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Aviation History
1951
1951 - 1586.PDF
17 August 1951 201 THE DESIGN-STUDY Shortening the Tedious Preliminary Work in the Conception of a Modem Aircraft By H. K. MILLICER, M.Sc, A.F.R.Ae.S. IN the heroic era of aviation, the thin end of the scientificwedge had not been inserted into aircraft design, and themachines came into being largely by "eye," or to put it rather more euphuistically, through the artistic qualities of the makers. Those without artistic ability were forced to rely on "by guess and by God" methods. Today, matters are different: no one with any knowledge orappreciation of the subject would deny that most of our great designers of aircraft are artists, but it would be foolish to claimthat the artist is a better designer of aircraft than the engineer. The truth of the matter is that the finest aircraft are conceived byengineers who, willy-nilly, have a strong element of the artist in their make-up. Artistry, however, is a tenuous quality which cannot easily bedenned in finite or objective terms. Thus in this article the artistic element of aircraft design will be accorded recognition, but nothingmore. It is with the rather more prosaic and definitive aspects of "engineering" the concept of an aircraft that we shall concernourselves. The economic strictures of life today have virtually banished the private-venture design, and most aircraft come intobeing as a result of designers and their staffs carrying out a proper design-study to a given specification before starting the actualdrawing of the machine. By "design-study" is inferred the setting- up of a design policy, which is followed logically by work of asemi-research nature which, in turn, leads eventually to the development of an aircraft shape. A good design-study is based on the best theoretical andstatistical data available, and freakish or doubtful singular evidence should be eschewed. After certain assumptions have been made,it becomes necessary for the designer to do his utmost to achieve them. For example, if the intention is to attain a percentagestructure-weight of 25 for a military aircraft, it is necessary to strive extremely hard to get it, enforce strict weight-control in thedrawing office, and impress upon the stressman in charge that it is chiefly his responsibility to keep the weight down on every com-ponent, no matter how small or insignificant. On the other hand, if it is intended that the aircraft shall have particularly lowdrag-characteristics, the shops must be instructed and supervised in the manufacture of wings and tail surfaces in which the designcontours are maintained with high fidelity. This involves the special selection of light-alloy sheets of minimum wayiness,machining rivets, rubbing down and filling and, too, the elimina- tion of air leakage on all joints. In this last connection, it is ofpoint to note that de Havillands ensure that all panel and structural joints on their aircraft are sealed, and by this means reductions of15 to 20 per cent in profile drag can be achieved. Every design-study should have as its aim the creation of thesmallest and most compact aircraft for a given duty. An old designer used to tell the author, "there is no need to carry airinside your aircraft structure." Aircraft size is reducible as a function of the reduction of profile drag and, of course, by theapplication of high-lift devices, as they permit wing and tail areas to be reduced to minima. A generous use must, therefore, be madein the design-study of low-drag aerofoils and joint seals, and of high-lift flaps and slots. On the structural side of the design-study,long, slim sections, structural offsets and cut-outs should be avoided if possible. It sometimes occurs that a beautiful aero-dynamic shape cannot be reproduced in practice owing to struc- tural difficulties. Aircraft design is essentially an art of com-promise, and it is manifestly impracticable to pursue an idea which, for example, involves a structural weight penalty whichoutweighs aerodynamic gain. It is considered to be good design policy to make a generousallowance for large, high aspect-ratio control surfaces, toeether with control runs involving the minimum of friction. In addition,ample ratios between control-column and control-surface angular movement are worth while in order to decrease mechanically thepilot's stick forces. Many otherwise good aircraft have been spoilt at the outset bv such things as having too short a tail arm, too greatan amount of friction in aileron control, insufficient rudder area, too small a rudder pedal travel, and so forth. Another thing to be decided at the start of a desisn-studv is thetype of undercarriage to be used. Tricvcle undercarriages offer the greatest ease of taxying and landing and should, therefore, be used for all aircraft other than, perhaps, racing or research types, thoseaircraft designed to operate from very rough surfaces, and radial- engined trainers, a characteristic of which is that the wheelbascbecomes dangerously short. It has been established that, if an aircraft is initially designed for a tricycle undercarriage, it need notsuffer the weight penalty resulting from such a choice. (Ref. 1 ) For airscrew-driven aircraft, it is sound policy to provide for thelargest diameter and largest solidity slow-turning airscrew that can be accommodated. In this connection, an especially designed air-screw, together with a well-chosen reduction gear ratio, can shorten the take-off run by half, can increase the initial rate of climb by30 per cent, or can enable a high-drag aircraft to fly safely on one engine. It may not be generally realized that, for a given rate ofclimb on one engine, as much as 5 per cent of the payload in a twin-engined aircraft can be gained for every 1 per cent increasein airscrew efficiency. The fact that such criteria as power-required curves, stabilitycalculations and so forth, take so long to calculate and are, in any case, too cumbersome to use in preliminary work, frequentlyprecludes the designer from being able to carry out a proper design study. So much time is taken up that he is often forced to some-thing approaching pure guesswork, albeit the guesses are based on a knowledgeable foundation. In order to relieve this difficulty, TABLE I: WEIGHT BREAKDOWN FOR AIRSPEED AMBASSADOR Structure Marnplane, including centre-section ... Fuselage, including floors Tail unit ... Landing gear Nacelle structure and (airing (aft of bulkhead) Flying controls Total structure unit ... Power Installation Engines, dry Mountings, cowling Engine cooling system ... Oil cooling system Oil in engines, coolers and airscrews Engine accessories, exhausts, air intakes, etc. Airscrews complete Total power installation Fuel and Oil System Permanent fuel tanks and supports ... Fuel piping system and fuel in pipes ... Oil tank and supports Oil piping system and oil in pipes Hydraulics Pneumatics Electrics ... Total fuel and oil system Auxiliary Power Service* Total auxiliaries Manufacturer's bare weight... Accessories De-icing Fire precautions Instruments (engine, navigational and flying) Auto-controls Radio and radar Fixed galley equipment Drinking and washing water, de-icing and other fluids Furnishings, sound-proofing, air conditioning and other equipment Weight empty Disposable Load Removable galley equipment Crew, including maps and other navigational"! instruments *• Stewards Passengers and luggage ... Mail and freight , Fuel and oil J All-up weight 50,000 Weight(Ib) 6.530 4,765 980 2,415 1,010 350 16,050 6,370 540 330 270110 1.130 1,580 10.330 210 350 90 70 720 iso 30 750 1,030 28,130 354 230 100 180 550 240 176 3.360 33,320 220 750 15,710 'ercentsg*a.u.w. 13.06 9.53 1.96 4.83 2.02 0.70 32.10 1X74 1.080.66 0.54 0.22 2.26 3.16 20.66 0.42 0.70 0.18 0.14 1.44 0.50 0.06 1.50 2.06 56.26 0.71 0.46 0.200.36 1.10 0.48 0.35 6.72 66.64 0.44 1.50 31.42 100.00
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