6 inches
4 ft
yes.
airplanes are the same thing as hovercrafts... but only hovercrafts have a louder engine
on a cushion of air
yes
hovercrafts
Why are you asking where to find hovercrafts? But I would have to say, that scientists are trying to build some, or aliens are, but you should have known that already.
Very good conditions!
walk
They both fly I think 
"Hovercrafts" are available now and have been.
Usually gasoline or diesel fuel.