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#31
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Backwash Causes Lift?
Le Chaud Lapin wrote in
ups.com: On Oct 3, 8:39 am, Tina wrote: You might also want to think carefully about airfoil shapes, since wings can provide lift when flying inverted. Any theory that does not support inverted flight is obviously flawed. Actually I did because every book I read about flying skimped on the subject. I'm going to hop over to MIT's OpenCourseWare later this week and download their most basic course on aero/astro. Benoulli's principle is toss around as if it were facecloth, but I'm getting the feeling that no one is really doing the physics. Well, NASA have and they wholeheartedly support Bernoulli , as do an Airbus engineeer and a Boeing Engineer of my acquantence. NASA used to have a whole web page debunking the Bernoulli deniers, but it sems to be gone now. It's pretty simple in how it relates to how the airplane flies from one standpoint and complex from another, but at the end of the day it's how your knowledge makes the airplane perfomr that counts, and the only thing tha's going to teach you that is flying an airplane guided by a good instructor. If you try to understand it at a molecular level, you've got a problem, as nobody really undertands lift completely (Here's anthony's chance to go all creationist on us now) But the neat thing to do is to hold your hand out of a moving car's window, and feel the impact pressure on its surfaces as you tilt it in the airstream. It's not that the hand is being "sucked" up, you don't feel suction on the top surface, you feel push on the bottom one. Any theory you develop had better be consistant with those observations. Someone with more time than I have might like to start with the fact that air weighs about .08 pounds per square foot near sea level, and crack some numbers to show how that deflecting that mass can result in lift even if the lifting surface has some funny shapes.- Hide quoted text - Yes it is. In fact, I was having this discussion with someone who claimed that it *was* Benoulli's principle only. I made the following diagram to try to illustrate my point. View in fixed width | inverted | | table | |--------------------| |--------------------| | upright | | table | The Bernoulli people often describe air flowing above the a table being faster than air below a table, and therefore, pressure is reduced. Hmmm... what happens if the horizontal velocities above and below a table are both essentially 0? If you place an inverted table on top of an upright table so that the table tops are mated, then have a machine, with a tremendous amount of force, on the order of 14.4lbs/in^2 of force, yank the inverted table upward, in one quick jerk, I contend that the lower table will be strongly inclined to follow by jumping updward, obviously due to pressure beneath it. So any type of rarefication on one side of a doubly-pressurized surface that is free to move in direction that is perpendicular to the surface, will, indeed, move, if pressure is reduced. And this is why, I am pretty sure, that if I were to search the web, one would find people who are fanatical about the leading edges of wings, in the most minute detail, because it is not simply the length of the top of the wing that matters, but the amount of pinching, and the distribution of air as it flows backward from the pressure point. IMO, that pinching results in displacement of the air above to make it effective go backwares, causing rarefication. That all sounds fairly sound, but it's too esoteric to relate to handling an airplane well. Don't forget, handling, and it's handling that is the aim of the knowledge you seek, is primarily a right hand brain operation. If you try to fly with too much of the left included, you are going to fly like a chicken on crack. Bertie |
#32
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Backwash Causes Lift?
On Oct 3, 10:34 am, wrote:
Both Newton and Bernoulli are correct. Even inside a pipe the static pressure drops as velocity increases. That's why your bottom table jumps as you yank off the top one: you accelerated an airflow. And in generating lift there's a displacement of air. Can't escape that at all. It is the removal of air from above the table that causes the lift. If a person sucks on a straw, thus removing air from the inside of the straw, the fluid rises into the straw from the container do to the air/ fluid system outside the straw. This is the same phenomenon that is occurring in my table scenario. In fact, I could enclose the entire table scenario inside a tube, and cause the bottom table to rise up off the ground. This has nothing to do with the velocity or acceleration of air. The stagnation point on a leading edge isn't right at the front. It's slightly below the wing, and as AOA increases it moves back underneath quite a bit. It's not all intuitive, you see, and that intuitive understanding of some of this stuff is where people get all messed up and think they have the answers that have escaped all the other experts all these years. We run into this attitude rather frequently in the flight training industry. It tends to make the student unteachable. I do rely on intuition to figure things out, but most importantly, I'd rather actually understand, than except shallow explanations. I never attempted to contradict either Bernoulii or Newton. What I keep saying is that I have seen too many situations where someone will rattle of "Bernoulli's Principle" and not really understand it themselves. For example, as I mentioned earlier, I am reading Jeppesens Private Pilot manual, and there are clearly errors in concept the manual (energy being created by engine, for example), even though Jeppensen probably has access to as many Ph.D. laureates as they want. -Le Chaud Lapin- |
#33
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Backwash Causes Lift?
"Dan Luke" wrote in
: "Le Chaud Lapin" wrote: Obviously, any air above the wing can only result in a force downward on top of the wing. The only force causing the plane to want to move upward comes from beneath the wing. The effect of any air above the wing is to cause rarefication above the wing, resulting in lower pressure, thereby giving the 14.7lbs/in^2 (plus) to do its work. That "reaction" coming from downward movement of air seems just plain silly to me. Then why does the wing stall and cease lifting when flow separates from the upper surface? Actually, that's not the definition of the stall, seperation occurs just after the stall with most airfoil/planform combinations and in most flight situations, ordinarily. Some wings will have seperation at the stall, but I've never flown one. The defintion is an abrupt loss of lift when the critical angle is reached. Seperation usually occurs immidiatly after (*but not always, for instance, deltas will continue to have smooth flow way below he point they will actually keep flying) This is not to be confused with the back siide of the drag curve, BTW. Having said all hat, there are some reputable design texts that define stall as the point at which the bubble breaks down and buffet occurs and as far as I know, this doesn't disturb engineers (of which I am not one, BTW, so take this all with a large grain of salt) any more than a "po- tay-to, po-tah-to" argument would. Bottom line is you're interested in keeping your airplane from going down and the point at which the wing ceases to do what you would like it to do is the point at which you're most interested. Make sense? If it does I must not have explained it well. Bertie |
#34
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Backwash Causes Lift?
Bertie the Bunyip wrote:
like a chicken on crack. Bertie Funny...I suspect you know what that looks like. ; ) -- Message posted via AviationKB.com http://www.aviationkb.com/Uwe/Forums...ation/200710/1 |
#35
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Backwash Causes Lift?
Le Chaud Lapin wrote in
oups.com: On Oct 3, 10:34 am, wrote: Both Newton and Bernoulli are correct. Even inside a pipe the static pressure drops as velocity increases. That's why your bottom table jumps as you yank off the top one: you accelerated an airflow. And in generating lift there's a displacement of air. Can't escape that at all. It is the removal of air from above the table that causes the lift. If a person sucks on a straw, thus removing air from the inside of the straw, the fluid rises into the straw from the container do to the air/ fluid system outside the straw. This is the same phenomenon that is occurring in my table scenario. In fact, I could enclose the entire table scenario inside a tube, and cause the bottom table to rise up off the ground. This has nothing to do with the velocity or acceleration of air. The stagnation point on a leading edge isn't right at the front. It's slightly below the wing, and as AOA increases it moves back underneath quite a bit. It's not all intuitive, you see, and that intuitive understanding of some of this stuff is where people get all messed up and think they have the answers that have escaped all the other experts all these years. We run into this attitude rather frequently in the flight training industry. It tends to make the student unteachable. I do rely on intuition to figure things out, but most importantly, I'd rather actually understand, than except shallow explanations. I never attempted to contradict either Bernoulii or Newton. What I keep saying is that I have seen too many situations where someone will rattle of "Bernoulli's Principle" and not really understand it themselves. For example, as I mentioned earlier, I am reading Jeppesens Private Pilot manual, and there are clearly errors in concept the manual (energy being created by engine, for example), even though Jeppensen probably has access to as many Ph.D. laureates as they want. Yes, well, you obviously need to write a good old fashioned, angry, frothng at the mouth letter to Jeppeson. I'm sure Elry will be suitably rattled. Bertie |
#36
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Backwash Causes Lift?
"Kloudy via AviationKB.com" u33403@uwe wrote in news:79253d6018083@uwe:
Bertie the Bunyip wrote: like a chicken on crack. Bertie Funny...I suspect you know what that looks like. ; ) Nah,I just have a good imagination. Though if oyu send me some crack I do have some chickens I could try it on. Bertie |
#37
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Backwash Causes Lift?
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#38
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Backwash Causes Lift?
On Oct 3, 11:28 am, Bertie the Bunyip wrote:
"Kloudy via AviationKB.com" u33403@uwe wrote innews:79253d6018083@uwe: Funny...I suspect you know what that looks like. ; ) Nah,I just have a good imagination. Though if oyu send me some crack I do have some chickens I could try it on. Bertie )))! -Le Chaud Lapin- |
#39
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Backwash Causes Lift?
On Oct 3, 10:34 am, wrote:
On Oct 3, 8:15 am, Le Chaud Lapin wrote: Actually I did because every book I read about flying skimped on the subject. I'm going to hop over to MIT's OpenCourseWare later this week and download their most basic course on aero/astro. Benoulli's principle is toss around as if it were facecloth, but I'm getting the feeling that no one is really doing the physics. Lemme see: People have been building flying machines since the late 1800's, about 125 years now, and none of them have been interested enough in the phenomenon of lift to do the physics? How old are you, anyway? Many of the contributors here have been flying much longer than you have likely been alive and have studied this in detail, and some of them might even have doctorates in the subject. The subject of lift has been beaten to death on this forum and if you Googled it you'd find some good information. I want to be clear. I did not me to say "no one" is doing the physics. Obviously there are aero/astro scientists all over the world. What I mean to say is that there seems to be a lot of *pilots* who are using Bernoulli's principle somewhat carelessly, IMO. Some of these people are CFI's. Please don't ask me to name individuals, but I know with certainty that there are at least 2 living, breathing CFI's who do not understand where 29.92 Hg comes from, or does not understand it well enough to make it make sense to a student. The might have understood it at one point, but they don't now. I know because I asked them. My feelings about teaching is that if you are not very certain about something, you do more damage than talking about it. Of course, this leads to the conundrum of having to explain to a student why a plane stays in the air without providing erroneous information. If I were a CFI, I would simply say that the aerodynamics result in pressure below plane is sufficient to counteract pressure above planes for force of gravity. If they wanted to know more, I'd direct them to book on aerodynamics. Can you fly without understanding many of these things? Certainly. But personally, I would feel a lot better in a cockpit if I did. The more I know, the more confident I am, and if something goes wrong, the added perspective will allow me to quickly eliminate those things which I am certain is not root of problem. This reminds me of incident about year ago when I was fixing a neighbor's car, and another neighbor kindly ask me if we needed to borrow jumper cables, even though she had heard the engine turn over with no problem many times. Someone who understands how automobile works would have known that it is highly unlikely that there was problem with battery with such robust cranking. [Turned out to be fuel line]. -Le Chaud Lapin- |
#40
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Backwash Causes Lift?
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