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Aviation History
1920
1920 - 0743.PDF
-,;.V» T»»*,T" " "' -'* 1020 MODEL AER6PL^N!;S BVFJCAMM < All communications to be addressed to the Model Editor. A stamp should be enclosed for a postal reply *" Model Firms at the Forthcoming Olympic Aero Show. IVxeSsrs. A. E. Jones, Ltd., 52 High Street, New Oxford Street, London, W.C. 2. (STAND 21).—As is doubtless patent to all our readers, Messrs. Jones were pioneers in establishing a model aeroplane accessory business in this country. A full range of their goods will be shown (a glance at their catalogue will show this to be a comprehensive one, and anticipates the most minute requirements of the model maker), including r.o.g. machines, both tractor and canard, built-up fuselage models (a reproduction of pne of which is A popular model biplane by Messrs. A. E. Jones, Ltd. here given), scale models, wood of all sections, wire, air- screws (both two and four bladed, in carved and bentwood). elastic, an interesting form of hollow spar with elastic enclosed, varnish, lubricant, wheels (disc and tangent spoke), gears and gear brackets, winders, soldering outfits, and a full range of text-books suitable for the beginner. One con- gratulates this firm on their enterprise in the world of model aeronautics, and upon the issue of such a complete catalogue which, fully indexed as it is, should prove an almost indis- pensable vade mecum to the modellist. A copy can be obtained for sixpence, which amount is allowed off the first purchase exceeding 25. 6d. The D.A.P. Engineering Co., 185-197, Replingham Road, Southfields, London, S.W. 18. (STAND 9).—This well- ,known firm, founded by Mr. D. A. Paveley in 1907, are showing A compressed air model by Mr. D. A. Paveley scale models of full-size aircraft, miniatures and larger sizes, reproduced in correct colours and markings, compressed air plants, compressed-air-driven models, rubber driven models, and propellers, accessories and all materials. A visit to this stand will prove of interest and help to our readers. An illustration is given of their compressed-air-driven tractor biplane. We referred in greater detail to this firm in our issue of June 3rd. Those Competition Cups I SHOULD be glad if those in possession of the cups formerly belonging to the now defunct K.M.A.A. would communicate with me without delay. I have made somewhat exhaustive enquiries regarding them, but up to the present they have proved unavailing. Perhaps if this catches the eye of Mr. Akehurst, he will communicate with me. It seems extraor- dinary that the whereabouts of a number of valuable cups, such as the " Wakefield," " Gamage," " Model Engineer," " Johnson," etc., cannot be readily ascertained. If there is no likelihood of a resuscitation of the K.M.A.A. a new body should be formed. Meanwhile the stimulus to model aviation which competitions for these cups would undoubtedly give is withheld, and the cup winners, if they still hold these cups, should return them to the Secretary of the R.A.C., Clifford Street, W. 1. As a sort of anticlimax, an offer comes from a reader who is willing to help swell a fund for a cup or other trophy for power-driven machines. " I am prepared to start the sub- scription list with £1 is. od., and others willing to add should let me know the amount they are prepared to give. It would then be possible to arrange some rules in conjunction with one or two ' power-driven ' modellers for an annual competition." Air-screws Further Considered THE pitch of the air-screw bears a definite relation to the speed of the model; for instance, if the speed of the model is 15 m.p.h. or 22 ft. per second, it would obviously be in- correct to fit an air-screw whose pitch multiplied by its revolutions per second fell short of • this distance. Now, the pitch of a screw is found by multiplying the tangent of the angle of the tip of the screw by the circumference of the disc swept by the propeller. If, therefore, the angle of the tip is 30 deg., by looking up a table of natural tangents (to be found in any good engineering pocket-book) we find that the corresponding tangent is .57 approximately. If the diameter of the air-screw is 12 ins., the circumference of the disc swept by it will be 3 X 12 — 37TS — 37-42- Multiplying this by the tangent, .57, we obtain 22.32 ins. as the pitch of the screw. Now, the speed of the model is 22 ft. per second. Therefore, dividing this by 22.32 ins. gives the revolutions per second of the screw in order to obtain the correct flying speed, but we must first make a deduction for " slip." Slip is expressed as a percentage of the theoretical pitch, and represents the amount of the actual pitch lost due to the yield- ing nature of the air. For instance, an air-screw with a theoretical pitch of 24 ins. may only give an effective pitch of 18 ins., wherefore there is 6 ins. slip — -£j — 25 per cent. It is usual to make about 30 per cent, deduction for slip to be on the safe side, hence the effective pitch is 70 per cent, of 22.32 = 15.62. Now, dividing 22 ft. ( = 264 ins.) by 15.62 thus, —£- = 16.9 = revolutions per second at which the screw must be driven. Thus, in order that the model may fly most efficiently, a screw with a theoretical pitch of 22.32 ins. revolving at 16.9 revolutions per second, or 101.4 revolutions per minute, must be fitted. If the slip does not amount to 30 per cent., the model may fly a trifle faster than 15 miles an hour; but it is not thought that any model air-screw has an efficiency of 70 per cent. Indeed, full-sized air-screws seldom equal this, so the speed, if anything, will be under this. Slip can only be determined by actual trial. For example, suppose in the case we have outlined above, the screw had been wound 500 times. Theoretically, therefore, the model would fly 500 x 22.32 ins. = 310 yds. If the model is adjusted to fly straight, and the actual line of flight be measured, it will be found to measure something short of 743
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