Parakeets
Posted by: tfabris
Parakeets - 29/01/2007 22:19
A quick google didn't show me the answer to this one, but it seems like it'd be a standard physics-class puzzler. Anyone got a definitive answer?
Assume you have an airplane that can only carry 100 pounds of cargo.
Suppose its cargo today needs to be 200 pounds of parakeets.
Can you solve the problem by keeping at least half the parakeets flying around in the cargo hold the entire time?
Does the problem change depending on whether the cargo hold is pressurized or vented to the outside air?
Posted by: lastdan
Re: Parakeets - 29/01/2007 22:56
the cargo hold needs to be pressurized or the dozen or so cats you let loose might flip out.
( man, I really hope the answer has nothing to do with Snakes On A Plane )
Posted by: MarkH
Re: Parakeets - 29/01/2007 23:51
Are they frictionless parakeets...?
I think you're in trouble unless the parakeets can match the performance of the plane (speed, rate of climb, etc). If they can then yes, keeping them in the air reduces the take off weight. If not then no, as the plane has to convey energy to the parakeets to get their velocities to match.
Regards
Mark
Posted by: tanstaafl.
Re: Parakeets - 30/01/2007 00:26
Can you solve the problem by keeping at least half the parakeets flying around in the cargo hold the entire time?
There Ain't No Such Thing As A Free Lunch.
The parakeets fly by pushing air downward with their wings, giving a transfer of momentum from the air to the parakeet, lifting the parakeet and pushing the air downwards. The air does not continue going down forever -- it hits the bottom of the cargo hold, transferring the downward momentum imparted by the parakeet's wings to the cargo hold, restoring equilibrium and giving the effect of increasing the weight of the air by the amount that the parakeet's weight was effectively "reduced".
This applies to a closed system such as the cargo hold. In an open system it would be different. For example, if you put a box with an upward-facing propeller on a postage scale, the box would weigh less when the propeller was turning since at least some of the air pushed downwards by the propeller would not hit the weighing surface of the scale.
tanstaafl.
Posted by: DWallach
Re: Parakeets - 30/01/2007 01:56
Now if you loaded the parakeets up with helium before the plane departed...
Posted by: jimhogan
Re: Parakeets - 30/01/2007 01:57
Quote:
Assume you have an airplane that can only carry 100 pounds of cargo.
Why does this sound like a Microsoft interview question? 
To which i would respnd: What is any self-respecting bird doing on an *airplane*?
Posted by: lectric
Re: Parakeets - 30/01/2007 02:55
Wasn't this a concern for Lindbergh? Only it was a fly's weight he was pondering the whole flight, if memory serves correctly.
edited to correct CL's name.
Posted by: lectric
Re: Parakeets - 30/01/2007 03:03
Damn, can't confirm or deny that statement.... the word "fly" is just a LITTLE too common for google to handle, especially when linked to any type of plane.
Posted by: jimhogan
Re: Parakeets - 30/01/2007 03:06
Quote:
Now if you loaded the parakeets up with helium before the plane departed...
But then only dogs could hear them chirp.
Posted by: Neutrino
Re: Parakeets - 30/01/2007 04:34
Isn't a birds wing shaped like an airfoil? Or perhaps more correctly stated, isn't an airfoil shaped like a birds wing? In which case after the bird has gained forward momentum is not some of the lift caused by the low pressure region developed on the top surface of the wing? I would aslo contend that a box with a rotating propeller on it would indeed weigh less as not all of the lift is developed by downward force. Some of the lift is developed by the low pressure created on the top of the prop. Generally speaking an airfoil does not produce lift via downward force but by developing a negative pressure above it's top surface.
Posted by: webroach
Re: Parakeets - 30/01/2007 05:28
Quote:
Damn, can't confirm or deny that statement.... the word "fly" is just a LITTLE too common for google to handle, especially when linked to any type of plane.
Google-Fu + linguistics ("a fly") = the answer.
Well, to the Lindbergh question, anyhoo.
Posted by: frog51
Re: Parakeets - 30/01/2007 06:35
The answer still stands - anything you do inside that closed system is not going to help the plane fly. Attaching all the birds to the outside of the plane, however - well, that's another story.
Posted by: wfaulk
Re: Parakeets - 30/01/2007 12:26
Quote:
Generally speaking an airfoil does not produce lift via downward force but by developing a negative pressure above it's top surface.
I'd argue that that's saying the same thing in a different way, but, regardless, the same argument applies. If the bird is "creating" a low pressure area above its wing, it's eventually pulling down on the top of the cabin.
Posted by: Neutrino
Re: Parakeets - 30/01/2007 13:36
Of course, that's correct, I should have seen it.
Posted by: tfabris
Re: Parakeets - 30/01/2007 17:32
Quote:
For example, if you put a box with an upward-facing propeller on a postage scale, the box would weigh less when the propeller was turning since at least some of the air pushed downwards by the propeller would not hit the weighing surface of the scale.
Okay, so let's take that same concept and make it a bit more clear...
I have a radio controlled helicopter, and it's sitting on a very accurate scale with a very large "plate" on top (or whatever they call the bit you sit things on). The plate is much larger than the helicopter, large enough so that the downwash from its props will definitely hit it.
I hover the helicopter 1cm over the center of the scale. Is the weight registered on the scale exactly the same as the weight when the copter is landed?
If not, how is that different than a parakeet flapping its wings in the cargo hold (assuming the cargo hold is not pressurized)?
Posted by: wfaulk
Re: Parakeets - 30/01/2007 17:56
If the cargo hold is open to the outside, then, no the parakeets won't add weight. But if it's open to the outside, they also won't be traveling due to the speed of the plane. If it's simply not pressurized, but closed, that doesn't make any difference, as air isn't going to randomly move in or out of the plane's hold. The more open you make it, the less weight it will have to carry, but the fewer of the birds it will carry.
Posted by: bbowman
Re: Parakeets - 30/01/2007 18:29
I would say that the system is still open. Now if the top of the scale were an air-tight container inside of which the helicopter did its hovering, then I wold assume that the weigtht would be the same as if it where landed.
For the parakeets, tie the feet of nearly 100 to the top of the plain, feed then uppers and let them help the plane fly. If they are not burned out and die from overwork, then you may have 200 at the destination.
If you do this type of scenario, would they actually cancel the weight of the parakeets inside? If so, the you could have 200 in the plane and 100 helping to give lift to the plane!
Posted by: TigerJimmy
Re: Parakeets - 30/01/2007 19:39
Yes, in theory, the scale will register the weight of the helicopter, assuming all of the air put in motion by the rotors will impact the scale, and assuming the helicopter is not moving vertically.
In practice, it won't register the total weight, because there are losses to the fluid momentum (the air), because it disturbs the surrounding air, creating vortices and also because of the viscosity of the air resulting in heat loss. In addition, the lift varies spacially as the roters spin, so there will be fluctuation on the scale readout. But, neglecting these losses, and taking an average of the reading, the scale will register the weight of the helicopter.
If the helicopter is climbing, the scale will register more, if decending then less.
And yes, the birds add to the weight of the airplane, even if they are flying.
Posted by: TigerJimmy
Re: Parakeets - 30/01/2007 19:46
This isn't really true. Even if the cargo door is left open, the flying birds exert a reaction force downward on the air. The air then exerts a downward force on the bottom of the airplane. Whether the plane is pressurized or not doesn't really change things. In equilibrium, all the forces sum to zero (Newton's 3rd Law).
For the bird to fly, some force needs to counteract gravity. This is provided by the air. The air transmits this force to the floor of the compartment. Basically, the bird's flying increases the air pressure under it. This pressure acts equally on the floor of the airplane and on the bird (but in opposite directions, summing to zero as required by Newton's 3rd Law for a system in equilibrium). The air pressing down on the floor has the same "weight" as the bird. Even if the door is open.
Posted by: wfaulk
Re: Parakeets - 30/01/2007 19:52
Well, by "open", I meant more than just an open door; I meant "unenclosed". But an "unenclosed cargo hold" is "outside".
Posted by: TigerJimmy
Re: Parakeets - 30/01/2007 21:37
If the bird is "outside", then it doesn't contribute to the weight of the airplane. If the bird is at home in a cage, it doesn't either. If the bird is flying freely in its naitive Australia, it dosn't either.
Posted by: JBjorgen
Re: Parakeets - 30/01/2007 22:04
Would that be 200 African parakeets, or 200 European parakeets?
Posted by: gbeer
Re: Parakeets - 30/01/2007 22:13
It all depends on if the birds are flying, from the front of the plane to the back, or from the back to the front. :P
Posted by: tfabris
Re: Parakeets - 30/01/2007 22:23
Quote:
Would that be 200 African parakeets, or 200 European parakeets?
*facepalm*
Posted by: lectric
Re: Parakeets - 31/01/2007 02:08
Quote:
Google-Fu + linguistics ("a fly") = the answer.
Niiiiiice google-fu. Why didn't I think of that?!
Posted by: webroach
Re: Parakeets - 31/01/2007 02:50
Quote:
Quote:
Google-Fu + linguistics ("a fly") = the answer.
Niiiiiice google-fu. Why didn't I think of that?!
That's what 8 hours of linguistics study each day'll do for ya. Ugh. 
Posted by: wfaulk
Re: Parakeets - 31/01/2007 14:32
Quote:
If the bird is "outside", then it doesn't contribute to the weight of the airplane. If the bird is at home in a cage, it doesn't either. If the bird is flying freely in its naitive Australia, it dosn't either.
Yes. That was my point. The less enclosed the hold gets, the less the birds will affect the plane, but the less the plane will affect the birds.
Posted by: blitz
Re: Parakeets - 31/01/2007 19:07
Quote:
A quick google didn't show me the answer to this one, but ...
If you had only Googled after you posted, you would have had your answer
Google
Posted by: tfabris
Re: Parakeets - 26/04/2007 15:15
Bringing back the thread: I just saw Mythbusters do this one. It was a kick to see it done for real.
Even in a non-pressurized system, whether using model helicopters or birds, the weight of the container did not change when the contents were in flight. Precisely because the downward pressure from the wings/rotors pushed downward on the floor of the container.
Posted by: andy
Re: Parakeets - 26/04/2007 15:21
I thought it was one of the least good Mythbuster I have seen. It was just a "myth" that didn't need testing. If I remember rightly that was the speed boat one as well, which was fairly pointless, I suspect the boat in the myth was fibre glass, whereas their test boat was a huge hunk of wood.
Normally Mythbusters is excellent, but there have been a few duff ones recently. I'm just about to watch this weeks one...
Edit:
Hmmm, Ninjas, I have a bad feeling about this.
Edit:
Don't worry, they are back on form
Edit:
Though it is a shame their walking on cornstarch and water lacked some scale compared to the definitive demonstration:
http://www.youtube.com/watch?v=f2XQ97XHjVw