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Old Mar 11, 2012, 10:43 PM
Grad student in aeronautics
United States, GA, Atlanta
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Quote:
Originally Posted by aeronaut999 View Post
"There is downward momentum in any air column behind the wing. There is zero momentum in any air column ahead of the wing, outboard of the trailing vortices, or aft of the starting vortex."
Unless I'm reading something incorrectly, I'm thinking this is just completely wrong.
Steve, I think if you'll read my previous post you'll find it agrees with what you are trying to say.
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Old Mar 12, 2012, 10:14 AM
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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.)

Quote:
Originally Posted by DPATE View Post
Steve, I think if you'll read my previous post you'll find it agrees with what you are trying to say.
Yes, it seems to be consistent with what I am saying.

I guess one question of interest is, during 1-G flight, if the plane's downwash, and/or other related effects associated with lift creation, is exerting a pressure on the floor of the box, does that pressure involve momentum?

Steve
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Old Mar 12, 2012, 10:36 AM
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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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Originally Posted by pmackenzie View Post
I would say it has to be different, since the total "weight" of the box is the same if the plane is in your hand or gliding along.
It would also vary as the plane did pushovers and pullouts, just as it would if a motorcycle was doing loops on a vertical circular track. Or if you are throwing a ball up and down. Different mechanisms in each case but the principle is the same.
Consider-- plane doing loops inside a large box, with a scale under the box. The loops are positively loaded throughout, in the aircraft's reference frame-- i.e. the wing is always making some amount of positive lift in the aircraft's reference frame.

If the scale read x when the plane was at rest, I think we agree that the scale will read x during 1-G flight, and less than x during 0-G flight (doesn't occur during the loops in this example), and less still when the plane is inverted at the top of the loops but pulling positive G's (in aircraft's reference frame), and the scale will read more than x during the pullouts near the bottom of each loop. Just as if a motorcycle were doing loops around the inside of the box.

I suggest that this is due to the pressure exerted by the downwash on the various surfaces of the box as the plane flies round and round the loops. I further suggest that the pressure exerted on the walls is due to the dynamic pressure created by the downwash-- as could be measured by a pitot tube or a scale. Not due to a static pressure increase, as could be measured with an aneroid barometer. Dynamic pressure is associated with momentum.

Now, maybe at some point very close to the wall, some of the energy of the downwash is changed from dynamic pressure to static pressure as the downwash interacts with the wall. I'm not sure about this. If so, make the box sufficiently large and move the measuring instrument sufficiently far from the wall, and I suggest we'll only measure a dynamic pressure increase not a static pressure increase.

I further suggest that the momentum of the downwash is conserved indefinitely (or at least till it starts to "feel" the wall?) , and in 1-G flight exerts a force equal to the plane's weight against the ground, no matter how high the aircraft is. Yes the dynamic pressure decreases but the cross-sectional area expands, so the total force that can be exerted against a sufficiently large wall or floor-- if there is one-- stays the same.

I'm not ruling out the possibility that the momentum of the downwash can be expressed in some form other than macroscale motion of the air --consider a directed pulse of sound, etc-- but there must be momentum involved, not just a static pressure.

I may have some details wrong and would appreciate hints for fine-tuning the description but the thought experiments of the aircraft circling a point over the earth, and the aircraft doing loops inside the box, suggest that this is how it must be....

Steve



Examples of using a scale to measure dynamic pressure:
http://www.flickr.com/photos/tom-margie/2515768971/


Well maybe that's a little off topic, the plate needs to sheltered from the suction of the airflow behind and also flush with the wall, if we want to measure dynamic pressure at the wall / floor-- if we want to measure dynamic pressure at some distance from the wall/ floor then we have a problem perhaps-- so instead install a streamlined pitot tube at the location of interest, pointing in the direction of interest, to sample the incoming dynamic pressure at that location....

Steve
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Old Mar 12, 2012, 10:39 AM
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( duplicate post)
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Old Mar 12, 2012, 12:11 PM
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Quote:
Originally Posted by aeronaut999 View Post
does that pressure involve momentum?
I think you're getting to the heart of the question. At a microscopic level, as I'm sure you know, pressure from a fluid on a body comes from the fluid molecules colliding with the surface of the body and, therefore, transferring their momentum. However, take this example.

When you are about to enter a room and do so by quickly opening the door, you can hear the other doors in the room slam shut if they were barely cracked open. The result is seemingly instantaneous. The pressure "information" travels at the speed of sound, not at the speed you were shutting the door, and not at the speed the air pushed by the door traveled.
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Old Mar 15, 2012, 09:05 PM
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Quote:
Originally Posted by DPATE View Post
I think you're getting to the heart of the question. At a microscopic level, as I'm sure you know, pressure from a fluid on a body comes from the fluid molecules colliding with the surface of the body and, therefore, transferring their momentum. However, take this example.

When you are about to enter a room and do so by quickly opening the door, you can hear the other doors in the room slam shut if they were barely cracked open. The result is seemingly instantaneous. The pressure "information" travels at the speed of sound, not at the speed you were shutting the door, and not at the speed the air pushed by the door traveled.
Thanks for the note--

Still thinking over what types of disturbances truly transfer momentum and which types don't-- a sound wave has a lot of energy but does it impart a momentum to the object against which it is directed? Not sure.

Do you have any insight into the question posed by posts 29 and 31?

Steve
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Old Mar 15, 2012, 09:19 PM
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I'll participate more in a few weeks. But I can give you one quick interesting fact. On an episode of myth busters they had an F-18 fly super sonic above a mobile home and it blew out the windows. I just hope everyone understands that the air contacting those windows was not the same air that was contacting the plane.
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Old Mar 16, 2012, 05:19 AM
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Quote:
Originally Posted by aeronaut999 View Post
Thanks for the note--

Still thinking over what types of disturbances truly transfer momentum and which types don't-- a sound wave has a lot of energy but does it impart a momentum to the object against which it is directed? Not sure.

Do you have any insight into the question posed by posts 29 and 31?

Steve
Generally waves are thought of as transmitting energy, not momentum.
But the medium the wave travels in (the air) has no net movement.

A speaker moves and generates a sound wave in the air.
That sound wave will cause a microphone to move, so "momentum" is transferred during the interaction.


Pat MacKenzie
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Old Mar 16, 2012, 09:13 PM
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Thinking-- the membrane of the microphone vibrates but doesn't undergo a net acceleration in any particular direction--

An actual shock wave may be different (would a flat board standing on edge be knocked it over, or might it rebound back to the original position if it wasn't broken by the initial energy of the shock wave (like the windows of the house)-- or a better example, a wheeled cart with a sail on it, would the shock wave get the cart rolling? Would the sound from a speaker get the cart rolling?

Quote:
Originally Posted by pmackenzie View Post
Generally waves are thought of as transmitting energy, not momentum.
But the medium the wave travels in (the air) has no net movement.

A speaker moves and generates a sound wave in the air.
That sound wave will cause a microphone to move, so "momentum" is transferred during the interaction.


Pat MacKenzie
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Old Mar 16, 2012, 09:21 PM
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Shockwaves:

Atomic Bomb Testing (1 min 22 sec)


If you watch there is initial movement away from the source, then everything gets sucked back in. So even in this extreme case there might not be net movement.
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Old Mar 16, 2012, 10:16 PM
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Check out the waves within a tuned exhaust system--
really interesting stuff
and as usual--- misunderstood
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Old Mar 16, 2012, 10:19 PM
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That is an example of waves in a moving fluid.
Net flow out the exhaust, but the pressure waves resonates in the chamber.
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Old May 08, 2012, 02:47 AM
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Downwash revisited

I've been thinking about the downwash / momentum issue some more. I guess I'm retreating a little from some (but not all!) aspects of my earlier thoughts...

Some key points:

1) If the air is exerting an upward force on the wing, the wing is exerting an equal and opposite downward force on the air. That is certain.

2) If we were in outer space, accelerating our spacecraft by shooting BB's or isolated gas molecules or whatever, out the end of a rocket nozzle, we could be sure that the momentum imparted to the BB's or gas molecules was just as large as the momentum imparted to the space craft.

3) However, when we create lift by pushing down on the air, there are forces that act against the air accelerating freely. NOT the mass of the air molecules (that's not a "force")-- -- rather, the inter-molecular forces created by shearing the airmass, etc. So just because we exert force X on the air, doesn't mean that we see a commensurate acceleration of the air molecules. And the wing doesn't care how much the air is accelerating. As long as the wing is pushing on the air it is happy regardless of whether the air is accelerating or not. Just as the lift force does not actually end up accelerating the aircraft upwards, (most of the time), because it is all used up opposing gravity. The air doesn't care whether the aircraft actually accelerates upward or not. As long as the air exerts an upward force on the aircraft it is happy, regardless of whether the aircraft actually accelerates upwards or not. The force on the air equals the force on the aircraft, no matter how much or how little either actually accelerates, and the happiness is mutual all around and Newton is happy too.

4) Example: if we were swimming in honey, we could hold ourselves up while imparting very little downward acceleration to the honey molecules, because the downward force we exert on the honey would be opposed so strongly by the viscosity (resistance to shearing) of the honey. F=ma, a = F/m, but the net acceleration of the honey is reduced by the fact that other forces arise that act in opposition to the force from our hands as we swim in the honey. That doesn't matter to us as we swim--as long as we are pushing on the honey, the honey is pushing on us the other way too, no matter how much the honey is actually accelerating in response to our push..

5) For a given lift force, just as the amount of momentum imparted to the fluid is less if the fluid is highly viscous, so too is the distance that the momentum will propagate before being dissipated into heat.

6) -- IMPORTANT-- The dissipation of the downwash momentum by shear forces etc does not violate the laws of physics, nor does it contradict the idea that a scale under a box (in which an aircraft or bird is flying) will "feel" the weight of the aircraft in a way that is varies according to the vertical component of the G-load (lift force) that the aircraft is creating,

7) Regardless of whether we are flying in honey or water, if we do some maneuver that makes us weightless, a scale under the pool will no longer detect our weight, and if we do some maneuver that involves a 2-G pullout from a dive, the scale under the pool will detect twice our normal weight. (Assume we are very heavy and very skinny, so buoyancy is negligible, for simplicity.)

8) The extreme ("most viscous") case is when we are standing on dry land. Again, the scale beneath us registers more weight when we are doing a high-G maneuver (e.g. accelerating upwards by un-bending our legs as we jump up). The other extreme case is when viscosity is zero-- we are floating in outer space, accelerating ourselves by shooting bb's out of a gun. If a scale happens to be in our line of fire, the scale will register more weight (force) when we are shooting a lot of BB's to accelerate rapidly, than when we are shooting only a few BB"s to accelerate slowly. So I'm still convinced that a scale under the box of air containing an aircraft does "feel" the aircraft's weight in a way that varies according to the vertical component of the G-load (lift force) that the aircraft is creating.

9) So, it's not appropriate to think of the aerodynamic creation of lift as being exactly equivalent to say, shooting BB's out of a gun in a vacuum. Here are some other ways that the air is different than
BB's-- see this link

section 3.6 here
http://www.av8n.com/how/htm/airfoils.html#sec-fluid

By the way this is a very good website
http://www.av8n.com/how/

Especially the section on airfoils and airflow
http://www.av8n.com/how/htm/airfoils.html#sec-airfoils

10) Key points in summary-- the earth does "feel" the downward push or shove that the aircraft exerts on the air molecules. This downward push or shove in turn pushes or shoves on the earth and makes the earth "feel" the weight of the aircraft. Put a gigantic box 5 miles per side and open to space on top, under the aircraft, and a scale under the box, and the scale WILL register the weight of the aircraft, and doubly so during a 2G pullout, and not at all during a 0G ballistic maneuver. However this downward push or shove exerted by the aircraft on the medium (air), and transmitted in turn to the earth, need not be expressed as a large downward momentum of the medium (air), especially if the viscosity if the medium (air or whatever other fluid we are flying in) is very high.

11) In the real world of course we do see a downwash behind a wing-- it has a huge and well-known effect on the stability and control of the aircraft-- it exerts a downward push or shove on the horizontal tail surface (if there is one). The downwash is also visible in smoke trails. Wake vortices are known to sink downward. Under the right conditions the tip vortices (containing the entrained downwash) can be seen to sink away from the main body of a jet contrail. Etc etc etc.In the real world it is easy to see that a downwash exists behind a wing.

12) Questioning the magnitude of the downwash is not the same as questioning that wing is exerting a downward push or shove on the air-- that is beyond any doubt, downwash or no. Newton's laws and all that.

13) One way to look at the downwash is to see it as kind of a failure of efficiency of the wing-- if we could eliminate tip vortices, etc, the wing would act as if it had infinite span and no downwash-- which is also the same as saying the air would behave as if it had infinite resistance to shearing, and thus did not permit the formation of a downwash? I'm a little hazy on how this all comes together but I think it does somehow.

14) I saw the downwash behind the wing of a hang glider this very morning.

Steve
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Old May 08, 2012, 03:31 AM
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I had difficulty understanding number 3.

I dissagree with number 5: " For a given lift force, just as the amount of momentum imparted to the fluid is less if the fluid is highly viscous"

Number 13 is, in my opinion, the number one cause of confusion. For an infinite hershey bar wing, there are no trailing vortices, but there is still the bound vortex. The bound vortex is associated with downwash behind the wing and upwash in front of the wing.

The downwash in the wake is the superposition of the downwash from the bound vortex and the trailing vortices.

A great way to learn about these concepts is to study lifting line theory. The model assumes a flat wake, which isn't quite how the wake behaves, but the basic principles are profound and unavoidable.
http://www.desktop.aero/appliedaero/...ftingline.html
I've seen this treated reasonably well in every aerodynamics book I've found since the 30's.
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Old May 08, 2012, 07:49 AM
Lift is cheap - Drag sucks
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(10) Are you proposing that the rudder does not work because it has no earth to push on?

Tom
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