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Aviation History
1960
1960 - 0102.PDF
102 FLIGHT, 22 January 1960 D.H.I2I PROGRESS REPORT ON BRITAIN'S 600 m.p.h. VISCOUNT REPLACEMENT THE first Airco D.H.121 is scheduled to fly in December1961, and six more aircraft will join the flight-test programmeduring 1962. Deliveries to BEA are expected to begin in the middle of 1963. Given the orders, a production rate of sixaircraft per month could be achieved. These new details of the 121 programme were disclosed byAirco at Hatfield recently. At the same time a general briefing on progress was given. De Havilland, who are leaders of the 121design and production team, in which Hunting and Fairey are participating, have "never been so certain" about so much detailthree years ahead of the time when a new project is due to enter service. Summing up the broad concept of the aircraft, P. F. L. Hall,technical sales manager of de Havilland Aircraft, described the 121 as "uncompromisingly a short-haul aeroplane—a 600 m.p.h. A progress report on the D.H.121's engineering systems, with special reference to blind landing, appears in the regular "Flight" Systems Survey feature on page 120. successor to the Viscount and the Convair 440 with very lowoperating costs." There was built-in stretch for increasing design range beyond the basic 1,000 statute miles without losing the costadvantages of short-range "optimization": by increasing weight from 105,0001b to 112,0001b, and basic take-off distance from6,000ft to 6,700ft, the range with full payload could be increased to beyond 1,400 statute miles. The Smiths blind-landing system, to be introduced first asautoflare and then as full autoland, was being designed in right from the start. Other general points made by Mr Hall were that at one stagesilencers were going to be fitted on all diree engines, but noise characteristics appeared to be so good that this would probablynow become an optional extra. It was expected that only the two side engines would be fitted with reverse thrust, which would beusable in the air for fast let-downs through icing conditions. There was no elevator as such: the 121 would have an all-moving tail-plane the trailing edge of which was geared to it and moved in the same sense. The offset, sideways-retracting nosewheel was economical innosewheel-bay space and the airstairs were stowed above it under the cabin floor. The front door was therefore unobstructed, andcould be specified as an emergency exit. The offset nosewheel was in line with the captain's seat so that steering should feel morenatural. The main undercarriage legs rotated as they retracted so that the four-in-line wheel groups would lie flat in the fuselageand occupy little space. The legs also extended during retraction, affording a wide-track mounting. The complete undercarriage wasof free-fall design. Redux, "an adhesive of which we have 15 years' experience"was used for the attachment of stringers and doublers. This pro- vided excellent crack-stopping qualities. Structural philosophywas "fail-safe with a very long safe life." M. J. Goldsmith of DITs sales engineering team explained thatthe 121 had a "very fast" wing. In practice the operator would fly at Mach 0.875 down to 23,000ft. Below this height an IASlimit of 360kt was applied, giving a maximum speed of 606 m.p.h. A speed higher than 360kt IAS was not chosen because it wouldhave resulted in a heavier structure, while the ride at lower altitudes would be uncomfortable. Anyway, for short range work the timesaved by going faster was negligible and 606 m.p.h. was considered fast enough. A certain amount of engine over-speeding was allowedby Rolls-Royce on the RB.163, affording a degree of power restora- tion for take off on hotter-than-standard days. Steady approachspeed was 127kt at an average landing weight—a moderate step up for the sort of pilot who had been used to Viscounts or Convair440s. The maximum landing weight was very high—95 per cent ofmaximum take-off weight. This gave a high "residual range," i.e., capability of landing and taking off again without refuelling (seeFig. 3). Economics were estimated according to the ATA method (see Fig. 5), the only variation from the ATA formula being theassumption of a ten-year depreciation life with a 15 per cent residual value. It was felt that this was justified as the aircraftwas likely to last longer than the present seven-year, ten per cent formula. Cost per seat mile, with 82 mixed passengers over a 1,000-milestage, was down to about Id, a figure which had been sought for a long time. With 100 economy passengers the cost was downto around lc per seat mile. Economics were not unduly sensitive to changes in cruising altitude and varied only about 2 per cent fora 10,900ft deviation—a factor which operators would find very useful when forced by ATC to fly at the altitudes for which theyhad not flight-planned. The Airco D.H.121's "return on investment" was comparedwith that of competing aircraft. It was shown that an operator with a fleet of D.H.121s could expect over 8 per cent higher returnwith a given traffic uplift than with any other fleet. The factors in this study were a constant traffic for all competitors over an800 statute-mile stage, a US domestic revenue rate and 60 per cent load factor. ATA direct costs were assumed to be equal to indirectcosts, and annual utilization for jets was not allowed to exceed 45,000 40.000 TOTAL STATIC THRUST-LB. 35.000 30,000 25.000 Dcsiqn Requirement -Balanced Field Lenath - 6,000tt. at Sea Level. I.S.A.+ 10°C | Desiqp Requi 600 m.p.h ement S 0125,000 t. > • 45.000 40,000 35,000 30.000 25.000 1.200* 200 400 600 800 1,000 STAGE DISTANCE-STATUTE MILES Fig 1. The "balanced thrust" case tor three engines 3,000 hours, although more was accepted for a slower turboprop.The rate of obsolescence was again a ten-year life with 15 per cent residual value for the jet aircraft and the standard ATA seven-year,10 per cent values for the turboprop. John Wilson, D.H. operations engineer, spoke of the airlines'need for better regularity. Not only was irregularity annoying for the passenger, it exacted a big economic penalty from theairline. The only way of making an appreciable advance in this direction was by automatic landing. Such a system had to bedesigned for pilots of average ability; it was not true of the D.H.121 as it perhaps was of the bigger jets that pilots of above-averageability would be selected. The standard aircraft would be delivered in 1963 with autoflare equipment. After certification there would
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