Aircraft Physics Question

So just to complicate things, the OP doesn't state if the plane fuselage is enclosed and pressurised, or is open to atmosphere (eg a C130 with the ramp down). In the later situation, the plane will be lighter once the birds have taken off (perhaps not by the full 5,000kg). Fact.

 

Wait - different "mediums" respond to gravity differently? Can you explain? I thought all things were affected by gravity in exactly the same way.

Yes, true. All "weigh the same" answers are assuming the "plane" is enclosed. It doesn't matter if it is pressurized or not.

 

I'm not one to quote the MythBusters as scientific evidence - but it is so obvious, and they have done the work - so why not? They have done the experiment. Episode 77.

 

Cheesy - Let me ask you this.

Let's say LouB is flying his 747 (MTOW, say 1 million pounds).

The 747 is loaded with 2 million pounds of birds.

It starts rolling down the runway for takeoff. But it can't, because it is too heavy.

Are you telling me (assuming the 747 gains appropriate speed) that if the 2 million pounds of bird all start flying halfway down the runway, the 747 will then be able to take off?

 

Visualize a full-boat 1960s Cadillac convertible on a moving-company scale,top down. A 500-pound bird hovers over it and then perches on the back of the front seat. After a short roost,it lifts off and leaves. I say that the only time that the weight of the car goes up is when the bird is actually perched on the seat (such scales register in 20-pound increments).

Others mentioned an open-cockpit plane. Birds flying in the cockpit can't increase the weight of the plane. Closing the cockpit won't change anything.

The San Francisco area has a brilliant scientist and inventor named Bill Wattenberg who hosts a radio call-in show. I bet he would know the answer right away and be able to explain it convincingly.

 

As others have noted correctly, Newtons third law applies, for every action there is a reaction that is equal and opposite. Just because it is hard to measure doesn't mean it isn't there.

We've all seen the effect of the downwash of air and the results of that downwash on the ground when an airplane flies very low. There are plenty of pictures of that on youtube so I won't bother finding one where a plane flies low over the water, you can see the ripples on the surface from the air being compressed under the aircraft, wing in ground effect, and air cushion vehicles are other examples of how building pressure under the aircraft supports it.

Now step back and realize that as an aircraft flies higher, the area effected under it becomes larger, and the pressure increase becomes smaller. Imagine a cone of higher pressure that spreads out under the aircraft. Since the higher you go the area of the cone increases as a square of height, the amount of pressure on the ground from an aircraft passing over it decreases as a square of the height too, and it therefore falls off very quickly.

The USAF actually tired to measure this increase in pressure to try to detect aircraft flying over the DEW line back in the cold war. What they found was that the amount of pressure increase that they could measure was about the same as the sound, that is, if they could hear the aircraft they could measure the increase in pressure of the aircraft on the ground.

The birds are no different, if there is no lid on the box, an open cockpit, or an open end on the fuselage the results will change very slightly, but the fact remains that most of the increase in pressure is reacted by the floor of the aircraft and that is what is supporting the bird. Just like an air cushion vehicle, which presses down on the ground below it, there is an increase in pressure that is the reaction of the lift and that is reacted by the surface right under it.

In the example of a bird over a moving car, the downwash of the wings of the bird will press down on the car before it lands, and will press down as it takes off, so there will be an reaction (an increase in weight on the wheels of the car) before it lands. As the bird lifts and flies higher, the force on the car decreases as a square of the height once the cone under the bird exceeds the area of the car.

The best way to think of this is to think about an air cushion vehicle. Pressure is increased under the vehicle and it lifts off of the surface, but the pressure increase times the area is equal to the weight of the vehicle. An air cushion vehicle on a scale (provided the scale is bigger than the vehicle) will weigh the same whether it is flying or sitting on the scale. Same thing with the birds, they have to press down on something, and if there is a closed surface below them, that's where they are supported.

 

Sorry, I guess I don't speak Feline. I'm more of a dog kinda guy.

 

no one is disputing Newton's work, a formula was posted above, use it... it will help explain where some are going wrong in making false assumptions... for more detailed explanation throw in some tensor analysis of the examples...

 

Can we discuss loss of airspeed in a downwind turn next?

 

is that when the wind "grabs" you when you turn base?

 

If Newton and the Mythbusters' test say it's so, end of discussion.

Weighs the same. period dot.

 

Yes! Exactly. It grabs you and then puts you in a "wind pocket".

 

Here's a link to the mythbusters show.....

[ame=http://www.youtube.com/watch?v=in34DVzk-rw]Birds in a Truck - Mythbusters for the Impatient - YouTube[/ame]

 

I am still amazed by this display of truly abominable physics. Where does the helium GO when the balloon deflates?

You are suggesting that it DISINTEGRATES and the mass disappears, and then inexplicably the mass returns when the air fills the void.

That is not what happens when a balloon bursts - the contents of the balloon is still there, it just suddenly loses the pressure it had in the balloon and dissipates explosively into the outer air until the pressure equalizes. No mass is created or lost.

 

use a sky hook, makes finding it easier when lost

 

Can you give us a clue about how the Mythbusters segment is wrong then? We obviously need to learn.

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nothing disappears or inexplicably reappears, all can be accounted for... air is comprised of several elements, which is diluted by the lighter weight helium, you can calculate the change in mole weight from the resulting dilution within the "container"/ plane... "standard air" has a fixed distribution of elements which can be altered by things as helium from a balloon or pollution... for calculation purposes, air at a standard temperature and pressure as well as composition is used... a bursting helium creates changes other than "pop or a poof" :=)

 

you can start by using the formula in an earlier post, one of the values in the formula is zero... by Newton's own findings... fill in the blanks and you have your answer... you can use your own values, the result will not change

Newton findings deal with realities, not perceptions which most of the discussion is about, which lead to false assumptions and conclusions

 

So what is shown on that mythbusters segment is incorrect?

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Changing the pressure of a gas does not change the mass or weight of the gas. It changes the volume and the temperature. This is fundamental Boyles Law - 9th grade High School physics.

 

pressure cannot change the weight of the elements that make up a sample of a gas, it does allow for more molecules to be stored in a given space, changing the weight of the container by holding more molecules... leading to the use of STP to maintain constant distribution of the molecules within a given space or volume ( per liter or per cu ft )

 

The volume of the airplane hull did not change. The mass of the atmosphere inside the airplane hull did not change. The mass/weight of the airplane did not change.

There WAS NO CHANGE.

 

Holy crap - So you think the mass of a closed system changes if you pop a balloon inside a the system? This, plus all your other nonsensical posts about this topic, leads me to think that if you paid for school you should ask for your money back.

If we agree the aircraft cabin is a closed system and not accelerating, then the weight of the system is exactly the same whether birds are sitting on the floor or being supported by the gas contained within the closed system (i.e., flying). This is elementary physics and there is no simpler - or more complicated - way to explain it.

A steady-state (i.e., no acceleration) vector diagram of the forces external to the closed system is IDENTICAL whether the birds inside are flying or not. Period.

 

Weight stays the same. End of story. Nothing else to see here. Let's move on.

 

Forgive me, but how big is the balloon? Is it the size of the cabin? Would there be more helium then air?