Andy Blackburn wrote:
At 18:48 02 August 2005, Eric Greenwell wrote:

Tactically, one should take off with full ballast early
enough to have a
chance of completing the flight, and drop whatever
it takes to stay up.


That's one approach - but keep in mind that the difference
in climb rate between full and empty (in a 45-degree
bank) is less than 50 fpm.

The _sink rate_ in calm air might differ only that much, but I've seen
much bigger differences in climb rate in contests, due to the width of
the thermal and the ability to maneuver. In addition, the glider with
the lighter wing loading can often continue up another 400-500' in good
lift, while the heavier can't. And this is with climb rates in the 2-4
knot range, not just really weak conditions.

I've experienced these differences in gliders only 1 pound/sq ft apart
in wing loading. It's not necessary to be empty to have a worthwhile
climb advantage in modest conditions.


Furthermore, the McCready-derived XC speed differential
for full versus empty water is 6-9 knots. The actual
difference with streeting, etc. may be greater. That
amounts to about an hour less time on course with water
versus without. To break even without ballast

I wasn't suggesting Tim fly without ballast, only that a partial load
might be a better choice if he could start 30-60 minutes earlier.

you'd
have to make about 80 miles before you could get started
on course with ballast.

If you flew 6 hours in good conditions, you'd have an extra 36-54 miles
over the empty glider. If it got started an hour earlier in the
conditions we often have, it could make that 50 miles pretty easily by
starting downwind.

I'm thinking this would only be true if the day developed
with either very weak (0.5-1.5 knots climb, dry) or
very narrow thermals for a very long time (1-2 hours).
Under those conditions I don't think you're making
80 miles even if you have Helium in your wings.

I'd recommend taking tows until you can stay up with
full water.

I think it depends very much on your weather and your glider. I'm sure
that's good advice for Tonopah and other strong areas, but I don't think
it will work here in the Pacific NW. Tim will need to experiment some,
and, I hope, report back to us in a year.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA