# Theory of flight **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [March 5, 2010, 3:14pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416 "2010-03-05T15:14:04Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Don\_Hood](https://avatars.discourse-cdn.com/v4/letter/d/e8c25b/32.png) [@Don\_Hood](https://boards.straightdope.com/u/Don_Hood)\ **Post date:** [March 5, 2010, 3:14pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/1 "2010-03-05T15:14:04Z")
I have been taught the flawed theory of lift (air traveling faster over the top cteates a low pressure area…etc.). I also know that while building and flying models, I have built planes that have NO airfoil (flat on top and bottom, blunt leading edge). They all fly with no significant problems. As long as the plane maintains a positive angle of attack, it flies… I also fly model planes that have a symmetrical airfoil (curvature on the wing is the same on top as it is on the bottom). Hmmmm, how does the air know which way to “lift” the wing? The only way to get the plane into the air is to increase the angle of attack. Just my personal point of view. Don ---
**Author:** ![Anne\_Elk](https://avatars.discourse-cdn.com/v4/letter/a/22d042/32.png) [@Anne\_Elk](https://boards.straightdope.com/u/Anne_Elk)\ **Post date:** [March 5, 2010, 4:38pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/2 "2010-03-05T16:38:27Z")
At top speed a Piper Cub’s airfoil (Clark Y) is actually operating at a negative angle of attack. What was once a high cambered, flat bottomed, high-lift airfoil becomes a semi-symmetrical, lower lift airfoil. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [March 5, 2010, 8:37pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/3 "2010-03-05T20:37:19Z")
Hi, **Don Hood**. When responding to a column, it is helpful to provide a link. Unfortunately, the board software does not do this automatically. > [@](#): > > I have been taught the flawed theory of lift (air traveling faster over the top cteates a low pressure area…etc.). Technically, that part is not flawed. The flawed part is the erroneous description that the air packets going over the wing must for some reason match up with the air packets going under the wing. In fact, they go _faster_. However, going faster does equate to less pressure. > [@](#): > > I also know that while building and flying models, I have built planes that have NO airfoil (flat on top and bottom, blunt leading edge). They all fly with no significant problems. As long as the plane maintains a positive angle of attack, it flies.. > I also fly model planes that have a symmetrical airfoil (curvature on the wing is the same on top as it is on the bottom). Hmmmm, how does the air know which way to “lift” the wing? The only way to get the plane into the air is to increase the angle of attack. Yes, flat airfoils will fly with positive angle of attack, and so will symmetrical airfoils. When you give a symmetrical airfoil (flat or curved) a positive angle of attack, you are creating the non-symmetrical airflow conditions that allow lift. Nonsymmetrical airfoils provide two bonuses. 1. They provide positive angle of attack to the wings while the fuselage is horizontal. 2. They make the wing performance zones smoother and reduce the conditions that create stall. Symmetrical airfoils (vs flat ones) also do the latter, but nonsymmetrical shape adds the first item. ---
**Author:** ![Contrapuntal](https://avatars.discourse-cdn.com/v4/letter/c/e274bd/32.png) [@Contrapuntal](https://boards.straightdope.com/u/Contrapuntal)\ **Post date:** [March 5, 2010, 8:43pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/4 "2010-03-05T20:43:55Z")
> [@Irishman](#): > > Hi, **Don Hood**. When responding to a column, it is helpful to provide a link. Unfortunately, the board software does not do this automatically. [Let me do that for you.](http://www.straightdope.com/columns/read/2214/how-do-airplanes-fly-really)😉 ---
**Author:** ![rwjefferson](https://avatars.discourse-cdn.com/v4/letter/r/a6a055/32.png) [@rwjefferson](https://boards.straightdope.com/u/rwjefferson)\ **Post date:** [September 5, 2010, 11:03pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/5 "2010-09-05T23:03:32Z")
> [@Don\_Hood](#): > > I have been taught the flawed theory of lift (air traveling faster over the top cteates a low pressure area…etc.)… The only way to get the plane into the air is to increase the angle of attack. > Just my personal point of view. > Don Just yours and mine and newton’s and bernoulli’s et al. A wing attacks the relative headwind with its highest point to its trailing edge. Yes, the angle of air mass departure means more than the angle of attack. Inertia causes air to separate above the trailing wing. Inertial differential in turn causes lift by acceleration in verse pressure differential. A wing forces air downward; air in turn forces the wing back and up. peace rwjefferson ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 6, 2010, 12:00am UTC](https://boards.straightdope.com/t/theory-of-flight/531416/6 "2010-09-06T00:00:07Z")
> [@Irishman](#): > > Nonsymmetrical airfoils provide two bonuses. 1. They provide positive angle of attack to the wings while the fuselage is horizontal. You can do that with any airfoil, simply by controlling the angle at which the wing attaches to the fuselage (typically known - at least in the US - as the angle of incidence). The advantage of non-symmetrical airfoils is efficiency: If you plan to do most of your flying rightside-up (as is the case for most winged animals and machines), it makes sense to use an airfoil that is efficient in that mode of flight. Such an airfoil will pretty much necessarily be less efficient if you wish to fly inverted - but that’s almost always an acceptable tradeoff. ---
**Author:** ![dzero](https://avatars.discourse-cdn.com/v4/letter/d/5f9b8f/32.png) [@dzero](https://boards.straightdope.com/u/dzero)\ **Post date:** [September 6, 2010, 12:31am UTC](https://boards.straightdope.com/t/theory-of-flight/531416/7 "2010-09-06T00:31:12Z")
Thanks for resurrecting this thread. I never would have seen the article otherwise. favorite quote: > [@](#): > > We engineers are happy if we’ve got enough practical knowledge to build flying aircraft. The rest we chalk up to magic. that reminds of a joke about Xeno’s paradox. A physicist and an engineer are asked ‘if you are across the room from a beautiful woman and you are allowed to move towards her 1/2 of the distance at a time, will you ever reach her?’ The physicist says ‘no, of course not’. the engineer says ‘no, but I’ll get close enough.’ ---
**Author:** ![rwjefferson](https://avatars.discourse-cdn.com/v4/letter/r/a6a055/32.png) [@rwjefferson](https://boards.straightdope.com/u/rwjefferson)\ **Post date:** [September 7, 2010, 10:35pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/8 "2010-09-07T22:35:55Z")
> [@Xema](#): > > You can do that with any airfoil… Who needs an airfoil? observation - A sphere with the top spinning forward rises. question - What forces this lift? bernoulli or anti-bernoulli? peace rwj~ ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 7, 2010, 11:39pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/9 "2010-09-07T23:39:04Z")
> [@rwjefferson](#): > > A sphere with the top spinning forward rises. I think it’s the other way 'round - bottom should be moving forward. Here’s a [NASA video](http://www.grc.nasa.gov/WWW/K-12/airplane/cyl.html) For another example, think about looking down from above on a pitcher (probably left-handed for this example) throwing a baseball with clockwise spin: it will curve to the pitcher’s right. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 7, 2010, 11:45pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/10 "2010-09-07T23:45:34Z")
Sorry - it’s a diagram, not a video. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 8, 2010, 1:36am UTC](https://boards.straightdope.com/t/theory-of-flight/531416/11 "2010-09-08T01:36:11Z")
The problem with the traditional ‘partial vacuum’ principle is that it implies that low pressure above a wing causes it to rise. The minor pressure difference between the top and bottom of a wing wouldn’t provide much lift. The pressure difference is a side effect of air moving faster over the top of a wing. When the faster air stream from the top meets up with the slower air from the bottom, a vortex forms as the faster air wraps around the slower air, with an overall downward direction. Equal and opposite reaction to the downward direction of the vortex creates the lift. Wings generally could be called devices for throwing vortices toward the ground. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 8, 2010, 5:51am UTC](https://boards.straightdope.com/t/theory-of-flight/531416/12 "2010-09-08T05:51:58Z")
> [@TriPolar](#): > > The minor pressure difference between the top and bottom of a wing wouldn’t provide much lift. I think you’ll find that in unaccelerated flight the average pressure difference is in fact (Weight of plane) / (Wing area). > [@](#): > > The pressure difference is a side effect of air moving faster over the top of a wing. I would call it a direct effect - they are two aspects of the same thing. To fly, a plane must continuously generate an upward force equal to its weight. This requires the action/reaction of moving lots of air downward, which must result in a pressure difference between wing top and bottom. The real problem with the traditional ‘partial vacuum’ explanation is that it fails to mention what produces the pressure difference (the wing moving large amounts of air downward). So it seems as if some sort of magic is happening. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 8, 2010, 1:24pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/13 "2010-09-08T13:24:11Z")
> [@Xema](#): > > I think you’ll find that in unaccelerated flight the average pressure difference is in fact (Weight of plane) / (Wing area). That would be the ‘wing loading’. The characteristics of flight tie together in the different measurements. > [@](#): > > I would call it a direct effect - they are two aspects of the same thing. To fly, a plane must continuously generate an upward force equal to its weight. This requires the action/reaction of moving lots of air downward, which must result in a pressure difference between wing top and bottom. > > The real problem with the traditional ‘partial vacuum’ explanation is that it fails to mention what produces the pressure difference (the wing moving large amounts of air downward). So it seems as if some sort of magic is happening. Direct effect’ is a good way to put it, but I’d reverse the cause and effect, the pressure differential is caused by the difference in air speed, causing the downward movement of air, resulting in lift. I’ve seem several simple explanations, that imply, or directly state, that lift is the result of a higher pressure under the wing lifting in the manner of buoyancy. Sometimes vertical arrows are shown under or over the wing, with no mention of the downward air movement or vortex generation. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 8, 2010, 9:50pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/14 "2010-09-08T21:50:59Z")
> [@TriPolar](#): > > …with no mention of the downward air movement or vortex generation. Yes, this is a notable weakness of many standard explanations of lift. If you talk only in terms of velocity changes and pressure differences, you’re ignoring the elephant in the room: great gobs of air are being continuously shoved downward. When teaching, I explain this by reference to a helicopter: most people have seen video of one hovering over water, so they have an idea how much air must be thrust downward to keep it aloft. And most people can readily accept that a helicopter’s rotors are simply wings moving in a circular path. Though harder to visualize, the exact same thing is of course happening when fixed-wing aircraft fly. And I think the “stays aloft by shoving air downward” concept is highly intuitive: If you throw someone into water, he will pretty much instinctively try to stay afloat by shoving water downward. While he’s doing this, you could choose to analyse the pressure difference above and below his hands, and no doubt find it to correlate well with the upward force he’s producing. But the straightforward explanation is that the force that’s keeping his head above water is produced by pushing water downward. ---
**Author:** ![rwjefferson](https://avatars.discourse-cdn.com/v4/letter/r/a6a055/32.png) [@rwjefferson](https://boards.straightdope.com/u/rwjefferson)\ **Post date:** [September 9, 2010, 9:50pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/15 "2010-09-09T21:50:01Z")
I agree. Holy Science Authority hasn’t a clue. > [@Xema](#): > > > [@rwjefferson](#): > > > > A sphere with the top spinning forward rises. > > I think it’s the other way 'round - bottom should be moving forward. Here’s a [NASA video](http://www.grc.nasa.gov/WWW/K-12/airplane/cyl.html) > > For another example, think about looking down from above on a pitcher (probably left-handed for this example) throwing a baseball with clockwise spin: it will curve to the pitcher’s right. The last time I checked, baseballs were relatively rough. What if a just smooth enough topspinning ball was observed to lift? What forces this levity? newton or bernoulli? peace rw~ ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 9, 2010, 10:12pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/16 "2010-09-09T22:12:19Z")
> [@rwjefferson](#): > > The last time I checked, baseballs were relatively rough. What if a just smooth enough topspinning ball was observed to lift? What forces this levity? newton or bernoulli? > > peace > rw~ Well it might be Coanda, but I’m not positive that fits the exact definition. Its the effect of the ‘adhesion’ between two substances, the surface of the ball, and the surrounding air. As the ball moves through the air (or as a cylinder rotates as shown in the Nasa diagram), air is going to adhere more to the top of the ball, which is moving in the same direction as the air, than the bottom of the ball, which is moving in the opposite direction. The air over the top speeds up, under the bottom slows down, and we are back to the basics of aerodynamic lift. BTW: Xema, maybe you should start with the head above water example. People might have fewer misconceptions about that, and you can focus on Newton’s third. ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [September 9, 2010, 10:20pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/17 "2010-09-09T22:20:24Z")
> [@rwjefferson](#): > > What if a just smooth enough topspinning ball was observed to lift? A ping-pong ball is reasonably smooth, and a shot hit with topspin definitely drops. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [September 9, 2010, 10:56pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/18 "2010-09-09T22:56:23Z")
> [@Xema](#): > > > [@rwjefferson](#): > > > > What if a just smooth enough topspinning ball was observed to lift? > > A ping-pong ball is reasonably smooth, and a shot hit with topspin definitely drops. Whoa, wait, topspin, the top spinning in the direction of the ball’s motion, opposing the airflow? Cancel my last post. Yes, negative lift. Sorry. ---
**Author:** ![rwjefferson](https://avatars.discourse-cdn.com/v4/letter/r/a6a055/32.png) [@rwjefferson](https://boards.straightdope.com/u/rwjefferson)\ **Post date:** [January 8, 2011, 11:20pm UTC](https://boards.straightdope.com/t/theory-of-flight/531416/19 "2011-01-08T23:20:54Z")
Adhesion is a state of relative viscid friction. Viscid friction means topspin carries a layer of airmass around under toward the trailing side. Adhesion means airmass departs a topspin back and upward. Newton states action means in verse equal and opposite reaction. Bernoulli means inviscid. Air flows faster over the top of a ball spinning topside into the relative wind. Bernoulli states air flowing faster over a surface means lift. A relatively smooth enough topspinning ball demonstrates lift. Is this lift due to Bernoulli of Newton? peace 🙂 ron~ oh yes in deed allison wonderland there is such a thing as a relative headwind ---
**Author:** ![Xema](https://avatars.discourse-cdn.com/v4/letter/x/9de053/32.png) [@Xema](https://boards.straightdope.com/u/Xema)\ **Post date:** [January 9, 2011, 12:34am UTC](https://boards.straightdope.com/t/theory-of-flight/531416/20 "2011-01-09T00:34:06Z")
> [@rwjefferson](#): > > A relatively smooth enough topspinning ball demonstrates lift. Questions: 1. Are we in agreement that when a soccer ball, golf ball, tennis ball, baseball or ping-pong ball is hit with topspin, it drops more quickly than without the topspin? 2. Are you saying that a ball more smooth than any of these might rise when launched with topspin? 3. If so, can you point to any examples of this? [Next page](https://boards.straightdope.com/t/theory-of-flight/531416.md?page=2)