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Without going into the math, to obtain TAS from
two-way GPS ground-speed runs, 1mph error will come from the following
wind-speed/directions for the listed TAS: 200 mph TAS, 20mph@/87deg.; 250
mph TAS, 25mph@ 63 deg.: 300 mph TAS, 25 mph@ 78 deg. So don't sweat the wind
speed if the forecast is for less than 25 mph. Use the forecast wind and
direction and fly a GPS TRACK into and
with the wind, then average the ground-speed results. Don't forget to
convert the forecast wind direction, which is relative to true north, to
magnetic north. Nothing could be simpler! Here's something else. Note your
altitude reading on the ground, then fly a low pass down the runway and check
the altitude you get when flying at some estimated height. If it's not the same,
you've got an in-flight static error, which you can correct with a slight
obstruction ahead of the static port for an altitude error higher than
estimated or just behind the port for an altitude error lower than
estimated. About 1/32" obstruction will give about 5mph error at 200
mph.
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