# how airplanes fly. **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [February 9, 2020, 1:51am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658 "2020-02-09T01:51:10Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![john\_price](https://avatars.discourse-cdn.com/v4/letter/j/c67d28/32.png) [@john\_price](https://boards.straightdope.com/u/john_price)\ **Post date:** [February 9, 2020, 1:51am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/1 "2020-02-09T01:51:10Z")
I read your recent article about how airplanes fly. I want to add my thought. Should we start as follows: A plane is on the runway. Air pressure at 35 pounds per square inch is pushing both up and down on the wings. As the plane accelerates down the runway the configuration and position of the wings (and perhaps the horizontal tail as well) causes the air pressure on top of the wings to gradually diminish until the difference between the air pressure on top of the wings falls ( in pounds per square inch) to a level where the pressure on the bottom of the wings minus the pressure above the wing X the total area of the wings in square inches surpasses the weight of the plane, then the plane lifts off. If this is correct, then the question is what causes the air pressure above the wing to diminish. Maybe one factor is the inertia of the air above the wing which would slow down the air filling the partial vacuum developing above the wing. ``` John Price ``` ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [February 9, 2020, 2:43am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/2 "2020-02-09T02:43:33Z")
Referring to this column, I’m guessing: [How do airplanes fly, really?](https://www.straightdope.com/columns/read/2214/how-do-airplanes-fly-really/) by SDSAB member **aerodave** , July 12, 2005. ETA: > [@john\_price](#): > > I read your recent article . . . > John Price Not exactly recent, though. These Straight Dope articles are all reprints. Anyhow, welcome to the Straight Dope Message Board, **John Price**. Be a good boy, don’t piss off the mods 🙂 and enjoy the ride! Some more ETA: Hang tight for a bit … I’m going to hunt up some cites that I’ve seen before … There are all kinds of theories about how airplanes fly, and it seems that it’s even less certain than we though back 45 years ago when I was first learning this stuff… ---
**Author:** ![Senegoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/senegoid/32/6606_2.png) [@Senegoid](https://boards.straightdope.com/u/Senegoid)\ **Post date:** [February 9, 2020, 3:09am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/3 "2020-02-09T03:09:47Z")
Oops – Looks like the cite I was trying to find is, in fact, the same one that **aerodave** gives at the end of his article. If you click through the pages, it has several pages showing _wrong_ theories and some pages showing some of the better current theories. > [@aerodave](#): > > NASA’s Glenn Research Center has an educational site that covers these topics (and lots of others, like propulsion) at a basic level, with interactive Java applets. The incorrect theories of lift start at: [http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html](http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html) Back in the day, we all learned the Bernoulli theory. But I don’t recall ever hearing about the “equal transit time” idea. As far as I can figure, that was somebody’s idea of what the Bernoulli theory was all about. But I can still believe the Bernoulli theory, at least partly, and I can’t see any logical reason why one needs to imagine an “equal transit time” theory to go along with it. ---
**Author:** ![Crane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crane/32/3495_2.png) [@Crane](https://boards.straightdope.com/u/Crane)\ **Post date:** [February 9, 2020, 3:57am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/4 "2020-02-09T03:57:04Z")
We know in detail what an airfoil does but not so much about how it works. Modelers have found that a wing works about the same if you turn it around and fly with the trailing edge first. You do not need an airfoil envelope. Flat plates and curved plates work well as do wings with a curved airfoil for the first third of the wing and a flat plate from there to the trailing edge. And, check out Magnus effect or Flettner aircraft. They have virtual airfoils. Some great stuff on You Tube. ---
**Author:** ![steepone](https://avatars.discourse-cdn.com/v4/letter/s/977dab/32.png) [@steepone](https://boards.straightdope.com/u/steepone)\ **Post date:** [February 9, 2020, 6:12am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/5 "2020-02-09T06:12:27Z")
> [@john\_price](#): > > I read your recent article about how airplanes fly. I want to add my thought. Should we start as follows: A plane is on the runway. Air pressure at 35 pounds per square inch is pushing both up and down on the wings. As the plane accelerates down the runway the configuration and position of the wings (and perhaps the horizontal tail as well) causes the air pressure on top of the wings to gradually diminish until the difference between the air pressure on top of the wings falls ( in pounds per square inch) to a level where the pressure on the bottom of the wings minus the pressure above the wing X the total area of the wings in square inches surpasses the weight of the plane, then the plane lifts off. > If this is correct, then the question is what causes the air pressure above the wing to diminish. Maybe one factor is the inertia of the air above the wing which would slow down the air filling the partial vacuum developing above the wing. > > ``` > John Price > > ``` Slight nitpick, the air pressure would be 15 pounds per square inch, not 35 ---
**Author:** ![octopus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/octopus/32/3716_2.png) [@octopus](https://boards.straightdope.com/u/octopus)\ **Post date:** [February 9, 2020, 7:24am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/6 "2020-02-09T07:24:09Z")
Airplanes fly by pushing down air. It’s an F=ma type of thing. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 9, 2020, 10:59am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/7 "2020-02-09T10:59:11Z")
I believe the Bernoulli explanation is at last (mercifully) falling out of favor, and the Newtonian one is (deservedly) becoming more common. To summarize: planes fly because their wings deflect air downward, which (in accordance with Newton’s laws of motion) produces an upward force (equal to the weight of the plane when it is in unaccelerated flight). The best way to grasp this is to observe a [helicopter hovering over water](https://www.youtube.com/watch?v=UHM4M1ssiFU): The rotor blades (which clearly are wings) keep the machine airborne by pushing a huge amount of air downward, made obvious by the way the water surface is disturbed. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [February 9, 2020, 12:55pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/8 "2020-02-09T12:55:03Z")
The Bernoulli explanation _is_ the Newtonian one. Bernoulli’s equation is just F = ma as applied to fluids (as opposed to discrete objects). What’s thankfully falling out of favor is the explanation which is frequently taught as “the Bernoulli effect” but which has nothing to do with it, nor with any other aspect of reality. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 9, 2020, 1:33pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/9 "2020-02-09T13:33:35Z")
> [@Xema](#): > > The best way to grasp this is to observe a [helicopter hovering over water](https://www.youtube.com/watch?v=UHM4M1ssiFU): The rotor blades (which clearly are wings) keep the machine airborne by pushing a huge amount of air downward, made obvious by the way the water surface is disturbed. Yep. A helicopter stays aloft because it pushes air _ **down** _. An airplane stays aloft using the same principle: it pushes air down. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [February 9, 2020, 2:38pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/10 "2020-02-09T14:38:22Z")
Re: Shape of wing. People have been studying wing shape for over a century now. And your standard airliner wing cross section is still basically the same as it was in the 1920s. This shape provides more lift at cruising speeds than a flat wing and therefore saves fuel. Sure you can fly a plane with flat or inverted wings, but you’re going to burn more fuel. It’s absurd to deny this reality of wing shape. The standard shape increases lift! ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [February 9, 2020, 3:14pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/11 "2020-02-09T15:14:28Z")
> [@ftg](#): > > Re: Shape of wing. > > People have been studying wing shape for over a century now. And your standard airliner wing cross section is still basically the same as it was in the 1920s. > > This shape provides more lift at cruising speeds than a flat wing and therefore saves fuel. > > Sure you can fly a plane with flat or inverted wings, but you’re going to burn more fuel. > > It’s absurd to deny this reality of wing shape. The standard shape increases lift! IANA aerospace engineer. But it’s my understanding that a “barn door” shaped wing can (and will) provide lift by deflecting air down, but there are problems when the wing has a simply flat geometry (i.e. no airfoil shape): - The bottom surface of a flat wing _will_ deflect air down, thus providing lift. But the air won’t follow the profile of the _top_ surface, and thus the top surface won’t deflect air down and provide lift. - The air above a flat wing is very turbulent, thus producing drag (and inefficiency). An airfoil is shaped so that, in addition to the bottom surface, the top surface of the wing _also_ provides lift. The top surface is shaped such that the air wants to follow the profile. (I am not sure how this occurs, exactly, but it does.) And if the air follows the top surface of the wing, it will be deflected down. In addition, the air is trying to “pull away” from the top of the hump on the top surface of the airfoil, but is unsuccessful in doing so. So it creates a slight vacuum between the air and the top of the hump, thereby providing additional lift. One more thing: in addition to being inefficient, a barn door shaped wing would have a terrible stall angle compared to an airfoil. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 9, 2020, 8:29pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/12 "2020-02-09T20:29:25Z")
> [@ftg](#): > > It’s absurd to deny this reality of wing shape. The standard shape increases lift! I think the right way to say this is that airfoil shapes are solutions to the problem of making wings efficient. If you don’t care about efficiency, you can produce large amounts of lift with a crude approach, up to and including “barn door” wings. And note that there is no standard shape - airfoils vary a lot in their details, according to the job(s) the wing is expected to do. Airfoil research is ongoing - future wings will not be the same as today’s, which in many cases are quite different from those of the past. ---
**Author:** ![Bryan\_Ekers](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bryan_ekers/32/183_2.png) [@Bryan\_Ekers](https://boards.straightdope.com/u/Bryan_Ekers)\ **Post date:** [February 10, 2020, 12:02am UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/13 "2020-02-10T00:02:06Z")
Most of the time, pretty well. ---
**Author:** ![Jasmine](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jasmine/32/2964_2.png) [@Jasmine](https://boards.straightdope.com/u/Jasmine)\ **Post date:** [February 10, 2020, 12:42pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/14 "2020-02-10T12:42:38Z")
It is the speed of air flow that produces the lift. Bernoulli’s principle, which says that if air speeds up the pressure is lowered. Thus a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift. ---
**Author:** ![Crane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crane/32/3495_2.png) [@Crane](https://boards.straightdope.com/u/Crane)\ **Post date:** [February 10, 2020, 1:20pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/15 "2020-02-10T13:20:37Z")
Lift is a function of area and angle of attack, not airfoil shape. The airfoil shape determines drag. The air that is deflected down at the trailing edge of the wing does not produce lift. The angle of deflection is inversely proportional to the aspect ratio of the wing. A long, thin wing produces the same lift as a short wing of the same area, but has little or no downwash. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 10, 2020, 10:07pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/16 "2020-02-10T22:07:57Z")
> [@Jasmine](#): > > … a wing generates lift because the air goes faster over the top creating a region of low pressure, and thus lift. This is pretty much the standard “Bernoulli-based” explanation. It’s not wrong, but it’s a bad explanation for two reasons: It encourages the listener to overlook the important fact that to create that region of low pressure, an enormous amount of air must be continually deflected downward. It amounts to explaining the unknown (how a plane flies) in terms of the unfamiliar (Bernoulli theory of fluid flow). By contrast, Newtonian action-reaction is familiar - something every toddler encounters every day: “When I push on the chair, it moves away from me, and I move away from it.” ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [February 10, 2020, 10:17pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/17 "2020-02-10T22:17:31Z")
> [@Crane](#): > > A long, thin wing produces the same lift as a short wing of the same area, but has little or no downwash. Regardless of aspect ratio, no downwash = no lift. The short wing acts energetically on a relatively small volume of air, thus embedding a relatively large amount of energy in the wingtip vortices that are an inevitable consequence of lift. The long thin wing acts more gently on a relatively large volume of air, with consequently lower induced drag. But there ain’t no such thing as (unaccelerated) winged flight without downwash. ---
**Author:** ![somepumpkin](https://avatars.discourse-cdn.com/v4/letter/s/7feea3/32.png) [@somepumpkin](https://boards.straightdope.com/u/somepumpkin)\ **Post date:** [February 12, 2020, 12:34pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/18 "2020-02-12T12:34:15Z")
I don’t have a dog on this catwalk, but I have been reading you guys’ discussion with interest. So, of course, the internet gods provided this article on my home page this morn. From Scientific American, it discusses that “No One Can Explain Why Planes Stay in the Air”. > [@Sci...American](#): > > What Anderson said, however, is that there is actually no agreement on what generates the aerodynamic force known as lift. “There is no simple one-liner answer to this,” he told the Times. People give different answers to the question, some with “religious fervor.” More than 15 years after that pronouncement, there are still different accounts of what generates lift, each with its own substantial rank of zealous defenders. At this point in the history of flight, this situation is slightly puzzling. After all, the natural processes of evolution, working mindlessly, at random and without any understanding of physics, solved the mechanical problem of aerodynamic lift for soaring birds eons ago. Why should it be so hard for scientists to explain what keeps birds, and airliners, up in the air? > **[No One Can Explain Why Planes Stay in the Air](https://www.scientificamerican.com/video/no-one-can-explain-why-planes-stay-in-the-air/)** > > Do recent explanations solve the mysteries of aerodynamic lift? ---
**Author:** ![octopus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/octopus/32/3716_2.png) [@octopus](https://boards.straightdope.com/u/octopus)\ **Post date:** [February 12, 2020, 2:49pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/19 "2020-02-12T14:49:11Z")
> [@somepumpkin](#): > > I don’t have a dog on this catwalk, but I have been reading you guys’ discussion with interest. So, of course, the internet gods provided this article on my home page this morn. From Scientific American, it discusses that “No One Can Explain Why Planes Stay in the Air”. > [No One Can Explain Why Planes Stay in the Air | Scientific American](https://www.scientificamerican.com/article/no-one-can-explain-why-planes-stay-in-the-air/) If the wing was entirely flat and perpendicular to the ground and pushing air like a bulldozer out of the way but generating no lift would it be surprising that there would be a pressure difference in the fluid on the two sides of the wing? On one side you are compressing the fluid on the other side you are pulling away from the fluid. Put the same flat wing at an angle to the ground other than parallel, while in motion, and you still are pushing fluid forward and down while pulling away from a fluid on the opposite side of the wing. ---
**Author:** ![Crane](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crane/32/3495_2.png) [@Crane](https://boards.straightdope.com/u/Crane)\ **Post date:** [February 12, 2020, 3:10pm UTC](https://boards.straightdope.com/t/how-airplanes-fly/847658/20 "2020-02-12T15:10:54Z")
If downwash produced lift then long thin wings would produce less lift than short broad ones. They don’t. Lift is a function of area. If lift was created by pushing down on the air then parking at the end of a runway to watch 747s take off would result in you car being flattened by the supporting column of air. I’ve done it and my ears did not even pop. [Next page](https://boards.straightdope.com/t/how-airplanes-fly/847658.md?page=2)