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Aviation History
1949
1949 - 1129.PDF
714 FLIGHT JUNE IOTH, 1949 JOINT CONFERENCE Two Further Important Papers : Experience with the Turboprop and the Outlook for Flying Boats N his paper entitled " The Propeller Gas Turbine in Service" Mr.-*- Owner started by saying that it had to be freely confessed that muchmore experience must be gained before the turboprop could occupy its rightfulniche in its appropriate field; never- theless, a certain amount of " takingstock" of achievement seemed justifi- able. Without question, the mainadvantage of the turboprop lay in the improvement in propulsive efficiencyfrom take-off up to moderately high speeds which it afforded over the jetengine. This had greatly simplified its development, flight testing andearly operational use, and furthermore it could be fitted without serious re-design to a comparatively large number of existing aircraft. These considerations led to the selection of the Lincoln heavy bomber for the initial flight testing of the Theseus, on the grounds that aircraft and spares were readily available and the structure was adequate for the power and speed expected. The initial flight develop- ment testing was so satisfactory that, at the instigation of Air Marshal Sir Alec Coryton, C.S. (A), two further conversions were put in hand substantially to the same specification, so that they could be operated under Service transport conditions by R.A.F. Transport Command. In relating experience with the Theseus, Mr. Owner devoted attention to a brief description of the engine and then went on to consider bench testing. Most of the bench testing that had been done so far was of a development nature. It was found in the early days of the Theseus that there was a tendency for performance to vary considerably between engines built to the same specification, and it became the practice to carry out a performance curve immediately prior to what- ever test was scheduled. This not only enabled performance to be checked against the required standard, but also in time established the identity of the variables affecting per- formance; the most critical factor was found to be variation in the turbine stator throat areas. The lecturer postulated that the technique of bench testing should avoid devoting engine running time to the develop- ment of components which could be tested separately. In the case of the Theseus, this policy had proved so fruitful THE first paper presented by a British1 delegate et the joint I.Ae.S.-R.Ae.S. Conference in New York was " The Propeller Gas Turbine in Service," read by the author, Mr. F. M. Owner, M.Sc, F.R.Ae.S., M.S.A.E., chief engineer (engines), Bristol Aeroplane Co., Ltd. In last week's issue of Flight there appeared a condensed resume of this and other British papers which were pre- sented. Here, and on the succeeding pages the papers delivered by Mr. Owner and by Mr. Keith-Lucas, B.A., A.M.I.Mech.E., F.R.Ae.S. (chief designer, Short Bros, and Harland, Ltd.), respec- tively are abstracted in considerably more detailed form. 13O Na Va12-d— TO Ha COMPRESSOR RPM. AIRCRAFT TRUE AIR SPEED (m.|>.h.) AMBENT STATIC TEMP °C»bs AIRSCREW HP AIRSCRE Pa AMBIENT STATIC PRESSURE (H/sq in) Tt-O- ZCi that relatively little development workon bearings, combustion chambers and so forth needed to be carried out on theengine. Due to the nature of the power/r.p.m. curve associated with turbo- props, in which the s.h.p. increasedapproximately as the fifth power of the r.p.m. in the region of full throttle,it was essential that speed measurement instruments should be sensitive.accurate and reliable. For performance calibration, an oscilloscope had beenadapted for the purpose of measuring r.p.m. This consisted of a tuning forkvibrating at a constant frequency of 1,000 c.p.s., this frequency beingmixed with one phase of the three- phase tachometer on a circular timebase. Due to the turbulent nature of the airstream entering a turboprop, it was almost impossibly diffi-cult to measure mass flow with any degree of accuracy. It had been found quite recently, however, that the most accuraterecord of air mass-flow was that obtained from a pitot-static rake situated at the outlet from the jet nozzle. Temperature and Thrust Regarding the measurement of temperature, Mr. Owner con- fined himself to the observation that it was still extremely difficult to assess the true mean turbine inlet temperature, outlet temperature and metal temperature, and that much more work must be done on these problems before temperature readings could be interpreted with confidence and accuracy. As regards power measurement, although the power developed at the airscrew shaft could be obtained accurately enough, the magnitude of the residual jet thrust had to be obtained indirectly. At Bristol, this was estimated by means of a pitot-static rake at the jet nozzle outlet. The error in thrust measurement affected only a small percentage of total power so that resultant error on the total power was quite small, being of the order of i per cent of total ground-level power for a 10 per cent error in measurement of thrust. By December, 1946, i.e., some 17 months after the engine first ran, sufficient progress had been made to embark on a comprehensive programme of flight testing. Piston engines were retained in the inboard power plants as this permitted • economies in the provision of Theseus engines with an adequate safety margin during deliberate scheduled tests covering engine blow-out con- ditions. Mr. Owner went on to say that in choosing parameters for the flight test programme, instead7 r --- I of running the engine at fixed intervals of height,forward speed and engine speed, it was decided Va N' to run at fixed intervals of height, ._ ... and ,_ ..,- (Ta)M: (Ta)'A (where Va is the true aircraft speed, N the r.p.m., and Ta the ambient absolute temperature °C).Va The three values of -:^, .„• chosen were 10, 15 and 20 m.p.h., and these covered a large field within the stalling and maximum speed of the aircraft at all heights. By this method, it was thus possible to derive the non-dimensional groups of the various observed quantities and plot a series of graphs for various values of N , Va . ,. • 1 and ,lr. „,. A specimen non-dim ensionai Fig. I. Typical non-dimensional grid plot for airscrew horsepower. () ()plot for horse-power is shown in Fig. 1. From this, and similar curves, the performance of theengine at any height, forward speed and r.p.m. in I.C.A.N. conditions could be derived by substitu-tion and extrapolation, as illustrated in Fig. 2. The full calibration of the engine up to20,000ft at 300 m.p.h, at five height intervals took less than 15 hours of scheduled flying, T 18
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