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Aviation History
1950
1950 - 0834.PDF
54* FLIGHT, 4 May ig50 THE LIFE of a TURBOPROP How Reliability is Achieved: Mr. R. N. Dorey's Lecture to the S.A.E.in New York v>AV at. Installed in the Vickers Vis- count, the Rolls-Royce Dart will be the first turboprop to go into airline service. THOUGH the Americans have not yet brought theturboprop type of power unit to a state of develop-ment comparable with that obtaining in this country, they are nevertheless, showing a great deal of curiosity. It was to be expected, therefore, that Mr. Dorey's paper at the S.A.E. Aeronautic Meeting in New York would arouse a good deal of interest, for it gave a comprehensive picture of the behaviour of the Rolls-Royce Dart under conditions of extended service. As manager of the Rolls-Royce power plant and flight development section, Mr. R. N. Dorey, O.B.E., B.Sc.Eng., F.R.Ae.S., has been closely associated with the develop- ment of the Dart and its installation in the Vickers Viscount. A slightly abbreviated version, of the first part of his paper follows; Part II will appear next week. As a member of the Rolls-Royce team of engineers (said Mr. Dorey) I have responsibility for, among other pro- jects, the development of the Dart turboprop engine. This paper is written around its past, present and future development. The views I express are entirely my own and do not necessarily represent the official view or policy of Rolls-Royce, Ltd. The Dart—designed by Mr. L. Howarth, whose turbine experience goes back to 1942—-is a two-stage centrifugal- compressor engine of 1,400 b.h.p. at sea level, weighing (complete with oil, oil tank, oil cooler, air intake, nose de- icing and starter) 993 lb. Its first serious application will be to the Vickers-Armstrong Viscount airliner. In accept- ing responsibility for the Viscount commitment we have, we believe, undertaken a task which has not previously been attempted in any other country, namely, to launch an engine for airline use without any previous military background. For an aircraft of any type certain economics of the engine are necessary to enable the airframe manufacturer to prepare figures showing the costs of operation, overhaul times and anticipated life. We have committed ourselves to a warranted time of 1,500 hours for the major parts and, depending on experience prior to actual airline use, we are thinking in terms of 500-hour overhaul periods. As we see the picture to-day the following is our antici- pated trouble list in order of magnitude : — (1) Flame tubes and nozzles. (2) Nozzle guide-vanes. (3) Turbine blades. (4) Bearings and air seals. (5) Gearing. (6) Air and oil joints. _ , (7) Making two engines alike. The Dart has over 6,000 hours of test-bed and hangar development-running behind it. The total flying time is nearly 3,000 hours, including nearly 2,00a -engine hours in the Viscount itself. Design was started in early 1945 and the first engine ran in the summer of 1946. The first complete type test (model test) was run in 1948 at a rating of 1,000 b.h.p., followed in September of the same year by another test at 1,250 b.h.p. Darts fitted to the Viscount have a full C. of A. and an overhaul life of 250 hours, this figure being decided with a yiew to obtaining experience. Four engines have just recently reached this figure. Others, at the 1,400 b.h.p. rating, are at present under development. [The author then dealt with his "trouble list" item by item, as follows, illustrating his points with slides.] (1) Flame Tubes.—The problem [as emphasized by slides showing flame, tubes after lengthy periods of service] is one of cooling the skin. The general effect of a hot portion between the head and skirt is cracking and buck- ling. We have found that steady running conditions similar to those of the test bench does far less harm to the tubes than the fluctuating conditions of flight. The essence of long life is to keep the skin temperature down, and in designing a tube to run at a lower metal temperature, several conflicting requirements have to be met. No increase in jet-pipe temperature must take place, no change to nozzle guide-vane temperature traverse— assuming this was correct beforehand—and no increase in pressure-drop. The skin-cooled tube appears to meet all these conditions but extended mechanical tests have still to be done. Tubes are at present manufactured in Nimonic 75 material and this is still the best we know. Tests are also proceeding with ceramic coatings on the inside of the tube. I am very optimistic that the use of ceramic as a protective reflecting layer may open up the possibility later of using a less expensive material while still further im- proving the life. My own objective in this connection is to eliminate the flame tube entirely and coat the air casing itself with an insulating and/or reflecting material. With afterburning the gas temperature is some 900 deg F (500 deg C) in excess of that inside the engine and combustion is controlled without any flame tube. Temperature traverse and skin cooling is. of course, a problem. Flame-tube life, is also affected by the type of fuel used and by burner design and spray cone angle. Our early attempts at endurance running on the Dart produced carbon deposit on the burners, the effect of which is to deflect the spray on to the skin, with catastrophic results. The original burner and the latest development after more than three times the running time are illustrated (Fig. 1). We are now satisfied that we have eliminated the burner
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