The list of units
Helicopters are essentially air transports that can carry one unit to any square within their operational range, regardless of enemy units. Helicopters may carry foot soldiers only -- no mechanized units allowed.
A city needs oil and rubber in its Strategic Resource box to build a helicopter.
The concept that was to become the modern helicopter was first explored by artist/engineer Leonardo da Vinci. He made sketches of a rotor-driven aircraft in the early 16th century. It was not until 1939, however, that the first practical helicopter design was developed. Helicopters have a distinct advantage over fixed-wing aircraft in that they can take off and land vertically. This makes it possible for helicopters to operate in many places that would be too small to accommodate a runway. Today, helicopters are used in civilian roles as rescue vehicles and by law enforcement agencies. All branches of the military utilize many different types of helicopters, from giant cargo and troop carriers to fast and maneuverable gunships used as air support for ground troop operations.