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Bernoulli's principle and the 3rd law of motion

  1. Aug 6, 2010 #1
    I know that a faster moving fluid creates a lower pressure which can give lift to an airplane. Although I do not know what the opposite reaction is during lift.You have the upward force on the wing, but where is the downward force that should be opposite being exerted.

    Does the downward force on the air after it goes over the wing account for most of it, and is this explained in Bernoulli's formulas?
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  3. Aug 6, 2010 #2


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    Bernoulli's formulas define a relationship between pressure and speed when a fluid or gas accelerates from a higher pressure zone to a lower pressure zone with no change in total energy (and no change in temperature).

    The intergal sum of pressure differentials above and below a wing correspond to lift, but some of the pressure differential is due to normal acceleration of air (acceleration perpendicular to direction of travel) in the immediate vicinity of a wing, which isn't covered by Bernoulli (unless you consider the total flow of all affected air). There are also issues with the boundary layer and turbulence.

    Newton's third law always holds, forces between objects only exist in equal and opposing pairs. The aircraft exerts a downwards force onto the air, coexistant with the air exerting an upwards force onto the aircraft. Out of ground effects, the reaction of the air in the immediate vicintity of the aircraft is a downwards acceleration in response to the downwards force. The aircrafts reaction depends on the sum of lift, drag, thrust, and gravity.
  4. Aug 6, 2010 #3


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    A pressure differential along a streamline makes a particle accelerate along that streamline, in the direction of lower pressure.

    The lower pressure itself is, typically generated by the forced CURVING of the flow, i.e, centripetal acceleration.
  5. Aug 6, 2010 #4

    I fly single engines, and each time I'm about to land behind a commercial plane, tower tells me to watch for "wake turbulence". We have to fly over this most often invisible flow of air, because it can shake a smaller aircraft out of control, especially if it's caused by the heaviest airplanes.
    Last edited: Aug 6, 2010
  6. Aug 6, 2010 #5
    the weight of the plane is the downward force.
    Also, it is favoured in 'nature' to be drawn towards a lower pressure. say for instance, u suck out air in a water bottle, wheres the walls of the bottle going to go? inside. so the greater pressure is pushing on the walls.
    Like on a wing, where the greater pressure, under the wing, pushes upwards against the weight (which is spread somewhat evenly throughout the area of the wing).
    sorry if this is what you already knew, i hope it answers questions. let me know if u want to know anything about the horiz forces.
    ps-only reason why i answered this way is because i quickly skimmed the topic and posts and didnt see anything on the weight.
  7. Aug 6, 2010 #6

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    No, it's not. Newton's third law always applies to two different objects. The third law counterpart to the upward lift force exerted by the airflow on the plane is, as rcgldr already mentioned, that the plane turns the airflow downwards.
  8. Aug 6, 2010 #7
    well, in that case, I havent hit my third yr in aerospace yet so, please, do continue (ill be subscribing to this thread :)
  9. Aug 6, 2010 #8


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    The weight of the plan equals the downwards force in level flight, but the force can be greater if the angle of attack or speed is increased (such as a high g turn) or less if the plane is in an accelerating dive. The force is related to speed, angle of attack, and shape of the wing and aircraft, and is independent of gravity. A pilot has to adjust angle of attack (via "trim") for speed and altitude so that the upwards force from lift counters the downwards force from gravity to result in level flight.
  10. Aug 6, 2010 #9
    Take an airplane that is flying through the air, the air that is generating its aerodynamic force is made up of still air. This air is not moving over the wing the wing is moving through the air. For this air to experience low pressure as a result of its increased speed it has to increase its speed. In order for it to increases speed it has to have speed. The way the wing sets this still air in motion is by pulling on it and pushing on it (measured in pressure). Some pull is from viscosity. After this air has been set in motion its pressure may go down but it would not be in motion unless the wing pushed or pulled on it, and the air pushed and pulled back causing pressure.

    The downward force on the air after it goes over the wing does account for most of the lift and has little to do with Bernoulli. The shape of the top of the wing does not cause low pressure it just causes the low pressure that is there to be pulled downward. Hold the curved back of a plastic spoon into the downward flow of water from the kitchen faucet, it takes a little force to get it out because the spoon is diverting some water away from the flow as a result of its curved shape (Newton). Now hold a plastic knife up to the flow at zero angle of attack and there is no noticeable force, although I am sure there is some that is the result of the low-pressure flow (Bernoulli). What the flow lacks in low pressure it makes up for in viscosity, this experiment works just as well with a less viscous air, hose.
  11. Aug 7, 2010 #10


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    The downforce exerted by the aircraft onto the air, is transmitted through the air as an impulse, eventually resutling in the air exerting that downforce onto the surface of the earth. The air set in motion related to that impulse experiences a higher than ambient pressure as that impulse moves downwards.
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