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Aviation History
1963
1963 - 0710.PDF
• .- . • j • FLIGHT International, 9 May 1963 BOEI NG 727 ... The result of this investigation is seen in Table 3, from which it is clear that, on the basis of existing operating and certification rules, a twin-engined 727 would be markedly inferior to a three- or four-engined 727 in weather reliability (and also in range for a given payload and field-length). At the same time, most of the twin-engined studies tended to show a d.o.c. slightly lower than that attainable with larger numbers of engines. This was especially the case with the early parametric studies. When "actual" studies were undertaken, using available or projected real engines, the twins showed up less well, as a result of the very advanced power- plants sized to match the three-engined aircraft. On an "actual" basis, the best threes came out on the bottom line of the d.o.c. plot, some 5 per cent better than the "actual" twins, while all the fours tended to price themselves out of the picture in the bracket 6.5 to 12.5 per cent above the base line. By the late summer of 1959 Boeing's president,William M. Allen, and a technic al team had held discussions with the main US airlines, and obtained the latter's views as set out in Table 2. In September Boeing concluded that three engines was a better choice than either two or four. But the question of how to hang the engines was still wide open. Use of an odd number of powerplants meant that at least one must be mounted on the rear fuselage. Aircraft with two wing pods and one engine at the back appeared very promising. They were superior to the ail-aft configurations in their basic balance, irres pective of class of seating or freight-hold volume. But this arrange ment had a number of drawbacks. One was that the wing-mounted engines were too far inboard to relieve wing bending moment in the manner achieved in the 707, and the pylon struts were less effective in acting as fences. Moreover, there were weight and maintenance penalties in having engines so far apart each coupled to a host of pipes, ducts and cables. Finally, the ail-aft 727s appeared likely to offer a quieter interior, especially on take-off. During the early parametric studies Boeing had looked at almost every conceivable method of mounting three engines on the rear fuselage. Some of these are illustrated in the lower left corner of page 677, and they included 727s with two engines on one side and one on the other, 727s with butterfly tails or with twin fins (the two latter arrangements facilitate the installation of an engine on the upper centreline), and even a triple package inside the rear fuselage fed by a boundary-layer intake. The decision to go ahead with the detail design and development of the 727 was taken in May 1959, and the construction of the first of three mock-ups was begun in July. The aircraft configuration finally adopted was settled in August, and all other solutions were terminated by the start of 1960. There remained the choice of the engine. Pratt & Whitney had offered to build a completely new turbofan sized to the required thrust level of 12,0001b to 14,0001b. Rolls-Royce and Allison jointly offered an uprated4'version of the engine which has since become known as the Spey, under the designation ARB.963. Boeing had extensive experience with both Pratt & Whitney and Rolls-Royce, and were happy to work with either. But the major US airlines had experience of only one of these companies. United, one of the two original customers for the 727, had not then operated a single Rolls-Royce engine (UAL is today one of the largest users of Darts and Avons in the world), and they expressed a preference for the US firm. Eastern, the other original customer, was firmer: Eastern insisted on the Pratt & Whitney engine being chosen. The JT8D was accordingly selected to power the 727 in August I960. Large orders by Eastern and United were announced in December of that year. Boeing accordingly went into production with an airliner which, although it is essentially a completely new aeroplane, draws heavily upon 707 experience and actually incorporates many parts common to the 707 and 720, It is an oversimplification to say that the upper bubble of the fuselage is the same on all three aircraft, although, after extensive weighing of the pros and cons of three widths of fuselage, the 727 was made the same width as its four- engined predecessors. This particular study was virtually a foregone conclusion, because Boeing had not only invested more than $3m in tooling for interior furnishing—to say nothing of airframe structure—matched to the 148in width but had previously almost lost the 707 as a commercial transport owing to their natural desire to build the latter to the same 144in width as that 683 of the KC-135 tanker already in production. Four inches can mean so much. Naturally, Boeing have adhered to the 707 structure and systems wherever this has been possible without much penalty; but, in fact, the only major part of the 727 actually constructed in 707 jigs is what is known locally as "the cab section" (i.e., the flight deck). The upper lobe of the body is shorter than that of any 707 or 720 variant, the lower lobe is much shallower, and the completely new air-conditioning layout incorporates an overhead duct above lowered ceiling panels. The flight deck has been made almost a facsimile of that of the 707/720, the forward entrance doors are identical in all three Boeing jetliners, and there are numerous common system components and, especially, such furnishing items as window-surround forgings, dished-plastics window trim, baggage racks and passenger-service units. But a true index to the degree of "newness" of the 727 is afforded by the fact that one customer—Eastern—will hold 3,088 normal 727 spares, and of these 895 have part-numbers identical to items alreadv held for the 720. Airframe A study of the 727 airframe reveals that, while detail design engineering is of an exceptionally painstaking nature, the actual structure is remarkably conventional (compared, for example, with that of the VC10 or One-Eleven). There are no integrally stiffened skins at all, and the number of machined parts has everywhere been held to a minimum, although the main wing skins are milled in the flat to taper them across both chord and span. Although the primary structure of both main and nose gears (undercarriage units) is of high-tensile steel, the main-gear beams in the wing are massive forgings in aluminium alloy. Almost the only other parts of the structure made of steel are the tracks and carriages for the leading-edge slats and trailing-edge flaps and detail fittings around the engines. At the start of 727 manufacture Boeing found it difficult to buy steel meeting their stringent specifications. Today all their suppliers of structural steel use vacuum melting and heat-treatment to achieve 240,0001b/sq in with more than 20 per cent area-reduction. Large components like main-gear legs are all contour-forged; practically nothing is hogged out of a rough billet. Nylon clamps and Teflon bushings are extensively used for hydraulic and fuel pipe attachments, respectively. Supports and brackets are generally of aluminium, although some ten per cent are of magnesium alloy. All magnesium parts are cast, sheet and extrusions being excluded (even the Kriiger leading-edge flaps are magnesium castings). In secondary areas extensive use is made of honeycomb sandwich in aluminium alloy and glass-fibre. Non- perforated metal honeycomb construction is used for the fixed part of the wing behind the main torque box, the flight and ground spoilers, the fore flaps, the trailing edges of the mid (main) flaps, the portions to the rear of the spar of the aft flaps, and the tabs on all the control surfaces. Glass-fibre honeycomb is used for the underwing access panels forward of the front spar, the wing/body fairing, the wing-root trailing-edge fillet, the fin tip fairing and the rain gutters at the junction of the fin and body. Structurally the main plane comprises a centre section of the same width as the fuselage joined by multiple peripheral bolts to port and starboard wings. Aerodynamically the wing is a refinement of that of the 720, with slightly reduced dihedral, less sweep—32° at the basic quarter-chord line—and an increased sweep inboard similar to that introduced by the leading-edge "glove" on the 720. As a fuel tank, the wing is divided into three—left outer, right outer, and the linked inner sections and centre cells. The 727 wing is small enough for chordwise splices to be elimi nated entirely outboard of the root at the side of the body. The main box is built up around two spars, ribs, skins and stringers. The upper skin, subject to compressive or mild tensile loads, is in 7178ST aluminium alloy, and it is stiffened by stringers attached by aluminium rivets (steel rivets are disliked by Boeing, because they do not expand in their holes and tend to create local high stresses). The under skin is subject to high tensile and fatigue loads, and is manufactured of 2024ST (a 24ST material) again stiffened by riveted stringers. This under skin is divided into front, centre and rear sections, the middle skin incorporating 18 elliptical holes for tank access and four smaller holes for access to the booster pumps. Maximum thickness of the skins is approximately 0.75in, and the finished panels are milled away to 0.060in outboard. In the centre section, which extends from the centreline out to
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