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Aviation History
1957
1957 - 0181.PDF
FLIGHT, 8 February 1957 183 By-pass Assessment A Plain Marts Guide to the Rolls-Royce Conway Rolls-Royce by-pass —the Conway. IT requires a rare clarity of mind to expound with simplicityupon technical subjects. But last week, at the Institute of theAeronautical Sciences in New York, two Rolls-Royce engi- neers, Mr. A. C. Lovesey (assistant chief engineer, military types)and Mr. L. G. Dawson (chief project engineer) presented a lecture which was a model of its kind. Their subject was The By-passEngine for Transport Aircraft, and rarely has the raison d'etre of this type of engine been stated so lucidly. The lecturers opened their talk by reminding their audiencethat the turbojet engine is now an accepted propulsion unit for transport aircraft (orders have been placed for over 300 jettransports). It seemed to them only a matter of time before the turbojet—operating blown flaps and deflected jets—enteredthe field of short-range transport aircraft as well. Long-range stratosphere cruising speeds of 500 kt have beenmade possible by the high thrust: weight ratio of the turbojet engine combined with aerodynamic advances of the air frame.Cruising speeds above M = 0.9 are likely to require greater effort on aerodynamic improvement than upon engine design: theDouglas and Boeing high subsonic transports will be with us for many years. But have we [asked the lecturers] found the best type of turbo-jet for this field of operation? The turbojet engine has made steady progress in the direction of better specific weight, betterspecific fuel consumption and greater thrust per unit frontal area. But the improvement of all three factors together isbecoming more difficult; the design of an engine for a special duty is becoming an interchange between the three performanceparameters. An improvement in specific consumption, for example, may usually be achieved only at the expense of higherspecific weight. In long-range transport aircraft the weight of fuel carried—about half the take-off weight and about four times the payload— is high, and for such aircraft the specific consumption will bethe most important characteristic of the engine; for short-range operation engine weight becomes more important. Consider the ways [continued the lecturers] in which a lowspecific fuel consumption may be achieved in a turbojet engine. As an example we have assumed a pressure ratio of 12:1, acompressor efficiency of 0.87 polytropic and a turbine expansion efficiency of the same value. The intake has been assumed tocause no loss. What are the principles involved? In the case of the jet engine air is inhaled, compressed, itstemperature further raised by burning fuel and then expanded through a turbine where sufficient energy is extracted to drive thecompressor. The pressure aft of the turbine is greater than that pertaining in the intake and, by expanding the gas to atmosphericpressure, energy is made available for propulsion. The gas is converted into kinetic energy by expanding it in a propelling nozzle; the thrust produced depends upon the massand the velocity of the gas. For a given mass of air, the more fuel burned the higher will be the temperature aft of the turbineand the greater will be the pressure and the higher the velocity generated after expansion. This produces a greater thrust. For the long-range transport we are concerned not only withthe thrust produced but with the efficient conversion of the heat energy of the fuel into mechanical energy. This is repre-sented by the jet velocity and the best use must be made of it to propel the aircraft. The conversion of fuel energy into mechanical energy isgoverned by the thermal efficiency of the basic cycle, and is a function of the pressure ratio and the combustion temperature.The measure of conversion of the jet energy into propulsive effort is the Froude (propulsion) efficiency and is normally written interms of the jet velocity. Or it may be written in terms of thrust per lb of air: jet velocity flight speed or 1 + thrust per 1b x g: flight speed It appears that a high propulsive efficiency requires a low jetvelocity and a low thrust per lb of air. There are two fundamental relationships required to under-stand the problem: the relation between propulsive efficiency and thrust per lb (given above) and the relationship between specificconsumption, thermal efficiency and propulsive efficiency. This is : flight speed ft/sec s.f.c. lb/hr/lb/thrust = 4,000 x thermal x propulsive efficiency In Fig. 1 the propulsive efficiency, thermal efficiency, specificconsumption and turbine inlet temperature are plotted against thrust per 1b of air for a typical high-pressure-ratio jet engine.It will be seen that the propulsive efficiency falls off with an increase in thrust per lb of air or an increase in turbine inlettemperature. But thermal efficiency rises with increases in flame temperature, since the compressor and turbine have efficiencies ofless than 100 per cent; if these components had no losses the thermal efficiency would depend on the compression ratio alone. The point to notice is the opposing influence of the thermaland propulsive efficiencies on fuel consumption. It will be remembered that the specific consumption curve is the inverseof the product of the thermal and propulsive efficiency curves. From the same graph it is interesting to note the extremely low Fig. 1. Typical performance curves for a simple turbojet. I4OOK>8 - I2O0O-7 COMBUSTION IIOO TEMPeWTURfK Fig. 2. Thrust per Ib of airflow vs. engine weight (constant thrust). HAIM WIGHT \O 20 4O SO THRUST/LB 7O 8O Dnl55»
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