Bernoulli's equation is BS!

[drgondog]

I was being a bit flippant, but the general gist of the lecture was that Bernoulli's equation should not be applied to the questiion of why a wing creates lift. My underatanding is that according the equation, airflow separates at the leading edge of the wing and meets at the trailing edge, and as the distance travelled over the curved top of the wing is further than that across the botom of the wing, pressure is lower on top and the wing creates 'lift'.
Unfortunately there are a couple of problems with this. Firstly, what is the imperative that required the two airflows to meet gain at the trailing edge of the wing? If molecules a and b separate at the leading edge what compells a to speed up so it can meet b at the trailing adge. Apparently, nothing. Secondly, as I posited earlier, how can a plane fly upside down? if Bernouli's equation explains lift and inverted aircraft should head down, irrespective of the angle of attack.


To be fair neither Bernoulli or newton ever aplied themselves to the question of lift or created equations to explain that particullar phenomenum. I don't have the link to the particular lecture I was watching, sorry, but a quick search of the web provides heaps of argument on Bernoulli v Newton regarding lift. One point of concensus seems to be that the nice simple diagram of airflow separating over a wing and meeting at the other end is wrong. The actual factors that produce lift are much more complex.

stick with Bernouli and consider that the mathmatical treatement Requires that airflow separating at leading edge must 're-connect' at the trailing edge.

 

[CobberKane]

If you want to prevent a wing from producing significant lift, you attach something to spoil the air over the UPPER surface of the wing. If the action/reaction theory were correct, this wouldn't have much of an effect.

If you tack something on the upside of a wing shouldn't that divert airflow upward, thereby counteracting lift as per newtons second law?

drgondog - do you mean its best to stick with Bernoulli because it's an elegant explanation, even though the observational evidence doesn't support it?

This is great. For our next thread, let's reconcile quantum theory with relativity!

 

[michaelmaltby]

"... do you mean its best to stick with Bernoulli because it's an elegant explanation, even though the observational evidence doesn't support it?"

Surely there's a little more to "lift" than just Bernouli .... like .... air is a fluid and therefore subject to the laws of fluid dynamics ..... high pressure inclined to move to low pressure, air flow behaving like a fluid (layered), the faster a fluid traverses the less pressure it exerts, etc. etc. Bernouli's principle can be demonstrated by the venturi effect whereby airflow through two facing (opposing) airfoils forces them together ... IIRC .... long time since my RCAF Reserve Airframe Tech days ..

MM

 

[gumbyk]

If you tack something on the upside of a wing shouldn't that divert airflow upward, thereby counteracting lift as per newtons second law?

This is great. For our next thread, let's reconcile quantum theory with relativity!

not when its a 1" square piece of timber, it spoils the airflow.

And, would that be special or general relativity?

P.S. Bernoulli didn't say anything about wings, just speed/pressure differences.

Incidentally, the 'particle flowing over the top meeting with the particle flowing under the wing' is a fallacy (Equal Transit Time Fallacy) AFAIK, the speed of the air flowing over the top is too fast for the flow underneath to catch up.

 

[CobberKane]

Personally, I think wuzak might be on to someting with his observation that airliners flap their wings. Another possibility is that the earth blows.
If I accuse Bernouli of being a nazi, I wonder if the moderator would close this thread?

 

[buffnut453]

When I hold the tip of the handle of a spoon and then move the curved back of the spoon into a flow of water from the tap in my kitchen, the spoon is pulled into the water flow not kicked out. I'm no aerodynamicist nor a mathematician but methinks Mr Bernoulli was onto something even if it is not a sufficient explanation for the totality of lift.

Ofcourse, there is Bernoulli's Second Theorem, that every object in the world is covered with small particles, called Bernoullis, that exert small amounts of force omnidirectionally. Since objects have Bernoullis on all their surfaces, the effect of the Beroulli particles cannot be detected. However, when an aircraft wing is moved through the air, the Bernoulli's on the upper surface of the wing are blown off because, as we've all been told, the air moves faster over the top surface than the bottom surface. This creates an excess of Bernoullis under the wing which then push the plan upwards. When the aircraft slows down, more and more Bernoullis are able to catch up with it and they affix themselves to the upper surface, increasingly balancing out the Bernoulli Distribution Quotient (DBQ where 1 is equality) until the aircraft can no longer fly.

Simple, innit!?!?!?!?

 

[CobberKane]

If Bernoulli's equation explains lift, then a wing with zero angle of attack will still generate lift, right? Is that the case? Do helicopter rotors require an angle of attack?

 

[wuzak]

If Bernoulli's equation explains lift, then a wing with zero angle of attack will still generate lift, right? Is that the case? Do helicopter rotors require an angle of attack?

It depends on the profile.

 

[GregP]

This thread is funny.

Lift is due to magic, sort of like air conditioning versus heat pumps. People tell you there's heat in the air even on cold days and that's how heat pumps work. That, of course is stupid. There is no heat in the air on cold days. That's why they're called cold days. If there's so much heat in the air on colds days, how come you don't see heat pump salesmen frollicking on the beaches in New Jersey in February? Huh?

What really happens is the heat pump has demons that go around and steal heat from babies, cats, dogs, and people with a lot of blankets on their beds.

You should get a heat pump before your neighbors do. Bernoului was a heat pump salesman before they were popular. Newton tried to poo-poo heat pumps and espouse air conditioners ... but that was before the toilet was invented, so who cares?

Back then, some Frenchmen would step over the border and relieve themselves on German soil. The Germans would retaliate, but they lived a bit from the border, so they rode horses to get there. When they relieved themselves, it was worse because of the horses. War resulted and the French and Germans fought many wars over nothing but crap. Natrually, there was a lot of horse effluvia to deal with, too.

Sir Robert Crapper, inventor of the toilet, probably knew why wings work, but it went to the septic tank with him. His invention removed the major cause of wars between adjacent countries, but had the side effect of polluting the rivers. It was found that the Reynlolds Number of the floating efluvia created lift in the rivers and caused the stuff to bump into swimmers and generally move along the top of the water.

You couldn't ride a bicycle across it, but it still interested a couple of inventors in Ohio, Orville and Wilbur Wright.

Later the world invented Me 109s and Spitfires, but nobody ever questioned Mr. Bernouli until now ...

Can anyone come up with a relevant reason why Bernouli was wrong? If so, I submit your heat pump will burn out on you soon due to a low pressure across the top of the fan wing. And it SHOULD. The Physics are easily explained by rifts in the space-tiime continuum, which agrees with Mr. Newton and doesn't contradict Mr. Bernouli.

Really, get a heat pump before your neighbor does so yours will get in the habit of taking heat from him before his starts takling heat from you.

 

[Marcel]

The lift of a wing consists of 2 factors:
1. Bernouilli effect
2. Angle of attack

A good wing uses both. AoA is not the only factor, if that would be true, then every aircraft could easily fly inverted, which is not the case. Using only AoA means a very inefficient wing, needing a lot of power to stay in the air. You need to increase the AoA to be able to fly uside down, which incleases aerodynamic drag. Therefore this demands more of the structure and power. Also getting a too high AoA will give you turbulence on the wing, messing up the lift.

 

[oldcrowcv63]

Two things to observe (if you have a good pitching arm and like tea or coffee with cream). First is Buffnut's spoon. But put it in coffee or tea with a dab of unmixed cream added. Gently move the spoon forward in the coffee so the unmixed cream acts somewhat like fluid streamlines. You will see a little vortex (whirlpool) leave the trailing edge of the spoon/airfoil. It's due to the difference in velocity between the upper and lower surface caused by the curvature of your 'spoon/wing.' The bernoulli effect made visible.

Second thing is to just throw a curve ball. (if you can't do it anymore and I really could never do it well, then just think about it.... Gosh that reminds me of something else I find difficult or impossible to do anymore and spend time thinking about ) A ball isn't a wing or even a flat surface but it moves in an arc through the air. That's because it has a spin, or so I learned as a pup. The spinning ball creates a speed differential on opposite sides of the baseball and causes lift-a force to move it in the direction of the higher speed relative airflow (the surface of the ball that happens to be approaching the direction of ball's general motion through the airmass. Now that that has been typed I decided to check it. Turns out there is as much disagreement on whether the Bernouli effect is occurring in the case of the curveball pitch as in the issue of aircraft lift. Bummer... life was more fun when you learned something and it stayed put instead of bouncing around like a ping pong ball or golf ball which some say really does show the lift due to the Bernoulli effect more faithfully than the baseball.

 

[wuzak]

People tell you there's heat in the air even on cold days and that's how heat pumps work. That, of course is stupid. There is no heat in the air on cold days. That's why they're called cold days.

Actually Greg, there is heat in the air on cold days. Just not as much as on warm days. Temperature is the measure of the average heat in a system. Heat is energy.

I suppose there would be no heat in the air if the air was at absolute zero (-273C).

 

[wuzak]

Two things to observe (if you have a good pitching arm and like tea or coffee with cream). First is Buffnut's spoon. But put it in coffee or tea with a dab of unmixed cream added. Gently move the spoon forward in the coffee so the unmixed cream acts somewhat like fluid streamlines. You will see a little vortex (whirlpool) leave the trailing edge of the spoon/airfoil. It's due to the difference in velocity between the upper and lower surface caused by the curvature of your 'spoon/wing.' The bernoulli effect made visible.

Second thing is to just throw a curve ball. (if you can't do it anymore and I really could never do it well, then just think about it.... Gosh that reminds me of something else I find difficult or impossible to do anymore and spend time thinking about ) A ball isn't a wing or even a flat surface but it moves in an arc through the air. That's because it has a spin, or so I learned as a pup. The spinning ball creates a speed differential on opposite sides of the baseball and causes lift-a force to move it in the direction of the higher speed relative airflow (the surface of the ball that happens to be approaching the direction of ball's general motion through the airmass. Now that that has been typed I decided to check it. Turns out there is as much disagreement on whether the Bernouli effect is occurring in the case of the curveball pitch as in the issue of aircraft lift. Bummer... life was more fun when you learned something and it stayed put instead of bouncing around like a ping pong ball or golf ball which some say really does show the lift due to the Bernoulli effect more faithfully than the baseball.

That is called the Magnus Effect.

And can even be used for sailing vessels

 

[oldcrowcv63]

I was being a bit flippant, but the general gist of the lecture was that Bernoulli's equation should not be applied to the questiion of why a wing creates lift. My underatanding is that according the equation, airflow separates at the leading edge of the wing and meets at the trailing edge, and as the distance travelled over the curved top of the wing is further than that across the botom of the wing, pressure is lower on top and the wing creates 'lift'.
Unfortunately there are a couple of problems with this. Firstly, what is the imperative that required the two airflows to meet gain at the trailing edge of the wing? If molecules a and b separate at the leading edge what compells a to speed up so it can meet b at the trailing adge. Apparently, nothing. Secondly, as I posited earlier, how can a plane fly upside down? if Bernouli's equation explains lift and inverted aircraft should head down, irrespective of the angle of attack.
To be fair neither Bernoulli or newton ever aplied themselves to the question of lift or created equations to explain that particullar phenomenum. I don't have the link to the particular lecture I was watching, sorry, but a quick search of the web provides heaps of argument on Bernoulli v Newton regarding lift. One point of concensus seems to be that the nice simple diagram of airflow separating over a wing and meeting at the other end is wrong. The actual factors that produce lift are much more complex.

These are excellent questions and my understanding is that in the first case the imperative is due to the incompressibility of the air. It essentially forces the air flowing over the wing to behave as though it were in a pipe. The video shows the case of a biplane with the upper wing removed. In other words the air flow on the upper surface is essentially moving through a 'throat' or constriction.
The answer to the second question is more subtle. Look carefully at the various airfoils in the video with streamlines surrounding them. Even a symmetric airfoil with a nonzero angle of attack a geometry is created wherein an air parcel must traverse a longer distance across the upper surface than below. Inverting it merely forces the aircraft to assume what amounts to a higher angle of attack to create the same positive lift situation. I mean, if you are flying inverted, you have to push the stick forward to attain a positive angle of attack and stay in level flight whereas level flight in an upright aircraft has an angle of attack built into the wing structure and a more generally neutral stick position can be maintained. I've never flown inverted (on purpose and with my hand on the stick) so I can't speak as a pilot with that experience but would certainly defer to any pilot who had actually done so.

 

[oldcrowcv63]

Actually Greg, there is heat in the air on cold days. Just not as much as on warm days. Temperature is the measure of the average heat in a system. Heat is energy.

I suppose there would be no heat in the air if the air was at absolute zero (-273C).

I think Greg was pulling our collective legs.

 

[oldcrowcv63]

That is called the Magnus Effect.

And can even be used for sailing vessels

Yes, that was the rather startling realization I came to on checking my curveball story. yet you can still find a significant number of sites that use the bernoulli effect as a contributing factor even acknowledging the Magnus effect. It's far too messy if you ask me. I like my phenomena explained by one theory and one only and preferably in single syllable words.

 

[GregP]

Pulling your collective legs? With a certainty ... even on days of absolute zero.

Funny you should post a pic of the Alcyone. I was talking to Eddie Paul a few weeks ago and was asking him about his days as Cousteau diver. I asked where the Calypso had really been sunk and he asnwererd, "Which time?" I professed infnorance and asked what he meant.

He said, and I quote, "Please remember, these were Frenchmen. Before the diving saurcer was loaded, before the food was loaded, before the diving equipment was loaded, they loaded the wine. Several times they got a bit wine-relaxed and sailed the Calypso into the dock on autopilot and sunk ... right at the dock! It won't show up on the films, but it happened. You didn't want to be in the nose viewport when they were scheduled to dock!"

 

[oldcrowcv63]

Actually looks like Cobber was getting some good dope on a more sophisticated explanation of the theory of flight:

How Airplanes Fly

Turns out the velocity differential does not, as he stated, require that initially neighboring air molecules become reacquainted at the wing's trailing edge. oh well, back to school... I know Parsifal doesn't like revisionist history. I wonder how he feels about revisionist science? Doesn't seem like a big problem considering it won't be that long before many of us are revised out of existence. Then such things just won't matter. My happy thought for the evening.

 

[GregP]

Being serious, I think the lift generated by a wing is a combination of downwash and lower pressure on the top of the wing. People who wonder why the wing can lift when inverted should really look at the angle of attack when inverted and look at the stagnation point. When inverted the "top" of the wing (usually termed the bottom), has a longer path for the airflow due to the stagnation point moving downward and backward along the airfoil. The inverted angle of attack is usually MUCH steeper than when upright for a cambered wing section. Both phenomena, downwash and lower pressure, contribute to lift.

If you want to check this out, do a simple test in a wind tunnel. Take a wing section and cut it down the chordline and place the top half of the airfoil in the wind tunnel on top of a load cell such that air cannot get under it, but the upper curved section is in the airflow. It will generate lift easily in the presence of airflow, so Mr. Bernouli is certainly correct ... but probably isn't the whole explanation since you can also easily do the reverse and put the bottom half of the airfoil on the bottom of a load cell such that air cannot get over it, introduce airflow, tilt the airfoil upward, and measure a lift due to downwash.

The obvious answer is that lift can be produced by both phenomena at the same time. Their vector addition is the total lift.

Quantizing this stuff is tough when the wing is whole, but I believe it can be done by interested parties. It is easier when done in a liquid wind (fluid) tunnel since liquids don't need to move nearly as fast to produce results. Water is a good medium.

Remember to calibrate the load cells and all sensors traceable to the NIST (in the U.S.A. ... elsewhere use the International Bureau of Weights and Measures) so your results are valid and unimpeachable and be prepared to validate your calibration.

Acutally, this HAS been done but it is buried in government reports that have numbers instead of titles. If anyone can unlock the number puzzle, maybe we can all read it. If not, then get to your wind tunnel and DO it!

 

[CobberKane]

Actually Greg, there is heat in the air on cold days. Just not as much as on warm days. Temperature is the measure of the average heat in a system. Heat is energy.

I suppose there would be no heat in the air if the air was at absolute zero (-273C).

Actually, wuzak, at absolute zero there would be no air in the air. Air is a mixture of nitrogen, oxygen and carbon dioxide, plus some trace gases, in a gaseous state. If a given volume of air were reduced to absolute zero it would consist of these components in a solid state, plus some vacuum
So there