They both (some airplanes do) use propeller's.
airplanes are the same thing as hovercrafts... but only hovercrafts have a louder engine
They both fly I think 
Airplanes move through the air in a similar fashion to the way boats move through the water.
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.
They are similar in that both serve a purpose, and some insects have wings like airplanes. Other than that, they are unalike.
yes.
6 inches
4 ft
Old Airplanes and new are similar because they work under the same principles. Creating enough lift to become airbourne. To do so a plane needs to be light enough with enough thrust and enough surface area of the wings to create the lift