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 Post subject: Quadcopter flying inside a box is weightless - common sense
PostPosted: Fri Aug 01, 2014 1:21 pm 
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Location: Somerset, UK
This was a silly argument at work, its obvious to me that if you weigh a box with a quadcopter flying in it, your just weighing the box as the quadcopter isn't sitting on the bottom of the box - its flying.

A couple of people at work insisted the box weighs the same whether the quad is flying or not, so I made a video to test this.



Last edited by jamesb72 on Sat Aug 02, 2014 2:16 am, edited 1 time in total.

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 Post subject: Re: Quadcopter flying inside a box is weightless - silly arg
PostPosted: Fri Aug 01, 2014 6:57 pm 
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LOL

Someone has too much time on their hands i think!!

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 Post subject: Re: Quadcopter flying inside a box is weightless - silly arg
PostPosted: Sat Aug 02, 2014 12:44 am 
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Funny - how could you not agree with your argument ?? The couple of people who disagree woundnt happen to be senior management by any chance ???


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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Sat Aug 02, 2014 6:48 am 
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But now what if that box were being carried inside an airplane? Would the weight of the airplane decrease by the weight of the quad when it lifted off? ;) :lol:

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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Sat Aug 02, 2014 7:59 am 
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Your messing with my limited intellect ! - presumably that would depend on whether the plane was on the ground or flight ?? Hmmmm


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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Mon Aug 18, 2014 6:14 am 
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I'll bite. The weight of the box containing the quad should be the same regardless of the method that the quad is supported inside it. Weather the quad is supported by a cushion of air or its own feet, the mass of the system is the same.

I can't explain the results of that experiment though, where the measured box mass seems to decrease as the quad is flying. Seems counter intuitive to me.


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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Tue Aug 19, 2014 4:50 am 
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Subsonic wrote:
I'll bite. The weight of the box containing the quad should be the same regardless of the method that the quad is supported inside it. Weather the quad is supported by a cushion of air or its own feet, the mass of the system is the same.

I can't explain the results of that experiment though, where the measured box mass seems to decrease as the quad is flying. Seems counter intuitive to me.


I agree that the mass of the system is the same, so the reading of the scale should be the same when averaged over time.

However, think about a cat in a box. Again, the total mass is the same but the cat can exert forces that move the box and/or apply forces that can be felt by someone holding the box. So I could see fluctuation in the scale reading as the center of mass accelerates. Maybe it has something to do with how the scale reads a fluctuating signal? (always min point?)


Mythbusters tested something similar with birds and couldn't detect a change in weight:

http://mythbustersresults.com/episode77

A truck carrying birds will be lighter if the birds fly around than if the birds sit/stand.

busted

Adam and Jamie constructed a large box and placed it on top of scale and then filled it with captured pigeons. Then, the Mythbusters activated a special contraption that would force the pigeons to fly into the air, but they could not detect any discernible difference in the weight of the box. They then placed a model helicopter inside the box and had it hover above the ground, but this method also failed to produce any results. The Mythbusters theorized that the air being displaced by the birds’ wings and the helicopter rotors was pressing down the box, which is why there was no change in the overall weight.


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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Mon Sep 01, 2014 6:44 pm 
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That's it? We're done? Sheesh, if this was posted in rcgroups, it would be 20 pages long with constant what-ifs, alterations, conditions, etc until the final argument no longer represented the original question! Of course, I refer to the infamous airplane on the infinite treadmill discussion (why did I waste my time responding in that thread?). I'm surprised nobody asked if placing the quadrotor upside-down and applying full-throttle would make the closed box appear to weigh more!

Obviously, Chris and Bobby are vetting the membership here. Oh sure, they claim that it's because of Go Daddy that the new membership requests are delayed. ;)

- Jim


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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Mon Sep 01, 2014 8:30 pm 
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Jim, I like to say that we have the "kinder, gentler" membership here.
Or to put it another way, no riff-raff allowed! LOL :lol:

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 Post subject: Re: Quadcopter flying inside a box is weightless - common se
PostPosted: Tue Sep 02, 2014 7:45 pm 
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Going have to try this. I think the copter is lifting against the lid. You can see it suctioning to the lid after takeoff. Need to try a bigger / taller box. In the case of a cargo carrying airplane, if you invent an anti gravity ray and lift all the cargo, the video suggests that you will save fuel. Something does not add up with that math.

Taking the thought a little further. The Birds in the Myth-busters and the Micro quad have stored kinetic energy as they sit still and allow gravity to pull them down against the earth / scale. When that kinetic energy is converted to flight to fight gravity the object reverses the effect of measuring mass on a scale. The video makes more sense. Under the conservation of energy, the anti-gravity ray would expend more energy then you would save in fuel cost. (Of course that depends on the efficiency of the systems generating the power so it may still be possible; after we invent the anti-gravity ray. :-) Ooooo the possibilities.


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