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Aviation History
1955
1955 - 1479.PDF
7 October 1955 This "Flighf photograph, studied in conjunction with the drawing opposite, gives an excellent idea oi the Ultra-light's compactness. Note the small rotor diameter, which has enabled a direct tilting-head type 0/ control to be employed. Fortunately, there was an obvious powerplant already available from a British manufacturer. This was the Turbomeca Palouste, re-engineered by Blackburn and General Aircraft, Ltd. The original French engine first ran several years ago and is now a reliable and well-proven unit. The Blackburn engine not only has provision for a far more comprehensive range of accessories but employs a turbine machined from Nimonic 90 alloy, as a result of which the engine can run hotter and give somewhat more power than its French prototype. An oversize centrifugal compressor is employed, the excess delivery from which is bled off from the central casing of the engine surrounding the annular combustion chamber; pure air is extracted, heated only by com- pression and untainted by any combustion products. At the point of delivery from the engine the extracted air has a pressure of about 41 lb/sq in gauge at maximum r.p.m. of 35,000. It would, of course, have been quite possible so to design the rotor as to obtain sufficient tip drive from this airflow alone. In the French S.N.C.A.S.O. Djinn, the nearest equivalent to the Fairey helicopter, this is in fact the case, the rotor producing something like 100 horse-power at maximum power using air alone. Previously (with the various types of Ariel helicopter) S.N.CA.S.O. had attempted to inject fuel into airjets at the tips of the rotor blades and thereby increase the total rotor horse- power. This line of development was, however, abandoned several years ago, partly on account of excessive noise but, we believe, chiefly owing to difficulties experienced in metering the air and fuel flows along the rotor blades, and in obtaining efficient propulsive jets. Undeterred by S.N.C.A.S.O.'s decision, the Fairey Company set out to employ tip-burning, using a weak mixture at the pro- pulsive jets, thereby securing cooler running and reducing the noise problem and at the same time providing the aircraft with considerable development potential. For the best all-round efficiency it would be desirable to have, at the tip of each blade, a propelling nozzle with an adjustable orifice, so that the cross-section of the jet could be decreased when operating on air alone. This arrangement is, in fact, somewhat impracticable, and in the Fairey helicopter the orifice size and the whole air system are designed to reach peak efficiency at maximum power. Operating conditions are, however, efficient and stable over a wide range of power, and the use of tip burning is not reflected in increased back pressure at the Palouste. Were the rotor to run on air only—i.e., without tip burning—a special nozzle would be required. The actual control of power is obtained in the following manner. On the end of the collective-pitch lever is a conventional twist- grip throttle which, operating through the Palouste constant- speed unit, varies the engine r.p.m. and hence the air delivery and pressure. Movement of this throttle grip also turns a cam which operates control regulators for the up-jet fuel supply. The Palouste is started electrically and the air delivered bled to atmosphere through a discharge pipe immediately above the engine and under the control of a manually-operated blow-off valve. When the latter is closed the air is delivered through a large lagged supply-pipe leading to the rotor and thence through the rotor blades themselves to the propulsive jets at their tips. Fuel—kerosine, from the same tank as that which feeds die Palouste—is then fed to the rotor by a low-pressure pump, and metered at the head itself. Flow of fuel down the blade is, to some extent, discontinuous; but the droplets of fuel coalesce at the injection nozzles under the centrifugal load to form a con- tinuous spray. The latter is then ignited by sparking plugs in the miniature combustion chambers. The life of the latter cannot yet be fully determined, but it is significant that all flying has so far been on one pair. At full power some 250 gas horse-power are available from the Palouste, but the Fairey helicopter does not need anything like so much. Were the full potential power available to be employed, the maximum weight could be greatly increased and a multitude of jobs tackled. Operation of the tip jets has been found to be as easily con- trolled and flexible as is that of the Palouste itself. Combustion can be sustained over a wide range of pressure ratios and mass flows. When power is reduced from the maximum value, the fuel supply to the tip jets is cut down at a proportionately greater rate so that the mixture strength is weakened for cruising flight. The pilot has a choice of engine speeds between 35,000 and 25,000 r.p.m., the latter corresponding to the flight-idling position. Even with the uirottle twisted right down to the flight- idling stop, the tip jets remain alight. The rotor horse-power cannot, therefore, be reduced to zero in the air; but the power available at the flight-idling condition is less than that needed for level flight and the aircraft can therefore make normal descents at this setting. For really rapid descents, however, it is possible to open the blow-off valve and thereby cut off the supply of au- to the rotor—thus, in effect, putting the machine into auto-rota- tion. Fully auto-rotative descents have not yet been carried out, but no difficulties are expected. Under development is a revised form of fuel regulator interlinked with the blow-off valve in order to provide single-level control of rotor horse-power from 100 per cent power down to zero. In the design of the rotor, the great reserve of power available has enabled the Fairey designers to consider criteria other than pure performance in selecting the design parameters. The low diameter of 28ft has been made possible by operating at high r.p.m. and accepting a fairly low mean operating lift coefficient. The Djinn, on the other hand, has a rotor designed principally upon performance factors, with considerably greater diameter and operating CL and, owing to the use of tip jets employing Concluded on page 592)
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