FlightGlobal.com
Home
Premium
Archive
Video
Images
Forum
Atlas
Blogs
Jobs
Shop
RSS
Email Newsletters
You are in:
Home
Aviation History
1960
1960 - 0103.PDF
FLIGHT, 22 January 1960 103 ,O.OOOr 140,000 23 3O,OOOh 20,000 H3O.OOO 120,000 Tttottd Won «itk — 2 2 I.E. Doictt. 10,000 RtcommtMtd CniiM MM. Arcratt Wetyit HlOOOO 100,000 LB 45O 5OO 55O 6OO 65O 7OO TRUE AIRSPEED -KRK Fig. 2. Normal operating limit and design speeds, ISA Comtntkwl Win g No L.E. Doicet O-8O O82 084 OB6 O 88 O9O CRUISE MACH NUMBER Fig. 4. Landing max. lift coefficient and cruise Mach number Paylood' 85 passengers and baqqaqe Reserves : standard ! Lonq - range cruisrnq procedure — New Ybrk - Cleveland- Chicooo Paris - Barcelona - Madid O 2OO 4OO 6OO BOO IOOO I.2OO I.4OO INITIAL STAGE DISTANCE-STATUTE MILES Fig 3. Typical example of the 121's high "residual range" then be a period of getting to know the system, after which today'saverage minima of 200ft and half a mile might be reduced to 100ft and i mile. The ultimate objective was full all-weatheroperation with automatic landing, and this should be achieved by 1970.But before either could be attempted it was necessary to clear autopilots for use close to the ground. At present, manufacturershad to demonstrate a recovery from the worst case of an autopilot runaway or "hard-over" to establish minimum height at whichautopilot could be used, and this was about 200ft above the ground with present equipment. An autopilot for use in auto-landing obviously had to be completely cleared for use right down to the ground.Regarding the case for three engines (Fig. 1), Mr Hall men- tioned first the matter of cost. For the 121's thrust requirementof 30,3001b, leaving out for the moment the question of balancing take-off thrust with cruise thrust, the engine first-cost picturewas roughly as follows: four engines, £210,000; three engines, £180,000; two engines £155,000^and, of course, the fewer theengines the greater were the savings in indirect costs. However, for the 121's design range of around 1,000 miles, thequestion of relating teke-off thrust to cruise thrust requirements suggested that a twin-engined aircraft would be overpowered onthe cruise; and a four-engined aircraft, with engines designed for the cruise case, would be overpowered on take-off. Furthermore,a higher aspect ratio for a twin-engined aircraft would be neces- sary for the one-engine-out take-off case, and this was a furtherconsideration weighting the case against twin engines. Aspect ratio of the three and four engined designs would be six. It wassuggested that for the market at which the 121 is aimed, three engines showed a three per cent cost advantage.On the method of installation (Fig. 6), every precaution had been taken to protect the airframe from damage in the unlikelyevent of an engine structural failure. There was also always the problem of compressor blades being shed and passing forwardsout of one intake into another. This would be virtually impos- sible with the 121's three-engined layout. All the engines werewell above the ground from the point of view of possible debris ingestion (wing flaps protected the intakes from stones thrown up by the main wheels); and even in a belly landing the sideengines were 2|ft clear of the ground and were unlikely to come in contact with spilled fuel. So far as fire was concerned (seeFig. 8) the rear engine was within a self-contained fireproof enclosure, subdivided into hot and cold zones. Mounting structureof all engines was designed to 12g: even assuming such a decelera- tion the centre engine still had to pass through three strong struc-tural members before it could enter the fuselage. An important advantage of the clean wing, made possible byrear-mounted engines, was that full flap area without cut-outs could be used, which was not the case on wings with under-wing pods(these had to be larger and therefore heavier than necessary for the same flap area and approach speed). Also the problem ofreverse thrust in the air—there being a much less severe asymmetric case—was simpler. Furthermore, areas of the 121 subjected tojet-noise damage were much less than with the under-wing pod designs which had considerable areas of wing structure exposedto jet noise effects. Accessibility of the centre engine was excellent: two hourswas estimated for an engine change, and less for the outer engines. Intake loss for the centre engine was estimated to be 2 per centon take-off and rather less on the cruise. There was a boundary- layer diverter, and intake efficiency had been proved from windtunnel results as "very satisfactory." The intake was heated by air bled from the centre engine to melt ice and snow. At highangles of attack, the angle of incidence of the side engine intakes was one half that of the wing incidence because of the down-flow from the wing. Summing up the aerodynamic aspects, Mr Goldsmith said thatin designing the 121's wing, half-models had been used in the Hatfield high-speed tunnel giving Reynolds numbers of about1,000,000. Full-span model tests had been made in the ARA tunnel at Bedford, and DH were very pleased that the resultshad been as expected. About 40 models had been made and testing had occupied two and a half years. The wing was ofvarying-camber design, an incidence of 4° at the root being washed out to — 1 ° at the tip. There was negative camber at the root; itwas here that there was the only interference problem—unlike wings with engine pods. The cruising CL was lower than that onlarger jets, making variations in leading-edge profile uncritical. The wing could remain in service for a long time without deteriora-tion; DH had "not gone in for such exotic profiles as certain Fig 5. Direct operating cost per seat mile JA.T.A. Mithod .ilk lOjn Dtptcut COST PER SEAT MILE I 6< - CENTS 2OO 4OO 6OO BOO tOOO I2OO STAGE DISTANCE-STATUTE MILES
Sign up to
Flight Digital Magazine
Flight Print Magazine
Airline Business Magazine
E-newsletters
RSS
Events
