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Old Aug 23, 2012, 11:29 PM
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Old Aug 24, 2012, 06:30 AM
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Originally Posted by Tim Green View Post
The fact that the airflow is the same, in this one instance, is a coincidence of position
So the airflow is the same at every point in the air for a helicopter hovering in ground effect and a helicopter tethered at the same location with the same rotor RPM and collective. One can actually calculate the rate of momentum transfer if you know the velocity at every point in the airflow. If the airflow is the same in both cases, it should be evident that the calculated rate of momentum transfer will also be the same. Yet you are trying to suggest that in one case there is net momentum transfer and in the other case there is not. I would say the "coincidence" you refer to is a rather inconvenient coincidence for your position. Understaaaaaand the physics.
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Old Aug 24, 2012, 08:43 AM
Grad student in aeronautics
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I love it!

Understaaaannnddd the physics!


Steve, good find. Anyone that shrugs something off because it uses "complex mathematics" gives me serious doubts. Also, they didn't have computers in the 20's and 30's.
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Old Aug 24, 2012, 01:19 PM
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Old Aug 24, 2012, 01:27 PM
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Originally Posted by Tim Green View Post
Hey,

Did you guys happen to notice, that NASA took the Bernoulli explanation off of their kiddie page, as well as the adult portions of their web sites?
The NASA web page has an "adult" section? How did I miss that?
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Old Aug 24, 2012, 02:33 PM
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Old Aug 24, 2012, 03:12 PM
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Originally Posted by Steve Anderson View Post
I found this in two minutes. Apparently you just need to have better than K grade intelligence to find and UNDERSTAND IT. Again, note the first sentence “The part of the theory about Bernoulli's equation and a difference in pressure existing across the airfoil is correct.”


http://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html

• The part of the theory about Bernoulli's equation and a difference in pressure existing across the airfoil is correct. In fact, this theory is very appealing because there are parts of the theory that are correct. In our discussions on pressure-area integration to determine the force on a body immersed in a fluid, we mentioned that if we knew the velocity, we could obtain the pressure and determine the force. The problem with the "Venturi" theory is that it attempts to provide us with the velocity based on an incorrect assumption (the constriction of the flow produces the velocity field). We can calculate a velocity based on this assumption, and use Bernoulli's equation to compute the pressure, and perform the pressure-area calculation and the answer we get does not agree with the lift that we measure for a given airfoil.
I agree with this too. And I agree with NASA when they say, over and over, that lift is due to turning a fluid, air, downward.

Bernoulli's theorem's are correct. They just aren't the cause of lift. Quit taking only what you want, from NASA - take it all - and you'll be smarter, maybe.

BTW - apparently you missed the link, on the first sentence on that page, to the Newtonian explanation of lift.
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Old Aug 24, 2012, 03:16 PM
Launch the drones ...
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You guys are tiring me out, with your tag teaming - got to take a break for a bit. You guys try devouring each other for a while, as you were before I returned to this thread so recently.

And here's that NASA link to their explanation of lift ...

http://www.grc.nasa.gov/WWW/K-12/airplane/lift1.html
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Old Aug 24, 2012, 03:24 PM
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Originally Posted by Tim Green View Post
You guys are tiring me out, with your tag teaming - got to take a break for a bit.
Enjoy the break. Hope you learn some Physics while you are away.

Turning air downward locally does NOT imply that you are adding net downward momentum to the air.
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Old Aug 24, 2012, 04:50 PM
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Originally Posted by Tim Green View Post
Quit taking only what you want, from NASA - take it all - and you'll be smarter, maybe.
You might try taking your own advice.

J.P.
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Old Aug 25, 2012, 10:40 AM
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Originally Posted by ShoeDLG View Post
Enjoy the break. Hope you learn some Physics while you are away.

Turning air downward locally does NOT imply that you are adding net downward momentum to the air.
Doesn't matter what happens to the air after it exits the system. But how the air exits the system is everything. And for a chopper without a plate (a system), the air leaves the system with a ton of downward momentum. So the system lifts, in response.

This is reality. Unfortunate for some, on this thread.

Goodness gracious, I love this - haven't had this much fun in ages.
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Old Aug 25, 2012, 12:34 PM
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Naw- the efficiency simply changes -
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Old Aug 25, 2012, 09:05 PM
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no where on the nasa, What is Lift? page is there any mention of downwash. Instead, is says the air is turned. But even the Nasa Lift from Flow Turning isn't very specific about what turning means.

can someone, Shoe, explain turning better?

greg
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Old Aug 25, 2012, 09:40 PM
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Turn ----
Big TV alert
the air always turns somewhat when disturbed
wht's the big secret?
none
Having read thru a recap of countless assessments of what causes lift and what are the byproducts resulting from a body causing pressure differences - I find it all quite amusing
Ferreting out the obvious seems to be the goal for a number of writers .
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Old Aug 25, 2012, 09:43 PM
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Originally Posted by Tim Green View Post
Doesn't matter what happens to the air after it exits the system. But how the air exits the system is everything. And for a chopper without a plate (a system), the air leaves the system with a ton of downward momentum. So the system lifts, in response.
You are restricting your attention to a subset of the air influenced by the helicopter (the air within the "system"), and shifting your definition of the system to suit the conclusion you wish to reach.

In both of the cases discussed above, there is a plate. In one case the helicopter is tethered to the plate, in the other case the helicopter is hovering at the same location above it. You agree that the airflow in both cases is exactly the same, yet in one case you see the downward momentum of the air changing, and in the other case you see it staying the same.

This accounting is manifestly sloppy, and provides no basis for drawing any connection between the aerodynamic forces on the helicopter and downward momentum transferred to the air.

The basis of the disagreement in this "discussion" is very clear to me. You read a book that offered an alternative, simplified explanation of lift that didn't require you to grasp any complexity ("wings shove air down"). You now believe that this explanation should replace the idea that a rigorous investigation of the wing's interaction with the air may require a (sometimes complicated) set of conservation laws or "field equations".

You are emboldened by the author's suggestion that facility with analysis tools does not ensure understanding: "to be able to calculate something is not the same as understanding it". This is a true statement. However, just because you are able to compute something doesn't mean you don't understand it. There is also a very important converse of this statement that applies here: "calculating something incorrectly is a sure sign that you don't understand it." A well-known researcher in Computational Fluid Dynamics (Robert MacCormack if you want to add him to your list of accomplished aerodynamicists who are confused about lift) used to tell us: "you don't truly understand something until you can simulate it." Over the years I have come to appreciate the truth in this statement. Trying to simulate a physical process accurately over a range of conditions forces you to come to terms with details you inevitably miss through less deliberate "cogitation".

Your inability to reconcile the fact that the rate of momentum exchange is the same in the tethered and untethered cases should give you cause to reexamine your analysis. I have been around long enough to know that it will just make you more vociferous in your theology.

I have provided an accounting showing that the rate at which a wing transfers downward momentum to the air is not (in general) equal to the lift, but instead depends on the boundaries of the air the wing is flying through (post #420 in this thread). Neither you, nor anyone else here has shown that this accounting is flawed. Please, provide an accounting (other than "look at how this air right here is moving") that shows that my analysis is incorrect.
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Last edited by ShoeDLG; Aug 30, 2012 at 09:47 PM. Reason: added request at end
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