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Aviation History
1948
1948 - 0313.PDF
MARCH 4TH, 1948 FLIGHT .267 Aircraft Design Procedure Neither Works of Art nor Works of Engineering Can be Produced by Committees AIRCRAFT design has become a very complicated busi- ' /\ ness, and few people outside the industry have any very clear idea of what is involved, or why it takes so long to produce a new type of aircraft. Last Thursday the Royal Aeronautical Society was given an outline of the evolu- tion of the design of an aircraft, by Professor R. L. Lickley, B.Sc, D.I.C., F.R.Ae.S., whose special task at the new College oS^Beronautics at Cranfield is to initiate students into the merceries of aircraft design. As we feel that many of our readers will take a more charitable view of the aircraft designer if they understand something of his difficulties, we are pub- lishing a fairly extensive summary of Professor Lickley's lecture. It might be mentioned that his new title does not. indicate that he is merely ah academician. For many years he was on the Hawker technical staff under Mr. Sydney Camm, and when he speaks of the maze of complications by which the designer is pestered, he does so from very real and very practical experience. ; Professor Lickley pointed out that the design of an aircraft isa long and tricky business, in wh'.ch compromise is constantly required. As a result, aircraft design is still an art rather thana science, and likely to remain so. Pie likened it to the creation of a picture from a jigsaw puzzle, with the added complicationthat some of the pieces change shape while waiting to be fitted into the picture. There are two main ways in which the design of an aircraftbegins: In one, the designer has an instinctive feeling that a certain design is the best for a particular purpose; in the other, a rigidspecification is presented by an outside body, giving little overall freedom of design. The former, the lecturer thought, results inthe best design, ft used to be known as a private venture, or P.V. The rising costs have made it difficult for a firm to undertakeprivate ventures. When either the Air Staff or an operating company requires anew aircraft, the respective branches of the Ministry of Supply issue to selected firms a statement giving main requirements suchas speed, range, disposable load, and so on. From this, the project branch of the firm can start investigating various designs, the best' of which will eventually be submitted as a tender. A typical preliminary requirement might be as follows: — Proposed General Requirements A commercial aircraft is required for general airline duties. Itshould carry two crew and 20-24 passengers with an allowance of 45 lb per passenger for baggage. It is of major importance that it should have very good take-offand landing characteristics and be able to meet all I.C.A.O. require- ments when using airfields with 800-yard runways. The performance is to be the best possible, bearing in mind theabove take-off and landing requirements and the cruising speed should not be less than 180 m.p.h. at JO.OOO feet at 50 per centof take-off power. Pressurization is not required.4 t should be capable of cam-ing full load on a stage distance 200 miles. Full fuel allowances must be made on this distance STAGES IN DEVELOPMENT Staje 1. Development of design. 2. Specification stage. 3. Mock-up 4. Main design work. 5. Prototype completion. Section of firm Design Investigation ofrequirements. Preparation oftender. Specn. accepted.Lines being developed. W.T. work, etc., start-ing. Completion oflines. Investiga- tion of structure,layout, etc. Wind tunnel tests. Main structural details. Drawings to becompleted. De- tail require-ments agreed. Completion ofstressing. Experimental Discussion* onmock-up. Methods ofmanufacture. Manufacture of mock-up. Start on jigs. Jigs completed.Main structure started. Functional mock-ups. Prepn. of test specimens.Completion of building. Testsprior to flight. Production Discussions onproduction, jigs, materials, etc. Continuance of 4. Study of Exp. shop methods. for taxi-ing, take-off and landing, together with satisfactoryallowances for adverse winds, hold-ups in control zone, and so on. These should not be less than 200 miles at cruising speed andii hours at minimum fuel consumption speed. The maximum stage distance required is 500 miles. ' • The design should be such that the possible utilization will notbe less than 2,000 hours a year. The initial rough stages in the project office are exemplified inthe sketch designs. During the preparation of .the tender; much project work is doneby the firm, and may represent thousands of hours of work and the preparation of many drawings. At this stajge, decisions mustbe taken which can make or mar the whole design. They include such matters asaircraft shape, wing loading andhigh-lift devices, and, most im-portant of all, choice of powerplant and estimate of all-up weight. The shape ofthe aircraft allows the designer .toshow his indi- viduality. Oncethe general lines are decided upon,they are rarely altered except indetail. It is not always realized how great a responsibility rests on the designer atthis stage. The full requirements are not known, but the designer must use his experience and instinct to produce something whichwill be the best for the ultimate duties of the aircraft. Coupled closely with the overall shape is the wing plan form,the decision to have small wing areas and expensive flaps, or large areas and simple flaps. Next comes the choice of power plant. This was difficult enoughwhen there was only the piston engine. Now, with jets, airscrew- turbines, reciprocating types and compound engines, the choice needsgreat forethought and skill. The designer is in the hands of the engine man in such matters as engine weights, powers, and specificconsumption. The desire to believe the optimistic figures is hard to fight down. The best -designs are those in which the designerhas guessed right as to the best future power plant, and has tempered the optimism of the engine firm with more conservativeestimates. The over-optimism of the engine people on the future of airscrewturbine units has led to a difficult situation in this country with regard to new civil types. Greater faith in the piston engine mighthave given very different answers to the operators' problems. Accurate estimation of the all-up weight is a fundamental problemin the project stage. Bad weight estimation can destroy the machine's chance of success. In the past, insufficient attentionwas often paid to this, and fantastic estimates were often submitted with tenders. There is still a saying that an aircraft goes intoservice 20 per cent heavier than the tender-estimate. This is not the manufacturer's fault; equipment changes and goes up in weight;power plants go up in weight. A strong weight section, with plenty of data and in close touch with the project section, can producea much more realistic estimate than was often used in the past. The next stage is represented by the submission of the tender.At this stage the committees get to work, and it is difficult for one closely connected with design to speak objectively of thismatter. Neither works of art nor works of engineering can be produced by committees. They are, however, necessary in smallquantities and at long intervals, to ensure that the contractor is provided with a firm contractual agreement; that he has all thehelp he needs (technical and otherwise); and to check at the end that the final design bears a close relationship to the specification.When tenders have been submitted, there is a meeting of what might be called the tender-acceptance committee, composed ofrepresentatives of the M.o.S. and the user, but not normally includ- ing representatives of the design firm. This committee decideswhich tender is best and what changes can be asked for or inflicted on the designer. This is followed by a get ting-together of theprospective purchaser and the designer, to hold a bargaining match— the purchaser tb see how much extra he can get and the designerto stand firm bv his existing tender which, after all, represents his best ideas. However, the firm probably needs the order, and thedesigner may be forced, against his better judgment, to accept amendments which do not improve the design. On the basis ofthis meeting a full specification can be drawn up, and this represents the contractual requirements. In most cases it is a nevdisource of argument from then on. To bf continuued)
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