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Aviation History
1936
1936 - 0097.PDF
JANUARY 9, 1936. FLIGHT. 47 Helping the Prospective Air-line Operator to Choose His Fleet : A Simple Formula Which Takes into Account Speed, Power and Cost Per Passenger By E. N. B. BENTLEY, A.F.R.Ae.S. THE business man who intends to run an air transport company on a commercial basis is not directly inter ested in such things as percentage structure weight, the relative merits of fabric or stressed-skin cover ing, and so on. He wants to know the income and ex- penditure account; how many paying passengers the aero plane can carry, and how much it will cost to carry them. The technical marvels by which these results are achieved, however much the aeronautical community may admire them, are taken as a matter of course. Almost all the expenses of operating an air line are dependent on the type of aeroplane used, and what the air-line manager wants is a convenient method of com paring the various aeroplanes on the market. His choice will, of course, vary according to the operating conditions on his proposed air route. A short ferry service over the sea or an estuary will be most efficiently operated by a type of aeroplane different, for example, from that required for a long-distance air route over high mountains. But when he has decided on the general requirements of his particular case, he wants to be able to compare the merits of the different types of aero plane which appear suitable. The ratio of income to ex penditure is the important factor from the operators' point of view, and I will call this the "Commercial Efficiency" of the aeroplane. Income, broadly speaking, means passengers, because very few air lines rely entirely on mail or goods, and in any case passenger accommodation can always be con verted to the less exacting demands of letters and parcels. But the number of passengers carried is not the only thing by which the efficiency of the transport aeroplane is to be assessed ; speed is also very important. A fast aeroplane may be able to make two journeys, carrying a load of paying passengers, in the time that the slow one makes one journey. This is not so unlikely as it seems, and it does not mean that the fast aeroplane must be twice as fast as the slow one, as the following example will show. An aeroplane flies 300 miles at a cruising speed of 100 rn.p.h. against a head wind of 30 m.p.h. ; the average speed over the ground is 70 m.p.h., and the time taken is just over 4J hours. The aeroplane which can cruise at 17° rn.p.h. under the same conditions has an average speed over the ground of 140 m.p.h., and therefore does the journey in half the time of the slower aeroplane. The advantages of the fast aeroplane are that it is much less handicapped by adverse weather conditions, and can often do one more stage in the day, carrying its load of paying passengers (the important point) than the aero plane with the low cruising speed. This means fewer can cellations of service and a better adherence to the time table, both of which points make for a more popular ser vice. From the point of view of income, the efficiency- of the aeroplane must therefore be considered to be propor tional, not to capacity, but to passenger-miles per hour. The item on the other side of the balance-sheet, namely, expenditure, is divided into two main parts: Standing arges, which are not affected by the amount of flying done, winning charges, which depend on the number of hours In actual practice it will be found that the running ASSESSING "COMMERCIAL EFFICIENCY" (Flight photograph.) charges amount to about twice the standing charges for the year. This is, of course, only an approximate figure, and if an exceptionally big mileage is done the propor tion will be higher; con versely, if the rate of obso lescence is high, the propor tion will be lower. The standing charges are the insurance of the aero plane and the obsolescence charges. If the aeroplanes are going to be replaced by new ones every four years, then the obsolescence rate is 25 per cent. ; if they are replaced every five years, then the rate is 20 per cent. Both the insurance and the obso lescence charges are proportional to the cost of the aero plane, and are independent of the speed. The Commercial Efficiency of the aeroplane, with respect to the standing charges, can be expressed by the ratio; passengers carried per £1,000 cost of the aeroplane. The Formula The running charges consist almost entirely of fuel and maintenance costs, which are proportional to the total horse-power of the engines. The Commercial Efficiency of the aeroplane relative to the running charges is expressed by the ratio: passengers carried multiplied by the speed of the aeroplane (i.e., passenger-miles per hour), divided by the total horse-power of the aeroplane. The prospective air-line operator has therefore a simple formula which will give him the relative Commercial Effi ciency of the aeroplanes which he is considering : — PxV P ~HF+ J where P = number of passengers carried. V = top speed in miles per hour. HP = total horse-power. /=number of thousands of pounds which the aero plane costs. I have worked out the Commercial Efficiency of several aeroplanes by this formula, and it will be seen that the first term (running charges) is about twice the second term (standing charges), which shows that the formula also gives the correct proportion between the two charges. Before looking at the results obtained from this formula
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