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Aviation History
1973
1973 - 1569.PDF
FLIGHT International, 7 June (973 almost certainly be equipped with auxiliary propulsion in the form of two Pratt & Whitney J60 turbojets. Both in fact will have provision for the auxiliary propulsion if the company so decides. The lift engine on both aircraft will be a UACL PT6T Turbo Twin-Pac, rated at 1,800 s.h.p. The S-69s will have two counter-rotating, coaxial, three- blade rigid rotors employing the ABC principle, which is designed to give increased speed and manoeuvrability without departing too far from the present state of the art in rotor design. The idea is that by adopting twin sets of counter-rotating rotors and making the blades as rigid as possible, the blade napping (and lift loss in the advancing sector) of conventional rotors can be avoided. Quite substantially increased lift can be generated across the whole rotor disc. The blades are designed to maintain a constant angle of attack in the retreating sector and to generate no lift, which in a conventional rotor would be required to balance that from the advancing sector. As might be expected with counter-rotating main rotors, the usual anti-torque tail rotor and all of its asso ciated moving parts and gearboxes have been eliminated on the ABC research aircraft, and as can be clearly seen from the accompanying illustrations a twin-fin layout has been adopted which has conventional aircraft-style elevators. Power savings, reduced maintenance and in creased reliability are quoted by Sikorsky as advantages of this layout. Dimensions of the S-69 are: rotor diameter, 36ft; fuselage length, 40ft 9in; overall length- including blade, 41ft 5in. From this it can be seen that the rotor blades are not as long as they would almost certainly be on a conventional rotor, giving rigidity additional to that arising from their being hingeless. Shorter blades have increased drag in the hover and clearly involve higher rotational speeds. It seems likely therefore that they will have swept- back tips (present, for instance, on the S-67 blades) to avoid compressibility problems. While rotor drag difficulties may be encountered at the low end of the horizontal speed scale, the S-69 is essentially a high-speed research vehicle and trials with the rotor in the 40ft X 80ft Nasa wind tunnel at Ames have demonstrated the aerodynamic and structural performance at speeds up to 345 m.p.h. The ABC programme is being conducted under US Army contracts worth $13-2 million. The military designation of the S-69 is XH-59A. It seemed at one time that the S-69 airframe would be substantially the basis for that of Sikorsky's proposal for the US Army's Uttas (Utility Tactical Transport Aircraft System) requirement, but things have changed and there is now little or no resemblance between the two. Company- designated the S-70, Sikorsky's Uttas (in competition with a Boeing Vertol design) is a tailwheel aircraft with large aft-sliding doors which are claimed to permit a squad of 11 troops to board within four seconds and leave within three. The rotor-blade spars are titanium with a glass- fibre skin, and the tips are again swept. The S-70 has elastomeric rotor heads which require no lubrication and are said to eliminate more than 80 com ponents compared with earlier designs. The gearboxes are grease-lubricated rather than by oil; the latter type ' needs more maintenance and is more susceptible to battle damage. The rotor head also includes bifilar vibration absorbers which have been shown on the S-67 to give a 40 per cent reduction in maintenance man-hours. The principle of the canted tail rotor on the design, according to Sikorsky, is to give increased vertical lift and greater flexibility in e.g. limits. The tail rotor itself is, like that of the S-67, made up of two composite-material beams clamped together to form four blades. This arrange ment eliminates all rotor-head bearings and is expected by the manufacturer to cut the weight of the tail rotor by about 40 per cent. Blade pitch control is to be achieved through the inherent flexing characteristics of the com posite spar. The aircraft has a gross weight of 15,8501b or so and carries 11 troops plus a crew of three. Rotor diameter is 53ft, about 5ft more than that of the UH-1 currently in service, allowing one aircraft to be carried in a C-130, two in a C-141 or six in a C-5A without any disassembly. The second of Sikorsky's two ABC research helicopters will have two P&VV 60s for auxiliary propulsion, mounted as seen in this artist's impression. Thrust of this engine is in the 3,0001b bracket and may take the S-69 up to speeds of more than 300 m.p.h. While the Uttas was the only type present at Paris in model form on the Sikorsky stand, the company's proposal for the AAH (Armed Attack Helicopter) was in evidence in the form of a photograph. Choice of contractors to build competitive prototypes has yet to be made, so it remains to be seen whether the S-71 will see the light of day. Nevertheless, the company will clearly be drawing heavily on its experience gained with the S-67 Blackhawk gunship. The AAH is to be considerably more "austere" than envisaged when the Lockheed Cheyenne was being developed, but as Sikorsky division president Wesley Kuhrt says: "In the rotary-wing business, it's easier to scale down than up." Meanwhile the Blackhawk is now being modified to test the so-called fan-in-fin concept which, with its variable-pitch, ducted fan, is being developed by UAC's Hamilton Standard division with the approval of Aerospatiale, on whose fenestron rotor the design is based. The whole project is being undertaken under a $2-1 million contract from the US Army's Air Mobility Research and Development Laboratory at Ft Eustis. On what perhaps seems a more mundane front, Sikorsky expects to begin construction soon of two proto types of the CH-53E heavy-lift helicopter for the US Navy and Marine Corps. This three-engined growth version of the CH-53D is designed to be able to lift a 16-ton external load or carry 9-3 tons internally. In amphibious opera tions, Sikorsky claims that the CH-53E would be able to airlift 93 per cent of a Marine division's combat items, compared with the 36 per cent that current helicopters can cope with. The entire propulsion system, including the seven-blade, 79ft-span main rotor and the four-blade, 20ft-span tail rotor, recently completed a 250hr endurance test cycle, which confirmed the ability of the uprated main gearbox to accept the combined 11,570 h.p. at take-off of the three engines. Wind-tunnel tests have also been carried out with the main rotor blades and the tail rotor which, like that of Uttas, will be canted to 20°. First flight of the air craft is scheduled for March 1974; after preliminary evaluation the second stage of development would begin with the building of two pre-production models. A pro duction decision is not expected before early 1976. Shortly before the Paris Show, Sikorsky announced that it had been awarded a US Naval Air Systems Command contract to study four helicopter concepts for future Navy and Marine use: the ABC rotor, the pure helicopter, the compound helicopter and the so-called RVR (Reversing- Velocity Rotor) system. A similar contract has been awarded to Boeing Vertol to study four other designs, including the tilt rotor. The main targets for the research are anti-submarine and medium assault transport heli copters for about 1985. The US Navy is eventually planning to replace its ASW SH-3s in its HXA programme and the Marines are planning to replace medium-lift CH-46s in the HMX programme. At first Sikorksy will project different aircraft for the two services, then assume use of the same dynamic com ponents in different airframes, and finally look at a single aircraft to meet both requirements.
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