Why do tandem jumpers use a drag chute? If I recall my physics properly, they should fall at either the same rate (in a vacuum) or slower rate as a single jumper due to the added drag of the 2nd body so why use a drag chute? Maybe to help stabilize them?
TIA.
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The bodies are falling in a manner that does not double the drag. That's my guess. Stabilizing also prob... No telling what the newb is gonna do when they start falling.

Agreed. I'm saying 2 bodies will increase the drag (not double it). I can see stabilizing the fall but I've always heard the use of the drogue chute is the decrease the fall rate which to me sounds incorrect.

Update: Even Wikipedia (not the best source, I know) indicates the purpose of the drogue chute is to slow down a tandem jump. <img src="/forums/images/graemlins/confused.gif" alt="" />

Basic physics isn't my strong point, and fluid dynamics even less, but I can tell you my test cases from about 700 solo jumps, and 150 tandems as the instructor in command.

As you jump your body is falling through a volume of fluid, in this instance air. There is force from the skydiver moving through the volume of air, let simplify this and call it wind resistance.

Our terminal velocity is when our acceleration due to gravity equals the force from falling through the air.

You can alter terminal velocity by changing mass, or you can alter terminal velocity by changing wind resistance. We put weight belts with lead shot on skinny girls with no rear ends so that they can fall at a rate faster than their normal terminal velocity for formations with other people. We also alter our wind resistance by bring in our arms and legs, or changing our altitude (belly to earth versus head down).

Even though we are falling through the volume of air, it as if we have wind coming up from the earth. We call this relative wind, but it isn't actually wind like an updraft or upper winds. This is why you can simulate free-fall in a vertical wind tunnel. Blowing actual air past a body can impart the same force as falling through a body of air on a real jump.

With no drogue tandem terminal is much faster than your typical solo skydiver terminal velocity, fast enough to have opening shock that can injure the jumpers and blow up equipment. The reason being is that we have on much more mass than a solo skydiver, but only slightly more surface area exposed to the relative wind since we are attached front to back.

I can free-fall stable without it, but it does help stability. This is only a secondary purpose of the drogue. A tandem instructor needs to be able to free-fall stable with no drogue, you need to be stable when you deploy it so that it doesn't tangle with the pair.

Here is a second actual proof that I have done in free fall:
If I jump with a regular regulation tennis ball it falls way to slow and then second I let go of it I fall away from it. If I put a small slit in a tennis ball from the same container and fill it with lead shot I can add enough mass to get it to fall at a speed that a skydiver can match. Same surface area, same wind resistance, but the extra mass from the lead shot leads to a much faster terminal velocity.

Thanks Doug. The call helped as well. It makes sense to me that the divers mass x gravity will affect the amount of force pushing the air out from under the skydiver(s). More mass = greater force pushing the air allowing a heavier sky diver or pair of divers to fall faster.

For others reading... yes I know in a vacuum 2 things will fall at the same speed regardless of weight or their dimensions/drag. <img src="/forums/images/graemlins/kewl.gif" alt="" />

This begs two questions----

1) Why would you jump out of a perfectly good airplane?
2) Why would you jump out with a bed sheet?

1. You haven't seen our airplane.
2. We have upgraded to nylon sewn by hippies in basements.

A skydiver exits the airplane with his parachute on. He falls at a terminal velocity of about 120mph. He opens the parachute (note that his weight did not change, but his cross sectional density is drastically decreased) and now falls at about 17mph (I take my figures from an old Switlick silk parachute from ww2). The cross sectional density of tandem jumpers is not quite the sum of the two weights, as the air flows around the lower jumper and impinges on both to some extent, but the net result is a higher csd. In parachute training, they sort the jumpers by size as much as possible when doing a mass jump, heaviest out the door first, to avoid the big heavy guys from landing on and collapsing he long floating light guys' chutes on the way down.

Air resistance gets in the way of a lot of newtonian physics. This is one case. Artillery ballistics is another. The fall of shot can not be predicted by newtonian physics if you are not in a vacuum. The math is very hairy. The Navy invented mechanical analog computers using cams and rollers to solve the problem. Look up Siacci and Vernet.