Hawker Siddeley Harrier and Yakovlev Yak-38. An unmanned aerial vehicle (UAV) does not suffer the problem of pilot attitude. The Dornier Aerodyne is of ducted-fan configuration similar to a coleopter, and a test UAV flew successfully in hover mode in 1972, before development was discontinued. Another contemporary UAV project was the NSRDC BQM-108 that was developed by the United States Navy; although work was discontinued almost immediately after its single successful test flight. During the 1970s, several studies and wind tunnel models were made of a tail-sitting version of the General Dynamics F-16 Fighting Falcon that was intended for use on board ships; however, it was decided not to pursue further development of the concept due to the large thrust requirement involved, as well as the need for extensive apparatus to handle take-off and landing. In the present At present, most of the tail sitter projects or proposals fall under the category of unmanned aircraft such as Bell Apt or Northrop Grumman Tern. List of tail-sitters See also Thrust vectoring Tiltrotor Tiltjet Tiltwing Coleopter VTOL References Citations Bibliography Allen, Francis J. "Bolt Upright: Convair's and Lockheed's VTOL fighters". Air Enthusiast (Key Publishing) Volume 127, January/February 2007, pp. 13–20. ISSN 0143-5450. Brown, Kevin. "The Plane That Makes Airfields Obsolete." Popular Mechanics, 133(6), June 1970, pp. 80–83. Taylor, John W. R. & Michael J. H. Jane's Pocket Book of Research and Experimental Aircraft. Collier Books: New York, 1977 . Wilson, Stewart. Combat Aircraft since 1945. Fyshwick, Australia: Aerospace Publications, 2000. . Winchester, Jim. "Lockheed XFV-1 Salmon." Concept Aircraft: Prototypes, X-Planes and Experimental Aircraft. Kent, UK: Grange Books plc., 2005. . Tailsitter aircraft Aircraft configurations