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Old Nov 18, 2011, 06:56 AM
Launch the drones ...
Ashtabula, OH USA
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Originally Posted by BMatthews View Post
With the plate flat and taped to the legs it is like you just tied the heli down to the ground. With it folded and slotted down between the boxes the boxes are now like the ground and it lifts normally.
Sorry - the above statement makes no sense to me. It's not like it's tied to the ground, because the plate is light enough that the copter can lift it (provided Bernoulli kicks in, which it doesn't). The ground cannot be lifted - BIG DIFFERENCE.
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Old Nov 18, 2011, 06:59 AM
Launch the drones ...
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Originally Posted by Brandano View Post
Incidentally, for each blade of an helicopter rotor, the amount of upwash is the same as the downwash. The flow through the rotor is not the airflow the single blade sees. The rotor generates lift, that is used to push the column of air downwards. But you can't consider the rotor system as equivalent to a wing.
A rotor is a rotating wing.

And upwash is a figment of the imagination - stand behind a prop.
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Old Nov 18, 2011, 07:01 AM
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Originally Posted by JetPlaneFlyer View Post
Tim,
As already pointed out, NASA did not say lift is the result of "shoving air down".. They said it's the result of "turning air", not the same thing at all. Fact is that the air in front of a wing is turned upward then as it passes the wing it's turned downward, and then (in 2D flow) it turns again to level out to the same height as it was before being disturbed by the wing in the first place, no net downwash.

You can quite clearly see this if you follow the streamlines in NASA's 'Foilsim' 2D flow simulation program (taken from the same site where you got your miss-quote): http://www.grc.nasa.gov/WWW/k-12/airplane/foil2.html (also see attached screen shot)

3D flow would look a little different, you would have some downwash in the wake of the wing, the amount of downwash being dependant on Cl and aspect ratio. This downwash would be matched by upwash around the tips (again no net downwash)
Wind tunnels aren't large enough to show downwash - so the engineers factor in what's called "wall effect", and predict the downwash from what they can see and measure in the wind tunnel. Except when testing props, etc., where the downwash is easily measured.
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Old Nov 18, 2011, 07:04 AM
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Originally Posted by Sefco View Post
As an airfoil starts from rest, fluid particles adhere to the surface to create what is known as the no-slip condition. Because of the no slip condition and the viscosity of air, momentum (kinetic energy) is transferred from the airfoil to the surrounding flow field. As the airfoil begins to move at an angle of attack, fluid particles in the immediate vicinity of the airfoil move along with it. The particles on the bottom surface are essentially “pushed” forward into other air molecules causing an increase in pressure, while the molecules on the upper surface are “pulled” along creating a void where they once were and a decrease in pressure.

It is helpful to think in terms of the Ideal Gas law, PV=nRT. On the lower surface, molecules are pushed along so that more molecules (increasing n) occupy a fixed volume (fixed V) and accordingly Pressure must increase (increase P). Likewise, on the upper surface molecules are pulled away, hence less molecules (decreasing n) occupy a fixed volume (fixed V) and the pressure must decrease (decrease P).

Now here is where it get’s interesting: Molecules from both the upper and lower surfaces rush along and attempt to fill the void that was created in the wake of the upper surface. As the idiom states Nature Abhors a Vacuum. Molecules from the lower surface will attempt to move from the high pressure of the bottom surface to the low pressure of the upper surface by accelerating around the SHARP trailing edge. It would require an infinite velocity to make the U-turn around the trailing edge and as a result a vortex is created as the fluid attempts to go around the trailing edge. This is known as the starting vortex and it separates from the trailing edge as the airfoil moves away and eventually dissipates due to viscosity.

Kelvin’s circulation theorem states that the circulation around a closed curve moving with the fluid remains constant with time. In other words the circulation created by the starting vortex must be accompanied by an equal and opposite circulation around the airfoil itself. The system will reach equilibrium when the magnitude of the circulation around the airfoil is sufficient to move the rear stagnation point to the SHARP trailing edge. This is what is known as the KUTTA CONDITION, read it over thousands of times if you must, it is the TRUE foundation of modern aerodynamics.

An important property of Vortices is that they form lines called vortex lines, that must start at fluid boundaries or form closed loops. In the case of the finite wing, the vortex line creates a closed-loop comprised of the circulation around the finite wing connected through the tip vortices that span all the way back to the starting vortex. This is essentially just a large vortex ring. The Kutta-Joukowski lift theorem states that the lift created is directly proportional to the circulation. So say an airfoil increases it’s angle of attack, then more circulation is needed to move the rear stagnation point to the trailing edge, more circulation = more lift. An interesting side note here is that with a change in lift a starting vortex would be shed from the rear trailing edge that was equal in magnitude to the circulation required for the change in lift.

Bernoulli’s equation is a neat way of accounting for the conservation of energy. The drop in pressure requires an increase in velocity in order for energy to be conserved, nothing more, nothing less. A common technique for calculating lift is integrating pressure measurements over the upper and lower surfaces of the wing. This technique would not work if Bernoulli’s principle did not apply.

In terms of your helicopter plate experiment, aircraft fly because the wings possess surface area for the pressure to interact with. In your case the upper surface of the rotor creates a low pressure and the lower surface of the rotor creates a high pressure. By attaching the cardboard, the upper surface of the cardboard now interacts with the increased pressure generated on the lower surface of the rotor and produces a net downward force equal and opposite of the thrust being created by the rotor. As BMathews stated previously, if you were to measure the force in the landing gear it would be equal to the lift being created by the rotor.

I saw a similar situation in a cartoon where a character in a boat blew directly into his own sails to generate thrust, when in reality he would go nowhere, much like your helicopter with the cardboard.

Also Google Vortex Ring State in regards to downwash generated by a helicopter.

This link is also great reading.
http://www.df.uba.ar/users/sgil/phys...ewton_lift.pdf
Wrong - here's why ... If you change the shape of the plate attached to the skids, so it sends the air in a particular direction, rather than in all directions, the copter will move in the opposing direction. Proving once again, that the downwash air is the energy, that lifts the chopper. (Because it can be used to move the chopper in a direction other than vertical up)
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Old Nov 18, 2011, 07:08 AM
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Originally Posted by DPATE View Post
Great example Brandano. Books do not have to be peer reviewed like journal articles.

One thing I have learned from this discussion is that the larger, bolder, and more colorful the text, the less I care about what I'm reading.

Sefco, thanks for the great explanation of a starting vortex, I agree that helps give understanding to how the steady state solution becomes the steady state solution.

Tim, the reason you will not be able to understand our explanations of downwash is because you say "stand behind a prop and feel the downwash". Let me make this clear. You are feeling "prop wash". This is not directly equal to the downwash produced by the props in any way. If you want to use the propeller for an example of downwash, you need to draw a diagram and show the local flow over a propeller section, because that is how you examine the downwash produced by it.

Bruce, I think you have clearly stated the flaw that I (and I think most of us) see in Tim's reasoning. Until Tim agrees that the flow leaving the helicopter is the same in the two cases (folded vs unfolded white plate), there is no way this discussion can proceed.
A prop is a rotating wing.

The flow leaving the rotors is the same in either case - and it's irrelevant to the direction of the thrust vector of the chopper. The direction of the lift, or thrust vector, is dependent on what the chopper does with all that air.

A. chopper lets air continue down - lift happens.
B. chopper vectors air in another direction - the chopper responds by moving in the opposing direction - whatever that is.

These are facts proven via experimentation - words cannot undo these facts, no matter how many you use - if you cannot wrap your mind and eyes around experimental data - so be it.
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Old Nov 18, 2011, 07:09 AM
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Originally Posted by HX3D014 View Post
interesting Video.
I wonder if the blades flex just the same amount, or not, in both experiments ?
Blades should flex same, either way. They are providing air flow, no matter what chopper does with that air.
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Old Nov 18, 2011, 07:12 AM
Launch the drones ...
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Originally Posted by Windrider53 View Post
Here is a question for the OP if he comes back. If lift is due to air pushed down, how does a biplane work? If a plane weighs too much for it's wing area and you add a second wind and it flys, how is the bottom wing not blocking the downward air from the top wing for a net lift gain of zero?

Note: lift gain over having just one wing.

Oh, and don't try to try to tell me it depends on where the two wings are in relation to each other. Look at examples of the Beach Stagger wing to the many WW1 and pre WW2 biplanes. The wings are all over the place with basicly the same results.
Then why don't planes with lots of little wings fly well? Because they do get in the way.

Biplanes use two wings for the drag - slows them down, which allows them to do stunts other planes find difficult. Biplanes are also for nostalgia.
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Old Nov 18, 2011, 07:16 AM
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Originally Posted by stardustertoo View Post
Hey, just thought of one other thing that I am curious about in regards to your vacuum behind the wing idea Tim. Wind tunnel testing has shown that the low pressure area also extends some way ahead of the leading edge. To me this would shoot your idea down in flames, but maybe you have a reason for that.

Like I said, I really am truly curious about this theory. Never heard it before so I want to hear more about it.
Not a theory - common sense. If you have a truck, standing still. and then that truck moves forward, air will move into the area behind the truck - this can only happen if there's a low pressure area there, as air never moves from low pressure to high pressure.

This is true for anything that moves in a fluid.
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Old Nov 18, 2011, 08:17 AM
Grad student in aeronautics
United States, GA, Atlanta
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Quote:
Originally Posted by Tim Green View Post
NOTE: Here's the link to the helicopter experiment, showing Bernoulli cannot lift a copter alone ...

http://rcuvideos.com/video/BladeCPBootStrap-wmv
Let make this clear so that we can all agree. It's useless discussing science with you until we can agree on what we see.

When the plate is unfolded, it is roughtly the size of the box. The helicopter is unable to ascend.

When the plate is folded and slotted into the box, the helicopter is able to ascend.

Now, since the plate was laying flat on the box, the geometry is essentially equivalent in both cases. Do you or do you not agree that the air flow is the same in both cases (until the helicopter changes its position by ascending)?
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Old Nov 18, 2011, 08:48 AM
Launch the drones ...
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Originally Posted by DPATE View Post
Let make this clear so that we can all agree. It's useless discussing science with you until we can agree on what we see.

When the plate is unfolded, it is roughtly the size of the box. The helicopter is unable to ascend.

When the plate is folded and slotted into the box, the helicopter is able to ascend.

Now, since the plate was laying flat on the box, the geometry is essentially equivalent in both cases. Do you or do you not agree that the air flow is the same in both cases (until the helicopter changes its position by ascending)?
Of course I don't agree - in one case the airflow's deflected by the chopper, due to the plate it has attached to its skids. And in the other case, the chopper doesn't deflect the airflow, due to the plate being folded.

The ground is irrelevant to the experiment. You'll get the same results if you put the chopper on a grating.
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Old Nov 18, 2011, 09:18 AM
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Not a theory - common sense. If you have a truck, standing still. and then that truck moves forward, air will move into the area behind the truck - this can only happen if there's a low pressure area there, as air never moves from low pressure to high pressure.
Yes, i agree with that, HOWEVER, a truck is shaped entirely different than an airfoil. The "trailing edge" of the truck is rather square, whereas an airfoil is shaped to provide a smooth transition from front to back to minimize that "vacuum" as you call it, for effeciency. See, thats why I am curious.

Furthermore, you still did not actually answer the question I asked, regarding the low pressure being present somehat ahead of the leading edge.

Also, I know you were out for a bit and maybe missed them, but I had a couple other questions there regarding laminar flow airfoils and some airfoils have a negative AoA being the zero lift angle. Maybe you could have a peek back at them and let me know what you think. I am interested in you low pressure sucking the air in faster idea.
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Old Nov 18, 2011, 09:48 AM
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Originally Posted by stardustertoo View Post
Yes, i agree with that, HOWEVER, a truck is shaped entirely different than an airfoil. The "trailing edge" of the truck is rather square, whereas an airfoil is shaped to provide a smooth transition from front to back to minimize that "vacuum" as you call it, for effeciency. See, thats why I am curious.

Furthermore, you still did not actually answer the question I asked, regarding the low pressure being present somehat ahead of the leading edge.

Also, I know you were out for a bit and maybe missed them, but I had a couple other questions there regarding laminar flow airfoils and some airfoils have a negative AoA being the zero lift angle. Maybe you could have a peek back at them and let me know what you think. I am interested in you low pressure sucking the air in faster idea.
Sorry, but that question didn't interest me. Why? There are pressure gradients whenever air moves. Proves nothing. I like experiments that isolate Newton from Bernoulli, as these are the most helpful to my understanding of lift.

Sure a truck is shaped differently from an airfoil - but I was trying to use an example everyone's seen for themselves - but the concept's the same - move something forward in a fluid, and a low pressure area will develop behind it. This something includes wings, props, rotors, fans, birds, balls, etc.

Start with a plank, laying flat (at 0 AoA).
Tilt the plank up a bit (positive AoA)
A low pressure area will form on the top of the plank (remember the truck).
That low pressure area accelerates the air moving over the top of the plank.

How else would the air over the top of the plank increase it's velocity?

And remember, the air under the plank will slow down a bit, due to the higher pressure under the plank, once it's tilted up. (For understanding the air under the plank, we don't use a truck analogy, instead we stick our hand out the window of our car while tooling down the freeway, to feel the higher pressure in front of an object (our hand) moving in a fluid (the air).)
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Old Nov 18, 2011, 10:07 AM
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Sorry, but that question didn't interest me. Why? There are pressure gradients whenever air moves. Proves nothing
Didn't interest you??? Well it interested me, that why I asked it!! You are the one who has this theory that you are trying to push, so why not answer the question so people who are interested can wrap their heads around it. You really are not helping you case at all by only answering the questions you want, and ignoring the rest.

Quote:
How else would the air over the top of the plank increase it's velocity
Well, the way I understand it, by tilting that plate up and giving it a bit of AoA, you are doing a very similar thing to a normal airfoil, causing the air to be turned. And that turning action is where that acceleration comes from. Just like your very first link in your very first post explains.
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Old Nov 18, 2011, 10:22 AM
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Originally Posted by Tim Green View Post
I like experiments that isolate Newton from Bernoulli, as these are the most helpful to my understanding of lift.
i like the comment, i'd like to understand the relationship better too. But doesn't newton describe how force affects a mass, while bernoulli describes how a force is created?

after reading some texts, i see that the airspeed across the surface of an airfoil is not constant, so the arguments used to counter bernoulli using only airfoil surface length differences seems flawed.

my take is that bernoulli explains how an airfoil can pull/pu on the air to generate a force (presumably more efficiently that just a rudder). That force not only balances the weight of the aircraft, but as newton says, accelerates the air causing it to move. Can't see how the wing can generate a force without moving air.

what happens to the air after that is irrelevant ... unless, of course, that airflow impacts a part of the aircraft (e.g. thrust reverser, flat plate).

greg
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Old Nov 18, 2011, 10:52 AM
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Originally Posted by ciurpita View Post
what happens to the air after that is irrelevant ... unless, of course, that airflow impacts a part of the aircraft (e.g. thrust reverser, flat plate).

greg
You said it both ways - and you cannot have it both ways. Either it's relevant, or it's not. And so far, experiments prove it's always relevant. Always - an object moves opposite the direction of any air it exhausts.


Someone took a box. Cut three holes in the front, and put the box on wheels. Then they took a fan, attached it to the middle hole in the box, such that the fan was blowing into the box. Bernoullists would claim that the box would move forward, pulled by the fan. But the experiment showed the box moving backward, propelled by the air it was expelling through the other two holes.

Block the two exhaust holes, and the box doesn't move at all. Bernoulli alone, cannot move the box.

This experiment, plus the chopper with the plate - prove Bernoulli ain't lifting nuttin.

No one on this forum, has come up with a chopper experiment, showing Bernoulli only lift, or any Bernoulli lift. But we can come up with an infinite number of experiments showing the following ...

A. No have lift without throwing air down.
B. The direction of an object's thrust vector is determined by the direction of the air leaving the object. (not the direction of any props, rotors, fans, etc., moving air into the object)

And rotors are wings - which is why fans and choppers make this work so easy - if you don't disregard the experimental data.
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