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How can aeroplanes fly?

  1. Apr 11, 2004 #1
    Can someone explain to me why aeroplanes fly? By considering the wing, I have read something stating that if air two particles must start and finish at the same time (travelling along the wing), the top particle must have a greater velocity, so there is lower pressure above, therefore a pressure gradient is set up so there is a net force upwards?
     
  2. jcsd
  3. Apr 11, 2004 #2
  4. Apr 11, 2004 #3
    yes but in the passage the expert says that "necessarily flows both over and under the wing" but he does not explain why, or even do the particles travelling in the route above and below arrive at the other end of the wing at the same time. Can someone explain?
     
  5. Apr 11, 2004 #4

    Doc Al

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    I know of no law of physics that says that air particles together at the front of the wing must end up together after passing over and under the wing. It's baloney. This kind of argument is used to defend the so-called "Bernoulli" explanation of airplane flight: since the particles travel different distances in the same time, the particles of air going over the wing (over the hump, presumably) must go faster and thus have less pressure.

    A better (and simpler) way to understand how wings can generate lift is via Newton. The wing, due to angle of attack and the Coanda effect, deflects air downward causing the air to push back on the wing, providing lift.

    Find more info here: http://www.aa.washington.edu/faculty/eberhardt/lift.htm
     
  6. Apr 20, 2004 #5

    rj

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    i would definitely agree with you Doc Al
    Lift is created because of the angle of attack of the wing and not because of the so called 'bernoulli' explanation.
    The wing is made in such a shape so that the coanda effect can take place.
    even if there is a high angle of attack the air will still flow over the upper surface of the wing and that maximizes the downwash and creates more lift.
    so the coanda effect is nothing but a phenomenon that allows air to flow through the upper surface of the wing.
    Imagine, when u put your hand outside the window of a car that is travelling
    at a high speed and when the hand is twisted slightly upward (that would be the angle of attack of your hand to the direction of the wind ) your hand will move upwards. THAT upward force is lift.
    Now the wings of an aircaft is like your hand which is outside the window of a car. so due to the angle of attack a lift is created and the aircraft flies.

    Now... some physicists say that lift is created due to a combination of both - the bernoulli's theorem and the angle of attack.
    they say that lift is created due to the bernoulli's explanation only till about mach 0.3.
    i still have to think about that.

    I think what i have written is right coz right now i am in a defence academy and a couple of years from now i should become a fighter pilot.
     
  7. Apr 20, 2004 #6

    russ_watters

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    Oops, I mised this one before.

    No, angle of attack is not something you should look at when explaining how an airplane generates lift: most airfoils generate lift even at zero angle of attack. This inevitably leads to some confusion that can be avoided by ignoring angle of attack.

    Its really quite simple: air flowing over the top surface of the wing travels faster due to its curvature than air over the bottom surface. Faster air has a lower static pressure. The wing is literally sucked up by a pressure difference of its own creation.

    Doc, I remember a similar thread that used a description similar to your link and I really find it insufficient. Things like this:
    Ok... "useful," fine - but very misleading unless you first teach what the real angle of attack is: geometric aoa. Understanding why a wing can produce lift even at zero geometric angle of attack is critical to understanding lift.
     
    Last edited: Apr 20, 2004
  8. Apr 20, 2004 #7

    Doc Al

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    Hump Theory??

    Interesting... everything I've read suggests the opposite.
    I don't buy this. Wings with hardly any curvature routinely generate sufficient lift to fly*. How can acrobatic planes fly upside down? I think the curvature is important in deflecting the air about the wing, but I don't think you can do a simple calculation along these lines: top air goes farther in same time, thus it goes faster, thus less pressure via Bernoulli. This somehow assumes that the air moving over and under the wing transits the wing in equal times. Not true.

    *I read that the Cessna 150 has a wing with an upper surface just 1.6% longer than the bottom. It has no problem flying at speeds of around 55 mph, for which Bernoulli predicts a lift of about 40 lbs---but the plane weighs 1600 lbs. :eek: (I pulled this from here: http://www.geocities.com/galemcraig/)
     
  9. Apr 20, 2004 #8

    enigma

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    Unfortunately, that is a (often repeated) gross misrepresentation of the Bernoulli principle.

    Bernoulli is nothing more than Newton's second law applied to low speed incompressible flow.

    When you put an obstacle in the way, some of the flowstreams need to divert to get out of the way. Wings are designed in such a way that the stagnation point of the flow is farther toward the bottom of the thickness of the airfoil, so the compression acts more on the upper surface than the lower. From there, it's nothing more than mass continuity and the Venturi effect.

    It's a pet peeve of mine, but I don't feel that N3 is the best way to explain flight. Yes, air gets diverted downward, and yes it is common sense that if the flow goes down, the wing must go up. Still, N3 doesn't explain the mechanism of what is causing the lift: The only ways for ANY force to get transmitted to ANY (macro scale) body are pressure, friction, gravity, and electromagnetism. The three which are taught in aerodynamics classes are the first three; flow divergence is brought up only as an aside... you can't measure how much force was given to the wing, because it's difficult to impossible to measure the air as a whole.
     
  10. Apr 20, 2004 #9

    enigma

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    When the plane is upside down, the flow diverts differently around the wing when it is inverted. They still generate lift, but they certainly don't produce as much lift as if they were right side up.

    no, no, no. Air moving over the top of the wing usually travels the chordlength FASTER than the air on the lower surface. From the website you referenced:

    It's not so much the curvature as it is the object in the way of the flow. The curvatures are designed how they are to avoid flow seperation near the end of the wing.
     
    Last edited: Apr 20, 2004
  11. Apr 20, 2004 #10

    LURCH

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    Because aerobatic planes use a "symetrical wing", and generate their lift by angle of attack, as do fighter planes. This, however, constitutes a very small fraction of the world's air traffic. If you've ever travelled by passenger aircraft, it is Bernoulii's Principle that kept you aloft.
     
  12. Apr 20, 2004 #11

    Doc Al

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    Can you give an example using Bernoulli's Principle to calculate lift? I'd like to know what assumptions you would have to make.
     
  13. Apr 20, 2004 #12

    enigma

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    You can't. You need to integrate the pressure and shear distributions over the length of the wing.
     
  14. Apr 21, 2004 #13

    russ_watters

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    ...except for planes designed to be aerobatic - which have symmetrical (or nearly so) wing cross sections and thus perform exactly the same inverted as upright.
    To expand: THIS link shows static pressure on/around an airfoil. The profile is extremely important for the performance of the airfoil an its ignored if you just talk about mass flow.
    Experimentally, its a piece of cake: take pressure measurements along the surface of the wing and integrate. To just calculate it based on airfoil shape and airflow - ugh, thats why I switched from Aero to Mech-E. IIRC, there is no exact solution, but you can model the wing as a bunch of flat plates and add up the pressures.
     
    Last edited: Apr 21, 2004
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