No One Can Explain Why Planes Stay in the Air (2020)

11 points by ohxh 2 years ago | 20 comments
  • injb 2 years ago
    There's a very famous book called Stick and Rudder that does a good job of explaining this. A plane stays up because of Newtonian reaction. The Bernloulli principle and its associated airfoil shape should be thought of as an efficient way of displacing air downwards without the appearance of the wing being angled upwards.

    IOW a classic airfoil that appears horizontal is really best thought of as a flat surface that's angled upwards. When a plane with a wing like that inverts, it really does "lift" downwards, and it takes considerable elevator correction to keep level.

    If the wing had a symmetric airfoil section, then it needs to be visibly angled upwards in order to fly; but on the other hand when upside down it doesn't take as much elevator input to stay up as the classic asymmetric version.

    • Mageek 2 years ago
      The title is complete clickbait. It should say “no one’s short intuitive explanations of why planes stay in the air are fully correct”
      • azalemeth 2 years ago
        Exactly. Like many things in physics there are simple answers, there are correct answers, and there's a sliding scale between the two.
        • karmakaze 2 years ago
          Not 'fully correct' there are also simple/correct answers and complex/incorrect answers.
      • entropicgravity 2 years ago
        Just as no one can explain how bicycles stay up once they get moving. Or why hot air rises but it's always cold in the mountains when they should be hot from all that rising hot air.
        • wayeq 2 years ago
          And why all these windmills don't keep us cool
        • musicale 2 years ago
          We know why paper airplanes stay in the air. They are lifted up by hot air from the teachers they are thrown at.
          • morpheos137 2 years ago
            I'd say reaction force is a sufficient explanation. The wing compresses the air column below it. The pressurised air generates buoyancy sufficient to dynamically support the air craft while moving at a high enough speed.
            • p_l 2 years ago
              And unless you cross over to supersonic regime it's completely wrong (^_-)
              • morpheos137 2 years ago
                Ok. Why? See if this is an educational discussion it makes sense to support one's argument, not just say no, you're wrong...I supported my argument with a plausible explanation.

                If planes are not supported by the air beneath them pushing up why don't they fall out of the sky?

                • p_l 2 years ago
                  Sorry for a bit flippant answer, but I was on phone and the long answer is, well, long. Going to try to make a somewhat summary of it.

                  EDIT: Wanted to add a link to NASA's page about it, but unfortunately the links changed and I can't find the old (ca. 2000) site material.

                  You have a bunch of responses with Bernoulli's principle, but they are incomplete.

                  Essentially, the lift is higher pressure of air on the underneath of the wing, creating a force pushing the wing away. Now, how do you get said increased pressure?

                  Bernoulli's law shows that air moving with higher speed will give you effective lower pressure compared to air moving slower, and this is main source of lift in subsonic regime.

                  Now the secret is "why is the air under the wing moving slower than above it?" and the answer is much more complex than high school level physics textbooks try to say.

                  When airflow leaves the airfoil and mixes again, it creates a vortex rotating in the same axis as wing. If you were looking at a plane flying to the left, the vortex would be rotating counterclockwise. This is called sometimes a "bootstrap vortex", because it's rotation induces a vortex around the wing (in the hypothetical viewing angle, it would rotate clockwise). When you combine the induced vortex movement with forward movement of the plane, you end up with air underneath the wing moving slower compared to aircraft, and the air above the wing moving faster, giving you the conditions for bernoulli's law to provide you with pressure differential necessary for generation of lift.

                  Differences in Angle of Attack both impact parameters of the bootstrap vortex, impact pressures on the wing (but less than people would think, at least in the range where you get usable lift instead of stalling), and most importantly decide the rotation direction of the bootstrap vortex - which is why symmetrical wings work so long as you have positive angle of attack - and why flat plane doesn't work (because it doesn't create a good bootstrap vortex, just random turbulence).

                  • rawgabbit 2 years ago
                    There are simple experiments you can perform to prove the correctness of Bernoulli’s principle. If the math cannot accurately describe completely what is happening, then it is our math that is lacking. Technically the air is less dense on top which generates lift. And yes planes do fall from the sky when they stall due large angles of attack which disrupt the laminar flow.
              • 2 years ago