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Aviation History
1957
1957 - 1605.PDF
AIRLINERS OF THE WORLD ELECTRA Lockheed Aircraft Corporation California Division), Burbank, California A FTER many years of paper studies embracing a host of jetA\ and propeller-driven projects, Lockheed finally evolved the *. J^- Model 188 Electra as the aeroplane which they believe tobe the optimum for intense short and medium-haul operations. American Airlines had been conducting similar studies and it wastheir formal specification—which was submitted to several major American manufacturers—which resulted in the actual Electradesign. Lockheed claim that it is "the only turbine-age passenger transport designed to meet all requirements of short and mediumhaul operation . . . partner to the long-haul turbojet aeroplane." It is exceedingly clean and well laid out. Numerous detailchanges have been made to the design since it was described in our 1956 review of airliners, and these changes are commentedupon in the ensuing paragraphs. AIRFRAME All primary structure has been designed inaccordance with fail-safe precepts and incorporates multiple load paths and reduced stress levels. As an example, no part of theskin of the pressurized fuselage has an operating stress greater than 12,000 lb/sq in. In all cases the Lockheed designers haveadopted the optimum structure without regard to tooling costs and it is partly this which has resulted in the break-evenproduction being exceptionally large. Fuselage The Electra differs from most modern transports inthat the cross section of the fuselage is a true circle over most of its length (because, say Lockheed, "it is the strongest and lighteststructural shape for the required function"). Most of the structure is 7075 aluminium alloy with 2024 aluminium alloy used for skinpanels and bulkhead webs subjected to tension loads through pressurization. Tension-type break joints are located at station 200,just behind the flight deck, and station 1118, the rear pressure bulkhead. The simple frames are generally spaced 19in and com-prise lipped channels alternately facing fore and aft, and notched to receive the rolled-section stringers- Straps are then addedaround the frames between the stringers and the skin to form tear-stopping load paths. The complete constant-section pres-surized hull is built from four quarter sections (top, bottom and two sides), the floor beams joining the bottoms of the side members. At stations 570 and 695 are strong frames aligned with thewing spars. The stub-wing portion of the fuselage is a straight section of box beam which separates the hydraulic and electricservice centres and extends 65in on either side of the aircraft centre-line. The front pressure bulkhead is a kinked and braceddisc and the rear pressure dome is convex. Wing Of neat, laminar-flow design, the Electra wing is basedon a primary box beam consisting of two spars and upper and lower surfaces built up from integrally stiffened machined panels.These panels are of 7078 alloy on the upper surface but most of the remainder of the wing structure is in 7075 alloy. Port and starboard wings are built separately and are boltedto the stub-wing box which extends just beyond the fuselage frames. Each wing box has considerable chord; the front of eachbox is straight, and parallel to the transverse axis of the aircraft, and the rear of the box is kinked at the inboard end of the aileron and has acute taper. Fail-safe integrity is enhanced by various points of detail design.The vertical plate webs are divided into upper and lower sections which are joined by a double row of rivets. Each web is thenstiffened by extruded booms and vertical angles riveted on. Between the widely spread spars is a network of closely spaced ribs eachbuilt up from Warren girders fabricated from angle sections with a peripheral member notched to match the integrally machinedskins. The latter have a ruling width of some 20in and contain about six spanwise stiffening webs. Joints between the machinedplanks are of the lap-type with double rivet rows. The front three panels are unbroken from root to tip and additional panels arebrought in at the root to fade out on the taper of the rear spar. Behind the rear spar the secondary structure comprises an outerskin stiffened by a bonded inner sheet provided with cut-outs and stiff eners. Notes on the flaps, ailerons and de-icing are provided later inthis account. The leading edge incorporates thermal anti-icing and is designed to suffer minimum deformation as a result of hailor other impact damage- All structure ahead of the wing box inboard of the outer engines is designed to hinge downwards toprovide access to systems. Tail Unit Lockheed state that the tail structure "is designedto withstand at ultimate strength the yawing moment loads result- ing from an outboard engine turbine failure coupled with simul-taneous failure of all engine and propeller safety devices and adverse pilot reaction under critical altitude, speed and temperatureconditions." Northrop Aircraft have had prime responsibility for the design and manufacture of the Electra empennage and numer-ous detail changes have been incorporated since the original configuration was projected. Basically the structure is conventional with spars, ribs and skinin a ruling material of 7075 alloy. The vertical fin has two main spars growing out of the fuselage frames, the front spar beingraked backwards some 6ft above the upper line of the fuselage. Incorporated in the fin are structural-plastic V.O.R. windows andprovisions for hot-air de-icing. Each half-tailplane is joined to the fuselage outboard of the root fillets and picks up on a shortcentre section built as an integral part of the tail cone. The two principal spars are at 19 per cent chord and at the trailing edgeof the tailplane itself and the ribs are lattice girders, as in the wing, which carry heavy extruded stringers on which the skin issupported. The rudder and elevators have thin skins stabilized by thin "waffle" sheet bonded on, as shown in the large drawing. Undercarriage Designed and manufactured by Menasco, theElectra gear is both sturdy and neat, and is quite conventional. Each main leg is a single forging braced by pin jointed upper andlateral arms on either side fabricated in heat-treated steeL This assembly is carried outside the wing box on large forged trussribs each just outboard of the inboard nacelle on each wing. The drag strut has a Y fork, at the junction of which is a bungee-loaded
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