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Backwash Causes Lift?
Ο "Le Chaud Lapin" έγραψε στο μήνυμα ups.com... Hi, Student pilot here, self-teaching using the Jeppensen Private Pilot Kit after taking ground school. .................................................. there is more to it than the way it is being described in context of flying.] -Le Chaud Lapin- Hot Bunny ?????????????? ROTFLMAO |
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Backwash Causes Lift?
You wrote
On Oct 2, 9:57 pm, Le Chaud Lapin wrote: Hi, Student pilot here, self-teaching using the Jeppensen Private Pilot Kit after taking ground school. I read in the book that combustion "creates" energy, which is technically not true, but I decided to ignore it since the pictures are sooo pretty. Now, in Chapter 3, section about airfoils, it actually says: "In addition to the lowered pressure, a downward-backward flow of air also is generated from the top surface of the wing. The reaction to this downwash results in an upward force on the wing which demnstrates Newtons' third law of motion. This action/reaction principle also is apparent as the airstream strikes the lwoer surface of the wing when inclinded at a small angle (the angle of attack) to its direction of motion. The air is forced downward and therefore causes an upward reaction resulting in positive lift." IMHO, the latter part of this paragraph is correct, but the former part is wrong. 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. You would do well to think in terms of differential pressure. If one can by some means cause the pressure on the upper surface of the wing to be 1 psi less than the pressure on the lower surface, there would be an upward force on the wing of the order of 144 pounds per square foot (my physics training, a million years ago, was in english units). 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. 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. |
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Backwash Causes Lift?
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. 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. -Le Chaud Lapin- |
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Backwash Causes Lift?
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. 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. 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. Dan |
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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- |
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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 |
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Backwash Causes Lift?
Le Chaud Lapin writes:
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. Describe the errors in a letter and send it to them. Good technical publishers are always willing to accept corrections. |
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Backwash Causes Lift?
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Backwash Causes Lift?
jimp wrote I think the main issue is that it doesn't require a Phd in physics to fly an airplane and the explanations of lift, stall, drag, etc. for pilots tend to be highly simplified, and rightfully so. A full explanation that would satisfy a physisicist would likely cause exterme eye glaze in the average pilot. If one want's that level of insight, I would suggest they go read a good aerodynamics text and not expect to find it in a couple of paragraphs in a USENET posting. Exactly. To learn to fly, it is important to understand aerodynamics to the point of knowing what you can do to mess them up, and what you can do to fix things. Only the most basic understanding of the subject is needed to achieve these goals. -- Jim in NC |
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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- |
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