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Aviation History
1949
1949 - 1134.PDF
JUNE IOTH, 1949 FLIGHT 717 Joint Conference . for a landplane and flying boat at about 140,000 lb a.u.w.Both aircraft were designed for the England-Australia route, to fly at about 250 m.p.h. at 20,000ft and carry a payload of22,000 lb over a still-air range of 3,310 statute miles. The designs were taken to about the normal tender design stage,and it was found that the boat had an appreciably lower •••structure weight, viz., 30.3, against 31.9 per cent for theJandplane. However, the latter had the lower drag and there- fore required less fuel, but against this, the all-up weight ofthe boat was just less than that of the landplane, although the latter was 10 m.p.h. faster. Nevertheless, this higher speedhad to be offset against the additional maintenance required on the tyres, undercarriage and retraction mechaifism. It was clear that the choice of landplane or flying boat wouldrest on such factors as the location of the respective bases, and the overhead costs of operating them. The indirect costswould be the deciding factor rather than the direct costs, which were so obviously very nearly equal. This meant thatattention had to be turned to the larger sizes to see the whole picture, but it was worthy of note that at 140,000 1b the"point of equal weight," i.e., the point at which, to accom- plish a given duty, the all-up weight of the Lmd plane andflying boat were equal, had been passed. Extrapolated Comparisons In making comparisons at extrapolations of twice and fourtimes the weight of the original aircraft, Mr. Keith-Lucas stated that it had been assumed that the laws of dynamicsimilarity would apply. In that the basic structure weight percentage increased as the linear dimensions, it paid to keepthe latteT as small as possible. However, the laws of dynamic similarity assumed that aerodynamic forces were proportionalto weight, but this was not borne out in practice. For com- parative purposes, therefore, the power had been taken asapproximately that necessary to give the required speed. At the 270,000 lb category, the comparison showed thatthe flying boat with fixed floats was just the lightest of the three designs; the landplane was faster than the flying boatwith retractable floats, but only by 3J m.p.h., whilst the flying boat with fixed floats was 3 m.p.h. slower than its counterpartwith retractable floats. It was a nice point to decide whether the 3 m.p.h. would be worth the additional maintenance costs. The 540,000 lb figure was the maximum selected for con-sideration, not because it represented the maximum which could be envisaged or would be practical (there was, indeed,no reason why flying boats should not be built at twice this weight), but simply because there seemed no need to extendthe investigation much beyond the "point of equal speed," i.e., the weight at which the cruising speed of the flying boatequalled that of the landplane. This point was just passed in the comparison, providing that the wing-tip floats of theboat were made to retract. Throughout the investigation, it had been the deliberatepolicy when in doubt to give the benefit of the doubt to the landplane. Repeat calculations had therefore been made afterreducing the structure weights of the flying boats respectively by 0.5 and then 1 per cent of the all-up weight. Theseresulted in the point of equal speed being reduced from about 500,000 lb roughly to 350,000 lb and 250,000 lb respectively.Whilst, therefore, it would be unwise to quote a definite weight at which the performance of the flying boat was equal to thatof the landplane, it could be deduced that the performance was very nearly the same at weights between 250,000 lb and500,000 1b, and that the flying boat would become relatively faster as weight increased. Design Assumptions The lecturer then devot'-d attention to the effect of designassumptions, dealing firstly with speed and range, and showed that an increase in speed would, of course, favour whicheveraircraft was the faster, that is to say, the landplane in the smaller sizes, and the flying boat at weights above the pointof equal speed. The same argument applied to an increase of range, provided that the payload carried was still sufficientto demand a two-deck cabin. As payload capacity was a func- tion of floor area rather than cabin volume, as size increased itbecame increasingly difficult to provide sufficient capacity; a double-bubble cabin would therefore be required on a 270,000-lb aircraft for all ranges up to about 5,000 miles. At the larger sizes, the circular-section cabin became Jess efficient and itsadvantage over the double-bubble type became less. The Bristol Brabazon and the Saunders-Roe Princess would offera most interesting comparison when they became operational. Turning to a consideration of higher speeds, Mr. Keith-Lucas stated that the use of the jet engine which, for economy, must operate at high altitudes, brought with it other factorsof comparison, the chief of which was the drag rise due to Mach number. Contrary to popular expectation, the SR/Ai jetfighter flying boat had demonstrated that the hull of a flying boat with normal chines and steps was very nearly, it not quite,as good as a conventional fuselage at high JVlach numbers. What difference there might be was unimportant because thedrag rise would almost inevitably occur on the wing first. High Mach number had one other important effect on thecomparison, viz., that it caused a decrease in the maximum lift coefficient and so might limit the lift coefficient whichcould be used for cruising. Wing loading would then be dictated by considerations of cruising and not landing, whichwas important in relation to the effect of varying the wing loading. The depth of a flying-boat hull had usually been dictatedabout equally by airscrew clearance and landing flap clearance requirements. The deletion of the airscrew would not there-fore ease the problem appreciably unless something were done about the flaps, too. It had already been noted that the wingloading would have to be moderate on account of the cruising lift coefficient; high-lift flaps would not therefore be necessaryto mitigate high landing speeds. Flap drag was not important on a flying boat because of the excellent braking effect ot thewater. All that was necessary was to be able to steepen the approach path, which could be done by dive or brake flapswhich, in any case, would be necessary for the descent from altitude. Regarding wing loading, the lecturer stated that occasionswere certain to occur when, for some reason, the aircraft could not reach, or could not be accepted at, the scheduled destina-tion; it had then to be able to land elsewhere. Alternative landing places would be few and far between for the large land-plane with high wing-loading. No such problems arose with the flying boat. Even a forced landing on land was a muchsafer proposition than for a landplane. In consequence, the wing loading of a large boat could be greater than for theequivalent landplane. To be capable of landing at a reasonably wide choice of aero-dromes, the landplane must have a landing distance from 50ft of not more than 1,600 yd under I.C.A.N. conditions, and forthis the wing loading had to be limited to about 68 lb/sq ft. Making a new comparison on this basis between landplane andflying boat showed that the largest landplane was over 20 m.p.h. slower than the rival flying boat. Economic Aspects Turning to considerations of economics, the lecturer thoughtthat the cost of fuel was likely to be about 20 per cent of the total cost of operating the aircraft under consideration. Thefuel consumed could be taken as directly proportional to the reciprocal of the speed, and the difference in fuel cost expressedas a percentage of the total operating cost was therefore hardly more than 0.5 per cent in all cases, and was reduced to zeroat something like 500,000 lb. At the point of equal speed, the weight of the flying boatwas less than that of the landplane. The prime cost of the airframe, the insurance premiums and so forth should, there-fore, be less than for the landplane'. On this basis, the point of equal economy could justifiably be expected to fall some-where between the point of equal speed and the point of equal weight. Mr. Keith-Lucas made the trenchant observation that, until ,now, landplanes had often operated with a hidden subsidy in the form of aerodromes built for military reasons. This wasparticularly true in Britain and parts of the British Common- wealth. The additional cost should, he suggested, be includedin the overall accounts of the British Airlines, which were owned by the State, so that any profit and loss would haveeventually to come back to the individual taxpayer. The new safety requirements of I.C.A.O. looked as if theywould demand runway lengths greater than were dictated by military necessity and" many of the existing aerodromes wouldbe rendered obsolete for civil aircraft. It should then be re- considered whether flying boats or landplanes, and conse-quently waterways or new aerodromes, would provide the more economic solution. Large aircraft of 300,000 to 500,000 lb were bound to comeso as to obviate the congestion at the terminal airports as the volume of traffic increased. They also showed improvedeconomy of operation over the small types when they could be operated at the same load factor. The question was whetherwe could afford the capital expenditure to realize these advan- tages unless we used the runways which Nature had supplied.Better docking facilities were wanted for flying boats, but even the best dock which could be imagined was likely to cost on'ya fraction of the equivalent aerodrome.
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