How is it possible to fly inverted???

[UnAnswerd]

On the other hand, with a chambered airfoil at a negative AoA but still with a positive Coefficent of Lift, bernoulli's principle makes only sense. The airflow above the surface speeds up, thus decreasing pressure. The high pressure below the wing tries to 'unite' with the low pressure above the wing and therefore produces an upward force.

As a student pilot, I REALLY want to at least know what keeps the aircraft from falling out of the sky. What was mentioned above is basically what I've read in a number of textbooks. Not only have I read this information, the principle does seem to make sense, and I will get quite confused if anyone disputes the theory stated above. The "low pressure" idea is really how I prefer to look at it. It works the same way in a carburetor. Fuel is literally lifted upwards from the float-bowl despite gravity. Air rushing through the carburetor throat creates an area of low pressure, thus allowing atmospheric pressure to push the fuel upwards. I respect all opinions in here, and the fact that I'm just a student. Nevertheless, I respectfully do not understand how this theory can be disputed, even though it may not be the entire factor contributing to the production of lift.

 

[mzaharis]

As a student pilot, I REALLY want to at least know what keeps the aircraft from falling out of the sky. What was mentioned above is basically what I've read in a number of textbooks. Not only have I read this information, the principle does seem to make sense, and I will get quite confused if anyone disputes the theory stated above. The "low pressure" idea is really how I prefer to look at it. It works the same way in a carburetor. Fuel is literally lifted upwards from the float-bowl despite gravity. Air rushing through the carburetor throat creates an area of low pressure, thus allowing atmospheric pressure to push the fuel upwards. I respect all opinions in here, and the fact that I'm just a student. Nevertheless, I respectfully do not understand how this theory can be disputed, even though it may not be the entire factor contributing to the production of lift.

Conceptually, that is more or less what's going on, and that is a good place to start. Just understanding that, by decreasing the cross section of a flow, you speed up the flow and decrease its pressure (the basics of Bernoulli), you have gained an aerodynamic understanding that escapes most people. It's just that you have to go through a few more layers of detail to see how it ultimately results in lift on the wing, which is a much more complex process than what can be communicated in a couple of paragraphs in a web forum. Unfortunately, most non-aerospace engineering texts omit those layers of detail, and end up with an explanation that, while flirting with a few truths, is inaccurate.

Some worthwhile links have been provided by some others and myself. Print them out, spend a Sunday morning reading them over coffee instead of the morning paper, and you may soon know more than your instructor regarding teh aerodynamic mechanisms for generating lift (just don't tell him that! ;-) )

 

[VNugget]

On the other hand, with a chambered airfoil at a negative AoA but still with a positive Coefficent of Lift, bernoulli's principle makes only sense. The airflow above the surface speeds up, thus decreasing pressure.

True that there's a higher speed and lower pressure, but if the higher speed causes the lower pressure, then what causes the higher speed?

 

[mzaharis]

BTW, if you want to really bake your noodle, go to the source. The definitive source book on airfoil behavior is a book called "Theory of Wing Sections" by Ira H. Abbott and A. E. von Doenhoff, a couple of NACA scientists (not NASA - it was written in the 1949, I believe, when people didn't think of space travel - NACA stood for "National Advisory Committee for Aeronautics", and was the precursor for NASA). It was probably on the bookshelf of the guys who designed your airplane's wing. It is still a must-read for all aerospace engineers. It was the first textbook to really describe in detail the behavior of airflow around an airfoil.

It is EXTREMELY mathematical by non-engineering standards, but it also has a great many useful charts describing the lift behavior of numerous symmetrical and cambered airfoils. That alone makes it worth the $13.97 it costs. If not, find a local university engineering library, and there's a good chance that they'll have a copy, particularly if they have an Aero E department.
http://www.amazon.com/exec/obidos/ASIN/0486605868/qid=1097121280/sr=2-1/ref=pd_ka_2_1/103-1791045-7591024


EDIT - The book, while written in 1949, was re-released in 1959, after NASA was created. Therefore, you will see some references to NASA in all contemporary versions of the book.

 

[VNugget]

(I was SUCH a geek - oh, wait, I still am). I had a fun time working with a FORTRAN program that did pretty much the same thing as FOILSIM, but without all the pretty real-time animations. It had been written by a grad student as his thesis, and contained considerably more advanced math than I understood at the time. It took airfoil coordinates, and determined the pressure and velocity distributions based on a circulation theory of lift model (and the circulation vortex

Check this out, I think you'll like it. Except it's pretty clunky... to enter an airfoil other than the default NACA 0012 (just click "compute" twice) you have to go to the Examples section, copy a set of coordinates, and paste it into the applet (be sure to click "clear" first.)

 

[mzaharis]

Check this out, I think you'll like it. Except it's pretty clunky... to enter an airfoil other than the default NACA 0012 (just click "compute" twice) you have to go to the Examples section, copy a set of coordinates, and paste it into the applet (be sure to click "clear" first.)

Thanks. It's well past my bedtime (wife's in bed, grumpy that I'm typing away - stupid me), but I definitly will get around to playing with it. Thanks for the heads-up.

If you want a really good database of airfoils, go to the University of Illinois Applied Aerodynamics Group's website. This site is maintained by Michael Selig, one of the world's best low-speed airfoil designers.

http://www.aae.uiuc.edu/m-selig/

An interesting article on Selig

http://www.uiaa.org/urbana/illinoisalumni/utxt0202b.html

 

[Mr. Cole]

True that there's a higher speed and lower pressure, but if the higher speed causes the lower pressure, then what causes the higher speed?

Vnugget,

The world may never know. Actually, I'm only kidding. I think many of us understand circulation theory. Place a rotating sphere or cylinder in a fluid and the velocity of the fluid in the direction of revolution moves faster, sorta like walking in the same direction as a moving walkway. The flow in the direction opposite to the revolution is slowed, like walking against the walkway. This produces lift on the object through the Magnus effect. But most wings don't have this type of circulation about them in order to impose the velocity differential.

The greater the camber of the wing or higher the AOA, the greater the velocity increase over the wing. The change in camber is a change in direction of the airflow, which implies an acceleration. Now an acceleration doesn't necessarily imply a change in the velocity magnitude, it could be just a change in the direction of the flow. So maybe that's not the answer. But wait, doesn't the top of the cambered wing sorta look like the bottom half of a venturi throat? In other words you take a pipe that narrows then expands, but only take the bottom half. You're still narrowing the flow of the air over the wing more than beneath, hence an increase in velocity. As I mentioned before I'm a physics and engineering guy, but my expertise is not aerodynamics and I'm trying to learn so please refute any glaring inaccuracies.

Dave

 

[mattpilot]

But wait, doesn't the top of the cambered wing sorta look like the bottom half of a venturi throat? In other words you take a pipe that narrows then expands, but only take the bottom half. You're still narrowing the flow of the air over the wing more than beneath, hence an increase in velocity. As I mentioned before I'm a physics and engineering guy, but my expertise is not aerodynamics and I'm trying to learn so please refute any glaring inaccuracies.

That is exactly how it was explained in my adv aero class, and i find this to be the most plausible answer.

 

[Mr. Cole]

That's good to hear. For the first time in a while I thought about the question after reading Vnugget's post and tried to think of all the physical reasons I could for why that was the case.

Dave

 

[VNugget]

Vnugget,

But wait, doesn't the top of the cambered wing sorta look like the bottom half of a venturi throat? In other words you take a pipe that narrows then expands, but only take the bottom half. You're still narrowing the flow of the air over the wing more than beneath, hence an increase in velocity.

Dave

Actually, that was also what was going through my mind, but I had a brainfart and was thinking about it backwards, myself, when I asked that rhetorical question.

Blah.

 

[profile]

Chris Carpenter, in Flightwise, does a nice job of explaining what causes the air to accelerate over a wing. Chris is the head of Aerodynamics at the Royal Air Force College, and wrote the Flightwise books to aid students who had undergrad degrees in fields outside of Aero to understand the field. He also wrote them such that one can understand them even without a significant mathematical background (he designed them so you can skip the math in the text without losing the ideas). Flightwise is probably the best set of books I have seen for a complete understanding of the concepts without getting a degree in the field, available thru www.amazon.co.uk (not on the U.S. site for some reason). Shipping is very inexpensive thru the Royal Post. Two books, first is basic Aero, the second is all about Stability and Control. Well written, not dry, and he is very good at putting the concepts across. Good for those that want to know "why".

For pure Aero types, hard to beat John Anderson's books to really be able to crunch the numbers (mentioned earlier).

 

[mzaharis]

Vnugget,

The world may never know. Actually, I'm only kidding. I think many of us understand circulation theory. Place a rotating sphere or cylinder in a fluid and the velocity of the fluid in the direction of revolution moves faster, sorta like walking in the same direction as a moving walkway. The flow in the direction opposite to the revolution is slowed, like walking against the walkway. This produces lift on the object through the Magnus effect. But most wings don't have this type of circulation about them in order to impose the velocity differential.

The greater the camber of the wing or higher the AOA, the greater the velocity increase over the wing. The change in camber is a change in direction of the airflow, which implies an acceleration. Now an acceleration doesn't necessarily imply a change in the velocity magnitude, it could be just a change in the direction of the flow. So maybe that's not the answer. But wait, doesn't the top of the cambered wing sorta look like the bottom half of a venturi throat? In other words you take a pipe that narrows then expands, but only take the bottom half. You're still narrowing the flow of the air over the wing more than beneath, hence an increase in velocity. As I mentioned before I'm a physics and engineering guy, but my expertise is not aerodynamics and I'm trying to learn so please refute any glaring inaccuracies.

Dave

Actually, wings DO have circulation. It is just that it is not as intuitive. This image represents a wing without circulation (Kutta condition not satisfied). This wing is generating no lift and represents how the airflow around a wing would be if air had zero viscosity:

http://www.av8n.com/how/img48/nocirculation.png

A pure circulatory flow looks like this:

http://www.av8n.com/how/img48/circulation.png

Sum the two, and voila, you get the classic flow pattern seen around an airfoil, which sastisfies the Kutta condtion (which, in effect, says that a viscous flow cannot "turn the corner" around a trailing edge, like a theoretical inviscid flow would).

http://www.av8n.com/how/img48/normal.png

In regards to the "chicken and egg" posed by vnugget, aerospace engineers talk of a "starting vortex" that begins the circulation. Some people argue that this is a mathematical obscurity, borrowed from electromagnetic theory (most notably the author of the interesting book "Stop Abusing Bernoulli"), but others would argue that it is a real physical phenomenon.

The website of the author of "Stop Abusing Bernoulli"
http://www.geocities.com/galemcraig/

From that website, an interesting reason why we get the incorrect explanation in so many sources:

Dr. Theodore Von Karman, a most prominent aerodynamicist in mid-20th century, once told his assistant: "When you are speaking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth."

Once again, my favorite description of lift phenomena:
http://www.av8n.com/how/htm/airfoils.html