Skyguy11 From , joined Dec 1969, posts, RR: Reply 1, posted (10 years 2 months 4 weeks 6 hours ago) and read 8767 times:
Would it be affected?
That depends on your frame of reference. If you are the pilot, no it wouldn't be affected.
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!).
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.
Saintsman From United Kingdom, joined Mar 2002, 2065 posts, RR: 2 Reply 3, posted (10 years 2 months 4 weeks 4 hours ago) and read 8734 times:
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 From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 4, posted (10 years 2 months 4 weeks 4 hours ago) and read 8726 times:
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.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 5, posted (10 years 2 months 4 weeks 3 hours ago) and read 8707 times:
. Finally, they are again retarded
I thought they were continuously retarded...
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.
Boeingflyer From Germany, joined Sep 2003, 7 posts, RR: 0 Reply 6, posted (10 years 2 months 4 weeks 1 hour ago) and read 8661 times:
"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.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 7, posted (10 years 2 months 4 weeks 1 hour ago) and read 8660 times:
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.
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.
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.
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.
Lstc From Canada, joined Jun 2003, 320 posts, RR: 0 Reply 8, posted (10 years 2 months 4 weeks 1 hour ago) and read 8638 times:
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.
Rendezvous From New Zealand, joined May 2001, 511 posts, RR: 0 Reply 13, posted (10 years 2 months 3 weeks 4 days 9 hours ago) and read 8216 times:
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!
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 15, posted (10 years 2 months 3 weeks 4 days 3 hours ago) and read 8150 times:
. 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)
IMissPiedmont From United States of America, joined May 2001, 6244 posts, RR: 36 Reply 16, posted (10 years 2 months 3 weeks 3 days 9 hours ago) and read 8010 times:
"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.
Is grammar no longer taught is schools? Saying "me and her" or some such implies illiteracy.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 17, posted (10 years 2 months 3 weeks 3 days 3 hours ago) and read 8007 times:
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.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 18, posted (10 years 2 months 3 weeks 3 days 3 hours ago) and read 8001 times:
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?
FredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 19, posted (10 years 2 months 3 weeks 3 days 2 hours ago) and read 7988 times:
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...
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Ikarus From United Kingdom, joined Jan 2001, 3524 posts, RR: 2 Reply 20, posted (10 years 2 months 3 weeks 3 days 2 hours ago) and read 7997 times:
what's a G&T?
Message too short
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7E7 From Australia, joined Aug 2003, 159 posts, RR: 1 Reply 22, posted (10 years 2 months 3 weeks 21 hours ago) and read 7810 times:
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 From United States of America, joined Jul 2001, 7028 posts, RR: 22 Reply 23, posted (10 years 2 months 3 weeks 12 hours ago) and read 7756 times:
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.