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Skydiving General Ignorance

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mr2mk1g
93554.  Fri Sep 15, 2006 7:35 am Reply with quote

It's quiet on here this week so here are some random, Generally Ignorant ideas that come to people on the subject of skydiving.

(ok so perhaps the average person might not of heard of many of these before but almost any jumper has been bombarded by such questions, and similar, on many many occasions by people who don't jump - perhaps they are obvious to most, but maybe some will catch a couple of you out, and besides it's quite here this week).


Q1. What is the terminal velocity of a skydiver?


Q2. How far back up does a skydiver go when his parachute opens?


Q3. True or false: Skydivers do not have to breathe in freefall because O2 molecules are hitting them at such high speed and regularity that they are absorbed directly through the skin.


Q4. True or false: When jumping out of a plane, you feel your stomach turn over, just like when riding a roller-coaster. Then answer why?


Q5. What is the statistical probability at which parachutes fail to correctly deploy? (no, you're not expected to know but it would be interesting to hear what the average person thinks the probability is; it might surprise you).


Q6. What should a skydiver do if their reserve parachute fails? (ok, so this one I guess is more of a joke).

 
Ameena
93559.  Fri Sep 15, 2006 7:51 am Reply with quote

Hmm lemme have a go...I know bog all about skydiving so these are mostly guesses I've made up ;).

Q1. What is the terminal velocity of a skydiver?
- We got told in Physics, back at school, that Terminal Velocity is 120mph. Whether or not this also applies to skydivers I have no idea.

Q2. How far back up does a skydiver go when his parachute opens?
- Umm...does he?

Q3. True or false: Skydivers do not have to breathe in freefall because O2 molecules are hitting them at such high speed and regularity that they are absorbed directly through the skin.
- False, surely...?

Q4. True or false: When jumping out of a plane, you feel your stomach turn over, just like when riding a roller-coaster. Then answer why?
- Umm...false, because as you jump, you've still got the plane's momentum so are still moving "forwards" as well as beginning to go down?

Q5. What is the statistical probability at which parachutes fail to correctly deploy? (no, you're not expected to know but it would be interesting to hear what the average person thinks the probability is; it might surprise you).
- I have no idea, but I'll guess at umm...errr...one in every five hundred fails to open. Or a thousand. Or some other big number...unless it's surprisingly often, in which cast one in fifty ;).

Q6. What should a skydiver do if their reserve parachute fails? (ok, so this one I guess is more of a joke).
- Panic and go "Aaaaahhhh!"...or try and swim through the air to grab on to someone else ;).

 
Southpaw
93584.  Fri Sep 15, 2006 8:38 am Reply with quote

Quote:
Q2. How far back up does a skydiver go when his parachute opens?


I don't think he goes back up, but if you're next to a skydiver (or the cameraman is if you're watching it on tv) when he opens his 'chute, whereas you keep yours closed, it will look like he shoots up - it's just your relative motion.

 
grizzly
93635.  Fri Sep 15, 2006 10:24 am Reply with quote

Quote:
Q1. What is the terminal velocity of a skydiver?
- We got told in Physics, back at school, that Terminal Velocity is 120mph. Whether or not this also applies to skydivers I have no idea.


I don't know what the terminal velocity is but I do know that you reach it within the first 5 seconds of freefall at that for the rest of the time you are generally decelerating as the air becomes denser.

 
Tas
93638.  Fri Sep 15, 2006 10:31 am Reply with quote

Quote:
Q6. What should a skydiver do if their reserve parachute fails?


Aren't you supposed to put your arms out (in a star shape) to increase air resistance and thus slow your descent?

(Although landing like that may be a tad painful, I'd have thought).

There is a QI bit somewhere about an RAF Lancaster crewman who bailed out without a 'chute (as it had been destroyed by flak). The Lanc' was on fire, so he thought it'd be a better way to go. He ended up falling through a tree, and landing in a snow drift. Broken leg and arms (along with cuts and bruises) from a fall of 10000 feet plus. Not too bad, I guess!

:-)

Tas

 
mr2mk1g
93783.  Sat Sep 16, 2006 5:25 am Reply with quote

Ameena wrote:


Q1. What is the terminal velocity of a skydiver?
- We got told in Physics, back at school, that Terminal Velocity is 120mph. Whether or not this also applies to skydivers I have no idea.


This answer would not only have the klaxons going, but of all the numbers out there, this is the one that Stephen would have on a card. [grin]

 
mr2mk1g
93784.  Sat Sep 16, 2006 5:29 am Reply with quote

Quote:
Q4. True or false: When jumping out of a plane, you feel your stomach turn over, just like when riding a roller-coaster. Then answer why?

- Umm...false, because as you jump, you've still got the plane's momentum so are still moving "forwards" as well as beginning to go down?



Quote:
Q2. How far back up does a skydiver go when his parachute opens?

I don't think he goes back up, but if you're next to a skydiver (or the cameraman is if you're watching it on tv) when he opens his 'chute, whereas you keep yours closed, it will look like he shoots up - it's just your relative motion.


These two answers are absolutely right I am always surprised however by how many people believe the opposite.

 
BondiTram
93793.  Sat Sep 16, 2006 6:22 am Reply with quote

Q1. Terminal velocity is zero. Everyone has to stop sometime.

Q5. One in 50. Which is why I won't do it, except just after someone has plummeted.

Q6. Say shiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiit. And hope for the best.

 
mr2mk1g
93824.  Sat Sep 16, 2006 10:53 am Reply with quote

BondiTram wrote:
Q1. Terminal velocity is zero. Everyone has to stop sometime.


Nope, it's not a play on words.

 
grizzly
93825.  Sat Sep 16, 2006 11:01 am Reply with quote

mr2mk1g wrote:
BondiTram wrote:
Q1. Terminal velocity is zero. Everyone has to stop sometime.


Nope, it's not a play on words.


A shade short of the speed of light? Well if you are falling into a black hole perhaps.

 
Ameena
93834.  Sat Sep 16, 2006 11:58 am Reply with quote

Terminal velocity is the highest speed you can reach when falling, at which point you're prevented from accelerating by air resistance...well, that's what I understand it as anyway.

 
dr.bob
94054.  Mon Sep 18, 2006 4:31 am Reply with quote

Surely terminal velocity will depend on your altitude. It's the point at which the acceleration due to gravity is exactly countered by the deceleration due to air resistance. Since air resistance is proportional to the density of the air, and the density of air changes with altitude, the terminal velocity will also change with altitude.

If you're being very picky, you could argue that gravitational acceleration will vary with altitude as well, but not nearly as much as air density.

Another thought. Presumably your air resistance is also changed by your position when falling. If you spread out your arms and legs in the classic sky diver position, I'd imagine your air resistance is greater than if you tucked in your arms and legs and fell straight down, so that would also affect your terminal velocity. And, of course, once you deploy your parachute, your terminal velocity will change rather dramatically.

 
mr2mk1g
94084.  Mon Sep 18, 2006 7:29 am Reply with quote

ding ding ding ding ding

And there we have the correct answer - on both points no less.

As you point out; as air density alters a given object's terminal velocity, all things being equal, a skydiver will in fact be gradually slowing down as they fall towards the earth.

Skydivers therefore make use of two different speed indicators "true airspeed", which would show a gradual slowing down and "skydiver airspeed" which automatically adjusts for this known effect and provides a constant speed readout so that comparisons may more easily be made.

Your second point touches on the fact that terminal velocity a function of an objects mass vs. its drag. In this instance that means how much surface area a skydiver presents to the wind (as it's not that easy to rapidly loose or gain weight during a short freefall).

In the classic skydiver spread-eagle position this would equate to somewhere around 120mph, (hence the klaxons and card higher up), though as weight also comes into it, this speed would be different, (sometimes substantially), for a 9stone girl and a 14stone bloke.

Then you have skydivers who like to fall in positions such as this: http://www.skydiveelsinore.com/calendar/chicksrock/images2/headdown-2.jpg who are probably falling at somewhere around 160+mph. The world speed skydiving record is in excess of 325mph.

In fact, almost all competent skydivers would be able to make use of a range of fall rates from anywhere as low as 90mph right up to 180mph, with particularly skilled individuals being able to significantly exceed even these figures.

Certainly any mention of Galileo, cannonballs or feathers would have also produced a klaxon as, of course, the well known experiment he is supposed to have conducted only applies to objects falling in a vacuum.

 
grizzly
94086.  Mon Sep 18, 2006 7:49 am Reply with quote

What's the name of the specially designed suit that skydivers can wear in order to propel them forward at significantly higher speeds whilst falling (it has webbing between the arms and legs)?

I expect that this could achieve a much lower downward speed (although their airspeed would be nearly as high, just in a different direction).

 
mr2mk1g
94089.  Mon Sep 18, 2006 8:05 am Reply with quote

grizzly wrote:
What's the name of the specially designed suit that skydivers can wear in order to propel them forward at significantly higher speeds whilst falling (it has webbing between the arms and legs)?

I expect that this could achieve a much lower downward speed (although their airspeed would be nearly as high, just in a different direction).


They are called a "wingsuit". http://www.blahimmelblogg.se/images/20050908172525_wingsuit2.jpg

There are various makes and models from a number of manufacturers. The technology behind them is currently moving forward very quickly and each year there are new suits released which allow even greater performance than had hitherto been achieved.

Today an average skydiver could, with a little practice, achieve average forward speeds in the region of 90mph and have an average descent rate as low as 40mph. They would also be able to achieve level flight or climb for very short distances under the right conditions.

Recently wingsuit flyers managed to fly from North Africa to Gibralta and at present, there are even a couple of teams working on landing a wingsuit in the near future!

 

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