FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1955
1955 - 0884.PDF
882 FLIGHT LOW-CONSUMPTION TURBINES Some Rolls-Royce Achievements: Mr. Lombard's Lecture at the Anglo-American Conference ALTHOUGH "there is no aspect of turbine engines morerestricted by security than that of fuel consumption,"Mr. A. A. Lombard, F.R.Ae.S., chief engineer of the Rolls-Royce aero-engine division, has prepared a paper on this subject, and it was read before the Anglo-American (R.Ae.S.-I.A.S.) conference in Los Angeles this week. The opening words above, incidentally, are a quotation from Mr. Lombard's introduction. It was, said Air. Lombard, not possible to avoid introducingaircraft performance considerations when attempting to define the optimum engine for a given application. For example, theeffect on payload or range of haying a heavy or inefficient engine was not great in short-range aircraft, but at higher ranges theeffect became catastrophic. One of the objects of the paper would be to stress the importance of light weight, as well as low s.f.c.(specific fuel consumption), in engines for long-range aircraft. Study of curves of s.f.c. and specific weight for various Rolls-Royce gas turbines (plotted against an historical time-base) suggested that further improvements would be small. It washighly likely that every general type of gas-turbine configuration had already been considered, and that—unless something basichad been overlooked—further progress would depend largely on detail refinement.The lecturer went on to assess the relative importance of specific weight and consumption. At a fixed altitude and speed, it wasquite simple to express range as proportional to a formula involving both parameters. Another important relationship was that existingbetween the two parameters; if, for example, a one-per-cent change in specific weight brought k per cent improvement in s.f.c. thenthe result of: (per-cent decrease in s.f.c.) minus (k times per-cent increase in powerplant weight) had to be positive, if the modifica-tion was to be worth while. It was found that the primary variables affecting k were time and altitude, a reduction in the former andan increase in the latter having the effect of reducing the fuel required, and hence making engine weight of greater relativeimportance. Two of Mr. Lombard's illustrations were plots of k against stage-time for selected cruising altitudes assuming a cruis-ing speed of, in the first case, 550 m.p.h., and in the second, over Mach 2. It could be deduced that, for similar stage-lengths, therelative importance of specific engine weight and fuel consumption would not differ greatly between the two types of aircraft. Two other methods of assessing k were also described by thelecturer: the selection of aircraft cruising speed and altitude to suit the particular engine and the choice of total engine/fuel loadsto suit the particular engine. In the latter case, the lightest engines were used to increase wing loading and hence cruisingspeed (up to any limit set by Mach number). Briefly, if engine and airframe were truly optimized, then a percentage change inweight was worth approximately half the amount in percentage consumption.Turning to the assessment of the effect erf design variables, it was assumed that a basic design was available which was as goodas current knowledge could make it. The established methods of increasing engine performance (the effect of which on cycleperformance was generally familiar) were then examined from the point of view of weight, and hence range. Increasing the compressor pressure-ratio meant increasing thenumber of stages and probably, in the limit, adding a turbine stage also. Owing to the temperature rise it was necessary tomake the extra stages of steel; furthermore, the flame tubes would run hotter (for the same design standard of cooling) involvingincreased weight, and, where bleed air was used for sealing or cooling, the reduced cooling effectiveness would again requireweight increases, and the greater cooling flow needed would detract from performance. The weight increases were of less importancein turboprops, where the weight of the main compressor/turbine assembly was a low proportion of the total engine weight. A plot of specific weight and consumption against pressure-ratiofor two combustion temperatures and two flight durations showed flat optima. It could be concluded that in going above pressureratios of the order of 8:1 an increase in range of only 3 to 5 per cent was possible; and, for the best conditions, 12:1 seemed tobe the practicable limit. A similar picture could be drawn for the turboprop (Fig. I). It could be seen that, in this case, worth-while gains were possible, and that a figure of 350 deg C tempera- ture rise (much in advance of existing designs) appeared a suitableoptimum. For a pure jet engine, increasing combustion temperatureincreased the jet velocity and thus reduced propulsive efficiency, so offsetting the gain in basic cycle efficiency. On the other hand,in a turboprop there was every advantage in increasing combus- tion temperature. Unfortunately, there was a debit side in engineweight, owing to the arduous combustion-chamber conditions; further, there might be a performance loss as a result of increasedcooling airflow. A plot for a turbojet (Fig. 2, below) showed a flat curve for a 5 hr flight, but a debit was incurred at 10 hr. Itseemed from these curves that little would be gained by increasing combustion temperature above about 1,050 deg K. For the turboprop (Fig. 3) range continued to increase withflame temperature, particularly at short ranges where engine weight was more important. Although the percentage increasein range fell off at high ranges, for a constant range the results implied an almost constant addition in payload. Thus, at longrange the percentage increase in payload was much greater. For the turboprop, therefore, one could see high pressure ratiosand high temperatures ahead. Cooled turbines would be essential, and much progress had been made in their development. Theproblem was to engineer an air-cooled blade with adequate mechanical strength; also, the cooling had to be accomplishedwith the minimum amount of air, in order to avoid exorbitant penalty in performance. There was a fine balance in selectingthe right flow of air that could be used for cooling with high flame temperatures if inefficient expansion of this air was not tointroduce a performance penalty. This might involve a multi- pass system, in which the cooling air flowed over the maximumsurface area at as high a velocity as was possible with the available pressure drop. A prerequisite was very efficient aerodynamicdesign cf cooling-air passages. Considerable work was being done in attempting to increasecomponent efficiency. Some increase in compressor efficiency might result from reduction in stage loading and speed, but wouldimply weight increases. Some weight reduction might result from the use of transonic stages without too much loss in effi-ciency; but the greatest gain would probably result from the introduction of new materials, such as titanium alloys. Manywere of the opinion that, were combustion chambers treated as scientifically as the compressor and turbine, large reductions in Fig. 1 (left). Airscrew-turbine performance at 400 m.p.h. T.A.S. at 36,000ft, assuming 1,150 deg K P 55 combustion temperature. Fig. 2 (centre). Turbojet performance at 550 m.p.h. T.A.S. at 36,000ft, assuming a compressor temperature-rise of 300 deg C. Fig. 3 (right). Turboprop performance at ^ 0-5 400 m.p.h. T.A.S. at 36J000H, assuming a compressor temperature-rise of 300 deg C. •% H°-45 O5O cl O*45y " 25 -I1-5 PR 9 | s ESSURE ATlfS'? 12 | " . ••• ' — 15 | — .- - = 2P | .2hr _5hr :1Ohr "S09P .c ^0-85 u 'JO' 1 14 s S- 1-2 +2% J + 1% V 5 P ———— —a- — — — "--^ y— ' ' —-*" 1OSr •s ^ 2-5 o 2 I 1-5 +10% + 5% 1-5% —•— s. k — — 2hr \ *•' i ' k 1Oh£ — —• i ! 250 3OO 350 4OO COMPRESSOR TEMP RISE (deg C) 940 96P 980 1O0P 1O2O 1O4O 1P6P COMBUSTION TEMP (deg K) 1OOO 1O5O 1100 115O COMBUSTION TEMP (deg K)
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
