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Aviation History
1957
1957 - 0115.PDF
FLIGHT, 25 January 1957 115 OPERATIONAL ANGLES on the B-47 By COL. RICHARD £. EVANS, U.S.A.F., H.Q., STRATEGIC AIR COMMAND THIS notable study of the principal operational aspects of a high-performance bomber was first published (under the tide "Operational Aspects of the SAC Transition to Jets") as a paper presented to theInstitute of the Aeronautical Sciences, and later appeared in the Insti- tute's journal, "Aeronautical Engineering Review," by whose permissionwe reproduce it here, in slightly abbreviated form. Readers who are not technically inclined should not allow themselves to be deterred bythe graphs: Col. Evans's paper is an unusually skilful blend of the instructive and the entertaining. His remarks will recall a previous"Flight" article on handling the B-47, by the Editor, in our issue of August 20, 1954. IN 1955 the last Strategic Air Command B-50 Medium BomberWing converted to B-47 jet bombers. Last year saw the de-activation of the first B-36 Heavy Bomber Wing and the activation of the first two B-52 Wings. This year, the KC-135jet tanker will start to replace the KC-97. The Strategic Air Command has matured from prop-driven adolescence to jetmaturity. There have been growing pains, but the rewards and pleasures associated with maturity are usually worth the effort,and such has been our experience. In November 1951, Major Gen. Frank Armstrong and the menof the 306th Bomb Wing, stationed at MacDill A.F.B. near Tampa, Fla., were out on the ramp in force, awaiting the arrival of theworld's first combat jet bomber. The Wing Commander, Col. Michael McCoy, was delivering No. 1. It was a historic occasion.Probably Napoleon, returning to Paris, felt no greater elation than did Mike. It would have been a great day—except for GrowingPain No. 1. We had spent millions of dollars and millions of manhourspreparing for this moment. A new 10,000ft runway had been laid; special new ground power units to start the jet engines had beendesigned, built and delivered; new U.H.F. radio equipment had been installed in the tower; and a multi-million-barrel JP-3 tankfarm had been built and jet fuel had been moved across the Gulf of Mexico. Jet-engine mechanics had received special trainingat Scott A.F.B., 111., and at the Boeing factory at Wichita, Kan.; veteran B-29 navigators had received high-speed navigation train-ing at Mather A.F.B., Calif.; and battle-seasoned, 32-year-old pilots had each received $85,000 worth of radar, navigation andbombing cross-training and a preliminary jet check-out in the T-33. The great effort was justified because one of the new jetbombers would carry to an enemy of the United States in a matter of hours as many megatons of destructive power as all of theAmerican and British bombers in World War II carried to Germany, Italy and Japan in a four-year campaign. We had waited long for this great moment, secure in the beliefthat everything possible had been done to ensure the success of the 306th Bomb Wing's first jet mission. As Mike approachedthe runway, six jet engines left characteristic black trails to mark his path. The bicycle landing gear touched down smoothly, andmany saw the drag chute for the first time as it deployed and quickly caught the air. And then it happened! An alert fire chiefhad seen the "smoke" trailing the bomber, and where there's smoke, there's fire, of course. He raced out in front of a startledWing Commander and signalled him to stop. In a flash, a fire truck was on each wing spewinp Foamite into six brand newJ-47 engines. . . . The first lesson we learnedwas to get off the ground— . in a hurry. Engines Nos. 1 and6 were held at 52 per cent dur- ing ground operation to drivethe alternators. Numbers 2, 3, 4 and 5 were varied from 40per cent, the idle position, to 80 per cent during taxi. The ALinUDf (1000 FT) - k Fig. 1. Fuel consumption (leyel ,;.-„- flight). -- FUR CONSUMPTION (IB/MM) iffl*- average consumption rate on the ground was 150-200 lb of fuelper minute. The B-47, at its average weight and flying at its best cruise altitude (Fig. 1), uses fuel at about this same rate.This means, then, that 1 min on the ground is the rough equi- valent of 1 min of cruise; 1 min of cruise is 7 miles. Fifteenminutes' delay at the end of the runway means 100 miles less range. Conventional equipment had not placed such high premiumon ground operation. We found it necessary to secure air traffic control clearance, including climb-out instructions, before startingthe engines. The introduction of surveillance radar during this period greatly facilitated the clearance problem at bases whereit was installed. Empty, the B-47 weighs about 85,000 lb. It can more thandouble this weight with fuel and payload, going as high as 220,000 1b. The wing tanks each carry about 11,000 lb of fuel.The balance of the load is distributed, longitudinally, in the fuselage. Great care must be exercised in limiting and distributingthe load for take-off and to keep the e.g. within limits while dropping bombs and while using and replenishing fuel. The six J47 engines furnish 36,000 lb of thrust at sea level.Water-alcohol can be used to augment this thrust by 17 per cent, the equivalent of a seventh engine. Thirty-three solid fuel rocketsprovide the thrust of five or more engines, bringing the total available thrust for take-off up to 75,000 lb. With this power welifted a 160,000-lb B-47 off the runway at MacDill on a hot day in 3,300ft. With water only, an airplane of the same weightrequired 6,000ft. Another, without water or A.T.O., used 8,500ft. A.T.O. was rarely employed—it was and is considered an expen-sive emergency measure. Water was avoided for routine operation in that frequent use resulted in some additional sheet-metal workalong the trailing edge and in the flap area. Runway temperature and field elevation as factors affecting thetake-off roll assumed greater significance when we converted to jets. Even with 10,000ft runways, it was necessary—in Floridaand Arizona, for example—to restrict the fuel load in order to get off with safety. A 170,000-lb airplane, taking off 2,000ft abovesea level, at 40 deg F, needs 7,600ft; increasing the temperature to 80 deg F increases the take-off run to 10,000ft; and an increasein elevation of 1,000ft increases the take-off distance by another 1,000ft. If the engine exhaust gas temperatures are low, if thereis a tail wind, an uphill grade, large amounts of water on the runway, an engine failure, or a miscalculation, the airplane willnot get off. To provide a margin for error and a reasonable safety factor,the maximum take-off weight is determined by the "critical field
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