To be on the safe side, never approach those at less than three feet/1 metre. Preferably, keep twice that distance.
The news report was about a new Jackass-like trend in Europe, of young people playing "matador": they stand on a train track, and when the train arrives they move away at the last possible moment. Problem is, not only are they sometimes too slow (Darwin awards, anyone?), but sometimes they are still too close to the big thing and get sucked in against the train... or under it!
Another stupid "train game", slightly off-topic, consists of climbing on top of wagons. But since they are under a cable with a few thousand volts, by standing they get too close to it, and an electric arc forms, zapping them for good. What a way to go!
Regarding cancer, I'll be straight and blunt: I'd much prefer simple prevention. Same with Aids, in fact. (That was harsh, wasn't it? ;-)
Alas, there's a growing worldwide epidemic of cancer. I'm absolutely convinced it's from the countless unknown/untested/clandestine chemicals being released in the environment or added in the food.
"Say, Bert, was I once again betrayed by my special brand of humour?"
Nah. Just enjoying the tease! :)
I prefer to enjoy coffeese myself.
There are just too many varieties of tease to choose from...
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