nigel99 501 #26 May 14, 2006 Could you clarify this as I am a little confused. If the air density is lower = higher speed doesn't that mean that the air resistance is the same? I would have expected intuitively that if the air density is lower then you could expect to open at a higher speed without damage?Experienced jumper - someone who has made mistakes more often than I have and lived. Quote Share this post Link to post Share on other sites
obelixtim 150 #27 May 14, 2006 Fair enough, thanks for your reply....I appreciate your reasoning, and I also appreciate your willingness to acknowledge another point of view.....I also appreciate your not going down my throat about being a little abrupt and rude to you...I apologise if thats how I came across......but sometimes a point has to be made rather bluntly to convey its importance.... Anyway, its a good illustration of how information can be slightly distorted.....it happens all the time, and not just to you....often to people with a lot of jump experience who should know better than to perpetuate myths...... Jump numbers alone don't always convey accurately an in depth knowledge of the sport.... Better to look at the time someone has been in the sport.....the old dinosaurs usually are the most accurate, mainly because they've been there, done that.....got the T shirt......and deserve to have their point of view heard...... Its irritating though, to see people trying to reinvent the wheel, or try something that has been discredited many moons ago.....often at painful cost to some individual....and there are plenty of flash Harry's who try to do just that, then take offence when taken to task.... Your attitude impresses me as a person who is willing to take on board a little criticism.....there are not too many people like that in skydiving...believe me..... You'll do OK.......and learn lots with that approach.... Blue skies......My computer beat me at chess, It was no match for me at kickboxing.... Quote Share this post Link to post Share on other sites
dzjnky 0 #28 May 14, 2006 nigel, I'm not an expert, but here's my 2 cents worth... Decreased air density at high altitude would result in reduced drag, as evidenced by greater terminal velocities at higher altitudes, so yes, you would be able to open at somewhat higher speed without damage to your equipment. Unfortunately, that aspect is not significant when compared to other things. [geek alert on] Kinetic energy is related to the square of the speed, so doubling your freefall speed quadruples the kinetic energy that must be dissipated by the opening parachute system. More energy to be dissipated = increased chance of exceeding design limits and damaging the system. Another way to look at it is that when your parachute opens at any given altitude, your speed will decrease by the same percentage. In absolute numbers, however, you are slowing down many more miles per hour, so the G forces involved to slow you down that much will be much greater. An example might help: at "normal altitude" belly-to-earth terminal velocity might be 120 MPH, suppose that under canopy you fall at 10% of your freefall rate. That would mean that vertical velocity under canopy might be 12 MPH, so deployment results in a deceleration of 108 MPH. If you were at high altitude, where your belly-to-earth terminal velocity was 180 MPH, then I would speculate that your vertical velocity under canopy would be 18 MPH. While that is still a 90% reduction in speed, it is also a reduction of 162 MPH. Assuming that the deployment duration is the same, then you will undergo about 50% higher G-forces in the high-altitude deployment. [geek off] I'm sure there are holes in this argument, but I think it is close enough to illustrate the basic physics of the situation. Quote Share this post Link to post Share on other sites
MB38 0 #29 May 14, 2006 Thanks, I really appreciate it. I'm glad this thread has gone the way it has. It's certainly cleared some stuff up for me... and I know I'm not the only person out there who doesn't know jack.I really don't know what I'm talking about. Quote Share this post Link to post Share on other sites
billvon 3,006 #30 May 15, 2006 >If the air density is lower = higher speed doesn't that mean that the >air resistance is the same? Yes - BUT - even if all the timing works out exactly the same during the opening (which it usually doesn't) you are still decelerating from 140mph to 10mph at 16K, instead of 120mph to 9mph at 2K. That means more force overall and a harder opening. Quote Share this post Link to post Share on other sites
