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Aviation History
1934
1934 - 0221.PDF
FLIGHT, MARCH 8, 1»S4 Cruising Speed inm.p.h. 100 .. 110 120 130 140 150 160 170 Cruising H.P. per1,0001b. 28-2 330 39 0 46-5 54-0 64-0 750 87 0 Hours of Flight 5-00 4-55 416 3-84 3-58 3-333-12 " 2-95 Fuel and Tanks, PercentageGross Weight 8-4 9-0 9-8 10-7 11 -6 12-7 14 1 15-4 Gross H.P. per 1,000 lb. Weight of Motor Units 62 ! 15-5 62 15-562 15 5 62 15-5 72 : 18 0 85 21-2 100 25-0 116 ; 29-0 Weight Less Gross Pav Load 69-9 70-571-3 72-2 75-6 79-985-1 90 4 Gross Pay Load 30 1 29-528-7 27-8 24-4 20 114-9 9-6 NetPay Load 24 1 23-623 0 22-2 19-5161 11-9 11 Factor of Usefulness 24 1 2H-027 (i 28-9 27-2 24-2190 13 1 TABLE I (2) obsolescence; (3) aerodrome charges; (4) general officeand traffic expenses; (5) part of insurance. (b) Running costs, which include: (1) Fuel and oil;(2) upkeep of aircraft; (3) upkeep of engines; (4) inspec- tion; (5) wages of crew; (6) part of insurance. The standing charges depend almost entirely on thefirst cost of the aircraft, and this is proportional to the gross weight. It is convenient that aircraft structures andaircraft engines cost practically the same per lb., so that it does not matter if the horse-power is relatively high orlow. Standing charges therefore depend on the gross weight of the aircraft. Running costs for aircraft with the same relative horse-power a.lso depend on weight only, other things being equal. High powered craft cost more for upkeep, as engineupkeep per lb. is more than for aircraft; also the higher- powered craft use more fuel and oil per mile. It is im-possible to allow for this difference in a general way. Fuel varies in cost in various parts of the world, and it isdifficult to get reliable figures for upkeep. Fortunately it is not of much importance, for I shall show later thatthe most economical speed of flight is obtained with engines oi a horse-power that is settled from considerationsof take-off and flying with one engine stopped, except that twin-engine aircraft intended to fly on one engineonly need a much lower horse-power weight ratio than three or four-engine aircraft, and therefore need specialconsideration. Consequently the variation in the horse- power weight ratio is too small to affect our general con-clusion seriously, viz., that running costs are proportional to weight. As a first approximation, we may make the verj' con-venient assumption that the total cost of operating an air liner is directly proportional to the gross weight. We needonlv make a reservation that this applies to aircraft whose weight horse-power ratio does not differ much from 15 lb.per horse-power. We have seen that the earning capacity is directiv proportional to pay load x cruising speed. If Chnv orse oqUprrwnt m •W t MO ISO ifiO Spaod --. Mites per Hour 29 76 23 24 *2O "- m 13 12 \/ t / • — — FIG2 O M u / 1 9 C \ V — 4\\ > MO 150 m | hti we express pay load as a percentage of gross weight, it follows directly that the commercial value of an air liner is directly proportional to the ratio Percentage pay load x gross weight X cruising speed gross weightfor this is the earning capacity divided by running costs. The term gross weight can be cancelled, so the commercialvalue of any aircraft is directly proportional to two factors—percentage pay loadXcruising speed. This I pro-pose to call the Factor of Usefulness. Expressed another way, it is a. number which when multiplied by the grossweight of the aircraft in tons, gives the work in ton miles that caii be done in an hour. Determination of Economic Speed I have, in finding the cruising speed for which to designto get the highest Factor of Usefulness, made a number of assumptions which 1 believe represent the best modempractice. I have calculated the pay load for a number of aircraft of different cruising speeds, keeping the rangein still air the same—500 miles. I have assumed that the aerodynamic qualities of the aircraft are unchanged byaltering the horse-power of the motors, and that the efficiency of propulsion is the same. The aircraft charac-teristics are fairly correct for an air liner of gross weight from 10,000 to 30,000 lb.; actually the assumptions arelargely based on the experience of design of aircraft of 20,000 lb. The design assumptions I have made are asfollows. Design assumptions: (1) Range in still air is constantand is 500 miles; (2) net pay load excludes crew and special passenger equipment; (3) an allowance of 25 percent, of the pay load is added for passenger equipment; (4) wing loading is 15 lb. per square foot for a monoplaneof fairly high lift coefficient; (5) span loading = gross weight/span2 = 2.5; (6) parasite drag at 100 m.p.h. =25 lb. per 1,000 lb. weight; (7) structure weight = 42 per cent, gross weight; (8) Crew and their equipment, includ-ing wireless = 4 per cent, gross weight; (9) motor units, complete with startinggear, fairing, silencers, airscrews, piping and oil coolers = 2.5 lb. per declared h.p.(Note.—•" Declared horse-power " means the maximum horse-power for a 50-hourrun at normal speed, and not the maximum horse-power that can be obtained. Britishtype test horse-power is usually 10 per cent, less than maximum permissible horse-power); (10) fuel and oil at cruising speed = 0.54 lb. per b.h.p./hour. Tanks forfuel and oil = 0.06 lb. per b.h.p./hour. Total of fuel, oil and tanks = 0.60 lb. perb.h.p./hour. (11) propulsive efficiency of airscrew = 75 per cent.; (12) cruising h.p.must not exceed 75 per cent, of declared h.p.; (13) minimum type test h.p. for take-off = 62 h.p. per 1,000 lb. These assumptions refer to a four-enginedunbraced monoplane of good streamline shape. The undercarriage must either beretractable or else extremely well faired. The wing loading of 15 lb. per sq. ft. isconservative compared with modern Ameri- can practice. The figure of 25 lb. dragper 1,000 lb./weight conforms witk the 221
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