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Aviation History
1946
1946 - 0255.PDF
FEBRUARY 7TH, 1946 FLIGHT 141 SUPERFORTRESS DEVELOPMENT 1 i^^i^b^ ^ T • tan, 124ft. 7in.) / the consequent reduction in fuel storage space and com- plication of wing structure, and so Model 334 was Boeing's last attempt at using submerged engines. The twin tail was also abandoned, as any small gains in arcs of fire were countered by a lessening of structural efficiency. Boeing engineers were beginning to realise that the generally improved bomber they were after would not be obtained by such devices as freak engine installations, and they set MODEL XB-29 (Span, 141ft. 3in.) with the Model 333 and used the same Allibon engines, only this time they were completely submerged in the wings, the airscrews being carried forward of the wing leading edges on long shafts. The sumps of the outer engines protruded below the wing skin and were enclosed in streamlined blisters. Unfortunately, it was soon found that the power of the Allison engine fell off very rapidly at height, and it was decided to try a different engine installation. Both the Wright and Pratt and Whitney engine com- panies were working on the development of '' flat'' type engines that could be completely submerged in the wings of large aircraft, and so Boeings designed a project for each engine. Model 333B used four 1,850 h.p. Wright 1800 engines and had an airframe identical with that of the Model 333A. The larger engines put the estimated loaded weight up to 52,180 lb. and gave a top speed of 364 m.p.h. at 20,000ft. Unfortunately, Model 333B had an estimated range of only 2,500 miles with one ton of bombs, as the submerged engines took up space normally used for fuel tanks. As America has always been "Pacific war- minded," where distances are great, a range of 2,500 miles was hardly sufficient for a heavy bomber. Submerged Engines Unsuccessful The project with Pratt and Whitney 1800 " flat" engines—Model 334—was basically similar to Model 333B, but the wing was redesigned to increase fuel capacity, and thistype had an estimated range of 4,500 miles. By this time, March, 1939, European air forces were being re-equippedwith new and deadly monoplane fighters, the Hurricane and Spitfire with eight machine-guns, and the Messerschmitt 109with cannons and machine-guns, being outstanding. Consequently, the question of defensive armament becameof prime importance, and Model 334 was designed with twin fins and rudders in an effort to improve the arc offire of its eight machine-guns. This further enhanced the already-clean lines of the fuselage and, combined with#the improved efficiency of the wing, permitted an increase in bomb-load of one ton (to 7,830 lb.) and put the estimatedtop speed up to 39c m.p.h. at 20,000ft. It was found, however, that the completely submerged flat engines didnot improve aerodynamic efficiency sufficiently to justify to work developing instead a new aerofoil that wouldgive greatly improved all-round efficiency. The prototype Consolidated XB-24 Liberator, with its high-aspect ratioDavis wing, must have helped to convince them that this was the wisest policy to pursue, and the next pro-ject had a wing very reminiscent of that on the XB-24. Also, as in the XB-24, the main wheels of the tricycleundercarriage were now designed to fold sideways into the wings instead of into the nacelles. The project designed around the new wing was designatedModel 334A, but it was such a radical redesign that it should really have borne a new Model series number. Itwas the first '' blood ancestor '' of the present Superfortress, with a high single fin and rudder and a long dorsal fin. Bythis time the Wright 3350 engine was coming along well, and so Boeings decided to use four of these engines on thenew project. It was obvious that production engines would develop over 2,000 h.p., and it was estimated that theproject would have a speed of 390 m.p.h. at 16,oooft. and a useful 5,333 miles' range. The years of painstakingdevelopment work were beginning to show results and, although Model 334A was not built, Boeings knew thatthey were working on the right lines. Late in 1939 the U.S.A.A.F. asked several aircraft com-panies to submit designs for a larger long-range bomber, indicating minimum requirements for performance, ioad-carrying capacity, weight, range and armament. This was put out in the form of a formal specification in January,1940. The first few months of the war in Europe had been sufficient to convince the U.S.A.A.F. that their existingbombers and fighters were hopelessly out of date, and conse- quently Boeings were given plenty of encouragement indeveloping their heavy bomber designs. The Aerodynamics Unit was still busy developing im- DATA ON THE SUPERFORTRESS TYPES. Project Type No. MODEL 316 (March, 1933) MODEL 322 (June. 1938) MODEL 333 (Jan. 26. 1939) MODEL 333A (Jan. 27. 1939) ...MODEL 333B (Feb. 21, 1939) ... MODEL 334 (March 4, 1939) ... MODEL 334A (July, 1939) MODEL 341 (Aug., 1939-Mar.. 1940) MODEL 345 (March-April, 1940) tXB-29 (June, 1940-Feb., 1942)... *B-29 (July, 1943) Engines 2,000 h.p. Wright R-3350 1,400 h.p. Pratt & Whitney R2I80 1,150 h.p. Allison 1710 1,ISO h.p. Allison 17101,850 h.p. Wright 1800 1,850 h.p. Pratt & Whitney 1800 2.200 h.p. Wright 3350 2,000 h.p. Pratt & Whitney R-2800 2,200 h.p. Wright R-3350 2,200 h.p. Wright R-33SO 2,200 h.p. Wright R-3350 Wing Span ft. in. 157 0 108 7 109 0 108 6III 0 120 0 135 0 124 7 141 3 141 3 141 3 Length ft. in. 109 2 75 5 80 8 80 880 8 83 4 80 0 85 6 93 0 98 2 99 0 Gross Wt. Ib. 89,900 53,100 48,600 48.60052,180 66,000 66,000 85,672 112,300 114,500 135.000 Wing Loading Ib./sq. ft. 31 35 34 34 34 40 47 64 .65 66 77 Range with 2.000 Ib. Bombs miles. 4,000 3.600 3.420 3,0002,500 4,500 5.333 7,000 6,950 5,333Re- stricted Maximum Speed 24B m.p.h. at 15,000 ft. 307 m.p.h. at 25,000 ft. 307 m.p.h. at 15,000ft. 328 m.p.h. at 15,000ft.364 m.p.h. at 20,000ft. 390 m.p.h. at 20,000ft. 390 m.p.h. at 16.000ft. 405 m.p.h. at 25,000ft. 382 m.p.h. at 25,000ft. 382 m.p.h. at 25,OOOft. Over 350 m.p.h. Max. Bomb Load Ib. 9,928 5,800 5,8005,800 7,830 7.830 10.000 16,000 20,000 20.000 The dates given are those on which preliminary design drawings were completed, with the exception o* :— t Date of development of prototype. * Date of first flight.
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