Pigeons flying in a truck on MythBusters

robert Ihnot

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?

momentum_waves

Were the flapping birds attached to the truck?

lpfr

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.

Huckleberry

Oh no, here we go again.

The truck would have the same mass and weigh the same. there is another similar question in this thread. http://physicsforums.com/showthread....light=juggling

lpfr

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.

momentum_waves

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.

robert Ihnot

They were not able to get out of the truck, but not physically attached to anything.

robert Ihnot

IN this case the cage is the entire truck.

robert Ihnot

On this problem, the matter of air flow was entirely ignored.

momentum_waves

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.

lpfr

No, the classical problem is with the bird(s) hovering free in the cage.

robert Ihnot

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.

lpfr

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:
[tex]mv=Ft[/tex].
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.

andrei.cenja

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 !

russ_watters

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.

andrei.cenja

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 ?!

rcgldr

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).