Expert: Mark Janus - 12/12/2009

Question
I have noticed that when  calculating the perimeter of rectangular parachute canopies
with different side proportions the one with the least amount of perimeter
is the most like a square which is the also the most symmetrical object. Because of thaqt
I'm wondering why the shape that is the most symetrrical will spill the
least amount of air or have the most amount of drag.In other words why does the shape with the least
perimeter spill the least air, and why is that shape the most symmetrical
shape.For the question please include the involved laws or principles of
geometry, aerodynamics, and or physics. If you know any other sources about
this please tell me so I can read more.


Answer
Basically you have high pressure air on one side trying to get to the low pressure side...  the more "edge" you have, the more opportunity the air has to get to the side (equalize the pressure)
The pressure difference times the "side" area gives the force (opposing gravity)

For a drag chute, the air will only spill over the edge and the "drag" is proportional to the area, minimizing the edge to area ratio would lead you to a circle...  For a given planform area (i.e. the area of the chute), actually a circle gives the least perimeter length...  

A parafoil on the other hand is basically a wing and functionally behaves like one...  it requires forward motion to keep it inflated and actually generates lift (so it's basically a glider)...yet it still operates on a similar principle of pressure difference (between the top and the bottom) and with a parafoil the air does not spill from the front, just from the sides and back....  They are more maneuverable also...

Hope this helps...
mj