FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1957
1957 - 0709.PDF
FLIGHT, 24 May 1957 715 AIR FREIGHTING The Cargo Business and its Equipment By ALAN H. STRATFORD, B.Sc. (Eng.) A.F.R.Ae.S. M.I.Ae.S.* WHENEVER special freight aircraft, such as the Bristol170 Freighter, have been operated instead of adaptedpassenger aircraft, their marked advantages have been demonstrated at once. Steadily the breed of the freighter has beenimproved, and it is probable that in today's Lockheed Hercules C-130 military transport and in tomorrow's Armstrong WhitworthFreightercoach two aircraft are being developed which closely approach the ideal in their own fields of operation.As a basis for design, it is clear that speed for its own sake has no importance for the pure freighter. Only in its effect on revenue-earning capacity per unit of time, and on high productivity, will it be of value. Speed is of purely economic value to the freighter,and its prestige value is unimportant. In the convertible passenger role speed may be more significant, but a freighter cannot beexpected to compete for first-class passenger transportation on the blue-riband routes of the world. In fact, freighter design can beclosely linked with that of high-density, short-range passenger transports in which low operating costs are of more value thanthe highest performance. This consideration gives turboprop power a considerable attrac-tion. It probably provides the best application for this type of powerplant. Undoubtedly the jet engine may score on weight,simplicity of installation, and low cost, and the piston engine may win on low cost and low-altitude fuel consumption. Nevertheless,the turboprop is probably the best all-round proposition. Especi- ally is this true if high power and pressurization are essential,and when the highest speeds are not necessary. The turboprop's present showing is better than could have beenforecast five years ago. Weight, specific consumption and overhaul life have improved considerably in this period. First costs are stillhigh and there is a great need for larger-scale production and the lowering of manufacturing costs.The high output of the turboprop engine and its optimum economy at a high percentage of maximum power give adequatetake-off and climb power and provide cruising speeds which are within 100-150 kt of jet speeds. Moreover, it is economic overa wider range of aircraft-weight, altitude and temperature. Water- methanol injection can be used to give adequate take-off power inhigh ambient air temperatures. Current turboprop engines have good prospects of extensive development.Take-off performance of the turboprop is superior so long as suitably sized power units are available to enable the optimumtwo- or four-engine layout to be selected. Four engines have now become essential for long ovcrwater stages, and they give morefavourable payloads for the engine-out case on climb and take-off. The four-engined layout is now widely favoured and higherservice regularity and ease of ferrying back to base with one engine out are advantages.Probably the main point of contention in the design of aircraft for freighting is the provision or otherwise of a pressurized cabin.In this respect the piston-engined transport operating at altitudes of 10,000 to 12,000ft with a simple, robust structure of low weightand cost, and without the complications of pressurization, has something to commend it. The DC-4 and Bristol Freighter aretypical. The lack of pressure-tight doors with large-span openings is an immense advantage—as on the S.N.C.A.N. Noratlas and theFrye Safari. Their full-width doors take advantage to the full of a simple structure with rectangular fuselage frames. With nopressurization a rectangular cross-section can of course be used, and a notable gain in usable volume is achieved. It is verydoubtful, however, whether the full advantages of the turboprop freight aircraft can be realized without pressurization. Themoderate degree of pressure differential required for current turboprop engines (certainly not exceeding 6 Ib/sq in) need pro-vide no obstacle to the design of full-width doors, although some weight penalty must be incurred. It must be remembered thata wide range of foodstuffs, livestock and so forth need air con- ditioning, and pressurization is essential if the freighter is to beconvertible to passenger work. Hence moderate pressurization, heating, refrigeration and humidification will probably be neces-sary for the fullest flexibility of the most advanced freight trans- ports if they are to operate on a world-wide basis. It has been the practice of the U.S.A. military authorities for *Civil aircraft adviser to the chief designer, Sir W. G. Armstrong Whitworth Aircraft, Ltd. The first part of Mr. Stratford's article tvas published in "Flight" last week (May 17). ". . . it is probable that in today's Lockheed Hercules C-130 military transport and in tomorrow's Armstrong Whitworth Freightercoach"— one version of which latter aircraft is shown in the drawing above— "two aircraft are being developed which closely approach the ideal in their own fields of operation." many years to specify level-loading floors to suit the height ofmilitary trucks (about 4ft). The extension of this concept to the civil field in the later types of freight aircraft has been an importantstep forward. The height of the sill of the main hold on the DC-6A is 8ft 9in. In the Noratlas this is reduced by more thana half. The low-level hold required for ease of access from trucksdetermines to a great extent the high-wing arrangement that has now become normal for freight aircraft. This design feature,with full-width access at the front (Bristol Freighter and Safari) or at the rear (Noratlas and Lockheed Hercules 130) is now charac-teristic of the freight aircraft in its civil or military role. For it is in easy access to the hold and rapid loadability that the highutilization of the pure freight aircraft is maintained. Particularly is this so on medium and short stages. The passenger will loadhimself, and disembark on arrival at his destination, but freight must be loaded and unloaded. This can be a time-consumingprocedure if it is not well organized and is not assisted with the right handling equipment and unobstructed holds and fittings.With current transport aircraft it takes about half an hour to load or unload 5,000 lb of freight. Loading normally takes longer thanunloading, since more precision is called for unless careful prepara- tions have been made or special techniques are used. Many methods have been developed for preloading cargoes toreduce time spent on the ground. One such scheme is used successfully in New Zealand by the Straits Air Freight Expressto ferry cargo between the North and South Islands by Bristol 170s. Known as the Cargon, it moves pre-packed loads on railsinto the hold of the aircraft to minimize handling and to reduce turn-round time. A weight penalty of only 550 lb is incurred. Onreally short flights a quick turn-round is essential. Few kinds of cargo are as ideal as the motor-car traffic which Silver City havedeveloped so successfully since the war. They achieve a 12-min turn-round on their cross-Channel vehicle ferry. In the U.S.A.a universal pallet has been proposed (Ref. 4) on which pre-loaded goods can be moved on the ground or in the air. The aircraftrequires parallel tracks to take the pallets, which incorporate flush tie-down fittings, and are furnished with their own wheels andcastors. Such equipment requires wide-access, level holds. It makes it possible largely to dispense with the costly groundequipment—such as fork and power lifts and the special toboggan shutes and rolling carpets—often necessary in quantity to meetextensive operations. Undoubtedly the loading of freight aircraft could develop alongother lines. It may be recalled that the pre-war Junkers G.31 could be loaded through the top of the fuselage, and the DouglasC-124 Globemaster can be loaded from below the floor: a large floor section behind the wing can be lowered and raised by meansof winches. The Lockheed Speedpak which, it was claimed, could be attached to the under-fuselage of the Constellation in threeminutes and could carry four tons of freight in a volume 33ft long by 7ft wide and 3ft 6in deep, had a speed penalty of only 10 m.p.h.This scheme had much the same object as the large holds on the Vanguard with far more flexibility but with lower performanceand weight efficiencies. Quite unlike this was the German Fieseler Fi333, whose detachable cargo hold 24ft X 7ft x 6ft could bepreloaded and which could be flown without its hold attached. This original design came perhaps too early for effective use inservice. These ideas differ essentially in their conception. The Speedpak attempted to modify the passenger aircraft as and whenrequired. The Fi333 was an aircraft designed essentially for load carrying. They both accepted the importance of preloading andthe object of the design had much in common with the Cargon
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
