Just caught a little bit of a local airshow on TV, and right now they’re showing some skydivers. Now, I understand the basics of how a parchute work. The jumpers are, of course, doing some acrobatics. My question: One of the things that they’re doing is (what looks like) flips - doing something with the parachute so that they do a vertical flip, with the parachute as the axis. This momentarily puts the jumper above the parachute. How does this work? I’d think that it would make the parachute collapse, but apparently, it doesn’t.
Bonus question, as I’m listening to the horrid commentary: When talking about the forces a pilot experiences, what’s the difference between G’s and “negative G’s”?
Part 2: Accerleration forces can be measured in units of “G”, equal to the typical gravitational acceleration on Earth’s surface. 1 G is approximately = 9.8 meters/sec/sec or 32 ft/sec/sec.
Forces have a quantity and a direction. In aviation, positive Gs are defined as the direction from the top of the plane towards the bottom, or from the pilot’s head towards his/her butt in a seated position. There are also side-to-side and fore-aft Gs, but they are, other than in an accident, pretty minimal. So except in technical discussions, people are talking about head-to-butt or butt-to-head forces.
Note these are defined in the reference frame of the aircraft, not the Earth. So an airplane manuevering upside down which is experiencing posiitve Gs is experiencing G forces from the pilot’s head-to-butt, which is the same direction (at the moment) as earth-to-sky.
So in a typical loop manuever, the Gs are positive all the way around since centrifugal forces are always towards the outside of the circle, towards the bottom of the plane.
The same manuever flown with the plane starting out upside down, so the wheels are towards the middle and the top of the tail towards the outside would result in all negative Gs.
Part 1: Not my area. Sorry.
Former skydiver here.
Parachutes do not do flips of the sort that you are describing, however a tight turn definitely LOOKS exactly like that from the ground. Parachutes turn in a tight corkscrew fashion, turning and diving down at the same time (the sharper the turn the steeper the dive), with the skydiver always hanging directly beneath the canopy, roughly perpendicular to the bottom surface. It’s a little hard to describe in text but fiddle with a playing card as the canopy and a pen as the skydiver and you’ll get the idea.
In a steep turn, the leading edge of the canopy can be pointed almost straight down, with the surface of the canopy (your playing card) perpendicular to the earth. The suspension lines will be almost horizontal, with the skydiver at the end of the lines - from his perspective he’s sitting in the harness normally (like you’d look in a swing), but from your perspective his body is also horizontal, parallel to the earth’s surface. Intentionally stalling the canopy and then letting up on the brake toggles will produce a similar dive, it’s quite fun when done at a safe altitude.
My old middling-performance PD-210 could do some pretty fast turns. More modern canopies with zero-porosity fabric, elliptical/semi-elliptical planforms and high wingloadings are faster yet - the electronic automatic activation device I have on my reserve is set so that it won’t trigger at descent rates less than something like 70mph vertically, to avoid accidentally releasing the reserve during a steep diving turn under a fully open canopy.