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Old Mar 09, 2012, 06:38 PM
I don't want to "Switch Now"
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Originally Posted by aeronaut999 View Post
But in that case, what would be holding the earth down, preventing the earth from rising up toward the plane as it flies in circles?

(Conclusion-- no planet can exist whose atmosphere is an ideal fluid-- or flight can never be invented on such a planet? )

Steve

I could have been more clear.
I meant you don't have to worry about the effect of gravity on the downwash/upwash.
You do need gravity to require there to be lift in steady state level flight of course.

You could consider the case of flight within a hollow sphere. No net gravity there, so you could go in straight lines but to turn you would need to generate lift.

Pat MacKenzie
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Old Mar 09, 2012, 07:28 PM
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Let me start by apologizing for the fact that I don't have the time to really dive in to this.

I said that I don't think the air molecules are traveling all the way to the ground, but I do agree a force is transmitted, just as Bruce was talking about. Pressure can be transmitted without macroscopic motion. Like sound waves, or, similarly, shock waves.
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Old Mar 09, 2012, 11:20 PM
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Given sufficient altitude, one would think that the downwash would be dampened/absorbed by the viscosity of the very large fluid body that is the atmosphere. Like has been said, conservation of momentum is not broken just because the pressure wave does not reach the ground. The energy is transferred to the atmosphere through macro movement, heat, and etc.
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Old Mar 10, 2012, 05:35 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
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Since we are rationalizing flight in disciplines other than aeronautics:

It's a signal to noise issue. While it is true that the airplane exerts a force on the earth and, for that matter, the sun and the fringes of the universe, that force is small. It is especially small when compared to the local impact of clouds, turbulence, air mass movement and thermal activity. Any raising of the level or mass of the atmosphere or change in the net mass of the earth by the movements of the airplane are masked by numerous local effects which can be lumped as noise.

The airplane operates in a local air mass. For your rationalization to work, the tiny impact of trimming the ailerons for level flight would have to be transmitted through the atmosphere without modification the the surface of the earth. Not possible. To little signal in the presence of too much noise.

It's an airplane, not an anti-gravity machine.

Tom

Oh yeah - how does the rudder work in your pushes down on the earth concept? Does it circle the earth and push on the other side?
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Old Mar 10, 2012, 11:47 PM
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I have to suggest that it's rather assumptive as well. We don't actually need the earth to react against to produce lift.

The lift of an airplane is created by reacting with the air to produce an accelerative force in some direction. Usually this is against the pull of gravity. But the plane does not need to push against the earth to develop its lift. Take the earth away and stick the plane in a big sealed globe of air and it would still fly just fine. It just would only generate positive lift when actually pulling G and generating lift to turn a corner. So the earth is not needed to generate lift. And if it is not needed to generate lift then it sort of throws your conservation of momentum proof into doubt if not negates it.


The key is that an airplane in flight only needs to react to the air to develop any lift it needs. It does not need to push against the earth or anything else but the mass of air around it.

For example as a plane flies a tight high G loop and goes over the top while still generating positive G. What is it pushing against then? Only the air.
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Old Mar 11, 2012, 02:17 PM
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Originally Posted by Tom Harper View Post
Since we are rationalizing flight in disciplines other than aeronautics:

It's a signal to noise issue.
That argument doesn't do it for me. If the plane circles long enough, and is continually pulling the earth upwards by gravitational attraction, so the earth is continually accelerating upwards, then momentum is not clearly conserved-- heck, we even have relativistic concerns to worry about (the earth will start to approach the speed of light as seen from our original inertial reference frame).

Maybe each airplane that is in flight is always accompanied by another aircraft in flight on the opposite side of the earth? Have we discovered a new phenomenon here-- quantum coupling on a non-quantum scale? (Just kidding of course-- but serious about the basic idea of this thread..)

Steve
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Old Mar 11, 2012, 02:19 PM
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(Edit May 2012-- my thoughts on this matter have now changed significantly-- I'm still convinced that the earth "feels" a downward push from the wing of an aircraft in flight, equal in magnitude to the weight of the aircraft, which is also equal to the upward gravitational attraction that the aircraft exerts on the earth, but I no longer believe that this downward force need involve any specific amount of downward momentum of the air (downwash). For more, see posts 58, 61, and 72. End edit.)

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I have to suggest that it's rather assumptive as well. We don't actually need the earth to react against to produce lift.
The point is not that the airplane "needs" the earth to make lift. The point is rather that over the long run, the earth "needs" to feel the plane's downwash in some form or another, or else it will accelerate upwards due to the pull of the plane's gravity, and momentum will not be conserved.

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For example as a plane flies a tight high G loop and goes over the top while still generating positive G. What is it pushing against then? Only the air.
I completely agree. And see my previous response, immediately above. With the loop, the reason that momentum is conserved, is that the extra G's in the pullout end up making the extra downwash that holds the earth in place. I'm sure we could flesh this out in more detail in a more rigorous argument involving a continual series of loops.

The plane doesn't "need" to make an earth-directed downwash in order to make aerodynamic lift, e.g. at the top of a loop, where the downwash is aimed upwards. But over the long run, the earth "needs" to feel the downwash, in order to not accelerate upwards under the gravitational pull from the plane. Thus, over the long run, lift is always accompanied by an earth-directed downwash, which exerts a downward force on the earth equal to the upward pull of the plane's gravity on the earth.

A key point is that the plane is flying in small circles above a point on the ground, not circling the whole earth-- in the latter case the plane would not pull the earth upwards in any particular direction over the very long run.

Steve
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Old Mar 11, 2012, 02:38 PM
Lift is cheap - Drag sucks
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You neglect rudder action. It has no earth to push against.

Tom
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Old Mar 11, 2012, 02:39 PM
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Originally Posted by slow2by2z View Post
Given sufficient altitude, one would think that the downwash would be dampened/absorbed by the viscosity of the very large fluid body that is the atmosphere. Like has been said, conservation of momentum is not broken just because the pressure wave does not reach the ground. The energy is transferred to the atmosphere through macro movement, heat, and etc.

I don't necessarily disagree with the idea that the downwash may be converted entirely into pressure, but I do think that this pressure must exert a net downward force on the ground. The downwash cannot be dissipated in some way such that it is never "felt" by the ground. Maybe when motion is converted to heat in the atmosphere, it always ends up increasing the pressure at the ground? Even though we are not increasing the weight of the air column above that point on the ground? I am not completely clear on this. (But see rudder example below-- sidewash from rudder must be dissipated in a way that is NOT ultimately "felt" by the ground.)


Steve
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Old Mar 11, 2012, 03:04 PM
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rudder etc

(Edit May 2012-- my thoughts on this matter have now changed significantly-- I'm still convinced that the earth "feels" a downward push from the wing of an aircraft in flight, equal in magnitude to the weight of the aircraft, which is also equal to the upward gravitational attraction that the aircraft exerts on the earth, but I no longer believe that this downward force need involve any specific amount of downward momentum of the air (downwash). For more, see posts 58, 61, and 72. End edit.)

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Originally Posted by Tom Harper View Post
You neglect rudder action. It has no earth to push against.

Tom
The rudder shows that it is not fundamentally necessary for the downwash (sidewash etc) from an aerodynamic surface (airfoil etc) to exert a "push" against some other solid surface in order for lift to be created. Which is consistent with creation of lift during a negative-G pushover. (Flying upright, push stick forward to generate say -2 G's.) So we can agree on that.

However, the original contention of this thread is that over the long run, the earth must "feel" a downward force equal to the upward pull of the plane's gravitational attraction on the earth. This certainly suggests that lift always involves some sort of downwash or pressure wave that will be tangibly "felt" by a nearby (or even very distant) surface, if there is one.

A key point may be that the thought experiment of what force is "felt" by the earth's surface even if it is very very far below the plane, is fundamentally different than the thought experiment of what force is "felt" by a horizontal wall of finite dimensions that is very very far away from a deflected rudder. I'm not sure about this. For example does heating the atmosphere due to kinetic energy of downwash or sidewash end up increasing atmospheric pressure, which would be noted as downward force on the ground, but not noted as a sideways force on a vertical plate of (infinite?) dimensions at some great distance from a deflected rudder? But that doesn't make sense because that suggests that as the rudder's sidewash is dissipated, it heats the air, and to be consistent, we would have to say that that heating also pushes downward on the ground, just as does the heating from the wing's downwash. That doesn't make sense.

So, I guess I am saying that in some way or other-- downwash, pressure, whatever-- the downwash from a wing does end up pushing down on the ground in a real, physical way, exerting (over the long run) a force equal to the plane's weight. Whereas a sidewash of the same magnitude (plane vertically banked and pulling one G, or deflecting a gigantic rudder to make a one-G sideload) is dissipated in a way that may ultimately heat the air or whatever but does not press down on the ground (does not exert a downward force on the ground). The argument is not fully fleshed out at this point but I'm sticking to my position! Remember the argument is based on conservation of momentum which we really can't dispute.

It appears to me that the downward momentum of the downwash must indeed be conserved all the way to the ground in some form or another. Just saying that the downwash is ultimately disipated into heat doesn't cut it, because the same is true of the sidewash from the rudder, yet the ground needs to feel the wing's downwash, and the ground needs to not feel the rudder's sidewash, so something is fundamentally different about the way the ground feels the two effects.

If we simply postulate that the momentum of the downwash (and sidewash) is conserved indefinitely (or at least until collision with a solid surface) even as we do other things along the way (like heat the air), then the riddle is solved I think. If the plane could somehow fly along in a straight line with the rudder deflected, continually shoving (sidewashing) air to one side, then we would have another conservation of momentum problem. But we don't-- because producing a net sidewash will always make the flight curve, so the direction of travel will not be a straight line.

Considering the rudder issue has strengthened the case that the momentum of the downwash (or sidewash) is in fact conserved indefinitely as it passes through the atmosphere, for me at this time at least! Maybe the right word is not "momentum" but rather the "ability to exert force"-- including like a directed pressure wave (key word directed)-- is there such a thing? Is this just a form of momentum? I suppose so.

Key point-- I'm talking about "the ability to exert force" on a very large object. Like the ground. Granted the area of the downwash increases , so the net downforce measured on a plate of some fixed size decreases, as we move the plate progressively further below/ behind the aircraft.

Steve
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Old Mar 11, 2012, 04:02 PM
I don't want to "Switch Now"
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Trying to bring the earth's reaction into the mechanics of lift generation is nothing but a distraction. it won't help to understand what is going on in any meaningful way

I have a friend who always wants to bring Quantum mechanics into the discussion of physical phenomena.
So from that perspective should we be analyzing lift from the point of view of Quantum electro dynamics?
Or classical physics and use electromagnetics since it might be interactions between the shells of air molecules and those of the wing's atoms?

You could spend a lot of time with any of these approaches, but none of them will help you to design wing sections, planforms, models etc.

Pat MacKenzie

FWIW the weight of the plane is continued to be felt as pressure on the earth, much like a fish in a tank adds to its weight whether it is swimming or resting on the bottom.
All it has to do to do this is increase the average air pressure by a tiny amount since the surface area the pressure acts on is so huge.
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Old Mar 11, 2012, 04:04 PM
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Trying to bring the earth's reaction into the mechanics of lift generation is nothing but a distraction. it won't help to understand what is going on in any meaningful way
We disagree on this.... it seems to be shedding light on whether the momentum of the downwash is conserved all the way to the ground, or not-- we can disagree on whether or not that is meaningful but it is meaningful to me!

Keep in mind that we are staying within the confines of classical Newtonian mechanics here.... surely a worthy subject for "Modelling Science"....

Steve
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Old Mar 11, 2012, 05:45 PM
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FWIW the weight of the plane is continued to be felt as pressure on the earth, much like a fish in a tank adds to its weight whether it is swimming or resting on the bottom.
All it has to do to do this is increase the average air pressure by a tiny amount since the surface area the pressure acts on is so huge.
It seems to me that a balloon increases the air pressure of the whole planet ever so slightly by displacing the top of the atmosphere upwards. Likewise a fish, in it's bowl. I'm trying to think of exactly how this concept of pressure equates to downwash-- imagining a fan blowing down against a surface-- here the "pressure" is clearly acting in one direction and therefore associated with momentum in one direction-- affecting the ground in a way that a horizontally-blowing fan (like the rudder's sidewash), stirring and heating the air but not blowing it at the ground, would not.

Yes I certainly agree the effect is very dispersed.
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Old Mar 11, 2012, 06:05 PM
Lift is cheap - Drag sucks
Tom Harper's Avatar
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Steve,

OK - so let's go back to an earlier example:

The downwash angle is determined by the coefficient of lift Cl

The Cl is determined by the angle of attack

Lift then is proportional to velocity squared v

Using a flat plate section at an angle of attack A and some area S set v so the lift is 20 lbs

Now double the chord of the wing at the same angle of attack and velocity

The downwash angle and velocity will remain the same because the Cl and v have not changed

However twice the chord will produce twice the lift from the same amount and angle of downwash


If lift was in anyway dependent on downwash the downwash would have to have doubled or changed it's angle


Tom
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Old Mar 11, 2012, 06:31 PM
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Tom -- Bruce, I can't believe that you guys continue to take part in this silly discussion.

Downwash - sidewash - upwash - dragwash - propwash --- all are the effects - the result - of aerodynamic forces, not the cause of them.

Momentum is conserved in a closed system - so if you fly your helicopter inside a box there is no net force on the system. Not so in an open system - an aircraft in flight is an open system, - so the aerodynamic forces on the aircraft produce local downwash - upwash - sidewash dragwash propwash - jetwash ad nauseum

yak yak yak yak -- this thing must have gone to about sixty pages of just trolling and blather. --- almost as bad as downwind turns.

unsubscribed again
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