Disappearing Weight?

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bigblockz8

Pre-takeoff checklist
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I was wondering if, somehow, just in theory, if I am flying a 737 and my 15,000lbs of pax all jump in the air at the same time, don't I lose my payload and 15000lbs momentarily? That split second loses 15000lbs right? They aren't supported by the aircraft, they are up in the air within the aircraft. Now in a 172...same thing but less CG range to mess with. Just wondering. This is also assuming that I am level at say 4000ft msl/4000agl on a standard day and at a 1G state.

I get that when they all land there will be one heck of an issue but I'm talking about while they are suspended in the air. That brief second= lighter aircraft correct?
 
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I don't have enough room to stand up and jump in a 172.
 
You also have to figure that the downward forces form the passengers will increase when the push off the floor
 
bigblockz8 said:
I was wondering if, somehow, just in theory, if I am flying a 737 and my 15,000lbs of pax all jump in the air at the same time, don't I lose my payload and 15000lbs momentarily? That split second loses 15000lbs right? They aren't supported by the aircraft, they are up in the air within the aircraft. Now in a 172...same thing but less CG range to mess with. Just wondering. This is also assuming that I am level at say 4000ft msl/4000agl on a standard day and at a 1G state.
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Haven't you ever done the butt hop/adjusted your seat in the 172? You can feel the airplane "jolt" just a tiny bit. I imagine you'd get a more noticeable effect with heavier equipment.
 
What happens if you do it on a treadmill?
 
What is the point of this question?

From an engineering point of view, airplanes are designed to support a static load and a transient load. That is, each parts load will be in a "steady +/- oscillatory" form, for maximum peak loads and minimum peak loads. Ever hold an accelerometer and jump? You'll probably see about 2G. All you do by jumping momentarily is induce a very large oscillatory load on the floor and airframe. So, thanks for adding a "cycle" to the parts and reducing the fatigue life left. :p

Not to mention, what do you think will aerodynamically happen, the second 15,000 pounds "disappears"? That thing will be horribly out of trim.
 
There would be no difference in the weight the airplane "sees" when all your passengers jump up in the air simultaneously.

They are supported by the air, the air is in the airplane.

Lindbergh (well, Jimmy Stewart in the movie about the same) wondered the same thing, about a fly that was in the cockpit with him in the Spirit of St. Louis. Did the plane weigh less when the fly was buzzing around vs when it wasn't flying...

Mythbusters tested this very scenario. Busted.

It's easier to comprehend when you remember that air acts as a fluid... an invisible fluid.

If you float something in a glass of water, versus putting an object of the same weight at the bottom of the vessel, does the weight of the entire vessel change?

Nope.
 
bigblockz8 said:
I was wondering if, somehow, just in theory, if I am flying a 737 and my 15,000lbs of pax all jump in the air at the same time
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It's a nice physics problem. I'll explain it step by step, assuming the flight controls always remain adjusted the same way, as they were originally in straight and level flight.

Break it up into steps:

First, before they jump, in straight and level flight, the airframe has a zero net force in the vertical direction (lift up - weight down = 0). No net force, means no acceleration of the airframe (Newton's 2nd law).

Second, as they begin their jump, the PAX apply an extra downward normal force on the airframe that was previously absent. During this brief time interval, the airframe has a non-zero net force in the downward vertical direction (lift up - weight down - extra normal down < 0). So now the airframe will accelerate downward (Newton's 2nd law). Anyone who remained seated would feel it.

Third, once the PAX are no longer in contact with the airframe, the extra downward normal force is no longer present. Besides that, there's one more thing that's different - the weight is now less (as the PAX are now in their own parabolic trajectory in space, not contacting anything, they no longer apply any forces including their own static weight to the airframe). So as long as the PAX are not in contact with the plane, there is a non-zero net force in the upward vertical direction (lift up - reduced weight downward > 0). So now the airframe will accelerate upward. Again, anyone who remained seated during this entire exercise will feel it.

I could continue to when the PAX impact again on the plane, but that was such a long explanation that some readers probably already feel like the girl in the right rear seat of that video.
 
This is a classic word problem. A pilot is hauling a crate of birds and realizes he's over the weight limit. So he comes up with a plan to get all the birds flying in the crate at the same time.

It doesn't work, but it's a fun problem.
 
mattaxelrod said:
This is a classic word problem. A pilot is hauling a crate of birds and realizes he's over the weight limit. So he comes up with a plan to get all the birds flying in the crate at the same time.

It doesn't work, but it's a fun problem.
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Jumping passengers and flying birds are distinctly different physics problems, particularly when the question about the jumping passengers asks what weight (presumably) the wings are supporting during the time period the passengers are in free-fall.
 
This should be trivially easy to demonstrate. Get a couple of people in the bed of a pickup and jump. Put scales under the wheels if you want. The physics don't change when ypu leave the ground, mass is mass, inertia is inertia, etc. The airplane doesn't KNOW it's an airplane. But hey, I'm no professor.
 
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The passengers don't need to be jumping up and down for you to feel it. They can be engaged in much subtler activities.
 
I can't answer the question but I can say that with my 1941 Taylorcraft trimmed for level flight I could lean forward and pick up 5 miles per hour airspeed or lean back and lose it. Thats 10mph variation via weight shift. Also opening the left door caused the plane to turn right and the right door made it turn left. Seems backwards to me but who knows what evil lurks in the hearts and minds of men?
What does this have to do with the original posters question??? not much but about as useful in real world flying.

Frank
 
I slide my seat back in the Skylane on smooth VFR days in really long cruise legs and then have to roll in a little more nose-down trim.

Lots of legroom compared to the commercial cattle cars if no one is sitting in the back seat. :)

On the way back from Nebraska late last year I was playing the game of seeing how long it'd hold an altitude and a heading with no inputs other than me leaning forward and back in the seat.

I couldn't quite lean side to side enough. Had to cheat with rudder pedals once in a while. ;)

I found I was slightly left wing heavy so I stuck my legs over in the passenger side rudder area. That worked pretty good but I had to keep bringing my left foot over to push on the right rudder pedal ever so gently every seven or eight minutes.

The incredible complete lack of turbulence most of the way back was very nice. I had my own easy chair in the sky for an hour or so. Stared down at my landmarks gliding by, mostly airports. Looked real hard to see if I could spot any traffic from 10,500'. Only saw one airplane all day, way down there and of course the constant stream of jet contrails on a stable cold day way above and the occasional kerosene burner making them.

Truly a gorgeous flight that day.
 
TangoWhiskey said:
There would be no difference in the weight the airplane "sees" when all your passengers jump up in the air simultaneously.

They are supported by the air, the air is in the airplane.

Lindbergh (well, Jimmy Stewart in the movie about the same) wondered the same thing, about a fly that was in the cockpit with him in the Spirit of St. Louis. Did the plane weigh less when the fly was buzzing around vs when it wasn't flying...

Mythbusters tested this very scenario. Busted.

It's easier to comprehend when you remember that air acts as a fluid... an invisible fluid.

If you float something in a glass of water, versus putting an object of the same weight at the bottom of the vessel, does the weight of the entire vessel change?

Nope.
Click to expand...
Jumping passengers are not supported by the air.
 
Why don't you just drop the airliner suddenly and everyone including the food carts will float. You'd be really light then!
 
What happens if everyone jumps in the air and the airplane is on a treadmill?

airplane-treadmill.png
 
bigblockz8 said:
That brief second= lighter aircraft correct?
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Perhaps but like someone noticed the floor of tha aircraft will also be displaced down a the same time so aircraft will also be 'heaver' for a brief moment, I suspect the end result will be null. Whatever passengers will do inside the center of gravity of the whole system will not change its trajectory even an inch, this is according to Mr.Newton.
 
NoHeat said:
Second, as they begin their jump, the PAX apply an extra downward normal force on the airframe that was previously absent. During this brief time interval, the airframe has a non-zero net force in the downward vertical direction (lift up - weight down - extra normal down < 0). So now the airframe will accelerate downward (Newton's 2nd law). Anyone who remained seated would feel it.
Click to expand...
Don't forget that the downward force from the jumping is unlikely to be directly over the center of lift. So the jumping will create a pitching moment as well. Assuming the center of jump force is aft of the center of lift, this will create a pitch up force, countered almost immediately by a pitch down force while the jumpers are mid air.

Reminds me of the "hazing" submariners give/used to give new watch officers: having five or six crew members run from bow to stern and back again. The watch officer would chase the elusive bubble until he caught on...

-Skip
 
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