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Aviation History
1954
1954 - 0691.PDF
140 r 120 20- ONE TWO THREE ENCINE ENGINES ENGINES POWER FOR 600 ft/min R. of C. "T •TAKE-OFF (COMPOUND) CRUISE 3x1 HOUR 2x1 HOUR 2.CRUISE P TAKE-OFF xCRUISE 2x1 HOUR •1x TAKE-OFF (EMERGENCY) 1«1 HOUR OPT SPEED V V MAX. MAX. DOMAIN OF THE HELICOPTER... backward take-off (Fig. 2), Mr. Hafner considered the various types of power unit in helicopter use. Those used for tip drive were: (1) The ramiet and the pulse-jet. Both these engines were extremely light, partly intrinsically and partly because they replaced blade ballast. On the other hand, their fuel consumption was very high. (2) The p-essure jet. This_ had a lower fuel consumption but was heavier and more complex in its installation. (3) Turbojets. These units had been proposed for installation at the rotor blade tip, not only for large slow-rotating rotors, but also for smaller ones. Such an arrangement combined the advantages of low weight, simplicity and low fuel consumption, but the main difficulties were in the high centrifugal and gyroscopic forces to which the engines were exposed. The power forms for shaft-drive were: (1) The piston engine combined with a clutch. This power unit was economic in consumption, but was heavy and involved complexity in installation. The power characteristics of the piston engine did not, unfortunately, suit the helicopter and this incompatibility was aggravated in multi-engined types. (2) The free gas-turbine. This engine had higher fuel consumption than the piston engine, but it was lighter and its installation was much simpler. Its main advantage, however, lay in the fact that the only driving connection between the power producer (the gas generator) and the rotor-driving turbine was the gas stream. Not only did this eliminate the clutch of the piston engine, but it also served as an infinitely variable gear reduction over a useful range of speed ratios, without appreciable loss in efficiency. A consideration of weights and fuel consumptions for various types of power plant (see Fig. 3) showed that the light tip jets had the advantage in short-range flying, whereas the elaborate but more economic installations were superior for the longer ranges. The Three Basic Types.—Turning to the three basic types of helicopter denned earlier in his paper, Mr. Hafner proceeded to describe the distinctive features of these respective machines: The Pure Helicopter.—This type of machine relied wholly on its rotors for sustentation as well as propulsion. The weight- carrying rotor or rotors remained substantially horizontal and the axial component of airflow was never very large. The critical limitations of the rotors were in the tip speed and the tangential advance ratio. The top limit was formed by that forward speed corresponding to the highest tip Mach number that could be achieved without the development of shock. The low-speed limita tion resulted from the stalling of the rotor blade, which depended Fig. 3. Specific weight (total weight of power plant installation and of fuel consumed per maximum continuous h.p.) against flightdura- tion, for various types of power plant. I 2 FLIGHT DURATION (hr) FLIGHT, 12 March 1954 315 P*J£- C0NICAL OBSTRUCTION SURFACE SLOPING I M 10 FROM 300 FT. DIAMETER CIRCLEx c I SAFETY HEIGHT ">..WRflH-** SAFETY MARGIN * - rzi Fig. 1 (left). The power requirements of a pure helicopter of moderate cruising speed, expressed as a percentage of that needed for hovering and shown with variation in speed. Fig. 2 (above). The procedure for backward take-off at 60 deg, intended to reduce the danger of engine failure after take-off in a built-up city area. on the blade loading factor. The practicable maximum cruising speed, in the case of the typical pure helicopter, was of the order of 247 ft/sec =146 kt at sea level. There were, however, other limitations which had to be con sidered. A study of these factors showed that compromise had to be made between the conflicting desiderata, and much depended, therefore, on the specification against which the project was made. Thus, for instance, a single-engined helicopter must have a satis factory autorotating performance to permit forced landings to be made. This requirement restricted the disc-loading and, to some extent, the blade loading and the tip speed. In multi-engined helicopters, however, where this argument did not arise, the optimum disc-loading was of the order of 6.0 lb/sq ft. Summing up, it could be said that the speed envelope of the pure helicopter was controlled mainly by, (a) the design of the blade tip and, (b) the blade loading, and speed increases beyond certain limits were bought at progressive expense in power. The Compound Helicopter.—There were many forms of com pounding the rotor with other aerodynamic devices in order to advance the performance of the helicopter, but by far the most effective means was the stub wing. Such a wing had no effect at hovering or during slow-speed flight, but as speed was gained it progressively supplemented the rotor thrust until, at a given maximum speed, a worthwhile percentage of the all-up weight was taken by the wing, the rotor being correspondingly unloaded. The effect and the value of this action could be studied by refer ence to the previous example of the pure helicopter. If it was now assumed that a given stub wing was capable of carrying one half of the a.u. weight of the pure helicopter, at its maximum design speed, then the blade loading factor was reduced in this flight condition to half the previous limiting value. Thus the speed of the helicopter was increased without change either in the rotor blade area or rotor profile power. The rotor coning angle was also reduced and this diminished vibration; alternatively it would permit a reduction in blade weight. On the other hand, if no speed increase was desired, the stub wing could be used to enhance the economy of the helicopter, as it then permitted an increase of the blade loading factor by two, without ill effect, but with the advantage of a reduction in profile power. Allowance must be made, on the debit side, for the drag and the weight of the fixed wing. There was, however, a certain net gain, because the fixed wing was more efficient in the production of lift at speed than was the rotating wing, and by mounting the undercarriage to the wing a considerable weight saving was achieved. There were, nevertheless, certain limits which must be observed in the unloading of the rotor. The helicopter was basically con trolled in flight through its rotor, or rotors, and a loss in rotor thrust meant a loss in some of its control. If, therefore, the rotor unloading assumed considerable proportions, it was neces sary for the rotor control to be supplemented by a wing control. In addition, there was the problem of propulsion. In a pure helicopter, sustentation and propulsion were obtained together by a small forward tilt of the rotor vector. Compounding the rotor with a stub wing would, however, decrease the rotor lift whilst at the same time increasing the aircraft drag. This meant that the rotor thrust vector had to be given a considerable for ward inclination which, at a certain stage, began to introduce aerodynamic complications. These difficulties might to some extent be delayed by additional compounding with an airscrew or with a turbojet. A high degree of rotor unloading might some times be combined with a reduction of rotor speed in order to reduce rotor profile power; this, however, produced high advance ratios and, with them, the danger of reversed flow flutter. The Convertible Helicopter.—Although enthusiastic supporters of the convertible helicopter had been active for some time, serious endeavours in this field with a chance of practical success were a comparatively recent development. There was wide scope for
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