From: grg@boo-berry.ai.mit.edu (Gregory Galperin)
Newsgroups: rec.skydiving
Subject: Re: CYPRES altitude readings
Summary: All our world's turbulent.
Keywords: turbulence pressure physics-weenies
Date: 2 Oct 92 17:22:24 GMT
Organization: MIT Artificial Intelligence Lab

In article <7003@lhdsy1.lahabra.chevron.com> yzarn@lhdsy1.lahabra.chevron.com (Philip Yzarn de Louraille) writes:
>In article <1992Sep24.230949.9565@leland.Stanford.EDU> eap@leland.Stanford.EDU (Eric Perozziello) writes:
>>>>
>>>>I don't know how much pressure difference there is between back
>>>>and front of a jumper falling at 100MPH (any good physicists out there?),
>>
>>I would say the pressure difference is your wieght/ your body surface area
>>into the wind. (Is this over simplified?)
	Yes, this is way oversimplified; it might be true if the air were
solid, but fluid dynamics messes the whole thing up.  See below for more.

>> Now, does anyone know how
>>pressure varies with altitude? I assume it's linear (within the region
>>of air we generally jump in). 
>
>The air pressure decreases exponentially with altitude.
	Yes, it's exponential, but the difference in pressure between
your back and your front will never be detectable on your altimeter,
if that's what the question implied.

>The pressure difference has to do with your speed also. The matter gets
>complicated because of turbulences generated by your "passsage" through
>the air. Past a certain speed (I do not know what it is...) it is not
>possible to reliably predict the pressure reading directly behind you.
>
>Does someone know what speed is the transitional speed (in average for a
>skydiver) from laminar flow to turbulence??
>Do we even pass it?
	YES.  The skydiver is NEVER in laminar flow during ANY
skydive.  I think that walking fast might put you into a turbulent
regime; jogging almost certainly does it.  If pressed, I could do the
Reynolds number calculation to figure this out for sure. (But is
anybody really _that_ interested?)  We should thank turbulence for all
our fun.
	By the way, the equations we use to figure out drag, terminal
velocity, lift, etc.  (e.g., (.5)(rho)(Area)(velocity^2)(drag coeff))
assume a turbulent regime.  The equations should be proportional to
velocity rather than the square of velocity for laminar flow.  (Now
there's a fact to impress your friends, eh?)
	So, there's messy, turbulent air behind you (probably
averaging out to lower than ambient pressure for that altitude because
of bernoulli effect of your slipstream), compression in front of you
giving a higher than ambient pressure, and "skin effect" all around
messing both of these up in ways depending on where you're measuring.
Do you still trust your altimeter?

	Everybody says that "the altimeter reads high when you're on
your back."  This is generally true, but something to think about
might be that it probably reads *low* when you're on your belly; your
falling makes a pressure wave in front of you, where your altimeter is
usually riding, making it think you're at a lower altitude.  I doubt
the altimeter manufacturers did (or even could, since it would vary
widely) correct for this.  So don't be all that scared if there's a
1000' difference between what's indicated when you're on your back and
on your belly; it probably means the altimeter's several hundred feet
off your actual altitude in each case (with the real answer somewhere
in-between).

just be half as scared?----+
                           !
                           +-----=> Greg Galperin