(published on 10/22/2007)
The short answer to your question is that, after a certain point, doubling the size of a parachute does not affect the time of fall (or your velocity).
A really amazing experimental explanation of this is here:
https://www.youtube.com/embed/4CdbBHzw8yQ
The parachute very rapidly acquires almost its terminal velocity, reached when the air friction is enough to just cancel the force of gravity. The friction is proportional to the area. If the parachutes are made out of the same cloth, their masses (and hence the gravitational forces) are also just proportional to the area. If there's an extra weight on the parachute, the fall will slow as you make the parachute bigger until the parachute's own mass is large compared to the other mass.
The time is approximately proportional to the inverse of the terminal velocity, so it's approximately proportional to Area/Mtotal = A/(Mp +Mw), where A is the area, Mp is the parachute mass and Mw is the other mass. Once Mp is much bigger than Mw. this reaches a constant. That means that the velocity for a parachute with twice the area (and twice the mass) is going to be the same as for the original parachute. If, on the other hand, the parachute were very light compared to the load, then doubling its area would approximately double the time to fall.
Hope that helps!
John +mbw
(published on 11/13/2010)