User:The Bushranger/X-18

The  Hiller X-18, also known as the "Propelloplane", was an experimental aircraft built by the Hiller Aircraft Corporation for the United States Air Force. Intended as a testbed for tiltwing technology in the development of practical V/STOL aircraft. Proving troublesome in testing, a near accident resulted in the grounding of the aircraft, and only limited further testing was performed before the program was cancelled.

Design and development


Intended as a testbed to prove the feasibility of a large tiltwing aircraft, the X-18 was constructed by Hiller Aircraft Corporation under a contract awarded during 1956. The first large aircraft to make use of the tiltwing concept,, and only the second tiltwing aircraft built in the United States, the X-18 made extensive use of parts from existing aircraft in its construction, with the aircraft's fuselage being that of a Chase YC-122, the engines and propellers being sourced from the Navy program that had produced the Convair XFY and Lockheed XFV programs and fitted to a newly designed wing.

In addition to its twin turboprops, a small turbojet engine was mounded inside the aircraft's cargo hold, with its exhaust ducted under the tail to assist in pitch control at low speed.

Operational history


The X-18 conducted its maiden flight on November 20, 1959 from Edwards Air Force Base, and flight testing quickly proved that the aircraft suffered from a number of significant issues. The aircraft proved to be susceptible to wind gusts when the wing was rotated, its surface acting like a sail. In addition, the turboprop engines were not cross-linked, so the failure of one engine when in vertical flight mode would inevitably result in a crash. In addition, altitude and roll control authority were insufficient when in the hover.

On the 20th and final flight in July 1961, the X-18 suffered a failure in its propeller pitch controls while attempting to convert to a hover at 10000 ft, and the aircraft entered a spin. The aircraft's crew was able to regain control and successfully landed, but the X-18 never flew again, having never made a full conversion to vertical flight.

Despite this, ground testing of the tiltwing concepts continued. Eventually a VTOL Test Stand was constructed, on which the X-18's vertical takeoff and landing and hover control was to be tested. One engine run was successfully conducted to the full 15-foot wheel height on the test stand; however, the program was cancelled on January 18, 1964 before further testing could be conducted, and the X-18 was cut up for scrap.

Lessons learned
Despite its failure, the X-18 program did contribute to the knowledge of the problems faced in vertical flight, especially when using the tiltwing concept; these lessons included the fact that cross-shafting between the engines was mandatory in order to avoid loss of control in the event of an engine failure, while direct propeller pitch control was mandatory for precise height and lateral control during VTOL and hover. This knowledge was employed in the successful development and flight tests of the LTV XC-142A a larger and more successful tilt-wing VSTOL transport.