The gravitational principle, on the basis of which the propulsion system of a UFO is created, is proved by the details characteristic of its movement in the atmosphere of our planet. In particular, such as "hovering" over objects of interest to them, descent according to the principle of "falling leaf" or "pendulum" and two-stage departure.
Based on our ideas about technology and by analogy with our earthly flying vehicles, we should reason as follows. While hovering over an object, both planes and helicopters act on gravity, or rather, try to resist it indirectly: either due to the energy of the jet stream ejected from the nozzle of the aircraft, or due to the lift generated by the helicopter propeller. Moreover, both processes are accompanied by all known side processes - the roar of engines, jets of fire and clouds of smoke.
When a UFO hovers, these effects are absent, therefore, it must be assumed that UFOs resist gravity, acting on it in some other way, more effective and direct. Reports of UFOs came both from American astronauts during their flights to the moon, one of whom - McDevent - photographed a strange egg-shaped object from near-earth orbit, and from sailors who mentioned UFOs emerging from the water. All this speaks about the versatility of the UFO propulsion systems, allowing them to move without problems in various environments.
Of all the principles of motion conceivable today, allowing you to move the way a UFO moves without taking into account the difference between media, the first principle that comes to mind is gravitational. Therefore, it can be assumed that, by hovering, the UFO balances the Archimedean force directed upward and trying to displace the UFO from the atmosphere with the same magnitude force induced by it and directed downward.