Piasecki H-21 "Workhorse".
The designer, Frank Piasecki, was the first person in the United States to hold a commercial helicopter license. Piasecki Aviation changed its name to Vertol in 1956 and is still designing aircraft as a Boeing division. (The CH-46 "Chinook" was a later well known version).
The H-21 was developed in 1949 and five were delivered to the Marine Corps to test as utility aircraft. It quickly earned the nickname "Flying Banana" due to its odd shape. The aircraft is oddly shaped to prevent the rotors from interfering with one another.
The H-21B was powered by a single Wright R-1821 radial air-cooled engine that developed 1425 horsepower. The engine transmitted power to a mid-transmission and then longitudinally to the fore and aft rotor transmissions. It has cruise speed of 90 mph and service ceiling of 19,200 feet. The maximum gross takeoff weight was 15,200 pounds. The flight controls were hydraulically operated. The original blades were constructed of wood on a metal spar and later blades were of all-metal construction. The overall length was 52 feet with blades folded and 86 feet with rotors turning. The minimum crew was one pilot.
These early helicopters were used as utility and rescue aircraft in the Korean War. They could seat 22 troops in removable web seats mounted along the sides of the cabin. They could be fitted with inflatable pontoons on the wheels allowing the aircraft to land on water. Considered to be the first heavy lift helicopter, a modified UH-21 made the first nonstop transcontinental helicopter flight on 24 August 1956.
Most USAF H-21's were retired from service in the early 1970's.