# Thoughts on the caged birds in a plane chestnut.

Thoughts on the "caged birds in a plane" chestnut.

Someone recently asked in the notes and queries section of a British newspaper, "would the weight of a plane flying at a constant altitude change if its cargo of birds in cages were all on their perches or flying about in their cages?". I have since discovered that this is a version of a textbook staple re Newton's laws and the respondants to the question duly answered that the flapping of the unperched birds would be transfered to the floor of the plane, hence no change in weight. However...

1) If the birds were moving horizontally around their cages as well as vertically (as flapping birds surely must) would not the small lift derived from pressure differences due to the shape/angle of their wings account for even a small proportion of their flight NOT being passed on to the plane, hence a small reduction in the weight of the plane?

2) Would the change in position of the plane's contents caused by the birds getting off their perches effect the plane's cumulative centre of mass and hence how its weight would be measured in terms of pull from the earth's centre of gravity?

I appreciate that the whole thing is a thought experiment to demonstrate Newton and could never be practically measured but am interested to know if there is an official answer. Good conversation starter down the pub I've found.

Danger
Gold Member
The 'thrust' of their wings is in addition to the lift that holds them up, and is transfered to the plane laterally through the air. The lift component is still equal to their weight.

russ_watters
Mentor
1. To say it another way, a wing in lift still follows Newton's law - in fact, that's the other way lift is commonly described. For a bird to generate lift, air must be pushed down. Here's both Newton and Bernoulli: http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html

2. There would, of course, be instantaneous effects caused by the bird jumping off it's perch or landing back on it. These are completely irrelevant to the problem, since the problem specifies a bird in flight. But if you want, yes, a bird (or a person) jumping up and down inside a plane will make it go up and down. The oscillation aside, the plane will not deviate from it's flight altitude.

It boggles my mind how many times we get this question. No real knowledge of aerodynamics or Newton's laws is required to answer it. It should be completely intuitive. Maybe it is just that people overthink it?

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Kurdt
Staff Emeritus
Gold Member
It boggles my mind how many times we get this question. No real knowledge of aerodynamics or Newton's laws is required to answer it. It should be completely intuitive. Maybe it is just that people overthink it?
I think when people see a question like that they feel there must be a counter-intuitive answer otherwise it wouldn't be asked. An interesting quirk of psychology I suppose if somewhat frustrating at times.

Well, so how about digging an area right below the bird so that the the air flow would not act on the plane but get out through that area?

rcgldr
Homework Helper
air must be pushed down. Here's both Newton and Bernoulli:
http://hyperphysics.phy-astr.gsu.edu/hbase/fluids/airfoil.html
What that article leaves out is that air is accelerated downwards from above and a bit in front of a wing, so the air is pulled down from above as well as pushed down from below.

It also helps to explain how a gas exerts it's weight inside a container. It's done via a pressure differential, where the pressure decreases with altitude within the container so that the net pressure differential at the top and bottom surfaces of a container result in a net downwards force exactly equal to the weight of the gas inside the container.

When a flying (or gliding) object is inside the container, and there's no net vertical acceleration, then the object increases the pressure differential so that the downforce from the gas equals the combined weight of gas and object.

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russ_watters
Mentor
Well, so how about digging an area right below the bird so that the the air flow would not act on the plane but get out through that area?
Once you open the system, all bets are off. It may change things for the bird to be flying, but the airflow over the opening would change things a whole lot more.

russ_watters
Mentor
What that article leaves out is that air is accelerated downwards from above and a bit in front of a wing, so the air is pulled down from above as well as pushed down from below.
Actually, no - it shows both and in fact you can see from the top streamline in drawn in the diagram that some air upstream of the wing gets directed up and over the wing before being thrown down behind it. In the this particular diagram, that's the bigger effect than air being directed down by the lower surface.

This isn't a great photo because of the shadow, but you can see that the streamlines start bending upwards well forward of the airfloil: http://instruct1.cit.cornell.edu/Courses/mae427/427_06_fm3/sec3_html/photos/P7022925.html [Broken]

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rcgldr
Homework Helper
What I was referring to is air well above the wing, about 11 feet or so in the case of a Cessna 172 near the mid wing. In this region, there's no upwash, which is localized to the area very close to the wing.