Why does fly fly?

  1. Why does fly fly? The theory based on Bernoulli's law fail to explain it since the wings of fly are flat, not curved like the wings of birds

    "An air parcel going over the curved top of the wing has to travel a longer distance, but it has to arrive at the trailing edge at the same time, hence it has to travel faster, and Bernoulli's law says that pressure decreases as speed increases."
  2. jcsd
  3. mgb_phys

    mgb_phys 8,952
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    Only soaring birds wings are curved - if you flap the wings everything is different.

    True but irrelevent
    Who says ?
    This is a common misconception about how aeroplane wings work - do a search on this site.
  4. chroot

    chroot 10,427
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    Airplanes do not fly (entirely) due to explanation given by Bernoulli's law, though it does contribute to the lift created by some wings. Bernoulli's law as a sole explanation of lift is one of the most enduring popular myths in physics.

    Aerobatic airplanes have wings with symmetrical cross-sections. Most airplanes with asymmetrical wings are also capable of flying inverted. Airplane wings generate lift mainly due to their angle of attack -- they push air down, and the reaction force pushes them up. Simple as pie.

    - Warren
  5. Flat wings or curved ones, it does not matter if u want to fly. Birds and flies alike push the air downwards creating lift, so the wing shape can be almost whatever you want it to be, even in nice sparkly pretty colors ( butterflies ).

    But flies cannot glide, that means if they hold their wings still they drop. Birds have curved wings so they can glide.
  6. Actually, I read about 20 years ago that insect wings are a bit stretchy and acquire camber on the downstroke. They're not flat during flight. They can, in fact, operate like hanglider wings. They are reoriented on the upstroke to cut through the air with the least amount of resistance and reposition themselves for the next downstroke.

    What's really cool is the inertial guidance systems of flies. Also, they can walk upside down on the ceiling by virtue of the hairs covering their "feet" which gain purchase in the microscopic irregularities of the surface.
  7. chroot

    chroot 10,427
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    The hair on my feet, on the other hand, isn't really all that useful to me.

    - Warren
  8. Andy Resnick

    Andy Resnick 5,785
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    Insect flight is only partially understood. YOu may be interested in looking through some of the referenced work here:


    and specifically this article and references:

  9. I know some drosophila who would disagree with this.
  10. That's just so "inside the box".
  11. rcgldr

    rcgldr 7,409
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    Bernoulli's "law" bascially states that when no work is done during an exchange between pressure and speed, the total pressure, the sum of static pressure and dynamic pressure (dynamic pressure is relative to speed 2) is constant.

    At the air + wing interface, work is done, so Bernoulli's law is violated. Away from the wing, where no mechanical interaction takes place (direct deflection from the bottom surface or void "creation" from the top surface), Bernoulli's law applies because no work is being done once away from the immediate vicinty of the wing.

    In the case of insects, and humming birds, the flapping of wings that rotate (sometimes via flexing) in coordination with the flapping, generates a downforce on the air coexisting with the accelerated air generating an upforce on the wings. A dragon fly has relatavely large enough wings that it can glide well, but a bumble bee, fly, or humming bird require constant flapping.

    The pattern of the flapping is different. Humming birds use a figure 8 pattern and dynamically change this pattern to control movment. Butterflies "clap" their wings at the top of the flap stroke.

    As mentioned in the Wiki article for smaller insects, the Reynolds number is smaller, and to the insect the air is "thicker" because of viscosity versus their size and weight (weight will ultimately be related to the required generated air stream speed to sustain flight).

    One aspect of the constant flapping of wings that is interesting is minimizing the amount of power required to sustain flight. The muscles involved are very elastic, like springs, and only require a relatively small amount of power to keep flapping. In many insects, the muscles are attached to an elastic membrane that is then attached to the wings as mentioned in the wiki article.
  12. Andy Resnick

    Andy Resnick 5,785
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    Do you talk to them as well?
  13. No. I lurk at a fruit fly forum.
  14. Jeff is right fly’s wings rotate. One way to think about it is that a fly is swimming through the air.
  15. I didn't read all the posts here, but I will give a very basic overview of what happens in low Reynolds number insect flight.

    First of all, the statement that flies have 'flat plate' wings is flat out wrong. Micro Air Vehicles (MAVs) of a size somewhat close to that of flies (most slightly larger - drag fly size), do have chamber to them.

    The motion of a flapping wing MAV is also quite complex. There is an upstroke and backstroke. Each stroke takes advantage of different things. On the up stroke the wing goes through the air rather cleanly. It then reaches the forward position and puts the trailing edge down. This causes a ton of drag. Then the backstroke begins and the drag caused during the transition actually helps provide more lift on the return stroke. Its a non-intuitive effect.

    My advisor is a leading expert on insect flight and navigation. The information above is from a talk given here by a guy from a lab up in Princeton.

    We have small wind tunnels with high speed cameras that track fly flapping motion.

    [1] http://www.terp.umd.edu/4.0/engineering/
    Last edited: Jun 4, 2009
  16. and that force is caused by a pressure distribution. Sorry, Bernoulli just got you.
  17. rcgldr

    rcgldr 7,409
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    Bernoulli's equation describes a relationship between static pressure and speed^2 (component of dynamic pressure). There are pressure distributions on a wing that do not correspond to the speed of the air as described by Bernoulli's law, because work is done. For example, in the vicinity of the the upper leading edge of a wing , you have a significant component of centripetal acceleration of air, that corresponds to a reduction in pressure with no change in speed.
  18. There's a good series called nature tech on discovery. I remember seeing a group at caltech that does flow visualisation on a large scale model fly in oil. I can't seem to find the video on youtube though.

    Their consensus seemed to be that flies generate most of their lift from a leading edge vortex that stays attached to the wing throughout the fly's wing flapping movement.
  19. Its not because work is done, it's because viscous forces are prevalent. Bernoullis law is just a special case of the Navier-Stokes Equations. The Navier-Stokes Equations always hold true, everywhere.

    Work being done would be an effect of a helicopter rotor or propeller blade when you are using momentum theory.

    (PS, I was giving warren a hard time about what he said for the forces on a wing. He's right, I was nitpicking).
    Last edited: Jun 4, 2009
  20. rcgldr

    rcgldr 7,409
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    I was being equally nitpicky about Bernoulli. Although the Navier-Stokes Equations hold in the real world (not sure how complicated turbulence makes this), classical Bernoulli doesn't because of the assumption that total energy is constant, or that total energy along a stream line is constant. The mass flow is constant, but the energy is clearly changed in the case of a propeller, rotor, or turbine, and although the amount of energy change is less with a wing, it's still there.

    The "work being done" issue is mentioned in this Nasa article on propellers, and exit velocity. Not covered is what is happening at the outer edges of the decreasing diameter funnel of the main air stream.

    "But at the exit, the velocity is greater than free stream because the propeller does work on the airflow. We can apply Bernoulli'sequation to the air in front of the propeller and to the air behind the propeller. But we cannot apply Bernoulli's equation across the propeller disk because the work performed by the engine (by the propeller) violates an assumption used to derive the equation."

    Last edited: Jun 4, 2009
  21. That is exactly correct. The NS equations are true - period. It is the NS equations that CFD solves.

    Thats called 'wake contraction'.

    That is called, 'momentum theory'.
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