Originally posted by russ_watters
wimms, the title of that website is "A New Spin on the Perceptions, Procedures, and Principles of Flight," ie much of what you find there is *NOT* conventional aerodynamics. Some of the things on the site such as the definitions of chord line and camber are the accepted definitions, but this should be noted:
Guys, if you go to laugh at me or argue me out, that's ok with me. But if you go on and laugh at someone who has enough insight into the subject to write a book for pilots, then excuse me, I don't trust you. Such attitude sounds like "anyone who doesn't precisely follow my textbook is a crackpot".
I admit I have inadequate understanding in the subject, but I've seen enough debates on that subject to at least know this: physics of aerodynamics is not finished and is not explained away by bernoulli alone.
The generally accepted definition is "The acute angle between the chord of an airfoil and a line representing the undisturbed relative airflow."
And oh, by the way, the AOA indicator on an airplane uses the generally accepted definition - so this guy will confuse pilots more than he will help them.
Are you actually pilot to say such things? He covers his definition here:
http://www.av8n.com/how/htm/aoa.html#sec-raoa-aaoa
"..we are free to choose how the angle-of-attack reference stick is aligned relative to the rest of the wing.
Throughout this book, we choose to align the reference with the zero-lift direction. That means that zero angle of attack corresponds to zero coefficient of lift. According to the standard terminology, the angle measured in this way is called the
absolute angle of attack.
Some other books try to align the reference with the chord line of the wing. The angle measured in this way is called the
geometric angle of attack.
If you try to compare books, there is potential for confusion, because this book uses ``angle of attack'' as shorthand for absolute angle of attack, while some other books use the same words as shorthand for other things, commonly geometric angle of attack. To make sense when comparing books, you must avoid shorthand and use the fully explicit terms.
Also note that are many possibilities, not just absolute versus geometric; the choice of reference is really quite arbitrary. It is perfectly valid to measure angles relative any reference you choose, provided you are consistent about it. (Aligning the reference stick with the fuselage is useful in some situations)
Using the chord as a reference works OK if you are only talking about one section of a plain wing. On the other hand:
On typical airplanes, the chord of the wing tip is oriented differently from the chord of the wing root. Which one should be considered ``the'' reference?
When you extend the flaps, the chord line changes. (See section 5.4.3 and section 5.5 for more on this.) Most books that choose to measure angle of attack relative to the chord line violate their own rules when the flaps are extended, and continue to measure angles relative to where the chord of the unflapped wing would have been. That is illogical and creates confusion about how you should use the flaps. This is one of the reasons why it is advantageous to think in terms of absolute rather than geometric angle of attack.
Thinking about geometric angle of attack would be advantageous if you were building an airplane, or conducting wind-tunnel research on wing sections. Engineers can look at a wing section and determine the geometric angle of attack. "
Hope that clarifies confusion I caused in this thread. I took his definition of absolute aoa as conventional. Enigma, some of your objections were to points from the book and came from this confusion.
As to commercial airplane aoa vs fighters, com-planes have cambered wings to reduce drag for more comfort. fighters don't have high aoa, at mach-1 that would cause pilot loose it and excess stress on craft, doesn't it? They need least air resistence for max speed. And besides, they have to have same stall performance upside down. no?
Also, I have skimmed the site and that's the only glaring innacruracy I see. There is some info on a barn door (for example), but though he explains that the MECHANISM is similar, he does *NOT* say that the resulting lift is the same. If you have a link to that 12 deg aoa thing, I'd like to see it.
http://www.av8n.com/how/htm/airfoils.html#sec-inverted-camber
"At small angles of attack, a symmetric airfoil works better than a highly cambered airfoil. Conversely, at high angles of attack, a cambered airfoil works better than the corresponding symmetric airfoil. An example of this is shown in figure 3.14. The airfoil designated ``631-012'' is symmetric, while the airfoil designated ``631-412'' airfoil is cambered; otherwise the two are pretty much the same.11 At any normal angle of attack (up to about 12 degrees), the two airfoils produce virtually identical amounts of lift. Beyond that point the cambered airfoil has a big advantage because it does not stall until a much higher relative angle of attack. As a consequence, its maximum coefficient of lift is much greater."
Seems like I jumped to unwarranted generalisation from symmetric airfoil to 'barn door'. Should have stayed at symmetric wing.
One other point: Nowhere does he say that there is a vacuum on the upper surface of the wing during a stall.
Yes, that was purely my own imagination. (but nowhere does he even talk about stall physics). My reasoning was that airstream detaches because air mass of stream is unable to follow the wing shape due to air inertia, leaving volume of depressurised air between wing and detached airstream. Still we may find thin vacuum between airstream and stagnant air.
wimms, I really recommend you read some ACCEPTED explanations on aerodynamics. Though most of that site is ok, the differences are enough that they will cause you lots of headaches trying to reconcile what he is saying with what everyone else says.
I'm not into highend aerodynamics. This site came up just to show off that wing lift isn't simple to express in terms of bernoulli alone. There are many opposing 'theories' around that are all in some way wrong, "pressure difference alone matters" included. As far as I understand, nothing in this book is NOT ACCEPTED explanations on aerodynamics.
In reality things are quite abit more complicated. See at the end.
