What Bernoulli Missed About Flight

GrokSoup writes: "How come planes can fly upside-down? The Bernoulli principle as applied to flight -- air moves faster over the top of the wing creates low pressure, sucking the plane upward -- always bugged me because it didn't explain inverted flight. Turns out I'm not the only one. The current New Scientist has an entertaining interview with Fermilab physicist David Anderson who explains why the Bernoulli explanation is only partial: lift from wing-shape is the least significant component of lift. Much more important is the wing's angle of attack."

Comments

Something that's bothered me for years

[Anonymous Coward]

When I was in Jr high, one of my best friends was into radio controlled airplanes. He'd spend weeks building planes and then we'd take them out to this park near my house to fly them. One day one of the adults who also flew planes was looking at my friend's plane and told him, "That will never fly kid, you put the wing on upside down!" Sure enough he crashed it shortly afterwards.

Funny thing, though. We had flown that plane at least a dozen times before and never had any problems until this "expert" set us straight.

Re:He's got the equipment

[K-Man]

I'm not sure what the Coanda effect is, but I believe it's just an expression of the role of viscosity in moving air around. To explain: when a fluid has no viscosity, it tends to flow very easily around sharp edges, etc. and back to its original position, say, after a wing has passed through it. When this happens, no lift, drag, or dynamic forces at all are generated - all the fun goes out of aerodynamics. However when viscosity is introduced, it puts a limit on how easily the fluid can deform to flow around objects - it restricts the velocity gradient between adjacent pieces of air, wing, etc. So when the fluid hits, say, a sharp trailing edge of a wing, it does not flow around infinitely fast and back up the other side to its previous height; it sheds off the edge at a lower point than it started, and a net force is generated.

If you think about it, this is the way just about all of us move air - we know that if we wave a hand through the air, a certain mass of air will not be able to get around the hand, and will have to be dragged or pushed along by it in some way. The same goes with a moving wing.

Re:Where do you get it?

[K-Man]

Remember that angle of attack is measured from the stagnation point on the front of the airfoil, to the trailing edge. The bottom of a wing can be horizontal, and it can still have a positive angle of attack.

If you bring the top and bottom airstreams together at the same height that they were divided, then there's no lift. K-12 textbooks are woefully off base in explaining that.

Re:He's got the equipment

[K-Man]

Sorry, nope.

The curved wing explanation assumes that air wants to reach the trailing edge of the wing at the same time, regardless of whether it went over or under the wing. This is not the case. The air travelling over the wing reaches the trailing edge before the air going under, and the amount of advance is determined by angle of attack, not the camber of the airfoil.

Read some of the websites referenced by other posts in this thread. They explain circulation in more detail, with wind tunnel photos.

Here is Anderson's article

[K-Man]

Excerpt from the article on lift. As I suspected, he's just popularizing the usual aerodynamics.

Almost everyone today has flown in an airplane. Many ask the simple question "what makes an airplane fly?" The answer one frequently gets is misleading and often just plain wrong. We hope that the answers provided here will clarify many misconceptions about lift and that you will adopt our explanation when explaining lift to others. We are going to show you that lift is easier to understand if one starts with Newton's laws rather than the Bernoulli principle. We will also show you that the popular explanation that most of us were taught is misleading at best and that lift is due to the wing diverting air down. Most of this diverted air is pulled down from above the wing.

Let us start by defining three descriptions of lift commonly used in textbooks and training manuals. The first we will call the Mathematical Aerodynamics Description of lift, which is used by aeronautical engineers. This description uses complex mathematics and/or computer simulations to calculate the lift of a wing. It often uses a mathematical concept called "circulation" to calculate the acceleration of the air over the wing. Circulation is a measure of the apparent rotation of the air around the wing. While useful for calculations of lift, this description does not lend themselves to an intuitive understanding of flight.

The second description we will call the Popular Description, which is based on the Bernoulli principle. The primary advantage of this description is that it is easy to understand and has been taught for many years. Because of its simplicity, it is used to describe lift in most flight training manuals. The major disadvantage is that it relies on the "principle of equal transit times", or at least on the assumption that because the air must travel farther over the top of the wing it must go faster. This description focuses on the shape of the wing and prevents one from understanding such important phenomena as inverted flight, power, ground effect, and the dependence of lift on the angle of attack of the wing.

The third description, which we are advocating here, we will call the Physical Description of lift. This description of lift is based primarily on Newton's three laws and a phenomenon called the Coanda effect. This description is uniquely useful for understanding the phenomena associated with flight. It is useful for an accurate understanding the relationships in flight, such as how power increases with load or how the stall speed increases with altitude. It is also a useful tool for making rough estimates ("back-of-the-envelope calculations") of lift. The Physical Description of lift is also of great use to a pilot who needs an intuitive understanding of how to fly the airplane.

This is an old debate/flamewar

[K-Man]

People have debated this on the net for years, and not without reason. The bernoulli effect got put into textbooks years ago, and everyone was convinced that a wing cross-section was some magical shape that produced lift without drag, or with little drag, and that simpler shapes, like a fan blade, simply didn't have the right stuff to hold up an honest-to-goodness airplane.

The truth is much simpler. Aircraft stay up by accelerating air downwards. The viscosity of air allows it to be pushed down by a flat object held at a positive angle of attack. Any such object will have a net difference in pressure between its top and bottom, referred to as lift (and drag). Making the wing teardrop-shaped makes the flow more laminar and reduces drag, but that's really just streamlining.

Re:Bernoulli Effect

[rcw-work]

Nobody flies 747's upside down because the plane is not designed to do stunts - I don't know what its negative G rating is but I do know it's only rated for 2.5G's positive G's (most aerobatic planes are rated for +/-12 - some properly trained humans can tolerate up to +9 for short periods, and some of the big roller coasters pull +2 G's now). If it's only rated for something like -1.2G's, flight upside down only leaves you with .2G's of wiggle room before the wings tear off.

Those big jets do fly upside down though.

For a long time...

[cr0sh]

I have known it couldn't be all Bernoulli, and that it had to be mostly angle of attack - why?

I am sure some of you have played with paper airplanes, as well as balsa gliders. If not, go out and buy or build one - notice how on the "el-cheapo" balsa gliders the wing is just a flat piece of balsa? Not much of an airfoil - but it does fly! Paper airplanes are even worse - they are typically folded in such a way that they actually have an almost reverse airfoil, with a notch on the upper edge...

Speaking of paper planes - how many of you still play with them? I know I do - over the years I have managed to fold damn near every possibility. I have one design for a stunt plane that I actually have gotten to do both tail slides and flat spins (one time, the plane didn't recover, and hit the ground in a flat spin - amazed me to see that in a paper plane, something most people will never see with a full sized plane and live to tell the tale).

BTW - Does anyone remember the Kline/Folgeman (sp?) wing? This was a wing that had a notch on the bottom - they had an article in Omni Magazine in the 80's about it, with paper planes to cut out and fly. They also had a book - and they built full size flying mock ups (as well as RC models). Supposedly, the wing was impossible (or near impossible) to stall, and could handle very well at low speeds. What ever happened to them?

Worldcom - Generation Duh!

Bernoulli Effect

[qwertykid]

The Bernoulli effect in itself is not strong enough to provide the lift for the aircraft. It merely is responsible for holding the airflow to the upper surface of the wing with a given angle of attack. I fly indoor model airplanes (rubber band powered free-flight) as a hobby, and i can tell you that we certainly do not have flat-bottomed airfoils. Our wings are simply ribs and spars covered on the top (covering the bottom also adds too much weight and decreases the effectiveness of the angle of attack). Without the bernoulli effect, we would not be able to have such an angle of attack in our wings without causing turbulence on the top of the wing when the airflow doesnt hold to the surface. this phenomenon disrupts the airflow over the top of the wing... most of us call it a "stall".

--qwertykid

Re:Bernoulli Effect

[peccary]

The pilots of Alaska Airlines flight 261 had their MD80 flying inverted for a while. They had problems with the horizontal stabilizers, and it turned out the plane was more stable upside down.

Trouble is, while you may be able to fly the plane inverted, you sure as hell can't land it that way.

Re:Bernoulli Effect

[jwillem]

Big airplanes, like Boeing 747, are actualy very slow and heavy objects. They have to use all possible advantages they have. That's why they have their wings shaped like that. And you can say whatever you want, but is isn't easy to get a boeing 747 flying upside down. Just try it sometime in some flight simulator.
It's just as qwertykid says; The airflow sticks to the airwing. If one should take flat wings, the angle of attack is too big, and the plane would stall (which means that the lift from the top of the wing is gone, the lift from the bottom is still there).
The solution to this, when building your own model-plane, is getting a better engine. Then the plane would fly faster, and the lift you would't get from the top of the wing is compensated with the lift you'd get from the bottom of the wing.
This is why boeing has its wings shaped like that. One can't compare this to an F16 that has to be as manouverable as possible, and has to be able to pull negative G's.
There are other solutions for getting the plane in the air, but this is the optimal one.

Re:He's got the equipment

[jaga~]

Err... I don't think that this is the issue. The author is agreeing that the previous perception still stands true and is technically correct, but is not the actual reason that lift occurs. Therefore, proving the differences in airspeed would not prove or disprove this theory. The issue at hand seems to be whether or not it is air pressure which has been the classic explanation as the cause of lift, or the effects of a 'Coanda' effect, which the article describes as some tendency for fluids to 'stick' to surfaces.. which the article says causes lift by making the wing exert a force to counteract this (i think?).. anyways, the technical calculations and methods for determining lift etc are the same, it is merely the physical question as to what is actually occuring.

at least thats what I got from the article...once I read the book I'll post again.

Another discussion of the issue

[Elbelow]

On one of the pages of William Beaty's Science Misconceptions site, there is a discussion of this issue with diagrams and further links.

Re:Finally, an explanation that makes more sense.

[Placido]

There's a gap between the wing and the flaps because it reduces turbulence. If there wasn't any gap there would be a huge amount of wind resistance and turbulence creating severe problems for the flight of the aircraft. The gap also allows the air from the bottom of the wing to flow over the top of the flaps creating extra lift.


Types of Flaps
Flaps come in several varieties:

• Plain flaps are mounted on simple hinges. The trailing edge of the wing simply pivots downward. Plain flaps are common on small aircraft because they're simple and inexpensive.
• Split flaps hang down from the trailing edge of the wing, but the top surface of the wing doesn't move.
• Slotted flaps work much like plain flaps. But they leave a gap between the flap and the wing, allowing air to flow from the bottom of the wing over the top surface of the flap. This airflow dramatically increases lift at low airspeed.
• Fowler flaps are the most complicated and efficient arrangement. They move backward and downward as they're deployed, increasing both the wing's area and its curvature. Large jet aircraft usually have Fowler flaps.

There's also leading edge flaps which are cool.


Pinky: "What are we going to do tomorrow night Brain?"

Bernoulli effect

[slaytanic killer]

I understand that the Bernoulli effect doesn't explain flight all by itself, but is there such a problem with air going faster over a curved surface? Isn't there more surface area, so the speed relative to the wing's shape is greater?

Time to pull out my Feynmans. ;)