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Aviation History
1947
1947 - 1591.PDF
SEPTEMBER I8TH, 1947 FLIGHT 313 rests between the use of a heat exchangerand high compression. Mr. Owner then went on to consider the therrnodynamicaspects of these alternatives, illustrating that, when high overall thermal efficiencyis the criterion, at speeds of the order of 300 m.p.h. it pays to take about 20per cent of the thrust power in the form of jet reaction as this reduces airscrewweight without otherwise materially in- fluencing the performance. On the score of engine layou* Mr.Owner postulated that easily the most important single factor is the high volumeflow to be dealt with a~ the intake and exhaust pipes, this beir^g of the order ofilb air/min./h.p., and every care must be taken to avoid loss of head in theintake by bends, etc. The need for avoid- ng bends in flow after compression isnot so acute. Another important factor controlling design layout is power at Jem of blade form might be found in thechoice of a'Mach Number approximately constant with radius for turbines as weilM, compressors, and he went on to say that by far the hardest technical prob-lem in turbine design lies in the match- ing of the turbine charaeteristics withthose of the compressor. A very direct approach to the problemo*" increased thermal efficiency is offered by the heat exchanger. However, twofundamental features impose restrictions on its design and utilization which make COMBUSTION Comparison between ideal efficiencies of gas-turbine and piston engine. Per- centage heat exchange means change in gas tem- perature through exchan- ger expressed as a percentage of the extreme temperature difference causing the heat flow. take-off. The airscrew may be drivendirect from the compressor-turbine sys- tem or by a free power-turbine, the onlyexponents of the latter system at present being Bristol turbines. Airscrew locationis a further question, and although no pusher turbine installations are known asyet the gas turbine lends itself conve- niently to a pusher layout with, however,an important reservation on the score of vibration. Referring to compressor systems, Mr.Owner remarked that any very marked increase in axial compressor performanceand efficiency is unlikely in the imme- diate future until such time as the MachNumber effect has been mastered. A point in which the airscrew-turbinediffers essentially from the jet unit is that considerable centrifugal and gyro-scopic moments may have to be trans- mitted through the compressor outerRising to the mounting points, in addi- tion to the torque reaction. Again, wherea free power-turbine is used to drive trie airscrew, a passage for the far-ait has tobe made through the compressor discs. This tends to heavier cons;'ruction andmay in future impose a lir/iit on the design. The effect of component efficiency onoverall efficiency makes it not surprising tfca4 all present-day turbines are of axialflow type, but good reasons exist for believing that, especially in the smallsizes, it might be well worth while in- vestigating the characteristics of the in-ward, centripetal-flow type with axial discharge, i.e., a centrifugal impeller withthe flow reversed. In referring to the design of turbine blades, Mr. Owner sug-gested that a useful solution to the prob- 5-9 &6O THE R IDE A 2O in / / / $\A / / ^= &^£?^ •—. s JTr» T 3-— <«SS-—' r < •A .- • X)NS1 - r- sA U6TANT VOLUME : neCIPROCATING ENGINE crcu ^ —^.— SSURE —f _- *-—• (TWI '\A MNE)( _ •«1E \ 4 5 6 7 . COMPRESSION RATIO 1O its practical realization somewhat ldiffi-cult, these features being, in order of significance, weight and volume. Twofurther parameters in addition to those of weight and volume may be specifiedas affecting the performance of the en- gine. The first of these is thermal ratio,and the second is effective pressure drop which may be defined as: exhaust pres- charge pressure drop , sure drop+ , . . • : fr- andr turbine expansion ratioit has been shown that whilst the pres- sure drop across the '' cold '' side is ofminor importance the pressure drop across the "hot" side must be reducedto a minimum of about 0.05 to o.r of the local atmospheric pressure. In deal-ing with heat exchanger form the lec- turer suggested that the ideal is possiblya series of smooth flat plates, an attempt at such a design being given in what hasbeen termed the cross-flow secondary sur- face exchanger. Such a design gives avery compact assembly but has so far proved difficult to manufacture; it wouldappear, however, to have a promising future when these problems are mastered.An alternative form of heat transfer sys- tem which has received extensive theore-tical investigation is the regenerative cycle type, for which attractively highefficiencies have been claimed. In this arrangement the charge and exhaustgases are caused to pass through the system alternately at a pre-determinedfrequency, so that the heat absorbed dur- ing the passage of the hot exhaust gasesis subsequently given up to a following and relatively cold charge, effecting ahighly efficient heat transfer method. In dealing with development aspectsMr. Owner remarked that one of the major difficulties with airscrew-turbinesin obtaining consistency of results lies in the fact that the useful power is actuallyless than half the power developed by the turbine. Hence small variations ofturbine and compressor characteristics can have a surprisingly large effect onthe useful power produced, thus exag- gerating the effect on such items as com-pressor fouling and ground level atmo- spheric dirt. Under the head of operation, the .lec-turer emphasized that to obtain optimum operating conditions for an airscrew-turbine there are three main factors which can be varied either independentlyor in some predetermined relationship to each other. These are : (1) fuel flow ;f2) airscrew pitch; (3) exhaust nozzle area, and in connection with the last ofthese, it was stated that variation in the ' area has two distinct effects on the opera-ion of a gas turbine: it controls the posi- tion of the operating point on the com-pressor characteristics and also regulates the division of output between the air-screw and the jet. Mr. Owner later went on to refer tothe problems of coupled units, and ob- served that the most important is con-cerned with vibration, due to the fact that several modes occur in the regionof the complete power plant and its support, which arise from varioussources. These are broadly attributable to the aerodynamic interference of the 2 3 4 5 6 7 8 9 10 11 12 COMPRESSION RATIO Effect of compression ratio on airscrew- turbine performance. All power is as- sumed as given to the airscrew, and the turbine adiabatic efficiency is taken as 85 per cent. contra-rotating airscrews giving riseboth to torsional and flexural vibrations, which may react on the natural frequencyof the airscrew-cum-reduction gear on its mounting. In addition there is thenatural frequency of the engines on their respective mountings, and the local fre-quency of the aircraft wing structure, ft is evident that the complete vibratorysystem in this region is extremely com- plex, and that only by actual runningof a truly representative system can a full exploration of any one particulardesign be made. To terminate his paper the lecturersuggested that, in connection with prob-
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