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Bernoulli vs Newton

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Bernoulli vs Newton

Old 06-18-2008, 07:59 PM
  #26  
Tim Green
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Default RE: Bernoulli vs Newton

Looking at the experiment that initiated this thread, it seems to indicate that the momentum of the air is the primary lift factor, since the plate under the helicopter could only negate the lift provided by the rotors due to the momentum of the air hitting that plate. One force cancels the other. Pretty neat experiment.
Old 06-19-2008, 09:39 AM
  #27  
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Default RE: Bernoulli vs Newton

ORIGINAL: Tim Green
IMHumblerO, the formula for gravitational acceleration doesn't explain what causes gravity, what gravity is. The formula merely allows us to mathematically model gravity. So, do Newton's forumlas, or Bernoulli's, explain lift? Or do they merely allow us to mathematically model certain observed phenomena when air moves over a wing? Without explaining the cause(s) of lift?
Yes, it's a rather bizarre coincidence that we now how to very accurately predict how gravity will act under given circumstances through the theories of Newton and Einstein, but that we do not really know what gravity is. Nevertheless, in the macroscopic world we have not yet made any observations that do not obey either Newton's or Einstein's theories. Until such observations have been made physicists and engineers will stick with these models.
Old 06-19-2008, 11:51 AM
  #28  
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Default RE: Bernoulli vs Newton

ORIGINAL: Shoe

What if instead, you increased the distance between the rotor disk and the ground plate (with fixed plate size)? I think most would also agree that beyond a certain critical rotor-to-plate separation, the helicopter would also be able to lift off.
I agree.
For any given rotor-plate separation, can you always make the plate big enough to keep the helicopter on the ground (assuming the plate doesn't get heavier as it gets bigger)? I think most would say "yes you can".
I disagree. It seems to contradict your first statement. What if the given separation were 60 meters?
Suppose that there were a scale under the foam plate in the video. Would the scale reading go down as you applied collective? If so, how would the size of the plate effect this? Could you make the plate big enough so that the scale reading wouldn't change at all as you applied collective?
I think (given the separation distance in the video) the scale reading would go down as the plates radius is reduced to less than that of the rotors radius. The scale would directly measure mg-ma.
What does this have to do with Bernoulli and Newton? Well, suppose that you COULD make the plate big enough so that the scale reading wouldn't change with collective. And suppose further that no matter how much you separated the rotor from the ground plate, you could always make the the plate big enough to keep the scale from changing with collective. This raises an interesting question about momentum. If the scale reading doesn't change, that suggest that the air exerts no NET vertical force on the helicopter-ground plate system. By Newton's third law, if the air exerts no net force on the helicopter-ground-plate system, then the helicopter-ground-plate system exerts no net force on the air. Newton's second law then leads us to the inescapable conclusion that the helicopter ground plate system can't be changing the NET vertical momentum in the air.

This should be troubling if you think that a helicopter flies by transferring momentum to the air. If the above is true, then if you consider a big enough piece of the earth's surface, the net downward force exerted by the helicopter on that surface is equal to the upward force on the helicopter (at any altitude). If that's the case, then the helicopter can't possibly be changing the momentum in the air. Yikes, that would certainly challenge my notions of how a helicopter flies!
I think that you could separate the rotor from a weightless plate enough that no matter how big the plate got the helicopter could still lift off and climb in altitude. Interplanetary space probes (Cassini, Magellan) make small course corrections using Newtons 3rd law and there is no air or planet to push against.
Interesting thoughts Shoe. I always enjoy your posts!
Old 06-19-2008, 05:25 PM
  #29  
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Default RE: Bernoulli vs Newton

It should be noted that rockets, jets, and propellers to not move because of "pushing against" air. They move by reacting to their acceleration of gas molecules, whether by combustion (rockets and jets) or by deflection (props). Rockets have more power when there is no air slowing down that acceleration.
Old 06-19-2008, 07:54 PM
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Tall Paul
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Default RE: Bernoulli vs Newton

I just put the heli and the plate on a scale..
Got about 2-1/2 oz of lift..
The heli weighs 12 oz.
http://rcuvideos.com/item/4875GZ10XC4N8430
Old 06-19-2008, 08:36 PM
  #31  
Tim Green
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Default RE: Bernoulli vs Newton


ORIGINAL: Red B.

ORIGINAL: Tim Green
IMHumblerO, the formula for gravitational acceleration doesn't explain what causes gravity, what gravity is. The formula merely allows us to mathematically model gravity. So, do Newton's forumlas, or Bernoulli's, explain lift? Or do they merely allow us to mathematically model certain observed phenomena when air moves over a wing? Without explaining the cause(s) of lift?
Yes, it's a rather bizarre coincidence that we now how to very accurately predict how gravity will act under given circumstances through the theories of Newton and Einstein, but that we do not really know what gravity is. Nevertheless, in the macroscopic world we have not yet made any observations that do not obey either Newton's or Einstein's theories. Until such observations have been made physicists and engineers will stick with these models.
Absolutely - we'll stick with these models. But my point remains - these models don't explain how wings work - they only allow us to predict what a wing will do. (pretty brash - huh?)

So, neither Newton's, nor Einstein's (who had some bizarre airfoil designs) nor Bernoulli's work will explain why a wing lifts. That requires observation, experimentation (like Tall Paul's neat helicopter experiments) and the application of logic.

Which I think you agreed with Mr. B., but with forums, one can never be sure - which is why I'm restating my point - and also, I've nothing better to do at the moment. Think I'll go shopping. Cheers.
Old 06-19-2008, 08:58 PM
  #32  
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Default RE: Bernoulli vs Newton

Tall Paul,

Very cool! What happens if you double the area of the foam plate?
Old 06-19-2008, 09:30 PM
  #33  
Tall Paul
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Default RE: Bernoulli vs Newton

Doubling the plate area?
I would expect maybe some decrease in the lift.
It's not going anywhere with the plate as it is, so I won't do much in that direction.
It's too hot in here to do anything with this now. (101 inside, 102 outside.. the a.c. is off and will be off for the summer. I'm really "thrifty".)
Ask again in November!
Old 06-19-2008, 10:42 PM
  #34  
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Default RE: Bernoulli vs Newton


ORIGINAL: CrateCruncher

....Rock,
I agree drag would occur in the vertical direction and could rob thrust but I'd bet the logging mod was to trim tare weight for more payload. Green wood is extremely heavy and non-essential weight in a commercial transport application can make a huge difference in total profit over the useful life of the equipment.
It's the "downal" area as well as the overall weight. I've seen other references to it but they were well in the past so I can't quote them. My buddy also mentioned that it was due to the exposed surface area as well as the weight.
Old 06-21-2008, 06:26 PM
  #35  
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Default RE: Bernoulli vs Newton

Shoe still has me thinking about Tall Paul's cool experiment. I remember as a kid I used to like the window seat just aft of the trailing edge on commercial flights because I could watch the terrain and control surfaces of the wing. Just as the plane touched down the pilot would nail the throttles open and huge clam shell deflectors would swing around to reverse the direction of the engine thrust and slow the plane. I still think the ground plate is merely a deflector that is redirecting the thrust of the rotor 90 degrees.

If this is right, the helicopter's lift will increase as the distance of the rotor to the ground plate increases even if you increase the size of the deflector proportionally. At some separation distance (1 m?, 2 m?) the ground plate should have no influence at all on the system. Any weight added as a consequence of increased distance or ground plate size could be subtracted from the new scale reading.
Old 06-21-2008, 08:25 PM
  #36  
timothy thompson
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Default RE: Bernoulli vs Newton

the downwash acted as a brake preventing liftoff. notice at one point in the vid one of his links holding ther training gear breaks and the heli trys to lift. this is the same as tying a heli to the ground. it wont move. the rotor downwash creates upward thrust unless it is attached to a plate that absorbs that force by pushing air from the ground
Old 06-22-2008, 12:00 PM
  #37  
Tall Paul
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Default RE: Bernoulli vs Newton


ORIGINAL: timothy thompson

the downwash acted as a brake preventing liftoff. notice at one point in the vid one of his links holding ther training gear breaks and the heli trys to lift. this is the same as tying a heli to the ground. it wont move. the rotor downwash creates upward thrust unless it is attached to a plate that absorbs that force by pushing air from the ground
.
YES!
Very observant!
Old 06-22-2008, 12:24 PM
  #38  
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Default RE: Bernoulli vs Newton

I just remembered this but I'm surprised that no one has mentioned it yet. It was CrateCruncher's post that triggered the foggy memory. He's right and as the spacing grows the effect of the plate reduces. At some point it won't matter how big the plate is the lift will be back to "normal"

Helicopters have their own unique version of ground effect. It results in a high degree of rotor wash recirculation. If someone checks on this they'll find diagrams with a torus or donut shaped vortex flow where the rotor wash is pushed down and out but then flows back towards the lower pressure region over the rotor disc and back into the flow. But without checking I can't remember if it adds to the lift as with a fixed wing airplane or detracts due to having a residual downwards flow component from the donut shaped vortex flow that occurs when in this ground effect. Actually it would appear to reduce it based on this test since as mentioned as the plate to heli distance grows the effect of the plate will lessen
Old 06-22-2008, 12:30 PM
  #39  
Tim Green
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Default RE: Bernoulli vs Newton

downwash = thrust (lift)
Old 06-29-2008, 07:07 AM
  #40  
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Default RE: Bernoulli vs Newton

I'm not convinced that it doesn't matter how big the plate becomes. Take a setup (meaning a combination of rotor-to-plate separation and plate size) that won't hover. Imagine you quadruple both the separation and the plate "diameter". Now imagine that you look at the setup from four times as far away. From this perspective, it appears that the separation and plate diameter stayed the same, but the ROTOR diameter got 4 times smaller. Getting back to the original setup where the helicopter wouldn't hover... would you expect to be able to make it fly by reducing the rotor diameter? (remember that part of the thought experiment was that the weight of the plate didn't change as it got bigger). I think you always can make the plate big enough to prevent hovering (at least in "steady state").
Old 06-29-2008, 10:33 PM
  #41  
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Default RE: Bernoulli vs Newton

The video is like jumping out of a plane with a parachute on to prove that Newton's calculation of gravity on Earth (9.8m/s2) is wrong. That's just MY take on it. The thrust from the prop makes the chopper go up, but it also pushes DOWN on the board. If the chopper is also attached to the board (as it was in the video)....it'll create a TOUCH more lift than downforce, which is visible when the chopper gets light on its skids. That difference is only there because the board doesn't absorb 100% of the rotor's thrust (because nothing does 100%).
Old 06-30-2008, 09:19 AM
  #42  
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Default RE: Bernoulli vs Newton

Perhaps we'll get some more data to argue when Tall Pauls lab finally cools off.
Old 06-30-2008, 11:00 AM
  #43  
Tall Paul
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Default RE: Bernoulli vs Newton

They ain't nuthin' preventing anyone else from doing anything more on this with their own equipment.
Just be consistent.
Old 06-30-2008, 12:21 PM
  #44  
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Default RE: Bernoulli vs Newton

This thread is still going on? In that case I will post an article written by Peter Garrison called The Bernoulli Brigade. This article was featured in the June 2008 issue of Flying magazine. It's interesting reading anyway.


The Bernoulli Brigade By Peter Garrison

It must be a perennial embarrassment to high school physics teachers that cheap balsa gliders - to say nothing of folded up pieces of paper, or butterflies - can fly. After all, it says right here in the official textbok that airplanes fly because air has to go a longer distance over the top of the wing than under the bottom, and so (because of some guy named Bernoulli) there is more pressure below the wing than above it.

This Bernoulli fellow, who lived several hundred years ago, was stating a simple fact of the physics of fluids (actually, fluids flowing through pipes) that he considered more or less self-evident. He would be no better known to pilots today than d'Alembert or Torricelli had his name not come to be associated with an appealingly simple - but unfortunately flawed - explanation of lift. Because in fact air particles marching past a wing are not like an ordered mass of soldiers - call them the Bernoulli Brigade - who part at the leading edge and rejoin their buddies at the other end. There is nothing to cause the upper-surface flow to arrive at the trailing edge at the same time as the lower-surface flow, and it doesn't. Actually, it gets there sooner.

But it is not even necessary that the distance air travels along the upper surface of a wing be greater than the distance it travels along the lower, as the folded paper airplane, the butterfly and the simple balsa glider show. If the angle of attack is small enough, all that is necessary is that it be positive; that is, it is the fact that the wing is at a nose-high angle to the passing air that is fundamentally responsible for the generation of lift. Curving the flat surface, adding thickness and shaping it like an airfoil are techniques for reducing its drag and allowing it to produce more lift and achieve a greater angle of attack without stalling; but the airfoil shape is not indispensable - it is a refinement.

What is remarkable about airfoils, however, is how well they work. They allow wings to multiply air pressure. The lift generated by an ordinary wing when it is just about to stall is about 50 percent greater than the pressure exerted by air striking a wall at the same speed. Think about that - it's really pretty remarkable. The force exerted by wing blowing "past" an object can be greater than the force exerted by that wind blowing "against" it, provided that the object has a certain shape. It's almost as if you could change your weight by making faces while standing on the scales.

The pressure of air blowing directly against a flat surface is called the dynamic pressure. It is about 25 pounds per square foot (psf) at 100 mph (I am using mph here, rather than knots, to preserve the easily remembered 100:25 relationship). The maximum lift of an ordinary wing, no flaps, is between 30 and 50 percent more than that. The lifting force is a function of the square of speed; at 50 mph it is a quarter of 25 psf, at 200 mph four times 25. The ratio between the dynamic pressure and the lift force just before the stall is called the maximum lift coefficient, and it is around 1.5 for plain airfoils. Good slotted flaps can push it above 3.0.

For single-engine airplanes weighing less than 6,000 pounds, federal regulations require a stalling speed of no higher than 61 knots. (This rule has to do with the chances of surviving a forced landing, which is considered more probable in a single-engine plane than in a multiengine one; the value of 61 knots - 70 mph - is arbitrary, a compromise between low landing speed and a reasonably small wing area.) The dynamic pressure at 61 knots is approximately 12 psf. An airplane without flaps can therefore weigh - theoretically at least - no more than about 18 pounds per square foot of wing area.

This is where flaps come in. A simple plain flap - something similar to an aileron that moves only downward - can add another 50 percent to the maximum force-multiplication of the wing, bringing the permissible wing loading up to 27 psf. A slotted flap can raise it to 35 psf, a multiple-slotted flap to nearly 40.

These figures are ideal ones. They suggest that a Cirrus or a Columbia with a gross weight of 3,400 pounds could make do with a wing hardly larger than the front door of your house. In fact, however, their real-life wing loadings do not exceed 25 psf. Where did the rest of the lift go?

The answer begins with the fact that wings have tips. The pressure difference between upper and lower surfaces causes spillage at the tips - this is the reason for the tip vortex - and robs the wing of 5 to 10 percent of its theoretical lift. Another loss occurs at the center of the wing, where the fuselage interrupts the airflow. The imaginary portion of the wing that lies within the fuselage - reported wing area includes this hidden part - produces, in reality, no lift. But changes in pressure are gradual, not instantaneous, and so the effect of the fuselage is to produce a dip rather than a sharp-edged gap in the spanwise distribution of lift. Depending on the fraction of the wing area that lies within the fuselage, another 10 or 15 percent of the potential lift may be lost there.

Next, flaps seldom extend over the full trailing edge of the wing. Usually, the outboard 40 or 50 percent of the span is reserved for ailerons. This outboard portion of the wing would produce only about a third of the lift, flaps up, because of the tip losses I mentioned before. But flapped wings produce their lift at a lower angle of attack than unflapped ones do - that's why the nose comes down when you put the flaps down - and so the unflapped outer portion of the wing never gets into its own stalling angle of attack, and it consequently yields less lift that it is really capable of.

Finally, the flap has its own tip losses. The accompanying computer-generated illustration shows a wing with a single-slotted Fowler flap - this one moves all the way back to the trailing edge before deflecting 30 degrees. It is close to its stalling angle of attack. The colors on the fuselage and wing encode pressures; colors tending toward the red, for instance on the upper surface of the wing, indicate low pressure, you can see the lifting force on the wing that is holding the airplane up. Green is neutral, purple is high pressure. The strings are streamlines - the paths followed by air particles flowing past the wing - and their colors, too, indicate air pressure. You can see that low pressure is not confined to the wing surfaces, but forms a kind of cloud - graphically, a pink glow - above the wing.

Pressure and velocity are inversely related; this is the physical fact that we associate with Bernoully. Thus, the reddish portions of streamlines indicate accelerated flow, and the blue-green portions retarded flow.

Think of the strings as columns of soldiers in the Bernoulli Brigade. They seem to have had a jolly time last night. (Parenthetically, "Taps," the mournful bugle tune played to summon soldiers to their barracks for the night, gets its name from the taps of beer barrels in the local pubs.) Columns of molecular soldiers are thworn into violent swirls by the ends of the flaps, each of which generates its own private tip vorted (whose core is visible, if you carefully pick your seat in a landing jet on a moist day, as a trembling gray rope). The flow over the middle of the flap veers inward above the wing and outward below it, producing a scissors like shearing at the trailing edge. The low pressure generated by the flap distorts the ranks of streamlines on the outer panel, pulling them inward, while the vortex at the tip of the wing is quite weak, indicating that not much lift is being generated out there.

It should be apparent from this picture why the traditional account of wing lift is wrong. Air molecules do not file past a wing in neat rows and rejoin their mates at the trailing edge. Not only do the upper-surface particles outrun the lower-surface ones, they go every which way while doing so. Streamlines that curve obliquely across the wing don't even see the airfoil as it was designed, but instead a whimsically distorted version of it. And this, in plainly visible form, is why a wing whose flap can theoretically carry 36 psf at 61 knots will, in reality, support only 25.
Old 07-06-2008, 06:02 AM
  #45  
BillHarris
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Default RE: Bernoulli vs Newton

What a wonderful thread.

It's discussions like this that convince me that aircraft fly because of hot air

Bill
Old 07-06-2008, 07:54 AM
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Nathan King
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Default RE: Bernoulli vs Newton


ORIGINAL: BillHarris

What a wonderful thread.

It's discussions like this that convince me that aircraft fly because of hot air

Bill
Yep, apparently pilots produce enough of their own hot air to defy gravity.
Old 07-06-2008, 01:00 PM
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Default RE: Bernoulli vs Newton

Thank you Nate, now I understand why there are always good thermals over our field
Seriously though, I very much enjoyed reading your post, it made some things clearer for this layman
Regards,
Pete
Old 07-07-2008, 08:26 AM
  #48  
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Default RE: Bernoulli vs Newton

Garrison outlines the "obvious".
Not making fun!! Just noting that many explanations of lift are simply goofy.
His is not.
If some of the studious types would just go lie by a creek and watch how watr flows around pebbles etc., -they MAY make the leap of realization that air and water share the same basic laws .
most basic :
They always take they path of least resistance.
Lift is simply the result of pressure difference.
The same thing happens when water moves a pebble.
Of course -being humans -we tend to over complicate by attempting to simplify.
The coptor on a plate is a classic example of the "obvious"
It ain't how much force is applied - it is how it is applied.
Old 07-07-2008, 09:04 AM
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Default RE: Bernoulli vs Newton

This is from the NASA website. It talks about swimsuit design for the US olympic team. The previous post talks about the similarities of water and air flow, so I posted it to show "Studious" types are aware of the similarities!

One area Wilkinson focuses on is drag reduction … how to make something propel though air or water faster and more efficiently. That was Wilkinson's task for Speedo … to test swimsuit material for drag.

"We evaluated the surface roughness effects of nearly 60 fabrics or patterns in one of our small low speed wind tunnels," said Wilkinson. "We were assessing which fabrics and weaves had the lowest drag. The tests have generally shown the smoother the fabric, the lower the drag."

But how can you test a material that's supposed to be used in water in a wind tunnel that uses air? It turns out that air and water obey the same physical laws of motion, so a wind tunnel can simulate similar drag a fabric would experience in water. Testing in air is also less expensive, simpler and offers more testing options than testing in water.

Old 07-07-2008, 09:24 AM
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Default RE: Bernoulli vs Newton

It works the other way too. I didn't have access to a wind tunnel so I once did drag tests on a model car in a swimming pool. I used an electric motor and measured the current draw. It turns out you can get a close approximation to air at a much slower velocity because the Reynolds number must be the same - higher density medium so lower speeds. After calculating the drag coefficient I then compared it to one derived from full-scale coast down tests at freeway speeds. They were within about 20% of one another as well as the manufacturers published number. Certainly not accurate enough for commercial work but cheap, interesting, and I learned something which was the intended purpose anyway.

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