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Aviation History
1958
1958 - 0079.PDF
FLIGHT, 17 January 1958 81 strated during field operation. It has generally been standard practice to rebalance and high speed a compressor rotor when a set of blades in any stage has been replaced. Military personnel have been telling engine manufacturers for years of the desirability of accomplishing compressor- blade replacement in the field. We have discovered that in addition to replacing compressor rotor blades in the factory test cells, we can change up to three complete stages in the field, with no detrimental effects on either performance or vibration. First-stage blades and blades in the rear stages have been replaced without removing the engine from its mount. This feature enabled us to accomplish a great deal more field service on the T58 than was possible with older engines. Major improvements were made in combustor durability between the 50 and 150 hour tests. Early liners showed the results of inadequate temperature distribution. Deterioration was gradual and resulted in general collapsing of the structure with resultant hot spots or blisters occurring on the combustor casing. Due to experience gained on develop- ment in other production engines, and the extremely conservative gas temperatures used in the T58, the design objective was to develop a liner whose service life would equal the allowable operating time of the engine between overhauls. The compressor diffuser and combustor liner were explored as a related single problem. Although it doesn't look greatly different from the previous liner, the improved pattern has seven major modifications. Positive distribution of primary and secondary air flow was achieved at the exit from the compressor. The cowl has been redesigned. The positioning of the liner in relation to the diffuser and combustor casing is more precise. The position, area and number of louvres and holes have been changed. Overall length and expansion provisions have been modified. The result is a liner insensitive to all but the most extreme cases of fuel-nozzle failures. One of these liners has now accumulated 330 hours of testing to the test cycle schedule, and shows no deterioration. Fuel controls and associated engine control parts h?ve historically been a major development problem. Helicopter rotor systems and drive shafts pose a far more severe problem to constant-speed control than does the propeller. The flexibility of rotor blades and their lag hinge attachments create a very elastic load, further aggravated by the backlash in the numerous gears of the transmission and reduction gears. The twist of the drive shafts increase the elasticity even more. The time constants of elastic rotor systems make it difficult for the engine control to sense the actual speed and load of the rotor blades. Computer studies coupled with rotor test-stand investigations helped us minimize the problems. The specific troubles of the first controls were that they had insufficient stability margin and did not permit consistent automatic engine starting. We are retrofitting our field engines with an improved fuel control. The improved control accumulated 1,250 hr of accelerated cyclic testing prior to initiation of the Model Test. A total of 15,000 hr bench testing was accumulated in the development programme; 15 controls were used prior to initiating the Model Test, and more than 4,000 engine hours were obtained. Some 635 hours were obtained using a new JP-4 fuel highly con- taminated with dirt, rust and lint which is frequently encountered in service. This fuel is much worse than any used previously in aircraft gas-turbine development. It quickly clogs filters and plugs the fuel nozzles. The dirt wears out bearings in the fuel pump and control and dogs metering orifices in the control. Nevertheless, fuel tanks in ships, 10 The TS3 in the Bell XH-40 has excellent accessibility. tankers, aircraft carriers, etc, gradually accumulate such dirt. Since the majority of the fuel used in the world is transported by ship, aircraft engines must be capable of operating on the fuel without the necessity for filters. A total of 17 engines were used from the first engine tested in April 1955 to the Model Test engine. Total time at the initiation of the Model Test was in excess of 5,200 hr. The test was run using JP-4 fuel and the engine accumulated a total running time of 181 hours, running perfectly at the completion of the test. Performance deterioration over the test was 1.4 per cent on MIL power, 2.1 on MIL s.f.c, 0.24 on typical cruise power and 0.4 on the corresponding s.f.c.; throttle burst times, oil con- sumption and similar factors remained generally unchanged. The small deterioration indicates that wear and dirt affect performance roughly independent of engine size. Curves (on page 83) illustrate data obtained from rotor stand testing and from flight testing in the S-58 and H-21 helicopters. We were pleased with the extremely close correlation of data. Note particularly how quickly and positively each engine automatically changes its power out- put to maintain a rotor condition set previously by the pilot. Driving helicopter rotor systems, 700 hours of ground and flight opera- tion have been accumulated. The engine compressor has been tested at simulated altitudes in excess of 35,000ft. The engine has been flight tested at 5,000ft altitude; at airspeeds in excess of 160 m.p.h.; in 19 16 18
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