Hey guys, Im fairly new in photography and need some basic info on exposure times. I've wanting to go down to my regional airport to take some pics of dash-8 taxing to the runway. This means that the sun would be behind me. could anyone tell me the exposure/ f-stop with a 50 mm F1.9 lens mounted on an OM-1 for the folowing situations:

1.noon, variable clouds, sun behind me.
2.noon, clear sky, sun once again behind me
3.dusk, sun already set in front of me.