The cheapest military plane in the world has now arrived in this country . The Americans call it the flying jeep , and it is used as an airborne observation post , replacing the old observation balloon of the last war as a spotter for artillery . It costs about ?130 to build , is a two seater aircraft with 65Hp engine and can land and take off on ordinary roads . This handy little machine answers to the name of the Piper Cub