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Aviation History
1955
1955 - 1109.PDF
PLIGHT, 12 August 1955 THE DESIGN OF SMALL HELICOPTERS . . . iilO £5 /4D00 /8D00 /12D00 FIRST COST Fig. 2 (left). Variation of operatingcost per hour with first cost. Utiliza- tion, 600 hr. A-Hillcr 360. B=S.N.C.A-S-O. Djinn. E = Hiller Hornet. Fig. 3 (right). Variation of operating cost per seat-hour vrith annual utilization. A-Hiller 360. B=S.N.C.A.S.O. Diinn. C=Two-secr gas-turbine helicopter. D=Three-seat helicopter with subsonic ramjets. E=Hiller Hornet. F=Three-seat helicopter with supersonic rotor and ramjets. 221 500 1000 1500 ANNUAL UTILIZATION (hr) for an operator to achieve the maximum possible annual utiliza-tion. Cmdt. Boris has said that for an agricultural operator 500 hours is a very good figure, whilst M. Vernieuwe2 has given theaverage figure for Sabena helicopters as 730 hours over the first two years of scheduled operation. Other operators have quoted figuresas high as 1,000 hours, and Mr. Masefield3 assumed a figure as high as 2,000 hours for his study of helicopter economics: in this lastcase it may be that the figure reflected B.E.A.'s hopes for the future rather than current achievements. It is clear from Figure 1 that utilization has a very powerfuleffect on operating cost per flying hour, but little on the relative economic efficiency of the various configurations. Of the designstudies, the ramjet helicopter is cheaper to operate than the gas- turbine-powered two-seater, for less than 900 hours of flying perannum, even though the ramjet design is for three seats. With supersonic rotor and ramjets the operating cost is by far the lowest,but since supersonic rotors are not a "current technique" in this country the curve serves only as a pointer to the future. It is of particular interest that neither the Djinn nor the gasturbine project are as cheap to operate as the ramjet designs, despite their much lower rates of fuel consumption. If the recentaward of a contract to build an Army ultra-light machine was made to Faireys for technical reasons, it is evident that a pressure-jet configuration must possess some considerable advantages which outweigh the greater servicing and operating costs.In Fig. 2 the operating cost of the three existing helicopters is plotted against first cost for a typical utilization of 600 hours.This illustrates very clearly the dominating effect of first cost, for although the characteristics of these machines could hardly bemore different, their operational cost variation with first cost is almost identical. Within the accuracy of the method a generaliza-tion can be made for almost all small helicopters: 0.41(FC)+1,800Operational cost, £ per hour = h 9-5 where FC = first cost and U=annual utilization.Fig. 2 also enables the monetary equivalent of one pound of basic weight to be calculated, and it is found to vary principallywith the parameter of first cost per passenger seat. In round figures one pound saved on the structure is equal to £100 reduc-tion in the first cost if the aircraft is to be used for carrying "ideal freight": that is to say, freight which can be divided to thenearest half-pound. Although such freight is undoubtedly to be found (bulk potatoes spring immediately to mind) it is often over-looked that for small helicopters freight nearly always comes in the form of large indivisible lumps, the most obvious examplebeing human beings. Provided that the helicopter can maintain the required performance with all seats occupied, a reductionof the structural weight by one pound is not worthwhile even if the first cost is increased by only a few pounds sterling. Purelyfor passenger transport the weight-saving must be 170 lb or nothing at all, although for freight carrying a smaller incrementwould probably be acceptable. This apparently brings us to the brink of a new science, "quantum economics", which is beyondthe scope and purpose of this article. _ In Fig. 3 the operational costs of Fig. 1 are divided by thenumber of passenger seats to obtain an index of passenger-carry- ing efficiency. The Hiller 360 is now more efficient than theHornet, but still less efficient than the three-seat ramjet design. The dominating factor is now the new parameter first cost perpassenger seat", and the approximate equation for cost becomes: 0AKFO+1JS00 9.5 Operational cost, £ per seat-mile = —-— + —- w.hs^..j££) = first cost, N=number of passenger seats and U— annual utilization. , . ,„ As an approximate index of operational efficiency first cost per TABLE Helicopter Three-seat (Subsonic Ramjet) Hiller Hornet bristol 173 Flk 3* Bristol 173 Mk 1« Hiller 360 Bristol 171* Two-seat (Gas Turbine) S.N.C.A.S.O. Djinn II First Cost (O 3,500 199.000 3,500 74,500 59,900 12,500 30,250 8,000 8,000 First Cost per Seat (£) 1,750 3,110 3.500 4,140 5,990 6,250 7,560 8,000 8,000 • Ref. 3. passenger seat" seems to apply to any helicopter, and in Table IIvalues are given for designs which cover the entire range at present envisaged. This table apparently indicates that the three-seat subsonic-ramjet-powered helicopter is operationally more efficient than Mr. Masefield's £199,000 Bealine Bus. This isprobably true for those operations for which the small helicopter is designed, but its maximum stage length of 50 miles is justabout the minimum contemplated in the B.E.A. studies'. More- over, the B.E.A. assumptions for the fixed annual costs of largehelicopters tend to minimize the effect of first cost by taking the long write-off period of eight years, an insurance premium of6 per cent of first cost (the current figure quoted in Ref. 1 is 22 per cent) and an annual utilization of 2,000 hours. Although these figures are undoubtedly attractive values to hopefor, they do not seem very closely related to the facts of the con- temporary situation for small helicopters. In an age when it isthe fashion to ask for larger and yet larger transport helicopters, powered by two, three or more free turbines, it seems that aprofit-seeking independent operator would be well advised to find and operate a humble three- to six-seat ramjet helicopter, whichwould enable him to operate stages up to fifty miles very much more cheaply than the large transport helicopters likely to becomeavailable in the next ten to fifteen years. It seems that, so long as the design is reasonably good, and the performance up tospecification, then the machine which costs least per passenger seat will generally cost the least to operate (within its useful range)however much the protagonists of turbine, gas drive and ramjet may rage together. It may be argued that the proviso about useful range pointsto a basic fallacy in the above reasoning. It is true, of course, that a pure-ramjet helicopter becomes prohibitively expensive for flightdurations in excess of one hour, but for long-distance work its place is taken by the ramjet booster compound helicopter.4 Inessence this is a ramjet helicopter which has a small turbine and propeller installed for use in forward flight. As explained inRef. 4, the boost ramjets are used for take-off, landing and low speed manoeuvring; for forward flight the turbine is started andthe power output of the ramjets reduced until at cruising speed they have ceased to function, and the rotor is auto-rotating in theclassical Gyrodyne manner. This means that there are two com- pletely separate systems for propulsion and sustentation, either ofwhich can be used to fly the aircraft if the other fails. Several prototvpes have already been built along these lines, and success-fully flown; but none of them achieves the extreme simplicity (and hence the high payload) of the ramjet boosted lifting system. First Cost and Weight. Hardly a year passes without someonepublishing new curves of cost in £/lb against some independent variable, and no doubt wise conclusions are sometimes reachedby their use, but not always in exploratory project work. The latest curve seen by the writer showed that, for a hundred units,airframe costs are about £10/lb. Applied to a typical Auster, with corrections for engine price, this gives a first cost of roughly
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