NASA says lift from air shoved down - can we trust NASA? - Page 2 - RC Groups
Nov 14, 2011, 12:44 PM
Sink stinks
Quote:
 Originally Posted by Tim Green Text book theory ... That the air over the top has farther to travel due to top surface being longer than bottom surface, so it speeds up before meeting at trailing edge, creating less pressure on top - voila - lift.
That is a common misconception that has nothing to do with Bernoulli. The only part relating to Bernoulli is the correlation of high speed - low pressure and vice versa.
 Nov 14, 2011, 03:03 PM An itch?. Scratch build. Re the plate and helicopter. Try fitting something like a CD disc behind the prop of a model plane and see if it still travels forward. There are plenty of planes with very large radial cowls that seem to fly quite well.
 Nov 14, 2011, 03:57 PM Cognitive dissonance ... Last edited by kcaldwel; Jan 27, 2012 at 06:10 PM.
 Nov 14, 2011, 04:10 PM Grad student in aeronautics Beautiful
 Nov 14, 2011, 04:44 PM Registered User Many have been taught what we may call the ”popular description of lift”, which is basically focused on the shape of the airfoil. The key point of this “popular” description of lift is that air accelerates over the top of the wing and because of the Bernoulli effect (which relates the speed of air to the static pressure) a reduced static pressure is produced above the wing thereby creating lift… This sounds convincing but there’s a missing part in this description that fails to explain why the air accelerates over the top of the wing. If the description based on the shape of the airfoil was the full story how could airplanes fly inverted? Actually, the shape of the airfoil is one of the least signinficant features when understanding lift. The principles of lift are the same for a wing flying right side up or in inverted flight, for a cambered wing, for a flat plate or for a barn door… One might also ask, if it was just the reduced pressure above the wing that caused lift, how fast must a Cessna 172 fly to lift its weight of 2300lb (1045kg)? Since the path length for the air over the top of the Cessna’s wing is just 1.5% greater than the length under the wing, using the “popular” description of lift the wing would only develop lift of about 2% of the Cessna’s weight at 65 m/h (104 km/h), which is slow flight for this plane. So, something must be missing with the “popular” explanation of lift. We’ve been led to assume that if the air is flowing faster its static pressure has been lowered, but this of course is not necessarily so. For instance, an airplane has a small port somewhere on its side of fuselage where static pressure is measured by instruments in order to show the altitude. If the static pressure changed by the air flowing, then indicated altitude would change as the airspeed changed by the propellers blowing air when the engines started … but it doesn’t. Thus, the fact that the air is moving faster doesn’t necessarily mean that the static pressure has decreased. So, if it’s not the air flowing faster that creates the low pressure over the wing, what causes the low pressure then? And it’s well known that the air always flows away from areas of higher pressure toward areas of lower pressure, thus the air over the wing top accelerates as it enters the lower pressure region (where the air curves toward the wing), whereas the air under the wing slows down as it enters the higher pressure region. So, one may say that the wings create lift by reacting against the air flow, driving it downwards, producing downwash. Last edited by funfly2; Nov 14, 2011 at 05:12 PM.
Nov 14, 2011, 06:07 PM
Registered User
Please note also how far back the stagnation point is at high angles of attack, and how on a portion of the airfoil the air actually travels in the opposite direction of the wind tunnel airstream, so strong is the pressure difference between the two surfaces of the profile

Quote:
 Originally Posted by kcaldwel A nice videos of wind tunnel test showing the difference in speed over the top and bottom, and how separation of the top surface flow causes loss of lift: http://www.youtube.com/watch?v=6UlsArvbTeo *snip*
Nov 14, 2011, 06:11 PM
Grumpy old git.. Who me?
Tim,
As already pointed out, NASA did not say lift is the result of "shoving air down".. They said it's the result of "turning air", not the same thing at all. Fact is that the air in front of a wing is turned upward then as it passes the wing it's turned downward, and then (in 2D flow) it turns again to level out to the same height as it was before being disturbed by the wing in the first place, no net downwash.

You can quite clearly see this if you follow the streamlines in NASA's 'Foilsim' 2D flow simulation program (taken from the same site where you got your miss-quote): http://www.grc.nasa.gov/WWW/k-12/airplane/foil2.html (also see attached screen shot)

3D flow would look a little different, you would have some downwash in the wake of the wing, the amount of downwash being dependant on Cl and aspect ratio. This downwash would be matched by upwash around the tips (again no net downwash)

### Images

Nov 14, 2011, 06:36 PM
Registered User
Quick question, just 'cause I am really curious now:

Quote:
 The vacuum comes first. Then the air speeds up
Quote:
 the air over the wing top accelerates as it enters the lower pressure region
Soooo, what is causing the "vaccuum" or lower pressure area then? Is it just magically there on its own?

Please do answer, I really would like to know how that works.

Thanks.
Nov 14, 2011, 06:43 PM
Launch the drones ...
Quote:
 Originally Posted by BMatthews 3) Back to your blocked heli flow. OK, it's blocked because there's a plate attached. But how is this plate different from the ground for a heli that is taking off? Simply it isn't.
Wrong, and the experiment shows it to be wrong. Look at the chopper with the plate not folded (no ground), and the plate folded (has ground). One way the chopper lifts, the other way it doesn't.

This clearly proves that the plate is different from the ground.

Until you admit what the experiment shows, you are blowing in the wind.
Nov 14, 2011, 06:46 PM
Launch the drones ...
Quote:
 Originally Posted by stardustertoo Quick question, just 'cause I am really curious now: Soooo, what is causing the "vaccuum" or lower pressure area then? Is it just magically there on its own? Please do answer, I really would like to know how that works. Thanks.
Common sense dictates that if you move into the wind, a low pressure area (a vacuum, if you will) is created behind the object moving into the wing. Same with a wing, tilted up. Do you understand now?
Nov 14, 2011, 06:48 PM
Launch the drones ...
Quote:
 Originally Posted by JetPlaneFlyer Tim, As already pointed out, NASA did not say lift is the result of "shoving air down".. They said it's the result of "turning air", not the same thing at all. Fact is that the air in front of a wing is turned upward then as it passes the wing it's turned downward, and then (in 2D flow) it turns again to level out to the same height as it was before being disturbed by the wing in the first place, no net downwash. You can quite clearly see this if you follow the streamlines in NASA's 'Foilsim' 2D flow simulation program (taken from the same site where you got your miss-quote): http://www.grc.nasa.gov/WWW/k-12/airplane/foil2.html (also see attached screen shot) 3D flow would look a little different, you would have some downwash in the wake of the wing, the amount of downwash being dependant on Cl and aspect ratio. This downwash would be matched by upwash around the tips (again no net downwash)
Anyone who thinks there's as much downwash as upwash in a chopper is simply ignoring reality. Please stand in front of a fan, then stand behind it, and tell me about the net downwash.
Nov 14, 2011, 06:52 PM
Launch the drones ...
Quote:
 Originally Posted by eflightray Re the plate and helicopter. Try fitting something like a CD disc behind the prop of a model plane and see if it still travels forward. There are plenty of planes with very large radial cowls that seem to fly quite well.
Tried it using a plate - they don't move forward. BTW, radial cowls aren't large enough to block all the air from the props.
Last edited by Tim Green; Nov 14, 2011 at 06:59 PM.
Nov 14, 2011, 06:53 PM
Launch the drones ...
Quote:
 Originally Posted by ciurpita is the airspeed across either top or bottom surfaces of the wing constant or are there sections where it is significantly greater than the average airspeed across the surface?
Good question. I don't know all the answers. What I do know is that the air below the wing slows somewhat, and that the air above accelerates - quite a bit. But I don't know the numbers.
Nov 14, 2011, 07:42 PM
Registered User
Quote:
 Common sense dictates that if you move into the wind, a low pressure area (a vacuum, if you will) is created behind the object moving into the wing. Same with a wing, tilted up. Do you understand now?
Hmm, interesting idea. I see where you are getting that when you take into account what we can see with non-aerodynamic objects, like say a 2X4. I would like to explore this a bit further with you if I might, I want to hear more about this. It is really intriguing me.

So, that does raise another question though.

With this theory in mind, can you explain how a Laminar flow airfoil is working?? If it is one of the true laminar flow foils, it should not have said "vacuum", especially if it is...say...in a slow descent to a lower altitude (reduced angle of attack).

Actually, while I am thinking of it, a Clark Y has a zero lift angle of something like NEGATIVE 4-5 degrees AoA (from a NASA paper on testing the Clark Y), so anything greater than that it will lift, which doesn't really fit well with your theory, can you explain that for me??
Last edited by stardustertoo; Nov 14, 2011 at 07:53 PM.
 Nov 14, 2011, 08:01 PM Registered User Hey, just thought of one other thing that I am curious about in regards to your vacuum behind the wing idea Tim. Wind tunnel testing has shown that the low pressure area also extends some way ahead of the leading edge. To me this would shoot your idea down in flames, but maybe you have a reason for that. Like I said, I really am truly curious about this theory. Never heard it before so I want to hear more about it.