It was sometime in the early 1980s. I had just drawn a crude airfoil and had given — I thought — a quite brilliant dissertation on Bernoulli’s principle to a roomful of Civil Air Patrol cadets.
“Any questions?” I asked, dropping the Piper Cub-yellow chalk into the aluminum tray that ran along the base of the chalkboard, then dusting off my hands on my dark blue uniform pants.
One teen tentatively raised his hand. I nodded for him to proceed and he asked, “Well how is it that airplanes can fly upside down, Captain?”
I. Had. No. Idea.
Now, my initial training was a long time ago. My first certificate was paper and was signed by Orville Wright himself, [Editor’s note: What William says here is exactly 50% accurate], so I might have forgotten some of what I was taught, but all these years later, I only recall being introduced to Mr. Bernoulli in my ground school classes.
And Bernoulli can’t explain why airplanes can fly upside down. Of course, I also thought he was Italian, and an ancestor of a company that makes my favorite pasta sauce — both of which are not true.
The sad thing is that flight training hasn’t come much farther in the intervening 40 years. Sure, the FAA has thrown poor Newton into the mix, but the average flight instructor — much less the average pilot — has a weak understanding of the forces of nature that let us cheat gravity.
And, of course, most CFIs still think Bernoulli is Italian.
Truth be told, the four-letter word LIFT is a lot more complex than we are taught in flight training. This, perhaps, is what prompted a retired Naval aircraft hydraulic and pneumatic mechanic to write a two-page letter saying that all the current theories of lift are wrong. Having worked professionally with both fluids and air, he felt that outside of closed environments they “do not behave alike.” He advocates that airflow is irrelevant, and it’s all about pressure.
And, our reader made the excellent, and on its face, obvious statement: “The only way to achieve flight is to push or pull the wings through the air. The wings travel through the air. The air does NOT travel over the wings. There is a difference.”
When I first read that, I thought, well, duh, of course. But then I got to thinking about virtually every diagram ever published of an airfoil in flight. They all show the air flowing over the wing. This, no doubt, is done for illustrative purposes, but has it led to a misunderstanding? Of course, sometimes air actually does travel over the wings, which still cause the wings to work just fine. There’s no shortage of YouTube videos of improperly secured airplanes taking flight when gusty thunderstorms pass through. So maybe that’s moot.
Of course, it’s not just about wings. All parts of an airplane generate lift, which is why both the nearly-wingless Gee Bee Model R racer and Santa’s Sleigh can fly. Both are what are called “lifting bodies,” aircraft that generate a great deal of lift from the fuselage rather than from the wings. Of course, for this to work, you need a ton of power. Either eight engines, in the case of Santa; or 800 horses in the case of the R-1 Racer.
But all of this got me thinking about lift. How it’s understood. Or not. And how it’s taught. Or not.
Now, I did confirm, by checking the dustiest book on my bookshelf, that in the 1980s, it was all about Bernoulli. My 1981 Jeppesen Sanderson Advanced Pilot Manual, says, “There is only one condition that must be present for an airfoil to produce lift — the air pressure above the wing must be less than the air pressure below the wing.”
To accomplish that, all that was required was for air to move faster over the top than over the bottom. And then the book introduced Dr. Bernoulli.
Now, Dr. B has been around a looooong time in aviation. But not completely without objection: 1944’s famous “Stick and Rudder” by Wolfgang Langewiesche is highly critical of what was then called The Theory of Flight.
“In the first place,” wrote Langewiesche, “Bernoulli’s Theorem does not really explain — the explanation is more puzzling than the puzzle! In the second place, Bernoulli’s Theorem doesn’t help you the least bit in flying.”
He even has an entire section called “Forget Bernoulli’s Theorem.” Instead, he is all about angle of attack, focusing on actionable intelligence usable in the cockpit, and the rest be damned.
But the FAA disagreed then (well, they were the CAA then) and still disagrees now, saying in the Pilot’s Handbook of Aeronautical Knowledge: “Modern general aviation aircraft have what may be considered high performance characteristics. Therefore, it is increasingly necessary that pilots appreciate and understand the principles upon which the art of flying is based.”
And how are they helping us understand these principles? Looking at Chapter 4 of the handbook, there’s a section called “Theories in the Production of Lift.” Note the plural. Here the FAA does admit that traditional theories don’t really do a good job in explaining “how lift is formulated.” But then they go on to spend the next five pages trying to do just that, still talking about Bernoulli (with the addition of Newton), but at the end they say, you know what? Just go read what NASA says. Literally, I’m not making this up. The NASA URL is the last word in the chapter.
So what do rocket scientists say?
Interestingly, the folks at NASA’s Glenn Research Center concur, at least in part, with our reader. On its Beginner’s Guide to Aeronautics website (targeted for middle school through undergraduate college students) the NASA folks say: “There are many explanations for the generation of lift found in encyclopedias, in basic physics textbooks, and on websites. Unfortunately, many of the explanations are misleading and incorrect.”
But then they proceed to spend more time bashing the common theories of lift — including the one I taught to the cadets — than they do explaining how lift works.
Here’s what they say on the latter: “Lift is a mechanical force generated by a solid object moving through a fluid. As it is a force, it is a vector quantity, having both magnitude and direction. It acts through center of pressure and is directed perpendicular to the flow direction.”
Allllrighty then. I’m glad we got that cleared up!
NASA, no dummies, go on to say: “The real details of how an object generates lift are very complex and do not lend themselves to simplification.”
I think that gets to the heart of the matter. For pilot training, we need a simple explanation as there is so much else to learn. If we even need to know how lift works at all. Maybe Langewiesche was right. Who cares if we can’t use it in the cockpit flight deck?
Me? To be honest, I couldn’t understand what our Naval aviator proposed any better than I could understand the FAA’s version or the NASA version.
Instead, I’ve decided to adopt aviation pioneer Percival White’s advice from his 1930 book “How to Fly an Airplane.” He wrote that while “no intelligent flying is possible without at least a smattering of aerodynamics,” he concedes that it’s a deep science, “which calls into play higher mathematics.”
Accordingly, he felt the “average student pilot does not need to go deeply into the subject,” adding, “When in doubt about what to do, fly fast!”
Amen.
Think in terms of the wing downwash angle, epsilon, and mass acceleration equations. Explains it all.
If you can’t prove it, it doesn’t exist.
There is no math problem that demonstrates why a plane flies, not one. Why I think it flies is just as valid as Bernoulli’s principle, which by the way did not prove flight. A barn door will fly, if you accelerate it fast enough. So will a house and a fence rail.
I think a plane flies because of the angle of attack and because of that viscous fluid we call “air”. Planes surf on that viscosity like bodies surf in the ocean.
If airflow above the wing is all that effective I wouldn’t think vortex generators and slats would be as effective as they are. The only thing improving the bottom are gap seals to reduce pressure loss.
I believe the relevance of upper and lower pressure was demonstrated decades ago in suspension bridge design. The bottom pressure lifted and destroyed them in winds.
This is fun! So let me understand this as a child.
If I stick my hand out of the car window, some times (If my hand is flat to the ground and edged to the wind) I can twist (maneuver) my hand and make it rise or drop depending on the Angle to the wind.
If I twist my hand flat to the wind my hand gets Dragged back without moving up or down.
The faster the car speed the stronger the Force up, down, and/ or back.
At slower speeds I mostly feel Drag.
So if I’m flying flight speeds, in 3 dimensions I can use smaller Angles to the wind to push the plane in a new direction or large angles to Drag the plane slower or perform acrobatics. At slow speeds Gravities Vector beats Bernoulli’s Theory.
Miami Mike has correctly analyzed “what makes an airplane fly” and it is money! If you want to prove the point just go into any FBO and state that you want to fly. I assure you they will not ask you questions about physics. They will in some fashion find out if you have the money. Happy flying everyone.
In a proper scientific discussion, one must first outline the assumptions and definitions. Herein lies all the hot air that everyone is expending on “lift’. The generation of wing lift is not the whole factor on what makes an aircraft or any other body overcome the force of earth’s gravity. Trying to limit such forces only to the Bernoulli demonstration of airflow over an aerodynamic surface sticking out the side of the plane doesn’t explain it. And for those who try to explain the lift of a wing as only such low pressure creating forces of such airflow, they forget the other kind of lift known as ‘flat plate lift.’ “Google it” as the modern geniuses say. And one more big factor on making aircraft ‘fly’ – all those spinning things: props, turbine blades and such. These, also have the airfoil considerations. Witness airplanes flying straight up, or managing a controlled landing with only one wing.
I have always felt Bernoulli was about 20% of lift and air smashing the bottom of the wing was 80%.
Popular Science had a article a while back with about 5 competing theories (leaving out radial momentum), the upshot is Bernoulli 0%, air smashing 40 to 60%, and the rest of the theories (FM) up to 80%. Or as Miami Mike so eloquently put it $.
Maybe up to 100%/0%. My paper airplanes with symmetrical (as in flat) wings always easily reached the other side of the room.
Bah humbug. All this is sophistry. What makes an airplane fly is money, period. Want to go faster? Bring more money. Want to go higher? Bring more money. Want to carry more ‘stuff”? Bring more money. Do you see a trend here? I sure do . . .
Best Regards,
Speaking as an engineer, ALL aerodynamics is based experimental data, NONE of aerodynamics is based on theory. All theories fail to fully explain aerodynamics. It works because we tested it and it worked. This is why if you modify the aircraft or operate it out of the design envelope, YOU are a test pilot. (and no that does not mean you are referred to as Chuck). For a beginning check out “The secret of flight” by Alexander Lippish on youTube (https://www.youtube.com/playlist?list=PLo5KJaXac_mUq2WJcP0rsJP7aXy-lQxOA) with 13 lectures. (Yes that Lippish)
The critical angle of attack varies with g-loading. So throw that rule away. Also, boys ride bikes, without ever having taken the physics course upon which the bike’s science depends. The obvious conclusion is that good operation does not always require good understanding.
Stick and Rudder is far and away the best reference ever created for the non-scientist. But if you need to explain things in those terms, the fact that the center of pressure works so well is convincing enough for me. The top of the wing surface is a bigger fan of Bernoulli’s principle.
The bottom of the wing is a strongest advocate for Newton’s third law. I listen to both views in the classroom, but ignore them both when I am flying. Nevertheless, it is a fun discussion that is no less worthwhile coffee talk than what controls altitude, pitch or power.
The point of both is that it gets you thinking.
I don’t know where to begin. I don’t know what to say. I don’t want to offend my compardres in aviation, and I don’t want to get kicked off this site for obnoxiousness. But…, garbage, garbage, garbage. Yes sir ree Bob, piled higher and deeper. Come on, man! Wake up.
Wings flies, produce lift, stall, and universally behave in the same manner no matter what their orientation to the earth ball, or flight attitude, just like you were…, er, taught. Exceed the designed maximum permissible angle of attack, it stops flying, it stops producing lift. Just because an aircraft is flying inverted does not change anything. Let me see if I can paint the picture for you.
Let’s say your chugging along in your 18 at around 600 knots on your way to grandmothers house. All is well, You’re kicked back in the cockpit, smoking a cigar and minding your own business, when…, all of a sudden your wing-man pops up about fifty feet off your starboard strobe [he’s supposed to be to port, but Junior never listens, and he didn’t do all that well in ground school, either]. As you give him a wave hello, he suddenly rolls 180 and is now flying inverted [upside down for the commentators above] just as happy as a clam. His 18 is also happy, very happy with its KJ designed wings generating just as much lift as they were before he decided to meet Jesus and go belly-up. In fact his wings, the aircraft being virtually identical [or nearly so] carrying the same load of fuel [or nearly so] being flown by someone just as mildly obese as you [or nearly so], are generating the identical lift that the wings on your bird are generating [or nearly so]. Get use to it, Buttercup.
It’s flying. Air is flowing over to ‘top’ of the wing generating life. It’s flying right along with you. It’s hanging right in there with your latitudinal, Even Steven. [You will note that Even Steven had to add a little bit of power to keep up [hold altitude] because he is really flying down [or so the plane thinks], and he has to push the nose, well, up [by pushing it down]. Huh? I thought I said he was flying level? I did! Stick with me on this. It is pretty simple.]
How does that work?
Angle of attack, Buttercup.
If you look carefully, his center-line of flight [pitch] is slightly elevated [relative to the horizon] which tends to push the aircraft up while the wings are still flying, but being inverted ‘lift’ the aircraft down. The two are balanced out by the pilot [this is not something for AI to get involved with…, it takes a pilot], and Junior’s aircraft is now apparently flying the same course, speed and altitude [straight and level] as you are, but with a slightly different attitude.
Since both of you are enjoying 18’s, the relative difference in longitudinal
center-lines are almost imperceptible [entirely attributable to velocity], but still different and can be observed. Do this in a pair of Aerobats and the difference will be amusingly wild, enough to make Wagstaff heave. But in either case, both planes are flying, the wings are lifting, and all is well in Wrightsville. In other words, nothing has changed since Kittyhawk or birds donned feathers. Wings always lift no matter where they are going so long as the angle of attack is not exceeded. Where the aircraft flies is the pilot’s business. How the aircraft flies is the plane’s business.
YHS,
CAPT. Johnson
Well, that explains it.
I always wondered why…when I‘d blow past a section of Hornets, in my 15, at 700 knots, usually after 2 x unobserved Fox II kills…one guy was always inverted.
I thought they were re-enacting the Mig-28 intercept scene from “Top Gun”; when they were actually flight testing gouge found in Chapter 1 of “Aerodynamics for Naval Aviators”.
Fight’s On,
Colonel Curran
Stand close to the approach end if a runway under a landing 747 you will notice considerable wind and turbulence shortly after the aircraft passes overhead. This is a result of high pressure forcing air downward beneath the aircraft. Reduced pressure above the wing can not be the cause of the high pressure beneath the wing.
A 2-dimensional flow, as simulated in a wind tunnel by extending a wing section from wall to wall of the tunnel’s test section, will generate lift with no net downwash. I.e., the upwash of airflow approaching the airfoil equals the downwash at the trailing edge. Yet there is a measured lift, due solely to a net differential pressure normal to the upper and lower surfaces. The only other force experienced in this 2-D case is a tangential shear, producing drag. Now shorten that same wing, a 3-D flow is introduced, producing tip vortices, due to the pressure differential & resultant span wise flows. Since the tip vortex on each wing tip is rotating in the same sense as the bound vortex of 2-D flow (only bent 90 degrees), there’s now a net increase in downwash. This is an effect of the wing producing lift, not a cause; but because we fly wings subject to 3-D flow, one can safely consider the downwash as the source of lift, if that “visualization “ helps the pilot “understand” how he flys the wing in it’s flight envelope. In any case, simply observe AOA limits in coordinated flight & enjoy the experience of flight!
So if the airplane only creates lift when top side up, then how does it ever land.
Answer, control inputs. Same as when the plane is top side down, control inputs.
Perhaps the best qualified contemporary aerodynamicist with a straight forward explanation for lift is Dr. John D. Anderson, Jr. Professor Anderson has authored numerous texts for aero engineering majors from low speed aerodynamics to hypersonic high temperature gas dynamics. See his Introduction to Flight text, section 5.19 for a treatise on various theories. Bernoulli wins!
I don’t know why the upside down disqualifies Bernoulli. When upside down, there is an angle of attack where the distance traveled is greater along what is usually the bottom of the wing then the usual top. More drag, sure less efficient but Bernoulli still applies.
Most aerobatic competition aircraft have a symmetrical airfoil about the chord line.
So, the wing flies the same right side up or upside down .
Yes, just like the symmetrical keel of a sailboat!
I suggest reading:
Theory of Lift: Introductory Computational Aerodynamics in MATLAB/Octave by McBain, G. D.(July 23, 2012)
Aircraft lift calculation is an extremely complex problem that is regularly solved by Boeing etc in their new aircraft design (using super computers).
For a good qualitative but correct discussion of lift , read lift related sections in :
SEE HOW IT FLIES book by John Denker (AVAILABLE ON THE WW WEB.)
An excellent text I believe all pilots should study.
Unless you have a good grasp of Physics including fluid mechanics, I suggest just flying and being assured the Navier Stokes equations for compressible flow that govern aircraft flight are not expected to be repealed by the creator.
Ok, I’ll wade in. Ultimately a wing generates lift by throwing air downward, just as a propeller generates thrust by throwing air backward. Having said that, the upper surface of the wing actually generates more lift than the lower surface. So in effect the top of the wing is sucking air downward to generate lift. You can attribute that to Bernoulli or to Coanda.