Stanley Hiller Jr.’s first helicopters featured dual, counter-rotating main rotors to eliminate the need for a tail rotor mechanism.  While mechanically efficient and extremely stable, this arrangement created some difficulties with regards to control.  Hiller experimented with creative approaches in improving the stability of single-rotor aircraft using the J-5 testbed in the late 1940s, leading to the Hiller 360 – best known as the United Helicopters UH-12, or as the OH-23 Raven in US military service.  This became by far the most widely produced of all Hiller helicopters, with over 2,000 produced from the 1940s through 1960s.

Certified in 1948, the UH-12 proved to be one of the most successful light helicopters in the first years of vertical flight.  Able to carry a pilot and up to two passengers in its most popular version, the UH-12 was able to operate at altitude and with a sling load.  Its distinguishing feature was the “Rotor Matic” control system, with small paddles attached to the rotor hub offset 90 degrees from the main rotor blades.  These paddles deflected in response to inputs from the cyclic control, enhancing the helicopter’s control.

French Army doctor Valerie Andre used UH-12 aircraft to develop methods of aeromedical evacuation during the conflict in French Indochina, an experience soon repeated on a larger scale by US Army OH-23s operating in Korea in the early 1950s.  Ravens were gradually supplanted as medical evacuation aircraft in Korea by the Bell 47-based Bell H-13, which could more easily carry heavier payloads in hot weather across the higher elevations of the Korean Peninsula.

While no longer in production, UH-12s remain in use as general aviation aircraft.

Specifications (OH-23D)

MTOW:  2,700 lbs

Powerplant:  Lycoming VO-435, 250 hp

Maximum Speed:  83 knots

Cruising Speed:  71 knots

Range:  171 nautical miles

Maximum Payload:  900 lbs