Parachute Loss of Lift

[Jaybird180]

I was talking with a coworker who was once a parachutist in the 82nd Airborne and he said in conversation that if another parachutist comes above you, it causes a loss of lift and you drop (dangerously) fast.

I asked how. He said he didnt know.

I theorize that the area above a normal canopy is a low relative pressure and under the canopy is high, thereby producing life similar to an airfoil (yes, I am aware that Author, Wolfgang Langewiesche disputes this as the sole reason an airplane flies). I hypothesize that the jumper at higher altitude caps the low pressure area, increasing the pressure until both sides of the canopy equalize, causing the loss of lift.

Can anyone shed some light on this?

 

[Jim Logajan]

From the Army Field Manual:

Stealing Air. A descending parachute causes an area of partial air compression immediately below the canopy and an area of partial vacuum and descending turbulent air above the canopy. This turbulent air extends about 50 feet above the canopy.
(1) A parachute falling into an area of partial vacuum (from a parachute below) does not capture enough air to stay fully inflated.

At the speeds that parachutes travel, air is acting effectively as an incompressible fluid. So the Army Field Manual references to "compression" and "vacuum" appear to be conceptually incorrect in trying to understand what is going on. Their reference to "turbulent air" gets closer to the truth - the air above a descending chute has inward, downward, and outward flows, and it is these flows that would affect anything above it. Typically there is a bunch of air above the canopy that is descending with it that causes the problem.