John Sinclair wrote:
I asked Dick Johnson, why couldn't I fly in calm conditions
(morning) and hold a given airspeed (say 60 knots)
and a given heading (say west) for 10 minutes, then
reverse heading to east (to cancel out any wind) and
then analyze the GPS trace to determine my ships L/D
at 60 knots. We have an accurate distance covered and
fairly accurate altitude lost, so why can't we crunch
the numbers?
Dick said the GPS info wasn't accurate enough. I thought
it was a good idea, but I defer to the master.

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.

If you use the GPS for the altitude instead of the pressure altitude,
you might have to fly off a 1000 feet or so of altitude, I suppose.
Maybe Dick was referring to GPS altitude?


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Eric Greenwell
Washington State
USA