A disk or cylinder could be a very efficient shape for a spacecraft if aliens from Planets with different gravities want to travel together. Flying saucer sightings frequently involve something rotating. If this is real Centrifugal power would have a similar effect to Gravity but the force would be different at different distances from the centre. Therefore different aliens that evolved on different planets or Moons with different gravities could all have homes somewhere in a flying saucer where the simulated gravity fits them. If a flying saucer picked up Humans somewhere in the craft would be an area that simulates the gravity of the Earth. What about flying saucers with a hub in the centre? Perhaps that’s imaginary. Perhaps the hub is for some alien machinery that we don’t understand. Areas in the disk where the simulated gravity doesn't suit any of the travellers could be used for storage and sections could be cleared if new aliens are discovered who need that level of simulated gravity.
To simulate gravity a rotating disk would probably need to be very big, say at least a hundred metres in diameter. A smaller, faster rotating disk could also simulate gravity but there would be significantly different gravity in different parts of the same room. Depending on the physiology of the aliens this could cause problems. The above can explain very large flying saucers or flying saucers in the sky where the real size is impossible to estimate. Smaller flying saucers might need a different explanation.
None of this shows that flying saucers are real but it shows that flying saucers are possible.
Problems with the "flying saucer" shapeEdit
There are problems with the "flying saucer" shape. The closest approximation that airplane designers have built is the "flying wing", and flying-wing airplanes have not been very popular. The large majority of airplanes are winged missiles. This shape cuts down on drag by reducing the airplane's direction-of-motion cross section. Interestingly, flying animals -- birds, bats, pterosaurs, and insects -- have a similar winged-missile shape.
This does not mean that there are no extraterrestrial spacecraft being operated in the Earth's atmosphere. It does suggest that they would resemble us Earthlings' airplanes more than the classical "flying saucer" shape.
Furthermore, the previous argument confuses different functions of spacecraft -- residence and exploration. A spacecraft for residence can be large and poorly designed for atmospheric travel, while one for exploration can be designed to enter and exit atmospheres. There's a close analogy with aquatic travel. A big ship often has small boats aboard it, boats that can be used to approach shorelines. So what we'd see in our atmosphere would be an aerial counterpart of those boats rather than the main ships.
Although human spacefaring has been very limited, some spacecraft have had parts that are specialized for different functions, like orbiting and landing. The combination travels to some other planet or moon, and the lander separates from the orbiter when there. The Apollo, Viking, Galileo, and Cassini spacecraft have all been orbiter-lander combinations.
Lpetrich originally wrote this section.