The trick to unloading troops or supplies from a ground vehicle is that the vehicle must be completely stopped before letting them go.. or they will just disappear.
C47.. yes over 800 feet above ground level, and less than 200 mph. Right over where you want to drop them just climb straight up and let them go.
On an AIRFIELD, drop troops over town, or supplies over the base
On a vehicle field or port, drop troops AND supplies over the base
Usually if I plan on taking the base, I'll fly around (clear of enemy planes) until we get the base, then just land there and up another (or if troops are disabled I resupply there).. you can resupply troops, but not field supplies. There really isn't much point to flying back to the base you took off from. If I feel there's a reasonable chance I'm going to die anyway, I'll fly my goon straight towards enemy planes and draw them away from the base (goons are like magnets to fighters) and give my guys a chance to get on their tails. Yes I'll die, but it usually scatters the hoardes. Even if you die, your troops will continue their march towards the map room (as long as you released them beforehand)
If I feel I can, I'll try to land my C47 (also known as a GOON) next to the town. When you can hit F3 to get an overhead view of the plane to make it easier to maneuver between buildings and all, and you can get that goon right up the the maproom if you're careful.
If I'm bringing supplies to the base, I just land it on the runway, let the supplies go when I come to a complete stop.. then end sortie and get my landing and my points, but of course you can't ALWAYS do that.
ALSO don't drop troops from a very high altitude.. they take forever to parachute down. If you drop from 25,000 feet it takes over ten minutes, and by that time either someone else has taken the base, or the town has come back up already.
OH! also.. in order to take an air base, *ALL* town buildings and town ground guns must be destroyed (except for one large black warehouse on the edge, which is indestructable). When you have troops, make sure you get a couple people on RANGE to verify all the buildings are down before you let go of your troops, because if even one building is up your troops will die as they enter the map-room.
Hope this answers some questions for you!