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Aviation History
1943
1943 - 1520.PDF
FLIGHT JUNE IOTH, 1943 VIM LMMK safe and comfortable without oxygen or cabin - supercharging. Mow the effect of raising the operat- ing altitude has to be examined. Certain of the inherent ad- vantages of supercharged - cabin operations, in the elimination of passenger discomfort and in making it possible to go above a substan- tial proportion of the storm and icing conditions, are obvious. Supercharging also has the merit oi making it possible to descend through the overcast at any de- sired rate, without having to con- sider the effect of rapid pressure change on the passengers, and it may thereby simplify the problems of traffic control. For some routes • the supercharged cabin will not only be helpful but indispensable; but in considering whether it is a feature to be accepted only when necessity dictates, or one to be eagerly espoused in every instance, its effect on economy must be considered. Weight of Pressure Cabin For the purposes of the following studies I have assumed —perhaps rather optimistically—that within a short time after the end of the war commercially satisfactory power plants will be available to produce enough power for cruis- ing and a reasonable rate of climb up to an altitude of 30,000ft. with a gear-driven blower. I have assumed that it will be possible to provide cabin supercharging for a total expenditure of weight in the cabin blowers, in the valves and* controls, and in the fuselage structure of 3.5 per cent, of the gross weight of the aircraft, and that the increase of weight in the engines and airscrews for high- altitude operation will be 0.2 lb. per take-off horse-power. I believe that those figures represent reasonable proba- bilities. Fig. 14, computed in the same fashion as Fig. 5 and upon the same assumptions in every respect except as to altitude, shows the cruising speed at 30,000ft. as a function of wing loading and power loading. The optimum wing loading if speed alone be considered is, of course, but little more than half the optimum for 10,oooft., and closely approximates 5 (V /100)2. tt £24 r a 4 SPEED ^* »T 30,000 f£IT \ ~—• i» _-= ^JJ^ :—~ """ -—> n POST-WAR] In Fig. 15 the take-off horse- power required per pound of gross weight is plptted against cruising speed for the two altitudes, assum- ing that in every case the optimum wing loading will be used up to, but not in excess of, 50 lb. per square foot. A third curve gives the differ- ence of the power-weight ratios, or the variation with speed of the amount of power pet pound of gross weight that would be saved by flying at jj|jjpooft. instead of 10,000. The last curve shows the total effect of high-altitude opera- tion on power-plant weight, allow- ing for a basic power-plant weight ofv 2.0 lb. per horse-power and for increase of weight for supercharging as previously assumed. If only uniform operation at cruising speed had to be con- sidered, the saving in power-plant weight by high-altitude operation should be sufficient to balance the additional weight of cabin supercharging provision at all speeds above 240 m.p.h. There are, however, certain additional factors to be taken into account. (a) Saving of fuel weight by high-altitude operation (assum- ing a uniform speed); (b) Inciease of structural weight due to lighter wing loadings for high altitudes; (c) The effect on block-to-block speed of the time lost in climbing to high altitudes. The first point is «in favour of high-altitude operation ; the second and third are against it. It would appear that if a high degree of economy is desired in a transport operation supercharging will be used only when weather conditions along the route are such that high-altitude operation will contribute notably to safety and regularity of service. If, on the other hand, it is not a primary objective to keep costs close to the iowest possible level, the operating advantages and the luxury of super- charging can be secured at a total expense of cost per payload ton-mile of 10 or 15 per cent, in the range of most probable operating speed. In the still more extreme case of determination to attain very high speed at almost any cost, high-altitude operation is a part of the specification for keeping the expense of the speed to a minimum. Long-range Flights Non-stop flights of more than ordinary length fall into two categories. The non- stop distance may be imposed by the lack of possible inter- mediate fuelling facilities, particularly on over-water flights; or it may be freely chosen, in the interest cf greater speed, economy, con- venience, or passenger com- fort. The insertion of an inter- mediate stop not required for The difference between the payload in operating at 30,000ft. and that in opera- ting at 10,000ft., plotted against speed for several lengths of flight. Proper allowance made for the effect J«O 26o seo 500 >2o of power loading and speedSPE "l"fMI ^ on fuel consumption. i / j / I •A ^y • G 16 -
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