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Aviation History
1940
1940 - 2042.PDF
JULY IS, 1940 CORRESPONDENCE The Editor does not hold himself responsible for the views expressed by correspondents. The names and addresses of ths writers, not necessarily for publication, must in all cases accompany letters. THE AIRCRAFT INDUSTRY Designing for Production I HAVE read with interest the articles in Flight by Capt.MacMillan and " Realist." There are many points in Capt. MacMillan's article with which I agree, particularly when he states that we have too many types of aircraft to make quan- tity production easy, and that is the great necessity at this time. With so many types, the tooling costs are excessive to the country, and the multiplicity of different sizes of tubes and extruded sections make production of the essential raw materials very costly. Generally it will be found that aircraft parts are designed from the designer's viewpoint only, whereas they should only be designed for ease of production, but, of course, in conjunc- tion with the design side. In any case, the production and tool departments should have the final say, and parts should be modified to suit their requirements. In this connection, one very important part of a particular aircraft comes to the writer's mind; this part is so difficult and costly to produce that it is the bottleneck of the whole production line, and the designer appears to have had no thought in his mind as to how the part could be made. Going farther into this matter it will be found that there are far too many different kinds of steel available to the air- craft designer, and these can be very considerably reduced to keep down production costs at the steel works. It will also be found that aircraft manufacturers have their own particular lists of standard parts which are required in their works for construction. This makes sub-contracting extremely compli- cated, because, instead of three or four main firms being able to place parts of similar design on an outside contractor, the contractor is faced with machining three or four different designs of parts to fulfil the same function in different aircraft. '' Realist'' also states that the supply of parts and com- ponents from sub-contract firms comes from so many scattered points that there would be little delay in production in the event of many of these being put out of action. I suggest that " Realist" has no practical knowledge on this subject, and he would be well advised not to make rash statements. In closing, the writer would like to make an appeal to any inspectors, Government or otherwise, to pass through parts which are serviceable and can be used at this critical period, even although they do not reach the high standard which we are accustomed to expect. The rejection of a part is so easy, but it takes a good sound engineering inspector who will be prepared to pass parts below standard, and accept full respon- sibility. SUB-CONTRACTOR. ASSISTING TAKE-OFF The Question of Porpoising IN your footnote to my article, " Assisting Take-off andAlighting," you query whether increased length, on the waterline would suffice to avoid porpoising. I think that there are strong reasons for supposing that it would, though it is not the only means available for dealing with the problem. I believe that porpoising is confined to stepped hulls where the buoyancy is unequally distributed about the working step. The necessities of take-off force this type of design upon the constructors of flying boats. A boat designed for unassisted take-ofl must have the major portion of its buoyancy right forward to counter the overturning moment due to the high thrust-line. The afterbody usually rises at an angle of 7-8 degrees aft of the main step so that the hull can be rocked back to the proper attitude for take-off. The afterbody is tapered off fine for aerodynamic reasons. When such a hull is planing it is balanced precariously on the main step subject only to the control of relatively small air forces acting upon the elevator. A wave striking the large forebody will pitch the hull through a large angle before the buoyancy of the afterbody suffices to overcome it. The air forces on the main plane exaggerate the tendency. Hence all the operating forces tend to instability. If only the alighting condition has to be considered, the distribution of buoyancy may be much better. As high water- drag is now advantageous rather than otherwise, the water- line may run the whole length of the hull. Sharp Vee sec- tions would cut into waves rather than ride them. The e.g. may be very much lower owing to lower placing of the engines and fuel tanks. All these factors serve to reduce the magni- tude of pitching forces and to damp them when they are initiated. A further possibility open to the designer of a hull for alight- ing only is to employ a hydrovane to control trim. As hydro- dynamic forces are very large at high speeds the vane could be very small. A hydrovane placed aft of the e.g. near the tail might be arranged to enter the water first on alighting and to exert a thrust tending to hold the tail down. This would be a very simple device and might be made to control trim during the alighting run quite automatically, since any tendency of the hull to dive would increase the angle of the vane and consequently the restoring force. In regard to your doubts about the possibility of dispensing with the v.p. airscrew, I would point out that if the aircraft is accelerated up to or above take-off speed by external power, airscrews of high pitch / diameter ratio can be used. These airscrews not only attain the highest efficiencies, but are effi- cient throughout a wide range of forward speed. Also, to an aircraft operating over water and capable of alighting at very high speeds, a high initial rate of climb is no longer the life-or-death matter that it is to the land machine. I am disposed to agree with your prediction as to the cruising speeds of future flying boats. I believe that an aera- dynamically efficient flying boat would displace the land-plane for commercial operation, because of its greater economy and safety. The land machine seems doomed to extinction by the appalling amount of ironmongery which it has to carry with it in order to effect a landing. F. W. KING' New Maiden. THE MYSTERY AEROFOIL An American Invention • •> y. •..';,• . I HAVE before me as I write an article entitled " MysteryAirfoil " ; it is the story of the Davis aerofoil and its inventor, David R. Davis. All readers of Flight will readily recall the phenomenal Con- solidated 31 flying-boat with its Davis wing and, more recently, the B-24 4-engined bombei of the same make. Until I read the story of the Davis foil I did not fully appreciate the fact that it was destined to go down in history as one of the epic inventions of our time with the most far-reaching effect on the design of aircraft. Here, indeed, is an enthralling and inspiring story: "A curve, he perceived, was not a dead geometric figure. IT WAS A LIVING, MECHANICAL, OPERATING THING. On the basis of this, he could not but depart from what now seemed an archaic process of obtaining the most efficient curve for an airplane wing—the so-called ' cut and try ' methods." Again, " A curve., he saw, was the product of mechanical action—rotation at infinite speed. ' A circle,' he said, 'is not an inert object. It is something going around—a radius arm (or lever arm) rotating about a centre.' " Having stumbled on this hypothesis, he discovered that the most efficient mechanical action to produce lift would result from the proper relation between the rotational speed of the radius arm and the translational velocity of the point about which the radius arm rotates. " Two factors must be borne in mind . . . one is rotation— MOVEMENT of the radius arm around the centre of a circle. The other is translation—movement of the centre itself. " Mark a dot on a piece of paper, designating it as the centre of a circle. Place the point of a pencil on that dot, then lay the pencil down to form a radius, or lever arm. Now turn the pencil clockwise and, at the same time, move the point in a straight line to the left (thus indicating movement of the circle's centre by the latter motion). The curve formed by the top of the pencil as result of, this simultaneous action is fundamentally part of a Davis airfoil profile—the product of two steady movements. " Steadiness is important. ' If you rotate a flywheel steadily it takes less effort to keep it moving,' Davis explains. 'If done jerkily more H.P. will be required. The same is true of translation. The centre of the circle must move at a (Continued on page 51.)
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