Aerodynamics - why wings create lift - current vs historical discussions

[rcgldr]

Regardless of the airfoil shape, the separation point is below the leading edge of an airfoil making lift because the lower pressure above draws the air from in front and below the leading edge backwards and upwards to flow over the surface.

It's not clear to me that this is always true, and at low Cl (such as in cruise), the stagnation point is effectively at the leading edge on pretty much any airfoil. Certainly any reasonable airfoil operating at higher Cl though will have the stagnation point well below the leading edge.

I was referring to the separation of flow point, which is well ahead of the stagnation point and below if the wing is generating lift.

m2-f2

Here's a better image, in this case the M2-F3. The AOA is high when landing, but this was a re-entry prototype that reached a max speed of mach 1.6 during tests. It does need a high AOA for landing, but at speeds around 400 knots, AOA and lift to drag ratio would be reasonable, probably similar to the Space Shuttle. In the case of the Space Shuttle, lift to drag is 1:1 at hypersonic speeds, 2:1 at supersonic speeds, 4.5:1 at sub-sonic speeds. These are somewhat low since the reentry to landing time is already 20 minutes, and reentry to landing distance is 5,000 miles. Two big turns in the shape of an S are done to scrub off speed and stay within reach of the landing zone.

 

[cjl]

I was referring to the separation of flow point, which is well ahead of the stagnation point and below if the wing is generating lift.

That isn't a meaningful distinction. The streamline that contains the stagnation point is also (by definition) the "flow separation" line. You're right that (at least for incompressible flow) this streamline will always be upward sloping for a 2D flowfield around an object making lift, so some air that starts below the wing will end up traveling above it.

I would kind of expect there to also always be some upwash in front of something like your M2-F3 example during flight in the incompressible regime, despite the much more complicated 3d flowfield. I just don't want to definitively make a statement that you couldn't have some weird 3d effects that would allow for lift generation without leading upwash. I can't envision how that would work off the top of my head though.

Also, just as an interesting counterexample (though this is way outside of the discussion above), in supersonic flight, the incoming flow will just split exactly where the leading edge is. Anything that starts above the wing stays above, and anything that starts below stays below. This is of course intuitively obvious though, because disturbances can't propagate forwards in a supersonic flow.

 

[russ_watters]

There are cases where the longer distance is on the bottom...

You haven't responded to my request for substantiation with regard to the lifting body, but here's evidence against, using the common example of an inverted airfoil:

As you can see, the stagnation point in both cases is underneath the "chin" thus the air flowing over the top does indeed follow a longer path/gets deflected more than the air flowing over the bottom. In both cases the flow just above the stagnation streamline curves up a lot as it approaches the wing, to flow over the top. And flow under doesn't have as far to bend down to flow over the bottom surface.

...with deference to @cjl 's point that since the shape is very non-uniform spanwise, the flow is going to be very 3 dimensional for a lifting body.

 

[rcgldr]

I was referring to the separation of flow point, which is well ahead of the stagnation point and below if the wing is generating lift.

That isn't a meaningful distinction. The streamline that contains the stagnation point is also (by definition) the "flow separation" line. You're right that (at least for incompressible flow) this streamline will always be upward sloping for a 2D flowfield around an object making lift, so some air that starts below the wing will end up traveling above it.

I should find a better term, since separation of flow normally refers to the detachment of flow over the upper surface of a wing. As for a description, the free stream flow well in front of a wing is horizontal, but the flow approaches the wing, the flow initially curves upwards, but then a bit closer to the leading edge, the streamlines separate into flows that will go over the wing and flows that go below the wing. Both of these transition points in the flow occur ahead of and below the leading edge.

As for the M2-F2 or M2-F3 at sub-sonic but fairly high speed (400 knots), the ratio of ambient pressure / pressure above the lifting body is probably less than the ratio of pressure just below the lifting body / ambient pressure, in otherwords, the pressure below deviates more from ambient than pressure above. This corresponds to a reduced lift to drag ratio (probably around the Space Shuttles 4.5 to 1), but as posted early, the Shuttle takes 20 minutes and 5000 miles from re-entry to landing, so a better lift to drag ratio isn't a goal here. One issue is that both the M2-F2 / M2-F3 / Shuttle have high landing speeds and need long runways to land.

 

[Aeronautic Freek]

Summary:: What is the real cause of lift, said to be true by current aerodynamics

My son and i were discussing aerodynamics and he brought up a paper from https://phys.org/news/2012-01-wings.html It seems that the latest discussions seem to completely discount the differential velocity of air flow as a cause of differential pressure, but point to a differential pressure (pressure gradient) caused by the force acting on the air being accelerated to cause the differential pressure. in other words, some of these papers and i don't know if they are outliers, are insinuating that it is the differential pressure causing the speed variance above and below an airfoil, and not the other way around.
thoughts??

Doug Mclean write one book where he only explain all these tons of false theories and missconceptions in aerodynamics.
Title of book is :Understading Aerodynamics : Arguing from Real Physics..
equal time theory
"bernulli" explanation
"Newtonian" explanation
coanda examples
wingtip vortices cause induced drag
etc
etc
etc
etc
etc
etceven in Aderson books ,there are some missconceptions..(water jet at spoon-test is not example of Coanda effect...)

 

[sophiecentaur]

all these tons of false theorys

OMG. So they're all wrong? Which is the right alternative?

PS I just watched the video. If our local experts in Maths can OK it then it seems that his explanation is fair enough. 'No downward Momentum' was a bit of a surprise for me.

Question is whether the pressure at the ground is high enough to measure for a plane at normal height.

 

[Aeronautic Freek]

OMG. So they're all wrong? Which is the right alternative?

PS I just watched the video. If our local experts in Maths can OK it then it seems that his explanation is fair enough. 'No downward Momentum' was a bit of a surprise for me.

Question is whether the pressure at the ground is high enough to measure for a plane at normal height.

Did you ask Math experts?

 

[sophiecentaur]

Did you ask Math experts?

I meant Maths Experts local to PF. Some PF members are no slouch when it comes to Maths.

 

[A.T.]

'No downward Momentum' was a bit of a surprise for me.

He says that "there is no downward momentum accumulated in the atmosphere" (at 45:10). This should not be surprising. How should the atmosphere as a whole accumulate downward momentum, with the ground in the way?

 

[sophiecentaur]

He says that "there is no downward momentum accumulated in the atmosphere" (at 45:10). This should not be surprising. How should the atmosphere as a whole accumulate downward momentum, with the ground in the way?

Ahh. Like a hamster on a hamster wheel when the wheel is accelerating. So it’s angular momentum?

 

[A.T.]

Ahh. Like a hamster on a hamster wheel when the wheel is accelerating. So it’s angular momentum?

No idea what you mean here.

 

[sophiecentaur]

No idea what you mean here.

The hamster imparts no linear momentum to the wheel but friction helps transfer linear momentum to the ground which allows the hamster to climb up a bit (angular momentum). Short term the hamster can derive ‘lift’ as it accelerates up the slope.
I can see an analogy.

 

[A.T.]

The hamster imparts no linear momentum to the wheel but friction helps transfer linear momentum to the ground which allows the hamster to climb up a bit (angular momentum). Short term the hamster can derive ‘lift’ as it accelerates up the slope.
I can see an analogy.

But a plane in level flight creates symmetrical counter rotating vertices, so their net angular momentum is zero.

 

[sophiecentaur]

But a plane in level flight creates symmetrical counter rotating vertices, so their net angular momentum is zero.

How is that relevant? The hamster could use two counter rotating wheels and my model would work. Also, is there no friction involved in the atmosphere?

 

[A.T.]

The hamster could use two counter rotating wheels and my model would work.

Then there would be no net angular momentum, so what was your point in bringing angular momentum up?

 

[sophiecentaur]

Then there would be no net angular momentum, so what was your point in bringing angular momentum up?

Whether or not I have understood the whole aerodynamics picture I can be pretty sure that the zero "net" angular momentum doesn't mean that the two counter-rotating hamster wheels and their individual angular momenta cannot provide the hamster with some lift. The fact that the net angular momentum is zero is not relevant to the two forces that give the lift to the hamster.

 

[A.T.]

The fact that the net angular momentum is zero is not relevant to the two forces that give the lift to the hamster.

Then how is angular momentum relevant at all here?

 

[sophiecentaur]

Then how is angular momentum relevant at all here?

Did you see the video?

 

[A.T.]

Did you see the video?

Yes

 

[FactChecker]

This video is an interesting and informative summary of the problems with simple explanations. I liked it enough to make a list of topics and times in the video so that people can look up their favorite erronious explanation:

[CODE title="Topics and times in the video"] 1. Outline: 4:23
2. Basic physics: 6:00
3. Lift explanations: 13:55
a) Bernoulli: 15:25
(1) Longer path, equal time: 15:40
(2) Streamline pinching and conservation of mass: 17:45
b) Downward-turning based (Newton): 19:50
(1) Newton's second and third: 20:34
(2) Coanda effect: 21:20
c) Bernoulli-versus-Newton "controversy": 23:00
d) Getting it right: 24:53
4. Vorticity and Biot-Savart: 28:30
a) Tip device drag reduction errors 33:28
5. Lift and momentum in 3D: 41:00
a) Trefftz-plane slice: 41:30
b) Momentum accumulation in the atmosphere: 44:49
6. Boundary-layer separation: 47:03
7. Base drag and Hoerner's jet pump: Ran out of time[/CODE]

 

[FactChecker]

OMG. So they're all wrong? Which is the right alternative?

I think it's better to say that all the simple explanations that people like are wrong or at best grossly incomplete. People do not like to accept the differential equations of fluid dynamics as an explanation. Even if people accepted DEQ, they would require infinite integrals and/or complicated boundary value assumptions.

Question is whether the pressure at the ground is high enough to measure for a plane at normal height.

I don't think that is an important question. Nobody would use that to estimate lift anyway. A model in a wind tunnel is the best way to directly measure lift. The validity that the total force on the ground equals the lift is not in question. The problem with that as a theoretical explanation is that it is like a platitude -- it seems profound, but it adds no insight into how lift works.

 

[Swamp Thing]

This one is pretty interesting and may be of interest to anyone following this thread

 

[cjl]

wingtip vortices cause induced drag

Ehh.. wingtip vortices are definitely related to induced drag. Certainly it's a complex interaction, but I would hesitate to try to separate the two too much.

He says that "there is no downward momentum accumulated in the atmosphere" (at 45:10). This should not be surprising. How should the atmosphere as a whole accumulate downward momentum, with the ground in the way?

There certainly would be accumulated downward momentum if the ground didn't exist though. Behind the aircraft, there absolutely is accumulated downward momentum. The only reason that doesn't happen is because the ground is in the way.

 

[Swamp Thing]

the ground is in the way

It doesn't necessarily have to be the ground, per se. If we think of a mini drone that is hovering several kilometers up, then it is creating a small column of downward air that accounts for the lift. But this column widens and slows down as it moves down. At some point, the undisturbed air around the column and at its base will have a mass much greater than all the air that the drone is causing to move. The mass (inertia) of all that stationary air could play the role of the earth, in that it is heavy enough to account for momentum exchange with a negligible velocity change of its own.

In other words, if the aircraft is high enough, we can reasonably draw a large closed surface that encloses it and say that FAPP, everything that's happening can be accounted for by the air within this surface without considering the existence of the earth. You need the Earth's surface only to account for why the air wasn't falling under gravity in the first place.

 

[cjl]

Negligible, but nonzero. Without something in the way like the ground, that can support a force without gaining momentum of its own, there will be accumulated downward momentum. As the drone hovers for longer and longer (or more and more aircraft fly overhead), the air will gain more and more downward momentum, and eventually you'll end up with non-negligible downward velocity unless you have a hard surface to absorb that downward force.

 

[Swamp Thing]

you'll end up with non-negligible downward velocity unless you have a hard surface to absorb that downward force.

That's hard to argue with. But perhaps that would only be moving the problem along one step, because then the Earth would have to acquire a negligible but non-zero velocity to balance the books. And maybe that is indeed the case, with the Earth and the air having their own little movements but maintaining a common CG that orbits the sun as per usual.

 

[cjl]

Sure, but the downward force eventually placed on the Earth by the downwash is counteracted by the gravitational attraction between the Earth and the plane in the first place.

 

[boneh3ad]

I don't think that is an important question. Nobody would use that to estimate lift anyway. A model in a wind tunnel is the best way to directly measure lift. The validity that the total force on the ground equals the lift is not in question. The problem with that as a theoretical explanation is that it is like a platitude -- it seems profound, but it adds no insight into how lift works.

*wind tunnel engineer slowly nods, grins*

 

[K41]

*wind tunnel engineer slowly nods, grins*

I agree with your point but I would like to add that people must always be aware that experiments are also difficult as well. I feel sometimes experimentalists just aren't appreciated in some parts of engineering.

In general, for example, PIV isn't great for near surface measurements due to the reduction in signal with window size. Multiple point measurements with Laser Doppler is possible but will take time and are complicated by the necessity of needing a kinetic stage to move the measurement volume accurately. In both cases you also need a method to introduce particles or droplets homogeneously in the air flow.

Using particles means that some particles will deposit on the surface of your model gradually over time, so you have to continually replace your model. You can use droplets but risk the droplets evaporating, particularly in high speed flows, so you may need to add things like glycol to the water to reduce evaporation - again more complexity.

In some cases you will need to ensure the upstream flow is uniform - which means you probably need a very long test section or a very small model. I also imagine small aerofoil models have problems with specifying the surface roughness in some cases, which I imagine makes studying transitional behaviour particularly challenging.

3D measurements are possible using PIV and Laser Doppler, but I imagine applying them to a wind tunnel, whilst possible, is not easy. This makes it difficult to study lift formation, particularly the "starting vortex" at the rear end of the aerofoil.

Big up to people like yourself who have to overcome these challenges and I am sure, many more!

 

[boneh3ad]

@K41

This is why I:

• build big tunnels,
• don't use particle seeding methods like PIV/LDV,
• work with supersonic flows, and
• get my graduate students to do all the tedious parts.