Bernouli's Irrelevant?

[eightyknots]

G'day all and Harro,Yes you can explain lift very well without knowing anything about Bernoulli. Bernoulli's theorems were base on Newton's Second Law. Lift is caused by turning flow or 'bending the air' if you like.

 

Google NASA Incorrect Lift Theories. The wing does not act like a venturi that to is a myth. The simplest wing is a flat plate and it works very well as up to about 8 degrees AOA. Lift quite simply is caused by AOA and wing camber (if any) which cause the air to be bent downwards invoking Newtons 2nd and 3rd laws. Any pressure differentials are a result of the wing physically moving teh air.

 

The air DOES NOT EVER meet up at the trailing edge if you are talking about the same 'particles' this is a myth called Equal transit time. On YouTube search how wings work Babinsky and watch the streamlines blow up this ridiculous myth.

 

The Kutta condition is a starting vortex flow at the trailing edge that detaches at about 3okts and is never seen again so it not relevant nor is Coanda which is in dispute as to whether it exists at all or is simply viscosity..

Here they are:

 

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

 

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

 

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

 

 

[completeaerogeek]

Here they are:http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

 

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong2.html

 

http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html

Yep they are in my first post as is the Cambridge University video. Now why can't CASA get this through their heads?????

 

 

[completeaerogeek]

And just to add support for Drifter's outlook ...We used to be taught that theoretically, a bumble bee can not fly! Lucky they don't know how to read the theory books then, eh! ;)

 

For those (like me) that like/need to understand the workings of a duck's rectum, the information is out there... but can be a bit of a liability sometimes. Horses for courses.

 

I would always like to think the designers of all our aircraft were well versed in both the theory and the practical. In my 'previous life' (decades ago), I've seen designs that frightened the socks off me. And yes, people died as a result.

 

Now I am happy to participate on the piloting side of things, so as long as the duck's healthy...

 

Bee safe!!! :thumb_up:

 

Harro

Yeah the old bumble bee chestnut was funny until they used high speed cameras and found that it used its wing in a figure 8 to create to create lift on both parts of the rotation. Kind of like a biplane!

 

 

[Yenn]

I don't know the theory of the venturi, but it seems to apply to a constriction in a pipe or similar. In a carbie you have a venturi and usually the inner dia is about 80% of the outer. It seems to be that the greater the constriction, the greater the pressure drop. If that observation is correct, what is the constriction at the top of a wing, with what would be equivalent to a massive diameter and a miniscule constriction.

 

 

[completeaerogeek]

I don't know the theory of the venturi, but it seems to apply to a constriction in a pipe or similar. In a carbie you have a venturi and usually the inner dia is about 80% of the outer. It seems to be that the greater the constriction, the greater the pressure drop. If that observation is correct, what is the constriction at the top of a wing, with what would be equivalent to a massive diameter and a miniscule constriction.

You have hit the nail on the head. There is no constriction above a wing which is why the venturi explanation is rubbish.

 

To use an analogy, cut a bucket in half vertically and see if it still holds water. Neither does the venturi explanation. Look at the NASA post on the page one of this thread.

 

Also the air actually slows down over a cambered wing surface-it doesn't speed up as in a venturi. Watch the Cambridge University Wind Tunnel video on page 1.

 

Have fun!

 

 

[kgwilson]

Single surface wings (hang gliders)

Just for the record, all except very low performance modern hang gliders have true aerofoil wings. This was begun back in the 1980s to improve performance by reducing drag at increased AOA. Some of the early examples were a bit stiff as with weight shift to enable turns the wings did not warp as easily. My first "Rogallo" style hang glider back in 1975 had an L/D of about 3:1. Increasing the aspect ratio and and aerofoil wing plus other aerodynamic factors introduced provide L/Ds of 13 to 17:1 these days. Bernouli doesn't concern me. I didn't even hear of the theory until I'd been flying for 30 years. As Ozzie says it is all magic anyway.

 

 

[turboplanner]

This thread has spun into the thread Debunking Lift "theories" still taught - which has been pretty much debunked itself.

 

Have a look at that thread to see get access to the United States Navy training on Bernoulli, and no Yenn they are not expecting you to fly a venturi - one example of the Bernoulli principle is the air speed indicator, another is a carby throat, but if you want to consider an aerofoil of the type we use, you consider Bernoulli's principle on a wing, not a veturi. I've linked a video showing several examples of the Bernoulli principle in action, including on a length of toilet paper.

 

The NASA "incorrect lift theories" got tangled up with the geeks hyperbole.

 

NASA said the "Longer path" and "Equal transit" theories which some people teach are incorrect, but the Bernoulli theory where the upper flow is faster so the pressure is lower is correct

 

 

 

[facthunter]

Some carburettors have two venturis, for better atomisation/calibration.

 

Also is the statement that the sum of the static and dynamic pressures is a constant, correct? Nev

 

 

[Guest ozzie]

The kid up the road from where i used to live reckons there is no such thing as 'Lift'. Aircraft are sucked into the air. With his theory the negative pressure on the top of the wing is producing more of the work than the higher pressure under the wing. More suck than lift. He puts up a pretty good argument. I told him he should go work for CASA

 

 

[Dafydd Llewellyn]

I've stayed out of this ridiculous argument, because I explained it all in an article that was written for the RAA magazine in March 2010. Ian, here's another reason to provide a subject index; this damn fool argument keeps cropping up.

 

Bernoulli bk.docx

 

Bernoulli bk.docx

 

Bernoulli bk.docx

[metalman]

The kid up the road from where i used to live reckons there is no such thing as 'Lift'. Aircraft are sucked into the air. With his theory the negative pressure on the top of the wing is producing more of the work than the higher pressure under the wing. More suck than lift. He puts up a pretty good argument. I told him he should go work for CASA

I'd really rather get sucked off( the ground) than pushed off ( the ground) ,but science rules ;-)

 

 

[aro]

I've stayed out of this ridiculous argument, because I explained it all in an article that was written for the RAA magazine in March 2010.

It may be a damn fool argment, but every attempt to explain seems to raise a few questions. In your article you say:

 

So if the flow (overall, if not immediately outside the airfoil boundary layer) is going to get to the trailing edge at the same time, the flow at the top has to travel faster than that at the bottom. Why must it – considered as a whole - get to the trailing edge at the same time? Because, if it didn’t, all the air above the wing would be dragged to the left (in the picture), compared to the air below, and you would not get the smooth flow at the trailing edge

Isn't this the equal transit time theory? But videos showing pulsed smoke e.g. in the other thread, clearly show that the flow doesn't get to the trailing edge at the same time. How does this work and why doesn't the air underneath "pile up" ahead of the wing? I don't know - but I guess some of it ends up flowing over the wing in the faster stream, as well as around the wing in the wider circulation patterns. In fact it seems like there must be less air flowing under the wing, to allow for the downward flow off the trailing edge.

 

 

[dlegg]

It may be a damn fool argment, but every attempt to explain seems to raise a few questions. In your article you say:

 

Isn't this the equal transit time theory? But videos showing pulsed smoke e.g. in the other thread, clearly show that the flow doesn't get to the trailing edge at the same time. How does this work and why doesn't the air underneath "pile up" ahead of the wing? I don't know - but I guess some of it ends up flowing over the wing in the faster stream, as well as around the wing in the wider circulation patterns. In fact it seems like there must be less air flowing under the wing, to allow for the downward flow off the trailing edge.

Watching the video, it clearly shows the airflow on top of the wing slowing down.....

 

 

[aro]

Watching the video, it clearly shows the airflow on top of the wing slowing down.....

That's OK. Since it's the difference in pressure that produces lift it just means the flow underneath needs to slow down more.

 

 

[Dafydd Llewellyn]

It may be a damn fool argment, but every attempt to explain seems to raise a few questions. In your article you say:

 

Isn't this the equal transit time theory? But videos showing pulsed smoke e.g. in the other thread, clearly show that the flow doesn't get to the trailing edge at the same time. How does this work and why doesn't the air underneath "pile up" ahead of the wing? I don't know - but I guess some of it ends up flowing over the wing in the faster stream, as well as around the wing in the wider circulation patterns. In fact it seems like there must be less air flowing under the wing, to allow for the downward flow off the trailing edge.

Look at http://galileo.phys.virginia.edu/classes/311/notes/aero/node2.html ; note that there is an upwash ahead of the cylinder, as well as a downwash behind it. The picture is for an "ideal" fluid; in reality there is a large separated wake on a spinning cylinder, but the same upwash and downwash effects occur, and so it will in fact generate lift. The same is true for an airfoil that is generating lift; that becomes very obvious when one uses a pivoting-vane pitot some way ahead of the wing leading edge. The air does indeed move faster over the top when lift is being produced. The process is not perfect, because air has a finite viscosity, so the molecules do not exactly match up at the trailing edge, but the effect is small compared to the lift it generates. If you're looking at smoke puffs passing close to the airfoil surface, this will be visible. The "Kutta-Joukowski" condition is an approximation; in reality the circulation is never quite strong enough to fully comply with this, and the discrepancy gets larger, the closer the airfoil gets to its stall angle; a wool tuft at the trailing edge will point rearwards at cruise, but will wrap itself around the trailing edge and point forwards as the stall angle is approached.

 

Remember that in an actual flight situation, the air as a whole is not dragged along en masse noticeably by the aeroplane passing through it; however the drag force on the aeroplane is equal (and opposite) to a change in momentum of the air - actually a small slice of air - so, yes, some air is dragged along, but viscosity slows it down; and the overall movement is vanishingly small; a cubic mile of air near sea level weighs around 150,000 tons, so an aeroplane weighing one ton, with a lift-drag ratio of 10, flying at 100 miles per hour through the middle of it, will apply a force of 224 pounds to that block of air for 31 seconds; if the force were uniformly distributed over the whole cubic mile, the air would gain a velocity of 224/150,000 ft per second every second; so after 31 seconds it would be moving at about 9/16 of an inch (14 mm) per second. So for practical purposes, the air stays still; and so it would in fact if the aeroplane had zero drag. The small difference between the velocity of the air passing below the wing, and that passing above it, produces a sheet of tiny vortices in the "slip" zone in the thin wake of the wing; the viscosity of the air quickly damps them out, so it's all over very quickly.

 

 

[Bruce Robbins]

Remember that wind tunnels are used to enable us to study and measure what is happening. In real life the air stays still and the wing moves.

 

Sometimes it's more important to just know what happens, rather than why.

 

 

[Bruce Robbins]

Sorry Aro, I type too slow. Dafydd said it better.

 

 

[kgwilson]

In real life the air stays still and the wing moves.

You obviously have never heard of wind.

 

 

[Bruce Robbins]

You obviously have never heard of wind.

Not so, I'm sure I've read something about it somewhere. :)

We fly aeroplanes, not kites, so unless you forgot to remove the tie-downs, then it's not wind that makes a wing work.

 

 

[Dafydd Llewellyn]

I see my arithmetic was wrong in post # 90; the density of air at around half a mile density altitude is about 92.5 % of the standard sea-level value, 0.0766 pounds per cubic foot. So the average air density in the hypothetical cubic mile of air will be 0.0708 pounds per cubic foot. A cubic statute mile contains 1.472 x 10^11 cubic feet, so it weighs 1.0422 x 10^10 pounds. One ton = 2240 pounds, so one cubic mile of air weighs about 4.6 million tons. So the end answer of my post #90 is not 14 mm/sec, but 0.45 mm/sec (Shouldn't do those calculations late at night . . .).

 

 

[Guest ozzie]

re

 

Not so, I'm sure I've read something about it somewhere. :)We fly aeroplanes, not kites, so unless you forgot to remove the tie-downs, then it's not wind that makes a wing work.

Wanna bet!

 

Relative wind how does thou blow relative wind.

 

 

[Bruce Robbins]

G'day Ozzie,

 

I'm probably getting into semantics, and definitely off topic, and I'm sure you know all this, but the important point to me is that it is the plane moving through the air, not the air moving relative to the ground, that makes the wing work. Quite a few students and enough qualified pilots to be scary still think of wind in respect of the ground and relate that to how the aeroplane is affected. ( " I need to hold rudder to counteract drift", "I get more lift when I turn into wind" etc etc etc)

 

Relative wind (which I prefer to call relative airflow) is the movement of air relative to the aeroplane only, with no reference to the ground or any other point of reference. In a dead still air mass ( no wind), an aeroplane flys nicely by moving through the air, thus generating what the pilot in his open cockpit feels as relative airflow.

 

Wind tunnels reverse this, and move the air past a stationary object, so it is easier to study. This must however distort the behaviour being measured to some extent, because the air now has movement and inertia that it does not have in real life. I rarely fly in 120 knot winds, but I often fly in a 120 knot relative wind. :)

 

That was the point I meant to make in the previous post. The behaviour shown in wind tunnels by streamlines and staccato puffs of smoke may not be the same as in real life. Daffyd and the aerogeek might be able to show the difference is negligible, but it must still be there.

 

Going back to my feeble joke. If the plane is attached to the ground by a rope in a 50 knot wind, you can sit in the plane and make it fly nearly as high as the rope is long. Cut the rope and the flying soon stops. It is not the wind that makes us fly, it is the movement of the aeroplane through the air.

 

 

[Dafydd Llewellyn]

In fact, all the wind does (in general) is make the ground move in a funny way.

 

 

[Bruce Tuncks]

Hi other Bruce, your paragraphs 1 and 2 were fine but you couldn't be more wrong with the next bit when you say that the air has no kinetic energy if there is no wind.

 

The air has kinetic energy with respect to the plane.

 

All motion is relative... why is the reference of the plane "worse" than the ground? I guess you would say that the ground is bigger.

 

Well the biggest thing around is the galaxy we live in, and our speed around that is 792,000 km/hr. Quite a lot of kinetic energy in that "still" air huh?

 

 

[SDQDI]

 

 

Yes I know it's a dead thread and I don't want to restart any arguments ( I just wanted to have the last say:wink:)

 

A longish video but it does cover quiet a few points and I found it interesting.