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Aviation History
1943
1943 - 1521.PDF
JUNE IOTH, 1943 FLIGHT 615 TRANSPORT AIRCRAFT traffic reasons involves loss of time and, in many cases, a sacrifice of regularity of operation. The loss of time in landing, refuelling, and taking-off and climbing back to cruising altitude is direct and reasonably predictable. The loss of regularity is less determinate. It depends upon weather conditions at the proposed intermediate points and upon the amount of progress that is to be made in continuing operations in spite of weather difficulties. One extreme instance is familiar to all of us. Under present conditions, transatlantic operations depending on New- foundland as an intermediate stop would suffer many can- 320 280 24O ZOO ISO \ \ \ —s, I \ s •—> n I ,—^—s]^—s — •Si *-* OCl -TO [)C|I .40^ :oo t.f._ M EEC 1 ! • 'GO M •mm r» u 5.17 O «00 KO ISOO »O0 ZOOO 2400 2BOO 3ZO0 3SOO «OOO *«00 WOO LENGTH Of NON-STOP STAGE (MILES) 1 1 \ 1 1 I 1 • I 20 to 5 4 3 2 I 0 NUMBER OF INTERMEDIATE STOPS IN 3,000 MILES cellations and delays which would be avoided in a rton-stop operation between the United Kingdom and the United States or the Canadian mainland. The use of the Azores as a refuelling point for seaplanes presents similar difficul- ties, due in that instance to water conditions rather than to weather. There are, on the other hand, many cases in which the climatic conditions at the potential intermediate stops are good enough so that no appreciable proportion of cancellations need be feared as the result of their use. In those instances the decision can be based on considerations of overall speed, economy, and operating convenience. Fig. 17 provides some of the data for a decision in such a case. For an assumed 5,000-mile flight in still air, and for block-to-block speeds of 160, 200 and 240 m.p.h. respec- tively, the cruising speed required to make good the desired overall speed is plotted against the length of the individual flight stage. Operation at an altitude of 10,000ft. is assumed, and the length of time actually spent on the ground at each stop is taken as varying from 10 minutes to refuel after a 500-mile flight, to 30 minutes for refuelling after one of 2,500 miles. Effect of Stops In Fig. 18 the study has been extended, for the particu- ' lar case of a block-to-block speed of 200 m.p.h. over a 5,000-mile distance, to an analysis of the variation of pay- load with frequency of stops. The major factors controlling payload under these conditions are: — (a) Increase of fuel load required as the length of stage is increased. (b) Increase of power-plant weight and structural weight with increase of cruising speed, as required over the shorter distances to make good the desired block-to- block speed. This analysis, like most of those that have preceded it, is based on a four-engined aircraft of about 6o,ooolb. gross weight and upon the best aerodynamic and structural char- acteristics that seem reasonably sure to be attainable in transport aircraft built immediately after the war. Specific fuel consumption is assumed at 0.42 lb. per -ii.p./hr. Take-off power is assumed to be governed by the two requirements that the average cruising power used throughout the flight must not exceed 50 per cent, of take- off power, and the cruising power in the initial stage, with the aircraft in its most heavily loaded condition, must not exceed 65 per cent, of take-off power. An aircraft which meets the latter requirement will normally have a rate of climb of at least 500ft. per minute on take-off power, and ability to accomplish the take-oft should, therefore, present no problem. Some empty weight might be saved by using a higher take-off power loading, but it would be at the expense of having to increase the proportion of take oft power used in cruising to such an extent that a considerable increase in specific fuel consumption in the early portion of flight would result. The requirement as here set forth is roughly equivalent to a maximum limit of 32 for the products of take-off power loading and square root of the span loading at take-off. For an aspect ratio of 12, the product of the take-off power loading and the square root of the wing loading at take-off would be limited to no. Reserve Fuel The fuel carried is computed on an assumption of flight against a 40-mile wind, together with an allowance for increase of distance due to detours or navigational errors or having to proceed to an alternate destination, of from 250 miles for the shortest distances to 500 for the longest, and an additional fixed reserve for approximately one hour of cruising operation. The combined effect of these several factors is to call for a fuel supply which exceeds the amount required to cover the designated non-stop distance at cruis- ing speed in still air by a proportion ranging from 55 per cent, for a 5,000-mile distance, to 175 per cent, when the distance is 300 miles. Fig. 18 also includes a curve of fuel load and one of the power loading required at take-off i:i order that the specified block-to-block speed may be attained. To allow for accommodation adequate for a long flight, whether it be made in a single stage or in several stages, the total weight of all those items which can be considered as directly proportional to passenger load has been taken to be equal to 50 per cent, of the payload. It is assumed that where intermediate stops are made, changes of crew will be pos- sible ; and the size of the operating crew carried in flight is, therefore, assumed to increase progressively with dis- tance from a crew of three for non-stop distances of 720 miles or less to one of nine for a 5,000-mile flight. Under these conditions the maximum payload at a given block-to-block speed would be carried by dividing the flight into stages of approximately 500-mile length. In view of the inconvenience of making any unnecessary stops and expense of maintaining the ground personnel at an in- creased number of points, the practically desirable distance between stops where economy is a factor of major import- ance would appear to range from 700 to 1,250 miles. For non-stop distances greater than 1,250 miles the economic penalty accumulates rapidly. » W 14 S m I it i-1 ./11 |-1 T — \\ • ! 1 ! - ' i 1''' — • - -- 1 N. 1 i t i ; 1 tr' T- 1 ! 1V X —1 1 \ — 1 FlC ___ tS *OO BOO iZQQ IGOO 20QQ Z400 Z900 J20Q 3WQ 400U *«©0 «O0 52<St> Payload for average block-to-block speed of 200 m.p.h. over 5,000 miles distance.
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