Parachute Loss of Lift

Jaybird180

Jaybird180

Final Approach
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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?
 
I didn't know that. I would have guessed the parachutist at the top would start accelerating faster; sorta like drafting in car racing.
 
I would think that if you got that close the lower jumper would loose lift due to the legs in his canopy.
 
Jaybird180 said:
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.

[snip]

Can anyone shed some light on this?
Click to expand...

Dunno. Will comment that based on a story from my faculty adviser (grad school), both the upper and lower parachutists drop at similar speeds.

His story was that they were on a demo jump out of a Huey and the jumpmaster had him go too soon after the previous guy. He said the training was to "walk" off the 'chute below and that it worked. His other comment was that he realized that he was told to jump too soon but his training was to go when told to so he went anyway. He too was 82nd airborne.
 
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Parachutes do leave a slight wake. The wake from a round is mainly vertical since they have very little forward drive. Squares leave a wake with more horizontal component. You can feel this if you get directly in it, but it you'd have to be REALLY close for it to create any significant problem.

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Maybe if they are using a circular chute and going straight down. The guys in the image below weren't going down particularly fast, but those parachutes have a good bit of forward movement as well; note the flag streaming behind the upper soldier.

77859509.jpg
 
In the duscussion he was referring to circular chutes
 
Back in the 70s when people still jumped round canopies, we were taught not to fly directly over another parachute for the reason stated. However, having done it, I can tell you that the effect is minimal unless you're close enough to touch the canopy below you with your feet.
 
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. The top parachute may partially collapse and drop below the other parachutist’s canopy until the force of unaffected air reinflates it. Then this canopy, being lower, “steals” the air from the canopy above; this causes the canopy above to partially collapse and the jumper to drop past the lower canopy. This “leap-frogging” action continues unless corrective action is taken by the parachutist.

From a 2012 Army accident report:

Suspected Cause of Malfunction - Incident: Both jumpers lost situational awareness of each other while preparing to land. As a result, the higher jumper drifted over the lower jumpers T-11 main canopy at approximately 50' AGL causing the higher jumpers T-11 main canopy to lose lift capability. Once the higher jumpers T-11 main canopy lost lift capability, he slid to and through the lower jumpers T-11 main corner vent causing the Low Altitude Corner Vent Entanglement.

If you see another jumper approaching, immediately attempt to SLIPAWAY!
 
BTDT many times. The description slipaway posted is accurate. Round parachutes are based on drag only and you have to be very close for it to occur (5-10 feet or so). It is really only dangerous near the ground or if you get entangled, but that is enough of a risk it's best to stay apart.

Square or elliptical parachutes are very different and are basically inflatable wings that fly based on the same rules of aerodynamics airplane wings do.
 
slipaway said:
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.
Click to expand...

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.
 
Jaybird180 said:
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?
Click to expand...

When the upper chutist's feet collapse the lower chutists canopy, you drop real fast...
 
slipaway said:
.

From a 2012 Army accident report:

Suspected Cause of Malfunction - Incident: Both jumpers lost situational awareness of each other while preparing to land. As a result, the higher jumper drifted over the lower jumpers T-11 main canopy at approximately 50' AGL causing the higher jumpers T-11 main canopy to lose lift capability. Once the higher jumpers T-11 main canopy lost lift capability, he slid to and through the lower jumpers T-11 main corner vent causing the Low Altitude Corner Vent Entanglement.
Click to expand...

I showed my coworker this thread. He said the lower jumper has right of way.
 
Cap'n Jack said:
Maybe if they are using a circular chute and going straight down. The guys in the image below weren't going down particularly fast, but those parachutes have a good bit of forward movement as well; note the flag streaming behind the upper soldier.

77859509.jpg
Click to expand...

Ah, another gold and black canopy of YOUR United States Army Parachute Team, the Golden Nights.
 
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