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Aviation History
1952
1952 - 1838.PDF
16 FLIGHT, 4 July 1952 PHHpp^P^ About Bristol Helicopters An early flight of the Type 173, first British commercial helicopter and first of its type to be capable of flying on either of two engines. Plans for the Type 173: Air Impressions of the 171 Mk 3 By THE EDITOR THE publication last December by B.E.A. of an outline specification for a large commercial helicopter was not only an indication of what the Corporation foresaw as its own requirement a few years hence; for many people it was also something of an introduction to the characteristics of commercial helicopters as a class. Two main departures from existing helicopter practice were the insistence on twin-engine (or multi-engine) safety, and the need for a considerably higher cruising speed than has so far been provided. For city- centre operation there is no doubt at all that the insistence on twin-engined safety is justified. B.E.A.'s proposed capacity of 30-36 seats will no doubt be approached by stages, just as the final design is to be exten sible by degrees up to 45 seats. In the Bristol 173 we see the first important step towards B.E.A.'s specification, and it has already been stated the the Corporation will carry out trial operations with a prototype of this aircraft. During a recent visit to Filton a great deal was learned about current thinking regarding operation and development of the Type 173. Mr. Raoul Hafner, Bristol helicopter chief designer, has an infectious kind of enthusiasm for his pets, and certainly it will be no fault of his if inter-city heli-buses are not operating before the end of the 1950s. Of the various comparisons of operations with helicopters and fixed-wing machines which have been made, the majority have attempted to fit the newcomer into an existing pattern and to fly it according to fixed-wing standards. As a result, the helicopter's applications and future have sometimes appeared to be unduly limited—the more so when it is dis covered that, for example, its structure-weight is currently some 5 to 7 per cent higher than for conventional types of airliner—say 72 per cent of all-up weight—and its initial cost relatively high unless large-scale production can be seriously contemplated. For the same reason that a helicopter is today expensive, however, Mr. Hafner suggests that it can be expected ulti mately to be cheaper than other types. The mechanical units (particularly transmission) represent a high percentage of the helicopter's weight and, in small numbers, are expensive. But as experience with large-scale production of piston engines has shown—and the two are comparable on this basis —good-sized production batches can be put through more cheaply than an equivalent unit weight of structure. In round figures a 10,000 lb a.u.w. helicopter having, say, 2,400 lb for payload might be sub-divided into two power units at 2,000 lb; transmission 800 lb; rotors, 300 lb; structure and basic equipment 4,500 lb. A comparable fixed-wing machine might be some 450 lb lighter, and the 800 lb transmission- weight would be represented in the structure and such things as flying controls and a heavier retractable undercarriage. On many occasions helicopters will be able conveniently to share facilities with other commercial aircraft at main airfields, but for city operation airstops will have to be built. These are expected to be simple landing areas, possibly circular, allow ing a 100-ft run into wind in any direction. The only facilities required will be those of a bus centre or railway station, such as the ticket office and passenger waiting rooms, and, in addi tion, lighting for night services. All servicing and refuelling is likely to be carried out at satellite stations around the city, which would also serve as diversion, emergency or holding fields. These would be directly comparable with bus garages or, on the railways, locomotive sheds and sidings. No refuel ling or servicing would normally be carried out at the city air- stops. Traffic control and navigation are expected to be relatively simple with the aid of the Decca Navigator and V.H.F. com munications. The whole pattern of helicopter operations normally being below that of the fixed-wing trunkline aircraft, there should be little or no interference at main airports. The helicopters would, if necessary, be able to fit in with the nor mal traffic pattern, and even with circuit procedures. With the provision of Decca and a blind-flying panel adapted to suit the helicopter's flying characteristics (and including a yawmeter) only the worse weather would inter fere with operations. Provided that the pilot could see the airstop lighting in fog or at night, after having reached a hovering point overhead at, say, 200 or 300ft, it is thought that he would be able to land safely. Taking off from an airstop, the pilot will head into wind and possibly climb backwards initially in order to keep the airstop in view at the most convenient position ahead for an emergency landing. Twin- or multi-engined helicopters are not, however, concerned with safety height so much as with safety speed, and in the case of the Bristol 173 prototype 65kt is the figure to keep in mind for optimum minimum speed. Below this speed failure of one engine inevitably results in some loss of height while this speed is regained or, alternative ly, necessitates an immediate landing. Above about 65kt engine failure need involve no loss of height and the pilot could continue to the nearest "garage" on the city perimeter. Not only would this somewhat larger area give him more margin for errors of judgment in a single-engined landing, but also (and obviously enough) the operators would not want to have an unserviceable machine perched upon an airstop above city buildingSTvith no repair facilities available. With forward speed a single-engine rate of climb of 250 ft/min is possible on the Bristol 173. Another important consideration is that of fuel reserves
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