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Aviation History
1965
1965 - 0035.PDF
21 HJGHT International. 7 January I96S HELICOPTER POWERPLANTS: THE WORLD SCENE ... On the other hand, the torque/r.p.m. relationship gives an unstable type of drive necessitating adequate power reserve. The free-turbine engine is more flexible, offers choice of output speed to suit con- ditions and gives an inherently stable drive. No clutch is needed. Because gas generator speed is proportional to power demand, rapid acceleration is more difficult to achieve, and variable vanes and blow-off are used. Helicopters operate in ice, rain, dirt and sand and, when hovering near ground or sea, the engines must withstand erosion, corrosion and foreign object damage. Much research is being carried out on intake protection. Everyday protection against foreign object damage is essential, but the power-loss penalty must not be too great. Effective anti-icing is generally achieved by compressor-bled hot air. Satisfactory erosion protection is difficult to achieve, but both axial and rotational momentum separators, felt filters and porous niters are all being investigated. Corrosion is extremely penetrating and the use in compressors of high-grade steels, titanium and protective coatings is being explored. Bristol Siddeley have laid down a technique of daily washing for turbine engines operating over sea, with excellent results. In the larger helicopters multi-engine powerplants must be considered. Twin-engined helicopters are quite common, but the Super Frelon has three Turmos and the Agusta AZ.101 has three Gnomes. The case for multi engines lies, of course, in engine-out capability. For hover cases, the loss of 50 per cent of the installed power in a twin-engined helicopter could lead to difficulties in recovery. Obviously, the greater the number of engines the lower the proportionate power loss, but cost and complexity must increase with the number of engines. Contingency ratings have been developed in the United Kingdom to provide a margin of power for the engine-out case. Front or rear drive from the engines depends on the relative importance of intake or exhaust losses in the installation concerned. For the Bristol Siddeley Coupled Gnome for the Wessex 2 and 5 the engine company has to supply a coupling gearbox as well as the twin engines. Present-day trends, however, favour all reduction gearing being incorporated in the helicopter rotor gearbox with engine supply finishing at the high-speed drive shaft. The excess power available demands an accurate light torquemeter and re- search on various types applicable to high-speed shafts is under way. Control In the helicopter powerplant, power is transformed directly into lift by virtue of the rotor. The method of control that has evolved is to allow the pilot to vary rotor pitch while rotor speed is main- tained within the narrow limits dictated by aerodynamic and mechanical considerations. In the piston engine days, the pilot man- ipulated both engine throttle and collective pitch lever and, by careful anticipation, maintained r.p.m. almost constant, but rotor governing was introduced with the turbine so relieving the pilot of a difficult task. In the Gnome free-turbine engine there is a simple flow- control fuel system, automatically regulated for height change, with the throttle set by an electric motor in response to signals from an electronic computer. The computer acts to maintain constant free-turbine speed no matter what load is imposed by collective- pitch change. Input signals to the computer are taken from free- turbine speed, compressor speed, gas temperature and air inlet temperature. Protection circuits safeguard against overspeed, overtemperature and surge. The rotor governor works on a droop law sufficient to ensure stability and allow adequate load-sharing in the multi-engine case. In order to cater for rapid manoeuvres a mechanical link can be introduced between governor datum and collective pitch, thus giving a degree of anticipation, and also eliminating static droop. The pilot can select a 15 per cent range of governed speeds. Transmission failure is covered by a separate overspeed governor which rapidly cuts off the fuel. There is no doubt that rotor governing systems have helped helicopter pilots, but any automatic device must have good relia- bility. Full-authority governing demands such reliability, particu- larly in single-engined helicopters operating in difficult conditions; and opinion is growing on the need for easy reversion to manual control in such single-engined types. The gas turbine-powered helicopter is now an important world- wide machine with large production orders and will continue as such for at least a decade. Design groups throughout the world Basic gas generator section of the GE T64, a unit which can serve as turboprop or turboshaft and represents the basic power units referred to by the author. Rristol Siddeley hold the production licence are studying types of aircraft to supersede helicopters, maintaining VTOL characteristics but offering greater productivity by increased forward speed. Experiments in the United States on the addition of turbojet thrust engines to Bell and Lockheed helicopters show that speeds in the region of 200kt are possible. This method, however, must be extremely wasteful in fuel consumption because of the poor efficiency of the turbojet at such relatively low forward speeds. Propulsive efficiency can be improved with turbofans, but the helicopter rotor imposes considerable stress and parformance problems at high speeds. Rotor loading can be reduced by fitting small wings, so partially eliminating Mach number effects on blade-tips. In Germany, the BSlkow 46 with Derschmidt lead-lag high-speed rotor system is being flight tested and promises speeds of 200kt. Another method, typified by the ill-fated Fairey Rotodyne, is the use of a rotor driven by hot or cold gas for take-off and by turbo-propellers for normal flight. In the United States the Hughes hot cycle research helicopter is a variation on this theme. Exhaust gases from two T64 gas gener- ators are ducted directly through the rotor head to tip-jets. A further step is the tilt-wing aircraft in which the whole wing, engine and propeller rotate through 90° to provide necessary vertical thrust for take-off and then back to the more conventional position in the transition to forward flight. The Chance-Vought Hiller Ryan XC-142A using four T64 engines is now in active flight development and promises to combine hovering performance with increased forward speed. Tilting rotor and tilting ducted-fan aircraft such as the Curtiss Wright X-19A and the Bell Aerosystems X-22A illustrate two other methods of approach. The flying crane helicopter is another type which could grow from machines such as the Sikorsky S-64. For the huge lifts needed one can consider the installation of jet engines at the tips of the rotor blades to give the necessary power drive, so eliminating the heavy reduction gearing otherwise needed. There is, therefore, a need for engines to grow, not only in their present form, but also in different forms for varying applications. Because development of new engine types requires large financial investment there is great pressure to develop successful engines in various growth forms. The proposed pattern of development for the Gnome engine takes the original 1,000 h.p. turboshaft in steps, by the use of a compressor zero-stage, a two-stage power turbine, and a cooled gas-generator turbine to much greater power output in the same scantlings and with reduced s.f.c. A co-ordinated General Electric/Bristol Siddeley programme is being pursued with exchange of components to make the programme possible. Again the growth pattern for the T64 will very materially improve the initial rating of 2,850 h.p. largely through the use of a cooled turbine and higher airflow. High gas temperature is essential for improving power and thermal efficiency of future turboshafts. At Turbomeca, the original centrifugal engines have been improved some 30 per cent in power and fuel economy by the addition of a high pressure-ratio single axial compression stage. Later developments, with the addition of another zero axial stage will give additional improvements of up to 15 per cent. Engine weight and first cost must be reduced, and items such as cast rotors are accordingly being developed. Heat exchangers are being examined in several countries, because high thermal ratio leads to a simple engine cycle. Considering the varied roles of helicopters, a "bolt-on" heat exchanger for use during long-range flights may be attractive. Helicopter engines are therefore being developed as basic growth units and in new forms to meet present and future requirements, offering lower weight, improved efficiency and reduced first cost.
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