A lot of UFO and flying saucer sightings, here in the UK, are Semi-Bouyant Recconaissance Drone, which just happen to be very large saucer shaped craft. I've had some involvement with winged drones, most of which are remotely piloted. Most used large Expanded Polystyrene dope skinned covered wings and were powered by the ubiqitous ROTAX 250cc twin cylinder 2 stroke pusher prop microlight engine.
Airspeed was about 100 to 120 Knots.
Needess to say, these were cheap and expenable 50 Pound payload delivery systems, remotely piloted from up to a hundred miles away. Just giant RC planes really. They could probally be used to drop leaflets, but never in the quanities of a Snow Goose.
They are all usually vehicle launched and usually fly at night, due to vulnerability to ground fire, slow speed, etc.
This automonus microlight type pilotless drone seems ingenious.
A box with an engine attached to a parachute. Slow but effective.