Western727
Topic Author
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Bernoullian Vs. Newtonian Lift

Thu Jun 07, 2001 8:53 am


Earlier in the year, we had a heated debate on whether airlplanes were held aloft by bernoulli's priniple, i.e. pressure differential (high pressure under the wing, low on top) or newtonian lift, i.e. wind striking the underside of the wing, pushing it upward.

Today, I attended the Harvard Commencement Class Day (incidentally, the speaker was Bono, the lead singer of U2, who gave an address which had nothing to do with Harvard, or class day, or graduation - I was a little put off - I mainly just watched Beech 1900s and Saabs Climb out of Logan). I had a chance to sit next to a Phd. in Aerospace Engineering. As one can imagine, a conversation about aviation struck up between us. I asked this man which of the two phenomenon was responsible for the production of lift. His answer was, with a laugh, bernoullian. In fact, he seemed a little concerned that I, a certified pilot, would even ask such a question, regarding it to elementary for even the most uninitiated neophyte.

So, to all those who insisted that I was "technically illiterate" for standing with common sense and the Jeppesen Manual..

I TOLD YOU SO!
Jack @ AUS
 
OldMan
Posts: 207
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RE: Bernoullian Vs. Newtonian Lift

Thu Jun 07, 2001 9:36 am

Western 727 I invite you to take a look at www.allstar.fiu.edu/aero/airflylv13.htm
Regards. Oldman
 
jjbiv
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RE: Bernoullian Vs. Newtonian Lift

Thu Jun 07, 2001 12:24 pm

Oldman -- That looks to be a dead link.
 
Buzz
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RE: Bernoullian Vs. Newtonian Lift

Thu Jun 07, 2001 10:01 pm

Hi Western 727, Buzz here. I've been a wrench bender for a few years, taildragger pilot just a little while.
Anyway i tend to be of the Newton persausion, with a brief nod that Bernoulli may have some effect (how's that for straddling the fence?)

But yesterday when helping freinds launch their DC-3 off a grass airstrip ...... downhill.... tall grass on each side (runway not much wider than the wingspan) i had to notice how much grass was being flattened -displaced as the wing went over it. Seems that the wing was walloping air molecules downward.
And i've been under a few helicopters, seems that they need to accelerate enough mass downward to balance out gravity.

But before we start another flame war, may i suggest getting some simple taildragger, flying aroudn for an hour at 1000 AGL or so and enjoy flying - sightseeing - smelling the farms below with your elbow out the window and a big grin on your face? That's the therapeutic part of flying.

g'day
 
speedbird002
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 1:18 am

I believe this is the link that Oldman was talking about,

http://www.allstar.fiu.edu/aero/airflylvl3.htm

(I was shocked myself when I saw it because it goes against everything I've been taught up to now!)
 
speedbird002
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 1:23 am

I believe this is the link that Oldman was talking about,

http://www.allstar.fiu.edu/aero/airflylvl3.htm

(I was shocked when I saw it because it goes against everything that I've been taught up to now!)
 
speedbird002
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 1:27 am

Sorry for posting the same message twice, but I didn't include the proper link.
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 2:52 am

I cant believe this is being discussed. How does a plane fly? Are you guys feeling ok? Lift is produced by making the air go faster across the upper surface than the lower. This causes a low pressure area which sucks the wing up. As I remember from the first week of A+P school, at least 75% of lift is generated by the upper surface of the wing. This works for props, heli rotors, control surfaces, anything on the plane. Bernoulli Vs Newton, that is funny..JT
 
Western727
Topic Author
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 4:18 am

Ok, so I looked at the website, and I found it quite interesting. I must say, it almost has me convinced. It made a lot of sense, and since it is from a credible source, (NASA) it seems valid. However, I still have one question. If the "downwash theory" (as I like to call it) is true, then the wingtip vortices in the picture (of the Cessna Citation just over the clouds) are rotating in the wrong direction. The website suggested that the vortices were caused by downwash. That would cause a vortex spinning in the opposite direction than the ones shown in the picture. The vortices appear to have rotating in a direction that would be caused by air moving from under the wing towards the top (because of low pressure on top of the wing, and higher pressure underneath). Perhaps I am just missing something, or interpreting the photo incorrectly. Someone please help me out here.

One more thing: The proponents on Newtonian lift lead me to believe, in earlier conversations, that the theory stated that lift was caused by air striking the underside of the wing (much like sticking your hand out a car window at freeway speed ) Now I see that whoever made that assertion was mis-stating the theory.

I didn't see this mentioned on the website: How does ground effect fit into all of this?

Oh, and also, I believe the original phrase that someone used was "technically ignorant." I don't consider myself that either.
Jack @ AUS
 
Monocleman
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 7:06 am

I'm just an amatuer, but it seems that common sense (or at least mine) dictates that the Newtonian principle comes in to play just as much as Bernoulli's. I sort of agree with Buzz, with exceptions at high speeds. Most lift, it seems, at takeoff, is from the rotation of the wing and the force of the air on the underside plane of the wing. At cruise, however, the aircraft's attitude is more or less level, and is at a higher speed. If lift was 100% Newtonian, then aircraft could not fly at a level attitude. In more high-lift wings or at higher speeds, however, Bernoullian lift is the only explanation. I have seen pictures (on this site) of of aircraft taking off, or at least achieving on takeoff, a nose-down attitude. This is Newtionally impossible, it seems, because the elevators alone could not produce enough downwash to accomplish this. I'm no PHD in aerodynamics like some guys out there, so please correct me if I'm wrong. (Really do, I'm a young'un and still have much to learn from all you out there.)

-Will
 
XFSUgimpLB41X
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 8:50 am

Ive always felt it was both-with newtonian producing much of the lift at lower speeds and bernoullian at higher.
Chicks dig winglets.
 
PerthGloryFan
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 08, 2001 11:29 pm

Hmm, not sure I'd want to fly in a machine designed by that PhD!
The only absolute is that there are no absolutes!
A completely flat non-airfoil wing will generate lift given the right airspeed and angle of attack - not very efficiently perhaps, but it will fly (eg, a simple kite)- no Mr B's effect there.
Bernoulli is all about the effect that fluids flowing from areas of high to low pressure create, but you must have a cause first.
Ground effect is something additional ...
But anyway as Buzz says - hey, when you're up there thoughts of the finer points of physics are replaced by more fundamental feelings.  Smile/happy/getting dizzy
PGF
 
fr8tdog
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RE: Bernoullian Vs. Newtonian Lift

Sat Jun 09, 2001 4:26 am

Personally I believe that the relationship between both laws are a variable ratio.
In order to develop the most efficient form of lift, you need both laws in varying degrees. Depending on the speed, power setting, shape of the wing, AoA and numerous other facters that are involved, lift generation from bernoulli or newton are both used in varying percentages.

Ground effect basically interfere's with the three dimentional flow about the aircraft. In order for this phenomina to occur the aircraft is within 1/2 of the wingspan to the ground, resulting in a reduction of drag from spanwise flow (wing tip vortices) , lower AOA due to less drag and from a "quasi" (easiest way to explain it) cushining effect of the downward deflection of air from the top and bottom of the wing.
 
JG
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RE: Bernoullian Vs. Newtonian Lift

Tue Jun 12, 2001 3:54 am

Western727,

It may have been me... but the statement was not directed a you. I copied a quote posted on a web site discussing lift and it was not referring to any person in this forum. It was part of that Bernouli/Newton discussion and how for simplification the aviation texts "dumb down" the science so that many can have a basic understanding of lift as they venture in to take their written exam. Obviously not all private pilot students have an advanced degree in aerodynamics. I have included the same quote below and also two additional links on the subject.

From the first link: (quote) Many years ago, a most famous aerodynamicist, Dr. Theodore VonKarman, instructed his assistant: "When you are talking to technically illiterate people you must resort to the plausible falsehood instead of the difficult truth." (From Stories of a 20th Century Life by W.R. Sears) (end quote)

http://www.geocities.com/CapeCanaveral/Hangar/6544

And, the second link is below. The NASA supported site referenced in a post above is also a good read. It seems your interest is peaked, enjoy all three sites.

http://www.monmouth.com/~jsd/how/htm/title.html

JG
 
aaron atp
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Blah

Tue Jun 12, 2001 12:12 pm

Arguing over this is useless. If you want to know how lift is generated you need to buy yourself a bookshelf of engineering textbooks and mathematical texts to back them up. Unless you are willing to do that, accept that simplified theories contain inherent shortcomings.

Any theory that does not, at a minimum include discussion of the bound vortex or Γ ("Circulation") (a rotating cylinder is usually presented as a good example) is over-simplified in the eyes of any engineer in my opinion. You shouldn't distinguish between Bernoullian and Newtonian theories, instead you should understand the relationships between the two. That doesn't mean assigning a ratio of percentages (25/75 is utter crap, and to go off an a tangent: even if it were that simple, it would greatly vary with AOA).

What it all boils down to: Pilots (and mechanics) do well with simple theories and airplanes do not fall out of the sky because the captain believed in an ignorant over-simplification of lift-theory. Those simple theories are all that pilots need to fly the airplane. Most continue to fly well even after forgetting the theoretical details. Just try to realize that "it is complex and my knowledge only scratches the surface." Then you won't be ignorant of your ignorance.


________________________________


Arguing over this indeed useless -since no one is going to change their understanding based on one post, but I will throw in my $.02 for the sake of interesting reading.


>>>we had a heated debate on whether airlplanes were held aloft by bernoulli's priniple, i.e. pressure differential (high pressure under the wing, low on top) or newtonian lift, i.e. wind striking the underside of the wing, pushing it upward. <<<

Certainly, in the air or in the water, we look at pressure differentials to understand what makes 'lift' happen. The thing is, as Buzz stated in regards to the DC-3 t/o, we have infallible evidence of a downwash. The lightbulb will come on when you understand the theoretics of (including the previously mentioned bound vortex) what produces the downwash.

I'll give you a hint, the low pressure area above the textbook airfoil isn't there to act like vacuum cleaner: it doesn't "suck" the wing "up."

The pressure differentials induce the circulation, the circulation causes the downwash (but also the upwash), and the mass of air in the net downwash is related to the mass of the aircraft (action-->reaction)

Angle of attack affects this circulation by increasing the pressure differentials, and thus increasing AOA makes the circulation stronger. When the airfoil stalls (loss of laminar flow at the critical AOA), the circulation breaks down, the downwash is reduced, and CL falls rapidly as AOA continues to increase. Without lift, gravity accelerates a/c downward.

Now there is at least one person who says the air does not circulate spanwise around the wings. He is right, each air molecules passes only passes the stagnation point once, but this discussion is taking place in a different reference frame and we are not concerned with the flow of a single molecule.

So in effect, you were probably still wrong in the eyes of the PhD -even though you argued Bernoulli.


Then again, everything I know could be wrong in the eyes of someone who knows more than I.

Mathematically, we toss around two or three equations regarding lift production, but in fact, the amount of Calculus involved in calculating lift beomes quite complex with page long equations involving intregrals et al.


http://www.desktopaero.com/appliedaero/appliedaero.html


aaron
 
fr8tdog
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RE: Bernoullian Vs. Newtonian Lift

Tue Jun 12, 2001 3:00 pm

Huh just reread that link for Lift theory as an airpump.
one thing that they forget to mention is the fact that there is a cancellation of energy from the upwards displacement of air by the downward flow on the aft portion of the wing, resulting a net reaction of close to zero.
but I could be wrong I am only human! ya know....

 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Wed Jun 13, 2001 7:42 am

Aaronatp, you are not in the bussiness, so go read another book. 25/75 is utter crap? Tell that to the faa. They approved the textbook. Im so sorry our puny minds cant work as well as yours. Are you an engineer? Did you design a wing? Or did you design a cupholder or an ashtray for a now extinct airplane made by a now extinct company. BTW are you really an atp, or just another "wannabe spotter"? 25/75 is a ballpark figure, and I have seen nothing yet to disprove this figure. By the way, when a wing stalls, it is because the airflow over the wing upper surface can no longer maintain laminar flow. This is why the plane looses lift. If it was held up by air beating against the bottom of the wing, there would be no stall. Yes lift will vary with aoa, but not by much. By the way, you must practice "your ignorance" to even write such a silly post..JT
 
JG
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RE: JT-8D

Wed Jun 13, 2001 11:19 am

JT-8D,

With all due respect, back off. Aaron atp speaks the truth. Lift is a system, not one theory, law or principle. Lift involves an wing passing through a fluid, not a bunch of little dots passing over the wing as represented by many aviation texts.

I AM in the business and wonder, with all your experience and your comment in your previous post, how you can believe that the FAA is the keeper of all knowledge. Open your mind just a crack.

Now, I can't say whether or not Aaron has designed a cup holder or ash tray... but I can say that your post was the silly one. Even more silly than your comments about rolling a 707.

Regards,
JG
 
aaron atp
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...

Wed Jun 13, 2001 12:54 pm

have a nice evening JT-8D.

aaron
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Wed Jun 13, 2001 7:38 pm

I can't believe this topic has popped up again. After two years of aerodynamics courses, I have never heard of anything dubbed "Newtonian Lift" - I do not say it does not exist, but it is definitely a term unheard of in my course. Neither is there such a thing as Bernoullian lift. Lift is a force normal to the chord of an airfoil - generated by the circultation the airfoil induces into the airflow.

The act of producing lift cannot be summed up in a few words. You could try and explain it with high pressure below the wing and low pressure above it (a valid explanation, and the most common way to compute values for supersonic airfoils) or by the vorticity of the air and the induced circulation (generally used for computations in incompressible flow, i.e. most subsonic airfoils).

You cannot explain lift simply by pointing out Bernoulli's equation for ideal flow, or by mumbling something about Newton.

Let's just look at why: Newton is all about forces. You can analyze the forces on the aircraft (weight, lift, drag, thrust) and exclaim they are equal. You can use him to find the thrust needed, given the drag, or the lift needed, given the weight. But in what way does he help you in finding the cause of lift? The Newtonian frame of reference takes forces and analyzes them - their cause is a side issue.
Now you could try and explain the forces by the downwards momentum induced into the air by the wing. That would use Newton's momentum equation. But: How does the momentum get into the air? It is not as if the plane flapped its wings, is it? Unless you have a downwards pointing rocket, you will need fluid dynamics to explain why exactly the air receives downwards momentum. And Fluid Dynamics is not just Bernoulli (his name is just the most well-known in the general population). In this subject, the names you are most likely to hear are Kutta, Joukowski, Runge, Prandtl, Meyer, Glauert, Blasius and a few others. Bernoulli's principle does not explain lift on its own. You will need all those people's theories to understand lift, but especially the first four.

But, fundamentally, for any subsonic airfoil, it is easiest to analyze and explain due to circulation. Even a flat plate airfoil produces lift only by circulation (provided it is inclined at an angle of attack) It is not "pushing the air down" - it pulls the air before the airfoil up, and pushes it down behind it, it pulls it along on the upper surface of the airfoil (speeding it up) and pushes against the flow direction on the lower surface (speeding it down). And, because vorteces cannot end (they are infinite), they curve towards free stream direction at the wingtips, forming trailing vorteces.
Lift depends on the shape of the airfoil (and how good it is at inducing circulation), and on angle of attack (a linear relationship until the stall regime is reached).

For supersonic airfoils, the preferred method of analysis is to compute the pressures behind each of the shock waves, and then do a force analysis based on the pressure along each surface of the airfoil.

If anyone is seriously interested, I suggest going into a library and taking out a good aerodynamics text book, and I hope your maths is up to shape, because you won't understand anything without a good knowledge of differential equations, partial differentiation, complex numbers and complex mappings. In our university, the standard bible appears to be Anderson's "Fundamentals of Aerodynamics" - even though I find it not too good at explaining things to someone new to the subject (i.e. I did not understand any of it two years ago...)

Regards

Ikarus
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 15, 2001 8:37 pm

JG, I think your post is the silly one. You seem to think that the "correct" answer is decided by how many people are on your side? Its not up to a vote, there is a correct answer, and an incorrect one. As for your comment on the quality of drawings used by the faa, I would trust those "little dots" more than any questionable info I see posted here. The faa is not the "keeper of all knowledge", and neither are you. Certainly Aaron is not either. As stated earlier, I have seen nothing here to change my ideas of how a plane flys. Theory is nice to talk about at cocktail parties, but in the real world, it doesnt add up. Open my mind a crack? There is a big differance between open minded and gullable. I dont believe everything I read, especially from some guy calling himself a pilot, with no credentials other than an all too familiar pilot "I know best" attitude. As for your last comment on the 707, go fly upside down..JT
 
Guest

RE: Bernoullian Vs. Newtonian Lift

Fri Jun 15, 2001 8:50 pm

You guys are all wrong. When I was in AFROTC back in college athey told us how airplanes flew - It was lifties that got underneath the wings and pushed it up. You mean to tell me that it was all a lie?
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 15, 2001 8:58 pm

Are they related to the Gremlins? Second couisins maybe?..JT
 
DC-10Tech
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 15, 2001 9:09 pm

I believe both concepts come into play. Just like sticking your hand out of a car window, your hand catches the air and forces it downward 'climbing' on the air if you will. (But then again, that's what is creating a low pressure area above your head...) both theory's seem very interrelated. As for the comment about some aircraft taking off nearly level, take the B-52 for example, its practiacally nose down on take off and landing, but if you look at the aircraft on the ground, the wings have a leading edge-high angle of incidence. If you take the DC-10, for example, its wings are practically level to the ground, and the aircaft actually flies slightly nose up during 'level' flight... Both houses have an argument, and its ignorant to bash the other.
Forums.AMTCentral.com
 
Guest

RE: Bernoullian Vs. Newtonian Lift

Fri Jun 15, 2001 11:24 pm

Kind of like the old "what controls airspeed - pitch or power" debate a few years back, eh?
 
VC-10
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RE: Bernoullian Vs. Newtonian Lift

Sat Jun 16, 2001 12:00 pm

If the press drops & velocity increases when a fluid passes through a venturi why is it that when 3 or 4 lanes of road traffic filters into one lane eveything clogs up ?
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Sat Jun 16, 2001 3:41 pm

Traffic has some similarities with fluid and some major differences. A fluid cannot be simulated as a collection of particles - not unless you choose 10^50 particles and compute all inter-relations etc. - i.e. simulating fluids as particle stream is generally hard and expensive.

So why does the traffic clog up? Because you want a certain distance before your car. Because, if someone changes the lane fast in front of you, you brake, forcing the guy behind you to do the same, and transmitting the event (slow-down) along the traffic behind you.

Fluids do not work that way. If you have a (subsonic) flow, the mass flow through a tunnel stays constant. So if you have a neck (or "nozzle") in the tunnel, the speed has to increase (mass flow = area*density*velocity). - Funnily enough for supersonic flows, the behaviour is exactly opposite: There expanding the cross-sectional area speeds up the flow, and narrowing down the tunnel slows the flow down (this is due the density changing and not being constant - i.e. compressible flow)


And now for JT-8D:
"Yes lift will vary with aoa, but not by much. By the way, you must practice "your ignorance" to even write such a silly post..JT"

That statement alone disqualifies you - lift varies linearly with angle of attack, until stall is reached. A symmetric airfoil, at 0 incidence, produces NO lift. A cambered airfoil does, but the lift production STILL varies LINEARLY with incidence.

So: Why does the air go faster above the wing than below the wing? If your answer is "Because the airflow above the wing needs to arrive at the trailing edge at the same time as the airflow below the wing" then it surely shows how your textbook is not more than a simplification for those who need not know the full complexity of the matter.

To everyone: Could we keep this discussion on a somewhat more civil basis? This is not the non-av forum....

Regards

Ikarus
 
VC-10
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RE: Bernoullian Vs. Newtonian Lift

Sat Jun 16, 2001 7:58 pm

Ikarus

I wasn't being serious  Smile Smile Smile
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 3:06 am

I was adressing the issue of the percentage of lift, and how it varies with aoa, not the actual quantity. I am disqualified for what?..Jt
 
JG
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RE: JT-8D

Mon Jun 18, 2001 3:49 am

JT,
Quite the pilot vs. mechanic chip on your shoulder, your ignorance continues to amuse me. Interesting web sites have been offered for continued education... One NASA supported site from oldman, and I shared two additional sites. I harbor no "us against them" feelings and I look forward to reading your next "silly" post. Email me if you wish to discuss this further.
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 3:50 am

Sorry JT-8D - did not mean to be quite as offensive.

I'm getting rather annoyed at this discussion, however. Why? Someone recently referred to the Civil Av forum as "idiot forum" - laughing at the ignorance in there. And yet, the people in this thread are guilty of exactly the same "crimes": People argue about things that they have no well-founded understanding of, and they argue it fiercely. This sort of surprises me.

I think I'll go through some of my textbooks this week, and maybe I'll try and write an article about lift generation for airliners.net. After all, this is the kind of issue that the entire article idea was originally designed for: To answer questions that interest us, and to answer them in a relatively competent way.

Now let's see whether I can find the time for it.... And the patience to actually cancel out the maths....

regards

Ikarus
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 4:01 am

Its just amazing how so many "students" find the guts to contradict people who have training, licences, and hands on experiance. When I was just starting in this bussiness, I knew nothing. I asked questions, and listened to the answers. The younger people here do not seem to share that courtesy or respect. As I have stated several times, I have no problems with the younger people commenting and asking questions. However, I will not argue technical questions with people who have probably never seen an airplane closer than the airport terminal window. Quote from all the textbooks you want to. The fact is that an indepth knowledge of air molecule behavior is not required, and usually just complicates a problem. When I speak to engineers from our company, and from Boeing, and from MD (RIP) we have no problems exchanging information. Theory is fine for impressing people who dont know any better. Fact is what gets the planes back in the air..JT
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 4:16 am

JG, if my posts are so silly, why do you continue to respond? I have no problems with pilots, as long as they stay in the cockpit, and leave when I tell them to. E-mail you? Why? What is there to continue? BTW, if I had a website stating that airplanes used anti-gravity generators, and said the data was obtained from nasa, would you be dumb enough to believe that too? Im looking forward to your next stupid post too. Tell me the theory about cows jumping over the moon..JT
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 7:18 am

JT-8D

So: Why does the air go faster above the wing than below the wing?

I'm really interested in your reply. After all, the topic is more or less "How is lift generated?" - not "How can we argue and insult each other?"

So: How is lift generated? Air faster above wing, slower below wing, pressure difference, yadda yadda yadda ..... But why is the air faster above the wing than below the wing? And WHY does the upper surface produce 75% of all lift (if it does)?

I'm eager to hear your theories.

Regards

Ikarus
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 7:25 am

Cuz I read it in a book..JT
 
bio15
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 7:28 am

This thread is getting a bit tense. Is there a way in which you can all conclude something about lift? I am interested and intrigued with this discussion but it's impossible if you guys keep bugging each other. JT, try not to get mad with everyone and take the lead on giving a concluding statement about what generates lift, I trust your knowledge! Just for the moment untill Ikarus writes his article!  Smile
I have thought a lot about the lift issue I thought it was "newtonian" lift. But Bernoulli's principle can't be left behind, and it's logical to think that both play important roles in lift, varying with the AOA and wing shape. right?

-bio
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 8:42 am

Bio, I didnt come to this forum to rewrite the "accepted" rules of flight. I shouldnt have even commented, but it just annoyed me to see people saying that everything that we have taken for granted for the last 75-80 years is incorrect. I can only go by what I was taught. And what I was taught was that the upper surface produces much more lift than the lower. This fact is supported several ways. Scratches and damage to a wing upper suface is critical, and must be repaired to fairly high standards, while the same scratch on a lower surface can just be painted over. All leading edge devices are designed to change airflow over the upper surface of a wing, not the lower. Spoilers are mounted on the upper surface, not the lower. On small planes, flush rivets are used on the upper surface of the wing, while (within reason) pretty much any kind can be used on the lower. Ill admit that this is a very non-scientific way of looking at the question, but it doesnt seem to me that the lower surface is nearly as important. The next question is why? Could it be because the upper surface is doing most of the work? If you had seen as many training films as I had to watch in school about why a wing stalls, you would see that the lower surface is not even a factor in a stall. Truth is bio, nobody here is an expert on this matter. I just wont accept that Im wrong on anything until its proven, and it still hasnt been proven to me..JT
 
bio15
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 9:47 am

Okay, guess I'll just wait for Ikarus's article  Smile (just kidding)
I'll research on the subject
thanks for responding.

-bio
 
PerthGloryFan
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 4:48 pm

JT-8D
but it just annoyed me to see people saying that everything that we have taken for granted for the last 75-80 years is incorrect
Not "incorrect" JT, but "incomplete".
It's the difference between science and its application as technology.
For example, any reasonably technically competent person can can apply the technology to assemble a working personal computer without necessarily having to know the molecular properties of the semi-conducting materials that make up the integrated circuits on the various printed circuit cards.
Same with building and maintaining airworthy aircraft - the science can be "simplified" (relative term  Smile )into technological rules that when applied correctly work for us.
Stay cool everyone - I don't see the need to attack this issue with obstinate fervour  Smile/happy/getting dizzy - there's plenty of knowledge out there for all of us to learn and share.
PGF
 
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 8:29 pm

Ikarus,

You wanted to discuss theory, well here it is.

When an airfoil experiences a fluid motion there exists a pressure pattern which is relatively equal. If there is a camber to the airfoil, there will be a difference in the pressure pattern of the upper surface from that of the lower surface. Furthermore if the airfoil is fixed at some angle to the airflow, the pressure distribution, hence the velocities over the surface will be altered. The net result of the static pressure distribution over the surface is a lifting force.

If you were to look at any text book which gives diagrams of the above, you will see that it is easy to surmise that the ratio of the force is 75/25, although I cant remember any figures to substantiate this ratio. So I guess that its fairer to say The ratio is approximately 75/25

The description above is taken from the Boeing Jet Transport Performance Methods manual. I’m pretty sure that at this stage, Boeings engineers really do know what they are talking about, needless to say when they are talking about lift, they do not mention Newton Laws. They do however devote a lot of attention to Mr Bernoulli.
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 8:52 pm

I agree that the upper surface of wings generally creates a higher pressure difference than the lower surface. I cannot say anything about the 75/25 ratio, but it sounds like a reasonable approximation to me. Just like the idea of the sum of all lift forces acting generally between quarter-chord point and 1/3-chord point, this is just a rule of thumb - not science.

Remember: It wasn't me who slagged off that argument, but AaronATP. I was merely shocked to read somewhere that angle of attack is only a minor factor. Angle of attack is EVERYTHING. Even a cambered airfoil will usually not be able to lift the entire plane without a positive angle of attack. (Well - that is a generalization: It depends on the weight of the plane, the velocity etc.)

Why do planes rotate before they lift off? Because that increases the angle of attack, thereby the lift, and it can leave the ground. If you look at planes, they usually point slightly upwards of the direction they are moving in (extreme examples are on rotation and prior to landing) - this is due to the necessary angle of attack.


Regards

Ikarus

 
PerthGloryFan
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RE: Bernoullian Vs. Newtonian Lift

Mon Jun 18, 2001 9:48 pm

Hmmm ... cambered upper surface ... eh?
Now would that be "upper" surface when the a/c was inverted or right way up?

hehehe  Smile/happy/getting dizzy

Just stirring to show that engineers can apply aerodynamic technologies using the physics of fluids and forces without many of us always understanding this underlying science. And of course there's trans-sonic and supersonic issues ... but let's keep this friendly eh!

I'm just so grateful that Messrs O & W Wright got it right 98 and a bit years ago so we are able to have this discussion.

PGF
 
aaron atp
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Ikarus

Tue Jun 19, 2001 2:14 am

Please realize that this 75/25 mostly deals with my disdain for assigning ratios. In effect, what this means is bernoulli and the upper half of the wing create 75% of the lift and air striking the bottom of the wing create the other 25%. AOA changess everything, that is why you can't assign such figures.
____________________________________

At low angle of attacks, frontal surface area above and below the stagnation point for an elementary airfoil (eg. NACA 2412) airfoil will be nearly equal, with possibly more frontal area above the stagnation point exposed to the oncoming freestream.

This would lead one to think that an airfoil of normal camber would produce negative lift (without circulatory flow) when the chordline is parallel to the flow due to this 'newtonian force.'

This equation which I have posted before gives a rough estimation of CL for the uninitiated. (I don't ever remember where it came from)

CL = (CL0 + 2pi(α )) / (1+(2/AR))

where:
CL0 is the lift from camber
2pi(α ) is lift from AOA "α" ("α" is AOA in radians)
AR is aspect ratio

CL0 will be zero for symmetric airfoils and positive for normal cambered airfoils.

_____________________________________

>>>I was merely shocked to read somewhere that angle of attack is only a minor factor.
where the hell did you read that? I did say this:

>(25/75 is utter crap, and to go off an a tangent: even if it were that simple, it would greatly vary with AOA). <

and

>increasing AOA makes the circulation stronger. <

Perhaps you misunderstood the goal of the second statement. CL does not increase primarily because of increased frontal surface area below the stagnation point, and the air striking the that area. It does however increase the downwash of the wing due to changes in zone pressure and circulation.

The second number (regards to the first) would vary from 0% for the lower surface at the zero lift AOA to 100% after reaching the critical AOA and thus flow separation and the breakdown of circulation.

-thus my statements stand.



aaron

 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Tue Jun 19, 2001 3:51 am

To PerthGloryFan, point taken, you are correct. To Aaron, if you reread my later post I clarified what I meant by my statement that lift doesnt change with aoa. Ill save you some searching. I was refering to the percentage of lift produced by the upper and lower surfaces, not the total amount. Your equasions are quite impressive, I havnt seen those since A+P school about 18 yrs ago. Ikarus, what are we fighting about then? You agree that the upper surface is what makes most of the lift, that was my whole argument right from the start. The whole 75/25 thing is a ballpark figure. Never in my 15 yrs in this business have I ever had to compute the lift that an airfoil will generate. Im not even going to bring up symetrical airfoils. That would probably be another bloodbath. I look forward to the next post--lol..JT
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Tue Jun 19, 2001 7:06 am

AaronATP - I read the statement that angle of attack does not matter in JT-8Ds post (he just corrected it, though) That is why I took part in this discussion more than I had originally intended

JT-8D: What we are arguing about? Out of my point of view, all I wanted to hear is WHY? This discussion is about WHY and HOW lift is generated. Quoting a rule of thumb figure, or mentioning low pressures, high pressures, circulation etc. in passing did not exactly explain that. They just point out some of the effects, but not the underlying principles and causes (and it is quite hard to explain those - it took me 2 years, and I'm still not sure I understand everything well enough: I can apply some formulae, but can I explain it to myself (and then to the forum)? We will see, I suppose)

Also, I found the tone in some posts (AaronATPs as well as yours) a bit startling. There's nothing like a healthy debate, but once it gets offensive, I think the "forum for industry professionals and knowledgeable amateurs" is not the right place....

Anyway: Somewhere way up there (before the serious mudfight) someone asked about ground effect. In our current course, we simulate it by assuming a mirror system of vortices. (The wing is simulated as one "horseshoe vortex" - a bound vortex along the span, and infinite trailing vortices following it at the tips). Then all we need to do is find the force the real and imaginary system of vortices cause at a point, or on the wing...
This works reasonably well, as a flat wall generally has the effect of "mirroring" the airflow....

And again back to Bernoulli and Newton: The bottom surface of the airfoil deflects the flow downwards. As does the upper surface. It does not beat it down by flapping the wings. It is simply the fact that at the trailing edge of any airfoil (not stalling), the flow leaves the airfoil parallel to the trailing edge. This is known as Kutta condition. It has little to do with Mr Newton or Mr Bernoulli. It is just the result of the airflow being not ideal (i.e. it is viscous). So, if you wish, start a debate about whether Kutta-ian, Bernoullian or Newtonian or whatever-ian principles are more responsible for lift. Fact is, lift cannot be explained using any single (easy-to-understand) theory. You can calculate around with forces and momentum (Newton) or pressures and velocities (Bernoulli) or rotation (Kutta-Joukowski) but all that is just scratching the surface: It's just calculations. It is not the sole true holy cause of lift. It's just what we use to calculate.

and tomorrow I'll try and read through my books again, and see whether I find a satisfactory explanation (not just a description of phenomena, side-effects and rules of thumb)

Regards

Ikarus

 
aaron atp
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RE: Bernoullian Vs. Newtonian Lift

Tue Jun 19, 2001 1:30 pm

OK, for the record, a few things.

Regarding my last post, I thought that Ikarus was referring to the content of my post when he wrote this:

>>>Remember: It wasn't me who slagged off that argument, but AaronATP. I was merely shocked to read somewhere that angle of attack is only a minor factor. Angle of attack is EVERYTHING

Second, people apparently don't appreciate my tone with the 75/25 thing... Please realize that it is _not_ about or really directed to who posted it. I have heard that for years and I think it is nonsense for at least 3 reasons, and it perpetuates theories like 'little bullets striking the underside of the wing forcing it upwards lift.' My apologies to JT-8D or Icarus if my tone was out of line.

Finally, as I have stated before, my degree is not aeronautical engineering. It is not my intention to mislead others into thinking it is so.


I should just say that what we are discussing is beyond what should be held as common knowledge and is unnecessary for pilots and mechanics. That's something I think we all know.


aaron
 
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RE: Bernoullian Vs. Newtonian Lift

Wed Jun 20, 2001 6:47 am

Ok I have to admit that i didn't read all the replies in the thread bit I have a question.

If lift was created by the Newton theory why is it that, if you forget to close the fuel cap on your wing and you take off this way all the fuel would get sucked out of the tank.

Bernoulli would explain that easily with the low pressure over the wing, but if lift was Newtonian what would explain this phenomenon?

Nicolas
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Fri Jun 29, 2001 7:15 pm

After having read through my textbooks (again), I suddenly realized that the theory dubbed "Newtonian lift" in this forum is actually really WRONG. There is downwash, but there is an equal and opposite UPWASH (on the other side of the trailing vortices and ahead of the wing), meaning that the wing does not push the air down. Lift is generated through a pressure difference.

Anyway: I have written an article on the subject, but in order to avoid embarassing my University if I got it wrong, I asked my aerodynamics professor to read through it. He said it'll take some time, as he is busy right now. But fundamentally, it should be available shortly...

Regards

Ikarus
 
JT-8D
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RE: Bernoullian Vs. Newtonian Lift

Sat Jun 30, 2001 2:16 am

Ikarus, who says your professor is right? I promised myself I wouldnt even respond to this thread anymore, but I had to say this. If he knew that much, he would be teaching tech schools for a manufacturer, or an airline. Might be a big payraise too?..JT
 
Ikarus
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RE: Bernoullian Vs. Newtonian Lift

Sat Jun 30, 2001 7:51 am

Why should he bother with a lowly thing such as a Tech school for a manufacturer or an airline? This is Imperial College of Science, Technology and Medicine - in any reasonable or official rating it comes out better than Oxford and only slightly behind Cambridge. Our professors are THE experts on their subjects, with some of them practically totally dedicated to Formula 1 and other areas of Fluid Dynamics. My personal tutor is Ferrari's aerodynamic expert. Payraise? Huh?

Well, I haven't heard his opinion yet. If he says it is wrong, I'll accept that (disappointedly). But if he says it is reasonably correct, I'll definitely be leaping with joy!

I do believe it would be hard to find anyone more qualified in aerodynamics than our lecturers....

Regards

Ikarus

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