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Aviation History
1938
1938 - 0363.PDF
FEBRUARY IO, 1938. FLIGHT. 129 Of the tendency to "drop a wing," Mr. Relf said that this could be completely cured by the use of wing'tip slots nd by "wash-out " of incidence. The latter method was not * tisfactory because it needed so much wash-out that there was serious disturbance of the lift grading at top speed. Slots introduced moving parts and did not give an entirely smooth surface, so that designers would prefer to cure the trouble by modification of the wing tip shape. It seemed likely that the thing to avoid was a wing-tip section which had a sudden Mr. Keif had attempted to classify the nature of the stall of the 78 aerofoils of N.A.C.A. Technical Report No. 460. A diagram shown indicated that three types of section had a gentle stall' very thin sections, very thick sections, and those of medium thickness which had a high centre line camber and far back position of maximum camber. The last were the most suitable from a practical point of view, but had the disadvan- tage of higher profile drag. Flight experiments were necessary, as stalling phenomena were sensitive to scale effect. A series of flight tests might enable a satisfactory tip design to be found. The present position as regards design precautions against flutter seemed satisfactory, but Mr. Relf was not sure how long it would remain so. Speeds were increasing rapidly, and since flutter could be broadly described as a state in which certain aerodynamic forces win in a fight with elastic forces, it could not be assumed without proof that the measures which suc- ceeded to-day would also succeed at the speeds of the future. At very high speeds mass-balancing of controls might not give sufficient precision Mr. Relf described a new technique in which the model is forced to oscillate at a. given frequency and with a given amplitude. The force is transmitted to the model through a short length of nickel tube, and by making use of the magneto- striction effect in nickel the variations of stress in the tube are converted into variations of electric potential, which can be recorded by a cathode-ray oscillograph. Thus, a time history of the force during a complete oscillation is obtained. Until flutter theory was firmly established the^ designer should continue to apply the present remedial measures, and should keep an eye open for any innovations in design which tended to increase the number of degrees of freedom in any possible mode of flutter There was great attraction in decreas- ing the numbef~o"f degrees of freedom, and one'development was the adoption of irreversible controls. Such a control, applied to the aileron, reduced the wing flutter to the simple case of flexure-torsional flutter, with only two degrees of freedom. The mechanism must, however, be perfect and free from backlash. Whether the same conclusion would apply to irreversible control of rudder and elevators was not yet known. A really good mechanical device might go far towards the simplification of design against flutter dangers. /' •• ff BIRTH of a BEARING How Aircraft and Other Ball and Roller Bearings are Made A RECENT mention of ball bearing production amongsome uninitiated people revealed that the basis of theirknowledge of the subject was a vague idea that moltenmetal was allowed to drip from the top of a tower on the lead shot principle ! A visit to the Birmingham works of British Timken, Ltd., demonstrates very quickly what a highly specialised work the production of ball and roller bearings really is. The firm is specially noted for its tapered roller bearings, which have a number of applications in aircraft. Small rings have several operations carried out on them by automatics, while larger jobs, up to 15m. diameter, are dealt with by No 5 Herberts. The largest horizontal lathe is the No. 20 Herbert, and for larger jobs still boring mills are used. Following machining operations the inner and outer rings are hardened and, to assure concentricity, are " jig-quenched," that is, quenched while clamped in true shape to prevent dis- tortion. Grinding and finishing come after degreasing. Outside sur- faces of small and medium rings are fed through centreless grinders, and, for the inside, Heald machines are used; in the case of parallel bores the machine gauges as it grinds. Many of the smaller rings are hopper-fed to the grinding machine, through which, in some cases, they are passed three or four times. For final grinding, rubber-bonded grinding wheels are used. During operations the parts are viewed at frequent intervals foi size and finish. In certain cases the inspection actually costs more than does the production of the part. Rollers of large sizes are forged hot from suitable billets, and smaller balls and rollers are cold headed. It is interesting that although the size of bearings made by British Timken varies from 55in. (a special order for a gun barrel boring lathe) to less than one inch, the method of production of the balls and rollers is practically the same whether large or small. Finished rollers are graded by ingenious American-designed machines Depending upon the depth of entry of each roller into a grading collet, an electric contact is made, causing a trap and guiding flap to open across the conveyor belt carry- ing the rollers. The rollers are in this way graded to a limit of 0.00025m. The finishing of steel balls—a process at which British Timken claim to be unsurpassed in the world—is simple in theory. The balls must first be annealed to relieve stresses set up in pressing them from wire into a rough sphere. Rough grinding is carried out with the balls held loosely in a circular groove with the grinding wheel under spring pressure turning horizontally below. For final grinding the balls are lubricated with paraffin while turning between vertical stones cut with a large number of concentric grooves of semi-circular sections. The final finishing and polishing operation is barrelling. This gives the well-known bright finish and accuracy to o.00005m. Essential parts to several types of bearing which have not so fai been mentioned are the cages for locating balls or rollers. These are produced by a series of neat and rapid operations trom strip, blanked, pressed, and then notched in pocketing machines, and finally having the edges ra/liused. For several of the bearings the inner and outer rings and cages are given a rust-resisting finish. S.B.A.C. Scholarship Vacancies A PPLICAT1ON forms from boys wishing to apply for this **• year's S.B.A.C. scholarships must be in the hands of the Royal Aeronautical Society by the end of this month. Last year five scholarships were awarded, and the number is unlikely to be less this year. The scheme, sponsored by the S.B.A.C., provides a four years' apprenticeship in an air- craft or engine works, facilities for attendance at technical classes, experience in the drawing office and research depart- ments, and a working wage. Applicants for the scholarships must be British subjects between 16 and 18 years of age and must have a school certifi- cate or its equivalent. Forms of application may be had from the secretary of the Royal Aeronautical Society, 7, Albemarle street, London, W.i. The Willingdon. Trophy T*HE Royal Aero Club has been informed by the Aero Club~ of India and Burma that the award of' the Willingdon Uiallenge Trophy will be made in May, 1938, and will be forwe year ending December 31, 1937. Jhe Trophy, presented by the Marquess of Willingdon, will•* awarded annually for the most meritorious performance Quring the year, carried out in any part of the world, by any."dian national or any person trained in India for six months n the calendar vear for which the award is made. Pilots should send in their applications to the Secretary, Aero Club of India and Burma, New Delhi, together with full details, and proof of six months' residence in India during the year. A Pobjoy—but Not from Rochester SOME confusion appears to have occurred during the lastfew weeks as a result of the fact that a new company which has recently commenced operations bears the title of "The Pobjoy Aviation Co." Actually, it has no connection with the old-established Rochester organisation of Pobjoy Airmotors and Aircraft, Ltd. It so happens that the founder of the new concern, which has premises at 100, Richmond Road, Dalston, London, E.8, also bears the somewhat uncommon name of Pobjoy— he is Mr. R. Pobjoy, but is no relative of Mr. D. R. Pobjoy. The London firm intends to market a light two-seater to appeal to clubs and private owners, and has made a sensible start by circularising clubs and others in order to find out what type of machine is most likely to appeal. The questionnaire enumerates a score of questions dealing with such things as seating arrangement, type of engine and horse-power, dimen- sions and performance instruments required, etc. Although the company have not yet co-related the replies, it seems likely that the type produced will be an inexpensive wooden low-wing side-by-side seater with an air-cooled engine of about 50 h.p.
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