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Jun 04, 2013, 09:20 AM
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Quote:
Originally Posted by Nereth
It's oversimplified, but let's say for arguments sake that was true for now, what of it?
Is not the force created by the action/reaction between the rotor blades and the air the same regardless whether you've a plate on the top of the heli or not?
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Jun 04, 2013, 09:26 AM
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Quote:
Originally Posted by funfly2
Is not the force created by the action/reaction between the rotor blades and the air the same regardless whether you've a plate on the top of the heli or not?
No, the plate changes the flow field. And I have to sleep, will deal with the rest of this tomorrow.

Edit: I should ask you to clarify, do you mean to ask if the net force vector applied by aerodynamic forces on the heli is the same, or do you mean the way that vector is made? My answer was assuming the latter - is that vector made the same way. It is not. It does have the same magnitude and direction if the heli is doing the same thing though. And now, sleep. Zzzzzz
Last edited by Nereth; Jun 04, 2013 at 09:35 AM.
Jun 04, 2013, 09:35 AM
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ShoeDLG's Avatar
Quote:
Originally Posted by funfly2
Let us say that the force/static pressure that reaches the scale is created by the action/reaction between the rotor blades and the air.
Is that correct?
Nope. If you go there you've already headed down a hopeless path in at least two ways.

1. The use of: "reaches the scale" implies that the pressure is being created somewhere else within the air and then convected or transmitted to the surface of the scale. In steady, subsonic flow, the pressure at the surface of the scale is "created" at the surface of the scale (simultaneously influenced by the the conditions throughout the entirety of the air and along the air's boundaries). You're ytrying to perpetuate the "sequential fallacy".

2. The video in post #1447 show pretty clearly that action/reaction between the rotor blades, the scale and the air results in different pressures on the scale than interaction between the rotor blades, the top plate, the scale and the air. It should be readily apparent why your proposal is a non-starter.
Last edited by ShoeDLG; Jun 04, 2013 at 09:41 AM.
Jun 04, 2013, 10:17 AM
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Quote:
Originally Posted by ShoeDLG
Nope. If you go there you've already headed down a hopeless path in at least two ways.

1. The use of: "reaches the scale" implies that the pressure is being created somewhere else within the air and then convected or transmitted to the surface of the scale. In steady, subsonic flow, the pressure at the surface of the scale is "created" at the surface of the scale (simultaneously influenced by the the conditions throughout the entirety of the air and along the air's boundaries). You're ytrying to perpetuate the "sequential fallacy".
If you rise the heli further away from the top of the scale the force on the scale will decrease and so the readings, this despite the force created by the action/reaction between the rotor blades and the air still is the same as when the heli was closer to the scale.
And if you keep rising the heli away from the scale you’ll come to a point where you barely can read any force applied on the scale, hence the expression “reaches the scale”.


Quote:
Originally Posted by ShoeDLG
2. The video in post #1447 show pretty clearly that action/reaction between the rotor blades, the scale and the air results in different pressures on the scale than interaction between the rotor blades, the top plate, the scale and the air. It should be readily apparent why your proposal is a non-starter.
The value shown on the scale is not necessary the same as the force created by the action/reaction between the rotor blades and the air as described above.
Last edited by funfly2; Jun 04, 2013 at 10:26 AM.
Jun 04, 2013, 10:32 AM
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ShoeDLG's Avatar
Quote:
Originally Posted by funfly2
And if you keep rising the heli away from the scale you’ll come to a point where you barely can read any force applied on the scale, hence the expression “reaches the scale”.
That may depend on the size of your scale.

It seems we agree that a scale below a helicopter will not, in all cases, indicate the helicopter's weight.
Jun 04, 2013, 10:44 AM
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Quote:
Originally Posted by ShoeDLG
That may depend on the size of your scale.
I guess if you climbed "enough" away from the scale you won't get any readings but "noise" from the surrounding air, even if you used an enormous scale...


Quote:
Originally Posted by ShoeDLG
It seems we agree that a scale below a helicopter will not, in all cases, indicate the helicopter's weight.
Well, I never questioned the readings on your scale, but how you interpret them...
However, as far as this experiment is concerned, the value shown on the scale is always a result of the action/reaction between the rotor blades and the air.
Jun 04, 2013, 10:52 AM
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ShoeDLG's Avatar
Quote:
Originally Posted by funfly2
However, as far as this experiment is concerned, the value shown on the scale is always a result of the action/reaction between the rotor blades and the air.
The reading on the scale changes when a plate is introduced over the helicopter.

If your statement is true, then the action/reaction between the the rotor blades and the air must also change when a plate is introduced over the helicopter (barring hysteresis/multivalued behavior).
Jun 04, 2013, 11:08 AM
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ShoeDLG's Avatar
Quote:
Originally Posted by funfly2
I never questioned the readings on your scale, but how you interpret them...
Do we then agree that:

If a large scale below a helicopter in ground effect accurately indicates the rate at which the helicopter is transferring downward momentum to the air, then the video in post #1447 shows that the rate at which a helicopter transfers downward momentum to the air is not generally equal to the helicopter's weight?
Jun 04, 2013, 11:08 AM
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Quote:
Originally Posted by ShoeDLG
The reading on the scale changes when a plate is introduced over the helicopter.

If your statement is true, then the action/reaction between the the rotor blades and the air must also change when a plate is introduced over the helicopter (barring hysteresis/multivalued behavior).
No, the force created by the action/reaction between the rotor blades and the air must be the same, the difference may be due to some part of that force doesn't reach the scale...

I've been thinking about that for a while and came to another suggestion:

When you put the plate over the heli, you prevent the air from above to freely flow into the rotor blades, so some part of the air that is pushed down by the blades returns back to top of blades instead of contributing to the force that reaches the scale.

You could try confirming this by introducing smoke around the heli's rotor blades and see the flow changing when you put on the top plate.
Last edited by funfly2; Jun 04, 2013 at 11:15 AM.
Jun 04, 2013, 11:22 AM
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ShoeDLG's Avatar
Quote:
Originally Posted by funfly2
When you put the plate over the heli, you prevent the air from above to freely flow into the rotor blades, so some part of the air that is pushed down by the blades returns back to top of blades instead of contributing to the force that reaches the scale.
Then your for your stament to be accurate, it should read: "the value shown on the scale is always a result of the action/reaction between the rotor blades and the air and a result of the action/reaction between the top plate and the air."

Sounds to me like you're suggesting the top plate might be modifying how much downward momentum the helicopter is putting into the air.
Last edited by ShoeDLG; Jun 04, 2013 at 11:42 AM.
Jun 04, 2013, 12:46 PM
Registered User

physics


From simple, basic, classic Newtonian* physics, the force eventually distributed on the Earth, assuming a flying aircraft with mass to which the wing is attached, and despite the difficultly of accurately measuring this widely distributed force** [NOT pressure] which has been quite diligently shown in this thread, both with physical and thought-experiments, AND assuming that the aircraft is not speeded up into orbit so that it falls around the Earth, NOR flies so high as to reduce the gravitational force between it and the Earth .... HAS to be the same as the force exerted by it on the Earth when the aircraft is stationary, landed and NOT flying ....


namely, its WEIGHT


But, please "carry on", gents!

[it is a fun game,methinks]


Lee



* two masses in free space which are mutally gravitationally atrracted will only separate if a mutual FORCE acts on both of them to make them separate, in so many words

** air being a viscous fluid, one might consider this to be a VERY "extended ground effect"
Jun 04, 2013, 02:34 PM
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aeronaut999's Avatar
Thread OP
Re the post immediately above: yes, that was the original point of this thread.

Otherwise, an aircraft flying a circle of 1 mile radius around the north pole, at 1000 feet msl, for a few million years, would perturb the earth's orbit through its gravitational attraction on the earth. Which just doesn't seem right...

Steve
Jun 04, 2013, 03:37 PM
Registered User
Quote:
Originally Posted by ShoeDLG
Then your for your stament to be accurate, it should read: "the value shown on the scale is always a result of the action/reaction between the rotor blades and the air and a result of the action/reaction between the top plate and the air."
I would like to reformulate it as follows:
The value shown on the scale is always a result of the action/reaction between the rotor blades and the air. The value shown may or not may be close to the heli's weight depending on how far the heli is from the scale and on possible strange objects placed so they may change the normal path of the airflow that affects the scale readings.


Quote:
Originally Posted by ShoeDLG
Sounds to me like you're suggesting the top plate might be modifying how much downward momentum the helicopter is putting into the air.
No, I assume that the top plate does not modify the downward momentum the helicopter is putting into the air.

The top plate prevents the air from above to freely flow into the rotor blades, this creates an abnormal under-pressure between the top plate and the rotor blades, so some part of the air that was pushed down by the blades turns back to the top of blades instead of contributing to the force that affects the scale, resulting in a lower value shown.
The downward momentum the rotor blades are putting into the air is the same, just part of the air is turning back seeking the under-pressure on the top of the blades without affecting the scale.

Obviously, this scenario does not occur when you remove the top plate.
Last edited by funfly2; Jun 04, 2013 at 03:56 PM.
Jun 04, 2013, 04:04 PM
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ShoeDLG's Avatar
So I think you are suggesting that in all cases, the helicopter is transferring downward momentum to the air at a rate equal to the thrust/lift. Serious question here: what makes you believe that?
Jun 04, 2013, 04:07 PM
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ShoeDLG's Avatar
Quote:
Originally Posted by funfly2
No, I assume that the top plate does not modify the downward momentum the helicopter is putting into the air.
The plate clearly modifies the reading on the scale. So you must also be assuming that the reading on the scale does not indicate the downward momentum the helicopter is putting into the air.


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