On a humid day, the lowering of the pressure over the wings can basically force the air temperature at that point to lower and reach dew point temperature, essentially forming temporary clouds that are whipped around by the moving air vortices.
Once the aircraft has passed that point, the temperature generally stabilises and conforms to surrounding air temperature, which usually dissipates the temporary condensation.
The 'twirly' bits on the wingtips is basically spillover from the high pressure under the wings to the low pressure above the wings, creating that mini tornado vortex. This is also the reason that many modern aircraft have those 'winglets' on the wingtips, to try and minimise these spillover vortices which can cause problems for trailing aircraft, as well as induce extra drag on the source aircraft.
https://s-media-cache-ak0.pinimg.com/236x/09/de/53/09de53063...
https://s-media-cache-ak0.pinimg.com/736x/0a/a1/80/0aa180ef6...
If you're near Austin, the Texas Memorial Museum on the UT campus had a mounted skeleton.
I think maybe the GP is more amazed about the (self-) generated lift and assorted scale. Seems like humans have a thing for hurling self-propelled heavy metal boxes at ridiculous speeds, whether on ground, in the air, or in a vacuum.
"and then he said, 'ounces'"
Similar phenomenon to boiling water at high elevations. Walking up the mountain isn't heating the water, but the boiling point drops as air pressure drops.
The fact that air gets deflected downward is a necessary (c.f. conservation of momentum) effect too. So it's not incorrect to do the analysis that way. But it's absolutely wrong to correct someone saying that "lift is pressure" with "lift is actually deflected air".
This is clearly demonstrated by this incident. Many tons of air deflected downwards by the A380's wings had enough force to flip the unfortunate plane below several times.
Yes it does. And those bullet impacts are the very definition of pressure.
The aerodynamics snobbery that you're trying to invoke is that those bullets also hit each other, so you can't make ideal gas assumptions and must model the whole system as a giant system of differential equations (c.f Bernoulli) if you want numbers out.
That doesn't change the fact that air pressure at the surface of the body is what causes the force on the body. That busybodies feel the need to argue against this bleedingly obvious point says some very bad things about the way the aviation community has tried to teach aerodynamics.
> Air isn't magic fairy dust that pushes on airfoils from a distance.
No. Like I said, it's a fluid that has pressure and velocity everywhere. At low enough Reynolds numbers, like for model gliders, you can even get away with treating it as a non-compressible fluid with decent results.
> Pressure is impact.
And contact between two objects is really about Pauli exclusion surfaces at the atomic level. So what?
My point was you can still find a lot real world issues for example wing icing. So, that you need to be aware of what you are and are not simulating.
pushing a mass of fluid downward is equivalent to creating a net downward pressure. its the same thing, just expressed in different terms. the unit of pressure is force per square inch, for you americans. by integrating over all the area, you get the force that you are talking about.
The air "sticks" to the surface of the wing and leaves the trailing edge in a partially downward direction i.e the air is deflected downward by the top of the wing not just the bottom.
For a quick demo of this, hold the bowl of a spoon under a running faucet and see how it pulls the spoon into the stream.
The "equal transit time" theory that you appear to be referencing here (air over the top takes a longer path, thus it must go faster and air under the bottom takes a shorter path thus it goes slower) is a complete fallacy. There is no mechanism in physics that requires the air to "meet up" at the trailing edge of the wing.
You can read the HN discussion here [2], since this appeared on the front page a week ago, much to my surprise [3].
[1]1 http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...
For aircraft lift, "equal transit time" is not easier to understand than "an angled wing pushes air down, because of conservation of momentum that pushes the aircraft up".
If pure 'flat plate' theory was valid, then all those aircraft speeding down the runway with the leading edge of their wings canted downwards 20 or so degrees would result in the airplanes simply being pushed towards the ground and never lifting off...
And Boeing, Airbus et al would build planes with flat slab wings mounted at a 45 degree angle to the airflow, because that would give the maximum lift by the 'flat plate' theory, wouldn't it?
Not that I disbelieve that a flat slab cannot generate lift - but that it is probably a very inefficient way to generate lift compared to the standard aerofoil shape.
[0] - https://upload.wikimedia.org/wikipedia/commons/thumb/3/3b/Bo...
Proof? Inverted flight on low power aircraft and gliders.
To whit, I've had the fortune to fly an old DH Tiger Moth biplane - on that little baby, when you approach the stalling point, you can actually see the the canvas on top of the bottom wing bulge and contort with pressure differential, and you can hear the sucking sounds as the airflow struggles to 'stick' to the wing. There is a little movement on the bottom surface of the top wing too, but not as pronounced.
I'd be interested to see in this thread, who here has actually studied aeronautical engineering, or flown actual aircraft, and who is relying on YT videos or a pure theoretical approach to come up with these theories?
Also interestingly, I believe most of the textbooks I used at flight school were filled with data from NASA and other US military branches with regards to flight dynamics etc., and here on this thread we see articles from NASA (albeit aimed at K-12 audience rather than trainee pilots) basically disproving their earlier academic research.
I studied Aerospace Engineering (PhD in Aerodynamics), and I stay out of internet conversations over "how" lift is generated. For me it is one of those topics that is just not worth debating. It seems that people get _really_ attached to their personally preferred theory of lift.
"Wings generate lift by changing the velocity of the flow around them" seems general and correct but the why part is pretty tricky without notions of continuity and conservation laws. And vorticity helps a lot, too.
http://ljjensen.net/Maritimt/A%20Review%20of%20Modern%20Sail...
I've contributed all that I wanted to say here, and am happy to bow out now and let the conversation take its course.
I think "It's complicated" and "Why do you need to know?" are often the only appropriate answers, as context is important. I've read some "aerodynamics for pilots" type books that from a research point of view I considered to be, to some degree, wrong. But ultimately they were _right_ in that they taught the pilot exactly what they needed to know.
In a way it reminds me of electricity. I know enough to design and make simple circuitry, but I know electrical engineers and physicists that could run rings around me at both the circuit design level and the "That's not how electricity works, you idiot!" level.
I don't think that is right. The tail wing has a different angle of attack than the main wing. On the order of one degree I think. This is so to give a self stabilizing effekt. When flying upside down you have to compensate heavily to avoid what would be a destabilizing effect.
Flying inverted in a relatively stable aircraft requires a lot of forward pressure because for the inverted aircraft's wing to have a positive angle of attack, the horizontal stabilizer has an even larger positive angle of attack. The pilot must counter this with the elevator to establish a stable ratio of lift to downforce.
Less efficient - but makes for symmetrical performance.
Fair enough, it is symmetrical, but still has a lot of shape to it - only time I've been inverted was in a grob 103, and was a passenger for that part of the flight, here's the cross section of it's little sister which is also fully aerobatic:
https://en.wikipedia.org/wiki/File:Grob_G_102_Standard_Astir...
My point in the gp post was that since one can fly inverted, the shape of the wing is not the only fact. I think you and I are saying the same thing.
You can push the air hard on the underside, but on the top side, you have to gently accelerate the flow downwards, or else it forms vortices and you suddenly lose a significant fraction of the amount of air you would otherwise deflect downwards.
Explanation of why aeroplanes fly to airhost: https://youtu.be/AaE9j7u3XJA?t=642
From Captain to Airhost: https://youtu.be/AaE9j7u3XJA?t=1031
From First Officer to Airhost: https://youtu.be/AaE9j7u3XJA?t=1458
Hence the high pressure under the wing and the low pressure on top of the wing. The very act of 'deflecting' millions of cubic metres of airflow generates high/low pressure points. Hence at the end of the day, we could argue that the pressure differential is what is causing the lift?
Disclaimer: Not an aeronautical engineer, just a former commercial pilot.
Deflecting air downward is the way they create that pressure difference.
Get a piece of A4 or Letter sized paper and try this experiment with it [0]. For best effect, hold both the edges closest to you in each hand and twist the front edge downwards to keep it straight and prevent the paper twisting which could inadvertently straighten it. (i.e. Not with one hand like he is doing in the video).
The Bernoulli principle with postulates the Venturi effect is about the only theory that can explain why the trailing edge of the paper moves UPWARDS when you blow across the top of the curved paper.
No one questions whether the Bernoulli and Venturi effects actually exist. But airplanes are heavy and it's pretty obvious that the Bernoulli effect does not produce sufficient lift by itself.
As proof that most of the lift comes from deflecting air, I offer the fact that airplanes can fly upside down.
Do the same experiment with the paper, but rest the trailing edge on a table. The air can no longer be deflected downwards, so the paper will not rise.
Another one to think about, at the air metal boundary the air is stationary on both sides of the wing. So why is there a pressure difference? Bernoulli's law is valid only along flow lines and is a consequence pressure difference required to accelerate (or decelerate) a fluid. Since in the laminar situation flow does not occur from the top surface to the bottom (minus around the tips), direct application is nonsensical. If you integrated all flow lines from all surfaces however, you would get the correct answer.
Not sure why people are looking at the situation as ONE Venturi plane. What I am trying to explain is that there is effectively TWO - and expanding one over the top of the wing leading to a low pressure effect, and a constricting one below the wing leading to a high pressure situation. I wouldn't say that both provide equal amounts of lift, but the paper experiment, and the 747 in the boneyard proves it to a point.
Extend the flaps and slats on that aircraft and I am sure the effect will be all the more pronounced.
And yes I've flown aircraft inverted too - it take a tremendous amount of forward stick to try and maintain level flight in that config to counteract the wings natural tendency to move towards the top surface.
People have pointed out the "Thunderbirds" F-16s flying in the "mirror" formation [1] and not displaying much difference in AoA between the upright and inverted aircraft, but supersonic fighter usually have a straighter, almost trapezoidal shaped wing cross section rather than a curved 'fish' shape. I am willing to bet the inverted pilot has a fair bit of forward stick on though.
[0] - https://www.youtube.com/watch?v=cHhZwvdRR5c
[1] - http://c8.alamy.com/comp/EG175E/little-rock-air-force-base-a...
[EDIT] This also happens with the Oberth Effect.
The pressure explanations and the Newton's Third Law explanations are not different theories of lift. They are just different ways of looking at the same thing underlying thing.
The motion of a wing through a fluid has to conserve mass, energy, and momentum. If you analyze lift by focusing on conservation of momentum, you get the Newton's Third viewpoint. If you analyze by focusing on conservation of energy, you get a pressure viewpoint.
See: https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html
The "Newton's 3rd. law" explanation treats the wing and its immediate surrounding as a black box: air flows in, and exits deflected downwards, so there has been some downwards-directed acceleration. It does not address details of that process, such as why the above-wing airflow usually generates the larger part of the lift, or even how the deflection occurs, though it is fairly obvious that some sort of deflection will result from driving an inclined plane through the air.
If I remember correctly, the trails from the outboard edges of the flaps are just like wingtip vortices. The airfoil's geometry suddenly changes, acting like a wingtip and creating a vortex.
> In addition to mitigating en route midair collision hazard, SLOP is used to reduce the probability of high-altitude wake turbulence encounters. During periods of low wind velocity aloft, aircraft which are spaced 1000 feet vertically but pass directly overhead in opposite directions can generate wake turbulence which may cause either injury to passengers/crew or undue structural airframe stress. This hazard is an unintended consequence of RVSM vertical spacing reductions which are designed to increase allowable air traffic density. Rates of closure for typical jet aircraft at cruise speed routinely exceed 900 knots.
Seriously, "unintended consequence"? It seems quite obvious in retrospect.
This incident brings the Reduced Vertical Separation Minima (RVSM)[5] into question. Strategic Lateral Offset Procedure (SLOP)[6] can be used used to avoid such incidents.
[1] https://www.youtube.com/watch?v=E1ESmvyAmOs
[2] https://www.youtube.com/watch?v=dfY5ZQDzC5s
[3] https://www.youtube.com/watch?v=uy0hgG2pkUs
[4] https://www.youtube.com/watch?v=KXlv16ETueU
[5] https://en.wikipedia.org/wiki/Reduced_vertical_separation_mi...
[6] https://en.wikipedia.org/wiki/Strategic_lateral_offset_proce...