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Aviation History
1955
1955 - 1595.PDF
712 FLIGHT, 4 November 1955 LOCKHEED ELECTRA A Preliminary Description of Americas First Turbine-Driven Airliner DURING the past three years the California division of theLockheed Aircraft Corporation have been steadilyadvancing the design of a four-turboprop airliner, now known as the Model 88 Electra. The company's studies in thefield of turboprop and jet transport go back much further than this, but it is only in the period named that actual design workon the Model 88 has been in progress. Lockheed have had their -hands very full with other work and, in any case, the continuedsuccess of the Constellation family (on the one hand) and the lack of any real American need for a new medium-range machine(on the other) have allowed the manufacturer plenty of time to arrive at what they hope is the optimum design. During the formative years of this project, Lockheed kept inclose touch with the requirements of likely customers—in particu- lar American Airlines;—and the final size and shape of the L.88is tailored to the busiest routes in the U.S.A. For a period of years Lockheed were undecided upon the position of the wing.Many studies were made of high-wing designs, and the possi- bility of developing an airliner from the C-130 Hercules militarytransport was explored at length. The final layout, however, is conspicuously conventional. A single-deck pressurized fuse-lage, with a constant circular section, is mounted above the wing, the latter being accommodated in a cut-out under the cabinfloor. Structure. Lockheed have emphasized that the structuraldesign of the Electra is straightforward and relatively simple, and that the working stresses have been fixed at exceptionally con-servative levels, particularly in components subject to stress- reversal or to widely varying loads. Forgings and integrallystiffened portions have been employed wherever possible. The fuselage is a true cylinder, apart from the wing cut-out,and is built up from a number of conventionally made portions. Almost the whole of the 104ft length is pressurized; the frontpressure dome is immediately behind the search radar installation and the rear pressure bulkhead is at the junction periphery of thetail unit. Design pressure differential is 6i lb/sq in. All interior furnishing is non-structural, and the floor is laid on multiple longi-tudinal stringers of rolled Z-section. Further description of the doors, windows and air conditioning is given under the sub-heading of "passenger accommodation." There are two tip-to-tip wing spars with a third spar carryingthe flaps and ailerons. Most, if not all, of the skin between the front and main spars, on both upper and lower surfaces, isintegrally stiffened. As in the L.I 649 Super Constellation, these skins are milled from large slabs of high-strength light-alloy, withstraight spanwise stiffeners. Not only has this made possible a considerable reduction in number of components but it hascleaned-up the structure, improved the expected fatigue life, made possible a great reduction in the required number of ribs and alsoeased tankage problems. The leading and trailing edges are of conventional rib/skin form, the former having thermal de-icing sufficient for the NACA. heavy ice condition.Double-slotted flaps are fitted, continuous beneath the engine nacelles. Each flap runs out on guides under the actuation ofelectric screw-jacks, and the increase in lift coefficient which is thereby gained is particularly important in allowing the Electra,with a wing loading of up to well over 80 lb/sq ft, to operate from existing 5,000ft runways. Tail structure is conventional, without integral skinning. Allcontrol surfaces are manually operated, with aerodynamic boost. Fin and tailplane are electrically de-iced. Powerplants. We have previously described the provision ofpower for the Electra as a "nigger in the wood-pile," owing to the fact that the engine assumed in the original projection of thedesign—the Allison 510—is not yet fully developed as an airline engine. Before describing the installation of this engine it istherefore worth adding a few notes on alternative units. The Electra is designed to use any modern turboprop in the3,000 to 4,500 h.p. class. Into these limits can be fined the Allison 510, the Rolls-Royce Tyne, the Napier Eland and—fullydeveloped but of earner conception—the Bristol Proteus. The Allison suffers from a number of fundamental deficiencies,which become serious when considering the engine from an airline viewpoint. At present, it is a constant-speed engine(13,820 r.p.m.) and even the ground idling speed is as high as 10,000 r.p.m., resulting in a high noise level on the ground.Allison have put a great deal of effort into perfecting the control system, and have brought the engine/airscrew combination to thepoint at which it can be accepted for service in the C-130 for the U.S.A.F. General handling, however, is still not as good as itshould be for an airliner. Another factor which should not be overlooked is that theAllison 501-D-10 is not yet ready to deliver its full design power in commercial service. This power is 3,460 sJi.p. (3,750 e.sii.p.)at sea level, i.s.a., at zero forward speed, and to achieve this the turbine inlet temperature has to rise to 1,780 deg F (1,320 deg K),making it one of the hottest engines yet flown. Development running is going forward with the 5O1-D-12 commercial enginewith a maximum rating of 3,015 e.s.h.p. at 1,625 deg F max. t.i.tcmp., but the Electra needs more power than this. For futuredevelopment, Allison hope to achieve well over 4,000 e.s.h.p. in the 501-D-8 and D-13, but it would seem that they will be hardpressed to reach this goal. From a number of aspects, the Tyne and Eland appear to fitthe requirements of the Electra very well. The former is an outstandingly efficient unit and will provide more than enoughpower. The latter is also well-engineered and efficient, and will shortly deliver considerably more power than the Allison, usingair-cooled turbine blading. The view of Lockheed, expressed directly to us earlier this year was that, although the AmericanAirlines' Electras were to be powered by the Allison 501, it should not be inferred that this would become the standard, or This new impression of the Electra, by a Lockheed artist, is cut away to show the 66 - passenger "cus- tom configuration." Seats are arranged tour-abreast. In tour- ist form, the aircraft would carry 85 pas- sengers in the five- abreast seats, while retaining the six-seat lounge in the rear cabin. Also illustrated are the positions of galley, cloakrooms and carry-on luggage bay (opposite buift-in loading stairway).
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