best way to measure actual polar of a glider?

On Jan 10, 4:16*am, bildan wrote:
On Jan 9, 8:03*pm, John Cochrane
wrote:

There is one untried principle so far. A little vane on a trailing
bomb could measure angle of descent. All it needs is accurate
measurement of heading relative to straight down. Angle of descent is
unaffected by smoothly rising or sinking air. Has anyone tried this?

John Cochrane

John, it's been seriously discussed several times - once in the
McCready/Carmichael group I mentioned before. *L/D is indeed the angle
between a horizontal plane and the free air flow. *However, for 40:1
it's around one degree which is a very small angle for experimental
measurement. *A long carbon fiber nose boom with a pitch vane could do
pretty well with the free air flow part. *The other part requires a
very good "stable table" inertial guidance system to provide a perfect
horizontal plane. *There's been a lag between the technology to
measure L/D this way and the ever increasing L/D to be measured.
Today it could probably measure 40:1 pretty well but anything more
than that would be lost in the 'noise'. *There are cheaper ways to
measure less than 40:1 L/D's.

Going back to the speed course idea. *There are huge basins in the
western US where pools of cold air collect in winter. *These pools of
heavy, cold air form under an intense inversion several thousand feet
thick which isolate them from air motions above it. *These cold air
pools appear to be utterly free of all air motion. *There are usually
perfectly flat dry lakes in the center of these basins where a glider
could land.

Laying out a carefully surveyed speed course where a glider could
enter the course at near redline speed and coast along well above
ground effect slowly losing speed in perfectly level flight wouldn't
be that hard. *A recording laser range finder altimeter could assure
the flight was indeed level. *An auto tow from the dry lake could
easily get a glider high enough to run the course. *It would cost so
little per flight that scores of runs could be done to reduce
'scatter' of data points.

I understand that the Germans now measure the L/D by comparison with a
standard calibrated glider in flight, to eliminate the effect of small
vertical air currents.

Derek C