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Aviation History
1949
1949 - 1068.PDF
JUNE QTH, 1949 FLIGHT 673 The Pressure-Jet Helicopter turned through 90 deg to develop a jet tangential to the tip path. In this instance, any reasonable variation in duct airflow chordwise was not too serious with regard to the formation of local hot-spots, the pressure drop across the burner was low, and the combustion problems were conven- tional. However, the design of the 90-deg elbow nozzle was difficult and resulted generally in large losses unless extreme care was taken. Further- more, it was difficult to avoid local hot-spots at the turn, and this might result in early material failure. In the second class of tip-burner de- sign, the burners were located normal to the duct airflow. In this instance, the relatively low-velocity air was mixed with fuel at each burner and combustion proceeded chordwise through the burner, thereby elimin- ating the problems associated with the 90 deg turn of the exhaust. This design, however, required careful attention to the chordwise gradient of duct airflow and, where a relatively large number of burners were used, might result in an excessive pressure drop across the burners. The duct losses, which came from the delivery of a quantity of air supplied by a compressor mounted in the fuselage and delivered to a burner installed at the blade tips, were difficult to evaluate and, in some designs, difficult to control. These losses were due to a drop in the air pressure during transit from the compressor to the tip and, consequently, in losses of power. The pressure drop was a function of the duct length, duct roughness, hydraulic radius, and to the square of the air velocity. Practical design considerations often dictated a weight-flow and pressure ratio compromise which would result in a reason- ably low duct air velocity to minimize duct losses. For conventional designs of the pressure cycle, a duct air velocity up to 200ft/sec permitted a reasonable compromise between duct losses and practical structural design. In the pressure cycle, it was important to know the variation of power output or jet thrust with pressure ratio and weight- flow, since, under certain conditions, large losses might occur. In Fig. 2 are shown results of an investigation of the relative change in rotor horsepower with a relative change in nozzle inlet pressure for various pressure ratios. At low pressure ratios, even a small pressure drop due to duct losses repre- sented a relatively large percentage of total pressure and resulted in a large power loss. At the higher pressure ratios, the effect of pressure drop on power was relatively small. Because the pressure drop in a rotor system containing many bends, flexible ducts at flapping or teetering hinges, and relatively rough duct surfaces was difficult to evaluate and control, con- servative design practice would indi- cate the use of the higher pressure ratios. Where range and endurance outweighed maximum lift, the higher pressure ratios had the additional advantage in lower fuel consumption, as shown in Fig. 3. Compressor power was transmitted as a given weight-flow of air at a given pressure ratio. The variation of -- B II INLET TOTAL TEMP = 98O*G. P =INLET TOTAL PRESSUREPo=AMEMENT STATIC PRESSURE ROTOR - FT./SEC 4OO 500 6OO ROTOR-BLADE NOZZLE ABSOLUTE-INLETTOTAL-PRESSURE RATIO Fig. 2. Curves of relative change in rotor h.p. with relative change in inlet pressure for various pressure ratios. 1O2 I-4I-K6 Fig. 3. Curves of thrust, pressure ratio and specific fuel consumption plotted against air weight-flow. pressure ratio with weight-flow for constant compressor power was thus of interest. This variation is plotted in Fig. 3, where, in addition, is: shown the effect on rotor horsepower of the variation in weight-flow when the compressor power remains the same. Although the design study from which Fig. 3 was derived was based on a single rotor system with a particular disc area, solidity, and tip speed, the effect on lift and fuel consumption due to variations of weight flow and pres- sure ratio were representative. It was apparent that for high lift and low endurance, the large weight-flow and low pressure ratio combination was best, if the pressure losses could be held to a small percentage of total pressure. Mr. Douglas concluded his lecture with a reference to rotor design. In general, it could be shown that for maximum jet-rotor power the weight- flow should be high for a given com- pressor power output, and that, for maximum lift, the solidity should be low. A low solidity, however, made for a small duct which, with a high- weight flow, produced a high duct-air velocity and high duct losses. At the higher weight-flows, the fuel consumption was relatively high. It was, there- fore, necessary to make blade layouts and design studies to determine maximum duct area consistent with blade strength and centre-of-gravity location. For a given- rotor solidity, the duct area was inversely proportional to the square of the number of blades, making the duct area of the three-bladed rotor only 44.5 per cent that of the two-bladed rotor. On the other hand, the two-bladed rotor needed a vibration isolation system in forward flight which added weight and complication. - • -• Blade Structural Design A limit for the amount of area within the blade contour that might be used for a duct was set by chordwise centre of gravity location. Since it was desirable to maintain the chordwise e.g. forward of the 25 per cent point With a minimum of leading edge balance weight, a compromise was necessary between duct area and weight, because increased duct size carried additional structure aft. Where high pressure-ratios and low weight-" flows appeared advantageous, the local effect of the high internal pressure on the blade structure had to be con sidered. Further, the high pressure- ratios also resulted in higher duct-air temperatures, which in turn necessi- tated an investigation of the materials used for blade structure at elevated temperatures. Whether a flapping or teetering blade was used depended on the number of blades, the complication of the flexible duct at the flapping or teetering hinge, and the rotor size. The three-bladed rotor would probably not be used in a low-pressure-ratio design because of the limiting effect of the duct area on power. The design of the flexible connection at either the flapping or teetering hinge was a vexing detail design problem which was easily overlooked and could be- come a source of recurring troubles. In a teetering rotor, it was con- venient to use a plenum chamber for the flexible connection from the com-
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