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Aviation History
1919
1919 - 1601.PDF
DECEMBER II, 1919 Setting Out Pitch Angles These angles may be found (as shown in Fig. 1) by erecting a line equal in length to the circumference of the disc swept by the airscrew and another equal to the pitch. Ify sub dividing this peripheral line as shown, the angle at the cor responding points of the blade are found. The angle at any point of the blade can be found in this manner. The drawing should, of course, be made to some convenient scale. The angles may be checked by the simple apparatus shown in Fig. 2, cutting cardboard or tin templates to the angles obtained by the method shown in Fig. 1, and erecting them at that distance from the centre to which they vie in the airscrew blade. In the light of this information it is absurd to imagine that a carved screw (if made without resorting to this method) should be more efficient than a bentwood. It is these angles which determine the shape of the screw ; the varying cross sections of the block necessary to ensure the variation of the pitch angle deciding exactly the position of the curved edge in relation to the parallel edges of the block from which the screw is being carved. If the reader makes a comparative test by carving two screws of similar diameters from two blocks of equal dimensions (in one instance adopting the method outlines, and in the other, resorting to guesswork), and then tries them on a model, he will be astonished at the difference in the power required to achieve the same result. For example, it may take 1,000 turns on a 3 ft. 6-strand motor to obtain 60 sees, duration with the incorrect screw, whereas the correct one may only take 800 turns on a 5-skein motor of the same length. Expressed in a more certain way, one may take 30 inch ounces of torque, and the other only 25 inch ounces. Measuring Torque Torque may be computed, as shown in Fig. 3, by winding the screw and letting its tip rest on the centre of an accurate balance-pan. The pressure ;s balanced in the counterpoise pan by means of small weights ; and torque is the product of the screw radius and the weight which just balances the pressure from the screw—in the example shown, 30 inch ounces. The thrust itself is the product of the weight of the mass of air driven back in one second by the screw and the speed (in ft. per sec.) at which the mass is moved ; the speed, of course, is determined by multiplying the pitch by the revolutions per second of the screw. It is generally accepted that better results are obtainable from screws having large diameter and moderate pitch (about 1 i times the diameter) than with screws of short diameter and long pitch—this applying to both bentwood and carved screws. I do not consider it advisable to exceed EI H " British Airships " THERE are singularly few books in English on the subject of airships, and for that fact alone the volume by Major George Whale, late of the R.A.F., is welcome. In the opening chapter Major Whale summarises in simple language the chief principles of design and the methods adopted in operating airships. He then goes on to trace the history and develop ment of airships in England and on the Continent previous to and during the War, giving brief descriptions of the charac teristic types which were evolved from time to time and outlining the results obtained and some reasons which lead to the improvement of subsequent vessels. His book is therefore one for the general reader rather than for the technical student. At the same time it will doubtless be found useful by those who are considering the possibility of utilising airships in any way, for it shows what has been done and what is possible with the various types of motor- driven lighter-than-air craft. In the concluding chapter Major Whale, discussing the future of airships, gives some figures regarding the load capacity of some of the latest types of rigids, such as the L70, R33 and R38, and indicates what may be accomplished with airships of even larger displace ment. Major Whale thinks that a small airship of the S.S. Zero type would appeal to many sportsmen who have hitherto sought their amusement and recreation in yachting, motoring or ballooning. He also points out the possibilities of utilising airships for such purposes as exploring the unmapped regions of the Amazon, searching for pearl-oyster beds, sponges, coral, etc., locating icebergs in the Northern Atlantic, policing deserts such as those in Arabia and the Soudan. The book is illustrated by about a score of photographs of typical British airships, but we think that in future editions the selection of photographs could be added to with advantage, bearing in mind that there is no illustration in the book of the latest British rigids such as the R33 or R34. The book is published by Mr. John Lane, and the price is 7s. 6d. net. WSR 45° pitch angle, when the pitch exactly equals the circum ference of the circle described by the screw ; the latter com mences- to lose efficiency once this angle is exceeded. In this latter respect the airscrew bears out the " critical-angle " theory of aerofoils which fixes a set angle of incidence, for a given section, at which the aerofoil will give a maximum lift. To be continued.) Bentwood Screws With regard to bentwood screws, I have obtained the best results with screws made to the proportions given by Fig. 4. Some ot the types much in vogue of late are shown in Fig. 5 ; it is not thought that much is to be gained by imparting freak shapes to the contour of the blades. The type given by a is my own type, and, of b and c, b is the most efficient. A pair of 12-ins. screws recently made by the writer (and with, which the duration mentioned in the previous issue was made) weighed 7T5? ozs. the pair ! Competitions With regard to my note last week, I wish to arrange a com petition (the rules and prizes for which to be announced subsequently) in which provincial aeromodellists may compete. I should be glad to hear from any interested provincial readers, as well as from the London clubs. The only difficulty seems to be that of authenticating the flights, but doubtless this could be got over. I should like readers to indicate their views with regard to the rules of the competition, in order that some working basis may be decided on. Illinois Model Aero Club The results recently to hand of the above club portends some surprising results next spring from America. W, D. Pease created a record for America of 133 sees, in the com petition for the Laura Weaver Trophy, which is awarded each year to the member putting up the best average per formance. It cannot be said that America previously encouraged model aviation, and it is interesting to note the progress the numerous clubs now formed there. Many of them possess members drawn from the English clubs, probably the most enthusiastic being Mr. W. P. Dean, previously of the Manchester club, and connected with model aviation even prior to the time of BISriot's Channel crossing in 1939. Replies to Correspondents H. S. and H. B. (Leicester).—Many thanks for your con joint letter. We replied direct to your queries. Why not form a club in Leicester yourselves ? The model you mention did within a second or so the same performance in four con- se ;utive flights in damp weather ; no sun, and a wind ! Hardly a matter of luck ? H H •' Cattewater Old Boys " Ex-OFFICERS and men who were stationed at the Sea plane Station, Cattewater, Plymouth, during the War, at a meeting at the Holborn Town Hall, on December 3, decided to form a club, to be called the " Cattewater Old Boys." Capt. A. T. Sketchley was elected President, and Mr. Bosher, Secretary, while the Committee consists of Messrs. Kalber, Murphy, Reid, May, Ogle, Mamstree, Balch and Courtneidge. The first dinner will be held in London at the end of January. All members who were at Cattewater during the War are asked to send their names and addresses to Mr. Bosher, 33, Langdon Road, Junction Road, Highgate, N. Experiments in Hovering Flight CAPT. HAMERSLEY, M.C., has been making experiments at Hamble in hovering flight. He took up the Avro " Baby " in a wind of about 40 m.p.h., faced the wind, and throttled down. By nicely calculating the respective Strengths of thrust and drift, he succeeded in remaining in the same spot over the aerodrome for about five minutes. The spectators say that it was a very curious sight. At Amsterdam he had allowed the Baby to be blown backwards, and landed on the aerodrome without turning ; but he wanted to see whether it was possible to actually hover over a spot for five minutes, and he succeeded. Air Raid and Rent AN action arising out of an air raid on March 8, 1918, was heard at the Bloomsbury County Court on Monday. A bomb fell on a flats building at Hampstead, and as a result of the damage the tenants had to go elsewhere. £13 was claimed for rent from one of them, Mrs. Steele. m Judge Bray told her that she might have an action for damages, and he advised her to consult a solicitor. There was no answer to the claim for rent, and she was ordered to pay £1 a month.
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