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As a thought experiment, imagine a plane with minimal fuel/appliances on board with hundreds of heavy passengers that make up say 10% of the weight of the plane.

Now imagine that they have a way to synchronize a jump where temporarily for 0.5s they are airborne in relation to the plane.

(NOTE: If idea of so many heavy passengers is troublesome to you, assume this is a cargo carrier with spring loaded "widgets" that have ability to be programmed to "jump" at once.)

Questions:

  1. Can flight dynamics be affected due to the jump at ANY phase of the flight?

  2. What % of the typical (cargo/passanger) plane weight is the actual cargo (excluding fuel)?

  3. Does the location of the "jumping" cargo matter? Tail vs. Cockpit

  4. What happens if all pax/cargo suddenly jumped to tail or cockpit section?

Everybody jump!

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I suppose nothing as the passengers would have the same momentum of the plane.. –  Fabrizio Mazzoni Mar 26 at 10:06
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I'm sure MythBusters would like to try that. –  Farhan Mar 26 at 13:18
    
...They all hit their heads on the cabin ceiling and learn that the passenger section of most airplanes isn't really all that big inside? (What? I'm 6'2 - I can't even stand up in some airliners!) –  voretaq7 Mar 26 at 15:31

1 Answer 1

up vote 7 down vote accepted

I will assume that the starting condition is level flight.

  1. yes. momentarily the assumption of "rigid body" would not be satisfied. Also, for that 0.5s the aircraft would not be in contact with 10% of the weight, assuming constant thrust and velocity, you would have 10% more lift than weight, making the aircraft rise a bit before descend again when all the passengers land again. Velocity would not be greatly affected, as the drag would remain unchanged (no external change in shape or large attitude variations)

  2. the difference between zero-fuel weight (ZFW) and Dry Operating weight (DOW) is probably what you are looking for. Airbus defines it as the "total traffic load". If you want to add catering, cabin equipment and the crew, you then take the difference between the ZFW and the Manufacturer's Empty Weight (MEW). As a percentage it can vary wildly based on the aircraft class (class 23 has much less cargo/passenger space that a class 25, for example)

  3. yes. think of a beam in equilibrium on a pivot located at half its length: if you push just above the pivot, the beam will not rotate, any other location will cause it to rotate and flip over. This does not mean that the aircraft might flip over, as the beam is an uncontrolled and unstable system, while the aircraft is controlled, but it is a comparison that can help visualize the effect

  4. I would define it more as shifting rather than jumping. This is a totally different scenario: the center of gravity at the end of this "jump" would be in a totally different location w.r.t. the start of the experiment, something that instead was not happening in the previous scenarios. By shifting the location of the center of gravity is likely that it might exceed the specified position limits and cause the airplane to be uncontrollable, to pitch up or down (depending if you shift the passengers backwards or forwards) and stall or nose-dive. On large aircrafts it is obviously not easy to achieve such unsafe scenario, as the cargo will be usually much heavier than the passengers, limiting the extent of the CG shift.

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I think this is wrong. Imagine you have only one passenger, with mass 1Kg. When this passenger jumps, they exert a downwards force on the cabin floor which is greater than the acceleration due to gravity. It must be in order for them to jump. Assuming that everything is rigid, the aircraft would experience a downward force exactly cancelled by the upward force experienced due to the temporary reduction in weight. Forgetting about fuel burn, the mass of the overall system does not change. If you could measure it, I believe that you would see down, up, down. –  Simon Mar 26 at 10:46
    
@Simon, it could be. I do not believe that the total net effect, istant by istant would be completely zero, there will most likely be some wiggling up and down (e.g. the downward accelleration you mention would win at fist, making the airplane move slightly down) If you can work out the equations, please make your answer, it would be interesting. –  Federico Mar 26 at 11:13
    
@Federico, let's not assume JUST level flight. Climbing and descending as well. I was originally inspired by the relatively recent crash of a 747 which crashed due to shifting load during take off which stalled it into ground. –  KORD4me Mar 26 at 12:15
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@KORD4me shifting load is a total different problem: you have the CG that moves and does not go back to its initial position. –  Federico Mar 26 at 12:21
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@KORD4me answer expanded, but it is my impression that that specific accident might merit a separate question/discussion –  Federico Mar 26 at 12:48

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