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motorheaddown

What if Baumgartner pulled high?

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I just sat through a high altitude training class sponsored by the military (I'm a civilian), and they claim pulling at higher altitudes results in higher G-loading at opening due to increased fall rate. I don't buy it. Dynamic pressure (i.e. what the body feels) is constant throughout the jump even though terminal velocity changes as a function of altitude. Consequently, I'd think the canopy should open normally with the same G-loading at any altitude once terminal velocity is reached.

Am I right?

Thanks,
-scott

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skydiverek


Do we know if the stated values in that post are true? In physics many many times things do not change linearly. So I am wondering if is true that if freefall is X times faster at altitude Y, then canopy descend rate is also X times faster. I guess it is.

I am also wondering if opening times will be the same, since air density is much lower, and fill the canopy cells with enough molecules might require more time. My assumption is that the extra time needed (at a constant speed) to inflate a canopy at a high altitude is compensated by the fact that the fall speed is not the same.

To summarize, I think the post is correct, but I am not an expert in fluid dynamics, and I would like to know if there have been experiments about that, or if an expert in fluid dynamics can confirm these assumptions.

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Deimian

I am also wondering if opening times will be the same, since air density is much lower, and fill the canopy cells with enough molecules might require more time. My assumption is that the extra time needed (at a constant speed) to inflate a canopy at a high altitude is compensated by the fact that the fall speed is not the same.


The air density is lower but you're falling faster. At terminal, your dynamic pressure (and therefore indicated airspeed) is going to be the same, so you're still moving by the same amount of molecules in that time. However, your true airspeed is higher so you have more kinetic energy to bleed off during the opening process resulting in a higher total shock.

Quote

To summarize, I think the post is correct, but I am not an expert in fluid dynamics, and I would like to know if there have been experiments about that, or if an expert in fluid dynamics can confirm these assumptions.


There has been a good amount of testing done, starting with the Army & Air Force post WWII through the 60s-70s with NASA joining in for the latter portion with their development of canopies for various spacecraft, landers & crew modules. I don't have specific references on hand but if I get a chance I can see if some of the historical work is online somewhere.

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excaza

I don't have specific references on hand but if I get a chance I can see if some of the historical work is online somewhere.



A couple sources are the Irvin Recovery Systems Design Guide AFFDL-TR-78-151, and the Parachute Recovery Systems Design Manual by Theo Knacke for the US military. Both are online... somewhere, and have been mentioned on dz.

Attached is a pdf I made of the Altitude effect section of the more recent Knacke manual.

It isn't ideal for our purposes but gives some idea of altitude issues.

The rate of increase of opening force with altitude does depend on the design of the parachute, and whether the load is light (like a person) or heavy (like a bomb) relative to the size of the parachute. The research shown only applies to unreefed round canopies. I would imagine that the reefing of ram air parachutes would decrease the sensitivity to altitude, but would not change the basic principles (like higher true speed and energy when up high) or that higher altitude openings get harder.

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Deimian


Do we know if the stated values in that post are true? In physics many many times things do not change linearly. So I am wondering if is true that if freefall is X times faster at altitude Y, then canopy descend rate is also X times faster. I guess it is.

Original models were with high drag/low lift round canopies, so that assumption is pretty defensible. Even square canopies, as they are deploying with brakes set, are at an angle of attack way past their stall AOA, and are therefore acting mostly as drag devices, not lift generating airfoils.

Quote

I am also wondering if opening times will be the same, since air density is much lower, and fill the canopy cells with enough molecules might require more time. My assumption is that the extra time needed (at a constant speed) to inflate a canopy at a high altitude is compensated by the fact that the fall speed is not the same.

To summarize, I think the post is correct, but I am not an expert in fluid dynamics, and I would like to know if there have been experiments about that, or if an expert in fluid dynamics can confirm these assumptions.

My explanation was qualitative, but not exactly quantitative. Parachute opening rate seems to be related to indicated airspeed, so the "low density" argument is countered by the "high true airspeed" factor.

I'm certainly not a fluid dynamic expert, but have read of the hard openings due to premature deployments on HALO jumps. The physics were explained in that article and seem very common sense to me too. :)

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pchapman

*** I don't have specific references on hand but if I get a chance I can see if some of the historical work is online somewhere.



A couple sources are the Irvin Recovery Systems Design Guide AFFDL-TR-78-151, and the Parachute Recovery Systems Design Manual by Theo Knacke for the US military. Both are online... somewhere, and have been mentioned on dz.

Attached is a pdf I made of the Altitude effect section of the more recent Knacke manual.

It isn't ideal for our purposes but gives some idea of altitude issues.

The rate of increase of opening force with altitude does depend on the design of the parachute, and whether the load is light (like a person) or heavy (like a bomb) relative to the size of the parachute. The research shown only applies to unreefed round canopies. I would imagine that the reefing of ram air parachutes would decrease the sensitivity to altitude, but would not change the basic principles (like higher true speed and energy when up high) or that higher altitude openings get harder.

That's an awesome reference, thank you!

Now, if you pay attention to figures 5-59 and 5-60, it looks like the opening force indeed does increase with the altitude, but no linearly. For instance, in figure 5-59, 28-FT, nylon: At 7000 feet the opening force is around 1600 pounds. At 40000 feet (~5.71x 7000 feet), the opening force is somewhere around 5200 pounds (~3.25x). The 28-FT silk chute opening force is somewhere around 4.1x.

I was just curious, now my curiosity has been satisfied, thank you!

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Jees, es super-hi, you never pull above fi thousan feet.
He could have die that day.
I'm standing on the edge
With a vision in my head
My body screams release me
My dreams they must be fed... You're in flight.

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You are totally wrong. The increase in speed would blow the canopy apart. It is a good thing Felix didnt pull high. At past the speed of sound, he wouldn't have had a chute ( except for the reserve. I have travelled at high speed from 7 miles up, and I would hate to have to use either chute at anything over 20,000 ft. That is when you get ground rush, and a chute wouldnt open in one piece at that high. I have been to 43,000 in a chamber, but they didnt have any way to simulate an opening shock at that altitude, but the did simulate the freefall , rapid descent.When jumping at Quito Ecuador, we went to 17,500 feet to get a 30 second delay, so with the land at 9500 ASL, we would be pulling at another 2000 ft above that. No noticeable difference than jumping in Ontario.




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motorheaddown


Interesting... Looks like I'm wrong.

Thanks,
-scott

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I just sat through a high altitude training class sponsored by the military (I'm a civilian)




This should have given you your first clue. They generally don't just make up training information.

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CSpenceFLY

This should have given you your first clue. They generally don't just make up training information.


No sense discouraging inquiry so casually. There is nothing wrong with wanting to understand what you're being taught. Though this isn't one of them, there are more than a few examples of modern scientific dogma that have gone quite far with little to no backing. Military training is not exempt.

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