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Aviation History
1944
1944 - 0332.PDF
172 FLIGHT .FEBRUARY 17TH, 1944 JET VERSUS AIRSCREW we may quote the gas turbine locomotive of the Swiss Federal Railway. In this unit, comprising an axial-flow turbine and an axial-type rotary compressor and fitted with one heat exchanger to pre-heat the air delivered to the combustion chamber, the turbine develops approxi mately 10,000 h.p., but about 7,800 h.p. is absorbed in driving the air compressor. Thus only 2,200 h.p. remains for useful work. Given a practical turbine-compressor unit, the functions can be varied within certain limits and will be determined by the duty for which it is intended. Thus, in the case of aircraft of the future, the conditions laid down for operation will determine, amongst other things, the form of propulsion to be adopted. Obviously, whether the duty is to be military or civil, long-range or short-range, high speed or medium-speed, high-altitude or normal-altitude, and similar factors, will exercise an important influence on the ultimate choice Eventually the criterion is not the specific efficiency of the power plant but the overall efficiency of the complete aircraft. Thrust-Drag Ratio The problem resolves itself into the thrust which can be made available to overcome the aerodynamic drag of the aircraft as it travels through the air. Due to its rela tively small dimensions and light weight resulting from its high operating speed, the turbine may be conveniently submerged in the wings or fuselage of an aircraft, with an extremely beneficial effect on the aerodynamic shape of the structure. The aircraft can be smaller, lighter and of much cleaner outline with, of course, a consequent reduc tion of drag. This improvement will apply in a different degree whether jet propulsion or airscrew propulsion is employed, but the advantage is more marked in the case of the jet- propelled machine. This factor is of such importance that, despite the lower thermal efficiency of the jet-propulsion plant as compared with a normal engine and airscrew, it is possible for a jet-propelled aircraft to have a better performance. At low speeds and low altitudes jet propulsion has a relatively low efficiency. It follows, therefore, that due io the inefficient thrust of the jet at low speeds, the take-off and the rate of climb are not comparable with those of an aircraft employing an airscrew. To overcome this defi ciency of thrust horse power when taking off from the air field, thrust augmentors have been proposed. These may take the form of fans or screws to increase the mass of air passing through the plant. Once a jet-propelled aircraft is airborne it can climb at high speed; but note that there is a difference between speed of climb, rate of climb and angle of climb. The efficiency of the turbine plant rises rapidly as the air flow through the blower and turbine is accelerated as the for ward speed of the aircraft increases. Also, just as the thrust horse power rises with speed, the consumption of fjiel tends to fall because the necessary mass of air for efficient combustion is now passing through the plant. Since the rate of fuel consumption is particu larly high at low speeds Hitherto it has not been revealed that the tail of the Campini jet propelled monoplane :s completely detachable. The object appears to be to enable inspection of the effect of fuel burning in the annular mixing channel. and lovv altitudes, there must be a limitation of. the operating range of the craft unless useful load is reduced. These facts suggest that at the present and immediate future stages of development, jet propulsion will be con fined to ultra-high speed, short-range aircraft or special high-speed craft operating at very great altitudes. One may deduce from such considerations that the advent of the turbine-compressor type of power unit challenges the reciprocating engine rather than the airscrew. An airscrew is able to restore the thrust necessary for a quick take-off and a high rate of climb. It will be apparent that to apply airscrews to the new turbine-compressor unit suggests very interesting possibilities. In combination, they hold great promise for aircraft intended for operation at normal flying altitudes. Factors Governing Selection In the consideration of airscrews or jet propulsion of air craft, the governing factors are speed, altitude, range and size of airfield for take-off. Decisions on these ppints arc of paramount importance to manufacturers. Until airline operators have definitely settled upon their specifications of performance required, including speed, altitude, carrying capacity, degree of comfort and non-stop ranges, it will not be possible for either aircraft or engine manufacturers defi nitely to fix their plans for peacetime programmes. It is a long and difficult process. Jf In the light of present knowledge, what conclusions can be drawn from these observations? If large passenger transports with pressurised cabins flying at 500 m.p.b. at very great altitudes are the aim, then jet propulsion will almost certainly be employed. On the other hand, if opinion is .confirmed that transports of the future will cruise at about 250 m.p.h. at the lower altitudes, which will not necessitate pressure cabins, then turbine-driven airscrews may well be chosen for propulsion. It must, of course, be presumed that the rate of fuel consumption can be improved as development work proceeds, and that metallurgists can assist by the production of better heat- resisting steels for the turbine blades. An objection to the use of airscrews with turbines is that a rather complicated and weighty reduction gear becomes necessary due to the high rotational speed of the turbine— over 10,000 r.p.m. for example. The disadvantage should be reduced materially by the adoption of the counter- rotating airscrews, at present undergoing development and test. They furnish adequate blade area with a reduced diameter and could thus be operated at higher rotational speed without exceeding the permissible linear speed of the blade tips. They possess the further advantage of smoothing out the slipstream and consequently improvinpf the air flow over the control surfaces of the aircraft, ^ Therefore, taking the long view of possible lines of development, the student of gas turbine design should not confine his attention solely to the jet-propulsion system, but rather consider the turbine-compressor unit with one or other of the alternative applications discussed. The possibility of combining the use of the two forms of turbines, with and without airscrews, on a single aircraft, such as a large flying-wing, will not be overlooked.
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