How to use a graphic flight computer to find ground speed and true heading<br>Align the wind direction under the " true index " mark by rotating the azimuth plate. If the winds are from 330 ° at 20 knots, align 330 under the true index. Align the grid so the center grommet is over one of the heavy lines.
The center grommet must be upsidedown
( z1 obligate :ARG2 ( z2 upside :op ( z3 grommet :mod ( z4 center ) ) ) )