A sheet of plywood is likely to ‘flutter’ though - and this would probably exert sudden forces on your hands.
The O.P. said
If you hold two corners in each hand, you are holding all four corners.
By the time you’ve knotted the corners securely under your crotch, what you’ve got left is little more than a sack, and you’re inside it.
To add to this, the very smallest parachutes, the high-performance square ones, are specifically designed to not just create drag but to act as an airfoil or wing (IOW convert downward vertical motion into survivable & controllable horizontal motion). Also if you had a bed sheet approaching sufficient size to generate sufficient drag, the forces on it would instantly cause bed sheet fabric to shred.
Lesson: Don’t use a parachute from the Acme™ Corporation…
Nope. The inflation of the sheet when it catches air will rip it right out of your hands. You’ll have to reach a certain speed for the sheet to inflate and opening shock will be significant. I don’t know if anyone has the hand or arm strength to hold on.
Plus as others have pointed out a bedsheet is too small. Fubaya was right, a lot of experienced skydivers use parachutes in the 100-200 sq. ft. range (When I was an active skydiver, my main canopy was 190 sq. ft.). A bedsheet would have to be quite a bit larger than it is, and made out of something strong like nylon to survive opening shock.
I could see this happening too in Singapore with an Indonesian maid.
I’m sure I remember seeing a video of someone jumping from one of the ‘twin towers’ holding onto something ‘sheet like’?
Anyway it happened the way you say, in that it got ripped out of their hands almost immediately.
Tested that as a kid. Doesn’t work. 
aids in the cleanup -
Not as dense as those two! 
I used to skydive a lot and that’s a pretty accurate range - with older uncoated F111 nylon you’d go for a loading of around 1psf for the average jumper. When coated zero-porosity fabrics hit the market and canopies could handle much higher wing loadings, manufacturers began moving to elliptical and semi-elliptical wings that were loaded up around 1.5psf (and probably higher nowadays with very experienced canopy flyers). The smallest main canopy on the market when I stopped jumping (years ago) was 77 square feet which is incredibly tiny.
As noted above, ram-air canopies are actually inflatable wings - lots of forward speed and plenty of lift (compared to older rounds and the like) - so you’re not just dropping straight down.
I saw that too. Within seconds it was ripped from his hands. Probably a table cloth.
And property values are sky high.
It didn’t work out for her because she was a housemaid, not a nanny. :smack:
Yeah, it’s insensitive, but I couldn’t possibly be the only one who thought of it.
I’m not sure what that means. I think the reason silk and later nylon were chosen is the strength to weight ratio is much higher. Cotton has to be fairly thick and thus heavy to hold together, meaning a much larger parachute for the overall product.
A canvas sail would be strong enough, but weigh more than the person it is holding. And folded would be bigger than the person trying to carry it.
2 corners each hand = 4 corners.
[QUOTE=Valgard]
I used to skydive a lot and that’s a pretty accurate range - with older uncoated F111 nylon you’d go for a loading of around 1psf for the average jumper. When coated zero-porosity fabrics hit the market and canopies could handle much higher wing loadings, manufacturers began moving to elliptical and semi-elliptical wings that were loaded up around 1.5psf (and probably higher nowadays with very experienced canopy flyers). The smallest main canopy on the market when I stopped jumping (years ago) was 77 square feet which is incredibly tiny.
[/QUOTE]
Thanks. So what I gather is material coatings reduces the airflow through the material, thus increasing the drag per surface area. Combine material improvements (i.e. more drag per area for less weight) with geometry design improvements (airfoils vs round) lead to the performance increases in parachutes. And the eventual design of the “flying squirrel” suits.
If you hold 2 corners in each hand the sheet can not expand enough above you. You won’t have enough surface area of the sheet catching the wind. A parachute has a harness and lines so the chute can open to its maxim size. The bed sheet is already too small and if you hold the corners it is much smaller.
So, assuming a sheet would work, which thread count(s) would be best?
Related question…
I have occasionally wondered, if a person had jumped from the twin towers on 911 using a door in a kind of a surfboard fashion, would they have had a reasonable chance of survival?
I was thinking of tying all four corners together and holding on to the knot.
No.
However they wouldn’t have had a better chance of survival if they stayed in the building.