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Aviation History
1955
1955 - 0063.PDF
63 Dawe Type 614 sensitive valve voltmeter; note the linear scale. (Right) In this view of the control room of a Rolls-Royce turbojet test-house can be seen (on shelf at top left) a Dawe Type 613 valve voltmeter modified for routine vibration measurements on production engines. instrument itself and include the addition of integrating circuitsso that the valve voltmeter can act as a fault detector for unbalance in later stages of engine testing.For all tests, ordinary seismic vibration pick-ups are mounted at critical points on the engine. Such pick-ups supply a voltage tothe valve voltmeter which is directly proportional to the velocity of the vibrating surface of the engine. This is the reason for theintroduction of an integrating circuit, since displacement (s) equals J V dt or J § dt. By suitable calibration, which is referred tolater, the instrument scale can be marked directly in peak-to-peak or r.m.s. displacement in thousandths of an inch. For researchtesting, however, the amplification of the instrument is deliberately varied from test to test so as to obtain the maximumsensitivity of reading. For this reason, changes of reading for different conditions are invariably recorded in such work. Taking a typical research and development engine, such as theOlympus, measurements of change of vibration amplitude may well be taken, first, on the very-low-frequency vibration causedby turbulence in jet-pipes and by general slow movements in the engine mountings. Variation of the tuned circuits connected tothe voltmeter would then permit study of the vibrations at much higher frequencies caused by the out-of-balance of the turbineor compressor. For routine testing, further modifications are advisable, becausefor this type of work the instruments have a more restricted purpose and are used by less skilful personnel. All such instru- ments (at least one for every test bed) are calibrated directly indisplacement in thousandths of an inch, a built-in integrating circuit being necessary for this purpose. The amplifier gain isalso reduced slightly and the whole construction made sufficiently robust to ensure that it is very reliable for long periods under thesevere vibration encountered on test runs. For routine as well as for special vibration testing a numberof vibration pick-ups are distributed round the engine. Particu- larly important points on turbojets are on the turbine and com-pressor casings. Pick-ups at these points will give a good idea of what is happening to the rotors. The pick-ups are connected, in turn, to the modified Dawevalve voltmeter and the readings noted. From the results, a curve of rotor speed against vibration displacement is plotted atthe beginning and end of the test period. A comparison of these curves will show at once any deterioration during the test run.Another good test of the quality of the engine is ensured by checking that at no point in the test does the vibration exceed apredetermined level considered acceptable at the particular operating speed.The outstanding point about this set-up is that no zero adjust- ment of the meter is required; the operator has only to turn theselector switch and note the reading on the directly calibrated linear scale. Moreover, all the instruments described are basedon standard designs, so that manufacturing costs are minimized and it is possible to add to the installation at short notice. AUTO-NAV COMPUTERS FOR R.C.A.F. TWO new automatic navigation computers have been developedin Canada for the R.C.A.F. One, called the R Theta com- puter, was designed by W/C. J. G. Wright, D.F.C., and developedand produced by P.S.C. Applied Research, Ltd., of Toronto. The other, called Position and Homing Indicator, is a moreadvanced version of the first and was developed by Computing Devices of Canada, Ltd., of Ottawa. Both are independent ofradio and therefore cannot be jammed, and are light and com- pact. They will give the pilot of jet aircraft a course and distanceto reach a selected point when, in the first computer, the pilot sets a desired position on the instrument, and, in the second,when he selects one of a series of preset points by pushing a button. The R.C.A.F. has adopted the R Theta computer for serviceafter 200 hours of air testing in various types of aircraft, and has handed the design over to the U.S.A.F. for comparative evaluationwith a similar American device. Details have also been given to the R.A.F. WORDS ON WEATHER INTERNATIONALLY agreed changes in the surface andupper-air codes used in synoptic weather reports and aero- nautical forecasts came into effect on January 1st. These changes, the first major ones of this nature since six years ago, were accom- panied by some modifications in weather recording and the pre- paration of synoptic charts. The revised Handbook of Weather Messages, published by H.M.S.O. for the Air Ministry Meteorological Office, comprises a complete guide to the current approved practices. A new feature of this second edition is its loose-leaf layout, enabling periodic amendments to be added.Part one, in course of preparation, gives details of meteoro- logical reports, forecasts, warnings, etc., issued by radio in theUnited Kingdom and from overseas. Part two, recently pub- lished at 4s 6d, lists the various codes and specifications used in compiling weather messages, and part three (in the press) containsinstructions to observers on the use of the messages and codes. The whole publication is designed to serve as a concise referencebook for observers at synoptic reporting stations. The new codes and specifications relating to surface observa-tions (land and sea) are also reproduced as an appendix to the second edition of Instructions for the Preparation of WeatherMaps published by H.M.S.O. for the Meteorological Office at Is 9d. This describes how to decode and plot surface weathermessages, and is useful both for amateur meteorologists who receive the morse broadcasts from Dunstable and plot their ownweather charts and for pilots and navigators who have to read weather charts plotted in the conventional symbols. A copy of this new edition will be supplied as part of thepresent edition of Meteorology for Aviators, by R. C. Sutcliffe. A more specialized aspect of meteorology is dealt with inGeophysical Memoirs No. 93, Variation of Wind with Time and Distance, by C. S. Durst, B.A. (H.M.S.O. for the MeteorologicalOffice, 4s 6d). The author discusses the variation of upper wind with time and place, as calculated from the radio and radar windascents made in recent years. The treatment is by vector statistics. FOR PRODUCTION EFFICIENCY "Material Handling in Works Stores (The Fork Truck andPallet System)," by L. J. Hoefkens. Published for "Mechanical Handling" by Iliffe and Sons, Ltd., Dorset House, Stamford Street,London, S.E.I. Illustrated. Price 18s (postage 6d). OF particular interest to the managements of all but the smallestfactories, this is the second edition of a book dealing with modern stores systems, with particular reference to the fork-lifttruck and pallet technique. The emphasis is on the use of these systems in the medium and light engineering industries, and theauthor, who is stores controller in a large group of engineering companies, describes in practical terms the results of introducingthe new techniques to a particular plant; he traces the progress of the products from the raw material stage to the finished article.
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