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Aviation History
1957
1957 - 0169.PDF
8 February 1957 171 DESIGN FOR PRODUCTION An Important Paper by the Chief Designer of Armstrong Whitworth Aircraft CURIOUSLY, there have been very few published paperson the specific subject of the actual design, planning,scheduling and general domestic programming of a new aircraft design. All the more valuable, therefore, is an exceed- ingly thorough assessment of the subject by Mr. E. D. Keen, B.Sc, F.R.Ae.S., A.F.I.A.S., chief designer of Sir W. G. Armstrong Whitworth Aircraft, Ltd. In this and the following pages we summarize Mr. Keen's paper, which was presented as a section lecture to the Royal Aeronautical Society on January 29. The title of the paper [said the lecturer in his introduction]implies that, in general, aircraft design is not fundamentally suitable for production and of course, for various reasons, designfor quick and easy production must be the result of compromise. In practice optimum design and easy production never seem tocoincide exactly. Refinement of design for production covers many broad conceptions. In its narrowest sense the revision of detail design to allowfabrication by more economical methods of production offers a wide scope for economies in time and cost, and in between thesetwo extreme interpretations of the title of the paper pros and cons of various basic types of construction offer fruitful fields fordiscussion. The foundation on which the grand plan for instituting a pro-duction run is based must be the promised dates by which the 1 WINGS 2 FLAPS & AILERONS 3 FUSELAGE 4 TAILPLANE & FIN 5 RUDDER & ELEVATORS 6 ENGINE INSTALLATIONS 7 FLYING CONTROLS 8 SYSTEMS 9 ELECTRICS 10 MOCK-UP 11 TEST&RICS 25 50 75 100 I O 10 TIME SCALE drawings will be issued. By reference to records of previous air-craft with suitable adjustments for size factor, complication factor, operational role, etc., estimates must be made of target dates forthe issue of sufficient drawings to produce the first prototype. The author's firm is now using the Hollerith system of recording andanalysing D.O. records. In Fig. 1 the horizontal lines give the period over which drawings will be issued, the left-hand endsgiving the dates of issue of the first drawing and the right hand ends the dates on which sufficient drawings are available to buildthe first prototype component. The top two lines also have marked on them the percentage of the total number of drawings to beissued by the given dates so that by use of a normal type of progress sheet the actual achievements can be continuously compared withthe estimates. Correct phasing of these components is essential if valuable timeis not to be wasted. There has been too much of a tendency in the past to tackle the apparently easy things first. Thus, tailplanes, fin,rudder, elevators, flaps and ailerons, because of their relative simplicity have been drawn out and issued as quickly as possible,thus apparently impressing the production departments with how quickly the design office is getting on with the job. How muchbetter to start on the difficult production tasks first; the large components which require more complicated jigs and tools, morefloor space and, what is far more important, contain more equip- ment, always the botde-neck in any aircraft production line. Thus,in die author's view, the smaller components should be left until later when there is less likelihood of alteration and to allow aproper emphasis by the production departments on the difficult components first. Meanwhile the production departments havebeen working on similar lines and have produced an estimated aircraft delivery programme (Fig. 2.) Each part or assembly is given a priority to indicate its positionin the fabrication cycle. The priority of any part is determined by its proximity or remoteness to the completed aircraft. Normallythis is the next lower priority to the assembly in which it is fitted, but in the case of parts with an exceptionally long "make" cycletime, i.e., longer than 10 weeks, a still lower priority number may be allocated. Although the early priorities (1 and 2) are mainlymachine shop and press shop parts, and main assemblies carry the later priorities (4, 5 and 6), there are many small parts whichhave later priorities if they are needed at a later stage of assembly. Each part or assembly has one priority only, although it may beused at two or more stages in fabrication. In these cases the priority is determined by the earliest requirement. The days when the sole object of the workshop organization wasto concentrate manufacture on those parts required for the first prototype in order to get a shiny new aircraft which will just aboutfly on the earliest possible date (and then claim a new record) are now fortunately disappearing. Manufacturers as a whole andproduction teams in particular are beginning to realize that a prototype aircraft without a pressure-sealed structure or air-con-ditioning equipment, restricted in its speed range by non-com- pletion of structural or resonance tests and the like, is more of aliability than an asset. Further, this contraption then goes to the S.B.A.C. Show where it obviously doesn't fly as fast as it shouldand because of its early appearance emphasizes in the public mind the very long time between this and the time when productionaircraft are in service. Large structural test specimens now amount to two additional (unequipped) aircraft and the production depart- Fig. 1 (above). A typical time-scale for the issue of drawings for a first prototype. The diagram is explained at the top of column 2 on this page. Fig. 2. A typical programme' of manu- facturing priorities, indicating the flow of details and sub-assemblies to the first-flight stage. The rectangles indicate start of detail manufacture and the triangles completion thereof; the circles locate the completion of stage assembly.
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