May 22, 2012, 08:17 PM Registered User Germany, BW, Stuttgart Joined Mar 2012 1,081 Posts Truth Squad, your excellent advice is noted. There is some irony in the use of a helicopter hovering in ground effect to illustrate momentum transfer. In the video, the helicopter is pushing down on the air with average force of about 4.25 oz. (Newton's Third Law suggests the force exerted by the helicopter on the air is equal and opposite the force exerted by the air on the helicopter). When the helicopter is over the glass plate, the average force that the plate exerts on the air increases by about the weight of the helicopter. If the helicopter and the plate are the only two things exerting a force on the air (and it's not hard to imagine an extension of this situation where that truly is the case), then the air experiences no unbalanced force. Newton's Second Law suggests an object experiencing no unbalanced force will experience no change to its momentum. If momentum exchange is the exclusive mechanism behind the generation of aerodynamic force, then it's curious that there appears to be a force acting on the helicopter in a situation where there isn't any momentum transfer to the air. But, you say, what this example shows is the helicopter is putting exectly the right amount of downward momentum into the air, and the glass plate is taking it back out. If instead of using the air as a medium, suppose the helicopter was creating an upward reaction force equal to its weight by shooting pellets down at the plate. I don't think there's any doubt in this case that the helicopter would be putting downward momentum into the pellets that is later taken out by the plate. Momentum exchange would clearly be the mechanism responsible for the upward force on the helicopter. Is this not essentially what's going on with the air? There is an important difference. If you consider all of the pellets (the ones still in the helicopter, the ones in transit, and the ones already on the plate), the center of gravity of this collection is continuously shifting downward (eventually all the pellets will have moved from the helicopter to the plate). A continuous downward shift of the center of gravity is an inescapable feature of any arrangement where there is a delay between adding net downward momentum into a collection of particles and taking it back out. The pellet example illustrates that a continuous downward shift of the center of gravity means that the helicopter will eventually run out of the thing it is putting momentum into (it will have moved all of the pellets - or air - below it). I suspect that you could run the helicopter for a very long time before you pushed all the air below it. This suggests that the INSTANT the helicopter transfers momentum to the air, all of that momentum is taken out by the plate. Another way of saying this is that the helicopter hovering over the plate never actually puts any momentum into the air at all. I suppose I could say that when I stand, I transfer downward momentum to the soles of my shoes, and that the floor instantly removes that momentum, but I don't think that's an accurate description of what's going on. The motion of the air induced by the helicopter creates an increase in the pressure on the plate. This pressure is what "holds the helicopter up", not the transfer of momentum to the air, because the momentum of the air never changes.
 May 22, 2012, 09:17 PM Ascended Master Palmdale, CA Joined Oct 2000 13,621 Posts More fun with helis, plates and scales. Heli flying over a plate on a scale.. http://www.youtube.com/watch?v=cTuIt...4&feature=plcp . Heli flying in a covered box. http://www.youtube.com/watch?v=nDnzV...3&feature=plcp . Heli lifting a plate attached to the skids. http://www.youtube.com/watch?v=wtIxL...5&feature=plcp . And again. http://www.youtube.com/watch?v=aF8A-...7&feature=plcp . Heli with a plate underneath, in the air. http://www.youtube.com/watch?v=IToJj...2&feature=plcp . Watching a heli approach a protrusion from a level surface, about a rotor diameter above the surface shows how the downwash being interrupted by the protrusion can deflect the helis flight path away from the protrusion. One can have a lot of fun with a \$50 helicopter...
May 23, 2012, 08:44 AM
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Quote:
 Originally Posted by ShoeDLG Another way of saying this is that the helicopter hovering over the plate never actually puts any momentum into the air at all.
This statement is so very, very, very wrong.
May 23, 2012, 09:38 AM
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Quote:
 Originally Posted by Tim Green This statement is so very, very, very wrong.
Good to know, because when I wrote it I though it was just very, very wrong.

If Newton is to be believed, then a net change in the air's momentum must be accompanied by an unbalanced force. When the helicopter is over the plate, the video shows the plate pushing up on the air with an additional force of about 4.25 oz. The helicopter is presumably pushing down on the air with about 4.25 oz. (its weight). So where is the unbalanced force that is causing the air's momentum to change?
 May 23, 2012, 09:49 AM Registered User United States, UT, Salt Lake City Joined Oct 2007 8,727 Posts You guys must be bored -or just having too much fun! Why not try measuring the low pressure above the heli as well as total pressure increase below On second thought - just weigh the helicopter
 May 23, 2012, 10:35 AM Grad student in aeronautics United States, GA, Atlanta Joined Oct 2010 526 Posts Shoe is looking at the entire air mass, saying that the net change in momentum of all the air affected by the heli and plate is zero. I think he would agree that locally the air in some places is changing momentum.
May 23, 2012, 12:19 PM
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Quote:
 Originally Posted by DPATE Shoe is looking at the entire air mass, saying that the net change in momentum of all the air affected by the heli and plate is zero. I think he would agree that locally the air in some places is changing momentum.
Exactly. The helicopter's interaction with the air will clearly impart some upward momentum to the air, and some downward momentum to the air. Unless the air is experiencing an unbalanced force, the net momentum imparted must be zero.

If it's accurate to characterize the aerodynamic force on the helicopter as being a result of the downward momentum transfer, then it is no less accurate to characterize it as being a result of the upward momentum transfer (and for some reason one direction is considered gospel and the other blasphemy).
May 23, 2012, 12:33 PM
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Quote:
 Originally Posted by ShoeDLG Good to know, because when I wrote it I though it was just very, very wrong. If Newton is to be believed, then a net change in the air's momentum must be accompanied by an unbalanced force. When the helicopter is over the plate, the video shows the plate pushing up on the air with an additional force of about 4.25 oz. The helicopter is presumably pushing down on the air with about 4.25 oz. (its weight). So where is the unbalanced force that is causing the air's momentum to change?
The force comes from the charge that was put into the model's battery. Where did you think it came from?
May 23, 2012, 12:42 PM
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Quote:
 Originally Posted by ShoeDLG Exactly. The helicopter's interaction with the air will clearly impart some upward momentum to the air, and some downward momentum to the air. Unless the air is experiencing an unbalanced force, the net momentum imparted must be zero. If it's accurate to characterize the aerodynamic force on the helicopter as being a result of the downward momentum transfer, then it is no less accurate to characterize it as being a result of the upward momentum transfer (and for some reason one direction is considered gospel and the other blasphemy).
Wow - So a chopper rotor blows air both directions at the same time. That's an interesting observation you're making chum. Very interesting. Well - I hear someone calling - got to go ...
May 23, 2012, 12:43 PM
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Quote:
 Originally Posted by richard hanson You guys must be bored -or just having too much fun! Why not try measuring the low pressure above the heli as well as total pressure increase below On second thought - just weigh the helicopter
Bored? Hmmmm. Not now. Maybe earlier ... though.
May 23, 2012, 01:13 PM
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Quote:
 Originally Posted by Tim Green The force comes from the charge that was put into the model's battery. Where did you think it came from?
Humor me chum. Provide an accounting of the forces acting on the air in this situation and show there's an imbalance.
May 23, 2012, 01:34 PM
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Quote:
 Originally Posted by Tim Green Wow - So a chopper rotor blows air both directions at the same time.
Just saying the motion of the rotor causes some air to go up and some to go down. Don't take my word for it though...

# Images

May 24, 2012, 05:37 AM
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Quote:
 Originally Posted by Tim Green Wow - So a chopper rotor blows air both directions at the same time. That's an interesting observation you're making chum. Very interesting. Well - I hear someone calling - got to go ...
The image below shows actual flow measurements near a lifting rotor disk (not in ground effect). The left side of the horizontal black line is the rotor hub and the right is the blade tip. Even out of ground effect, the air outside of the blade tip (and even some air inside the blade tip) is going up. So yes, a chopper rotor absolutely blows air both directions at the same time.

# Images

May 24, 2012, 07:11 AM
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Quote:
 Originally Posted by ShoeDLG Humor me chum. Provide an accounting of the forces acting on the air in this situation and show there's an imbalance.
I just did - the battery. Why can't you get that?
May 24, 2012, 07:28 AM
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Quote:
 Originally Posted by ShoeDLG The image below shows actual flow measurements near a lifting rotor disk (not in ground effect). The left side of the horizontal black line is the rotor hub and the right is the blade tip. Even out of ground effect, the air outside of the blade tip (and even some air inside the blade tip) is going up. So yes, a chopper rotor absolutely blows air both directions at the same time.
Well you are welcome to your physics - where fans apparently blow the same amount of air in both directions. Doesn't happen in my world though. Strange, how our worlds are so different.