jumping rounds

[ubazzo49 0]

I learned to jump...and do still occasionally jump...round canopies (MC-1B/C, GQ Aeroconical). Never jumped a square other than a few tandems. Anyway, with the bravos and charlies I was told pull down the rear risers as far as I could about 15-20 feet off the ground to help soften the landing. It works (I once made a stand-up quite unintentionally) but I don't know why. Can someone explain the physics/aerodynamics of this?

[markbaur 0]

The descent rate on MC-1 (commonly known as T-10) canopies is fairly low, but most folks jumping them have little experience with constant-speed descents, and a lot of experience with accelerating descents. As a result, they misjudge the timing of the landing, arriving just a bit later than they expect. Like stepping off a curb you didn't realize was there, even a small misjudgment can be jarring.
The reason why pulling the risers down works is because each time you try, you are making another jump and retraining your reflexes. It is the number of jumps, not the riser-pulling, that allows you to make the softer landing.
Pulling down the risers tends to make the parachute descend faster, but there is no appreciable rebound when you release them. The most effective way to use this rear-riser technique is therefore to pull down the risers immediately after the parachute is open, and to let up again right away. Do this on every jump, and pretty soon you'll be able to stand them all up (wind permitting.)
Mark

[jlmiracle 7]

I have never jumped a round. What is a Bravo and a Charlie?

Judy

Be kinder than necessary because everyone you meet is fighting some kind of battle.

[markbaur 0]

Quote

I have never jumped a round. What is a Bravo and a Charlie?

A basic T-10 is unmodified, no steering toggles. MC-1B ("Bravo") and MC-1C ("Charlie") are T-10s with holes cut in the canopy to provide steering and forward speed. (MC = Maneuverable Canopy; MC-3 is military Para-Commander.)
What civilian skydivers know as a "T-10" is either a T-10 that's been modified for sport use, with holes cut in like an MC-1B, or really is an MC-1B. Either way: big, round, green canopy.
Mark

[mjosparky 3]

By pulling down on the rear risers you move the apex hole of center thus slowing the vent of air. If the canopy is modified or steeable, you lower the rear of the skirt and raise the front of the skirt. This kills any drive produced by the mods. The result is a small decrease in desent rate. It was a common method of landing sport rounds such as the Piglet canopys.
Sparky

My idea of a fair fight is clubbing baby seals

[markbaur 0]

Moving the apex vent off-center does little to the descent rate. If the apex vent were closed, the air would simply spill out under the skirt. The descent rate under an unmodified T-10 is about the same as a MC-1, even thought the MC-1 has large holes cut in it.
Pulling down the risers does reduce forward speed, but since the speed is only about 5 mph to start with, there is not much effect here either.
Bottom line: the jumps spent learning these techniques are really just jumps retraining reflexes for a constant-speed descent.
Mark

[mjosparky 3]

Pulling the vent off center increses canopy pressure, thus slowing decent rate.
The holes cut in the T-10 is very low, close to the skirt and does not effect the canopy pressure higher up.
Sparky

My idea of a fair fight is clubbing baby seals

[markbaur 0]

The pressure inside a round parachute is the same, whether measured near the apex or the skirt (except for negligible differences caused by Bernoulli effect). It doesn't matter where the holes are; the same amount of air escapes from under the canopy, either through a vent (apex, drive, or steering), or through the big hole at the bottom, that is, under the skirt of an oscillating canopy. Moving the apex vent off-center just means that instead of the apex air going straight up and out, in has a small -- very small -- horizontal vector.
On a modified canopy (MC-1, double-L, 7-TU, etc), closing the apex vent entirely would have little effect on the descent rate, since the vent is small compared with the size of the drive and turn vents. Even on an unmodified canopy, there would be little effect, since the apex vent serves first as a means to allow air in during inflation, and secondarily allows easier manufacture.
Mark

[mjosparky 3]

You win, if it does'nt work now it probaby didn't work then.

My idea of a fair fight is clubbing baby seals