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Aviation History
1948
1948 - 1686.PDF
44° FLIGHT OCTOBER JTH, 194* and the new jets will have to work at very high altitudes.This implies a lot o£ research, both iu compressor design and in combustion; the compressor must provide the very largeamount of air required, and combustion must continue even at very high altitude. The variation in intake density providesan added problem for the compressor; although every effort is made to obtain a very high pressure rise per stage,in some cases the number of stages for a given compressor ratio has been increased purely in order to obtain a flattercharacteristic curve and consequently better operation at vary- ing densities. The need to improve fuel economy has broughtabout a general tendency to higher compression ratios. Other firms are preparing new jet models. Northrop-Hendy,at Los Angeles, seem to be particularly advanced in their design; they have an excellent team of young engineers and, apparently are preparing' a very powerful engine. At the other extreme, Boeing, of Seattle, have specialized in verysmall jets suitable for models and pilotless aircraft. From a conversation with the Boeing engineers it-appears that theystarted a few years ago with practically no previous know- ledge, and simply got on with the job; beside their well-knownmodel centrifugal-type jets, they are now developing an axial compressor. A rather disquieting series of accidents has recently happenedin America; they can most easily be explained by the fact that the turbojet engines have burst in flight. Although at thetime of the author's leaving no official report was issued, engineers generally attributed the accident to the second proto-type Northrop flying-wing eight-jet bomber to an explosion in a jet engine; apparently the aircraft disintegrated in the airand burst into flames before reaching the ground, which seems to substantiate this theory. .The wide use of cast materialsfor turbine bladings and the very high temperatures at which jets are run might provide an explanation. TURBOPROPS ALTHOUGH a very great deal of work is being done onturboprops in America, the day when they will be used normally either on commercial or on military aircraft seemsrather far away. Theie are two reasons for this. First of all, while it is relatively easy to make a reliable turbojet engine, itseems extraordinarily difficult to make a reliable turboprop. The trouble lies mainly in the reduction gear and in vibrationtrouble in the turbine and compressor blades. 11 seems strange at first that the reduction gear, which worksvery satisfactorily with the variable torque and the many vibrations of piston engines, should give trouble with a smoothrunning power plant of the turbine type; but in a piston engine the crankcase and the whole engine are rigidly built to with- Balancing the compressor for 0 very smell experimental Westinghouse gas turbine. Nonhrop-Hendy's Turbodyne has an annular combustion chamber between an axial-flow compressor and multi-stage turbine. stand the alternating stresses; while the rigidity of an aircraftturbine is very moderate and cannot easily be increased without prohibitive weight. Consequently, the whole machine is ex-ceedingly sensitive to any outside source of vibration; for instance, the blading vibrations are mainly due to frequenciesfed into the rotor by the reduction gear or, more often, by the airscrew ; every time a blade passes along the leading edge ofthe wing, an unbalanced force is originated and these are fed to the gears and to the blades. Vibrations can be set up evenby slight differences in combustion of the multiple combustion chambers. In turbojet engines, owing to the self-regulatiug propertiesand the variable speed, vibrations are very much less trouble- some, apart from the fact that there is no airscrew or reductiongear. But turboprops must work within very close range of r.p.m., and this causes trouble. For the same reason, the regu-lation of turboprops is more difficult than that of turbojets. All these problems have not found a satisfactory solutionand turboprops are consequently very erratic and unreliable; but even if they were reliable they would remain very unsatis-factory. The fuel consumption of the turboprop is still far too high to make it a really serious competitor of the piston enginefor moderate-speed aircraft. The trend in America is towards a definite improvement ineconomy by means of very high compression ratios and high temperatures. This trend has sometimes been carried toludicrous extremes; one model had apparently a compression ratio of 35 : i; needless to say, the compressor was not very ccessful! But the Americans are seriously contemplating theof compression ratios of 12:1; they are also working /erishly on some satisfactory form of lightweight heat ex-hanger, although the progress seems to be very modest. In the long run, the greatest improvement will probabljfcome from increases in maximum temperature, which will be discussed under "' Research." General Electric, Westinghouse,Northrop-Hendy, and other firms are all engaging in turbo- i>rop work. The general opinion is that the Northrop is a very e job indeed, and powerful enough to bring it outside thecompetition of even the largest piston engines. ROCKETS rockets a tremendous amount of work is being done, bothfor auxiliary purposes and as primary motors. Most of it; is, of course, highly confidential; but there is no doubt that alot of money and great ingenuity are being spent to improve the rocket engine. Some of the developments required by therocket may be of great assistance to the ordinary gas turbine. For instance, cooling the outlet nozzle of a rocket motor is avery ticklish problem indeed; and the metallurgical develop-' ments which were needed to solve it might be of great helpin providing better turbine blades. RESEARCH A COX SI DEK ABLE amount of research is of course carriedout in the laboratories of the firms. But the long-range research is made principally at the N.A.C.A., at university laboratories, and other similar institutions. The United States
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