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# Aircraft weight during straight and level flight ? watch

1. Good evening everyone , just a question thats been popping in and out of my head for around three weeks now.

This is just an example.

A 747 - 400 is on a cargo flight from LHR ( London Heathrow Airport ) to LAX ( Los Angeles International Airport ). This 747 - 400 is hauling a flock of birds , shall we just say they altogether weight 5,000 ibs , and the aircraft weighs 670,000ibs.

This 747 - 400 , is travelling at FL360 ( 36,000ft ) , and all of a sudden the flock of bird's break free and are now longer strapped to the floor in the cargo hold in boxes.

What happens in this situation , not what to do next like turn back divert or so , but to the physics of the aircraft.

From my current knowledge during straight and level flight lift is equal to weight , this the makes the aircraft weightless , So the aircraft literally weighs nothing , now these flock of bird have just took off while the aircraft is weightless , what happens ?. The aircraft can't get any lighter from my knowledge so , any suggestion's ?
2. If the plane is flying completley straight and level, wont it just pick up speed instead? Plus because of the extra speed and less weight the aircraft would try to rise up? Its a toughy! lol
3. Yeah possibly , it would speed up theere for increasing airflow ove the wings , which increase lift
4. What, so the birds are flying around free inside?

I think the fact that a plane is sealed and the fact that in order to lift off, the birds have to exert a downward force on the air in the plane, mean that it makes no difference to the overall mass of the plane. The mass of the plane is taken as the mass of the plane, the birds, boxes, people, air and everything inbetween, so the birds not being in contact with the plane makes no difference.

That's my guess anyway.
5. Erm.... theoretically it would probally rise. If lift is greater than mass, then the plane rises. However, compared to the weight of the aircraft and the power of the engines, the weight of the birds is negliable, so not much would actually happen. The birds are supported on the airfame to begin with, so the plane has to carry their weight, but if they start flying inside, then their own energy is used to create lift, meaning the engines and wings of the plane dont have to support them any more.

Lift = mass, but the plane isnt weightless, if the lift is 7 tonne, the plane still weights 7 tonne. In the case of a physics diagram, there would be no movement because forces are equal, it doesnt mean that mass = 0. If the plane weight drops to 5 tonne then the force upwards (lift) is greater than the force downwards (mass) and therefore the plane will rise. But you would need a very very large object to prove it.
6. Makes no difference. The cargo still has the same mass.
7. Surely momentum would be conserved so when the birds took off they would push the plane downward, could this balance the decrease in weight?
8. The real answer is, that the plane would rapidly de-pressurize as the cargo door has opened at 36,000ft and the airframe would probably break up and if it didnt, hoping that they managed to get their oxygen masks on, the pilots would have a very hairy 30 mins.
9. You'd think this thread would have attracted people that are competent in physics, wouldn't you? Apparently not, though, as only two posters have got it right and one of them has admitted that he is guessing.

The position of the birds makes no difference.
10. (Original post by Good bloke)
You'd think this thread would have attracted people that are competent in physics, wouldn't you? Apparently not, though, as only two posters have got it right and one of them has admitted that he is guessing.

The position of the birds makes no difference.
11. (Original post by GeorgEGNT)
You just quoted it. The position of the birds within makes no difference to the aircraft.
12. Ohhhhhhhhhhhhhhhhhhhhhhhhhhh, I feel slightly embarrassed now I've just actually got the point of the question now lol. I'll be off now.
13. lol , kl I have loads more question's about aircrafts a the physics i'll leave that for tomorow though need sleep.
14. This is similar to the question I always used to ask my dad when I was like 6. I'd ask if I jumped in the train why don't I get splattered to the end of the carriage, or if I jumped in an elevator when it was going down, why doesn't my head smack into the ceiling. He never knew

Err, but I know now, so I'll be the third to look at this thread who knew the answer
15. (Original post by Indievertigo)
This is similar to the question I always used to ask my dad when I was like 6. I'd ask if I jumped in the train why don't I get splattered to the end of the carriage, or if I jumped in an elevator when it was going down, why doesn't my head smack into the ceiling. He never knew
Intertia is similar to lift?
16. youve got alot of cooked birds thats what!
17. (Original post by Renal)
Intertia is similar to lift?
I dunno, do I?

I wasn't suggesting that anyway, I was suggesting the question was similar. Slight difference there, y'know.

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Updated: November 19, 2008
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