Benelli lift theory under question

Discussion in 'All Things Boats & Boating' started by Frosty, Dec 5, 2012.

  1. rwatson
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    rwatson Senior Member

    The hardest time of any bird-parents life is getting the "Lift= Coefficient of Lift" theorem memorized before the little sods have to take their first flight. Failed students litter the oceans around Guillemot nesting islands every spring.

    Knowing the math doesnt really explain how flight takes place, it just formalizes the process of deciding how big to make your wings.

    I suppose you only have to raise a sail, and feel the pressure of the wind as it heels the boat to understand flight.

    But, knowing the theory about lift from a curved surface inspires you to put some belly in the mainsail in light airs, push the boom out to almost right angles to the breeze, and enjoy the puzzled look on the face of the skipper in the boat next to you as you steadily pull away, after he has been successfully pacing you for the last half mile. Priceless !
     
  2. daiquiri
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    daiquiri Engineering and Design

    Great, I've learned something new today. My languages - Croatian and Italian. ;)


    Now, the lift... The only two means a fluid has to interact with a surface are:
    - the pressure
    - the shear stress.
    So it all ends up with making considerations on these two.

    Go take a walk outside on a windy day. You can feel the wind on your skin, and if it is strong enough, it will also take you some effort to maintain the upright position and to make your way forward. You can't see the deflection of the streamlines, nor you need to see it to get a feeling of the force it is exerting on you. Because it all goes down to the pressure and the shear stress, and these two you can feel very well, even if you never heard of Mr. Newton.

    That's the physical basis. All analogic stuff, no software or digital circuits required. Our skin is enough. :)

    And then there are mathematical models we use to express this physical observation through the numbers. So we have momentum theories, circulation theories, potential theories, Navier-Stokes equations and other means of rational analysis of the physical reality through equations and numbers, but they are all mutually interconnected and interdependent - because they all have to get back to the two physical mechanisms mentioned at the beginning.

    If you want to get an introduction to the concept of lift, how it is created and what tricks we use to mathematically model it, you could start with this short paper by Arvel Gentry:
    View attachment Arvel Gentry - Origins of Lift.pdf

    Cheers :)
     
  3. DCockey
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    DCockey Senior Member

    To a certain extent, circulation and vorticity and the associated theorems are mathematical constructs, albeit very useful ones. But I'm skeptical how much most people's understanding of lift is actually helped by talking about the "starting vortex", and then saying there are theorms which require a equal magnitude but opposite direction circulation about the wing, and that the circulation about the wing is what generates lift, as in Arvel Gentry's paper. One of the uses of the starting vortex concept, etc was to reconcile the vorticity theorms, invisicid flow assumptions, and lift. The apparent conflict of these was a major obstacle to the Cambridge based theoretical physicists of the early 20th century accepting what is now conventional airfoil and wing theory, and the starting vortex concept let them move forward. See Enigma of the Aerofoil by David Bloor (discussed in an earlier post) for more on this. It is entirely possible to model inviscid and potential flow around an airfoil or a wing without ever considering the "starting vortex" or circulation.
     
  4. Frosty

    Frosty Previous Member

  5. daiquiri
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    daiquiri Engineering and Design

    Yes, it is possible to do it, and in fact that's how it is done most of the time in steady-flow simulations. However, a starting vortex is a documented physical fact. Not easy to visualize in wind tunnel tests, because it requires an impulsive start of the airflow motion, not easily simulated in reality. But very easy to visualize with help of transient CFD simulations, like these ones:
    http://youtu.be/B-tcx5hTf_A
    http://youtu.be/V3Fz6QajeIw
    http://youtu.be/Vp0QS7YBudw
    Or by impulsively moving an airfoil with attached smoke tracers through the still air.

    By looking at the flow around an airfoil from a fixed reference frame, the circulation is apparently just a mathematical artifact (we can't notice any actual circulation of the flow around the airfoil). However, the observed existence of a starting vortex plus a fact that the Helmholtz 3rd theorem is effectively obeyed by real airfoils, gives us a hint that there must be something physical even in the concept of circulation.
    And in fact, when looked from a reference frame moving with the airfoil, this is what the flow looks like:
    http://youtu.be/j_J8kNodgBQ
    http://youtu.be/rLuJYHdWBJ8
    :)

    Cheers
     
  6. tom kane
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    tom kane Senior Member

    Do you all know that bumble bees are the highest flying insects? 40000 feet tested in lab conditions and many live on Everest way up there.
    And experts still tell us they should not be able to fly at all. It`s just a matter of moving the air particles in the right direction.
     
  7. daiquiri
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    daiquiri Engineering and Design

    This story keeps circulating around, but I have frankly never heard or read from any "expert" that they shouldn't be able to fly. Where does it come from?
     
  8. DCockey
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    DCockey Senior Member

    I completely agree that the starting vortex is real for an abrupt start. No argument there. But if airfoil starts at a zero lift condition and the angle of attack is gradually and continuously increased then there is no discrete "starting vortex", rather a sheer layer wake which of course can also be considered as a surface of continuous vorticity. And in a real flow that vorticity will be diffused and dissipated by viscosity.

    Consider an airplane cruising for a length of time. Is the best explaination of why the reason the airplane wing is developing lift is due to a "starting vortex" which occured many minutes or even several hours ago and hundreds or thousands of miles behind?

    Or consider a small airfoil in a very large, closed return wind tunnel. Assume the airfoil abruptly goes from its zero lift angle of attack to another angle of attack and then stays at that angle of attack. The airfoil will develop lift and reach essentially the steady state value of lift in the time it takes a particle move at free stream velocity to move a few airfoil chords. The airfoils stays at that angle of attack. After a few minutes or so the starting vortex will have been completely chopped up, diffused and dissipated by the turning vanes, fan blades and turbulence screens, but the airfoil will still be developing same amount of lift.

    I've done enough modeling of airfoil flows as distributions of vorticity to know that its a very useful concept. But the obvious question is why is the circulation the amount it is? That's a topic for another discussion.
     
  9. Frosty

    Frosty Previous Member

    The best place I saw that was Singapore . You could watch the plane land close overhead then as it stopped and tuned to taxi right at the bottom of the run way something whistled through the air and trees rustled ... But how has that got anything to do with lift when the plane is on the floor stopped.
     
  10. DCockey
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    DCockey Senior Member

    Sounds like a vortex; the origin of which was proably related to lift production. But even after the cause ends vortices will persist before gradually diffusing and dissipating.

    A similar phenomona are the vortex pairs sometimes seen coming off of a paddle or oar when the stroke ends. They can persist even after the paddle or oar blade is lifted from the water.
     
  11. Number4

    Number4 Previous Member

    https://www.wolfson.ox.ac.uk/~ben/zetie1.htm
    ''But how did the myth about bees not being able to fly start? When does the story date back to? J McMasters states that the story was prevalent in the German technical universities in the 1930s, starting with the students of the aerodynamicist Ludwig Prandtl at Gottingen. The story goes that a noted Swiss aerodynamicist, whom McMasters does not name, was talking to a biologist at dinner. The biologist asked about the flight of bees and the Swiss gentleman did a "back-of-the-napkin" calculation of the kind I described earlier, assuming a rigid, smooth wing and so on. Of course, he found that there was insufficient lift and went off to find out the correct answer.
    In the meantime, the biologist put the word around that bees could not fly, presumably to show that nature was greater than engineering, and the media picked up the story. The truth, then as now, wasn't newsworthy, so a correction was never publicized. The people I meet, therefore, continue to tell me that science is a load of crock because it once proved that bumblebees cannot fly. And they will not hear otherwise, especially not from a scientist.''

    I have heard it so many times from childhood I have never questioned it!
     
  12. Number4

    Number4 Previous Member

    Hi Frosty,
    What you are witnessing is the wake of the aircraft, the same as a boat.
    The aircraft displaces air the same as a boat displaces water.
    The wing with full drag flap has created a huge area of low pressure above it, causing all sorts of turbulence in the wake. The jet of air from a big turbo fan can pick up a van and toss it like a toy.
    There is an official time period for air traffic control to make light aircraft wait before landing behind a big jet. A little Cessna can be torn apart and slammed into the ground by the forces in a big jet wake.
     
  13. Frosty

    Frosty Previous Member

    With respect Diaquiri, not that I am afforded much,-- a Dinosaur is not only a big flying lizard from millions of years ago but it means today something that is old or obsolete even a person can be called a dinosaur that has old ideas and skills and is of no use to modern world.

    Dinosartheory.com considers these old worn out theory's

    Such as Bernoulli being responsible for discovering a wing lift when in actual fact he was born and died long before a plane was thought of.
     
  14. Number4

    Number4 Previous Member

    No No No No No No No No No Frosty!
    Crikey mate, nobody has EVER stated Bernoulli discovered lift.
    What he did was formulate a principle that describes the variation of density, pressure, and speed of flow of a fluid through a venturi.

    This principle CAN be used to explain a large amount of the lift created by wings and hydrofoils. However, there are also other factors involved, as we have discussed above. It is taught in text books that a certain interpretation of Bernoulli's Principle, the Equal Transit Theory, alone describes lift. It does not. This does not mean that Bernoulli's Principle is flawed.

    I read the NASA and dinosaur links you posted, and whilst these chaps explain very well what is wrong with other theories, I am not at all convinced that their answers are any better. The blogger you linked to just had no idea. Anybody can create a web page, even if they have no clue at all, it does not make it gospel.

    Bernoulli did not invent the carburettor, but his principle allowed someone else to invent the carburretor





    If Speed increases, then pressure decreases, in a constant flow.
    This is the principle.
    The pressure decrease causes fuel to be sucked into the air stream in a carb.
    The pressure decrease causes a plane to be sucked into the air.
    The pressure decrease causes L'Hydroptere to hammer along at 50 knots.
     

  15. daiquiri
    Joined: May 2004
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    daiquiri Engineering and Design

    Number4, thanks for the explanation of the bumblebee mystery. Now it is clear that the problem is not the lack of experts' knowledge, as the popular story wanted to show, but rather the lack of knowledge and honesty on the receiving side. Unfortunately that's also part of our human nature. The ugly part, ignorance and dishonesty, which has imo brought us to the verge of social and ecological collapse we are experiencing today.
    Cheers
     
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