There is actually quite a good reason. If the replacement air does not come from below then there would be a net downward flow of air. Eventually airplanes would push all the air to the ground and they would have nothing left to fly in.
Yes, that is a ridiculous straw man, but if the air does not come from below, then there will be a higher pressure below and a lower pressure above after the aircraft’s passage. In an open atmosphere, such a pressure difference will not be sustainable, and air will flow from the higher pressure zone to the lower pressure zone. Eventually, somewhere, air must flow upward to replace that which was thrown downward to generate the lift. With practical wing spans it will happen quickly and locally, and a vorticies will inevitably result.
Beyond that, I really am saying that vorticies are fundamental to winged flight:
Vorticies are the mechanism by which the air supplying lift to a wing can transport momentum downward without the requirement for a net downward flow of air…just as ocean waves allow flow of mechanical energy over long distances without net water flow. Without some other mechanism to replace this function of vorticies, there will be no lift. It may well be that some day a revolutionary advance will be made that allows a finite span wing to emulate an infinite span wing. Until that advance is made, wings will not create lift without vorticies.
Actually, such a mechanism is already well known. It goes by the name of ground effect. By transfering momentum to the ground, vorticies are not required.
Hmm, I guess it could be any number of reasons then. Usually they depart/land on 27. The only reason I could think of would be visibility or ceiling necessitating the use of 9.
For landing, 27 requires 700 foot ceiling and 2 miles visibility whereas 9 requires 400 foot ceiling and 1 mile vis.
Anywho, just guessing since I don’t know the actual weather conditions that day or any other ATC type things which might have been occurring.
But yes, usually planes will take off/land into the wind if at all possible.
I couldn’t edit my last post but I just wanted to add the point the vis/ceiling requirements are tailored for my company. Others may have different requirements.
Messrs. Kutta and Joukowski say otherwise. Any three dimensional wing is not infinitely long and therefore must create vortices as a direct consequence of producing lift. I don’t think it’s accurate to dismiss it as an undesired side effect; it’s more than that. It’s basically Newton’s 3rd Law.
Nice not to be a lone voice. It took me decades to come to my present understanding. Myself of 20 years ago (if I had internet access) would have been arguing with the me of today, and flaming at that. My understanding may yet evolve but many things are making sense to me today that didn’t those 20 years ago.
In some respects it is pedantic, but I really think advances may well result when it is more widely known exactly how wings work. Seeing that vorticies are not just a side effect, but a necessary part of creating lift is a huge step in that direction.
Tips or no, since you are moving an infinite amount of air, you only need to push it downward at zero velocity to produce whatever lift you need. Air thrown downward with no speed leaves no hole to be filled, and thus no vortex. For bonus points you get to create all that lift at zero airspeed…Shazam!
The fact that X=0 is a valid solution to many differential equations doesn’t make it useful.
Dayum, you’re right. I searched YouTube for “A320 engine sound” and got this video recorded by a passenger who thoughtfully violated the “no electronic devices powered up during takeoff or landing” rule. Those engines are pretty dang loud, alright, quite a bit louder than other planes I’ve been on.
Nitpick:
I don’t believe they specifically increase lift for a given angle of attack - but they increase the angle of attack at which stall begins, therefore allowing you to fly at higher angles of attack, and therefore at lower speeds.
Here’s a video of the maiden flight of the B787. You can see the wing flex at several points but you can really see how dramatic it is at 1:03 in the video where the angle is from dead astern.
As mentioned the plane is designed for this, nothing wrong with it at all but still distinctly more than you see on other planes.
True, but if I’m only at 4,000 and lose all power, I’m not really worried so much about “Can I maneuver” as I am about “Is there an airport around?” I’m just saying that for every 4,000 feet, you get 11.6 miles. So the chances of you losing an engine and not having at least a rinky-dink airstrip somewhere reachable is slim. Even if you had to land at a county airport, at least you’re not plowing through office buildings.
Am I misreading this, or is everyone overthinking it? Yes, you always (try to) take off into the wind. The pilot was saying that the wind was behind him, aka a tail wind. That’s why they were changing runways. “The wind behind us” is the opposite of “take off into the wind”. So where’s the confusion?
The runway had been changed TO the one with the tailwind. They’d been set to go from the into wind runway but it was changed for some reason and they were now using the downwind runway which that aircraft couldn’t use at that time so they had to wait while they worked out what they could do. It’s a fair question to ask why ATC would change to a downwind runway.
Incidentally, slats have been used for a long time. Some models of Tiger Moth had slats that automatically deployed at high angles of attack. They could be locked by a lever in the cockpit if you didn’t want to use them. Some ME109s also had them.
I guess then that we are not agreeing on the definition of a wing vortex. I agree that at some point the atmosphere has to equalize pressure. I disagree that that equalization has to occur around the tip of the wing as the wing passes through the air. That equalization could occur behind the wing, in which case, it does not swirl because there is no obstruction that the air is racing around to begin the angular momentum. A planetary-sized vortex to me is not a wingtip vortex. “Yes, that is a ridiculous straw man,” but if the air is pressed down to the ground, thereby raising ground pressure, and then spreads out accross the ground, and then presses up into the air in China, and then spreads out across the top of the sky and finally flows back to where the wing went past in Toledo, I don’t call that a wing vortex. YMMV.
pericynthion said:
Producing lift moves air downward. That creates a pressure differential in the air which has to equalize. Why is that equalization required to occur as a vortex in the (geometric) plane of the wing? Why can’t the swirl be 90 deg from the wing, i.e. the plane of the fuselage? Air pushed down by the wing then curls up and around from the backside.
The reason it does what it does is wrapping around the end of the wings as the wing is still in place. That creates an angular momentum, and thus the characteristic swirl. The direction of swirl is a consequence of wings having ends, not a consequence of air being pushed down that has to move back up. There are different possible patterns for that air to be redistributed, we get the one we do because wings have ends.
Kevbo said:
I agree that the nature of mixing air flows results in swirls, i.e. vortices. I don’t agree that those swirls have to be the direction we see them. Practically, though, it may be impossible to remove them entirely, simply because the air doesn’t wait to respond until later, but responds immediately. The pressure differential across the wing is mismatched and it is able to respond because the wings have ends. Ergo, the air wraps around the end of the wing, and that causes the swirls we see. Winglets change the geometry of the ends of the wings, and thereby disrupt that connection between the low pressure and high pressure. But only lessens it, because winglets have ends, too.
I just thought of an example to try to make clear what I mean. Consider a stream. Water flows down the direction of the gradient. Put a rock in the middle, and the water will wrap around the rock. Put a partial wall, and water will flow around the edge of the wall and make a swirl behind the wall as it fills in the “gap” i.e. low pressure zone. Now put the wall fully across the stream, you no longer get end wraps, but instead get an even flow over the top of the dam. The eddies (i.e. vortices) are no longer swirling in the plane of the stream, the mixing is spread evenly along the length of the dam and the vortices are each much smaller - you get turbulence instead of neat swirls.
Those engines are pretty dang loud, alright, quite a bit louder than other planes I’ve been on.