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Aviation History
1955
1955 - 0764.PDF
762 THE appearance of actual aircraft on the DC-3 replace-ment scene comes as a welcome change after the manythousands of words that have been expended on the subject over the past ten years. After living for so long on verbal and written pronouncements made by or directed at the operators, manufacturers, Press and public, it is indeed heartening to have tangible hardware in existence— especially the type of hardware that consists of almost- complete flying machines. This article is concerned with one such aircraft—the Fokker F.27 Friendship, soon to take the air. It is no new concept that "no such animal" as a true DC-3replacement exists; and certainly no modern aircraft complying with I.C.A.O. requirements can replace in every respect theeconomy and relative performance of the ubiquitous Dak. The new generation will have refinements, and, indeed, improvedperformance on certain criteria, but a compromise is called for somewhere. An order of importance must be" assigned for eachparticular aircraft, to such variables as payload, range, speed, take-off and landing performance, economy, versatility, simplicityin structure and in maintenance. The design staff of the Dutch Fokker company chose the middleroad for their contender in the DC-3 replacement field. Their specification was, however, based on two firm factors and onestrong preference. Market research and an analysis of the world's scheduled air services had pointed strongly to the need for a28/36-seater machine, with an optimum stage-distance of some 300 miles. To this basic specification was added the preferencefor the Viscount-proved Rolls-Royce Dart turboprop, mainly for reasons of favourable specific engine weight, improved main-tenance, lack of vibration, and decreased fire-hazard, in com- parison with modern piston engines. To obtain an up-to-date impression of the end-product of theFokker company's work on this project over the past four years, Flight recently visited Schiphol, successfully survived the com-pany's tight security-scrutiny (concerned entirely with the Dutch- built Hunters, it should be added) and was shown the prototypeF.27. During the period of our visit the fuselage, with centre- section attached, was moved—at six o'clock one morning—out ofits jig; considerable further progress has since been made, and the completed machine is likely to fly in August or September. Points of immediate interest in the Friendship are the advancedRedux-bonding techniques employed (to a greater structural extent than in any other aircraft) and the high proportion ofBritish components used, such as the powerplants and under- carriage. No one who visits the Fokker factory can be left in anydoubt as to the enthusiasm of all who designed and are building this promising feederliner, two of which are on order for K.L.M.An account of the development, design and construction of the F.27 follows, together with notes on the first four machines andthe tests to be made on them. Development It was, as mentioned, with the basic objective of providing asound DC-3 replacement that the Fokker design team started work. The new machine would be faster than the venerableDak, and cabin pressurization was required, yet the aircraft had to be simple, rugged and versatile. Market research had pointedto a 28-seater with a large freight capacity. The main reasons for choosing a high-wing layout were thoseof increased aerodynamic efficiency, faster ground loading and improved passenger view. And high wings, incidentally yetagreeably, were also in keeping with the Fokker tradition shown in the company's famous inter-war line of transports—includingAmelia Earhart's F.VII/3m Friendship of 1928. Landing and take-off performance was important, and so a high aspect-ratiowas adopted, together with a high-efficiency compound flap. The basic and detailed configurations of the Friendship Werearrived at after an extensive programme of wind-tunnel testing FLIGHT FOKKER F.27 Ho/land's Up-and-coming Feedlini which began in May 1951 at the National Aeronautical Laboratoryin Amsterdam. The original pressure-cabin design was of circular cross-sec-tion for most of its length. The incorporation of longitudinal beams beneath the floor to strengthen the fuselage in case ofwheels-u? landing produced a slight weight penalty, however, and this was overcome by increasing the radius of the cross-sectional arc below the floor, i.e., decreasing the depth of the under-floor portion. It was decided to use Redux metal bonding to as great anextent as possible, on flat and single-curvature sections, in order to save weight and labour. The two main applications were (1)in the laminated spar booms and wing skins, and (2) in the more conventional stringer/skin bonding on fuselage, wings, and tailunit. Design and Construction The F.27 Friendship is a high-wing all-metal monoplane,employing stressed-skin structures and metal bonding to a marked extent. The production version will be powered by two Rolls-Royce Dart 511 engines and will have 12ft-diameter Rotol airscrews; the prototype has Dart 507s and normal Viscount-type10ft airscrews. Fuselage. The fuselage, which is of stressed-skin, semi-monocoque construction, consists basically of two main assemblies, the front section and the main section. There is anabrupt change in section of the upper portion of the pressure cabin at the junction between the two, from the elliptical cross-section of the nose portion to the circular one of the major length of the cabin. As mentioned, the cross section of the main cabincomprises two circular segments of different radii, the cabin floor forming the boundary. The front fuselage is constructed of vertical frames andZ-section longitudinal stringers, covered by 0.6 mm, 24-S.T. light-alloy sheet as far forward as the front pressure bulkhead.This bulkhead carries the machined nosewheel bracket, a shaped nosewheel cover, and vertical and horizontal stiffeners. Forwardof the bulkhead is the moulded Fitreglass nose (incorporating frames and nosewheel doors of the same material), housing thesingle nosewheel and a taxying lamp. The stringers in this double-curvature section are riveted to theskin, as in the built-up longitudinal spar which runs along the top of the freight-door cut-out and forward to the cockpit canopy.The Redux-bonded framework of the canopy is riveted to the front-fuselage structure, and extends back to the bulkheadimmediately behind the pilots' seats. Behind the starboard side of this bulkhead the racked radio equipment and batteries arelocated; on the port side is the pneumatic equipment (the Friend- ship has no hydraulics); and between them is the cockpit accessaperture. The freight compartment behind the cockpit incorporates areinforced floor which is capable of accommodating loadings up to 100 lb/sq ft. The sliding freight-compartment door measures4ft X 3ft 7in and opens by moving inwards and upward between rails. Aft of the door, at the junction of front and main fuselage-sections, the change from elliptical to circular section (above floor level) is made by means of a shaped frame containing aperturesthrough which the control-runs emerge from the pressurized section of the fuselage. The "step" is illustrated below. The main section of the fuselage, as far back as the rear pressure Lett, the "step" in the pressure-cabin roof at the junction of the nose and main fuselage sections, looking towards the nose. Right, the outer port wing/fuselage attachment, comprising link fittings on the front spar of the centre-section.
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