Helicopter, comprising a base frame supporting at least a pilot's cab, at least a rotor (5, 6) and at least a propulsion device which comprises: - a first piston engine, for rotating a first engine shaft; - a second piston engine, for rotating a second engine shaft; and - a transmission unit comprising a drive shaft connectable to each of said first engine shaft and said second engine shaft via at least one free wheel mechanism and suitable for transmitting motion to said rotor (5, 6) of the helicopter; wherein: - said engines (7, 9) comprise a central engine housing, a first side engine housing and a second side engine housing, said first engine shaft being housed between said central engine housing and said first side engine housing and said second engine shaft being housed between said central engine housing and said second side engine housing; - said first engine shaft, said second engine shaft and said drive shaft are substantially parallel to one another; - said first engine shaft and said second engine shaft are of the same length; characterized by the fact that: - said transmission unit comprises: - at least a first drive pulley associated with said first engine shaft; - at least a first driven pulley associated with said drive shaft with interposition of a first said free wheel mechanism; - at least a first flexible transmission element closed on itself in a loop and at least in part wrapped around said first drive pulley and said first driven pulley; - at least a second drive pulley associated with said second engine shaft; - at least a second driven pulley associated with said drive shaft with interposition of a second said free wheel mechanism; and - at least a second flexible transmission element closed on itself in a loop and at least in part wrapped around said second drive pulley and said second driven pulley; - said first engine shaft and said second engine shaft are axially staggered by a preset distance; - said drive pulleys (20, 25) have a diameter greater than the centre distance defined between said engine shafts (8, 10), said first drive pulley being housed at least in part in a space defined between said second drive pulley and said central engine housing. - a first piston engine, for rotating a first engine shaft; - a second piston engine, for rotating a second engine shaft; and - a transmission unit comprising a drive shaft connectable to each of said first engine shaft and said second engine shaft via at least one free wheel mechanism and suitable for transmitting motion to said rotor (5, 6) of the helicopter; - said engines (7, 9) comprise a central engine housing, a first side engine housing and a second side engine housing, said first engine shaft being housed between said central engine housing and said first side engine housing and said second engine shaft being housed between said central engine housing and said second side engine housing; - said first engine shaft, said second engine shaft and said drive shaft are substantially parallel to one another; - said first engine shaft and said second engine shaft are of the same length; - said transmission unit comprises: - at least a first drive pulley associated with said first engine shaft; - at least a first driven pulley associated with said drive shaft with interposition of a first said free wheel mechanism; - at least a first flexible transmission element closed on itself in a loop and at least in part wrapped around said first drive pulley and said first driven pulley; - at least a second drive pulley associated with said second engine shaft; - at least a second driven pulley associated with said drive shaft with interposition of a second said free wheel mechanism; and - at least a second flexible transmission element closed on itself in a loop and at least in part wrapped around said second drive pulley and said second driven pulley; - at least a first drive pulley associated with said first engine shaft; - at least a first driven pulley associated with said drive shaft with interposition of a first said free wheel mechanism; - at least a first flexible transmission element closed on itself in a loop and at least in part wrapped around said first drive pulley and said first driven pulley; - at least a second drive pulley associated with said second engine shaft; - at least a second driven pulley associated with said drive shaft with interposition of a second said free wheel mechanism; and - at least a second flexible transmission element closed on itself in a loop and at least in part wrapped around said second drive pulley and said second driven pulley; - said first engine shaft and said second engine shaft are axially staggered by a preset distance; - said drive pulleys (20, 25) have a diameter greater than the centre distance defined between said engine shafts (8, 10), said first drive pulley being housed at least in part in a space defined between said second drive pulley and said central engine housing.