Bernoulli and Low Pressure

I would have to say that it is not a vacuum until the critical AOA and a stall occurs. The vacuum area is created when the airflow begins to separate from the surface of the wing just under the reverse flow of the rolling and is an area which the air can not get to which means it is a vacuum.
 
splash said:
an area which the air can not get to which means it is a vacuum.
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No, that's not a vacuum either. If it were, lift would increase enormously. Moreover, that would eliminate the positive pressure gradient that produces the separation in the first place.
 
Laymen's terms people associate a vacuum with suction is what I think is being considered by enroute. A person thinks of a vacuum and they picture that thing you push around the house that you suck things up with. For pure concept, they are right and I don't see any reason why you can't associate that suction to pulling up on a wing.

Just because we use a term that doesn't fit the scientific definition doesn't mean it can't be used. If it helps the student understand or even feel like they understand what is happening then it will allow that student to feel more confident and move on. As long as they know lift happens because of a difference of pressure other associations can't hurt unless they cause some form of negative transfer of learning, which I don't see any in this case.

After all, any explanation we give to re-enforce the true definition, lift is a difference of pressure, which cause no negative transfer of information and makes the student feel more comfortable with the true definition is a success in my book.


*Flame shield on*
 
tgrayson said:
No, that's not a vacuum either. If it were, lift would increase enormously. Moreover, that would eliminate the positive pressure gradient that produces the separation in the first place.
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Negative ghostrider. :D A vacuum is not a force but a simple place of nothingness!
 
I'll bite :D

shdw said:
For pure concept, they are right and I don't see any reason why you can't associate that suction to pulling up on a wing.
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Because we, as teachers, teach that which is correct, not - close enough.

Just because we use a term that doesn't fit the scientific definition doesn't mean it can't be used.
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Perhaps, but if a private pilot applicant exchanged the definitions for Angle of Attack and Angle of Incidence, what would the DPE think?

As long as they know lift happens because of a difference of pressure
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right there is the key. It is not a vaccuum and should be taught as such.

other associations can't hurt unless they cause some form of negative transfer of learning, which I don't see any in this case.
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ORLY?
*Flame shield on*
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Toasty? :D
 
splash said:
Negative ghostrider. :D A vacuum is not a force but a simple place of nothingness!
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You're joking, I hope? Lift is produced by a small pressure differences around the airfoil, less than an inch of mercury. Suddenly you have a vacuum and you have almost 30 inches of pressure differential. All the surrounding air would be applying a tremendous force attempting to fill the vacuum. Doesn't happen.

Regardless, the area behind the separated flow is filled with air from the bottom of the airfoil that flows around the trailing edge to fill it.
 
moxiepilot said:
private pilot applicant exchanged the definitions for Angle of Attack and Angle of Incidence, what would the DPE think?
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Given that the aerodynamic literature often uses the terms interchangeably (particularly the older stuff), the DPE would have to be very careful before deciding that the student was confused.
 
tgrayson said:
Pressure is a Newtonian effect....it's the momentum transfer of molecules hitting something.

But, again, it's meaningless to say the high pressure on the bottom is the "driving force" for lift. Did you know that at low AoA, the pressure is negative on BOTH sides of the airfoil? Which then is the true source of lift?
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I heard that too..it was stunning. and the differential wasn't much per my memory.

b.
 
yardman said:
I'm lost. I've never heard or read about Bernouli and Low/High Pressure being associated with Carburetors. Could someone flesh it out some more for me?
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Carbs are built to narrow airflow...thing putting your thumb over a garden hose...
That narrowing is called a Venturi....


http://en.wikipedia.org/wiki/Venturi_effect


As the air goes from the large volume area to the small volume area, it increases speed and loses pressure. (Coincidently that loss in pressure leads to cooling which is why a normally aspirated a.c. has carb heat)


b.
 
tgrayson said:
You're joking, I hope? Lift is produced by a small pressure differences around the airfoil, less than an inch of mercury. Suddenly you have a vacuum and you have almost 30 inches of pressure differential. All the surrounding air would be applying a tremendous force attempting to fill the vacuum. Doesn't happen.

Regardless, the area behind the separated flow is filled with air from the bottom of the airfoil that flows around the trailing edge to fill it.
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Grayson, lets keep it simple. I don't disagree with the airflow going around the trailing edge. However, at the separation point I believe there is a minute vacuum just above the surface. I have not done any research to prove this yet and I strongly use the word "believe" here for now.

What is the force you speak of? The tremendous force attempting to fill the vacuum (which is a force as you say) is applied to what and in what direction?

BTW, Im enjoying the debate.
 
Moxie: I didn't say teach it as a vacuum, I said teach it to the true definition and use a persons known concepts, in this case the idea of a vacuum being a big sucky thing for getting dirt, to explain conceptually what is going on.

Instead of ORLY, would you mind telling me what negative transfer of learning would occur from this approach? I am not trying to be snappy, I would just like to know if there is something here you see that I do not.

As for teaching it right the first time, there aren't four forces but we teach that don't we? If I sat here and worked through it I am sure I could give you 100's of more examples. We teach right enough so that when a student wants to improve their knowledge the new, more complex information, will not interfere with the original idea but instead build off of it. In some cases, like the four forces *cough* three *cough* we sacrifice some truth for simplicity purposes.
 
shdw, give me a day to think of a well thought out response and I will reply regarding the negative transfer. (I am really busy playing stoopid games on Facebook) :) :)
 
splash said:
at the separation point I believe there is a minute vacuum just above the surface.
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I think you may be misconstruing the nature of what "separation" means. There isn't a ripping away of the airflow from the surface of the airfoil, like what happens when you pull apart a piece of Velcro. The separation point is merely where the boundary layer comes to a halt and reverses. What causes it to reverse? Positive pressure, the very opposite of the vacuum that you postulate.

The pressure over the airfoil reaches its minimum point near the front of the airfoil, but the pressure increases steadily after that. This means that the boundary layer slows continually as it loses momentum against the increasing pressure. At some point, its velocity becomes zero. However, the air just above that point still has some momentum, but there is now a gap between the air that's flowing and the airfoil and that's what we mean by "separation".

If you indeed had a vacuum in this area, the boundary layer would not experience an increasing pressure gradient, but rather a negative one, which would accelerate the airflow and keep it attached. In fact, there are devices intended to do just that. There are little vacuum cleaners built into the wing which suck the air out of the area behind the separation point and keep the airflow attached.


The tremendous force attempting to fill the vacuum (which is a force as you say) is applied to what and in what direction?
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Haven't you heard the phrase "nature abhors a vacuum"? A gas will rush to fill an area of lower pressure and a vacuum is the lowest pressure possible. If you get between the air and the vacuum, the force is net difference between the pressure of the gas on one side and the gas on the other side. Since the vacuum offers no pressure at all, the force is at a maximum.
 
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