[lenadorap (OP)]

I think the reason is that all of them contain cyclotrone or something used for generation of kind of nuclear energy

[BenV]

I think it's more plausible that the idea of "flying saucers" has become well defined in the collective public idea of what an alien space craft should look like.

[RD]

or spherical ... http://www.thenakedscientists.com/forum/index.php?topic=37696.0  []

but seriously, the stereotypical "UFO" shape is consistent with lenticular clouds ...

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http://www.thenakedscientists.com/forum/index.php?topic=36369.msg337621#msg337621

[Airthumbs]

I think you should ask Geezer as he lives in one!

[Kryptid]

UFOs have popularly been idealized in the media as being saucer-shaped. However, if you go through reports there is actually quite a variety of shapes; triangular, cigar, crescent, spherical...

A cyclotron-type machine that is circular can be imbedded in a vehicle of pretty much any shape in principle. As an example, the lift-fan in the F-35B fighter jet is circular, but the plane itself is not. For minimizing supersonic drag, you'd want to go with the cigar shape (assuming they have some way of creating lift other than wings).

[joepierson]

In ancient greece, the UFO sightings were all discribed as flying chariots