Western727 From United States of America, joined Jan 2007, 643 posts, RR: 4 Posted (12 years 6 months 1 week 2 days 6 hours ago) and read 3450 times:
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..
Buzz From United States of America, joined Nov 1999, 697 posts, RR: 23 Reply 3, posted (12 years 6 months 1 week 1 day 17 hours ago) and read 3198 times:
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.
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3 Reply 7, posted (12 years 6 months 1 week 1 day 12 hours ago) and read 3171 times:
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 From United States of America, joined Jan 2007, 643 posts, RR: 4 Reply 8, posted (12 years 6 months 1 week 1 day 10 hours ago) and read 3163 times:
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.
Monocleman From United States of America, joined Apr 2001, 137 posts, RR: 0 Reply 9, posted (12 years 6 months 1 week 1 day 7 hours ago) and read 3153 times:
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.)
PerthGloryfan From Australia, joined Oct 2000, 751 posts, RR: 0 Reply 11, posted (12 years 6 months 1 week 15 hours ago) and read 3130 times:
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.
Fr8tdog From United States of America, joined Feb 2000, 120 posts, RR: 0 Reply 12, posted (12 years 6 months 1 week 10 hours ago) and read 3116 times:
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 From United States of America, joined Jun 2005, 0 posts, RR: 1 Reply 13, posted (12 years 6 months 4 days 11 hours ago) and read 3087 times:
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)
Aaron atp From United States of America, joined Mar 2000, 533 posts, RR: 2 Reply 14, posted (12 years 6 months 4 days 2 hours ago) and read 3075 times:
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.
Fr8tdog From United States of America, joined Feb 2000, 120 posts, RR: 0 Reply 15, posted (12 years 6 months 4 days ago) and read 3061 times:
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 From United States of America, joined Dec 2000, 423 posts, RR: 3 Reply 16, posted (12 years 6 months 3 days 7 hours ago) and read 3054 times:
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 From United States of America, joined Jun 2005, 0 posts, RR: 1 Reply 17, posted (12 years 6 months 3 days 3 hours ago) and read 3050 times:
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.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 19, posted (12 years 6 months 2 days 19 hours ago) and read 3043 times:
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...)
JT-8D From United States of America, joined Dec 2000, 423 posts, RR: 3 Reply 20, posted (12 years 6 months 18 hours ago) and read 3023 times:
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
Jetguy From , joined Dec 1969, posts, RR: Reply 21, posted (12 years 6 months 18 hours ago) and read 3018 times:
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?
DC-10Tech From United States of America, joined Jun 2001, 298 posts, RR: 3 Reply 23, posted (12 years 6 months 17 hours ago) and read 3015 times:
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.