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Old Aug 24, 2014, 05:37 PM
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Originally Posted by richard hanson View Post
it always has been - ANY action has a reaction of some type.
Well then you lost me with your byproduct comment.
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Old Aug 24, 2014, 06:18 PM
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The effect of a ceiling on the magnitude of rotor down wash is illustrated in the video. Putting a plate above the rotor dramatically reduces the downward force of the air on the scale.

RC Helicopter Hovering In Ground Effect Over Scale (1 min 28 sec)
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Old Aug 24, 2014, 06:25 PM
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Newton's Laws certainly don't require such a balance. Do you have any proof to show that this is the case?
Yes they do. If you want proof I suggest you read a high school physics text.
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Old Aug 24, 2014, 06:40 PM
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Yes they do. If you want proof I suggest you read a high school physics text.
Newton's Second Law states that an object (such as a volume of air) will experience a rate of momentum change equal to the unbalanced force acting on it. In the case of air supporting a lifting wing or rotor, Newton's Second Law requires the air to experience a rate of momentum change opposite the lift/thrust only if there is nothing else exerting a force on the air.

If, as you suggest, the air's rate of momentum change is always equal and opposite the aerodynamic lift acting on a wing, then it must be the case that things like the ground or a ceiling cannot exert any force on the air in the presence of a lifting wing. Do you believe this is the case?
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Old Aug 24, 2014, 06:47 PM
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When the craft is at the ceiling it has to be pulling as much air up and into the front of the prop as it is thrusting down. So there is no net down flow. The blades are simply lifting.

The ceiling effect may be the same as ground effect.

Tom
I do not agree. There is clearly a stream of air being forced down. But there is no discernable stream of air being forced upwards that would counteract the force in the downward stream.

The air above the rotors, and beside the rotors, is just air, before the rotor turns it downward. That new air from around the multi enters the region above the rotors is not relevant to the lift created by thrusting that new air downward.
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Old Aug 24, 2014, 07:00 PM
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And the air that is thrust downward exerts a lot less force on the scale when there is a plate or ceiling above the rotor.
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Old Aug 24, 2014, 07:28 PM
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And the air that is thrust downward exerts a lot less force on the scale when there is a plate or ceiling above the rotor.
Makes sense, since it requires less throttle to stick to the ceiling than it does to hover.
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Old Aug 24, 2014, 07:32 PM
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Newton's Second Law states that an object (such as a volume of air) will experience a rate of momentum change equal to the unbalanced force acting on it. In the case of air supporting a lifting wing or rotor, Newton's Second Law requires the air to experience a rate of momentum change opposite the lift/thrust only if there is nothing else exerting a force on the air.

If, as you suggest, the air's rate of momentum change is always equal and opposite the aerodynamic lift acting on a wing, then it must be the case that things like the ground or a ceiling cannot exert any force on the air in the presence of a lifting wing. Do you believe this is the case?
No. In analyzing this system you would draw a free body diagram of the aircraft. The only relavent forces are the forces acting on the aircraft and the opposite forces which the aircraft imposes upon the air. The only momentum changes to the air that are relavent to the analysis are those due to its interaction with the aircraft. If you properly define your analysis in this way there is no confusion about the correctness of a Newtonian explanation of the system.
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Old Aug 24, 2014, 07:37 PM
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No. In analyzing this system you would draw a free body diagram of the aircraft. The only relavent forces are the forces acting on the aircraft and the opposite forces which the aircraft imposes upon the air. The only momentum changes to the air that are relavent to the analysis are those due to its interaction with the aircraft. If you properly define your analysis in this way there is no confusion about the correctness of a Newtonian explanation of the system.
Makes sense to me.
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Old Aug 24, 2014, 07:47 PM
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No. In analyzing this system you would draw a free body diagram of the aircraft. The only relavent forces are the forces acting on the aircraft and the opposite forces which the aircraft imposes upon the air. The only momentum changes to the air that are relavent to the analysis are those due to its interaction with the aircraft. If you properly define your analysis in this way there is no confusion about the correctness of a Newtonian explanation of the system.
You say: "The only momentum changes to the air that are relavent to the analysis are those due to its interaction with the aircraft."

This is absolutely not the case if you are going to make any kind of statement about the air's rate of momentum change. If you are going to apply Newton's Laws to determine the rate at which the air's momentum is changing, then you must consider all the forces acting on the air.. including all boundaries.

Can you do an analysis where you just consider the interaction between the air and the aircraft? Certainly, but this restricted analysis will not allow you to make any statement about what is happening to the momentum of the air (at least not a correct statement anyway).
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Old Aug 24, 2014, 08:01 PM
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Originally Posted by ShoeDLG View Post
You say: "The only momentum changes to the air that are relavent to the analysis are those due to its interaction with the aircraft."

This is absolutely not the case if you are going to make any kind of statement about the air's rate of momentum change. If you are going to apply Newton's Laws to determine the rate at which the air's momentum is changing, then you must consider all the forces acting on the air.. including all boundaries.

Can you do an analysis where you just consider the interaction between the air and the aircraft? Certainly, but this restricted analysis will not allow you to make any statement about what is happening to the momentum of the air (at least not a correct statement anyway).
Am I missing something here? Kind of obvious that the air is changed from not moving to moving downward. It is a nicely isolated physics problem. The wing, one object, moves through the air, causing the air, a fluid, to move downward. This imparts an opposing lifting force to the wing.
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Old Aug 24, 2014, 08:15 PM
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As long as you leave it at that, you are mostly OK.

If you take it a step farther and say: "the air's rate of momentum change is always equal and opposite the lift force". Then you have gone too far.

How so? This is not, in all cases, the nicely isolated physics problem you suggest. The behavior of any element of the air is affected by the behavior all the elements of air that surround it. And the behavior of those elements is affected by the elements surrounding them. It is through this interaction between all the elements that the air's boundaries (be those boundaries the surface of a wing or the ground) determine the air's motion.

Suppose you were to look at the streamlines surrounding a 2D airfoil far from any boundaries, and then look at those streamlines again when the airfoil is near to a surface (like the floor of a wind tunnel). The shape of those streamlines will generally change in the presence of the surface. What causes the shape to change? The influence of the ground.

Any element of air is under the simultaneous influence of the ground and the wing in this case. If you neglect the presence of the ground, you will not predict the correct shape of the streamlines surrounding the airfoil, and therefore incorrectly predict the air's rate of momentum change.
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Old Aug 24, 2014, 09:43 PM
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As long as you leave it at that, you are mostly OK.

If you take it a step farther and say: "the air's rate of momentum change is always equal and opposite the lift force". Then you have gone too far.

How so? This is not, in all cases, the nicely isolated physics problem you suggest. The behavior of any element of the air is affected by the behavior all the elements of air that surround it. And the behavior of those elements is affected by the elements surrounding them. It is through this interaction between all the elements that the air's boundaries (be those boundaries the surface of a wing or the ground) determine the air's motion.

Suppose you were to look at the streamlines surrounding a 2D airfoil far from any boundaries, and then look at those streamlines again when the airfoil is near to a surface (like the floor of a wind tunnel). The shape of those streamlines will generally change in the presence of the surface. What causes the shape to change? The influence of the ground.

Any element of air is under the simultaneous influence of the ground and the wing in this case. If you neglect the presence of the ground, you will not predict the correct shape of the streamlines surrounding the airfoil, and therefore incorrectly predict the air's rate of momentum change.
What is your point? That reactive lift in a wing or rotor from the action of moving air down is not the only force in play during ground or ceiling effect?
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Old Aug 24, 2014, 09:48 PM
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And as a result, the air's rate of change, of momentum (in accordance with Newton's Second Law) will not be equal and opposite the aerodynamic force on the wing.
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Old Aug 24, 2014, 10:24 PM
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