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Aviation History
1948
1948 - 0147.PDF
JANUARY 29TH, 1948 fhe Way of Achievement FLIGHT Derwents were modified by cropping the blower rotors, were fitted with airscrews, renamed Trents and were then installed in a Meteor. From this flying experience—circa September, 1945—one of the most complex inherent prob- lems of this form of power installation emerged. Still (at time of writing) the only concern in the world to have flown an aircraft powered solely with airscrew-turbines, Rolls- Royce were the first to experience the fundamental diffi- culty of engine-airscrew control and, so far as is known, are the only people to have solved that problem. When the airscrew and compressor are mechanically linked and are driven by a common turbine, the exact con- trol relationship between the fuel-supply lever, or throttle, and the speed-control lever governing r.p.m. through air- sfifi^w blade pitch, is extremely delicate. This may be appreciated by realization that the power absorption capa- city of the compressor, being a function of its r.p.m., is dependent upon the speed-control lever, whilst the power available is dependent upon the throttle. In the original instance, when separate controls of these quantities were given the pilot, if he did not move the throttle consistently with movement of his r.p.m. control, the power available might easily be inadequate. He then had the unpleasant experience of finding that, for example, instead of his aircraft accelerating smartly forward, it checked sharply as though brakes had been violently applied ; disconcerting, to say the least. Now, however, the Trent /Meteor and the Dart are fitted • with a single power-lever control which ensures that there is always an adequate increase in fuel injection to cover the power demanded of the turbine by the airscrew and compressor. Temperature-Selection of Tnrust Another related development of gas turbine control which has been devised at Hucknall can best be described as the equivalent of boost control in a conventional piston engine. This sense applies in that control of the jet thrust is vested ia temperature rather than in r.p.m. As is well known, power output of the piston-type aircraft engine is governed by a combination of r.p.m. and supercharge, the pilot being provided to this end with a boost cont^gl. and a speed control, the latter selecting engine r.p.m?'v through the medium of airscrew blade pitch. The new Rolls-Royce gas-turbine control is based 0$ temperature., in that, although in the final instance contrql is a matter of governing fuel supplied, a temperature^sensitHi* govern- ing element is introduced into the control linkage so that Inference for thrust output is a texigerature scale and me pure yiayea oy ooservers in test f/ying /s second on/y to that of the test f>i/ots whose/risks and discomforts they share. Here are W. fffey and J. S. ienn/son who, with F. Toy/or, J. Porr/sh, F. Wil- kinson, and C. Chappell make up the Hucknall observers team. Load and vibration effects have been rendered far easier of study by the modern use of resistance strain-gauges. The Dart is here $t)own ready for such testing as a complement to the flight trials. not a speed scale. The very fact that thrust control is, however, a matter of rate of fuel supplied for combustion means that the pilot's control of the engine does actually govern r.p.m., but only indirectly. One of the more recent investigations has been concerned with the idling speed of a jet engine. The American Navy make the general recommendation that the idling condi- tion should be a fixed percentage of the maximum thrust. For the Nene, which is built under licence in the U.S. by Pratt and Whitney, this percentage is equivalent to a speed of 4,000 r.p.m. The Rolls-Royce idling figure for this engine is 2,500 r.p.m., at which, of course, the thrust is markedly less. The most critical single factor governing idling speed is the efficiency of the combustion chamber, for this decides the minimum air/fuel ratio which will support combustion ; this, in turn, decides the minimum compressor speed. ; ^~ «' Burner-Pressure Control It must not be overlooked that an engine may well be required to idle at any height if the desired flight condi- tion demands it, and as with increase in altitude air/fuel ratio naturally drops, the tendency is for the burners to 4^0 out. Rolls-Royce have devoted much attention to this irritating matter and have developed a control whereby a minimum burner pressure is maintained irrespective of altitude; as already noted, no height limitation is imposed on any R.-R. gas turbine. What is, perhaps, equally important is that if, for some reason, the burners do extinguish, it is now possible to re-ignite at all heights up to 20,000ft. An enormous amount of work has been involved in these developments. Tied-up with the subject of burner pressures is that of fuel consumption, a matter which exercises a good deal of Hucknall attention. It has been calculated—but not by Rolls-Royce who, in any case, are not prepared to agree that it is in fact correct—that, although given 100 per cent combustion and turbine efficiency, the best possible specific fuel consumption can be no more than 0.65 lb/hr/lb thrust; this item is included here only on the grounds of its being germane to an exceedingly topical issue. A fundamental fact which Rolls-Royce have estab- lished is that the working life of a jet engine is dependent almost solely on working temperatures. Rotational speeds in reasonable excess of normal maxima are not of themselves critical, but the reduction in engine life per thermal degree in excess of the normal maximum temperature is inordinate. This is illustrated by the fact that, whereas a working temperature of 700 deg C corresponds to a blade life of 1,000 hours at 16,500 r.p.m., if the temperature is increased to 720 deg C the life period is reduced to 350 hours.
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