Gene Whitt wrote:
Y'all,
Been many years on rec.aviation.student but even more years since
gliding. Unable to explain the 'why' of water ballast to increase
performance in gliders to argumentative airplane student.

I need a simple explanation in 25 words or less.

Gene Whitt

Try this:

The important points are the cruise speed goes up, the sink rate goes
up, but the increased sink rate is insignificant on a strong day.

For why the glide angle remains the same with different aircraft weights:

* the best L/D occurs at one particular angle of attack (AOA is the
important parameter here)
* for low weights, this AOA will produce a low airspeed; at higher
weights, higher airspeeds

For why this is beneficial, given the glider won't climb as quickly in a
thermal at higher weights; for example, with a 20% weight increase with
ballast:

* you get a 10% increase in cruise speed
* you get a 10% increase in sink rate, but that's only 2.5% decrease in
the climb rate (unballasted sink rate of 150 fpm while thermalling) on a
600 fpm day

If you get these points across, you can generalize the "best L/D" to
other AOAs, and talk about how we don't fly at the best L/D on a good day.

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

Eric Greenwell
Washington State
USA