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Comments on lift generation

  1. Aug 27, 2005 #1
    I have read a number of the forums on lift generation, and I would like to add some comments for feedback.

    I read the forum describing lift as the centripetal force required to curve the airflow (streamlines), and I must say this makes absolute sense, I myself have always believed that concept. However, I would like to add that this concept of lift has been around for a long time, since 1889, when the German engineer Otto Lilienthal first proposed it.

    There is no doubt that pressure distribution is the source of lift. However, one must remember that air (gas) pressure has its origins in Newtons Laws. Air pressure is caused by the incessant impacts of the constituent molecules. The molecules impact the surface (and each other) and cause a force to be applied to that surface, the surface in turn applies an equal and opposite force (Newtons third) to the molecule. In reality the impact "forces" are caused by the electrostatic forces between the eletrons in the molecules.

    A fluid element has mass to it, for instance at sea level a cubic foot of air has .0023769 slugs to it (standard day). This gives the fluid element a physical weight of .08 pounds. However, a cubic foot of air at sea level exerts a pressure of 2116 PSF (standard day) due to the translational kinetic energy (temperature) of its molecules, as described above.

    Since the fluid element has mass, in order for it to move in a curved path (centripetal acceleration) there must be a net force acting on it. This force is generated by the vertical pressure gradient in the flow. This requirement is Newtons second law for curved motion.

    Circulation is also required in order to move the stangnation point from the top surface of the airfoil to the trailing edge. Even if there was no circulation, the streamlines are going to curve, requiring vertical pressure gradients, but with the stagnation point on top of the airfoil, the resulting pressure distribution is symmetrical, hence no lift. So circulation is also required to move the stagnation point. So, you can see you have a combination of physics laws all working together to produce lift. Im just surprised that there is so much disagreement on this concept.

    Michael 1160
    Last edited: Aug 28, 2005
  2. jcsd
  3. Aug 27, 2005 #2
    I hope you dont mind, but I would like to pose another question here too if thats ok michael. I read on a nasa site that lift is not solely created by bernuli pressure, becuase an airplane can fly upside down. Also, a paper airplane is a flat wing. I recall it mentioning that the surface merely has to deflect the air down. It said something to the effect that if you follow two particles, one that goes along the streamline above the wing, and one below, the top one travels the most distance; however, it does not meet the particle traveling along the bottom portion of the wing at the trailing edge of the wing at the same time. So the air on the top of the wing does not travel further distance in the same amount of time. The time of travel between the top and bottom of the airfoil differs. Is there any truth to what I read?
  4. Aug 27, 2005 #3

    This is the equal transit times concept, and it is an absoulte myth. If you look at the stream pattern of a lifting airfoil, with the fluid elements approaching the airfoil, the fluid element that goes over the top of the airfoil will indeed reach the trailing edge long before the fluid element on the bottom does.

    Also, if you give an airfoil enough angle of attack, even a flat airfoil or an airfoil with positive camber flying inverted, the pressure on top of the wing will decrease as the fluid elements attempt to follow this "curvature"

    It is very similar to a vortex (like a hurricane or tornado) in which the low pressure core provides the pressure gradient (centripetal force) to keep the fluid elements moving in curved path.
    Last edited: Aug 27, 2005
  5. Aug 27, 2005 #4
    Thanks michael! So how does the plate appear to be "curvature" to the airflow? I know it deflects the air passing along the bottom portion down, but I dont see how the top can cause a change in flow.
  6. Aug 28, 2005 #5

    Curvature is just the rate of change of direction. I know it is easy to see how the bottom of a flat plate will change the direction of the fluid elements impacting it, so we will look at that first. There are only two ways that a fluid medium can impose forces on a solid body moving through it, pressure (normal forces) and friction (tangential) forces (we will ignore bouncy as that also appears in a static situation, besides for air it is negligible, unless you are a hot air balloon)

    Since air has mass, in order to change its direction, on the bottom of the "plate" there will be a "high" pressure on the bottom of the plate. This pressure will be higher then the pressure some distance away (vertically-down) from the plate. This pressure gradient is what causes the fluid elements to curve in the downward direction. Since pressure acts equally in all directions, it also pushes up on the plate.

    Now for the top of the "plate". The fluid going over the top of the plate will also see this as change in direction. Again. a force is needed to change the direction (curve) these fluid elements. As the fluid moves over the top of the plate, the surface of the plate effectively "drops away" (curves). This dropping away of the surface of the plate, relative to the fluid element, decreases the pressure, the presssure vertically -up from the plate is higher then the pressure right at the surface (vertcal pressure gradient), and the fluid on top of the plate also curves. Again since the pressure of a fluid acts equally in all directions (due to the random motion of the constituent molecules) this low pressure is felt on the surface of the plate. The difference in pressure between the top and bottom of the "plate" is lift.

    A flat plate is not a very good airfoil (but it still will work) because of the sharpness of the leading edge. This causes the pressure to drop very sharply (on the top) surface very close to the leading edge, this then causes a large "adverse pressure gradient" for the flow moving along the plate, and the flow separates (stalls) at a fairly low angle of attack. Separation causes a reduction in lift, and a large increase in drag. I hope this has helped.
    Last edited: Aug 28, 2005
  7. Aug 28, 2005 #6


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    Thanks for the Otto Lillienthal info, michael; I wasn't aware of that connection.
    As for topics of some controversy, and that has not been fully understood yet, the precise mechanism for the generation of circulation is perhaps the most important.
    Since an inviscid fluid can't generate circulation, the generation of circulation must be an effect of viscosity, which makes the whole problem rather nasty...
    Last edited: Aug 28, 2005
  8. Aug 28, 2005 #7
    Also, I meant to add that Bernoulli's principle, and all of Newtons Laws are implicitly buried in this whole flow field about the wing.
  9. Aug 28, 2005 #8


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    It certainly is, along with the conservation of mass law!
  10. Aug 29, 2005 #9
    theres a good video on the cambridge engineering dep. website about this at the mo.

    pretty much says what has been said...but video or text...depends how long your concentration span is :P
  11. Aug 30, 2005 #10


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    Saoist, do you have a link for that? I am always up for a read on this topic.
  12. Sep 1, 2005 #11
  13. Sep 4, 2005 #12
    Hmm...that's wierd, the quicktime plugin won't work for me in Firefox, yet it runs in IE fine. First time Mozilla has let me down :mad:
  14. Sep 4, 2005 #13
    My bandwidth isn't high enough for me to view this video properly, and I let it play all the way through hoping it would then play properly when I attempted to play it again. However, this was not the case.

    Does anybody know how I can download this fully instead of streaming it? This video is really going to help me with my IB Extended Essay (it's on How Angle of Attack Affects Lift for Cambered and Symmetrical Wings).

    Thank you,

    EDIT: I tried the other two videos and they both work fine. For some reason the one on wings doesn't want to play for me though.
    Last edited: Sep 4, 2005
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