Feb 26, 2012, 11:12 AM
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Discussion

# On the largest scale there is usually no downwash

Decided to start a new thread as the old one ( "NASA says lift from air shoved down - can we trust NASA" https://www.rcgroups.com/forums/show....php?t=1539175 ) seemed to be devolving...

I want this thread to specifically be not about Bernoulli vs downwash, but rather about whether there is always a downwash or not, or whether there is in fact an upwash in many cases, due to conservation of momentum--

EDIT 2/ 29-- the content from here through the three following posts represents a thought process in progress. To see the conclusion summarized in a more digestable form please go directly to post #12 ! xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx xxxxxxxxxxxxx
END EDIT

We'll grant that Bernoulli has his rightful place and stop bothering him--

Quote:
 Originally Posted by Crossplot For those that have an open mind and are interested. -- The Bernoulli EQUATION works just fine when associated with the centripetal acceleration of turning flow. The flow relative to the surface is not the relationship required for Bernoulli principle calculations. Refer back in this thread for posts 317, 353, 421, 426, 454.
Now backing off to a larger scale--

Consider this:

"Formally stated, Newton's third law is:

For every action, there is an equal and opposite reaction.
The statement means that in every interaction, there is a pair of forces acting on the two interacting objects. The size of the forces on the first object equals the size of the force on the second object. The direction of the force on the first object is opposite to the direction of the force on the second object. Forces always come in pairs - equal and opposite action-reaction force pairs."

( http://www.physicsclassroom.com/clas...laws/u2l4a.cfm )

So the wing exerts a downward force on the air and the air exerts an upward force on the wing.

However, does this really impose a requirement that there be a downwash? Just because the air experiences a downward force, does this mean that it must move down?

Argument: Whether or not there is a downwash, depends on what scale we look at. If look at the trailing edge of the wing, then of course there is a downwash. If we are in ground effect, we only have to expand our field of view a few meters to see that on this scale there is NO net downwash-- the air is not penetrating the ground. If we back off far enough, there will always be no net downwash, unless we maintain that the wing is in fact creating a net downward movement of the entire atmosphere, which seems ridiculous.

Counterargument-- this is not necessarily ridiculous-- is the entire atmosphere, and the whole earth with it, being pushed down some infinitesimal amount? On the planetary scale, as the aircraft flies above the ground, wouldn't the mass of the planetary system would be moved to a different point in space, unless the earth was displaced in the opposite direction?

Question: does Newton's third law say that each force is accompanied by an equal and opposite force, or that momentum imparted by one body to another is accompanied by an equal and opposite momentum imparted to the other body in the opposite direction, or both? What happens when something gets in the way and prevents the expected movement? (Well, then we just have another momentum transfer, to the obstructing body, as hinted above.)

What Newton had to say:
"Newton's third law

If a body impinges upon another, and by its force changes the motion of the other, that body also (because of the equality of the mutual pressure) will undergo an equal change, in its own motion, toward the contrary part. The changes made by these actions are equal, not in the velocities but in the motions [momentums] of the bodies; that is to say, if the bodies are not hindered by any other impediments."

(http://en.wikipedia.org/wiki/Newton's_laws_of_motion)

But there's more:

"Newton used the third law to derive the law of conservation of momentum;[32] however from a deeper perspective, conservation of momentum is the more fundamental idea (derived via Noether's theorem from Galilean invariance), and holds in cases where Newton's third law appears to fail, for instance when force fields as well as particles carry momentum, and in quantum mechanics."

(http://en.wikipedia.org/wiki/Newton's_laws_of_motion)

So at the end of the day we have to have conservation of momentum.

It appears to me that this means that-- on the largest scale--

1) If the aircraft is climbing, then the atmosphere and earth must be descending (there is a downwash)

2) If the aircraft is not climbing, then the atmosphere and earth must not be descending (there is not a downwash).

3) If the aircraft is descending, then the atmosphere and earth must be rising (there is an upwash).

Thoughts?

PS the downwash / upwash etc doesn't necessarily come from the wing per se-- consider also the propulsion system-- consider also the fact that a glider is always descending relative to the atmosphere, yet may climb in thermals or ridge lift, in which case the wing is creating a net downwash of the atmosphere by impeding the rising action of the thermal or ridge lift?

How is the upwash created during a descent? Think about-- air is forced downward relative to the line of the flight path through the airmass-- but still upward relative to the ground?

How can we explain in more detail the processes that cause there to be a net downwash when the aircraft is climbing, but not in level flight, and a net upwash when the aircraft is descending?

How can we explain the fact that the net aerodynamic force exerted by the atmosphere on the aircraft is unquestionably upward in most cases -- after all, gravity must be opposed by something, unless we are accelerating downward at a rate of 32 f/s/s -- so Newton says the aircraft must be exerting a downward force on the air, yet conservation of momentum says that for a given closed system (chunk of airspace, or entire earth and atmosphere), as the aircraft noses down to begin a descent, something (the air, the atmosphere plus the earth) must start moving upward for momentum to be conserved....

I'm at a loss as to the explanation for this...

PPS Just to be clear, when I say "the atmosphere and earth must be rising", I don't simply mean that is valid to use the aircraft's reference frame rather than the earth's reference frame. Rather, I mean that relative to some external reference frame-- say, an inertial reference frame established on the ground before the aircraft took off-- the atmosphere and earth must be displacing downward at some infinitesimal rate as the aircraft is climbing, and must be displacing upward at some infinitesimal rate as the aircraft is descending, so that the momentum of the whole system is conserved.

Steve

EDIT-- I thought of the solution. Not everything in the above post is quite correct after all. Can you think of the solution before reading more? What is the missing factor that we have not considered? End edit.
Last edited by aeronaut999; Feb 29, 2012 at 11:52 AM.
Feb 26, 2012, 12:38 PM
Registered User

# Solution-- yes there is a downwash on the local scale

Quote:
 Originally Posted by aeronaut999 So at the end of the day we have to have conservation of momentum. It appears to me that this means that-- on the largest scale-- 1) If the aircraft is climbing, then the atmosphere and earth must be descending (there is a downwash) 2) If the aircraft is not climbing, then the atmosphere and earth must not be descending (there is not a downwash). 3) If the aircraft is descending, then the atmosphere and earth must be rising (there is an upwash). Thoughts? PS the downwash / upwash etc doesn't necessarily come from the wing per se-- consider also the propulsion system-- consider also the fact that a glider is always descending relative to the atmosphere, yet may climb in thermals or ridge lift, in which case the wing is creating a net downwash of the atmosphere by impeding the rising action of the thermal or ridge lift? How is the upwash created during a descent? Think about-- air is forced downward relative to the line of the flight path through the airmass-- but still upward relative to the ground? How can we explain in more detail the processes that cause there to be a net downwash when the aircraft is climbing, but not in level flight, and a net upwash when the aircraft is descending? How can we explain the fact that the net aerodynamic force exerted by the atmosphere on the aircraft is unquestionably upward in most cases -- after all, gravity must be opposed by something, unless we are accelerating downward at a rate of 32 f/s/s -- so Newton says the aircraft must be exerting a downward force on the air, yet conservation of momentum says that for a given closed system (chunk of airspace, or entire earth and atmosphere), as the aircraft noses down to begin a descent, something (the air, the atmosphere plus the earth) must start moving upward for momentum to be conserved....
Consider something simpler-- like someone jumping up into the air.

It's not true that the moment they start descending, the earth starts rising.

At the moment of the jump, the person pushes the earth down, imparting a downward momentum to the earth, as the earth pushes the jumper up.

Yet when impacting again after the jump, again the jumper appears to push the earth down, as the earth exerts an upward force on the person to stop the descent.

So downward momentum appears to be transferred to the earth in both cases.

How can momentum be conserved?

Something is seriously missing from our calculations.

This is your last chance to solve the riddle, can you think what is missing before reading on?

Don't peek!

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Last edited by aeronaut999; Feb 26, 2012 at 12:47 PM.
Feb 26, 2012, 12:43 PM
Registered User

# Solution-- yes there is a downwash on the local scale (pt 2)

(Continued from pt 1)

Consider-- when the jumper is in the air, his gravitational force is pulling the earth upwards. Of course, this is true also when the jumper is not in the air, but his feet prevent the earth from moving upwards-- his feet are exerting an equal and opposite downward force on the earth. This is no longer true when the jumper is in the air. The upward acceleration of the earth in response to the jumper's gravitational pull, when the jumper is in air, is the missing factor in our calculations.

The same thing needs to be considered in regard to flight. The plane's gravitational attraction tends to pull the earth upward towards the plane whenever the plane's wheels are off the ground.

The wing always imparts a downward movement, as well as a downward force, to the local atmosphere. (Out of ground effect)

In level flight, the reason that this does not impart a downward movement to the entire global atmosphere, is that the ground is in the way. The reason that the ground (i.e. the planet) doesn't absorb the downward momentum of the downwash and start moving downwards at some infinitesimal rate, is that the plane's gravity is pulling the earth upward while the plane is in the air. So the net movement of the earth (and the atmosphere) is zero. On the global scale, there is no downwash in constant-altitude flight.

In climbing flight, as the plane's potential energy (height) is being increased, the net movement of the atmosphere (and earth) must in fact be slightly downward, so there is a global-scale downwash. In level flight, the net movement of the atmosphere (and earth) is neutral. In descending flight, as the plane's potential energy (height) is being decreased, the net movement of the atmosphere (and earth) must be upward. The case could be made more fully with some more math.

In ground effect, the case is a little more complex-- the wing still imparts a downward force to the atmosphere, but the downward motion is transferred to (blocked by) the earth, so we don't actually have net downward motion of the atmosphere even on a local scale (local being now defined as large enough to include the ground). If we are maintaining constant altitude in ground effect, the downward acceleration of the ground (earth) due to the momentum imparted by the downwash hitting it, is cancelled out by the upward acceleration of the ground (earth) due to the plane's gravitational pull on the earth.

Actually that's the same as we said above-- so really, all the same is true whether we are in ground effect or not. It's all a matter of scale. Locally, the wing always creates a downwash, defined as a downward momentum transfer to the air. The wing actually moves the air downwards. Globally, in constant-alititude flight, there is never a downwash, because the downwash is blocked by the ground. The ground would start moving downward, allowing the downward momentum of the downwash to continue in the form of an infinitesimal downward movement of the the whole earth, except for the fact that the aircraft's gravitational pull is pulling the earth upward just as hard as the downwash is pushing the earth downward.

Conservation of momentum shows that the balance of forces is slightly different in a climb-- where there is in fact a planetary-scale downwash and the earth is pushed every-so-slightly downward as the aircraft climbs and stores potential energy (height). Likewise when the aircraft is descending and releasing kinetic energy (height), there is a still a local-scale downwash, but the global atmosphere (and the earth) are actually rising ever-so-slightly in a planetary-scale upwash.

So on the local scale--meaning any scale that does not consider the interaction between the aircraft and the planetary surface, and the gravitational pull the aircraft exerts on the earth, and the way that the earth can move upward in response to this pull whenever the aircraft's wheels are off the ground -- yes an aircraft does always create a downwash in the creation of lift!

And on the global scale, there is not a downwash in constant-altitude flight-- the atmosphere as a whole (and the earth) is not moving downward. The wing is imparting no net downward movement to the air, because the air is blocked by the ground, and aircraft's gravity is pulling upward on the ground just as hard as the aircraft's downwash is pushing down on the ground, so the ground does not move downward and the downwash must end.

The reason that ground effect appears to present a paradox is that on this scale, we often consider the downward momentum transferred from the wing to the downwash to the ground to the planet, but fail to consider the upward pull of the plane's gravity on the earth. Therefore momentum appears to not be conserved.

Think about what we are really saying here--

In constant altitude-flight, the wing creates a local downwash as it creates lift, but the net action of the whole aircraft on the whole airmass creates no downwash. Because the aircraft's gravity pulls up on the ground, and therefore pulls up on the atmosphere, exactly as hard as the wing pushes down on the atmosphere. So you have a localized downwash at the wing, but zero net momentum transfer to the atmosphere.

Steve
Last edited by aeronaut999; Feb 26, 2012 at 05:10 PM.
Feb 26, 2012, 01:12 PM
Registered User

# Mechanics

Quote:
 Originally Posted by aeronaut999 ...It's all a matter of scale. Locally, the wing always creates a downwash, defined as a downward momentum transfer to the air. The wing actually moves the air downwards. Globally, in constant-alititude flight, there is never a downwash, because the downwash is blocked by the ground. The ground would start moving downward, allowing the downward momentum of the downwash to continue in the form of an infinitesimal downward movement of the the whole earth, except for the fact that the aircraft's gravitational pull is pulling the earth upward just as hard as the downwash is pushing the earth downward. Conservation of momentum shows that the balance of forces is slightly different in a climb-- where there is in fact a planetary-scale downwash and the earth is pushed every-so-slightly downward as the aircraft climbs and stores potential energy (height). Likewise when the aircraft is descending and releasing kinetic energy (height), there is a still a local-scale downwash, but the global atmosphere (and the earth) are actually rising ever-so-slightly in a planetary-scale upwash.
Detailed mechanisms-- in a descending glide, lift is slightly less than in level flight (because drag supports some of the aircraft weight.) This decreases the local downwash enough to allow the planetary-scale upwash. The gravitational attraction between the aircraft and planet has not substantially changed, but the downwash has decreased, so the planet can rise.

In a powered climb, again lift is slightly less than in level flight (because thrust supports some of the aircraft weight.) However, the powered climb is accompanied by thrust which involves some sort of propwash, jet wash, or rocket blast that transfers downward momentum to the ground (planet), so on the planetary scale there is a downwash (planet and atmosphere move infinitesimally downward as aircraft moves upward.)

Soaring (unpowered) climb in updraft-- by impinging on the thermal or ridge lift, the planetary circulation is upset in a way that pushes the atmosphere and earth infinitesimally downward as the aircraft climbs?

Soaring (unpowered) climb in downdraft-- ??

Etc

Steve
 Feb 27, 2012, 06:18 PM Registered User I apologize for dropping out of reading that early on. From the surface of the earth, there is air + plane above it. Average air pressure at ground increased by the distributed mass of the plane. Gerald
Feb 28, 2012, 11:04 AM
Registered User

# pressure

Quote:
 Originally Posted by G_T I apologize for dropping out of reading that early on. From the surface of the earth, there is air + plane above it. Average air pressure at ground increased by the distributed mass of the plane. Gerald
I was specifically interested in exploring conservation of momentum which this is not-- maybe should be a new thread-- but--

Sea level pressure 60" mecury

If you made a 747- shaped outline on the ground, how many inches of mercury would you have to pour into it to equal the weight of a 747?

Do I feel that increase when a 747 flies over my head? (How "distributed" is the weight?)

Is it different if I am standing on the runway as it takes off, or if it is at 30,000'?

Steve
Feb 28, 2012, 08:40 PM
Registered User
Quote:
 Originally Posted by BMatthews It would be interesting to take a basic weather barometer out to this landing path and see if it bumps at all as the same jet passes overhead. Considering that such barometers are easily able to measure down to ouces of difference per sq inch if the "column of air" idea is valid then I should be able to see some needle bumping as such a plane passes overhead.
In free air, the overpressure directly below any lifting object is
delta(p) = L / (4 pi r^2)
where L is the lift and r is the distance below the object. On the ground the overpressures from both the object and its image are superimposed, so on the ground the overpressure is twice as big as given above:
delta(p) = L / (2 pi r^2)

Both of these are "asymptotic" formulas, strictly valid only if r is significantly larger than the object's wingspan. In practice, 2x larger is more than enough for the above delta(p) to be very accurate.
Feb 28, 2012, 09:05 PM
Ascended Master
Standing at the Middle Marker for the runway 25 approach at Palmdale, the normal altitude of a plane on glideslope is 250'.
While there's no direct effect felt as a 747/L-1011/DC-10 passes overhead, the downwash that impacts the ground about the time the plane is touching down is both audible and physical.
The swirling of grass and dust on the ground is obvious.
Numerous "low pass" videos on the 'net show the effect on the ground of the downwash from fighters, especially.
At Mudd Lake, October 1977, Darryl Greenamyer's low altitude speed record flights we measured the sound pressures of the sonic boom at 600' off the centerline, with him flying at 50', Mach 1.28.
And after the plane passed, it raised a dust cloud some distance behind it.

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 Feb 28, 2012, 11:44 PM B for Bruce Sparky, with the big boys standing at the fence I was able to hear an odd ripping sort of sound about 15 to 20 seconds after thay had passed overhead. I assumed it was the tip vorticies growing and meeting and interacting. The grass around that area is all mowed and so I wasn't able to see anything in terms of waving despite it being a calm evening. Mind you if it was something that could ripple even tall grass I'd expect to feel it. I may be off in my estimate of the altitude as well. I do know that they looked DARN big and I could see individual hydraulic lines and other details inside the wheel well quite easily. Mark, thanks for chiming in. I'm assuming that since the final delta(p) would be in psi that L is the pressure in psi that I was suggesting. Otherwise the units to the whole equation become rather odd. If so then a "pulse" of even 2 psi variation as the plane passes overhead is going to be a mere 2/(2x3.14x200^2)... er... I don't know if the altitude should be in feet, yards or furlongs. Either way it would be small. Hardly more than a gnat's sneeze.
Feb 29, 2012, 12:23 AM
Registered User
Quote:
 Originally Posted by BMatthews Mark, thanks for chiming in. I'm assuming that since the final delta(p) would be in psi that L is the pressure in psi that I was suggesting. Otherwise the units to the whole equation become rather odd. If so then a "pulse" of even 2 psi variation as the plane passes overhead is going to be a mere 2/(2x3.14x200^2)... er... I don't know if the altitude should be in feet, yards or furlongs. Either way it would be small. Hardly more than a gnat's sneeze.
Pick whatever units you want. If L is in lbs and r is in inches, then delta(p) is in lb/in^2, or psi.
Feb 29, 2012, 11:19 AM
Registered User
Quote:
 Originally Posted by BMatthews Steve, I've tried to read through your three first posts in this thread and frankly you jump around with too many ideas to keep track of. And then you say in your last post that you wanted this to be about conservation of momentum. I'm afraid that from where I'm reading you broke your own goal early on. Or at least your ideas wandered around too much for me to follow.
Sorry about that, I'll post a summary below.

Quote:
 Originally Posted by BMatthews One point I did see was your bit about the man jumping away from the earth. You seemed to miss on that. When he jumps up he'll push the earth downwards in some infinitesimal amount. Law of action and reaction. Gravity whill act as a spring between the two to bring them back together.
Yes, the jumper's gravitational pull on the earth is critical to understanding how momentum can be conserved, and it is what I was missing in the first post on the thread. (PS note that we are expending energy in the jumper's muscles-- but still we expect momentum to be conserved as far as the interaction between the jumper and the earth is concerned.)

Summary--

If an airplane is flying horizontally and the engine is being used only to produce horizontal thrust, then momentum should be conserved in the vertical plane. In fact, even with a hovering helicopter where the engine is being used to produce vertical thrust, I believe that we should still expect momentum to be conserved in the vertical plane. And likewise when an airplane climbs and descends.

If momentum is conserved, the net upward or downward momentum of the whole system must remain at zero, from when the aircraft is on the ground, through takeoff, climb, constant-altitude flight, and landing.

But if an aircraft is continually producing a downwash, then it seems we have a problem. Take the simple case of constant-altitude flight. Is the ground some sort of "immovable object" that is immune to the laws of physics and isn't accelerated downward even the slightest bit by the momentum of the downwash? In other words, is the momentum of the downwash NOT transferred to the ground? How can that be? On the other hand, if the momentum of the downwash IS transferred to the ground, then we are continually moving (accelerating?) the earth downwards the whole time we are flying, which would appear to violate the law of conservation of momentum.

The solution to this problem, is that the reason the ground does not accelerate downwards as it absorbs the momentum of the downwash, is that the plane's gravity is pulling up on the ground just as hard as the downwash is pushing down on the ground. So the net vertical motion of the whole system (planet earth , including the atmosphere and the aircraft) is zero and momentum is conserved.

Similarly, conservation of momentum suggests that in descending flight, the ground rises ever so slightly as the plane descends. (We need to be careful in postulating how this could be-- in my coments above I think I proposed a mechanism that would cause the ground to accelerate upwards, not move upwards at a constant velocity-- that would be an error.) In climbing flight, conservation of momentum suggests the ground moves downward ever so slightly as the plane climbs.

The same should be true for gliding flight.

I'm not making any unusual claims about downwash, etc as viewed on the normal local scale. (I was initially led toward some unusual claims in my first post, but that was in error.) I'm simply saying that on the largest (planetary) scale, there is no net downwash (no net downward transfer of momentum), and momentum is conserved.

Kind of obvious now, but made me stumble for a bit there...

Steve
Last edited by aeronaut999; Mar 01, 2012 at 12:54 AM.
Feb 29, 2012, 11:58 AM
Ascended Master
Quote:
 Originally Posted by BMatthews Sparky, with the big boys standing at the fence I was able to hear an odd ripping sort of sound about 15 to 20 seconds after thay had passed overhead. I assumed it was the tip vorticies growing and meeting and interacting. The grass around that area is all mowed and so I wasn't able to see anything in terms of waving despite it being a calm evening. Mind you if it was something that could ripple even tall grass I'd expect to feel it. I may be off in my estimate of the altitude as well. I do know that they looked DARN big and I could see individual hydraulic lines and other details inside the wheel well quite easily. Mark, thanks for chiming in. I'm assuming that since the final delta(p) would be in psi that L is the pressure in psi that I was suggesting. Otherwise the units to the whole equation become rather odd. If so then a "pulse" of even 2 psi variation as the plane passes overhead is going to be a mere 2/(2x3.14x200^2)... er... I don't know if the altitude should be in feet, yards or furlongs. Either way it would be small. Hardly more than a gnat's sneeze.
.
At 600 feet, the bang from the boom was quite uncomfortable. The hippies that were directly underneath the plane came wandering by where I was situated, looking no worse for the wear, but it is difficult to tell, with them.
Playing in the house with the toy helicopters, it's fun trying to get them to approach a low obstacle. They tend to get near, then shy away as the downwash gets affected by the edge of the obstacle.
Feb 29, 2012, 12:19 PM
Registered User
Quote:
 Numerous "low pass" videos on the 'net show the effect on the ground of the downwash from fighters, especially. At Mudd Lake, October 1977, Darryl Greenamyer's low altitude speed record flights we measured the sound pressures of the sonic boom at 600' off the centerline, with him flying at 50', Mach 1.28. And after the plane passed, it raised a dust cloud some distance behind it.
One little detail, a lot of those low passes may be transonic, and there could be a fairly strong shockwave present causing the effects on the ground that you see. Certainly the F-104 supersonic pass would have a shockwave, and I would guess that the dust you witnessed was more from the shockwave than the downwash. Could be wrong though....I am just a lowly helicopter wrench and not an aerodynamicist.
Feb 29, 2012, 12:56 PM
Ascended Master
The bangs were real.
From the on-board observer panel in the F-104.. and a photo taken from a better vantage point than I had...

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