No one can explain WHY planes fly...

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by dragon2knight, Feb 5, 2020.

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  1. Feb 13, 2020 #121

    bmcj

    bmcj

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    I feel it necessary to correct his checklist...

    Step 7: add heat
    Step 8: extinguish flaming bunny ears
    Step 9: enjoy applause
     
  2. Feb 13, 2020 #122

    jedi

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    More of that "I have made assumptions that I will not bother to explain because it would bore you to death".

    The overall average movement is static as there is no large scale or measurable movement using common air movement instrumentation.

    There are half a dozen other but similar examples in post #116 that we can nit pick if anyone cares.

    The molecular movement that BBerson mentions is important when we get into the next phase of discussion (Bernoulli) as this is how the air molecules communicate with each other and get out of the way of the wing as the wing approaches.

    Next question please.
     
    Last edited: Feb 13, 2020
  3. Feb 14, 2020 #123

    wktaylor

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    As a long-time aero engineer, the precept of this thread is a bit like "how many fairies/angels can dance on the head push-pin... now prove-it without reference to web videos, articles and commentary". Some of these discussion points are surreal without a solid engineering science background. DEAL with this mystery... and have fun flying!

    Having said-that, here is an old engineering question to chew-over...

    The canary's in-a-truck scenario**.
    **or what ever small bird suits your fancy... and let's forget the obvious cruelty part of this question...

    A truck with an enclosed cargo compartment has been loaded with thousands of canary's in a few large cages. When loaded, they are all stationary on their perches. The truck is carefully weighed and it's determined that the birds [only] added weight is, exactly/remarkably, 500# [226.8Kg]. As the truck begins to roll/bounce the canary's all of sudden get frightened and ALL of them fly off their perches and swoop around in fear. Since they are still are fully contained with the cargo compartment, what happens to the vehicle weight... does it suddenly become 500# lighter [because they are all flying]?... or does the total weight remain, same since they are all still on-board [even though they are flying]?... or does something else happen?
     
  4. Feb 14, 2020 #124

    BBerson

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    Some of these discussion points are surreal without a solid engineering science background.*

    * from post above
     
  5. Feb 14, 2020 #125

    wktaylor

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    Another mystery... suitable for 14 Feb...

    Love is an endless mystery, for it has nothing else to explain it.” --Rabindranath Tagore
     
  6. Feb 14, 2020 #126

    henryk

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    +air EJECTION fenomenon ...
     
  7. Feb 14, 2020 #127

    cdlwingnut

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    Ok let me explain this to you. Lift comes from passenger fear, the more scared the passengers are the harder they lift up on the armrests therefore lifting the airplane. the engines are there to make noise which makes the passengers more scared so they lift harder on the armrests.
     
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  8. Feb 14, 2020 #128

    poormansairforce

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    LOL, I've got a post on here somewhere about passengers jumping up and down to get the wings to flap like a bird. Case closed.:D

    Edit: Oops, I forgot I'm not an aero engineer with a solid engineering science background so disregard the above as I'm not qualified to post here.:bow:
     
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  9. Feb 14, 2020 #129

    Norman

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    In post #118 you said "The air in a milk carton isn't static. The molecules are bouncing off each other and the carton at the speed of sound of those molecules"

    OK let's say that instead of a milk carton you have a rubber balloon in a very large room. As long as the balloon is sealed and at the same temperature as the room the molecules inside the balloon are zipping around at the same speed as those outside of the balloon bumping into each other and the wall of the balloon and exerting pressure through those collisions. In a closed container the pressure is equal in all directions so the balloon just sits there with no tendency to move unless something external acts upon it (it's static even though on a molecular scale there's a lot of movment). If you pop the balloon you now have a region of higher pressure in the room which will propagate outward at the speed of sound but the air that was in the balloon does not move very far. This can be demonstrated with a smoke filled balloon. When the balloon pops anybody in the room can hear the pressure wave and see the cloud of smoke. Eventually they'll also smell the smoke but that's carried around the room by turbulent mixing, not the pressure wave. This is analogous to electron signal speed vs drift speed. When you flip a switch in a DC circuit the signal travels around the circuit at a significant fraction of the speed of light but the electrons in the wire have an average velocity along the wire so low that if you raced an electron from the wall switch to a light you wouldn't have to walk very fast to win. So now let's toss a little balsa glider into that room. The glider produces some local movement of the air but it's not increasing the volume like that popping balloon did so the wake of that plane basically stays where it was created with a small drift velocity in the direction of the glider and dissipates through turbulent mixing long before it reaches the floor. [The net] momentum added to the air by the passage of the glider is horizontal. The weight of the glider is transferred to the floor by pressure not momentum.
     
    Last edited: Feb 15, 2020
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  10. Feb 14, 2020 #130

    Norman

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    Neither am I but I do it anyway. Mostly I just parrot what real engineers have told me.
     
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  11. Feb 14, 2020 #131

    Elmog

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    Basic understanding of "lift" was never an issue in aviation (early gliders). Refinement of the airfoil, though, has been an ongoing science that is proved in actual useage in the air over time, such as going from single surface airfoils to, say, a Clark Y airfoil. We keep trying new things until the newest outperforms the previous and then we build on that. If it can carry more weight, go faster, stall slower or achieve a better balance of these performance parameters, then we all celebrate the latest "breakthrough." An example would be when Cessna gave the 150 and 172 a new leading edge profile that tamed the stall characteristics a bit. A small change but a measurable step forward.
    Refinement and actual testing in the air rather than understanding "lift" is where the gains have been made in industry. While breaking down lift by using mathematics and theories is comprehensible by a few, the real work is being done in testing where success can only be measured by an improvement in performance over the previous.
     
  12. Feb 14, 2020 #132

    henryk

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  13. Feb 14, 2020 #133

    BBerson

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    Might be near horizontal at the trailing edge. The overall air mass under a hovering helicopter is definitely vertically down.
     
  14. Feb 15, 2020 #134

    Norman

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    Oops, replace "all of the" with "The net" and we're good. Stand a few feet outboard of a hovering helicopter with a smoke bomb and see where the smoke goes. It goes up and rotates. Some of it even goes back into the top of the rotor. All of the downwash of a wing ends up in the tip vortices because the downwash is what makes the vortices.
     
    Last edited: Feb 15, 2020
  15. Feb 15, 2020 #135

    jedi

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    I can understand the Canary Scenario. But I have never been able to resolve the following. Perhaps someone with one of those modern quad copters and a good camera system could film the results.

    I was flying along at 7,000 feet one day when a fly emerged from some unknown hiding place and began beating his head against the windscreen in a futile attempt to push the little Cherokee 140 just a little bit faster. After a while I started feeling sorry for the poor little thing working so hard and accomplishing so little while I sat enjoying the blue sky and warm sunlight. Besides, his buzzing and commotion was distracting and annoying.

    I decided he deserved to be set free. After opening the side window I finally coaxed him (or possibly her, I have no idea and suspect it does not matter) out into the cold cruel world. Now any human pilot would be delighted with all that free altitude but the fly had demonstrated his (excuse me mam, I just covered the ambiguity) intelligence with his instance on continuing to beat his head against the windscreen while I seriously tried to help him return to wherever he was attempting to go.

    Once he was out the window I began wondering about his fate. I had on occasion been caught too high on approach and had to work to get down. I know the feeling. I have never seen a fly power down the flappers and glide but they do make a nice powered descent and can maneuver to land upside down on the ceiling in the dark, a maneuver that Bill Stout thought was quite impressive, so I know he was quite capable.

    I know it was darn cold outside and the shock cooling would be similar to my ditching in the North Atlantic, not a pleasant thought. On the other hand he could hibernate while falling to a lower altitude (It was summertime) and awake to a nice warm environment.

    My concern was that his instinct was to keep flapping and thrashing as he did in the plane. If so it could take him hours to get to a reasonable landing field. I do not know if he had enough fuel on board to make it to a suitable landing site and that is a situation that I would not want to put any pilot in.

    I sure would like to know what happened to that fly. If any of you ever see him let me know how she is doing. Oh, and please take care of her for me and let her know that I love her.

    Happy Valentine's Day.
     
  16. Feb 15, 2020 #136

    jedi

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    And now a little Bernoulli nostalgia and background. Perhaps he will be able to answer the question of "how airplanes fly" even before the airplane was invented.


    About Bernoulli

    Daniel Bernoulli (1700 - 1782) was the second son of Swiss mathematician Johann Bernoulli. Daniel Bernoulli studied medicine, mathematics, fluid flow, and theoretical physics. He derived an expression for the pressure resulting from the molecular motion of a gas in a closed container but is most well known for his treatise “Hydrodynamica” published in 1783 which presented what is now known as Bernoulli's Equation.

    The Bernoulli equation relates the properties of fluid flow of in terms of speed, pressure, and potential energy with the mathematical expression:
    upload_2020-2-15_1-50-56.png
    The expression assumes steady flow along a streamline, constant density and no friction and describes the flow within a pipe where the flow is uniform across the diameter of the pipe.

    The equation was developed while studying the flow of blood within the body and applied to hydraulics, the dynamics of water flowing within a pipe.

    In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy.

    When the equation is applied to gasses there are two significant changes due to the low density of the fluids. The third term, density times the acceleration of gravity, times the height becomes very small and can be ignored or assumed to be zero. Furthermore, gasses are compressible and this fact is not accounted for in the equation. However, when the velocities are slow enough the dynamic pressure changes of the second term are so small, again because of the low density, that the density can be assumed to be constant. In aerodynamics, equations using this assumption are labeled incompressible flow. The errors due to the incompressible flow assumption are generally minimal up to two or three hundred knots or Mach 0.4 or 0.5 or less.

    You might say so what! That's ok say whatever you want. I intend to post my so what when I get around to it, not that anyone cares.
     
    Last edited: Feb 15, 2020
  17. Feb 15, 2020 #137

    BBerson

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    It seems the opposite would be true, like dynamic pressure. Can you explain why pressure would decrease?
     
  18. Feb 15, 2020 #138

    jedi

    jedi

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    That statement is from the internet where everything is known to be true. The URL and page contents are copied below. I see your point and am working on an answer but I am not sure I am clear on your question. See bold print [I modified] below. Are you referring to the
    "decrease in static pressure as the speed increases" or the "decrease in the fluid's potential energy." Refer to the equation and see if your interpretation of the word statement agrees with the equation.

    https://www.google.com/search?sxsrf=ACYBGNR4CdOqS3zzJbCtUuMJ5N4S3Wr6xQ:1581789365427&source=hp&ei=tTBIXpfxF8jY-wTSoLjABQ&q=bernoulli+formula&oq=bernoulli&gs_l=psy-ab.1.6.35i39j0l9.486.4404..10118...0.0..0.762.3476.3j0j1j0j2j1j2......0....1..gws-wiz.......0i131j0i10.6lPUtI0Q0Vw

    upload_2020-2-15_9-58-26.png
    upload_2020-2-15_9-58-26.png
    upload_2020-2-15_9-58-26.jpeg
    upload_2020-2-15_9-58-26.png
    upload_2020-2-15_9-58-26.jpeg
    More images
    [​IMG]
    [​IMG]
    [​IMG]
    Bernoulli's principle
    Description
    In fluid dynamics, Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid's potential energy. The principle is named after Daniel Bernoulli who published it in his book Hydrodynamica in 1738. Wikipedia

    Stay tuned:
     
    Last edited: Feb 15, 2020
  19. Feb 15, 2020 #139

    Dan Thomas

    Dan Thomas

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    [​IMG]
    Even in a straight pipe it's evident, though not as powerfully as in a venturi:
    [​IMG]

    It's not intuitive at all, but if this was not so, a turbine engine would not work. Not even a bit.
     
  20. Feb 15, 2020 #140

    BBerson

    BBerson

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    I know it works.
    But the question is why is air meeting a bottleneck in a tube so different than cars on Interstate 5 when reduced to one lane?
     

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