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Aviation History
1992
1992 - 0262.PDF
of the Cl-84. Four prototypes were built and demonstrated — "fairly successfully" — but the tilt-wing eventually lost out to the C-130 Hercules, ,says Kocurek. The similari ties between the XC-142 and the V-22 are not as great as those between the Cl-84 and XV-15, but the two aircraft are close enough for Kocurek to make his point. The XC-142 was powered by four T64 turboprops, while the V-22 is powered by two T406 turboshafts, but the power in stalled in both aircraft is equivalent — around 9,170kW. The tilt-wing had a VTO weight of 18,145kg, much less than the tilt-rotor's 23,000kg; but the XC-142's lower empty weight of 10,680kg, compared to the V-22's 15,700kg, translates into a higher payload — 3,210kg against 2,600kg for a 370km (200nm) mission, Kocurek calculates. HOVERING ARGUMENTS A tilt-wing was considered for the joint- services vertical-lift aircraft requirement which produced the V-22, but the long hover times required by some missions favoured a tilt-rotor. Kocurek does not dispute that a tilt-rotor is a more efficient hovering machine than a tilt-wing: instead, he uses his comparisons to illustrate that the penalty of hovering on propellers, rather than on rotors, is not onerous. "If we want to move people from point to point...the issue is vertical take-off capa bility, .not hover capability or, in particular, hover efficiency," Kocurek argues. "If you want to hover, buy a helicopter." Power loading is the traditional way of expressing hover efficiency, Kocurek says. Power loading is defined as aircraft weight divided by installed power — a more efficient aircraft requiring less powerful engines to hover at a given weight. Hover efficiency is also a function of disc loading: the aircraft weight divided by rotor (or propeller) disc area, larger rotors being more efficient. A VTOL aircraft with high power loading and low disc loading is the most efficient at hovering. Bell's Model 412SP, for example, a 5,440kg (12,0001b) twin-turbine, single- rotor helicopter, has a power loading of 2.9kg/kW (8.61b/shp) and a disc loading of 35kg/m2 (7.21b/ft2). The Cl-84 and XV-15 had lower power loadings, both around 1.35kg/kW, but the tilt-wing's disc loading, approaching 195kg/m2, was more than twice the tilt-rotor's 73kg/m2 and more than three times that of an equivalent helicopter. Ishida's initial figures suggest the 6,350kg TW-68 will have a power loading of 1.82kg? kW and a disc loading of 145kg/m2. While obviously less efficient in the hover than a helicopter, the TW-68 is not as far behind a tilt-rotor as the figures suggest, Kocurek argues. He points out that power and disc loadings apply to isolated rotors and do not take into account the effect of the wing on both the tilt-wing and tilt-rotor. Sponson fuel tanks - Rearward-retracting steerable nosewheel A tilt-rotor's wing stays horizontal while the nacelles tilt. In the hover, therefore, the wing is in effect a flat plate immersed in the rotor downwash. Kocurek puts the resulting vertical drag penalty on the XV-15 at 635kg-680kg. The similarly sized TW-68's wing tilts with the engines so that its aerofoil always points into the propeller wash. Kocurek estimates the vertical drag penalty at just 23kg. A tilt-wing achieves good "system" hover efficiency, he argues, because it does not suffer the vertical drag penalty of a tilt- rotor. More importantly, a propeller has better propulsive efficiency than a rotor in the cruise — where the TW-68 will spend most of its time — because of its higher disc loading, Kocurek says. He explains the designer's dilemma: "You want low disc loading to get high hover efficiency, but you want high disc loading to match the propeller to your cruise conditions. Hover is the more critical design area, [so] you end up with more propeller or rotor than you know what to do with in the cruise. The higher I can push disc loading in the hover, the better off I am in the cruise. Tilt-wing offers that." Tilt-wing technology has one other com pelling commercial advantage that results from the use of propellers rather than rotors, he claims. "Tilt-wing aircraft evolved from a turboprop technology baseline. In particular, the mechanical systems on the aircraft are turboprop-style and closer...to turboprop simplicity [than helicopter complexity]." Ishida's studies suggest the typical hourly cost of operation for rotors, for a 6,350kg- class aircraft, is around $140. The same cost for propellers is barely $3.50. That cost "...is for everything you screw on to the top of the mast", says Kocurek. "It doesn't matter whether the aircraft is a single or a twin, the cost per aircraft seems to scale with complexity. The differ ence in costs between rotors and propellers is a direct reflection of complexity. Com- 44 FLIGHT INTERNATIONAL 5-11 February, 1992
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