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Aviation History
1941
1941 - 0967.PDF
APRIL 24TH, 1941. 303 ^ AN "ELECTRIC FOUR" New Addition to the Rotol Range WITH the increase in power andperformance of modern aero-engines and aeroplanes the variable-pitch airscrew manufacturer has had to face many problems in design. One of his difficulties has been to provide effective blade area and so maintain efficiency at these higher powers. Up to now three glades have been adequate for all purposes, but with limitations of blade diameter, additional blades have had to be considered, and a fourth blade is regarded as a necessity for the higher-power engines now coming along. (The reason why blade diameter cannot be increased indefinitely is that tip speed is thereby increased, and efficiency of the airscrew decreases suddenly as tip speed approaches the speed of sound. Another reason why large diameter is undesirable is that it de- creases ground clearance or forces the undercarriage to be made taller.) . . One of the problems of the airscrew designer has been to produce a variable-pitch hub with more than three blades, the size and weight of which would not be exces- sive. Recently Rotol Airscrews announced a four-bladed hydraulically-controlled variable-pitch airscrew, the hub diameter and weight of which were actually less than the , equivalent three-bladed 35 deg. variable-pitch airscrew which is fitted to the latest Spitfires and Hurricanes. Now The blade angle ischanged by a battery- driven electric motorthrough a double- epicyclic reduction gearto a master bevel gear which meshes with abevel gear on each blade. \ the company's latest development is a iour-bladedelectrically-controlled variable-pitch airscrew. The advan- tages of this type of airscrew were fully set out in anarticle, "Variable-pitch Airscrews," on page I24d of our issue of August 15th, 1940, and the mechanical designof the Rotol electric three-blader, which is the same as that of the four-blader, was described on page 168 of ourAugust 29th issue. Both types have a booster for quick feathering. This raises the voltage so that the operationtime for turning the blades edge-on is reduced to a few seconds. De-icing fluid is conducted to the leading edgeof each blade by the " trough " system which can be seen in the illustration. , BOOK REVIEW "Jane's All The World's Airctaji, 1940," by C. G, Grey andLeonard Bridgman. Sampson Low, £3 35. FOR the thirtieth time, Jane's All The World's Aircraftmakes its annual appearance. This very useful book is up to the standard of past years with its complete catalogue ofeverything that flies and the engines that drive them as well. Its sections cover the historical side of military and civil avia-tion as well as the aeroplane and engine sections. A last pathetic section, for such is the present state-of airship work,P^Tf only one page. It will never be larger until peace comes once more. The photographs, drawings and silhouettes main-tain the book's high standard. Whether this will be the last issue which Mr. Bridgman willbe associated with we do not know, but it is believed that he had resigned before publication. Even if Mr. Grey's namewere not on the cover, those who are familiar with his writings would unfailingly recognise his touch. Mr. Grey will have itthat Russia is the U.R.S.S. and not the U.S.S.R., while it seems to us that the point is not worth bothering about in atechnical work such as this. However, if everything is to be pedantically correct we wonder why, a few lines farther on.Airacobra is spelled wrongly as " Aerocobra." The abbrevia- tion " J.A.T.B.," the last letter apparently a misprint, ratherjarred when found to stand for " Joint Air Training Plan," the Grey name for the Empire Air Tiaining Scheme, or, as it issometimes called, the British Commonwealth Air Training Pi.in. Mr. Grey's comment abcut U.S.A. " supporting the warfinancially to the last Englishman " got him excellent publicity l^'t will not cause any bigger ripple in international affairst!.;in his views on the ethnology of the Balkan Slavs which he ffiubl, it seems, display at all costs.Good features in the book are maps of the air relationship °- the parts of the British Commonwealth, the air routes in^ S.A., a table of comparative speeds at the same altitude of fe-itish, German and American fighters, and a table of German'• 'ilary aircraft and their characteristics. Accepting Mr. ^';i y's invitation, we point out a few small erre-rs which caught our eye: " exiotarce " ior "existence" in the preface;"Douglas DB-19" for " B-rg" on page 187c; and "twin- engined Excalibur " ior " tour-eagmed " on page 202c. Forthose who require a reference catalogue for aircraft, the book is essential; it has no competitors. Might we, however, pleadwith the publishers of this three-guinea book for a thumb index; it would facilitate reference very greatly. COMPOUND STRESS DIAGRAM (Continued from page h) same scale as that to which the other two sides have beendrawn. In the diagram the horizontal radius of the circle is thebase of one such triangle and its length to scale represents —. The perpendicular forms the second side and q is 2measured off on this in terms of p. The hypotenuse of this triangle, i.e.; a line from this height q to the centre of the circle gives us the solution of our equation in terms ol p. Hence to get the max. shear -stress it isnow only necessary to multiply the length of the hypo- tenuse by the applied direct stress p. The max. direct stress — — + 2 ——\- q2 is greater than the above by —, i.e., by the radius of the basic circle. - 2 yHence this is given by measuring the hypotenuse extended beyond the centre of the basic circle up to its circumfer- ence. This length is also in terms of p. Hence the max. direct stress is this length multiplied by p.
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