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Aviation History
1982
1982 - 0655.PDF
The key to X-wing is circulation control, which produces lift from an elliptical aerofoil by blowing air through a spanwise slot at fixed-wing altitudes, some 10,000ft higher than conventional helicopters, winning significant drag reductions and improvements in engine effici ency. Peak lift: drag ratio for a heli copter is about four; that for a tilt- rotor is about eight at 200kt, sea level, and nine at 250kt, 20,000ft. Tilt-rotor qualities have been dem onstrated by the Bell XV-15 testbed. These include a 301kt forward speed, a 21,000ft cruise altitude, 27,000ft ceiling, and a 70kt-wide helicopter- aeroplane conversion corridor. Mini mum speed for conversion depends on the nacelle angle, as this determines the downwash over the wing. Gener ally, the XV-15 can convert to and from helicopter mode at any speed between lOOkt and 170kt, although the tilt-rotor can accomplish a rolling short take-off with rotor axis tilted 25° forward. Bell sees no major obstacles to developing an operational tilt-rotor. Existing engines would be used, with the potential for later installation of advanced powerplants. Transmission ratings required to lift a 35,0001b medium-lift aircraft would be similar to those installed in Bell's 17,0001b 214ST — each nacelle would contain two turbines feeding a single trans mission. A composite wing would reduce the empty-weight penalty of tilt-rotor, and the stiffness-designed component is ideally suited to carbonfibre construc tion techniques. Tilt-rotor would also benefit from other rotorcraft techno logies, including electronic flight controls, composite airframe, blades and rotor hubs, and improved vibra tion reduction. For speeds above 350kt the rotor must be stopped, then stowed or used as a wing. X-wing takes off as a heli copter, with a four-blade shaft-driven rotor. At speeds between 170kt and 250kt the rotor is slowed, then stopped to form an X-planform wing with slender lifting panels. The aircraft can then reach 500kt, driven by an integrated lift/propulsion powerplant. The key to X-wing is circulation control. The blades have an elliptical section with rounded leading and trail ing edges. Air pumped into the hollow blades exits through narrow slots run ning the length of the upper surface at 5 per cent and 95 per cent chord. Blowing reduces drag and produces lift essentially independent of blade pitch and airflow velocity. In the helicopter speed range, cyclic pitch variation is replaced by cyclic lift modulation using trailing- edge blowing. Collective pitch is re placed by direct control of the engine- driven compressor supplying blowing air to the rotor—increased blowing equals more lift, for example. The rotor is not articulated, lacking even a pitch-change bearing. At high advance ratios and during conversion, where there is locally re versed flow over the retreating blade, air is ejected from leading- and trailing-edge slots simultaneously. In the transition to aeroplane mode, for ward speed is held constant while the rotor is slowed, then locked in any 45° position. Blowing switches to the trailing-edges of all four wing panels to provide pitch, roll, and lift control. A major advantage of circulation control is its simplicity. Cyclic airflow scheduling is controlled by a non- rotating valve and port assembly in the hub. The lack of moving parts allows the hub to be enclosed in a streamlined fairing to reduce drag. The same simplicity allows the easy implementation of higher harmonic blowing control. Circulation-control testbed Application of circulation control has awaited development of extremely stiff composite blades, both to control slot dimensions and to resist the aero- elastic divergence of forward-swept wings. Kaman has built and flown a circulation-control helicopter rotor on an HH-2 testbed. Results have been encouraging, with stall problems en countered at around 20kt, in the tran sition between vertical and edgewise flow, now traced to aerofoil design rather than to anything fundamental. Lockheed has built and tested X- wing windtunnel models, successfully demonstrating transition to aeroplane mode. The company was chosen be cause of its experience with rigid rotors, and the associated problems with unwanted gyroscopic precession. This causes the rotor to roll when the pilot commands a pitch manoeuvre, and was overcome on the Lockheed AH-56 Cheyenne attack helicopter by a moment-feedback control system. The company has now dropped out of the programme, with Boeing Vertol and Sikorsky picking up the techno logy under Defence Advanced Pro jects Research Agency contract, with the aim of building a manned X-wing demonstrator. The aircraft will be powered by a General Electric con vertible fan/shaft engine—a TF34 modified to produce both turboshaft torque and turbofan thrust with the At high speeds, the X-wing uses a conventional tail for directional control. With rotor turning, however, torque is reacted by vectoring power- plant thrust FLIGHT International, 13 March 1982 625
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