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Aviation History
1946
1946 - 2270 - 0243.PDF
DECEMBER 12TH, 1946 FLIGHT 649 AIRCRAFT PROPULSION screw or a jet, depends on the momentum per second of theslipstream. The kinetic energy of the slipstream is wasted energy, and the Froude efficiency of the airscrew orjet is 2Va/Va + Vj, where Va is aircraft speed and Vj jet speed relative to the aircraft. The airscrew also wastes powerbecause of the profile drag of its blades, and its overall effi- ciency is therefore lower than its Froude efficiency. The overallefficiency of a normal airscrew is usually about 82 per cent at top or cruising speed and 75 per cent when climbing, butat speeds greater than 450 m.p.h. the efficiency tends to drop. "There is a limit to the efficiency of jet propulsion for sub-. sonic aircraft, which proves to be a Froude efficiency of about .52 per cent at 36,000 feet and over. Choosing the ideal com- •japsion ratio for a gas temperature of 1,000 deg. C absolute, tips minimum fuel consumption works out to be 0.62 lb per',iour per thrust horsepower at 36,000 feet, and over, which is better than is likely to be obtained with a reciprocating, engine at that height. " Airscrews and Ducted Fans.—The advantages of the simplejet propulsion system aie very light weight of power plant and general simplicity. It has been seen that the Froude efficiencyof a simple jet will not in practice exceed 60 per cent. It is obvious that an increase in the mass flow for a given poweroutput will result in a decrease in slipstream velocity and a consequent gain in Froude efficiency; the lower the forwardspeed the greater will be the gain. The obvious way of increasing mass flow is to utilize anairscrew driven by a turbine that takes a fairly high percentage of energy from the jet. There will be a marked increase inthe overall efficiency of propulsion, up to speeds of about 400 m.p.h. '' Neglecting take-off and climb, it is seen that at high speedsonly small airscrews are needed, and at 400 m.p.h. and over the ' ducted fan ' scheme has advantages. The Aisual massflow through the fan is three times the gas flow through the turbine, so the total mass flow is increased four times; hencefor the same thrust and speed the kinetic energy loss in the slipstream is reduced to a fourth. '' The disadvantages of the ducted fan are its greater weightand its greater diameter. There is, however, a potential decrease of consumption of about 10 per cent at 500 m.p.h.,and this, for long-range commercial aircraft, might pay for the disadvantages. "Efficiency.—There is an optimum compression ratio forminimum specific consumption, which depends on a number of variables. In general, a pure jet unit needs a lower compres-sion ratio than a turbine airscrew unit, because with a jet the advantages of high thermal efficiency are offset by a high jetspeed, which means a low Froude efficiency. The compression may be by a centrifugal blower or by a multi-stage axial-flowunit. The former permits lighter power units, but the com- pressors are about 5 per cent less efficient and limit the com-pression ratio to about 4 to 1 at ground level. The axial-flow unit can have any compression ratio desired, but higher ratiosmean greater length. "Above about 5 to 1 compression ratio, axial-flow units aredifficult to start owing to stalling in some of the compression stages, and the result is that there is not enough power toaccelerate the unit to a working speed. This can be avoided by using two or more blowers in series, each driven by its ownturbine at an appropriate speed. It seems likely that a com- pression ratio of 12 to 1 at 36,000 feet will not be muchexceeded. '' An alternative to high compression ratio—only worth con-sidering for airscrew turbines—is the use of a heat exchanger \to transfer heat from the exhaust to the compressed air onf?fts way to the combustion chambers.- If 90 per cent thermal 'ratio can be achieved, the optimum compression ratio becomes3| to 1, and the thermal efficiency of the cycle approaches 48 per cent. It is, however, doubtful whether the increase inweight and complication will make such a system worth while for aircraft." Of power units with no moving parts, possible only at highforward speeds, the lecturers said: "This is jet propulsion at its simplest. The aircraft cannot be accelerated by this meansbelow 300 m.p.h., and unless supersonic flight can be achieved the efficiency—even at 600 m.p.h.—will be low." Turning next to the effect of temperature in gas turbines,it was stated that: "With existing materials and turbine design a maximum gas temperature of 1,150 deg. C absoluteis practicable, i.e., 877 deg. C. At this temperature the life of the blades is limited by creep under stress, but at a lower temperature—about 1,000 deg. C abs.—a big increase inlife can be expected, and it is usual only to use the higher temperature for take-off and emergencies. Temperatures of1,000 and 1,200 deg. C abs. give only slightly different efficiencies, but the higher temperatures increase the outputper lb of air per second and therefore make for lighter units. "Effect of Height on Power and Cruising Speed.—Owing tothe decrease in atmospheric temperature up to 36,000 feet the compression ratio of a blower at a given speed of rota turnincreases steadily and a blower designed for 5 to 1 ratio at sea-level will give nearly 7$ to 1 at 36,000 ieet, this beingthe chief reason why the specific cmisumpiion decreases as the height increases. "Owing to the increase in compression ratio with heightthe thrust of a jet unit falls off at a lower rate than the air density up to 36,000 feet, but afterwards (when the tempera-ture is constant) directly as the air density. The exact rate below 36,000 feet depends on the particular design, but it isgenerally between or0:8 and <r*-', where <r denotes air density- relative to sea-level. " The top speed of a pure jet-aircraft is fairly constant up tonear its ceiling, but there is generally a small increase between 30,000 and 36,000 feet. The economic speed, however, increasesin proportion to i/^rr, and there is an improvement in fuel consumption per mile as height is increased, largely owing tothe increase in Froude efficiency. With pure jet propulsion the cruising speed for the best range is 20 to 30 per cent abovethe speed of minimum drag. "When the turbine is used to drive an airscrew the gain inoverall efficiency with height is attributable not so much to the increased Froude efficiency as to the increased compressionratio resulting from the lower air temperature." The Discussion Dr. H. RICARDO, who opened the discussion, said that headmitted reluctantly that the piston engine was dead so far as military aircraft were concerned. He thought, however,that pistons would stay, and drew an interesting comparison between the recent introduction of the aircraft gas turbineand that of the marine turbine 50 years ago. The Royal Navy now employed turbines for most purposes, but this was nottrue of the Merchant Navy. Compounded engines were now also used in shipping. Commenting on the authors' figuresfor I.H.P./lb air/min and fuel-air ratio of 15:1, he said that these figures were given for the military engine of the four-cycle sparking-plug ignition type, but for civil use with lower supercharger ratio and slightly higher compression a figureof u h.p./lb air/min would be possible, and with fuel injec- tion and sleeve valves there would be increased economy, andfuel-air ratios of 16:1 could be used. An improvement of almost 10 per cent (in the region of 0.38 and 0.39 lb/h.p./hr)on the authors' consumption figures of 0.42 was, therefore, possible, with a piston speed of 2,000ft per minute. To replacelost take-off power, nitric oxide or water-methanol injection could be used. Mr. H. CONSTANT agreed with the authors' conclusions andthought that even 0.37 lb/b.h.p./hr would be possible with suitable compression ratio. He had hoped for more detail ofaccepted gas turbine performances, and added in lighter vein that the authors had apparently arrived at the right answer byintuition having been guided by pessimistic piston engine figures, hypothetical turbo-jet figures, and no figures deliveredfor airscrew turbines. Mr. DAVIS (Bristol Turbine Department) dealt with improve-ment of thermal efficiency and the Theseus heat exchanger. He showed (for the first time in public) a photograph of theheat exchanger unit and added that it could be removed from the airscrew-turbine when not needed. He proposed that anapproach be made to the ceramic industry with a view to stimulating research on materials which might be suitable forturbine components. Mr CHESHIRE said that the authors suggested that jetsmust compete with piston engines on a fuel consumption basis, but other factors had to be considered. The installed weightof a turbo-jet was about one-third of that of the corresponding piston engine so more fuel could be carried. The drag wasalso reduced to perhaps two-thirds. He believed that gas tur- bines could operate reasonably at lower speeds and altitudesthan those suggested, and named a compromised speed of 350 m.p.h. at which they could compare on a range and payloadbasis. He added that at the present time pure jet aircraft were capable of a 2,000 miles' rang* with a good payload.He doubted if the airscrew-turbine would be worth its weight and complication penalties, and questioned its noise and vibra-tion characteristics. He was sure that the case for the pure
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