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Pigeons flying in a truck on MythBusters

  1. Apr 19, 2007 #1
    A truck driver, it is said, once explained that when he was crossing a bridge and was overweight with birds; he just rapped on the truck and got the birds flying, so the truck was lighter!

    Of course, it was explained on MythBusters that Newton's Third Law does not work that way; and by putting 36 pigeons in a truck and getting them to flutter up; it was shown that the weight of the truck (goes up and down a little) but, really does not change.

    However nothing was said about air pressure. I presume that by design most of the outside air came in and out at the top of the truck. Certainly, since the birds did not suffocate, there was a source of outside air.

    What if the bottom of the truck was made of open slats covered with wire mesh? Then the air pressure could escape out of the bottom of the truck. What then would happen to the weight of the truck as the birds took flight?
  2. jcsd
  3. Apr 19, 2007 #2
    Were the flapping birds attached to the truck?
  4. Apr 19, 2007 #3
    Yes robert Ihnot, if the birds are in mesh cages, when they fly, the truck lightens.

    It has always bothered me that the drawings of this problems in physics textbooks, show always a bird in a mesh cage with solid floor. If the cage allows downward air flow the weight can diminish

    But if the cages have solid walls the weight does not change.
  5. Apr 19, 2007 #4
  6. Apr 19, 2007 #5
    Instead of saying "Oh not gain". Please read the question and the answer. It has nothing to do with juggling.
    If you do not see a difference between a solid walls and floor cage and a full mess cage, then, I can do nothing for you.
  7. Apr 19, 2007 #6
    How would the problem be viewed if, instead of many small birds, their same mass equivalent in large birds was used?

    Now consider the effect of the large flapping birds being attached to the perches, or not. Perform a force equilibrium balance.

    If the birds were very strong eagles, chained to their perches, then they could really go places. Only problem is where they should dump the truck.
  8. Apr 19, 2007 #7
    They were not able to get out of the truck, but not physically attached to anything.
  9. Apr 19, 2007 #8
    IN this case the cage is the entire truck.
  10. Apr 19, 2007 #9
    On this problem, the matter of air flow was entirely ignored.
  11. Apr 19, 2007 #10
    In a truck, if the birds maintained themselves aloft, their weight component on their perches would reduce - thus lightening the load on the truck.

    Now, if the bottom of the truck were solid, then the down-thrust from their wings would create a pressure distribution on the base of the truck. Multiply this by area & you have an instantaneous varying down-thrust. The value of this thrust could partly offset the weight (force) reduction.

    Again apply the same logic to the upper roof of the truck - if closed. It is all an instantaneous force-balance. Lateral thrusts would also impact the truck sides as the air begins flowing around the truck.

    A cfd study could be performed of the event, but, for this, the true boundary conditions are required. Have fun.
  12. Apr 19, 2007 #11
    No, the classical problem is with the bird(s) hovering free in the cage.
  13. Apr 19, 2007 #12
    They insisted despite some small variations, that the weight had not changed. The truck seemingly had solid walls top and bottom, though some of it seem plastic sheets. I think that air came into the truck from outside by their corner perches, where it was vented in from a downward spout.
  14. Apr 19, 2007 #13
    Let's talk history a little. This problem is a classical one and appears in the chapter concerning the conservation of linear moment or momentum as Americans call it.
    For a bird (or helicopter or plane) to hold in mid air, it should send a quantity of air downwards a given speed. The starting equation is:
    If F is the weight:
    [tex]F={m\over t}v[/tex]
    the first factor is the amount of mass that the bird should send downwards each second at the speed v.

    Now, if this downward air is stopped by the floor (and re-directed horizontally), the floor should exert a similar force upwards and it feels the same force downwards. This is the reason why, if there is a floor, the birds hovering do not lighten the truck.
    But if the downward air can continue until the road, through mesh cages and a mesh platform truck, then the downward force is exerted on the road and not on the truck.

    This is an idealized problem of physics, not a real problem for truckers. Either you suppose that the air is completely stopped o that the air goes through the mesh freely.
  15. Oct 12, 2008 #14
    Sorry for bringing this up again – and for being stupid, but something’s still bugging me …

    The mythbusters also tested a small chopper inside their “truck”. The chopper was hovering about 2-3 inches from the bottom of the “truck” and the “truck” weight did not change. OK, the down current generated by the chopper at that distance was obviously hitting the floor.

    What’s bugging me is that I can’t actually “see” the matter – in the “Oh, I see” sense.

    In all explanations I've heard, the floor was taking on the air pressure, therefore the weight of the truck remained constant. OK, but wouldn’t this explanation imply that a chopper (horizontal propeller) depends on a “floor” in order to generate thrust?

    Then what about a vertical propeller? Does a propeller airplane need a wall behind it in order to move ahead? Please point out why the situations are not equivalent.
    Thanks !
  16. Oct 12, 2008 #15


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    Staff: Mentor

    No, a helicopter does not require the ground to be there in order to generate thrust. One way or another, though, all the force is transferred to the ground. It just spreads out as the helicopter gets higher.
  17. Oct 12, 2008 #16
    If the ground or floor as the final recipient of the weight of the chopper plays an all-important part in this, shouldn't there be a fundamental difference in functioning principle between a horizontal propeller (chopper) and a vertical one (airplane) ?

    Does a chopper generate more lift because the force is transferred to the ground as opposed to an airplane propeller, which has nothing behind it to transfer the force to ?

    Where is the fundamental difference ? Because I can't see it, and it bugs me :-)
    And if there isn't any difference between them then it means that a chopper simply balances its own weight with the inertia of the air it preses downward, just like the airplane propeller generates forward thrust - only vertically. And that has nothing to do with the ground as support.

    This would in turn mean that if the chopper flies inside the box (large box for the sake of the argument) at an altitude from where the downcurrent is not felt any more at the floor level, the box should be lighter.

    I would have better about the mythbusters' test if the their box had been larger and especially taller with the chopper flying high enough to make the downcurrent at floor level negligible.

    So maybe I'm dumb, but the way I understand a chopper basically "clings" to a particular volume of air around it, supported only by the inertia of the air it moves with its blades, as opposed to using the air as a kind of pedestal. Am I that wrong ?!
  18. Oct 12, 2008 #17


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    Homework Helper

    The truck has to be a closed system in order for the weight to remain the same. If the truck has a vented top and bottom, then the downforce the birds exert on the air results in the air exerting a downforce onto the ground. If the truck is weighed with a small scale on each wheel, the reading on the scales will reduce when the birds are in steady flight. If the truck is resting on top of a very large scale, then the downforce transmitted through the air is also exerted onto the surface of the scale, and the scale's reading will not change.

    If the truck is a closed system, then the internal forces end up being applied to the interior surface of the truck, so the weight remains the same if the birds are in steady flight or resting on perches.

    Note that a helium balloon in a closed system results in the same weight regardless if the balloon is resting on the bottom, hoevering, or pushing against the top of the truck. The balloon hovers because it displaces air, increasing the pressure gradient inside the truck (higher at the bottom, lower at the top), so that the net downwards force always equals the weight of the air and model within the truck (if it's a closed system).
    Last edited: Oct 12, 2008
  19. Oct 12, 2008 #18


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    Staff: Mentor

    You are clinging to your misconception, not responding to what I said. I did not say the floor plays an important role in the lift of a helicopter, you did.

    Again, though the ground does not generally play a large role in the generation of lift, eventually the force reaches the ground.
    Again, it only feels negligible because it is spread out over a larger area.
    The helicopter is only supported by the air around it - but what happens to the air when it is forced downward?

    For that matter, what happens to the air a plane leaves after passing through it?

    In neither case does the air remain static, as if the plane/helicopter hadn't passed through. The air moves.
  20. Oct 12, 2008 #19


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    It's not just the floor, it's also reduction of pressure at the roof. It might help to understand how a gas exerts a downforce equivalent to the weight of that gas onto it's container. For example, put 80 cubic feet of sea level pressure air into a scuba tank, and the total weight increase by the 6 pounds of air inserted into the tank. There is a pressure gradient inside the tank, decreasing with altitude inside the tank, lower at the top, higher at the bottom. This pressure gradient results in a net downforce equal to the weight of the air.

    My example of a helium balloon model doesn't require any downwash of air. Assume the model has a deflated balloon and the helium in a high pressure tank. The helium is tranferred from the tank to the baloon, inflating the balloon until the model hovers. The inflation is allowed to continue until the model ends up pinned against the roof, exerting an upwards force on the ceiling. In all cases, the pressure graidient will be changed so that the net downforce on the container is always equal to the weight of the air and the model inside the container.

    The helicopter could be replaced by a model flying in a circular path, as long as there is no vertical component of acceleration. In either case, the pressure gradient inside the closed chamber is changed by the flying model so that the net downforce always equals the weight of the model and air inside the close chamber.

    If the truck isn't sealed, then some of that pressure gradient ends up in the air external to the truck. The total downforce onto the earth remains the same, but the weight exerted by the tires on the truck onto the ground will be reduced if the truck isn't sealed when birds or model is flying or hovering inside the truck.

    Regarding the helicopter flying low to the bottom, it would be in ground effect. This reduces the amount of power required, but it doesn't reduce the amount of downforce.
  21. Dec 4, 2008 #20
    Noob here. Hi.

    Also, a physics illiterate. I think.

    I haven't heard (and maybe I can't read with attention to detail either, bother) anyone talk about viscosity and the reduction of motion to heat yet.

    I'm an aviator (well, was, till having kids made that unaffordable), and one thing I know is that lift is not fundamentally dependent on shoving air in some direction to effect an equal and opposite reaction. Lift is very different than thrust. Obviously if you have a fan on a ship blowing air at a sail on the same ship, you're not going to get very far.

    But that's not what our pigeons on the truck are. To the contrary, they're very efficient flying machines that generate a lot of lift. The air they set in motion ends up turbulently being reduced to heat within a very small airspace. Consider the wake of a plane, to think of it differently.

    Here's a counter-question: You know the little planes you run on a tether, where you stand at the middle holding the tether as they go round and round?

    Consider the case of running two of these planes precisely opposite each other, so as to provide a balanced arc (no wobble). You're standing on a scale while flying the planes. How much does the scale indicate, compared to if you were standing on the scale alone, without flying the planes? Compared to if you were standing on the scale holding the non-flying planes in your hands?

    Based on what I read in this thread, I think some would theorize that if a large chunk of wood were atop the scale so that the airplane was flying over the wood, somehow there would be aerodynamic forces on the wood (pressing down) that would make this a zero sum on the scale. I vigorously dissent. That would mean that no energy was being reduced to heat in the plane's wake, and that's insanely not true.

    Viscosity. Thermodynamics.

    Heck, if the pigeons flew frenetically enough and the truck was made of a light plastic that could expand, perhaps on the merits of heated gas alone the thing would end up being lighter. ;-)
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