What Bernoulli Missed About Flight 38
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."
Re:Where do you get it? (Score:1)
So you agree with him that it's all about angle of attack if, as you imply, that velocity and angle of attack is much stronger then any Bernouli effect.
And you got your degree in aeronautics where?
Re:Where do you get it? (Score:1)
An airfoil will fly with 0 angle of attack. At the same time a perfectly flat wing can also fly, provided there is sufficient velocity and angle of attack.
The flat wing is less efficient than the airfoil and will therefore require more energy to make it fly.
I also disagree disagree with the post, you don't need an aeronautical engineering degree. Just common sense.
This disturbs me. (Score:1)
Planes can fly upside down because they increase the angle of attack so that Newton's lift (pushing the plane up) over comes Bernoulli's lift (which is pulling the plane down). You just pay a big price in air friction while you're doing it.
There, was that so hard to explain. I think not.
I'm not an aeronautical engineer, just a guy who like planes and has read and thought quite a bit about lift. I just find it hard to believe that 100 years of wing designers could be overlooking the basic principles of flight.
Something that's bothered me for years (Score:2)
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 (Score:2)
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? (Score:2)
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 (Score:2)
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 (Score:3)
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 (Score:4)
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:He's got the equipment (Score:1)
Now it all makes sense
Aric
Re:He's got the equipment (Score:1)
Re:Bernoulli Effect (Score:1)
Re:Bernoulli Effect (Score:2)
Those big jets do fly upside down [historylink.org] though.
Tacking.... (Score:1)
For a long time... (Score:2)
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 [worldcom.com] - Generation Duh!
Bernoulli Effect (Score:3)
--qwertykid
Re:Bernoulli Effect (Score:1)
Re:Where do you get it? (Score:1)
As the interviewee in the NS article concluded (and I've not got my copy to hand right now), Bernoulli's not guilty here. Daniel Bernoulli (gotta be SO careful getting the right one - not Jacques, Johann or Nicholas) died in 1782, and therefore never made any attempt to explain how fixed-wing aircraft got their lift.
What the article was saying, though, was that while Bernoulli's equations describe the airflow around a lifting wing correctly, and are therefore extremely useful, they don't explain the production of lift. That's a simple consequence of Newton's 3rd Law (not that Newton was 'guilty' either). What the interviewee said is that the airflow effects are a nicely measurable (and hence extremely useful) consequence of the reaction forces that cause lift in the first place. But not a cause in their own right
TomV
Wow (Score:1)
I am usually very pleased with the content of articles here, but this was obviously approved by a 9 year old.
Um, Hello? (Score:1)
Re:Bernoulli Effect (Score:2)
Trouble is, while you may be able to fly the plane inverted, you sure as hell can't land it that way.
Re:He's got the equipment (Score:2)
at least thats what I got from the article...once I read the book I'll post again.
Re:Bernoulli Effect (Score:1)
This would only tell you what happens when you follow the rules that were programmed into the flight simulator. You can't draw conclusions about the validity or the physical relevance of those rules. This being said, I admit I also think it would be very difficult to fly a 747 upside down.
Another discussion of the issue (Score:2)
Re:Finally, an explanation that makes more sense. (Score:3)
Types of Flaps
Flaps come in several varieties:
There's also leading edge flaps which are cool.
Pinky: "What are we going to do tomorrow night Brain?"
Experiment for yourself (Score:1)
It's pretty obvious from this simple test that the angle of attack is far more important than the Bernoulli effect in creating lift. It's also been pointed out a number of times,(Try this [amasci.com] for example) but somehow hasn't found its way into the science textbooks yet.
Sorry, it's really not big news.
Denker's explaination from "See How It Flies" (Score:1)
Re:Here is Anderson's article (Score:1)
I don't suppose you happened to notice who was the author of that page you used? Clue: Check who Anderson's co-author was. :-)
Bernoulli effect (Score:2)
Time to pull out my Feynmans.
He's got the equipment (Score:1)
1) Get a wing
2) Stick it in a wind tunnel
3) Attach airspeed measurement devices on the top and bottom (these have names, I'm sure)
4) Start the wind tunnel and measure the difference in airspeed between the top and bottom of the wing
5) Adjust the angle of attack and see if that makes a difference
Otherwise, this is all talk.
Dancin Santa
Re:He's got the equipment (Score:1)
The author seemed to disbelieve that airspeed above and below the wing differed. I'm surely not as qualified as he to quibble, but without more information it's hard to come to any sort of conclusion.
It's a very tempting theory, though.
Dancin Santa
Re:Bernoulli Effect (Score:1)
Dancin Santa
Re:Bernoulli Effect (Score:1)
Dancin Santa
Re:Finally, an explanation that makes more sense. (Score:1)
There are a couple of other points to take into consideration.
Firstly deploying flaps not only increases lift but also increases drag at the same time. Remember, aircraft also deploy flaps on approach to landing so that they may slow down to a manageable touchdown speed while maintaining enough lift to stay in the air. Imagine the length of runway required if a 747 had to land at anywhere its normal cruising speed!!
It seems reasonable to assume that it has something to do with the angle through which a given volume of air is deflected by the wing's angle-of-attack. If you hang a flap out from a wing it will produce more lift but as a side effect will also incur more drag. Therefore the whole thing is a complex interaction between the effects of lift and drag.
In a very simplified way, to produce a defined amount of lift you can either have a low drag wing which deflects air through a small angle at a high airspeed or you can have a high drag wing that deflects air through a greater angle at a lower airspeed. The effect of flaps is to convert one type of wing into the other during flight.
I also think that the Bernoulli effect is more important in the area of drag reduction rather than lift production. If the movement of air over the top of the wing is kept smooth and "attached" then the amount of drag for a given angle of attack is reduced allowing the wing to produce "more" lift by reducing drag (i.e. increasing its lift/drag ratio). Turbulent airflow is the enemy of lift.
Cheers
Evan.
P.S. The small Cessna's I fly also have electric Fowler Flaps so although they are common on large jets they are not restricted to them
When people say things without having any idea... (Score:1)
To explain why planes can fly upside-down and why this hasn't anything to do with Bernoulli's law, I must first explain this law: briefly,
The sum of the dynamic and static pressure in a fluid is constant
Lets take an example: assume you have a narrow pipe succeeding a large one, and water runs through them. Obviously, in the narrow pipe the water's linar speed will be higher. Therefore, it's static pressure will decrease.
Now, why do wings generate bernoulli force?
You must have noticed the jet planes' wings' vertical shape: it's like a round triangle, with the narrow angle ahead, the lower side horizontal, and the upper slightly upwards, something like this:
Jet planes rely mostly on this lift force. But we cant picture a jet plane flying upside-down, can we ? So, lets take a plane that can: a fighter plane. Now what's the difference between the wings of this two ? YES,you GENIUS, fighter planes have straight wings, like a sheet of paper, that dont get narrower near the end. Therefore, they dont generate Bernoulli lift! So why do they fly? well it's because the wings make a certain angle with the plane's flying direction.This presses air coming from the front under the wing generating extra pressure there, while on the upper side, the air gets scarce, generating less pressure. So, this is the lift fighter planes use. Still why do they fly upside down, as, in this position, the pitch of the wings reverses, generating pull-down force? Well, it's EASY !!! The pilot keeps the plane with its nose upwards, so that the wings get a right, positive pitch. This is all ! By the way: I've read this below: that adjustable-pitch side of the wing, called FLAPS: its pitch doesnt control the lift of the plane. It just helps the pilot lift the plane's nose, so that the wings' pitch increase, and they generate more lift.
SEE YA
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Re:Bernoulli Effect (Score:2)
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.
Ontario Science Centre (Score:1)
My memory may be playing tricks on me, but I think that there is an exhibit on this in the Ontario Science Centre in Toronto (if, of course, anyone is interested).
As a previous poster mentioned, there is a wing sitting in the path of an air current, and you can rotate it so as to change its angle of attack. When it reaches a certain angle it just takes off all of a sudden. Quite neat really.
Re:Finally, an explanation that makes more sense. (Score:1)