The paper addresses unmanned aerial vehicles (UAVs -aka drones) designed to be undetected by humans, i.e. stealth drones, or more generally, to have a minimal impact on living beings and environment, i.e. 'blue' drones, in the sense of preserving unaltered the blue color of the sky. Being 'blue' might be highly desirable in the future when drones will proliferate in large numbers; stealth is a special case, desirable to covert military and security operations. The paper explores general characteristics of blue drones and alternatives of making them, offering both system and component level solutions.

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