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Aviation History
1946
1946 - 0552.PDF
282 FLIGHT MARCH 2IST, 1940 THE CAS TURBINE IN COMMERCIAL AVIATION PISTON —*\ \APPROX.5O%TOBHP APPROX.?o9b MAX. R.R M 2O MAX.TAKt OFF BHR ° Fig. 1. Approximate picture ofcruising-power outputs for equal power of piston and turbine requires 21 per cent less installedpower, is 32 per cent cheaper to run, and can perform 16 per centmore work in a year. It is the sub- stantial saving in power unit weightper horse power possible with the turbine which is the chief cause ofthese improvements. In the private-owner class of air-craft, with engines of some 150 h.p., Mr. Clarkson did not look forany significant improvement, but the practical advantages would beavailable to the private owner and would amply justify the introduc-tion of the new prime mover. Ruggedness, low first cost and reli-ability would result in an engine of not very competitive economy andthe huge gear reduction would tend to make the engine heavier andbulkier than might be expected. Skipping several intermediate stages, Mr. Clarkson turned his attention to the long-rangetype of Empire airliners. He stipulated non-stop stages of 2,200 miles (against a 50 m.p.h. headwind plus a 450 miles'allowance for reaching an alternative airfield), entailing a still-air range of 3,000 miles. For the purpose of investi-gation he stipulated the following design requirements: Cruising altitude not less than 25,000ft; take-off undertropical conditions not more than 1,500 yards to clear 50ft; maximum wing loading for take-off 701b/sq ft; maximumwing loading for landing 551b/sq ft. A range of cruising speeds from approximately 300 to 500 m.p.h. was covered,and engines 1, 2, 3 and 4 of Table I separately investigated. Empire Airliners For the 300 m.p.h. case he took an aircraft of ioo.ooolball-up weight with 150ft span and 1,800 sq ft of wing area. In passing to the higher cruising speeds, more powerwas installed, and all-up weights allowed to rise. Struc- ture weights were suitably adjusted, and fuselagedimensions and passenger accommodation suited to the resulting pay loads. At the high-speed end reduction inspan was found to be desirable, the wing area remaining Fig. 2. A comparison betweena medium twin feeder-line aircraft designed for gas tur-bines (right) and piston engines. All-up weight 10,600 Ib. Installed power (t.o. Gross wing area Overall span Wing loading Fuel for 500 miles Cruising speed Pay load Ton-miles of pay load per gallon of fuel Ton-milei of pay load per hour First cost of complete aircraft Direct operating cost per ton-mile of pay load Turbo-screw.10,60 7 x 550 b.h.p. 390 sq. ft. 61ft.27 Ib/sq. ft. 131 gals.215 m.p.h. 3,000 Ib. 5.7288 £15,500 9.4d. Piston13,500 Ib. 2x700 b.h.p. 500 sq. ft. 69ft.27 Ib/sq. ft. 141 gals.(85 m.p.h. 3.000 Ib. 5.4248 £19,000 I3.9d. unchanged. All the aircraft are cruised at 15 per cent above the speed for maximum L/D at 50 per cent of take-off power for piston engines and 90 per cent maximum revolutions (45 per cent of take-off power) for turbines. Fig. 3 gives a rough idea of the size and weight of some of the power units installed at different cruising speeds. Fig. 4 -shows pay load attainable in aircraft of approximately con- stant size when-designed for differ- ent cruising speeds and powered with different types of engine. If an overall transition at 60 per cent of xie chord could be achieved at no cost in weight, point "A" would move to point " T." The aircraft represented at point "B' has four engines of 11,500 h.p. each. The power loading of the whole aircraft is under 31b/h.p.; aircrew efficiency has fallen to 70 per cent and will prob- ably fall rapidly with increasing speed. Furthermore, com- pressibility drag rise is beginning to set in, necessitating the incorporation of sweepback. It seems unlikely that the engines for this aircraft could be available within five years. 6O BO B.H.R I2O relativetake-off engines. 140 Fig. 3. Long-range Empire airliners. All-up weight100,000 lb, 150ft span, 1,800 sq. ft. wing area. The full plan view shows, on the port wing, radial air-cooled pistonengines totalling 4 x 2,250=0,000 h.p. Power-unit weight, 17.5 per cent, of a.u.w. Cruising speed, 280 m.p.h. On thestarboard wing, axial gas turbines driving airscrews, totalling 4x2,550 = 10,200 h.p. Power unit weight, 7.5 per cent.of a.u.w. Cruising speed, 300 m.p.h. The corresponding figures for the part-plan view on the left are : Port wingliquid-cooled piston engines totalling 16,000 h.p. Power unit weight, 25.5 per cent of a.u.w. Cruising speed, 370m.p.h. Starboard wing axial gas turbines driving airscrews, total 25,600 h.p. Power unit weight, 15.5 per cent of a.u.w. Cruising speed, 425 m.p.h. In the view on the right, sweepback has been used, and thepower units are gas turbines driving airscrews. Total power, 4 x 11,400= 45,600 h.p. Power unit weight, 26.5 per centof a.u.w. Cruising speed 515 m.p.h. Of the commercial type of aircraft with simple jet enginesMr. Clarkson said that its high-speed possibilities made it most suitable for long-range work, but it was technicallyleast suited to this duty on account of poor take-off thrust and high consumption. Assisted take-off would be required,and a certain austerity in passenger accommodation. Some acute problems might be encountered owing to the highMach number, but if the difficulties could be overcome, cruising speeds could be pushed beyond that of aircraft" B " in Fig. 5, as shown by the curves marked " J centri- fugal " and "J axial". Fig. 5 shows direct operating costs per ton-mile of payload, plotted against effective cruising speed. This is the cruising speed made good over a 2,200 miles' run in stillair after making allowance for climb and descent, and a quarter of an nour for stand-off, taxying, etc.
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