Fundamentals of Lift Video - Feedback Requested.

Discussion in 'Flight Following' started by Capt. Geoffrey Thorpe, Feb 13, 2020.

  1. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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    In the past I have created some videos on the topic of lift and some of the nonsense that is taught to pilots – but those tended to be on the long side – sorry, once I get going, it’s hard to stop before I’ve tried to explain the origin of the universe. Anyhow, it was suggested that I make some shorter, more focused videos – OK, fine. First one is recorded, but not yet listed on YouTube.

    What I would like, if you don’t mind, is watch it (below) and offer up comments / suggestions. I may consider re-recording. There are some things I am not happy about – the screen captures from the NACA video are pretty busy and the live markings don’t show up well for one. But then again, for a video that is going to earn 25 cents a month…

    Tentative series plan (suggestions welcome about this also)

    1: Fundamentals of how lift is generated. Intuitive, no math, not wrong.(below)

    2: What about Bernoulli? The actual B’s equation, how it is useful, does not explain velocity differences top vs. bottom. Requires theory such as circulation to really work. This is where the wheels fall off popular explanations – example – if you apply the popular “venturi” theory, lowering flaps on final should cause an airplane to crash into the ground. Hard. Some of the “Bernoulli” demonstrations also have nothing to do with Bernoulli.

    3: Airfoil shapes – Camber lines – lift vs drag. Thickness profiles – changes flow on both top and bottom. Compare camber lines, about 3 thickness values with Xfoil simulation. Other benefits of thickness somewhere in there.

    4: “Venturi” theory – Real airfoils do not look like the cartoon airfoils. Venturi flow does not correlate to flow around real airfoil (as shown by FAA video pushing this theory – that cracks me up.) Predictions using this so called theory are totally wrong.

    5: Circulation – should I tackle this? It’s a real theory. Whidden “Art and Science of Sails” does a nice job – I could crib some of that if I could find my book.

    6ish: At some point I should be able to get into the wind tunnel here at school – I have fabricated a set of wing models – flat plate, thin cambered, a couple NACA foils with different thickness. Not sure of details, but I have time to sort it out – right now there is a grad student in there working on his thesis – that’s gonna take a while.

    7: Dunno, might think of other things… open for suggestions.

     
  2. Walboy

    Walboy Cleared for Takeoff

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    Nice video. Looking forward to the circulation video. In a previous edition of the Pilot's Handbook of Aeronautical Knowledge, there was a brief discussion of the the Magnus Effect which has been omitted (at least from what I can see) from the current version. That idea as it relates to an airfoil never clicked with me. The common explanation was to drop a business card for example and watch it rotate as it falls. Never understood the link between that and an airfoil.
     
  3. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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    Thanks! And, yea, the connection is less than immediately obvious. But back in the slide rule days, there were methods to translate from the flow distribution around a rotating cylinder to the flow around an airfoil. I suspect, that most of the "Bernoulli" myths came from trying to apply Bernoulli's equation without getting into the circulation mess. See attached for a circulation based explanation (I've been poking at this for a while...)
     

    Attached Files:

  4. eman1200

    eman1200 Touchdown! Greaser!

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    good video but to give yourself more credibility I'd spell fundamentals correctly:happydance:
     
  5. Dana

    Dana Cleared for Takeoff

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    Please don't get into circulation. It, too, doesn't exist. Yes, you can use circulation theory to calculate how an airfoil behaves... but no aeronautical engineer actually believes in it; it's just a mathematical fiction that made calculations simpler in pre-computer days, and wasn't all that useful even then. My aerodynamics professor once spent half a class on it one day as an interesting historical tidbit... in 1978.
     
  6. Checkout_my_Six

    Checkout_my_Six Touchdown! Greaser!

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    wait....that's a flat wing? o_O
     
  7. eman1200

    eman1200 Touchdown! Greaser!

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    wait...…..you have access to a wind tunnel??? how in the world do we get me into that thing? I m u s t get into that wind tunnel...…….
     
  8. NoBShere

    NoBShere Pre-takeoff checklist

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    You have to tackle the theoretical question of a plane is traveling at a groundspeed of 120 kts with a tailwind of 115 kts. Will it fall out of sky or fly and why?
     
  9. 40YearDream

    40YearDream Filing Flight Plan

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    Have you read "Stop Abusing Bernoulli" by Gail M. Craig?

    And, if you haven't already, check out this website: secretofflight.wordpress.com

    In my personal experience, model airplanes will fly with a 'flat board' wing - but the larger ones don't fly as well as the smaller ones. They typically seem to be overly pitch sensitive, regardless of CG location. I think Reynolds number (relationship of inertia to viscosity) must be a factor - little tiny things generally operate at very low Reynolds numbers, where the inertia is less of a factor. Maybe something like a butterfly flies - closer to swimming than flying.
     
  10. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Touchdown! Greaser!

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    No, I have not read that.
    Yes, Reynolds number matters.
    A flat plate is a really lousy airfoil.
     
  11. Dana

    Dana Cleared for Takeoff

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    Flat wings have low Clmax and an abrupt stall, mainly due to the sharp leading edge radius. The low Reynolds number also contributes to the sharp stall, because the flow stays laminar longer, and laminar flow separates (stalls) sooner than turbulent flow.