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Aviation History
1959
1959 - 1747.PDF
832 FLIGHT, 19 June 1959 A 1,720-KNOT AIRLINER . . . suggested a swing away from pods, and to a layout similar to thatdepicted in Fig. 2. The duct was rectangular and the upper and lower surfaces might be taken as parts of the main wing surface.In design conditions the shock system from the intake wedge would be confined entirely within the duct; external wave dragwould thereby b<: eliminated and the entire installation would have the aerodynamic characteristics of a flat plate of zero thicknesscontributing to the lift. A possible layout of the combination powerplant was shown in Fig. 3 and this type of installation wasassumed in the subsequent analysis. In the investigation of engine performance it was possible toarrive at conclusions from plotting the total of engine + fuel weight against cruise Mach number. In all cases a total range of 4,000 n.m.had been assumed, together with a cruise L/D figure of 7.5. At M 2 there was little difference between the turbojet and the com-bination engine, but the curves diverged significantly at M 3 and above. It appeared that a Mach number of three was a good valueas a basis for a detailed study, as reasonable payloads might be expected without severe kinetic heating. It was next desirable to match the powerplant with an airframe,and the following requirement was assumed: stage-length, 3,100 n.m. against 40 kt headwind; balanced field-length 7,000ft;reserves for 20 n.m. diversion,* .stand-off for 45 n.m. at 20,000ft and 15 min at 5,000ft; unusable fuel remaining in tanks. *An allowance for being baulked at the last moment—by an accidenton the runway, for example. Fog at destination would be known in advance, owing to the short stage-time. In the remainder of his paper Mr. Jamison based his exposi-tion on a nominal aircraft—with a payload of 24,000 lb at a 3,100 n.m. stage with allowances—maintaining an unchangedconfiguration but introducing variations in wing area and aspect ratio, leading to associated changes in L/D and structure weight.The assumptions made for the powerplants were as follows: the intake was to be two-dimensional, of isentropic wedge form withvariable angle of attack, designed for M 3; the ramjet would havfe a combustion efficiency of 95 per cent, cruise tailpipe tempera-ture from 980 to 1,150 deg K, a cruise s.f.c. of 1.64 and a weight (including variable nozzle) of 0.68 lb/sq in of duct area; theturbojet had a design pressure ratio of 5.5, a take-off t.e.t. of 1,250 deg K, a cruise t.e.t. of 1,200 deg K, nominal t-o. thrust16,000 lb, nominal weight (including variable nozzle) 3,250 lb, weight per sq in of enclosing duct 1.74 1b, and s.f.c. at M 3 instratosphere 1.64; an afterburning turbojet would have the above assumptions plus 22 per cent increased weight and s.f.c. of 1.81at 1,200 deg K tailpipe temperature, 1.92 (1,400 deg K) and 2.1 (1,600 deg K). In the investigation of airframe aerodynamics, plots of L/Dagainst aspect ratio and wing loading suggested an aspect ratio of 2, and the derived values of peak L/D for this shape were thenplotted against start-of-cruise altitude, together with the associated wing loadings. By the same token, the L/D plot gave the totalnet thrust required at the start of the cruise, assuming the weight at this point to be 270,000 lb (allowing 30,000 1b fuel for take-off and climb). It was next possible to specify how many engines of each type should be installed and to obtain the aggregate air-flow and duct size, when Fig. 4 could be drawn. Points of cut-off on these curves represent the cruise altitudesat which the powerplants to match the cruise thrust would also meet the specified take-off performance. Mr. Jamison elabora-ted on this point by examining the effect of variations in cruise altitude, and combined the data to obtain a curve of total weight(engines + fuel + structure + systems) as a percentage of gross weight. A breakdown of component weights at the bestpayload point for each powerplant is shown in Fig. 5, which also tabulates the amount left for payload. The com-bination aircraft is significantly the best, and its payload of 9 per cent is equivalent to 27,000 lb, or some 130 touristpassengers. To complete the picture these figures were turned into direct PAVLOAD FUEL ENGINES STRUCTURE REHEAT I600°K(°l Fig. 3. A perspective sketch of the hypothetical powerplant system. The configuration is also illustrated on the previous page NO REHEAT T.E.T. I2OO°K REHEAT I2OO°K (2l6O°R) T.E.T. I2OO°K (2I6O°R) REHEAT I4OO°K (252O°R) T.E.T. I200°K (2I6O°R) REHEAT |6OO°K C288O°R) TEX I2OO°K C2I6O°R) COMBINATION ENGINE TEX I2OO°K (2l6O°R) onoN/mm TURBOJET REHEAT REHEAT I2O0TC I4OO°K (2I6O°R) (252OCR) Fig. 5. Effect on payload of alternative methods of propulsion, stipu- lating a given take-off to 50ft. The figures at the top of each column are the percentage payload • POINTS OF EQUALTAKE-OFF LENGTH TO 5O FEET D.O.C. (CENTS / SHORT-TON STATUTE MILE) O 5 IP 15 2O 25 3O Fig. 6 (right). A straight- forward comparison of direct operating costs for the five types of power- plant system studied. The superiority of the com- bination turbojet/ramjet arrangement is marked 50 60 70 80 90 START OF CRUISE ALTITUDE - K FT Fig. 4. Total weight of engines + fuel for a variety of turbine entry temperatures. The units along the abscissa are thousands of feet REHEATED TURBOJET I2OO°K (2I6O°R REHEATED TURBOJET I4OO°K (2S2O°RV
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