Real-World Versus Model Parachutes
The issues designers of real-life parachutes worry
about are sometimes the same and sometimes different
from the ones your students face.
Steering is something the students
might like to do with their chutes, but have no means
to do. Jumpers can deliberately spill air out of one
side of the parachute by means of cords attached to
the skirt of the chute. This makes the parachute fall
faster, but it allows the jumper to guide the direction
of travel, rather than simply going in the same
direction that any wind blows.
Deploying a packed 25 kg bundle of
cloth and cords into a fully inflated parachute is critical
in real life but not with your students -- many open
up their chutes before dropping them. Some real parachute
systems use a smaller pilot chute to pull out the main
Speed and load are
key differences between the performance of model and
real parachutes. People in free-fall drop from around
54-80 meters/sec (120-180 mph) -- their speed after
chute deployment slows down to 3.7-6.7 meters/sec. The
model parachutes your students will build fall at a
rate of 0.8-2 meters/sec or faster. Scaled to the size
of the chute, this means the models are proportionally
falling faster -- for a half-meter-wide model, this
comes to 1.6-4.0 diameters per second, whereas for a
7-meter-wide real chute, the real-chute descent speeds
above are more like 0.5-1.0 diameters per second.
This is not surprising, considering that air drag per
unit area is a function of the square of the speed --
more than 10 times as great at 6.7m/sec as at 2 m/sec.
If you were to make a model by scaling down both a chute
and its human passenger by the same factor with respect
to their linear dimensions (height, width, etc), for
example by a factor of 10 -- so that the chute area
decreased by a factor of 100 and the passenger weight
by a factor of 1000, and the loading per unit area of
chute decreased by a factor of 10 -- the descent rate
would not decrease by a factor of 10 but only by the
square root of that, or 3.17; so that in diameters per
second the model would descend 3.2 times as fast as
the real chute.
Moreover, the specifications for real parachutes would
never ask you to make something that descends as slowly
as the models, because the square-of-the-speed relation
comes into play here too -- to give a terminal speed
twice as slow, a canopy would have to have four times
as much area, which means that when it first opened
at the human’s free-fall speed, it would exert
four times as much force on the passenger -- a human
user would be injured or killed by slowing down too
abruptly, and in trying to bear the forces needed to
slow down the parachute system quickly the parachute
itself could rip and tear, which would lead to disaster
for the user.
Another difference that size makes involves multi-layered
parachutes. For a model parachute, the turbulence of
the bottom canopy would not interfere as much with a
second layer. For full-scale chutes this turbulence
extends a much longer distance downwind from the bottom
chute. This would make it impossible for a nearby canopy
layer to stay inflated as it descends.
Load has implications for stability, at both scales.
Students who make parachutes indoors and then test them
outside can sometimes find that a turbulent
gust of wind can cause the canopy to collapse.
One reason for a designer choosing to use more weight
rather than less is to maintain enough pressure in the
canopy to have it keep its shape when a gust hits it.
This was a major problem with the designs of the chutes
that were dropped from the top of the football stands
during a windy day.