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Aviation History
1949
1949 - 0872.PDF
MAY 12TH, 1949 FLIGHT 569 The Asymptotic Bomber 326,000 Ib at which this version is now flying. Admittedly, however, the take-off-weight condition is not a sound criterion of combat performance in the case of a 5,000-mile-radius bomber, so let us examine trie situation nearer the target. At the absolute target radius of 5,000 miles (after consump- tion of just over 60 per cent of the fuel load and immediately before the 10,000 lb bomb load is released), the gross weight is down to about 216,000 lb, and the most optimistic figure- juggling on our slide-rule indicates that the B-36B will then reach a service ceiling of approximately 39,000ft. We say " optimistic " because our figuring is postulated not only on a low parasite drag coefficient of 0.016, but on the assumption thai the Wasp Major R-4360-41 engines now installed in the B-36B can deliver a normal continuous output of 2,650 b.h.p. up to a critical supercharger height of 35,000ft. (The wet- boost take-off power, of coui.se, is 3,500 h.p.) The above-quoted drag coefficient assumes, for example, that all gun turrets are retracted, and while this assessment may be fair enough fo; an unmolested long-range mission, it is almost certainly on the low side under combat conditions. Such possible power dividends as in- take ram-effect and exhaust thrust- augmentation are considered to be nullified by the cooling drag, and this balancing of debit and credit is prob- ably on the optimistic side again. Air- screw propulsive efficiency at 35- 40,000ft has been estimated at 0.73 because, although this engine has a low airscrew- reduction gear ratio of 0.38, the blade-tip speed at full power and corresponding maximum aircraft speed is well into the supersonic range at M —1.1. (At cruising powers, however, the propulsive efficiency has been taken at 0.84, since M does not exceed 0.9.) But granted the B-36B can reach a height close to 40,000ft in the region of the target, this is really an escape ceiling rather than a combat altitude, since the manoeuvrability of a 160-ton bomber must be evanescent at its ser- vice ceiling. We need, also, to take a look at the speed-altitude chart (Fig. 1) to get a better idea of how such a long- range bomber is likely to approach the target. Studying first the maximum speeds, our figures show 290 m.p.h. at 20,000ft at the starting weight of 326,000 lb, rising to approximately 360 m.p.h. at 35,000ft at the taiget (W = 216,000 lb). In the latter condition, the maximum speed will drop off to about 340 m.p.h. if the aircraft is pushed up to its service ceiling. Therefore, 35,000ft can be taken as its best fighting altitude with the gate wide open. A long-range bomber, however, must perforce spend prac- tically all of its time cruising at its optimum-range speed, otherwise it cannot make its specified range, and, unfortun- ately, this is where the piston-engined bomber falls down, since the optimum cruising speed is way below the maximum. TARGET WEIGHT (BOMBS OFF) 2O6.OOO Ib. 2OO 25O " 3OO SPEED (m.p.h.) AERODYNAMIC DATA Characteristic Take-off gross weight Target gross weight (bombs 011)Target gross weight (bombs off) Landing «ros» weightWing area Wing spanAircraft efficiency fact 11 Geometric aspect rat-i >Effective aspect ratio ... Parasite drag coefficient (iniii.) Parasite drag area Aircraft lift/drag ratio (max.) Normal rated power (N -• 2.550 r.p.ni.)Supercharger critical Iieialit ... Airscrew diameter (3 blades)Airscrew rotational speed (near ratio. 0.381)Airscrew propulsive clticieiiev (V max.)Airscrew propiiNivc eflicieiicv (V emis.) ... ... . Specific fuel + oil consumption (aver.) —CONVAIR Symbol Wi W-. \V» W4 sll e A = bi/s AE = eA CDp f =-• CD,,* LID P H T)3 11 e B-36B Value 326.000 Hi 210,00011)200.000 lb 133.00011)4.772 sq ft 230ft0.77 11.18.55 0.016 70.4 si I ft 20.5 2,fi.">0 b.h.p.3."),000ft 19.0ft 10.2 r.p.s. 0.73-0.77 0 <l (.1..'in !b/ii.li.p./lir Aerodynamically, the aircialt must be flown at its maximum lift/drag ratio while, Ihermodynarnica-lly, the engine must be run at its most economical speed tor minimum specific fuel consumption. Thus, in all die long-range flights of the B-36 to date the average (i.e., the overall) cruising speed has been of the order of 230 m.p.h. Actually, this is somewhat lower than the cruising regime a ppearstv warrant, and it is possible that this figure can be improved to around 250 m.p.h. as the cruising technique is developed ' Incidentally, the B-36B is so heavily overloaded at the start that it cannot fly at its minimum-drag attitude until it has consumed some of its fuel and oil Ic4d. Oui own analysis indicates a maximum L/D ratio of 20:5,' but during the initial stage of a long-range mission it can achieve the requisite lift coefficient only by flying higher up the lift curve at a wing incidence beyond that of the aircraft maximum L/D—that is, the drag is above the minimum assumed in the Breguet range equation. Although in this particular case the increased drag is of no serious consequence, such a point is occasionally overlooked when juggling with M. Breguet's classic (and exceed- ingly useful) formula. Nevertheless, in view of the undeniable fact that the B-36B has recently flown a 9,600-mile mission with a TO,000 lb bomb load over the outward run, we can accept without quibbling the claim that it is now potentially capable of meeting its original specification range of 10,000 miles, albeit, with some extra 6,000 gal- lons of fuel in the bomb bay. Under the most favourable conditions (airscrew propulsive efficiency 0.84. specific fuel- plus-oil consumption of 0.50 Ib/b.h.p. / hr) an absolute maximum range of 10,600 miles can be shown to bo theoretically possible with a fuel tank- age of roughly 27,000 L'.S. gallons. Parasite There are a couple ol points to bear in mind in this connection. First, an absolute range of 10.600 miles repre- sents a practical service1 target radius o£ not more than 4,000 miles under nor- mal operational conditions, allowing for all the usual contingencies. The second fly in tiu- amber is also quite a bug— literally as well as figuratively. If a large part of the bomb bay is used for fuel cells (and it is difficult to see'how the above-quoted range can be achieved without them) what happens to the'bug on the flying trapeze, the McDonnell v-" XF-85 escort fighter? And even if such a parasite can still be accommodated in the bomb bay it needs a lot of imagination to conceive its effectiveness in the face of an overwhelming jet-interceptor attack from several quarters of the compass. Assuming the approach to the target is made at a weight of 216,000 lb, flying at a height of 35,000ft at maximum L/D attitude, the appropriate cruising speed is approximately 290 m.p.h.—which is a leisurely gait likely to bring consider- able joy to the heart of any aggressive jet-fighter pilot, especi- ally in view of the immense target presented. For remember that the bomber must stay on the job and slug it out, while the fighters (very much in the plural) can break off at will and then re-form. Moreover, if they keep it up long enough to harass the bombers into opening up their engines to full power for any length of time, the bombers may never get home to fight another day, because of excessive fuel consumption. The fighters, on the other hand, fall on native soil and may rise again—as witness the Battle of Britain. A counterfeit note has been injected into this hoary argu- ment by top U.S.A.F. admission that current jet fighters— presumably American—appear incapable of dealing with the B-36 at its highest bombing altitude, which apparently is claimed to be at least 40,000ft, although our analysis shows otherwise. It may be news to some people in England that R.A.F. and U.S.A.F. pilots have been interchanging their respective fighter mounts and the R.A.F. verdict on American jet fighters confirms these altitude shortcomings. Whether this is mainly due to high wing-loading or power deficiency at strato- spheric levels, or a combination of both, we cannot say at this time, but the finding seems to be reflected in the recent boost- ing of the B-36 programme and the cut-backs in the Republic F-84 and North American F-86 and F-93 jet-fighter production. The flaw in this U.S.A.F. thinking is that it is predicated on the thesis that " enemy " jet-fighter design has necessarily 35O 4OO Convair &-36B : speed performance with bomb-load of 10,000 Ib.
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