[bestclimb]

Originally Posted by jamesqf

Consider competition sailplanes, which are pretty much the ultimate in fuel economy. The cockpit has reclining seating and is usually quite tight, but is comfortable enough to sit in for hours, while weight (water ballast) is often added for increased performance.

While this is true about sail planes the correlation is not quite there.

The engine out glide ratio for a most planes stays the same across it's entire weight range (from nearly empty to full gross weight) more weight means the rate of decent is higher, but higher airspeed means you cover ground faster. So the distance traveled per altitude lost says the same.

So knowing that an aircraft goes just as far in a glide heavy or light why bother with the water ballast?

An airplane balances on the wing. The lever arm forward of the wing must equal the lever arm aft of the wing. you can do this a couple ways. Most aircraft have a smaller wing in the back providing down pressure to lift the weight forward of the wing. This situation is very stable from the pilot's perspective and easy to learn on. The down side is that by creating this negative lift we make induced drag. If weight is added aft of the wing we can balance the aircraft without the induced drag. If we add a bit more we can make the tail plane (horizontal stabilizer) make a bit of lift to make up for the parasitic drag it already has without inducing much more. This situation is very good for performance but is unstable. Water can be dumped and the plane made more stable.