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Aviation History
1952
1952 - 0221.PDF
25 January 1952 93 the use of a vapour as fuel would allow a shorter combustion chamber to be used, so reducing drag. A jet-driven rotor was the subject of a Fairey patent of July, 1946 (Fig. 12). The mass at the blade tips was greatly reduced by feeding the tip orifices with a fuel/air mixture previously compressed in the fuselage. In order to reduce the fuel consumption, the rotor jets were switched off in flight, the aircraft being propelled by an engine-driven air screw with the rotor autorotating. When the aircraft was to be used as a helicopter, the airscrew was disengaged and feathered, and the engine-drive transferred to a fuselage- installed compressor. This fed compressed fuel/air mixture to the hollow blades, within which were the combustion chambers and distributor vanes to deliver the efflux to the tip-jets. The fuel injection rate was varied with the pressure of the air delivered. The engine also drove a cooling-fan, part of whose output could be ducted to a directional nozzle at the rear of the fuselage. The main fuel supply could also be taken through a vapourizer (heated by engine exhaust) prior to introduction of the fuel to the compressor. Some of these arrangements may be seen from the accompanying drawing. It was pointed out that helicopter designs employing a fuselage-mounted gas turbine passing its efflux to the jrotor- blade tips presented a number of problems, owing to the high gas temperature required at the turbine and the con sequent excessive heating of the rotor hub and blades. Further, heat dissipated in cooling these items was largely wasted and, as a result of the high temperature and low density of the rotor gas-flow, the blade section required to be larger than the aerodynamic optimum. The consequent Fig. 12. The helicopter propulsion system patented by Faireys in 1946. The rotor jets are fed from a fuselage-mounted compressor, while an engine-driven airscrew is employed for forward flight. In 1948, a patent was filed in the U.S.A. by H. F. Pit- cairn, relating to an assisted-take-off/flight-refuelling apparatus. The patent consisted essentially of a subter ranean chamber in which were housed fuel tanks and a length of flexible hose. This hose was coupled to the heli copter, which then took off carrying the end of the hose with it, fuel being passed throughout. The hose was then re leased at a predetermined height. The final drawing shown in our precis of Mr. Hayward's lecture (Fig. 14) refers to a British patent of November, 1948, filed by the Autogiro Company of America. This concerned a control mechanism for jet-driven rotors, the chief feature being that a change in the operation of one jet influenced the operation of the other(s). In each blade tip was a suitable Fig. 11. The com plex blade pro jected by Stalker in 1946; the plan view shows the arrangement of the fuel-injection nozzles in the outer part of the • blade. large blade-surface area cooled the gas, wasting energy, and various joints in the ducting presented their own problems. Turbines mounted at the blade tips solved most of these problems but, on the other hand, involved undesirable weight concentrations at the tips and, as a corrollary, intro duced difficulties arising from gyroscopic reaction and blade inertia. To overcome these various disadvantages, the Fairey company patented a low-pressure system in 1947, shown in Fig. 13. This incorporated a small compressor operating at a moderate temperature. The compressor delivery was divided, one part of the flow being passed through a heat- exchanger to the gas turbine, and the other being ducted to the propulsion jets at the rotor tips, small ducts within the blades allowing the desired blade cross-section to be main tained. This thermodynamic cycle was, from the point of view of efficiency, almost independent of compressor delivery pressure and of the temperature in the combustion chambers. The working fluid within the blades was simply hot air at quite a moderate temperature, while the jet-nozzle outlet area was manually or automatically controllable as desired. Exhaust gases from the turbine were directed on to a rudder to enable the aircraft to be turned about a vertical axis when hovering, while, as a safety precaution, the rotor- blade pitch was arranged automatically to change for auto- rotation whenever the compressor delivery pressure fell below a predetermined minimum value. Fig. 13. The low-pressure-jet system developed by the Fairey Aviation Co., and patented in 1947. The principle is designed to overcome many rotor-jet problems by employing relatively low-temperature gas flow. jet nozzle the fuel supply to which was provided with a shut- off valve. This valve was solenoid-controlled, the solenoid circuit being made and broken by a sliding contact at.the rotor head which operated whenever pronounced leading or dragging of a blade took place. Thus, in the event of irre gular operation between the jets causing variation in rota tional speed of one or more blades, excessive movement between adjacent blades broke the solenoid circuit, closing the fuel entry valve for the jet on the "forward" blade. In this manner, a failure of one jet automatically shut off all the other jets, causing the rotor to go into autorotation and avoiding unbalanced drive conditions. Fig. 14. An American control system in which the fuel supply to rotor-tip jet nozzle is dependent upon.the angular position of each blade in relation to the other blades of the rotor.
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