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keithbar

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pchapman


But lets not make this too complex Calvin, or else we'll soon be talking about what happens if the 747 taking off has a treadmill in it facing backwards, with a bunch of pigeons on it, that suddenly flap upward, while holding helium balloons in their beaks. :P



Huh? The OP has one of those questions that can be effectively answered in a single sentence?

Science trolling. :):P

Quote

what happens if the 747 taking off has a treadmill in it facing backwards, with a bunch of pigeons on it, that suddenly flap upward, while holding helium balloons in their beaks.



Hmmm. Ok. Here we go.

Standard 747 takeoff.
treadmill facing backwards(no factor)
(x) pigeons flapping in the Y vector, creating airflow inside the cabin that exerts a force equal to the mass of the pigeons. (no factor, but it will vary over time for the individual accelerations of the pigeons made by their flapping)
helium balloons decreasing the inertial mass of the aircraft.

Answer:nothing

Science answer: It depends on the mass of the helium in the balloons.

The inertial mass and weight of the aircraft does not change if the pigeons are flying under their own power inside the cabin.

As the aircraft accelerates down the runway and the buoyancy-assisted pigeons start to fly inside the cabin, the aircraft will move forward as the birds, no longer being kept in place on the treadmill, will drift towards the back of the cabin (assuming they are not compensating for their visible environment). The helium balloons will exert a force on the pigeons in the vertical AND forward directions due to the buoyancy of the lifting gas and the new pressure/density gradient lines inside the accelerating aircraft. If the buoyancy force from the balloons is equal to the mass of the bird, it would be neutrally buoyant and would stay in place.

A helium balloon that is positively buoyant has to be about 20cm diameter. If you packed the 747 with as many pigeon balloon aircraft that would fit, it would be pretty heavy. But none of them could fly if there was no space left. Even if they could it would not change the problem.

Just be sure you tie down the treadmills:

https://www.youtube.com/watch?v=uIjO0sKBDDw

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Continuing on that tangent, paragliders also have an appreciable amount of inertial mass in the form of air in the cells. This is VERY noticeable when you fly different gliders, especially as some single-surface gliders have hit the market. There are no ram-air cells, so the wing itself loses a couple kg.

This may not seem like it matters but controlling the AOA of the glider is extremely important and an experienced pilot who had never flown a SS glider before will be very surprised at how fast the wing surges/dives and responds to brake input. With more than half the mass of the wing gone, it has no inertia. Fun stuff.

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Well played Calvin.

I applaud with halved coconuts, imported by African swallows.

Nice touch about the density gradient under acceleration, even if minor.

For the thread, that does remind me that for the skydiver under a parachute, the full mass is important for dynamic calculations, while for static forces -- for the forces on the lines -- one should technically subtract the weight of the air the skydiver displaces. We're buoyant in air, although just a tiny bit. Someone online calculated that a typical person's weight on a scale is about 0.2 lbs lighter than their actual mass. (Without getting into gravity variations on earth, an even smaller factor.)

Cool about those new single surface paragliders. The closest I've flown are single surface Paradactyl canopies, and their lift to drag ratios are so low that they are pretty sluggish dynamically anyway, even if they entrain no air within.


(CORRECTION TO MY EARLIER POST: D'oh, I used 2.25 in my calculation for air density kg per cubic meter when it should be 1.25. It's been a while since I engineered this stuff. So the air masses inside a parachute are lower than the original numbers -- but they can still be a few pounds.)

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Calvin19

Continuing on that tangent, paragliders also have an appreciable amount of inertial mass in the form of air in the cells. This is VERY noticeable when you fly different gliders, especially as some single-surface gliders have hit the market. There are no ram-air cells, so the wing itself loses a couple kg.

This may not seem like it matters but controlling the AOA of the glider is extremely important and an experienced pilot who had never flown a SS glider before will be very surprised at how fast the wing surges/dives and responds to brake input. With more than half the mass of the wing gone, it has no inertia. Fun stuff.



Hadn't heard of this, so I went googling: http://www.flyozone.com/paragliders/en/products/gliders/xxlite/info/

Many years ago, there were rumors of SS skydiving squares under development. I was visiting an out-of-state DZ one day and even met two guys who swore they had seen test jumps of one. But no one ever released one, and I've never seen photos, (unless you include the old Barrish Sailwing, and Delta II/Paradactyls).
"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

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ryoder


Hadn't heard of this, so I went googling: http://www.flyozone.com/paragliders/en/products/gliders/xxlite/info/

Many years ago, there were rumors of SS skydiving squares under development. I was visiting an out-of-state DZ one day and even met two guys who swore they had seen test jumps of one. But no one ever released one, and I've never seen photos, (unless you include the old Barrish Sailwing, and Delta II/Paradactyls).



I worked on single surface BASE canopies for two years ('13-'14) for a company in Boulder. Test jumping and all. Fun stuff. We paused the project, needs a LOT more time. I know there are a couple other companies that worked on these, I talked a couple times with them and shared ideas.

My first test jump on it was a Direct-bag slider up from a 182. It sniveled open and flew right off the bat. My words 3 seconds after the slider came down and it was flying stable were "you're fucking kidding me".


It's a different animal. But we made it fly. B|

-SPACE-

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pchapman


For the thread, that does remind me that for the skydiver under a parachute, the full mass is important for dynamic calculations, while for static forces -- for the forces on the lines -- one should technically subtract the weight of the air the skydiver displaces. We're buoyant in air, although just a tiny bit. Someone online calculated that a typical person's weight on a scale is about 0.2 lbs lighter than their actual mass. (Without getting into gravity variations on earth, an even smaller factor.)



Truth.

That is why using the terms "mass" and "weight" as completely separate concepts is important.

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Calvin19

***That is a funky-looking canopy!
Any other photos of it?
Did you land it?



Nope. It flies OK but It collapses very easily. Needs a ton of work.

I suspected they would collapse easily.
On the other hand, how quickly does it recover, since there are no cells to reinflate?
"There are only three things of value: younger women, faster airplanes, and bigger crocodiles" - Arthur Jones.

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ryoder

******That is a funky-looking canopy!
Any other photos of it?
Did you land it?



Nope. It flies OK but It collapses very easily. Needs a ton of work.

I suspected they would collapse easily.
On the other hand, how quickly does it recover, since there are no cells to reinflate?

Very fast recovery/re-inflation. Fast enough to where I considered landing it jump #1. Zero heading change(most important thing by far).

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keithbar

As the o.p. I can say. you guys lost me a while back.:P



No kidding keithbar, I was wondering how your original OP got to the ensuing conversation. What I got from your OP was the great idea of "I'm going home tonight, getting drunk, and weighing my rig." Fricking best idea I'll come across today!!!!!!!!!! Thank you, keep em coming:)

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