VFFman is walking down the street when he spots his arch-rival, HBman, master of the counterintuitive. HBman is cleaning his car.
On closer inspection, the car seems rather odd. It is shaped like an oil-drum on its side, mounted on four wheels. Besides that, the wheels are obviously go-cart in style: there is no propulsion to the wheels. Nor is there any sign of a propellor, nor of rockets or jets.
VFFman points out this lack of propulsion, mockingly (as he always does) to HBman. "Au contraire", replies HBman "it has a 3.6 litre V8 and runs very nicely, thank you." "But," comments VFFman, "your engine isn't connected to the wheels, nor to any other propulsion system." Walking to the front-end of the car, HBman says "But you have neglected THIS, my Bernouilli Disc!"On closer inspection, VFFman notes that the flat, circular face of the car is actually free to spin, rather like a propellor but with no blades. There is only a minute gap between this disc and the front bulkhead of the car, and this gap appears to be filled with lubricating banana oil."So?" says VFFman "Having a spinning circular face is not going to get you anywhere! You have invented a zero- efficiency propellor!" "Oh yeah?" Replies HBman, climbing into his car and starting the engine. The disc begins to spin and slowly, but with dignity, his Bernouillimobile starts to glide forward, neatly running over VFFman's toes as it inches forward."What the dot dot dot?" mutters VFFman to himself, and then a thought crosses his mind. "Could it be" he mutters to himself "that the front surface of the spinning disc, being in motion relative to the air, experienced a reduced pressure due to the Bernoulli effect? And could it also be that the rear face of the disc, being seperated from the front bulkhead by only a thin film of banana oil, experienced no such force? And could this Bernouillification result in a net force driving the vehicle forward? Egads, I think that may be it! Outbaked again!"-- Basepair, Apr 02 2005 A page on how airofoils work http://www.av8n.com...s.html#sec-spinners [zen_tom, Apr 03 2005] A page on the Bernoulli effect http://scienceworld...ernoulliEffect.htmlBernoulli effect is real, even though its importance in aerofoil lift is often exaggerated. [Basepair, Apr 03 2005] Bernoulli meets Magnus, to sail a ship http://www.grc.nasa...2/airplane/cyl.html"the propulsion force generated was less than the motor would have generated if it had been connected to a standard marine propeller!" [AbsintheWithoutLeave, Apr 04 2005] + just for the banana oil.-- DesertFox, Apr 02 2005 Why, thank you, [DesertFox] :-) Looks like I could have saved myself many words and one fishbone if I'd limited the idea to banana oil alone....-- Basepair, Apr 03 2005 I do enjoy the continued adventures of VFFman and HBman - but am not accustomed to this Earth thing you call the Bernoulli effect.
Normally I'd look it up myself, but I'm not feeling very researchy today, would you mind providing us with some explanationary links (preferably with nice animated graphics) please?-- zen_tom, Apr 03 2005 i'm not familiar with the bernoulli effect, but it sounds remarkably inefficient. doesn't get much more half-baked than that. croissant.-- Cthulhu, Apr 03 2005 Animated graphics are way out of my league. I am barely animated myself. However, the Bernouilli effect basically says that if air is moving over a surface, it exerts less pressure than static air. This is one way to explain why an aerofoil wing works (the air going over the curved top has further to travel, so has to move faster, so exerts less pressure above the wing than that below). If you want a simple demo, take a sheet of paper and hold the edge just on top of your bottom lip, leaving the paper drooping downwards. Blow hard (over the top of the paper) and it'll rise up towards the airstream.I'm assuming here that any small square you imagine on the face of the disc will experience a relative airflow (since that 'square' is moving as the disc rotates), and hence feel a reduced air pressure, effectively drawing the disc forward, oddly.-- Basepair, Apr 03 2005 [Cthulu] - wildly inefficient, I would imagine. HBmans's BernoulliCopter has yet to leave the ground, sadly....-- Basepair, Apr 03 2005 OK, I'll cope without the pretty graphics. However, I am going to put my intuition to the test and say that the difference between the curved surface of the aerofoil and the flat surface of HBman's device is that the air is moving in one example and static in the other.
i.e. it is the velocity of the gas or liquid under question that causes the pressure drop, rather than it's 'relative' motion to some other object.
Since static air is in evidence both in the front and at the back of the machine, there should be no difference in pressures, and the car will remain in situ, only to be engulfed by ants, bees and other roving insects attracted by the sweet, sweet aroma of banana oil...
Alas, I fear VFFman may be victorious this episode.-- zen_tom, Apr 03 2005 Ah, but Zen, the air *is* moving over the surface of the disc as it is over a wing. Imagine yourself ant-like standing on the surface of a 747 wing in flight - you feel a 350mph wind, apparently coming from the front of the plane. Now imagine yourself standing on any point on this disc (except the dead centre) - you will likewise feel a wind as the disc spins. The direction of the wind makes no difference (the lift it produces is at right-angle to the surface in either case). The basic point is that the front surface of the disc, like the surface of a wing, experiences an air flow across it.-- Basepair, Apr 03 2005 By the way, the curvature of the wing is not directly responsible for the Bernoulli effect (although, confusingly, it *does* add another cause of lift by a different mechanism) - in this context, the curvature is just a means of speeding up the airflow over the top of the wing (compared to that underneath it).-- Basepair, Apr 03 2005 And this is the question, is it relative motion that causes the effect, or absolute motion?
I would land on the absolute side (which is a very rare thing for me to do) and say that the pressure drop is a property of the atoms in the gas being more rarefied when at speed, than having anything to do with nearby spinning objects.
Ah, so is the curve a Bernoulli thing or not? If not then I need to go and do some research...-- zen_tom, Apr 03 2005 It's relative motion that counts. Basically, air pressure is the effect of momentum transfer from gas molecules to the surface as they rebound. When the air is moving over a surface, the "average" gas molecules hit the surface at a glancing angle, and there is less transfer of momentum. It makes no difference whether it is the air that moves or the surface. (If it did, relativity would have come to a grinding halt long before Einstein.)-- Basepair, Apr 03 2005 "There is a widely-held misconception that it is the velocity relative to the skin of the wing that produces lift. This causes no end of confusion." This quote taken from the link provided. There are some good bits further up the page (I linked to a part mid-way-through) that describe all manner of airofoilery.-- zen_tom, Apr 03 2005 A non-curvey-surface demonstration of the Bernoulli effect: place a drinking straw in a glass of your chosen beverage and blow across the top of it - the liquid will rise. (If you blow slightly downwards, however, you're blowing into the straw and will have the opposite effect. The Bernoulli effect *is* fairly weak, and it's difficult to demonstrate it in its 'pure' form. But it is real - I'll add links).-- Basepair, Apr 03 2005 [zen] thanks for the link. The problem with aerofoils is that they acheive lift through a whole slew of mechanisms, including 'reaction lift' (overall, the wing displaces air downwards as it moves through it, and hence displaces itself upwards). Although the Bernoulli effect is often cited as the cause of aerofoil life, it is a minor component. But, nevertheless, the Bernoulli effect per se is well established.-- Basepair, Apr 03 2005 I think the question here is 'what is causing the pressure drop'?
i.e. is it relative motion? or is it the property of a gas/liquid in motion?
I don't buy the atoms bouncing off at tangents. If that was the case, then a tennis ball dropped on a spinning record-player wouldn't bounce as high as one bounced on the ground. Experiment anyone?-- zen_tom, Apr 03 2005 [Zen] I take your point about the tennis- ball (though the real experiment would be to see if the record-player were displaced downward as much, not whether the ball bounced as high). My physics isn't good enough to understand why 'glancing' blows transfer less momentum than 'right- angle' blows (given that the component of the velocity at right-angles to the surface is the same). However, I believe that Bernoulli's law (apparently there's a law!) is well established. This leaves the question of whether it is caused by relative or absolute motion of the air, and I would argue that it *cannot* be caused by absolute motion of the air. All of my (inadequate) knowledge of physics tells me that, if a certain apparatus behaves one way when it is held against a wind of 100mph, it will behave exactly the same if it is on the back of a truck driving through still air at 100mph. It can only be the velocity of the air relative to the surface that matters. (For that matter, the whole of the earth's atmosphere is moving at huge velocities relative to the sun, and even faster relative to the distant stars, but we never have to worry about this in aerodynamic calculations!)-- Basepair, Apr 03 2005 OK, I'm going to imagine a hole bunch of air travelling westward at 100mph.
At the same-time, underground at Cern or wherever, there's a guy driving circuits at 100mph in his specially designed zero-turbulence car.
So both systems are equivalent (not sure about the zero turb car, but just for the argument)
The two air systems are essentially the same. If you point-measured the air-pressure at 1m intervals, you'd get a 3d matrix of equal values.
Ok, now we add an airofoil. If we point-test the airpressures now, we find a lower pressure above the wing - note, it's not at the surface of the wing, but in the area immediately above it.
If we set our point-pressure sensors to velocity mode we also see that the air is going faster above the wing than below it.
So the speed of the air has been changed by the addition of the airofoil, but the pressure drop doesn't happen at the surface, but in the region above the surface. The airofoil has sculpted the airflow around it to make this change.-- zen_tom, Apr 03 2005 The aerofoil does indeed sculpt the air (a very nice phrase, by the way), as a means of increasing the velocity of air over its surface. I guess an aerofoil is a nasty messy thing, and perhaps not the cleanest example of the Bernoulli effect. (Having said that, I note that the aerofoil works the same on the car as in the wind.) But now, try a second experiment. Instead of an aerofoil, we have a drinking straw standing dead vertically in a glass of water. In both cases (in the wind, and on the car) the liquid in the straw rises because the air moving (relatively!) over the top of the straw exerts a lower pressure.-- Basepair, Apr 03 2005 This makes me think of a special pipe tool that blows air (not vacuum) to lift up delicate silicon discs when the velocity is correct. This should theoretically mean that a tire-driven “bernoullimobile” with motionless disc, going fast enough would suddenly feel less resistence and a leap in speed.-- FarmerJohn, Apr 04 2005 Cool.As for the tennis ball dropped on a spinning record player, the ball will not bounce as high as on a stationary surface because some of the energy will be imparted in spinning the ball and some of the energy will be spent knocking it away from plum.
I think.-- 2 fries shy of a happy meal, Apr 04 2005 Lift Force: A force produced perpendicular to the flow of a fluid. It is predicted by Bernoulli's law for any obstacle which compresses streamlines at one boundary and compresses them on the opposite one, resulting in a difference in pressures which causes the lift force
Lift Force = one half of the Lift Coefficient times the area of the wing times the angle of attack (in radians) times ...wait...we don't need to go any further!!
The disc on the bernoullimobile has a zero angle of attack, so the Lift Force is also zero.
Very creative thinking however [Basepair] , so there's no way I'm boning you.-- ConsulFlaminicus, Apr 04 2005 [2 Fries] I disagree; the vertical component of motion will be identical for the ball dropped on the hard surface and for the ball dropped on the record player. The vertical component is independent of the horizontal impulse imparted by the player.-- david_scothern, Apr 04 2005 I suppose that makes sense. The turn table is adding its own energy to the equation.-- 2 fries shy of a happy meal, Apr 04 2005 My intuition tells me that this thing is going nowhere. Because there is no downdraft created, and nowhere for the air to displace to, there is no net force generated.
See zen's link.-- RayfordSteele, Apr 04 2005 Bernoulli states that for a constant TOTAL PRESSURE, a fast-moving fluid will have lower STATIC pressure than a slow-moving fluid.
Now, we have your spinning disk at the front of the HB-mobile. When measured from a point on the disk, the air in front will have the same static pressure as anywhere else. It will have a higher dynamic pressure, however. Unfortunately, since it's the static pressure that would do the work, the HB-mobile will not move forward or backward.
There is another effect that would come into play, though. Since the interface between the disk and the air is not frictionless, some of the air would be carried along with the disk. Centrifugal force (yes, I know there's no such thing, as it's simply an observed manifestiation of momentum in the presence of CENTRIPETAL acceleration) will move the air outward. Since some air has now been displaced away from the front of the HB-mobile, there will be a low-pressure region in front, which will initially pull the HB-mobile forward. Shortly thereafter, a circulation will develop from the disk rim back towards the center, and you'll once again have zero net thrust, if you're lucky. If you're unlucky, the forward-flowing component of the flowfield will entrain outside air, causing a net REVERSE thrust.
Sorry, HB-man loses again.-- Freefall, Apr 04 2005 I disagree. I see this as using the exact principle that centrifugal pumps use, but replacing vanes with friction. Think of your record player having ridges (molecular structure that causes friction) - the balls won't bounce very high upwards. I think it will be a very inefficient centrifugal pump, but will have a forward force nonetheless. Yes, there will be a circular current that will make this even more inefficient, but if it moves forward at all, then air will have been displaced backwards and the circular current will not form a perfect circuit.
If you'd like this to work much more efficiently, put a flat plate in front of the disk that's wider than the disk, with a hole in the center for air intake. Curve the outside edge around the front of your vehicle. Then, for much more efficiency, add vanes.-- Worldgineer, Apr 05 2005 [Worldgineer] that would be a fundamental redesign, turning it into a type of laminar flow pump, with the propulsive force reliant on the backwards deflection of the airflow from the 'pump', rather than any Bernouilli effect.-- ConsulFlaminicus, Apr 05 2005 I'm assuming the backside of the spinning disc is hidden from the wind.-- RayfordSteele, Apr 05 2005 Blimey! I go away for one day, and all thjis annotation happens! Wayyyy out of my depth with some of it, but:
re tennis-ball on turntable, I agree with David S and 2fries - the vertical component of the bounce should be unaffected by the turning of the turntable (I think). But this is an aside, sort of.
[ConsulF] //Lift Force = one half of the Lift Coefficient times the area of the wing times the angle of attack// - I'd always understood that the Bernoulli lift is independent from (though much less than) the reaction lift (which arises largely from the angle of attack), and that you can get aerofoil lift with zero angle of attack. In fact, this is why aerofoils are used rather than just flat slabs - to generate additional life beyond the a-of-a lift (I thought).
[Freefall] - I can't comment on your anno on Bernouilli effect, it's out of my depth. I suspect you're right as it's stated, but I think it still leaves a paradox. I guess the questions one would need to ask of a Bernoulliist are:
1) If I have a flat surface still air, and I then blow across that surface (ie, on one side only), does the surface experience a lift? My reading of Bernoulli says 'yes', and this fits with the experiences of (a) blowing across the top a straw (reduced pressure does lift liquid in the straw) and also (b) blowing across the top of a sheet of paper (it does get lifted 'into' the airstream).
2) If the answer to (1) is 'yes', then is there any difference between the surface moving through the air and the air moving across the surface? The answer to this can only be 'no', or relativity goes to pieces.
3) If the answers to 1 and 2 are 'yes' and 'no' respectively, then can a section of one face of the disc be considered equivalent to the flat surface described? The answer to this must be 'yes' (I'm setting aside the centrifugal/petal effects - they'll be real but a seperate matter).
4) If the answers to 1,2,3 are 'yes','no','yes', then does a disc spinning in free air experience a Bernoulli force on each face, tending to pull its faces apart? (Yes, given the answers to 1,2,3).
5) If the rear face of the disc is not exposed to the air, should there therefore be a net force acting forwards (effectively caused by the difference between the reduced pressure at the front of the disc and the 'normal' pressure at the back of the whole car). Presumably 'yes', if the answers to 1-4 are yes/no/yes/yes.
Now, the only one of these questions I'm not sure about is [1] - I don't understand the theoretical underpinnings of the Bernoulli effect well enough to answer from first principles. But all the sites I checked seem to say that a moving fluid exerts less lateral pressure than a static fluid, and the drinking-straw and paper- blowing experiments seem to agree with that.
I guess the point is that I *don't* feel that the car should work, but I can't see the flaw in the logic that says it should.-- Basepair, Apr 05 2005 [AWOL] Re the link - eppur si muove! (However, the Magnus effect is acting sideways on a rotating cylinder, which is not the same effect I was thinking of here. But interesting link anyway!)-- Basepair, Apr 05 2005 You're all forgetting one fundamental piece of this system: The banana oil. Since the (fluid) oil is in contact with the backside of the plate, it will also be subject to the bernoulli and what-have-you effects. Even as the plate creates low pressure at the air side, sucking the plate forwards, it *also* creates low pressure on the oil side, sucking the plate backwards. Intuition tells me the net force will be zero.
It is also worth noting, as a side comment, that the Magnus Effect only works if the cylinder is already in translational motion relative to the air. Therefore it is only suitable for redirecting preexisting forces and cannot actually generate any lift or other thrust by itself. In a sentence, the Magnus Effect can only serve to partially redirect a force from its original direction.-- 5th Earth, Apr 05 2005 I thought the preexisting force was friction causing air to flow in a spiraling outward direction across the face of the disc.-- FarmerJohn, Apr 06 2005 Aside from the (brilliant) use of banana oil, this is a silly example of yet another mis-understood physics principle.
Party on.-- moPuddin, Apr 06 2005 [5th Earth] //Even as the plate creates low pressure at the air side, sucking the plate forwards, it *also* creates low pressure on the oil side, sucking the plate backwards.// Ah, but, the force on the back of the disc will simply be pulling the plate and the front bulkhead of the car together, and the layer of banana oil (supplemented if necessary with some good old fashioned bearings) keep them just seperated. Hence, there's still a net force between the front of the car and the back of the car.
//as a side comment, that the Magnus Effect only works if the cylinder is already in translational motion// I agree comletely, but we're not using the Magnus effect (acting at right angles to the long axis of a cylinder) but a Bernoulli effect acting (putatively!) at right angles to the face of a disc. The Magnus effect was sort of a red herring...-- Basepair, Apr 06 2005 [moPuddin] //this is a silly example of yet another mis-understood physics principle// It may well be, and I sort of suspect it is. *BUT* I couldn't see why. Can you help clarify? The only moot point seems to be Question 1 in my earlier annotation. If I've got the answer to this one wrong (and if you can help me understand why) then the problem is solved and HBman goes away sadder but wiser.-- Basepair, Apr 06 2005 Yes, air moving past a stationary object will exibit the same behavior as stationary air with an object moving through it.
No, simply moving air past an object does NOT reduce the pressure.
Unconstrained air (ie air flowing around a vehicle) tends to have a constant TOTAL pressure. Total pressure is the sum of static pressure (what you'd feel from a non-moving reference) and dynamic pressure (the result of the momentum of the air).
Example: If you take a big flat plate (at sea level) and stick a pressure sensor in (not on) the surface, you will measure 14.7 psi. If you then took this plate and blew across the surface, you would still measure 14.7 psi. If you mounted it to the top of a car so that it was perfectly level and drove at 100 mph (ignore the disturbance caused by the presence of the car), you would STILL measure 14.7 psi.
There will be no change in static pressure caused by moving a flat plate through the air.
Read up on "pitot-static probes". These are used on aircraft to measure speed. Their function depends on the fact that simply moving an object past a body of air does not change the pressure, when measured from a surface that is parallel to the flowfield.
This flat plate will have a very inefficient centrifugal pump effect, but without some secondary constraint designed to force the airflow out of the toroidal circulation which will develop, you will get zero thrust from it.
Also: regarding the "molecules meeting up at the trailing edge" theory and the "longer path over the top" theory, I could show you water-tunnel photos that disprove both. Imagine a hypothetical airfoil which is a smooth curve on top, but which is very wavy on the bottom. The bottom would be longer, but the airfoil would still produce lift.-- Freefall, Apr 06 2005 [Frefall] //you would STILL measure 14.7 psi// Damn.
If that's the case, then all is lost, and I will join HBman for a beer so that we can console ourselves. I guess I don't understand Mr. Bernoulli after all, and this probably stems from a non- understanding of dynamic vs. static pressure. Woe is me. HBman 2: VFFman 1.-- Basepair, Apr 06 2005 [FF], what effect causes a piece of paper, when I blow on one side, to fly in that direction? How is this different than your plate on top of a car?-- Worldgineer, Apr 07 2005 The air has someplace to go. With the spinning plate, the only displacement would be from the 'centrifugal effect.'-- RayfordSteele, Apr 07 2005 Worldgineer, take a look back at what I initially said about Bernoulli. An unconstrained airstream will have a fixed TOTAL pressure, which is the sum of STATIC and DYNAMIC pressure. If you make the air faster (by blowing it through a nozzle), the dynamic pressure goes up and the static pressure goes down.
When you blow across the top of the paper, your lips are acting as a nozzle to speed up the air. Static pressure goes down, dynamic pressure goes up, but total pressure stays the same. Since the flow is parallel to the surface (thanks to effects discovered and described by a man named "Coanda"), the surface only "feels" the static pressure. Note, that since the airflow must curve downwards in order to follow the paper, there will be an additional reduction in pressure due to the momentum of the air. I won't go into that now, but you can find out more by looking up Coanda.
Since the air below is not moving (zero dynamic pressure), it has full atmospheric static pressure. Since the air above is moving, it has some dynamic pressure, and therefore less than full atmospheric static pressure. Hence, the paper rises.
Looking back at my car example, I'll admit that I was a bit off. If you blow across the top, you will measure less than full atmospheric pressure.
However, with this disk, you're still not changing the properties of the air in front of the HB-mobile. The air itself is not moving, hence it will have the same pressure as the air behind the HB-mobile.
I really wish teachers would stop teaching "Bernoulli" when describing why an airfoil produces lift. Reaction would be much more intuitive, and more accurate too.-- Freefall, Apr 07 2005 [freefall] //Looking back at my car example, I'll admit that I was a bit off. If you blow across the top, you will measure less than full atmospheric pressure.//Oh no! Now I'm more confused than ever. This means that the answer to Question 1 in my earlier annotation *was* yes after all, and I can't find any flaw in my reasoning on questions 2-5, which means that the car should move.At the same time, though, I agree utterly withyour statement that we're not changing the properties of the air in front (ignoring any centrifugal effect) and hence the car shouldn't move. So, help! Which one of steps 2-5 is wrong? I'm not arguing that the car has to move, I'm just trying to figure out where the reasoning is wrong.-- Basepair, Apr 07 2005 I've been trying to work-out a way to describe the paper-blowing effect - and will now attempt to do so by describing a shallow, but wide bowl/vat containing a high-viscosity liquid.
The liquid starts off in a static mode.... (Oh, for fixed-width annotations) +++++++++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ +++++++++++++++ [top-down view] Until a spoon sweeps along the middle, creating a 'stream' of liquid moving in a particular direction. Bottom to top. +++//|||||\\+++ +++//|||||\\+++ +++//|||||\\+++ +++//|||||\\+++ +++//|||||\\+++ +++//|||||\\+++ +++//|||||\\+++ Notice how the static parts of the liquid(+) blend into the flowing part of the liquid (/\) because the flowing part is 'getting out of the way' (||) So anything (a raisin for example) that is sitting at the edge of the stream will get sucked into it because the flow getting out of the way will have an apparent lower pressure than the static liquid behind it.
Likewise, a piece of paper separating a region of flow and a region of no flow, will experience a similar pull, suck, push or whatever because of the speedier air being blown out of the way.-- zen_tom, Apr 07 2005 [Zen Tom] That is interesting, and thanks. It makes sense to me, I think. However, I'm still not sure where that leaves us. It would suggest to me that the Bernoulli effect is sort of as I had imagined, and that the plate on the top of Freefall's moving car would indeed experience a measurable reduction in pressure (as he said in his last anno). But if that's the case, then the static plate in the 100mph wind would likewise experience the reduction in pressure, which means that any small square on the face of the spinning disc would too, which means the car ought to move. Damn my brain. What am I missing here (apart from common sense)?-- Basepair, Apr 07 2005 Perhaps I was unclear earlier. Blowing over the plate on the car (moving air past the plate) will show a reduced pressure, but driving the car (moving the plate through the air) will show full atmospheric static pressure.
By simply spinning the disk, you're not doing anything to the air. (viscous effects aside; that's been covered by the recirculating flow discussion).
If you put a pitot-static probe on your spinning disk, pointed into the direction of rotation, you would find that the probe does indeed measure higher dynamic pressure (the pitot part of pitot-static) because any part of the disk except for the exact center is moving relative to the air, but the static pressure would still be atmospheric.-- Freefall, Apr 08 2005 Perhaps if there was a goldfish involved...-- RayfordSteele, Apr 08 2005 [Freefall]//Perhaps I was unclear earlier. Blowing over the plate on the car (moving air past the plate) will show a reduced pressure, but driving the car (moving the plate through the air) will show full atmospheric static pressure."Now that IS weird, and I have to admit that I frankly don't believe it as stated- there is no difference in the two situations. For one thing, this means that you would measure a different pressure depending on whether you were driving east (with the Earth's rotation) or west (against it). There's something screwey here, or else I'm mis-reading your statement. Both are possible!-- Basepair, Apr 08 2005 I can't really say more than I've said already. I'd just be repeating myself. I have no idea whatsoever how driving with or against the rotation of the Earth has anything to do with this.
Aerodynamics is not simple, and is often non-intuitive. Some of this has to do with the various incorrect stated-as-fact oversimplifications that are presented early on, and a couple-paragraph statement on my part is just not sufficient to break through these preconceived notions of how things really work.
I'm certainly not perfect, and I readily admit my mistakes. That's evident in this discussion if you read through it. I'm not saying to take my word as fact, but I've given a couple pointers in the right direction if you want to go learn for yourself.
This thing will not work. It will move air around, but it will not provide any thrust in the configuration described.-- Freefall, Apr 08 2005 Here is the difference you seek:
With the paper, there is both the moving air, and the static air surrounding it. This is key, because the static air provides the static pressure baseline and gives the dynamic pressure some value to drop off from, causing the pressure differential and lift.
With the rotating plate, the entire air mass is still static with respect to itself, regardless of the plate's spin.
To differentiate, try this thought experiment: make the plate infinitely smooth, so that the air is not disturbed by the spinning plate. You can easily see why this would have no effect. Static air up against a plate which it can't in any way tell if it's rotating or not simply won't care.
However, if you were to blow across an infinitely smooth piece of paper, no doubt the effect would still work.
Another difference to think about: to the air, the spinning plate represents a wing with an infinitely long chord. Such a wing would never fly, because it would not allow the upper air to displace downwards and cause any action / reaction downdraft.-- RayfordSteele, Apr 08 2005 [freefall] I wasn't getting at you, just saying that I thought 'plate moving through air' and 'air moving past plate' had to be the same, or simple relativity goes out the window and you'd have to worry about some "absolute" frame of reference for all motion (and hence have to take into account the rotation of the earth, the motion of the sun through space, and all that - sort of an aerodynamic equivalent of Michelson- Morley). [rayfordsteele] your point about the *differential* movement of the air makes more sense to me, and I agree with your excellent thought experiment of the infinitely smooth disc. I also agree (from long-held gut feeling) that the car shouldn't move - but couldn't see the flaw in the reasoning. I get the feeling that the conventional explanation of the Bernoulli effect is wrong or missing something, since taking it at its face value leads to these contradictions. Either way, it's been fun.-- Basepair, Apr 08 2005 Relativity still holds, but you have to be sure to include the entire system in the analysis, which includes the air at the rear of the vehicle. If there's no net pressure drop between the air at the rear and the air at the front, (which would be doing the work), then it's not going anywhere.-- RayfordSteele, Apr 09 2005 [Rayfordsteele] Agreed - it's just that there's still some logical link broken between the Bernoulli law as stated and the car not moving. But, I suspect that this is not a problem we humble half- bakers can solve....-- Basepair, Apr 09 2005 BP, send me an email. There's a halfbakery yahoo group that we want to know if you want to join.-- RayfordSteele, Apr 09 2005 At my stage in life, any invitation is a surprise and a flattery. Email sent, if I've deciphered you're email address correctly.-- Basepair, Apr 09 2005 random, halfbakery