Natural Science Forum / Physics / General Physics / April 2007

No difference at all. As to the time, the acceleration due to gravity
is (about) 10 m/s^2, distance is 1/2 gt^2.
How long does it take to fall 10,000 meters (about 30,000 feet) ?

10,000 = 1/2 * 10 t^2
t = sqrt( 2000)
= 44 seconds.
Being sane, the pilot will pull out of the dive long before he hits the water
and before the plane's airspeed exceeds its permitted value as shown by
the airspeed indicator. Needless to say, the higher he starts the longer
he can maintain zero g, and he also gets zero g on the way up as
well as down. The limitation is airspeed, but the higher he flies the
thinner the air and the lower the indicated airspeed will be, so a plane
can travel with a ground speed of 500 knots and an airspeed
of 200 knots quite easily, commercial airliners fly at about 30,000
feet for that reason.  
 http://www.nappf.com/nappf_flight_instruments_files/image003.jpg

I am one of the lucky ones to have flown on the Vomit Comet, and here's
how it goes:

Each day consists of about 40-50 parabolic arcs, and the total flight
lasts for about 2-3 hours.  IIRC, it was 10 arcs in one direction, the
plane turns around, 10 arcs back, the plane turns around again... etc.

Each arc lasts about 3 minutes- a 2+ minute climb which is experienced
as a 2-g acceleration followed by 30 seconds of reduced gravity.  The
climb is when most people actually vomit- moving your head confuses your
brain.  Sometimes the turn is combined with a climb.

When the apex of the climb is approached, the engines are cut back and
the plane goes nose-down.  "reduced gravity" begins here, and the pilot
has three basic options, decided on prior to the flight.  The plane can
go for 0-g, 0.16-g (lunar) or 0.3-g (Martian).  There's an accelerometer
and readout, and the pilot controls the rate of downward fall (the
engines are cut back, not off) to keep the desired acceleration within
bounds.  This period lasts for about 30 seconds, at which point the
engines are brought back up, and the plane begins to climb. Total
vertical distance covered is about 20k feet.  My personal experience is
that "lunars" are more disorienting than free-fall.

The misconception that 0-g is acheived for part of the climb and through
the apex is normal, but incorrect due to air drag on the plane.  The
transition from 0-g to 2-g (and back) occurs within a second or so, so
safety is a concern- people like to goof around in free fall, but one
should not be upside-down, over electrical equipment, or otherwise
misoriented at the end of the dive. Apparently, the russians have a
version of the Super Guppy for cosmonaut training, and it's a problem if
someone shoots 'up' toward the top of the cabin, a good 30 feet above
the floor, and can't get back down in time.