At 18:30 28 December 2004, Eric Greenwell wrote:
How long ago did you ask him? GPS is much more accurate
in the last few
years, especially if using the WAAS ablities. But,
let's say you know
the distance to only +/- 100 feet (it's typically more
like +/- 30
feet), then flying only a mile (5000 feet) would be
a 2% error, or one
L/D point for a 50:1 glider. Good enough for us, I
think.

At 50:1 flying a one mile distance should yield an
altitude loss of 100', so I think a +/- 100' GPS error
in altitude could yield a 'measured' L/D of between
25:1 and infinity.

A single measured glide of 10 miles, assuming no other
errors from pilot inputs or net airmass movements,
would be good to only 10%, or +/- 5 L/D points, that's
a bit better, but you'd still need to average a bunch
of runs of 10+ miles to get any kind of accuracy. Best
done in dead calm air the way Dick does.

I have tried plotting speed versus glide angle over
a number of flights. You need to adjust for wind speed
and direction (estimated from thermal drift angle and/or
ground speed differences around a circle), then you
have to convert from TAS to IAS (2% per 100'). I plotted
about 30 points and realized that I had a very low
signal to noise ratio - that is the points made a big
cloud that sort of sloped in the expected direction.

I suppose with enough data points you could get a resonable
average L/D performance for any given speed, but I
did the experiment for a different reason, I was interested
in the VARIANCE in performance that I could expect
in real world conditions, including pilot technique
and long stretches of good/bad air.

If I know the difference between expected performance
and 99% worst case performance I know how much cushion
to carry on final glides - particularly as I get closer
to home, and the ground, and have fewer options to
recover. It also may help a bit to know how early you
can start a long final glide, though this is much more
subject to observable conditions like streeting and
average thermal strength and therefore less informed
by bulk statistical analysis.

9B