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Old Aug 22, 2012, 11:54 AM
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Originally Posted by Tim Green View Post
How can I answer, when I don't know if you agree to this or not - your question was do WE agree?
OK I'll get into the semantics. I believe that if you have a tethered helicopter and you set the RPM and collective of the helicopter such that the tension in the tether is zero, and you cut the tether, the helicopter will remain in the same place and the pressure of the velocity of the air at any point in the air will be unchanged. Do you agree with this?
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Old Aug 22, 2012, 12:22 PM
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Apparantly I am at the wrong tea party
I better go ask Alice ---
'I'm not really sure which parts of myself are real and which parts are things I've gotten from books.'
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Old Aug 22, 2012, 12:59 PM
Launch the drones ...
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Originally Posted by richard hanson View Post
Wayne Newton?
You apparantly did not read the explanation of basics.
Blades mean NOTHING --if they have no air supply and output path which allows them to work. A recirculating path does not accomplish anything
A demo is really not required in this case
It's all elementary
whatever - feel the physics dude
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Old Aug 22, 2012, 01:03 PM
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Originally Posted by ShoeDLG View Post
OK I'll get into the semantics. I believe that if you have a tethered helicopter and you set the RPM and collective of the helicopter such that the tension in the tether is zero, and you cut the tether, the helicopter will remain in the same place and the pressure of the velocity of the air at any point in the air will be unchanged. Do you agree with this?
Yes. The tether is a red herring in the above, as it's immaterial, since there's no tension on it.
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Old Aug 22, 2012, 02:19 PM
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A helicopter tethered to a plate cannot lift the plate because the plate prevents it from putting any net downward momentum into the air by deflecting the air outward. Correct?
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Old Aug 23, 2012, 12:44 PM
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Originally Posted by ShoeDLG View Post
A helicopter tethered to a plate cannot lift the plate because the plate prevents it from putting any net downward momentum into the air by deflecting the air outward. Correct?
Correct - Provided the plate is big enough, and attached to the skids (not tethered far below the chopper IOW).

SparkyPaul has posted a video of this several times - the chopper won't lift with the flat plate attached. But will lift with that same plate attached, so long as it's folded so it's not in the way of the air - video shows this better than I can type.

Aha - found the URL ... http://rcuvideos.com/video/BladeCPBootStrap-wmv
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Old Aug 23, 2012, 02:24 PM
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A helicopter tethered to a plate cannot lift the plate because the plate prevents the rotor from putting any net downward momentum into the air (provided the plate is big enough and the tether is short enough).

Imagine that the rotor's RPM and collective are set so that the tension in the tether is zero (or very, very small). The helicopter will not lift the plate (because the plate prevents the helicopter from putting any net downward momentum into the air)

Now imagine you cut the tether from the plate without changing the rotor RPM or collective. The helicopter will now be in a steady hover just above the plate, and the velocity and pressure at any point in the air will be the same as they were before the tether was cut.

If the pressure and velocity at any point in the air are the same before and after the tether is cut, then the air's net rate of vertical momentum change must also be the same. Before the tether was cut, the air's net rate of vertical momentum change was zero (the plate prevented the rotor from putting any downward momentum into the air). The air's rate of vertical momentum change is therefore zero after the tether is cut.

Once the tether is cut, the helicopter hovering (in ground effect) without changing air's net vertical momentum.
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Old Aug 23, 2012, 03:38 PM
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Originally Posted by ShoeDLG View Post
A helicopter tethered to a plate cannot lift the plate because the plate prevents the rotor from putting any net downward momentum into the air (provided the plate is big enough and the tether is short enough).

Imagine that the rotor's RPM and collective are set so that the tension in the tether is zero (or very, very small). The helicopter will not lift the plate (because the plate prevents the helicopter from putting any net downward momentum into the air)

Now imagine you cut the tether from the plate without changing the rotor RPM or collective. The helicopter will now be in a steady hover just above the plate, and the velocity and pressure at any point in the air will be the same as they were before the tether was cut.

If the pressure and velocity at any point in the air are the same before and after the tether is cut, then the air's net rate of vertical momentum change must also be the same. Before the tether was cut, the air's net rate of vertical momentum change was zero (the plate prevented the rotor from putting any downward momentum into the air). The air's rate of vertical momentum change is therefore zero after the tether is cut.

Once the tether is cut, the helicopter hovering (in ground effect) without changing air's net vertical momentum.
Not really - here's why ...

There are two systems in play - one is the chopper with plate, the other is just a chopper. The fact that the airflow is the same, in this one instance, is a coincidence of position - nothing more. Let's look at it more closely ...

First case - chopper without plate just above ground - it has lift. This system isn't deflecting its own air sideways - the thrust vector of this system is still downward.

Second case - chopper with plate, also just above ground - it has no lift. This system is deflecting its own air sideways. And it no longer has a downward thrust vector.

The air is being deflected sideways, either way - but in one case, an external item (ground) is doing the deflecting - so it has no effect on the system (chopper). In the other case, the system (chopper with plate) is deflecting its own air sideways, so the system has no lift.
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Old Aug 23, 2012, 04:13 PM
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Originally Posted by Tim Green View Post
First case - chopper without plate just above ground - it has lift. This system isn't deflecting its own air sideways - the thrust vector of this system is still downward.
Your analysis indicates that you are only looking at a local volume of air as it passes through the prop disk. Yes, this air is moving downwards. However, the question is about net momentum of the entire volume of air affected by the prop.

Consider a helicopter in a closed box such that the dimensions of the box are a few times larger than the diameter of the rotor. When the helicopter is in a steady hover, there will be an increase in pressure on the floor the the box equal to the weight of the helicopter, and the air will move in a toroidal fashion, it will have no net momentum (assuming everything started from rest). This, of course, is a result of Newton's laws.
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Old Aug 23, 2012, 05:24 PM
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Not really - here's why ...

There are two systems in play - one is the chopper with plate, the other is just a chopper. The fact that the airflow is the same, in this one instance, is a coincidence of position - nothing more. Let's look at it more closely ...

First case - chopper without plate just above ground - it has lift. This system isn't deflecting its own air sideways - the thrust vector of this system is still downward.

Second case - chopper with plate, also just above ground - it has no lift. This system is deflecting its own air sideways. And it no longer has a downward thrust vector.

The air is being deflected sideways, either way - but in one case, an external item (ground) is doing the deflecting - so it has no effect on the system (chopper). In the other case, the system (chopper with plate) is deflecting its own air sideways, so the system has no lift.
The question here is straightforward: is the air's net rate of vertical momentum changing or not? Because the velocity and pressure at any point in the air are the same in the tethered case and the untethered case, you can pick any volume of air you want (as long as it's the same volume in both cases), and the net vertical momentum of that air (and its rate of change) will be exactly the same. If there's no momentum exchange in the tethered case, there cannot be any momentum exchange in the untethered case. Either you don't understand what momentum is, or you're trying to pick different volumes of air to look at to support the conclusion you want to reach. You analysis represents nothing more than "faith-based accounting".
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Old Aug 23, 2012, 05:24 PM
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Originally Posted by DPATE View Post
Your analysis indicates that you are only looking at a local volume of air as it passes through the prop disk. Yes, this air is moving downwards. However, the question is about net momentum of the entire volume of air affected by the prop.

Consider a helicopter in a closed box such that the dimensions of the box are a few times larger than the diameter of the rotor. When the helicopter is in a steady hover, there will be an increase in pressure on the floor the the box equal to the weight of the helicopter, and the air will move in a toroidal fashion, it will have no net momentum (assuming everything started from rest). This, of course, is a result of Newton's laws.
Nope.
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Old Aug 23, 2012, 05:29 PM
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Originally Posted by ShoeDLG View Post
The question here is straightforward: is the air's net rate of vertical momentum changing or not? Because the velocity and pressure at any point in the air are the same in the tethered case and the untethered case, you can pick any volume of air you want (as long as it's the same volume in both cases), and the net vertical momentum of that air (and its rate of change) will be exactly the same. If there's no momentum exchange in the tethered case, there cannot be any momentum exchange in the untethered case. Either you don't understand what momentum is, or you're trying to pick different volumes of air to look at to support the conclusion you want to reach. You analysis represents nothing more than "faith-based accounting".
Shoe, shoe, shoe - are you dpate in disguise - cause you both believe in the impossible, with your net momentem stuff - that props blow air both directions equally.

I cannot imagine, what it must be like, to sit across from you at a table perhaps, and listen to you tell me that props blow air both ways, equally. To sit in front of someone, spouting nonsense like that - what do you say?
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Old Aug 23, 2012, 05:40 PM
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To sit in front of someone, spouting nonsense like that - what do you say?
Although it doesn't merit a mature response, I suppose I would say:

Based on your responses in posts #514 and #516, you yourself have all the information you need to see that there is no net momentum exchange in either case. You're just choosing to see only what you want to see (as you have been doing all along). I'm sure that a sharp high school student with no formal physics training would look at your convoluted analysis in post #518 and recognize it as pure nonsense.
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Old Aug 23, 2012, 07:00 PM
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I hope this doesn’t mean Helicopter Week is over already. I really enjoy it.

On another note. Since I have been learning aerodynamics from Tim I have made new friends at the loco airport. Before when I would try to talk to the pilots they would always blow me off. The other day I started talking about knockers and they all smiled at me and said they love to talk about knockers.

They told me about a lady that just got bigger knockers and now she’s in the mile high club. That’s pretty high right?

Anyway, thanks Tim.

























Not to be taken seriously. But really, have you read this thread?
LOCO airport
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Old Aug 23, 2012, 07:29 PM
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Hey,

Did you guys happen to notice, that NASA took the Bernoulli explanation off of their kiddie page, as well as the adult portions of their web sites? Wow - that happened only within the last few months. Apparently, they are trying to set the record straight - now, when you see the word lift in NASA web pages, it's a link, pointing to a page stating that lift is a Newtonan reaction from the wing turning the air downward. All NASA references to lift, link to this one page.

NASA's trying real hard not to confuse anyone - you guys ought to visit them.

audios amigos


And oh yeah - almost forgot - Feeeeeeel THE PHYSICS.
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Last edited by Tim Green; Aug 23, 2012 at 07:30 PM. Reason: feeling the physics
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