Here’s a bit more about fluid drag:
Fluid resistance is proportional to the speed v only at very low speeds where the dominant source of resistance is viscosity (the internal friction of fluids)At the speeds of ordinary objects in air (including bicyclists, tossed baseballs, airplanes in flight, and Cirri descending under parachutes), the force of fluid resistance is approximately proportional to v2, the square of the speed.[/list]When a Cirrus of weight W is descending under its parachute at a steady speed v, the downward force of gravity is exactly balanced by the upward force of fluid resistance. Using the v2 formula, this says that
W = kv2,
where k is a coefficient that depends on the shape and size of the parachute and the density of the air. Solving this expression for the descent speed v, we find
v = (W/k)1/2.
In other words, the descent speed is proportional to the square root (also known as the 1/2 power) of the weight W. As a rule of thumb, this means that every 2% decrease in the weight gives a 1% reduction in the descent speed.
This analysis also applies to those situations in which a person descends under a parachute without an airplane attached (amazingly, it does happen!).