A model aeroplane is used for presentations and testing. Model planes are also used for fun. They are used model-plane clubs, aeroplane company headquarters and studios. (Bedroom, in my house)
They were used in the first world war in submarines. The are used now also for submarines and also war tanks. The outer coating of the periscope for the submarines were firm and waterproof for...
The only answer that comes to mind is "Airships". Hydrogen, the lightest element, was used in airships, where excess weight was undesirable. However, accidents like the Hindenburg disaster showed...