Vertical takeoff and landing (VTOL) and short takeoff and landing (STOL) aircraft offer the possibility for landing in and taking off from downtown areas of cities, if appropriate sites can be found at reasonable cost. Passengers on the V/STOL aircraft could be transported directly to their final destination, or to a remote airport for a flight on a subsonic or supersonic airliner. The helicopter is one example of a V/STOL aircraft, but its current speed and fuel efficiency limitations prevent economic use for routine passenger service. Efforts to improve on the basic helicopter design to provide high-speed V/STOL travel has resulted in two practical designs: the tilt-rotor and the X-wing. Both designs were in R&D for military applications with civilian certification criteria in mind.
The X-wing aircraft accomplishes vertical or short takeoff with helicopter blades, and uses the blades as an X-shaped wing when cruising. The blades must stop in order to cruise, and the conversion from takeoff to cruise configuration has not yet been mastered. If successful, the X-wing aircraft could achieve higher cruising speeds than the tilt-rotor.
The science fiction X-Wing fighter was famous in the Star Wars movies. The actual X-Wing rotor flight test project was sponsored by NASA, the Defense Advanced Research Projects Agency (DARPA), and Sikorsky Aircraft. Developers of X-Wing technology did not view the X-Wing as a replacement for either helicopters (rotor aircraft) or fixed-wing aircraft. Instead, they envisioned it as an aircraft with special enhanced capabilities to perform missions that call for the low-speed efficiency and maneuverability of helicopters combined with the high cruise speed of fixed-wing aircraft. Some such missions include air-to-air and air-to-ground tactical operations, airborne early warning, electronic intelligence, antisubmarine warfare, and search and rescue.
The X-Wing project was a joint effort of NASA-Ames, DARPA, the U.S. Army, and Sikorsky Aircraft, Stratford, Connecticut. One of the two RSRA aircraft was later modified to the X-Wing and received limited testing at Dryden. The modified X-Wing aircraft was delivered to Ames-Dryden by Sikorsky Aircraft on 25 September 1986. The X-wing concept was under development by Sikorsky Aircraft, following R&D by NASA, the Army, and Sikorsky, and as of 1986 the first flight of a demonstrator was planned for 1990.
During high-speed taxi tests at NASA's Ames-Dryden Flight Research Facility (later redesignated Dryden Flight Research Center), Edwards, California, on 4 November 1987, the vehicle made three taxi tests at speeds of up to 138 knots. On the third run, the RSRA/X-Wing lifted off the runway to a 25-foot height for about 16 seconds. This liftoff maneuver was pre-planned as an aid to evaluations for first flight. At the controls were NASA pilot G. Warren Hall and Sikorsky pilot W. Faull. Following taxi tests, initial flights in the aircraft mode without main rotors attached took place at Dryden in December 1997. Ames research pilot G. Warren Hall and Sikorsky's W. Richard Faull were the pilots.
The contract with Sikorsky ended that month, and the program ended in January 1988. The X-wing stopped rotor concept was cancelled because of its extreme complexity when it was being readied to start flight development testing. During development, analysis and testing indicated that the X-wing design encountered high transient aerodynamic loads during the conversion from rotary to fixed-wing flight. Work on the X-wing indicated that the most severe loadings occurred during the last few revolutions of the rotor when it turns relatively slowly. Under these conditions, the rotation time-scale is much smaller than that of the flow, making a sequence of steady measurements at fixed rotor azimuthal positions an efficient and viable method to measure and predict the loads.
In 1992 a low-speed wind tunnel test was completed in support of ongoing conceptual design studies of the Stopped Rotor/Disk rotorcraft concept. A one-fifth scale model was tested in the NASA Ames Low-Speed 7- by 10-Foot Wind Tunnel #1 to evaluate the low-speed cruise performance. The primary test objective was to compare performance characteristics for three possible conceptual designs of the Stopped Rotor / Disk cruise configuration: the large hub fairing (disk) alone, the disk/extended blades configuration, and the disk/conventional wing configuration. Results showed that the disk/extended blades configuration was the most efficient in low-speed cruise. Other test objecives included making parametric changes by varying the geometry of the disk and by varying the extended blade incidence angles. Studies were also conducted to examine the aerodynamic interaction between the disk and a conventional wing. An examination was made into the effects of the disk on static longitudinal stability. The wake generated by the disk impinged on a T-tail of the model and thus degraded longitudinal stability. Alternative tail geometries are required in order to improve the concept's static stability.
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