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I Using the lift equation, can you make anything fly?

  1. Aug 22, 2018 #1
    If I have a contraption with wings that weighs 120 kg, the wings in total are 2 meter squared, and I make it go 60 km/h, will it fly? According to the lift equation (L = (1/2) d v^2 a CL), it should, since

    L=(1/2)*1.225*277.788*2*3.39 = 1176.789
    Weight = 120 kg = 1176.789 N
    L = lift = 1176.789 N
    D = density = 1.225 kg/m^3
    V = velocity = 16.667 m/s
    V^2 = velocity squared = 277.788 m/s
    A = area of wings = 2 m^2
    CL = coefficient of lift = 3.39

    If my calculations are correct, it should fly, right? Or am I forgetting something? I'm not really sure.
     
  2. jcsd
  3. Aug 22, 2018 #2

    russ_watters

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    Uh, well, sure, as long as you are ok with ignoring major considerations like stability/control, thrust/drag and structural integrity.

    ....and if you don't mind my asking, how much do you weigh...?
     
  4. Aug 22, 2018 #3
    I'm not sure but it's quite possible that the area is meant "projection as seen from the front", not the actual size of the wings.
    Also the CL looks quite optimistic but possible.
    How do you maintain the speed? The drag may be fairly high and should be computed as well.
     
  5. Aug 22, 2018 #4

    russ_watters

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    It's not; that's for drag.
     
  6. Aug 22, 2018 #5
    Thanks. I weigh 55 kg
     
  7. Aug 22, 2018 #6

    russ_watters

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    Not trying to make yourself fly, are you...?
     
  8. Aug 22, 2018 #7

    Bystander

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    William Devane's character, Major Phil Clark, in Red Flag the Ultimate Game regarding the F-4 Phantom, "You can make a brick fly if you put large enough engines on it," or words to that effect.
     
  9. Aug 23, 2018 #8
    Well I'm not neccesarily making myself fly, i was just researching the lift equation and I was just wondering wether it waspossible. So then I worked all this out and well now I have an answer :). It was hypothetical though.
     
  10. Aug 23, 2018 #9

    jedishrfu

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    Here's some useful information about the lift equation and how some common theories explaining lift are wrong or incomplete:

    https://www.grc.nasa.gov/www/K-12/airplane/presar.html

    There are several other tracks here for drag, forces and torque and stability.

    However, following the track for theories of lift they describe some incorrect theories:

    https://www.grc.nasa.gov/www/K-12/airplane/right2.html

    https://www.grc.nasa.gov/www/K-12/airplane/wrong1.html

    https://www.grc.nasa.gov/www/K-12/airplane/wrong2.html

    https://www.grc.nasa.gov/www/K-12/airplane/wrong3.html

    https://www.grc.nasa.gov/www/K-12/airplane/bernnew.html

    There's also a cool Paper Plane book by John Collins who holds the world record for longest paper plane flight. In the book, he describes the process of building the plane and the work that went into winning the Guinness award:

    https://www.amazon.com/World-Champion-Paper-Airplane-Book/dp/1607743884

    and more on John COllins and his paper plane:

     
  11. Aug 23, 2018 #10

    cjl

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    A lift coefficient of 3.39 is incredibly optimistic. Even with complicated multi-element airfoils, 2-2.5 is more typical as a Cl_max, although 3.4 is definitely possible. It's also worth noting that the drag at this Cl is likely to be quite high. You'd really want either more wing area or a higher speed to make this at all feasible.
     
  12. Aug 23, 2018 #11
    A similar statement applies to boats. From a Mercury outboard motor ad in 1964:
    upload_2018-8-23_14-54-7.png

    I would not want to try that boat on a windy day....
     
  13. Aug 23, 2018 #12

    CWatters

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    Sorry I couldn't resist...

     
  14. Aug 26, 2018 #13

    boneh3ad

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    Please note that "the lift equation" is an entirely empirical relationship, and ##C_L## actually incorporates a lot of really complex physics. Typically, ##C_L## is determined through some combination of experiments and computations for a specific shape or family of shapes. It's nearly impossible to just look at a random airfoil (or other) shape and guess its ##C_L##.

    In principle, you could make any object fly as long as you had enough thrust pointed in the right direction, but it's not always feasible.
     
  15. Aug 26, 2018 #14

    DaveC426913

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    Indeed. Stunt planes at air shows have engines so powerful they can turn vertical and simply hover. When they turn horizontal, they merely pull themselves through the air.

    In fact, because they do so many stunts upside down, their wings are symmetrical in cross-section (cambered top and bottom). The wings aren't needed for lift really, they're needed for stability and maneuvering.
     
  16. Aug 26, 2018 #15

    DaveC426913

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    WHAT SORCERY IS THIS
     
  17. Aug 26, 2018 #16

    CWatters

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    Did someone mention sorcery....

     
  18. Aug 26, 2018 #17

    jedishrfu

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    Aww you guys don’t know anything, the ETs have mastered lift best:

     
  19. Aug 27, 2018 #18

    cjl

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  20. Aug 27, 2018 #19

    DaveC426913

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    Yeah. I saw that. Clever.

    [ EDIT ] good lord! It's a kit!
     
  21. Aug 28, 2018 #20
    Actually the area used in the generic lift, drag relation of 1/2 x density x C x A x V^^2 is arbitrary since the coefficients of lift and drag are by experiment are referenced to the area dimensions selected. A classic text by Prandtl & Tiegens on Applied Hydro & Aerodynamics selected the largest projected area of the wing based on the chord and span for the representative area. Any representative area could be used and the determined coefficient would reflect that selection. Handbook of Fluid Dynamics by Streeter use the airfoil chord as a representative length for a unit wing span. Experimental work I have done required selections for various dimensions be made to determine the complicated unknown coefficients.

    You can launch a ballistic missile horizontal from a submarine but why would you want to?
    So can pigs fly with JATO.
     
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