View Poll Results: Is the statement in the first post of this thread true or false?
It is true 8 34.78%
It is false 15 65.22%
Voters: 23. You may not vote on this poll

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Jan 02, 2017, 01:46 AM
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ShoeDLG's Avatar
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Momentum Survey


Is the following statement true or false?

When a vehicle moves horizontally through the air, and the air exerts an upward force on the vehicle equal to the vehicle’s weight, Newton’s Laws require the air’s downward momentum to increase at a rate equal to the vehicle’s weight.
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Jan 02, 2017, 01:50 AM
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Jan 03, 2017, 03:47 PM
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I believe it is true, based on quite a nice explanation here:

https://www.grc.nasa.gov/www/k-12/airplane/momntm.html

As I understand it, we need to make assumption that the considered system is airplane+infinite amout of air. If the ground is added, then the downwash eventually generates a force equal to the airplane weight acting on the ground. This negates the force of gravity that the plane exerts on Earth, so that a flying airplane is not pulling the Earth up
Jan 03, 2017, 04:12 PM
B for Bruce
BMatthews's Avatar
This came up in a previous thread about lift and how or if the momentum eventually exerted a force on the earth itself connected through the air.

I also recall that someone pointed out that airfoils in wind tunnel videos don't show any sort of downwash behind the trailing edge because with a wing of infinite span (or with big end walls like in a wind tunnel to simulate the infinite span) that there is no downwash. Only on "real" wings with wing tips can downwash effects be shown. Yet wings in wind tunnels certainly do produce lift.

So it seems like there's more going on than just "air being pushed down".

Remember too that at one time it was all about measuring the pressure distribution over and under the wing. And the amount of lift produced was able to be shown to be equal to the summation of the pressure differential observed over the area of the wing under measurement. And it was that pressure differential that was seen as the provider of the lift observed....

Yet there can be only one.... or can there? Is one thing the cause of the other? Can either exist without the other? Well, in a wind tunnel we seem to have an example of one without the other. Or is it just that the momentum transfer that normally results on an "open wing" confined or changed into something different due to the effect of an infinite span wing?

I also wonder about this downwash effect and even about the connection back to the ground. Oh, I'm not saying that there is no downwash. But perhaps it's not as strong as it is made out to be?

I've stood at the fence at Vancouver Intl' while 747's and other big haulers pass about 250 to 300 feet over my head. It's a staggering sight. Yet I feel precisely zero effect as all that weight flies over head. Not a flutter of my ear drums or any slight pressure change other than the noise of the engines at near idle. Now I understand how the air can spread the effects out. But THAT close I'm amazed that I feel nothing at all.

Some time back I looked up the wing area and weight of a 747 and calculated that the wing needed right around 2psi average over the whole area to hold itself up in the air. Now 2psi is about what is needed to blow up a party balloon. That in itself was a pretty staggering figure. And it may explain why something that large could fly within that distance and not be felt only a couple or three hundred feet away.
Jan 03, 2017, 04:46 PM
Registered User
"I also recall that someone pointed out that airfoils in wind tunnel videos don't show any sort of downwash behind the trailing edge because with a wing of infinite span (or with big end walls like in a wind tunnel to simulate the infinite span) that there is no downwash."
BS.
If wind tunnels showed that much of an error nobody would have ever used them.
There are errors that have to be corrected for as the wings get close to the walls on a full model.
In wind tunnel talk, it's called "upwash", which confused me mightily till I had the instructor clarify the "tunnel talk terminology".
Usually they try to hold a model to a certain percentage of span relative to the tunnel wall. But be cause -at least at the tunnel I was at- they can shave it pretty close because the characteristics of the tunnel are well known and correction factors are plugged in.
Jan 03, 2017, 05:07 PM
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https://www.youtube.com/watch?v=3_WgkVQWtno

What I see here is pretty significant downwash that "evens out" behind the wing because:

1. Downward momentum is spread over bigger and bigger mass of air (due to the air molecule collisions), reducing vertical velocity and therefore deflection angle.

2. Supposedly, the tunnel has some bottom. The air acts on it, with the force equal to the lift force of airfoil. Therefore, the wind tunnel, as a closed system, generates no net force. Otherwise, that would be a reactionless thruster.

Quote:
Some time back I looked up the wing area and weight of a 747 and calculated that the wing needed right around 2psi average over the whole area to hold itself up in the air. Now 2psi is about what is needed to blow up a party balloon. That in itself was a pretty staggering figure. And it may explain why something that large could fly within that distance and not be felt only a couple or three hundred feet away.
A long time ago, I did a very rough estimation on the "air deflection angle". If we define the approx. "area of effect" of a wing as a box maybe 2 chords high, and calculate the mass of air flowing through it, then the vertical velocity impaired on the air turns out to be very small, on the order of fraction of m/s, if I remember correctly.
Jan 03, 2017, 06:55 PM
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Newton's law requires the forces to be in equilibrium, ie, that the plane exert a force on the air as large as that that the air exerts on the plane. But if this means that the plane should be accelerating the air mass downwards, by the same token the air mass should be accelerating the plane upwards.

Just to refine a bit the argument: If I am standing on a hard floor, and am subjected to gravity, I exert a force on the floor. By contrast, the floor exerts the same force on me, in accord to the 3rd Newtonian law. But neither me or the floor are accelerating in any direction. Or to be more precise, the reaction of the floor accelerates me upwards at the same rate as gravity accelerates me downward, and the two accelerations cancel out. the main issue with most people's understanding of the 3rd law is that most don't realize it describes an equilibrium state, and are misled by the typical picture of the two skaters.
Last edited by Brandano; Jan 03, 2017 at 07:03 PM.
Jan 04, 2017, 02:24 AM
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Right. To further un-simplify the issue and to bring it closer to what is going on when in flight, note that when standing, our muscles are constantly firing (another example is an acrobat balancing on a freely standing ladder), requiring energy and Entropy-loss. Further, the floor will eventually sag, depleting energy from forces holding the molecular structure "rigid". These forces are in essence no different from those of a jet-pack in "hover" or a juggler keeping objects in the air. So we simplify the "equilibrium" aspect and nullify it for convenience.

My arcane point is that there are continual momentum changes even in a stable system without requiring an obvious and overt net momentum change ....
Last edited by xlcrlee; Jan 05, 2017 at 10:50 AM.
Jan 04, 2017, 03:03 AM
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Quote:
Originally Posted by xlcrlee
These forces are in essence no different from those of a jet-pack in "hover".
In terms of momentum transfer, the forces in the cases you describe are fundamentally different from a jet pack in hover. In the static cases there is clearly no momentum transfer associated with the forces involved. In the case of a jet pack in hover, downward momentum is transferred to the propellant at a rate equal to the upward thrust.

While our muscles consume energy while we are standing, they are doing no work (they are not exerting a force through a displacement).
Jan 04, 2017, 04:40 AM
Registered User
Actually, just like servo jitter, there are actually very small movements, not just sideways, but up & down, also as the legs and body rock through an arc, the acrobat on a ladder being an exaggerated example; and don't forget normal micro & macro vibrations in the real world. The noise of the jet-pack illustrates these vibrations. It just seems different because we treat these cases that way.

The lift force is supposedly derived from the change in the momentum vector, its direction and even that as an instantaneous rate, which you have already shown can magically "disappear" over some distance. At least that's what I think you showed.

But in the practical sense, of course I give you right, Shoe!
Jan 04, 2017, 04:45 AM
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Quote:
Newton's law requires the forces to be in equilibrium, ie, that the plane exert a force on the air as large as that that the air exerts on the plane. But if this means that the plane should be accelerating the air mass downwards, by the same token the air mass should be accelerating the plane upwards.
Quote:
Just to refine a bit the argument: If I am standing on a hard floor, and am subjected to gravity, I exert a force on the floor. By contrast, the floor exerts the same force on me, in accord to the 3rd Newtonian law. But neither me or the floor are accelerating in any direction.
Exactly! Correct me if I'm wrong, but there are 3 quite different situations:

1. Aircraft is thrown into a volume of air in space. The aircraft accelerates upwards, the air-downwards. Momentum is conserved.

2. Aircraft is flying in volume of air, with gravity. The air is accelerated downwards, and the aircraft stays still. The momentum is NOT conserved because this is an open system - there is gravity coming from somewhere.

3. Aircraft flies over ground. This is the same as standing on the ground, the only difference is the air that acts as a sort of a cushion, transfering the force. If we follow a single air particle, it gets shoved down, creating force on the airplane, and then decelerates, creating force on the ground. Both actions may happen not directly, but through other particles.
Last edited by Davidz90; Jan 04, 2017 at 04:51 AM.
Jan 04, 2017, 09:39 AM
An itch?. Scratch build.
eflightray's Avatar
And the lower air pressure above the wing is ...... ?
Jan 04, 2017, 09:48 AM
Registered User
With a large enough volume of air, such that the air before and after the passage of the plane shows no disturbances, the momentum of the system is conserved. Therefore, the passage of the plane deflects as much air upwards as it does downwards. The system to be complete and analysable via the three laws of motion should also include the air that is dragged upwards in front of the plane, and the air that spills back upwards after the plane has passed to re-balance the pressure difference that the plane's passage has left. If we could safely ignore these we wouldn't need to be concerned with stuff like inducted drag.
Jan 04, 2017, 10:00 AM
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Basilisk lizard runs on water to catch a butterfly, Rate My Science (2 min 21 sec)
Jan 04, 2017, 10:25 AM
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Tom Harper's Avatar
I am confident that Newton is right, but that does not imply that a lifting wing needs to push on the earth for lift.

A vertical surface (rudder, sail, propeller) also creates 'lift'.

Brandano explains it very well.

Crane


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