Svilen Rangelov sports an impressive beard. It's eight years' worth of growth he says.

The beard dates back to when he and his younger brother, an aerospace engineer by training, formed Dronamics as Europe's answer to the emerging market for cargo drones.

He agreed with his brother Konstantin that they would only shave their beards after the first flight of the drone they've been building in their native Bulgaria.

At the time he established Dronamics big tech giants like Amazon were experimenting with drone deliveries to domestic addresses. But Mr Rangelov never believed in the concept of personal goods delivered by the drone.

The practical difficulties of flying a drone right up to someone's front door were obvious to Mr Rangelov. "We couldn't buy into the concept of small drones. We took a different approach."

This take on drone delivery will bear fruit when the prototype cargo aircraft takes to the air.