QANTAS747-438
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What Would Happen On A Plane If Every Jumped at Same time

Tue Sep 16, 2003 4:56 pm

I'm curious as to what would happen on a plane if for some hypothetical event were to take place where all the passengers stood up and jumped around as hard as they could? Would the plane be affected?

Also, in this same awful flight, what would happen if all the passengers went to one side of the aircraft?

Thanks!
My posts/replies are strictly my opinion and not that of any company, organization, or Southwest Airlines.
 
SkyGuy11
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 5:15 pm

First part:

Would it be affected?
That depends on your frame of reference. If you are the pilot, no it wouldn't be affected.

Why not?
The airplane still needs to support and carry the people. The fact that they are jumping means nothing, because they are still enclosed in the block of 'stuff' (air included) that the airplane supports. The same thing is true if you carry a bunch of birds and they all decide to fly around at the same time. Don't get me wrong, the pilot would still feel the vibrations and bouncing motions of the pax all jumping, but the airplane itself would be unaffected.

Would anything happen?
The only thing that would happen is that the airplane as a whole would lose a very small amount of altitude. Every movement an airplane makes is around it's CG. Change the CG inflight, and you change the movement of the airplane. If everyone jumps up at the same time, the CG keeps going in the same direction and altitude, but the CG is now higher in the plane, which means that the plane itself will lose a bit of altitude (to keep that CG in the same spot!).

Second question:
This would affect the lateral trim of the airplane. You might need to hold opposing aileron / rudder or trim off the control pressures. The same is true if the pilot only uses fuel from one wing for an extended period of time, the plane will have a rolling moment because the CG will be laterally displaced from center.

Ahhhh I'm bored!
.
 
POSITIVE RATE
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 5:21 pm

On some planes the CG is very sensitive. A Saab 340 pilot once joked that he could judge the weight of the female flight attendant everytime she walked up and down the cabin  Smile
 
saintsman
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 6:40 pm

Skyguy,

Intersting therory. If everyone jumped at the same time would the weight of the aircraft decrease and so therefore the lift would be greater than the weight resulting in an increase of altitude. Very hypothetical of course.
 
FredT
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 6:48 pm

If everyone jumped at once, at first they would accelerate the aircraft downwards as they propel themselves up. Action and reaction. As they leave the floor, the aircraft is momentarily free of their gravitational pull and will accelerate upwards, as it no longer feels the weight of the pax load. Finally, they are again retarded meeting the floor on their way down at which time they will accelate the aircraft downwards once more.

It is not the same as the question about what would happen if a load of poultry took flight. The pax would essentially be free falling, exerting no force on the aircraft or the air inside the cabin. The flying poultry would not be in free fall. They would exert a force on the air which would transfer to the airframe.

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Ikarus
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 8:01 pm

. Finally, they are again retarded

I thought they were continuously retarded...  Big grin


Wasn't there some Australian venture a while ago to have a clubbing plane that had an internal disco or something? Ask those pilots...

FredT's explanation is as far as I can tell correct. Now what would happen if the passengers happened to jump up and down repeatedly at a resonant frequency in the flight dynamics? They could cause a sustained oscillation of the flightpath, and, if the mode in question was not inherently stable, an escalating oscillation... At least, I think so.

Now if all pax went to the front or back of the plane, that would move the CG fore and aft, which would require a change in the moment (between tailplane and wing) so the plane would have to trim into a different angle of attack. If they moved from side to side, the effect would be much smaller (there is less of a moment arm) and the reaction would be less noticeable. If they did the same in a flying wing, it would again require a lot of trim, this time on the ailerons.

Regards

Ikarus
 
Boeingflyer
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 9:39 pm

"Now what would happen if the passengers happened to jump up and down repeatedly at a resonant frequency in the flight dynamics?"

The resonant frequency of an aircraft is very low (as far as I remember less than 2 oscillations/minute). So I doubt you could jump that slow. There might be more of a problem with the flight crew than with the aircraft.  Smile/happy/getting dizzy
 
Ikarus
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 9:51 pm

Boeingflyer: That's the long-term frequency.

If you look at the equations, you will find it's a quartic (longitudinal) or quintic (lateral) for (most) fixed wing aircraft. That means 4 (or 5) roots. 2 sets of complex conjugate pairs, so that means 2 frequencies. There is a short-period and a long-period frequency of resonance. The long-period one can be half a minute or even 90 seconds (the phugoid motion is the classic example). The short-period resonant frequencies are much much smaller, more like 1-2 seconds. So it would be feasible to jump up and down at (or near) that frequency on some aircraft, I believe. But on the other hand, the short period is usually a very stable resonant frequency, so probably even the small positive feedback of the jumping masses won't make it unstable.

Regards

Ikarus

Addendum: From an old trip report of mine:


Short period oscillation (Pitch):
When a plane is perturbed (disturbed away) from the steady (trimmed) state, and provided it is statically stable, it will attempt to return to the state it is trimmed to be in. Hence, if the pitch is increased for a very short moment, and the controls are released, the plane will immediately head back down again - and overshoot a little and come back up etc. etc. The result is a short period oscillation, that doesn't go through more than 1 or 2 periods before being dampened out.

Dutch Roll:
The Dutch Roll is the result of a perturbation in yaw. It is essentially the equivalent of the short period for yaw perturbations. However, as there are fewer and smaller vertical surfaces than horizontal ones, it does not stop quite as soon as the pitch oscillation. More interestingly, there is the secondary effect of roll: When the plane yaws to one side, the wing on the other side generates a bit more lift, and hence a roll motion is started. When the plane yaws back to the other side, the wing generates less lift and the roll changes direction. Hence, the yaw oscillation causes a roll oscillation as well. In a passenger aircraft, you can recognise that the plane if flying a dutch roll if you watch the wingtips and they trace an elliptic shape across the horizon. However, it is not too likely that you will see this a lot, as the pilot will suppress any such motions quite soon.

Phugoid:
The phugoid is the long-period oscillation in pitch. You might know that any vibration system has several resonant frequencies. Anologously, the dynamic system "aircraft" has different oscillation periods that are caused by perturbations. If the perturbation in pitch is big enough / lasts long enough, the plane will not experience the short-period oscillation described above, but a phugoid motion. This oscillation has a very long period (40-50 seconds) and it does not die down very quickly - it can continue for several minutes easily. You will probably not notice this much in a passenger flight, as pilots will automatically suppress this oscillation.



[Edited 2003-09-16 14:58:44]
 
LSTC
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RE: What Would Happen On A Plane If...

Tue Sep 16, 2003 10:05 pm

The fact that they are jumping means nothing, because they are still enclosed in the block of 'stuff' (air included) that the airplane supports.

Not so....I've done this to a smaller degree in an empty HS748. I jumped up and down in the rear of the airplane. It caused the airplane to pitch up and down pretty noticably.

I would say that the reaction of the aircraft would depend on the distribution of the people. If you think about it, its similar to what would happen if the aircraft suddenly pitched up then down rapidly. First you'd feed lots of weight on your feet (like you would as you started to jump). The you would feel unsupported momentarily as the aircraft experienced less than 1 G, then you'd feel lots of weight on your feet as the aircraft pitch neutralized, then you would feel normal weight.
 
captjetblast
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RE: What Would Happen On A Plane If...

Fri Sep 19, 2003 5:05 am

Proportionally, the same would happen to Earth if all the chinesse jump (and fall) at the same time: almost nothing.

 
Ikarus
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RE: What Would Happen On A Plane If...

Fri Sep 19, 2003 5:55 am

Captjetblast:

I disagree. The weight of all the Chinese, as proportion of the weight of the planet, is much smaller than the weight of the passenger load on an airliner.

Regards

Ikarus
 
cdfmxtech
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RE: What Would Happen On A Plane If...

Fri Sep 19, 2003 6:00 am

Coordinate this with the passengers on your next flight to see what happens, they can't possibly arrest all of you when you land!!  Big thumbs up

Or can they...
 
Ikarus
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RE: What Would Happen On A Plane If...

Fri Sep 19, 2003 6:03 am

If it's a flight from Paris and it lands in the US..... probably  Big grin  Acting devilish

But the logistics of it are baffling. there isn't enough floorspace for a good jump...

Regards

Ikarus
 
rendezvous
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Fri Sep 19, 2003 1:32 pm

Interesting theory I heard regarding the placement of toilets in aircraft. When the fasten seat belt sign is turned off, a large number of passengers get up to use the bathroom. Therefor the toilets are spread evenly amongst the cabin so you don't have everyone going to one end of the plane and screwing around with the CoG.

On the other hand, convenience comes to mind for placement of toilets!
 
IMissPiedmont
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Fri Sep 19, 2003 1:34 pm

The flight crew would be a bit annoyed. It would have no effect at all on altitude or speed as there is no change in weight.

It would be slightly funny though. Just make sure none of the passengers are wearing high heels, The resultant broken legs would not be funny.
The day you stop learning is the day you should die.
 
Ikarus
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Fri Sep 19, 2003 8:13 pm

. It would have no effect at all on altitude or speed as there is no change in weight.


As explained by FredT above, that is not entirely correct. The mass contained inside the aircraft would remain constant, but the weight force on the plane would not. While the passengers are airborne, their weight is not transmitted to the structure of the plane in any way, and just as they experience weightlessness on their parabolic jump paths, the plane experiences a reduced payload during their jump (and an increased one while they accelerate themselves upwards and when they impact the ground again)

Regards

Ikarus
 
IMissPiedmont
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Sat Sep 20, 2003 2:10 pm

"For every action there is an equal and opposite reaction." I have a hazy memory of that being the first law of physics. Therefore, the energy the passengers would use to jump upwards would be transmitted downwards to the floorboards. The bump a second later would indeed be felt but would have no effect on altitude, or attitude.
The day you stop learning is the day you should die.
 
Ikarus
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Sat Sep 20, 2003 7:53 pm

FredT's explanation is correct - so to repeat it:

The floorboards are connected to the fuselage. The fuselage would be pressed down as they propel their lardy carcasses up. The plane would lose a few centimetres of altitude. Now while the pax are airborne, as FredT described, the lift of the plane is constant, but the weight force of it is reduced (the pax are not transmitting any force to the structure, so for the duration of their jump, the plane is lighter by its pax load), so there would be a tendency of the plane to accelerate upwards to meet them. Then, the pax would hit the floor again, and transmit their weight, plus an impulse due to their impact speed, into the plane and force it back down.

Overall result: a very short, slight, oscillation in the flightpath. Altitude (and, as secondary effect, attitude) would be affected, slightly.

Regards

Ikarus
 
Ikarus
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Sat Sep 20, 2003 8:22 pm

Just had a thought.

If there was no time-lag in the (aerodynamic) reactions, would I be right in stating that the Centre of Gravity of the aircraft&pax would remain at a constant altitude, but the altitude of the plane as observed from outside would vary?

Regards

Ikarus
 
FredT
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Sat Sep 20, 2003 8:55 pm

Ikarus,
yup. No energy added above what would be added if it just went about the normal drag-countering flying business - so no net change in energy compared to the case where the passengers were sitting down, becoming retarded by G&Ts rather than being retarted by the floorboards...  Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
Ikarus
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Sat Sep 20, 2003 8:59 pm

what's a G&T?

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scbriml
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Sun Sep 21, 2003 5:29 am

Gin & Tonic (hic).

 Nuts  Nuts  Nuts
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7E7
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Tue Sep 23, 2003 1:39 am

Around the lines of the topic, I read a section in one of the magazines in 2001, from a flight attendant who worked back in the 70's on how F/O would say (joking) to the new flight attendant:'Do you hear this sound? The engines are out of synch! Go and stand between the two wings and jump up and down few times'
To the amazement of this ex-stewardess during a flight she took not a long while ago, she saw a young F/A jumping during the flight.

 
lehpron
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Tue Sep 23, 2003 11:10 am

i think for every brief second everyone is in the air, the plane it pretty much at operation empty weight with fuel and then at approximately overcapacity as well.

I do not think planes are designed so that passengers can go around it like an amusement park ride. Airliners are essentially air buses, not air ships, I know Carnival cruise ships have gyms. I wonder about the life cycles of certain components of the cabin, in terms of the limit in vibrations before replacement...Like so many parts have a million cycles to failure -- 200 people jumping once a second for 8 hours is over eleven times that.

Hell, why dont we dump a few hundred bird carcases into the freestream air ahead of the plane -- anyone can figure the damage there.

I dont think it should be so much of an effect of importance, just hope everybody understands when to stop.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
 
FredT
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Tue Sep 23, 2003 6:17 pm

Well, each jump would only be one cycle... and probably not near the loads generated by even light turbulence.

Easy to calculate with the acceleration in turbulence, no of pax and aircraft total weight known. I don't care that much though.  Smile

With one jump/sec for eight hours they will not need a gym!

And the bird carcasses is probably the definitive indication that this thread is going off on a tangent fast... Big grin

Cheers,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
gkirk
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Tue Sep 23, 2003 6:57 pm

It'd be damn hilarious!  Laugh out loud  Big thumbs up
Could just imagine a Pilot talking to ATC...
Pilot: Blah ATC, we have a slight problem
ATC: What is it?
Pilot: The passengers are jumping up and down
ATC: Stop serving them Red Bull, you know it gives them wings!
 Laugh out loud
When you hear the noise of the Tartan Army Boys, we'll be coming down the road!
 
Guest

RE: What Would Happen On A Plane If Every Jumped At Same Time

Wed Sep 24, 2003 1:02 am

You'd all look like idiots?

-Normal
 
747Teach
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RE: What Would Happen On A Plane If Every Jumped At Same Time

Thu Sep 25, 2003 11:19 pm

QUANTAS747-438: A few years ago I was on a flight from Juneau to Trinidad when that exact circumstance occurred. Somewhere over Guadalajara, that "Macarena song" came on over the entertainment system ("OOOOOOOH, Macarena"). It was innocent enough at first, just a few people tapping their feet. Then a couple of folks got up in the aisle and started to do the Macarena. As you can imagine, before long everyone in the plane was on their feet, jumping and dancing. The pilot came out and told everyone to stop, and was quickly hounded back into the cockpit. A very obese woman poked her high heel through a floorboard, so the flight attendants made everyone take off their shoes. The frenzy continued for an hour or two, then we changed to another old favorite, the Hokey Pokey ("Put your left foot in, take your left foot out . . ."). The pilot came out again, and was welcomed with open arms. We formed a conga line, and danced up one aisle, then down the other. I'm positive I saw the flight deck crew go by at one point. I'm sure you realize a few hours of this pandemonium would inevitably lead to the other favorite pastime of bored airline passengers . . . yes, galley cart racing. A cart is positioned in the aisle, and at a signal from the head flight attendant, the pilot would give the aircraft about 10 degrees nose up pitch, causing the carts to go careening down the walkway toward the toilets in the back. It was the job of the passengers seated along the aisle to keep the carts moving, sort of like curling. My cart was slow due to a wonky wheel, but my protest to the judges was to no avail. Hundreds of spilled drinks later, the dancing and singing finally spooled down, and flight attendants finally got the chance to pass out the meals.
You know, you just don't get on flights like that anymore. Sure was fun. Wish I could have found my shoes, though. The fat lady's high heels are making a blister on my foot. Regards,

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