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#1
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Thrusting or Sucking (where's Howard Stern when we need him.)
OK, what is the better explanation to give fledgling students. Should you say the wing deflects/pushes/thrusts the air down to hold the aircraft up ... or should you say the wing/airflow creates a low pressure area that sucks the wing/aircraft upwards. Like many people, Bernoulli was the initial and only explanation I was aware of ... but I now think it is easier and more accurate to explain that a wing/airfolil pushes the air downward. Yes, you do have pressure differences, but that is just an artifact of the process. A Bernoulli based explanation seems to create some inconsistencies. For example, boat and airplane propellers are basically identical devices and differ in engineering specifics primarily because they operate in different fluid mediums. Both employ "fluidfoils" ... and both produce "thrust" ... but wouldn't a Bernoulli view argue that the airplane prop is actually sucking ... and how would you use Bernoulli to explain the thrust produced by the boat prop. What about a Bernoulli view of SR-71 flight ops as it climbs from sea-level to 85,000 feet. thanks, |
#2
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Thrusting or Sucking (where's Howard Stern when we need him.)
You`re pushing air, water or whatever downwards. The result is a
pressure and shear distribution on a plane which creates both lift and drag. In reality you can only apply bernoulli on a slow (200 kts), airfoil shaped device because of compressibility and possible detatchment of the boundary layer. When you start adding slats, fowler flaps or start flying fast or very slowly (model airplane`s) Bernoulli is far beside the thruth. A propellor or rotor is simply a rotating wing, and works exactly the same. It both sucks and pushes the air afterwards. The Blackbird would probally not fly at all if you use Bernoulli. That`s it... Jarno Nieuwenhuize, The Netherlands. |
#3
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Thrusting or Sucking (where's Howard Stern when we need him.)
Stick the cambered side of a standard spoon under a running tap! Ease the cambered side of the spoon gradually into the running water until the flow ‘sucks’ the spoon into the flow. The faster the flow of water from the tap, the more "lift"… and the slower the flow the less "lift". You can demo the stall by reducing the flow of water until the spoon fails to sustain flight!! If you want to look like more of an idiot, blow on the back of the spoon whilst it's in the flow of water. It effectively demonstrates a few principles of fluid dynamics… even if it isn't as scientifically correct as it should be. You students won’t forget the demo. OK, what is the better explanation to give fledgling students. What's wrong with explaining both? There’s a lot of aerodynamics that requires a fairly comprehensive knowledge of both principles? Yes, you do have pressure differences, but that is just an artifact of the process. I'm not sure if I understand that sentence. Explaining 'deflection of air' alone without reference to pressure differential essentially means that the many reasons for a cambered wing are essentially ignored. Explain reasons why some wings are heavily cambered and why some are not and it qualifies your explanation. What about a Bernoulli view of SR-71 flight ops as it climbs from sea-level to 85,000 feet. My point exactly. What about a SFTOL aircraft as an example of the opposite end of the performance spectrum? Compare wings (L/D, span, chord, etc) and compare power/thrust/weight ratios. An example of an all-in-one wing might be an airliner with slats, slots, spoilers, and flaps etc that effectively create a new wing for a different purpose in different phases of flight. -- Marty Posted at www.flight.org |
#4
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Thrusting or Sucking (where's Howard Stern when we need him.)
Newton explains it better than Bernoulli. The FAA
written for PSEL no longer requires the Bernoulli answer about lift. Go to Amazon.com or Bookfinder.com and look for a book by Gale Craig, titled, 'Stop Abusing Bernoulli! - How Airplanes Really Fly '. (Search under Craig, Gale.) Gale is a physicist who enquired into this subject. Used copies are listed there, but the book is self-published and you can order fresh copies from him. He lives in Anderson, Indiana. Anyone who is interested in that address should write me and I will get it for you. At 17:00 09 January 2006, Marty wrote: Stick the cambered side of a standard spoon under a running tap! Ease the cambered side of the spoon gradually into the running water until the flow �sucks� the spoon into the flow. The faster the flow of water from the tap, the more 'lift'� and the slower the flow the less 'lift'. You can demo the stall by reducing the flow of water until the spoon fails to sustain flight!! If you want to look like more of an idiot, blow on the back of the spoon whilst it's in the flow of water. It effectively demonstrates a few principles of fluid dynamics� even if it isn't as scientifically correct as it should be. You students won�t forget the demo. OK, what is the better explanation to give fledgling students. What's wrong with explaining both? There�s a lot of aerodynamics that requires a fairly comprehensive knowledge of both principles? Yes, you do have pressure differences, but that is just an artifact of the process. I'm not sure if I understand that sentence. Explaining 'deflection of air' alone without reference to pressure differential essentially means that the many reasons for a cambered wing are essentially ignored. Explain reasons why some wings are heavily cambered and why some are not and it qualifies your explanation. What about a Bernoulli view of SR-71 flight ops as it climbs from sea-level to 85,000 feet. My point exactly. What about a SFTOL aircraft as an example of the opposite end of the performance spectrum? Compare wings (L/D, span, chord, etc) and compare power/thrust/weight ratios. An example of an all-in-one wing might be an airliner with slats, slots, spoilers, and flaps etc that effectively create a new wing for a different purpose in different phases of flight. -- Marty Posted at www.flight.org |
#5
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Thrusting or Sucking (where's Howard Stern when we need him.)
I think you can also obtain books that prove the earth
is flat and the Holocaust never happened. Oh and I forgot there are several that prove global warming :-) At 18:24 09 January 2006, Nyal Williams wrote: Newton explains it better than Bernoulli. The FAA written for PSEL no longer requires the Bernoulli answer about lift. Go to Amazon.com or Bookfinder.com and look for a book by Gale Craig, titled, 'Stop Abusing Bernoulli! - How Airplanes Really Fly '. (Search under Craig, Gale.) Gale is a physicist who enquired into this subject. Used copies are listed there, but the book is self-published and you can order fresh copies from him. He lives in Anderson, Indiana. Anyone who is interested in that address should write me and I will get it for you. At 17:00 09 January 2006, Marty wrote: Stick the cambered side of a standard spoon under a running tap! Ease the cambered side of the spoon gradually into the running water until the flow �sucks� the spoon into the flow. The faster the flow of water from the tap, the more 'lift'� and the slower the flow the less 'lift'. You can demo the stall by reducing the flow of water until the spoon fails to sustain flight!! If you want to look like more of an idiot, blow on the back of the spoon whilst it's in the flow of water. It effectively demonstrates a few principles of fluid dynamics� even if it isn't as scientifically correct as it should be. You students won�t forget the demo. OK, what is the better explanation to give fledgling students. What's wrong with explaining both? There�s a lot of aerodynamics that requires a fairly comprehensive knowledge of both principles? Yes, you do have pressure differences, but that is just an artifact of the process. I'm not sure if I understand that sentence. Explaining 'deflection of air' alone without reference to pressure differential essentially means that the many reasons for a cambered wing are essentially ignored. Explain reasons why some wings are heavily cambered and why some are not and it qualifies your explanation. What about a Bernoulli view of SR-71 flight ops as it climbs from sea-level to 85,000 feet. My point exactly. What about a SFTOL aircraft as an example of the opposite end of the performance spectrum? Compare wings (L/D, span, chord, etc) and compare power/thrust/weight ratios. An example of an all-in-one wing might be an airliner with slats, slots, spoilers, and flaps etc that effectively create a new wing for a different purpose in different phases of flight. -- Marty Posted at www.flight.org |
#6
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Thrusting or Sucking (where's Howard Stern when we need him.)
Here is a stab at a simple explanation -
The crux of the matter is that aircraft fly by the transfer of energy. Fluid dynamics and physics provide us with a variety of tools to model how this happens. A famous quote from IT applies - "All models are false, some models are useful" Depending on the situation one or another model may be more useful and/or accurate, but they remain models. We are generally concerned with low Reynolds numbers and laminar flow over low lift+low drag wings. In this regime the contribution of upper surface lift is apparently no less than 1/3 of the total, and at low angle of attack it may be greater than 50%. The net result of Bernoulli, Prantl et al is that a moving wing transmits kinetic energy to the air. As long as the air flows around the wing in a relatively orderly fashion, the reaction is lift (varying amounts of suck and blow), if the flow becomes chaotic (stalled) we get primarily drag.(Very simple.) Conversely - calculating how much will be created, and optimising efficiency over a range of performance requires serious computational power and a head for calculus. (very complex) Nyal Williams wrote: Newton explains it better than Bernoulli. The FAA written for PSEL no longer requires the Bernoulli answer about lift. Go to Amazon.com or Bookfinder.com and look for a book by Gale Craig, titled, 'Stop Abusing Bernoulli! - How Airplanes Really Fly '. (Search under Craig, Gale.) Gale is a physicist who enquired into this subject. Used copies are listed there, but the book is self-published and you can order fresh copies from him. He lives in Anderson, Indiana. Anyone who is interested in that address should write me and I will get it for you. At 17:00 09 January 2006, Marty wrote: Stick the cambered side of a standard spoon under a running tap! Ease the cambered side of the spoon gradually into the running SNIP -- Bruce Greeff Std Cirrus #57 I'm no-T at the address above. |
#7
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Thrusting or Sucking (where's Howard Stern when we need him.)
Don Johnstone wrote:
I think you can also obtain books that prove the earth is flat and the Holocaust never happened. Oh and I forgot there are several that prove global warming :-) "Climate Change" C'mon don, get with the times. "Global Warming" is so 1997. ;-) BTW, was the Earth flat before Magellan sailed around it? Shawn |
#8
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Thrusting or Sucking (where's Howard Stern when we need him.)
In article ,
Bruce wrote: Here is a stab at a simple explanation - The crux of the matter is that aircraft fly by the transfer of energy. Fluid Point of order: transfer of *momentum*. In order to fly, a heavier-than-air aircraft must apply a certain force to the air, transferring momentum to it. The amount of momentum transferred is constant across the aircraft's performance envelope, whereas the energy transfer varies. dynamics and physics provide us with a variety of tools to model how this happens. A famous quote from IT applies - "All models are false, some models are useful" Depending on the situation one or another model may be more useful and/or accurate, but they remain models. We are generally concerned with low Reynolds numbers and laminar flow over low lift+low drag wings. In this regime the contribution of upper surface lift is apparently no less than 1/3 of the total, and at low angle of attack it may be greater than 50%. The net result of Bernoulli, Prantl et al is that a moving wing transmits kinetic energy to the air. As long as the air flows around the wing in a relatively orderly fashion, the reaction is lift (varying amounts of suck and blow), if the flow becomes chaotic (stalled) we get primarily drag.(Very simple.) Conversely - calculating how much will be created, and optimising efficiency over a range of performance requires serious computational power and a head for calculus. (very complex) Nyal Williams wrote: Newton explains it better than Bernoulli. The FAA written for PSEL no longer requires the Bernoulli answer about lift. Go to Amazon.com or Bookfinder.com and look for a book by Gale Craig, titled, 'Stop Abusing Bernoulli! - How Airplanes Really Fly '. (Search under Craig, Gale.) Gale is a physicist who enquired into this subject. Used copies are listed there, but the book is self-published and you can order fresh copies from him. He lives in Anderson, Indiana. Anyone who is interested in that address should write me and I will get it for you. At 17:00 09 January 2006, Marty wrote: Stick the cambered side of a standard spoon under a running tap! Ease the cambered side of the spoon gradually into the running SNIP -- Alan Baker Vancouver, British Columbia "If you raise the ceiling 4 feet, move the fireplace from that wall to that wall, you'll still only get the full stereophonic effect if you sit in the bottom of that cupboard." |
#9
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Thrusting or Sucking (where's Howard Stern when we need him.)
Ken,
Howard Stern is on Sirius Satellite radio... Channel 100... first day today! Channel 101 replay for West Coasters ..... LOL |
#10
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Thrusting or Sucking (where's Howard Stern when we need him.)
Ken Kochanski (KK) wrote:
OK, what is the better explanation to give fledgling students. Should you say the wing deflects/pushes/thrusts the air down to hold the aircraft up ... or should you say the wing/airflow creates a low pressure area that sucks the wing/aircraft upwards. I suggest you tell them to hold their hand out a car window like a wing, and experiment with the angle of attack. Anything more complicated than that isn't going to help them fly a glider better. You don't have to understand the physics to fly well, as ras demonstrates repeatedly, and I don't see how explaining it with Bernoulli's theorem, or f=ma, or pressure differentials is an aid to flying. It's hard enough to get across the idea of angle of attack for stalling, much less Bernoulli or Newton. Like many people, Bernoulli was the initial and only explanation I was aware of ... but I now think it is easier and more accurate to explain that a wing/airfolil pushes the air downward. Yes, you do have pressure differences, but that is just an artifact of the process. And this illustrates part of the problem. Ken, whom I believe to be a good instructor, wants to explain it to the student, but he doesn't understand it either (I'm not suggesting I do, either). Nonetheless, his students can fly well, because you have to know what to do at the right time, and (fortunately) you don't have to figure it out from an explanation of the physics involved. Pragmatically, telling the student whatever explanation makes them happy is probably good enough, but maybe referring the really interested to good book like "Fundamentals of Sailplane Design" would be a good idea. -- Change "netto" to "net" to email me directly Eric Greenwell Washington State USA |
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