> Don't Gliders during distance races move water forward and or dump water to
> improve airspeed or accents?  I have heard of this technique but am
> unfamilier with the logic.
>

Gliders carry water as ballast, not to change the CG, but to fly faster.  The L/D ratio, glide polar curve, shifts to higher speeds with higher wing loading.  IOW, if the glider's best L/D is 44:1 it is still 44:1 at heavier weights - just at higher speeds.  Weight does not figure in when determining how far you can glide. The weight only determines how fast you'll get there.  Two identical gliders of different weights starting at the same altitude and flying at their best L/D speed will hit the earth at the same spot.  The heavier one will just get there first.  (This ignores wind effects).

Ideally, in a contest, you try to keep the water as long as possible dumping it just before landing.  A popular maneuver is to fly down the finish line runway at low altitude (25 feet) and red-line speed dumping water and then pull up gaining a 1000 feet or so for the pattern and subsequent landing.  Its called "putting out the fire on the runway".  The maneuver allows time to dump the ballast while maintaining enough mass-energy to get up to a reasonable pattern altitude.  Its great fun.

The only reason you would dump ballast earlier would be because it was a weak thermal day.  The penalty of carrying the extra weight is in reduced climb rates in thermals.  Some days are just weak enough so as to not support climbs in a "heavy" glider.  Its considered bad manners to dump ballast on a competitor in the same thermal below you. (g)

The water ballast, which is carried in tanks in the wings (similar to our fuel tanks), generally moves the CG forward.  To counteract this shift, many modern sailplanes also have a water tank in the vertical stabilizer.  Most competition pilots will fly with the CG at the aft limit.  The water in the vertical stab maintains this aft CG.  When dumping ballast, hopefully the tail tank will leak out at a rate to maintain the CG.  Problems could occur if flying at very high altitude since low temperatures could freeze the tail tank (and not the main ballast tanks).  I've often added antifreeze to the tail tank to keep the tail ballast liquid.  It would be a bad end to the day if the main ballast drained while the tail tank was still an ice cube.

Hal Woodruff
IV-P