HP-24 Project

On Feb 13, 11:57*am, Anthony W wrote:

Forgive my ignorance but what is the 200 pounds of water ballast for?

Carrying water ballast is common in soaring contests, and also for
long flights where speed is essential to achieving distance.

The increased weight increases both forward speed and sinking speed in
proportion, so that the maximum glide ratio does not change. The only
change is that increased weight increases the speed at which the best
glide is encountered.

My understanding is that this basically holds true right up to where
the speeds are great enough that you encounter transonic effects and
the rules start changing. I can't remember what sort of wing loading
that is, but I ran the rho-v-squared numbers back when Linda Wolkovich
called me about the Concrete Glider episode, and the numbers were
surprisingly large. It's theoretically possible to cast a ferro-
concrete version of a modern sailplane weighing several tons and have
it glide at the same ratio as its lighter composite bretheren. The
only problem would be launching it - the man-carrying, roll-off-
hilltop glider that Linda originally wanted wasn't going to happen.

At soaring contests, pilots evaluate the weather and ballast
accordingly. When the lift is predicted to be strong, they carry lots.
When its weak, they don't. Over the course of the soaring day, pilots
re-evaluate their situation and dump ballast as they see necessary.
They always (they should, at least) dump all ballast before landing to
reduce landing energy to the bare minimum.

Anyhow, when you see speeds up around (and sometimes over) 100 mph for
300 or 400 mile tasks at Nevada contests, you know those guys are
ballasted to up around 9.5 lbs/ft^2. That takes way more water than
200 lbs, but I figured I'd start there and then maybe test uphill from
there (or let others do it).

Thanks, Bob K.