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Okay, what is the official reason planes fly?

  1. Jun 6, 2007 #1
    It's very hard to find any answer on the net other than the common explanation of an airfoil using Bernoulli's equations.

    However, I have heard many times (some on this form) that this explanation is incorrect. Jeff_Reid has said it, and if I remember correctly chroot has some input on the subject as well. I'd expect danger to provide some info.

    So, what is the original explanation for why planes fly?
  2. jcsd
  3. Jun 6, 2007 #2


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    Normally it's taught to be mainly simple old, "Newton's 3 Laws," when taught correctly in school.
  4. Jun 6, 2007 #3
    In physics there are a number of cases where a single effect has differing valid explanations (e.g., magnetism is just the electric force from a different frame of reference).

    For lift the local mechanism is that the air pressure is higher beneath the wing than above. A more global explanation is that lift is the reaction to the wing redirecting airflow slightly downward. Both are true, but either one gives an incomplete understanding: the full explanation is the Euler equations (derived by combining "Newtons laws" and "just enough" properties of air.. of course there are additional properties and more complex effects like turbulence, viscosity, shock waves, etc, but none of those are necessary for flight).
    Last edited: Jun 6, 2007
  5. Jun 6, 2007 #4


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    Air planes fly because they accelerate air downwards, force equal mass times acceleration. It's complicated because as distance from the wing increases, the higher the volume of air affected, but the lower the acceleration. Airplanes also suffer from drag because they accelerate air forwards, just like a bus or a car on a highway (this is easy to seen when a vehicle's draft causes leaves or paper to follow along behind).

    How wings cause the air to accelerate downwards is where it get tricky, but one thing is common to all wings producing lift, an effective angle of attack that results in downwards acceleration of air. Most typical wings draw more air downwards from above the wing than push it from below. The classic curved top, flat bottom wing is often used as an example, but sometimes in a misleading way, so I offer this link to a photo of a lifting body glider with a flat top and curved bottom that glides just fine.

    lifting body.jpg

    Air foils can get tricky, but note that a flat board will provide lift, for example, small solid balsa model gliders. Efficient air foils are curved both above and below, depending on the intended speed. For aerobatic aircraft, symmetrical airfoils are used so inverted flight uses the same angle of attack as right side up flight.

    My response to the "Euler equations" thread linked to from above:

    My understanding, based on article from various air foil designers, mostly for Radio Control and full scale gliders is that laminar flow seperates from a wing at much lower angles of attack than turbulent flow. The smaller contest "hand launched" (discus throw methos) radio control gliders with 1.5 meter wingspans use turbulators (strips placed slightly behind and above the leading edge of wings) to break up laminar flow.

    Air moving around the top of a flat board will move faster if there's an angle of attack. Also that flat bottom wing has an effective angle of attack if it's produding lift. Then there this, a link to a picture of a lifting body, flat top, curved bottom, and it glides just fine:

    flat top curved bottom lifting body.jpg

    A wing is designed to produce a partial vacuum. This is why the air is accelerated downwards, to flow into the low pressure area created above a typical wing (except for that lifting body case I mentioned).

    Except that an aircraft peforms work on the air, so the total energy is changed. An aircraft in level flight will accelerate the air mostly downwards, producing a reactive lift force, and accelerate the air a bit forwards, producing a reactive drag force. This acceleration of air changes it's total engery, and reflect a net amount of work done on the air.
    Last edited: Jun 6, 2007
  6. Jun 6, 2007 #5


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    I guess if you are looking for the 100%, grade-a verified and stamped version of why airfoils work, you're not going to get a straight answer from a lot of people.

    The theory that makes the most sense is the one that combines effects due to Newton's 3rd law, Bernoulli and circulation. All three elements play a role.

    The issue of boundary layer and wings is not a cut and dry thing. Laminar BLs have less skin friction due to lower shear stresses. However they are unstable and will separate rather easily creating drag as well. Laminar BLs also have less heat transfer due to mixing. They are all trade offs, just like anything in any engineering application. What you want entirely depends on your application. Most airfoils in practice have a combination of the two anyways.
  7. Jun 6, 2007 #6


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    It is kind of frightening that our modern society depends so heavily on a principle of which we don't really have a clear understanding.

    I have visions that some scientist will finally nail down the real reason airfoils work, realise it doesn't work, and suddenly every plane will plummet from the sky.

    (Actually, now that I think about it, that's the perfect inspiration for an xkcd toon, which, if you haven't discovered yet, your life is incomplete.)
  8. Jun 6, 2007 #7


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    I wouldn't say that, Dave. Just because laymen don't understand all the complexities involved doesn't mean the experts don't either. It's the same as with a lot of the physics topics we discuss here. Nothing that was discussed here goes beyond the level of a 1st semester aerodynamics course. Bernoulli, circulation, and Newton's laws are all covered.
  9. Jun 6, 2007 #8


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    There are air foil programs and wind tunnels that help with the design of new airfoils, but the bottom line is actual implementation and testing. For existing air foils, the parameters are already known.
  10. Jun 6, 2007 #9
    none of the available reasons completely clarify the reason in all situations. they fail somewhere in some situations & end up contradicting their original reason.
    but all of 'em together, more or less, do help to understand the phenomenon
  11. Jun 7, 2007 #10


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    Still it's still basic Newtonian math. Regardless of the reasons at the microscopic level, circulation, Bernoulli effects, viscosity of the air, it all boils down to force = mass times acceleration. Airplanes fly because they peform work on the air, accelerating it downwards (lift) with the cost of accelerating it forwards (drag). The only issue is how to do this efficiently.

    Personally I prefer the "void principle" - a solid object passing through the air leaves a moving void behind it, which causes the air to fill in by accelerating towards the moving void. Design the moving void so it causes more downwards accelration than forwards acceleration and you have the basics of a reasonably efficient wing.

    The closed system example:

    So forgetting the details, wings produce lift by accelrating air downwards. As proof, consider a closed system with just air inside of it. The air exerts it's weight via a change in pressure versus altitude, so that the net pressure differential between the top and bottom of the closed system results in downward force exactly equal to the weight of the air. Compress 80 cubic feet of air, which weighs 6 pounds, into a scuba tank, and the combined weight of scuba tank and air increases by 6 pounds. Put a flying / gliding model inside the closed system, and as long as there's no vertical component of acceleration, the weight of the closed system is increased by the weight of the model. The mechanism for this is that the model increases the pressure differential within the closed system, by enough to that the net downward force of the increased pressure differential equals the weight of the model. The model is acting as an air pump (accelerating or pushing air downwards) in order to increase the pressure differential within a closed system.
  12. Jun 7, 2007 #11
    Very simply, the jets of plane provides the plane an acceleration with a vertical component of g (at constant height), higher (at taking off) and lower (at landing).
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