wrote:
Another question about high altitude gliding - My understanding is
that the potential energy available to the sailplane is height times
weight. The potential energy would not vary with altitude. The drag,
however, would be less because of the thin air. Therefore would the
sailplane travel farther for a given amount of potential energy
used?? I have very limited time in the cockpit of jets, but it
appeared to me that the fuel flow was much less at altitude while the
true airspeed stayed high. More miles for a given amount of energy.

The sailplane will travel farther for a given amount of potential energy
used at a given speed, if that speed is relatively high. To put it another
way, the thinner air means less parasitic drag, but it also means that the
wings have to work harder to produce lift, so it means more induced drag.
Whether this is a net gain or a net loss depends on where you are on the
polar. If you're going faster than best L/D, then increasing altitude
pushes you closer to best L/D, allowing you to cover more distance for
each piece of altitude. If you're already at or below best L/D, then it
starts to hurt instead of help.

If you vary your true speed with altitude to keep a constant indicated
speed then the ground you cover for your altitude stays constant, although
you'll cover it faster when you're higher.

On a dual wave flight at 25,000 feet I was warned about the danger of
high true air speed at altitude. We were cruising at a about 60 knots
IAS. I calculated that was a TAS of about 90 knots. I ask the
instructor if he thought our sink rate was what you would expect for a
Grob going 90 knots? He said no.

Ask him instead if the sink rate is what you would expect for a Grob going
60 knots, multiplied by 1.5. You're still at the 60-knot mark on the
polar, so your L/D is still right around your optimum. But you're sliding
down that hill 50% faster than normal. If your sink rate is, say, 2kts at
60kts at sea level then you'd expect to see 3kts at 25,000ft (as opposed
to the maybe 5kts you'd see at sea level at 90kts indicated).

--
Mike Ash
Radio Free Earth
Broadcasting from our climate-controlled studios deep inside the Moon