# How do airplanes fly upside down? **URL:** \ **Category:** Factual Questions\ **Created:** [January 14, 2004, 4:41pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395 "2004-01-14T16:41:32Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [January 15, 2004, 6:06am UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/21 "2004-01-15T06:06:52Z")
> [@Speaker for the Dead](#): > > Then you can use your flattened hand as a wing, and feel it get lifted upwards, pushed down, or stay resonably level. It’s fun. Ah, “aeropalmics” Or maybe “aeropalm-namics?” 🙂 GIF: [http://www.avweb.com/newspics/liftsuck\_fig1.gif](http://www.avweb.com/newspics/liftsuck_fig1.gif) A very good article (with a moving airfoil animation): > **[Lift Doesn't Suck](https://www.avweb.com/features_old/lift-doesnt-suck/)** > > Why does an airplane wing produce lift? It's neither wing curvature nor the difference in path length of air passing over and under the airfoil. And despite what you may have been taught, it has much less to do with Bernoulli and suction than it does... ---
**Author:** ![Andy](https://avatars.discourse-cdn.com/v4/letter/a/aeb1de/32.png) [@Andy](https://boards.straightdope.com/u/Andy)\ **Post date:** [January 15, 2004, 6:13am UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/22 "2004-01-15T06:13:08Z")
> [@](#): > > Say a piece of cardboard 4 inches wide, and 3 feet long. Align it parallel with the ground and move it through the air. No lift. Now, raise the leading edge up slightly, and move it through the air. All of a sudden, it takes off and heads into the air! At the right speed and pitch attitude, it will ‘skip’ across the air and stay level. Or get a kite. ---
**Author:** ![Vaeth](https://avatars.discourse-cdn.com/v4/letter/v/3d9bf3/32.png) [@Vaeth](https://boards.straightdope.com/u/Vaeth)\ **Post date:** [January 15, 2004, 6:20am UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/23 "2004-01-15T06:20:06Z")
From reading the above posts, it seems like the general consensus is that Bernoulli’s Equation is not an accurate model for explaining lift generated by the wings of a plane. I don’t know enough to have an opinion on this one way or the other but if the Bernoulli effect is not applicable to the wings of a plane, how then was it applicable to the design of one particular model of a F1 race car (by Lotus I think) which was later banned. In this F1 car, the designers came up with some special shape/surface for the underside of the car. This special shape/surface caused air to travel much quicker under the car than it did over the car which resulted in air under the car being at a lower pressure than air on top of the car. Having such an effect was equivalent to the F1 car literally being “sucked” downwards towards the road giving amazing road holding WITHOUT additional drag. Is that the same Bernoulli effect in action? i.e. causing a pressure differential across the top and bottom side of a surface so that the surface is “sucked” towards which ever side has the lower pressure? As a bit of trivia (can’t vouch for absolute accuracy): The Lotus(?) team tried to hide their new technology from competitors by pretending to keep the differential of their F1 car a big secret, leading almost everyone to assume that the Lotus car had such great road holding due to a new and improved differential. When the car was in the pit, pit crew would cover the back half of the car to prevent any possibility of competitors possibly working out how their “amazing new differential” worked. ---
**Author:** ![epepke](https://avatars.discourse-cdn.com/v4/letter/e/9de0a6/32.png) [@epepke](https://boards.straightdope.com/u/epepke)\ **Post date:** [January 15, 2004, 11:43am UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/24 "2004-01-15T11:43:23Z")
> [@Xema](#): > > Actually, it’s a bit more subtle than this - in a climb, what opposes gravity is a combination of lift and thrust. This could lead to a long discussion not especially germane to the question about inverted flight (very well answered by bbeaty, who is something of a crusader for the truth on this subject). Except that it isn’t quite correct. He is quite correct in stating that the introductory texts are not quite correct. They try to explain it in terms of the Bernoulli effect, which does assume an equal transit time. However, the Bernoilli effect falls out of only one, limited solution to the Navier-Stokes equations. If you assume unequal transit times, then you can laugh at the Bernoulli solution, but something else drops out of Navier-Stokes which also reduces pressure on the top of the wing. The thing is, though, the effect is pretty small, and easily overcome by a minor change in the angle of attack. The angle of attack is the most important factor in airfoil lift, but the effect of the asymmetry of the airfoil is not zero. I don’t know why early airplanes used asymmetric airfoils. Maybe they figured that, even if the effect was small, you might as well have it. Since then, of course, it’s become clear that an asymmetric airfoil also helps control turbulence over a more likely range of angles of attack than would a symmetric airfoil. It’s easy to show that a blunt leading edge and a sharp trailing edge is good for controlling turbulence, but showing the effect of asymmetry is harder. I don’t know if they had sufficient number-crunching capabilities back then or determined it empirically or just got lucky. ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [January 15, 2004, 3:31pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/25 "2004-01-15T15:31:08Z")
> [@](#): > > I, as everyone else, was taught that airplanes generate lift from bernoulli’s principle I too, was taught the Bernoulli Principle. It’s still being taught. It’s been extremely difficult for me to reconcile something I learned as a very young child, and which was taught to me by flight instructors, with the idea of “deflection” causing lift. Two years ago I sent a letter to Peter Garrison of _Flying_ magazine to get the straight dope on lift. Mr. Garrison is well-known for his technical articles and explanations of aerodynamics. Here is his reply, edited by me to fit this thread: > [@](#): > > I always find it frustrating to see people talk about the “Newtonian camp” and the “Bernoulli camp”. The only thing that divides these camps is an incomplete understanding of the physics. The phenomena are the same. The downwash is the result of the pressure difference. If the downwash did not occur, there would be no lift. There can be a lifting force without downwash; for instance, a bump in the floor of a wind tunnel experiences a lifting force. But no lifting work is done. As soon as the bump is raised above the floor and becomes a cambered solid, downwash occurs because of the pressure difference… > > It is interesting to observe that CFD programs calculate lift both by integrating pressure over the airfoil surfaces and by integrating the velocity of the downwash at a distance (eg 20 chord lengths) from the trailing edge. The numerical results are the same, because the same thing is being measured at two different points. This is not some mathematical artifact. If you instrumented a wing with pressure ports all over the upper and lower surfaces, and placed a rake of pitot tubes in the wake to measure downwash and momentum, you would get the same result (otherwise CFD would not be of much use)… > > You use the phrase “air striking the bottom of the wing.” This is neither Newton nor anyone else; it is nonsense. A wing is not a shovel. I take it that by this phrase you mean “deflection of the air stream by the wing” (as by a vane, etc). The problem with viewing lift as entirely a matter of air bouncing off an inclined surface is that it fails to account for the upper surface. What’s happening there? The “bottom of the wing” reasoning represents an attempt to convert the invisible behavior of a fluid flow into something more common and familiar, namely solids bouncing off solids. The wing does deflect air downward. If the air were visible you would see the downwash – you have probably seen pictures of airplanes flying just above clouds, with the downwash and tip vortices made visible. But that is all going on behind the wing. How does the wing “feel” the creation of downwash? By the pressure difference between the upper and lower surfaces, which is quantified by the Bernoulli relation. > > Incidentally, people often speak of high pressure below the wing and low pressure above. Actually, at small angles of attack pressure is low on both sides of the wing, and is only “high” at the leading edge and to some > extent near the trailing edge. > > I hope this is somewhat helpful. ---
**Author:** ![aerodave](https://avatars.discourse-cdn.com/v4/letter/a/f08c70/32.png) [@aerodave](https://boards.straightdope.com/u/aerodave)\ **Post date:** [January 15, 2004, 3:46pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/26 "2004-01-15T15:46:40Z")
The Bernoulli-as-an-explanation-of-lift argumtn came up in this thread recently: [http://boards.straightdope.com/sdmb/showthread.php?t=220857](http://boards.straightdope.com/sdmb/showthread.php?t=220857) Here’s what I had to say near the end of the pontification. > [@](#): > > Also remember that equal transit time is a requirement of hump theory, not of Bernoulli’s Principle (BP). Bernoulli doesn’t care whether the upper-surface air meets up with it’s lower-surface counterpart. The two halves of the airstream can do whatever they want relative to each other, as far as BP is concerned. Bernoulli is a perfectly accurate way of describing the flow over airplane wings…it has to be (at low speeds, anyway). The misconception is that Bernoulli necessitates equal transit times, and it doesn’t. Please read the rest of that thread for the whole discussion, which should prove enlightening. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [January 15, 2004, 4:05pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/27 "2004-01-15T16:05:11Z")
> [@audilover](#): > > Planes don’t deply flaps to lower altitude…they deploy flaps to increase lift at a given speed. this does haf the effect of increasing drag, as well, thuse causing the plane to lose speed and altitude. But the flaps are there so that the plane can fly at lower speed without stalling. (and it does this by increasing lift) When the flaps are lowered the pilot does not decrease airspeed. The nose is lowered to maintain the correct airspeed on approach to landing and this results in a faster rate of descent. ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [January 15, 2004, 4:42pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/28 "2004-01-15T16:42:48Z")
I haven’t heard it explained that Bernoulli necessitates equal transit times. The way I was taught (in both freshman physics and in flying school) is that the equal transit times idea is a logical necessity. Why it’s a logical necessity is never explained, but all of us freshmen just nodded our heads because it didn’t seem too far-fetched. Then, given the equal transit times assumption, it’s a simple idea to show how the Bernoulli equation can be used to calculate lift. Without the equal transit times idea, you can’t intuitively understand the Bernoulli explanation. It turns out that we know that the air on the top side of the wing goes faster, so much so that it arrives at the trailing edge _before_ the air on the underside. Why this happens is not intuitively obvious, although given that it does, Bernoulli’s equation would still apply and you could calulate life. The “Newton camp” exists not because we say that Bernoulli is wrong, but that it’s a very poor way to explain lift to the layman. First, you’d have to have an understanding of the Bernoulli equation (which is a conservation of energy equation, and COE is a poor way to explain things), then you’d have to know why air over the top goes faster, and by how much. On the other hand, saying that a wing produces lift because it blows air downwards is correct, and simple enough for everyone to understand. We’re not saying that Bernoulli is wrong, but it’s just a poor choice for explaining lift. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 15, 2004, 5:11pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/29 "2004-01-15T17:11:51Z")
[QUOTE=CurtC] The “Newton camp” exists not because we say that Bernoulli is wrong, but that it’s a very poor way to explain lift to the layman. [QUOTE] This is a good point. Minus the common “equal transit times” fallacy, the Bernoulli explanation is valid. But it’s a poor way to explain what’s happening. It amounts to explaining the unknown in terms of the unknown - not very helpful. For an analogy, consider a person treading water. You could explain what he’s doing in terms of the pressure distribution on his hands - higher on the bottom than the top. Or you could explain that he’s pushing his body upward by pushing water downward. Though each explanation is correct, they are not equally intuitive or useful. ---
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [January 15, 2004, 8:14pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/30 "2004-01-15T20:14:47Z")
> [@Vaeth](#): > > From reading the above posts, it seems like the general consensus is that Bernoulli’s Equation is not an accurate model for explaining lift generated by the wings of a plane. Bernoulli equation works fine with airplane wings, boat rudders, fans, helicopter blades, etc. (It really only fails during supersonic air flow.) “Bernoulli” is not a problem; the real problem only appears when authors wrongly claim that the lifting force is caused by the airfoil pathlength asymmetry. Or when they wrongly claim that a wing is like a half-venturi which experiences lift but without deflecting the air. The air above a wing really does travel much faster than the air below. But why? Most explanations never say. (And …the air above a tilted flat plate travels much faster than the air below, so “Bernoulli” explanations must apply to tilted flat plates, not just to airfoils with curved tops and flat bottoms.) > [@](#): > > In this F1 car, the designers came up with some special shape/surface for the underside of the car. This special shape/surface caused air to travel much quicker under the car than it did over the car which resulted in air under the car being at a lower pressure than air on top of the car. Having such an effect was equivalent to the F1 car literally being “sucked” downwards towards the road giving amazing road holding WITHOUT additional drag. > > Is that the same Bernoulli effect in action? Yes, but we should probably call it a “venturi effect” which is a special case. “Bernoulli” just says that faster fluids behave as if they have lower pressure. Period. “Venturi effect” adds some significant extra information: the fluid must flow through a decreasing pipe, and this produces forces on opposite walls of the pipe but without any net deflection of the flowing fluid. Wings don’t use any “venturi effect” except during ground-effect flight. In a venturi, the lifting force appears between two solid surfaces. But when an aircraft flys high, the force appears between a solid surface and the open air. There is no “constricted pipe” forcing the air to flow faster. Instead, the airfoil sweeps the air downwards, which would leave a hollow cavity above and behind it. Air flows fast above the airfoil in order to keep this hollow cavity full. ---
**Author:** ![bbeaty](https://avatars.discourse-cdn.com/v4/letter/b/c0e974/32.png) [@bbeaty](https://boards.straightdope.com/u/bbeaty)\ **Post date:** [January 15, 2004, 8:32pm UTC](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395/31 "2004-01-15T20:32:05Z")
> [@bbeaty](#): > > Instead, the airfoil sweeps the air downwards, which would leave a hollow cavity above and behind it. Air flows fast above the airfoil in order to keep this hollow cavity full. A critical factor in airplane wings is the inertia of the air. If the trailing edge of the wing did not give the air a descending motion, then our flow diagram would look like this, and there would be no lifting force: Flat plate, positive angle, zero lift (air w/zero inertia) [![](http://www.av8n.com/how/img48/barn20x.png) ](http://www.av8n.com/how/img48/barn20x.png) However, if we let the air have some inertia, then the flow pattern changes, and the exact same wing starts creating lift: Flat plate, positive angle, significant lift (air w/inertia) [![](http://www.av8n.com/how/img48/barn20z.png) ](http://www.av8n.com/how/img48/barn20z.png) See the difference in the two diagrams? Look at the trailing edges! To produce lift, the trailing edge of the airfoil must direct the air downwards. The same thing applies to conventional airfoils: Airfoil, positive angle, zero lift (air w/zero inertia) [![](http://www.av8n.com/how/img48/nocirculation.png) ](http://www.av8n.com/how/img48/nocirculation.png) Airfoil, positive angle, significant lift (air w/inertia) [![](http://www.av8n.com/how/img48/normal.png) ](http://www.av8n.com/how/img48/normal.png) Bernoulli is useful, but there’s a more fundamental rule: IF THE TRAILING EDGE OF THE AIRFOIL DOES NOT DEFLECT THE AIR, THEN THE AIRFOIL DOES NOT CREATE ANY LIFT [Previous page](https://boards.straightdope.com/t/how-do-airplanes-fly-upside-down/224395.md?page=1)